Selective catalytic reduction of NO over supported silver catalysts—practical and mechanistic aspects

Abstract

Selective catalytic reduction of NO by hydrocarbons (HC-SCR) is one of the promising technologies for removal of NO in exhausts containing excess oxygen, such as diesel and lean burn gasoline engines. Supported Ag catalysts, especially Ag/Al₂O₃, are thought to be the promising candidates for use in diesel exhausts, as confirmed by several reports on engine bench tests. The HC-SCR performance of supported Ag catalysts is very sensitive to the reaction conditions, especially the type of hydrocarbons and the addition of H₂. The control of reaction conditions would be key for practical use. The current research of supported Ag catalysts is reviewed from the viewpoints of practical use and the reaction mechanism, i.e., the reaction scheme, the role of surface adsorbed species, and the structure of active Ag species.

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1. Introduction

1.1 Automobiles as sources of NO_x emissions

Nitrogen oxides (NO_x: NO and NO₂) are major air pollutants that cause photochemical smog formation and acid rain. The emission of various nitrogen oxides into our atmosphere occurs on a massive scale. Worldwide, over 30 million tons of NO_x are vented to the earth’s atmosphere each year.¹ There has been a worldwide effort to discover improved solutions for the removal of NO_x emissions from stationary and mobile sources. Among sources of NO_x from automobiles, the demands for lean burn gasoline and diesel engines in vehicles are expected to increase because there are strong efforts towards reducing CO₂ emissions, according to the Kyoto Protocol to the United Nations Framework Convention on Climate Change.² The improvement of fuel economy and lower emissions of CO₂ is important in automobiles from now on. Fig. 1 shows the total CO₂ emissions from various types of automobiles relative to a gasoline engine.³ Among various engine technologies, a gasoline hybrid engine is the most effective technology on the market for the reduction of CO₂ emissions. Fuel cell engine cars do not exhaust CO₂ itself, although significant amounts of CO₂ are emitted from the transportation and production of H₂ from fossil fuels such as natural gas or petroleum. As a result, the total CO₂ emission is almost equivalent to gasoline hybrid engines. Diesel engines also show higher fuel economy than gasoline engines. Further reduction of CO₂ emission can be expected by the use of hybrid technology or bio-diesel fuels.

Fig. 1 CO₂ emission from various vehicles relative to gasoline engine cars. (Reproduced with permission from ref. 3)

Purification of diesel exhaust, i.e., reduction of particulate materials (PM) and NO_x, is the essential problem to be solved for energy efficient diesel engines. For gasoline engines, effective de-NO_x technology is already established by using a three-way catalyst, which is composed of supported precious metals including Pt, Rh, and Pd.^4,5 On the three-way catalysts, NO is reduced to N₂ by unburned hydrocarbons (HC), CO, and H₂, and HC and CO are oxidized to CO₂ simultaneously. The recent development of a CeO₂–ZrO₂ support significantly advanced the performance of the three-way catalysts.⁶ Since the air : fuel ratio is higher in lean burn gasoline engines and diesel engines, O₂ concentration is around 10% in exhausts. Although the target reaction is the reduction of NO by HC, undesired consumption of HC by combustion with excess O₂ occurs. To control these competitive reactions, new catalysis technology based on novel catalysts is desired.

1.2 Development of NO_x reduction technology for oxygen-rich exhaust

There are several candidates for de-NO_x technology for oxygen-rich exhausts.^5,7 For automobile diesel engines, the following three technologies are developing and coming onto the market: diesel particulate and NO_x reduction system (DPNR), selective catalytic reduction by urea (Urea-SCR) and selective catalytic reduction by hydrocarbons (HC-SCR). Toyota Motor Co. Ltd has already developed DPNR technology for simultaneous reduction of both NO_x and PM from diesel exhausts.^8–10 This technology is based on the NO_x storage reduction (NSR) catalyst system for lean burn gasoline engines.^11,12 The NSR system is performed with an engine that operates alternately under lean or rich conditions and is characterized as a time-resolved HC-SCR over an evolved three-way catalyst. The catalyst contains NO_x storage materials, such as BaO, which store NO_x as nitrates during lean conditions. The stored NO_x is released from the storage materials and reduced over noble metals by hydrocarbons and CO when turning the engine to rich conditions for a short period. In the DPNR system, stored nitrates also oxidize deposited PMs trapped on the wall of ceramic filter. This technology has been applied to commercial passenger cars from the end of 2003.

Urea-SCR is thought to be effective for large-scale diesel engine cars such as tracks and buses.^13–16 Daimler-Chrysler and Nissan Diesel Motor Co., Ltd. have adopted the Urea-SCR system for commercial tracks.^17,18 Daimler-Chrysler has announced that the Mercedes-Benz E 320 with the urea-SCR system named “BLUETEC” will be launched in the fall of 2006 in the US. The use of NH₃ is very effective to reduce NO_x to N₂ by using zeolite-based catalysts or vanadium-based catalysts, but it would not be possible to transport cylinders of NH₃ on automobiles. Therefore, aqueous solutions of urea ((NH₂)₂CO) are used as selective reduction agents. In the presence of H₂O, urea is hydroxylated to NH₃ and CO₂ according to the following equation

(NH₂)₂CO + H₂O → 2NH₃ + CO₂. (1)

Aqueous solutions of urea are non-toxic, non-corrosive and odor-free, can be relatively concentrated and therefore may well provide a safe and easy to handle reductant usable on-board for the removal of NO_x from diesel engines. The effective reduction of NO at low temperatures is the main benefit of this technique, however, it still has some problems which are yet to be resolved, such as the supply of urea solution, an appropriate injection of urea solution, and so on.

Selective catalytic reduction of NO by hydrocarbons (HC-SCR) in excess oxygen is also thought to be a potential method for removing NO_x from lean burn and diesel exhausts. In 1990, Iwamoto et al.¹⁹ and Held et al.²⁰ discovered that NO_x reduction under excess oxygen could be accomplished using hydrocarbons as the reducing agents over Cu–MFI, i.e., Cu ion-exchanged MFI-type zeolite. The use of unburned hydrocarbons in exhausts as the reductant is the main advantage of the HC-SCR system. Moreover, the HC-SCR system proceeds even in the presence of excess oxygen and the presence of oxygen is indispensable, which is a distinguishing characteristic of this system. Thus, this technology has the possibility of overcoming the disadvantages of the present reduction system, i.e., the three-way catalytic system.

Since the discovery of HC-SCR technology, numerous screening runs have been carried out and various types of HC-SCR catalysts have been reported, such as ion-exchanged zeolites, supported precious metals, and metal oxide-based catalysts, containing various active components such as Cu, Co, Fe, Ag, Pt, Pd, In, Ga, and Sn.^5,21–25 Ion-exchanged zeolites, especially Cu–MFI, show very high performance on the HC-SCR. From a practical point of view, however, they have some potential problems originating from the character of zeolites, i.e., instability in hydrothermal conditions that causes deactivation of catalysts. Supported precious metal catalysts, especially supported Pt catalysts, show high stability and high tolerance to sulfur oxides (SO_x) and water vapour. They show very high activity at lower temperatures, but the formation of N₂O and a narrow temperature range for NO_x reduction are problems still to be overcome. Metal oxide-based catalysts shows high stability and moderate tolerance to SO_x and water vapour. Amongst a number of catalysts reported, Ag/Al₂O₃ is thought to be the most promising candidate for practical use. Several reports on engine bench tests demonstrated the potential of Ag/Al₂O₃ in practical applications,^26–31 but low SCR activity at lower temperatures should be improved.

Despite the large number of reports on catalyst developments, the performance of the HC-SCR catalysts has not yet reached a practical level. This implies that there is a limit to the improvement in SCR performance through the design of the catalyst alone. Although most of the tests of HC-SCR catalysts have been carried out at fixed reaction conditions, it is possible to control the reaction conditions of practical automobile catalysts by controlling the engines. The NSR catalyst is an excellent example.^5,11,12 The NSR catalyst plays different roles under lean and rich conditions for the effective removal of NO_x. A drastic enhancement of the HC-SCR activity can be expected by using the HC-SCR catalysts under unsteady state reaction conditions or periodic operation. For the HC-SCR system, Iwamoto and co-workers proposed the intermediate addition of reductant (IAR) concept,^32,33 which is performed using hydrocarbons, such as fuel, injected into the exhaust gas. The injected hydrocarbons act as reducing agents for NO₂, which has been formed by the oxidation of NO in the exhaust. Since the concentration of unburned hydrocarbons in the diesel exhaust is relatively lower than in the case of gasoline engines, the IAR method is thought to be a preferable option for the HC-SCR system. The combination of a HC-SCR catalyst and plasma reactor is also effective for simultaneous reduction of NO_x and degradation of PMs.^34–43 However, the development of active and selective SCR catalysts is still the key technology for the development of these modified HC-SCR systems.

After Miyadera and Yoshida reported the high activity of SCR with ethanol over Ag/Al₂O₃,^44,45 supported Ag catalysts have attracted much attention. The higher hydrocarbons such as octane also act as effective reductants for HC-SCR over Ag/Al₂O₃.^46–49 In fact, good performances of Ag/Al₂O₃ catalysts have been confirmed by engine bench tests using secondary fuel injection and the use of ethanol as a reductant.^26–31 With regard to the catalyst design, the addition of metal additives, such as Pd and Rh, has been reported to be effective for the enhancement of the SCR activity of Ag/Al₂O₃.^50,51 Since real automobile exhausts contain water vapour and SO_x, the automobile catalysts should have tolerances to these atmospheres, which act as poisons to the catalysts. Ag/Al₂O₃ have been reported to have moderate tolerance to water vapour and SO₂.^{28–30,52–58} The low price of Ag is also attractive for practical uses. There have been various scientific investigations on the reaction mechanism and the effective species for HC-SCR. This review summarizes the practical and scientific research on the HC-SCR over supported Ag catalysts.

2. HC-SCR on supported Ag catalysts—strong dependence on reaction atmospheres

2.1 Oxygenated hydrocarbons as reductants

Ag-based SCR catalysts are known to be very sensitive to reaction conditions, especially reducing reagents for NO. In the early research on the HC-SCR catalysts, Ag-based catalysts were thought to be ineffective because Ag–zeolites show only poor activity to the selective reduction of NO by ethylene.²² On the other hand, Miyadera first discovered the high efficiency of Ag/Al₂O₃ catalysts in NO reduction to N₂ by oxygenated hydrocarbons, such as ethanol.^44,45,59,60 By using ethanol as a reductant, Ag/Al₂O₃ catalysts showed very high SCR performance, i.e., nearly 100% of NO conversion at around 623–723 K.^44,45 When 2-propanol, acetone, and ethanol were used as reductants for NO, the conversion of NO_x achieved above 80% at 523–673 K (SV = 6400 h⁻¹) even in the presence of 10% water vapour. The conversion of NO was significantly affected by the type of oxygenates, i.e., the conversion of NO was in the order of 2-propanol > acetone > ethanol > 1-propanol ≫ methanol at 350 °C, as shown in Fig. 2.

Fig. 2 Conversion of NO by oxygen-containing organic compounds over 2 wt% Ag/Al₂O₃ catalyst. Reaction conditions: 500 ppm NO, 1000 ppm (as CH_x) reductant (333 ppm), 10% O₂, 10% CO₂, N₂ balance, 10% H₂O, SV = 6400 h⁻¹. Symbols: (○) 1-propanol, (□) 2-propanol, (Δ) ethanol, (●) acetone, and (▲) methane. Reprinted from Appl. Catal., B, 2, T. Miyadera, Alumina-supported silver catalysts for the selective reduction of nitric oxide with propene and oxygen-containing organic compounds, pp. 199–205, © 1993, with permission from Elsevier.⁴⁵

The potential of ethanol-SCR over Ag/Al₂O₃ for practical use has been demonstrated through a diesel engine bench test.^30,31,61 He and Yu³¹ reported that the catalytic converter of Ag/Al₂O₃ showed an average NO_x conversion of greater than 80% in the temperature range of 300–400 °C. In the case of a space-velocity of 30 000 h⁻¹, the light-off temperature was 270 °C and the highest NO_x conversion reached 92.3% at 400 °C. Thomas et al.³⁰ tested SCR by various alcohols for the exhaust from 5.9 L diesel engine with 7.0 L Ag/Al₂O₃ catalyst system with a reductant injector. The light primary alcohols, such as ethanol, 1-propanol and 1-butanol, gave greater than 80% NO_x reduction in the temperature range of 300–450 °C and at SV = 50 000 h⁻¹. Recently, Kocat Co. Ltd demonstrated a practical de-NO_x system of ethanol-SCR over Ag/Al₂O₃.⁶¹ The critical improvement of the system has been made on the injection of ethanol reductant. The introduction of ethanol reductant into the catalyst bed and exhaust was performed by multiple installations and an injection manner to effectively remove NO_x, preventing ethanol from being oxidized midway in the catalyst bed.

2.2 Use of higher hydrocarbons as reductants

Although Ag/Al₂O₃ shows very high performance on ethanol-SCR, it is difficult to use ethanol on automotive SCR systems because of the inconvenience of attaching an extra tank for ethanol. The secondary fuel injection into exhaust gas, i.e., internal addition of reductant,^32,33 is thought to be an effective method for automotive diesel exhausts because an extra tank of reductants is not required and a lower concentration of unburned hydrocarbons than that of NO_x in diesel exhausts. When the secondary fuel injection into exhaust was employed, the catalysts are required to effectively use higher hydrocarbons as the reductant. Several researchers have focused on the effect of increasing carbon number ^47,62–64 or the use of higher hydrocarbons as the reductant ^{26,27,46–49,62–67} on various types of SCR catalysts, such as Co–ZSM-5,⁶² Pt-based catalysts,^63–66 Co-based catalysts,²⁶ and Cu/sulfated zirconia.⁶⁷ The systematic investigation on the effect of hydrocarbon reductants has been carried out by Burch and Millington by using Pt/Al₂O₃ and Pt/SiO₂ catalysts.⁶⁴ They reported that the SCR efficiency increases in the order i-paraffins < aromatics < n-paraffins < olefins = alcohols and generally activity increases with an increase in carbon number.

For Ag/Al₂O₃ catalysts, the use of higher hydrocarbons is preferable for both the activity and the water tolerance on HC-SCR.^5,47,68–70Fig. 3 shows the conversion of NO over 2wt% Ag/Al₂O₃ prepared by a sol-gel method in the presence of 2% water vapour when various linear alkanes from methane to n-octane are used as a reductant.⁴⁷ As the carbon number in alkanes increased, the temperature window of NO reduction shifted to lower temperature. Light-off temperatures for hydrocarbon conversion also shifted to lower temperatures with an increase in the carbon number. When n-octane was used as a reductant, sufficient SCR activity was achieved from 550 K. On the other hand, in the absence of water vapour, the n-hexane-SCR showed the higher activity at low temperatures than that of n-octane-SCR, as shown in Fig. 4. This is due to the suppression of the poisoning by deposited carboxylates and carbonate species by water vapour.⁴⁸ For SCR by n-octane and i-octane in the presence of water vapour, Ag/Al₂O₃ showed higher NO conversion to N₂ and selectivity than Pt/Al₂O₃ and Cu–ZSM-5. The nature of the Ag/Al₂O₃ catalyst is thought to be advantageous for HC-SCR of diesel exhaust with secondary diesel fuel injections before the catalyst bed.^28,29

Fig. 3 NO conversion to N₂ on an Ag/Al₂O₃ catalyst using various n-alkanes in the presence of water vapor. Conditions: NO = 0 or 1000 ppm, HC = 6000 ppm C, O₂ = 10%, H₂O = 2%, and W/F = 0.12 g s ml⁻¹ except for methane-SCR (W/F = 0.9 g s ml⁻¹). Reprinted from Appl. Catal., B, 25, K. Shimizu, A. Satsuma and T. Hattori, Catalytic performance of Ag–Al₂O₃ catalyst for the selective catalytic reduction of NO by higher hydrocarbons, pp. 239–247, © 2000, with permission from Elsevier.⁴⁷

Fig. 4 NO conversion to N₂ on Ag/Al₂O₃ catalyst using various n-alkanes in the absence of water vapour. Conditions: NO = 1000 ppm, HC = 6000 ppm C, O₂ = 10%, and W/F = 0.12 g s ml⁻¹ except for methane-SCR (W/F = 0.9 g s ml⁻¹).

The advantage of the HC-SCR by higher hydrocarbons can be attributed to the reactivity of hydrocarbons. Shibata et al.⁷⁰ reported that hydrocarbon structure had an influence on the HC-SCR activity: NO reaction rate increased with an increase in the carbon number in linear alkanes, and the rate in SCR by branched alkanes is lower than that by linear alkanes of the same carbon number. Fig. 5 shows the reaction rates of NO as a function of “mean bond energy” of the hydrocarbons, which is an average of all C–H and C–C bond energies in the hydrocarbons. Good correlations are observed on the HC-SCR by linear alkanes and branched alkanes over Ag/Al₂O₃, Ag–MFI, Cu/Al₂O₃ and Cu–MFI. It should be noted that the reaction rate was not dependent on the weakest C–H bond, which often contributes to the rate limiting step and controls the reaction rate in selective oxidation.⁷¹ The strong dependence of the HC-SCR activity on the “mean bond energy” means that the HC-SCR reaction takes place with multiple reaction steps concerting rate limiting. The slope of the correlation was greater in Ag/Al₂O₃ than in the other catalysts, indicating unique behaviour for Ag/Al₂O₃ which may be correlated to the changeable state of Ag, depending on reaction conditions. The mechanistic cause of these correlations can be rationalized by the reaction rate of the formation of surface oxygenated hydrocarbons, which has been revealed by in situ IR experiments, as reviewed in section 3.2.

Fig. 5 Reaction rate of NO in SCR by various alkanes over Cu and Ag catalysts as a function of mean bond energy of linear (solid symbols) and branched (open symbols) alkanes. Reprinted from Appl. Catal., B, 37, J. Shibata, K. Shimizu, A. Satsuma and T. Hattori, Influence of hydrocarbon structure on selective catalytic reduction of NO by hydrocarbons over Cu–Al₂O₃, pp. 197–204, © 2002, with permission from Elsevier.⁷⁰

2.3 Engine bench tests and tolerance to SO_x

Some research groups have investigated the possibility of using Ag/Al₂O₃ catalysts in practical applications for SCR by diesel engine bench tests. Nakatsuji et al.²⁷ have reported a maximum NO_x conversion of 75–45% over Ag/Al₂O₃ in engine bench tests using secondary fuel injection. Kikuchi and Kumagai also examined the use of diesel fuel (a cetane number of 57.9%, sulfur content of 0.005 wt%) as a reducing reagent of SCR over an Ag–Nb/Al₂O₃ catalyst.²⁶ The NO_x conversion of the Ag–Nb/Al₂O₃ catalyst was around 52% at 773 K, and kept above 50% after the reaction for 53.5 h. Eränen et al. demonstrated an engine bench test using a Volvo 2.5 L diesel engine with injection of Swedish MK1 diesel prior to the 2.5% Ag/Al₂O₃.^28,29 These reports have demonstrated the high feasibility of Ag/Al₂O₃ catalysts for practical applications. Aardahl et al.⁷² demonstrated a significant improvement in the SCR activity with Ag/Al₂O₃ coupled with a plasma fuel reformer in front of the catalytic reactor. In their system, a NO_x conversion of around 90% was achieved in the temperature range of 350–500 °C at space velocity of 22 000–28 000 h⁻¹. The promotion effect can be attributed to the on-site formation of oxygenated hydrocarbons in the plasma reactor, which was significant at lower temperatures below 350 °C.

Ag/Al₂O₃ is known to have moderate tolerance to water vapour and SO₂.^28,29,44 The sulfur tolerance of alumina-based catalysts strongly depends on various reaction conditions.⁷³ The deactivation is mainly caused by the formation of aluminium sulfate, however, the loss of SCR activity due to SO₂ is usually only limited to the initial deactivation to some extent and then the stable activity of alumina can be observed at low SO₂ level (<100 ppm).⁷⁴ Although some of the reports have indicated significant deactivation of Ag/Al₂O₃ in the presence of SO₂,^58,75,76 the tolerance to SO₂, which is strongly affected by Ag loading, reaction temperatures, reductants, additives, support materials, and so on, can be much improved by controlling the reaction atmosphere and appropriate catalyst design. Hickey et al.⁷⁶ indicated that the use of CeO₂–ZrO₂ supports confer improved sulfur resistance and activity at lower temperatures compared to alumina-based catalysts. Shimizu et al.⁷⁷ recently reported that SO₂ tolerance is improved by H₂ co-feeding into the C₃H₈-SCR reaction atmosphere as well as an increase in the Ag loading of Ag/Al₂O₃. The higher SO₂ tolerance is caused by the reaction of sulfates on Ag-containing sites with H₂, which results in the desorption of SO₂ to the gas phase or migration of the sulfates to the alumina surface. He and Yu³¹ confirmed the stable and sufficient activity in ethanol-SCR on the diesel engine bench test containing SO₂. Interestingly, a promotion effect of SO₂ on the HC-SCR activity of Ag/Al₂O₃ can be found in some cases. Angelidis et al.⁵⁶ reported that the addition of 25–100 ppm of SO₂ in the NO–C₃H₆–C₃H₈–O₂ reaction atmosphere at 480 °C results in an increase in the N₂ yield from 22 to 48–55% at the expense of the NO₂ yield and hydrocarbon conversions. They assumed that the promotion effect of SO₂ on the SCR activity is due to the following reasons; the increased acidity of the sulfated alumina support, the formation of R–SO_x species during the reaction, and some modifications of silver species in particle size or oxidation state.

2.4 Formation of by-products

One of the major drawbacks of Ag/Al₂O₃ catalysts is the formation of harmful by-products, i.e., CO, CH₃CHO, and nitrogen-containing molecules, such as NH₃, CH₃CN, and HCN. Due to the high toxicity of these by-products, several efforts have been made to suppress or decompose them. Miyadera made an attempt to remove by-products produced from ethanol-SCR over Ag/Al₂O₃, especially at lower temperatures below 350 °C. ⁶⁰ He found that a combination of other noble metal supported catalysts is effective. These harmful by-products were completely removed by a series of catalysts Ag/Al₂O₃ + CuSO₄/TiO₂ + Pt/SiO₂ placed separately in the reactor in this sequence.

Arve et al. carefully analysed the gas phase by-products in octane-SCR and toluene-SCR over Ag/Al₂O₃ by GC-MS and detected various types of by-products such as amines, ketones, and cracking products.⁷⁸ They also demonstrated that a cascade reactor consisting of Ag/Al₂O₃ and Cu–MFI is very effective for both the increase in NO conversion at lower temperatures and suppression of the formation of these by-products.

Eränen et al.⁷⁹ noted the production of a considerable amount of CO during the octane-SCR over 2 wt% Ag/Al₂O₃ catalysts, although the catalyst shows a maximum NO conversion of NO to N₂ of approximately 90% at 450 °C and an average NO conversion of 66% in the temperature range of 300–600 °C. Interestingly, the placement of a Pt-based commercial oxidation catalyst after an Ag/Al₂O₃ catalyst is very effective for the removal of CO from the exhaust by oxidation to CO₂, however, almost all activity in terms of NO conversion to N₂ was lost by the immediate connection of these catalysts. By extending the distance between the two catalyst beds, the NO conversion was significantly improved. They pointed out the importance of the residence time of the postcatalytic chamber and the contribution of the gas phase reaction after the Ag/Al₂O₃ catalyst initiated on the catalyst surface.

2.5 Modification of Ag/Al₂O₃ catalysts

There are not many reports on the modification of Ag/Al₂O₃ catalysts by additive elements. However, the addition of small amount of precious metals sometimes increases the HC-SCR activity of Ag/Al₂O₃. Since the insufficient SCR activity of Ag/Al₂O₃ stems from its low ability to activate hydrocarbons,^48,80 one of the roles of additives is the promotion of hydrocarbon activation. The addition of some precious metals, such as Pd and Rh, has been reported to be effective for the promotion of HC-SCR activity of Ag/Al₂O₃.^50,51 Sato et al. found that the addition of small amounts of Rh enhanced the activity of Ag/Al₂O₃ for the selective reduction of NO with decane at low temperatures.⁵⁰ Rh-promoted Ag/Al₂O₃ showed its rather high performance on decane-SCR even in the presence of 1–20 ppm of SO₂. Based on the catalytic activity for elementary reactions, it was suggested that the role of added Rh is to enhance the reaction between NO_x and decane-derived species, leading to NO reduction.

Modification of the support may also be effective for the promotion of HC-SCR activity. In the case of Ag–zeolite catalysts, the acid strength of zeolites plays an important role in controlling the SCR activity. As pointed out by Shibata et al., the acid sites of zeolites stabilize Ag⁺ ions and partially oxidized Ag species.^81–84 The strong support effect of zeolites is also reported by Seijger et al.⁸⁵ They found that NO_x conversion in propane-SCR of silver exchanged zeolites is ranked as Ag–Na-β > Ag–H-ferrierite, Ag–H-β, Ag–K-ferrierite > Ag–H-ZSM-5. Ag–Na-β showed 42% NO_x conversion at 300 °C. This may also be applicable to Ag/Al₂O₃. Shimizu et al.⁸⁶ recently investigated the effect of various additives (Zn, Mg, Y, P, and Na) to alumina supports of Ag/Al₂O₃ on C₃H₈–H₂-SCR. Adding Zn and Mg into the alumina support enhanced the conversion of NO to N₂, while adding Na and P significantly decreased the NO conversion. Comparing the rate of NO reduction and acid amount determined by NH₃-TPD, the contribution of surface acid sites of alumina supports to the promotion of the SCR activity of Ag was suggested. The acid–base property of supports should be the controlling factor for the HC-SCR activity of Ag/Al₂O₃.

2.6 Addition of H₂ into HC-SCR

The reaction conditions significantly affect the SCR activity of Ag/Al₂O₃. The most significant and surprising promotion effect can be obtained by the addition of a small amount of H₂. Satokawa, working at the Tokyo Gas Co. Ltd, first reported the promotion effect of H₂ on the HC-SCR activity of Ag/Al₂O₃ at lower temperatures.^87,88 As shown in Fig. 6, the addition of 909 ppm of H₂ into a C₃H₈-SCR atmosphere boosted the NO conversion at 673 K from nearly zero to ca. 50%. Interestingly, as shown in Fig. 7, the promotion effect of H₂ on NO and propane conversions was reversible depending on the presence of H₂ in the gas phase.

Fig. 6 NO_x conversion over Ag/Al₂O₃ at various H₂ concentrations. Feed: 91 ppm NO, 91 ppm C₃H₈, 9.1% O₂, 9.1% H₂O, and 0 ppm (●), 227 ppm (○), 451 ppm (Δ), 909 ppm (□), or 1818 ppm (◇) H₂ in He. GHSV = 44 000 h⁻¹. (Reproduced with permission from ref. 87).

Fig. 7 Effect of H₂ on the NO_x and C₃H₈ conversions at 673 K as a function of time over Ag/Al₂O₃. Feed: 91 ppm NO, 91 ppm C₃H_8, 9.1% O₂, 0 or 455 ppm H₂, and He as balance. GHSV: 44 000 h⁻¹. Reprinted from Appl. Catal., B, 42, S. Satokawa, J. Shibata, K. Shimizu, A. Satsuma and T. Hattori, Promotion effect of H₂ on the low temperature activity of the selective reduction of NO by light hydrocarbons over Ag/Al₂O₃, pp. 179–184, © 2003, with permission from Elsevier.⁸⁸

The promotion effect of H₂ was also observed in the SCR with saturated and unsaturated C1–C4 hydrocarbons.⁸⁷ The “hydrogen effect” was also observed over Ag–zeolite catalysts, such as Ag/MFI and Ag/BEA,^81–84 but it was not observed when TiO₂, ZrO₂, SiO₂, or Ga₂O₃ were used as supports (Table 1).⁸⁸ Interestingly, H₂ does not act as the reducing agent for NO,^88,89 but acts as a promoter for the NO reduction by hydrocarbons.⁸⁸ This remarkable “hydrogen effect” has been confirmed by Burch et al.,^90,91 Sazama et al.,^92,93 Arve et al.,⁹⁴ and Richter et al.⁹⁵

Table 1 Effect of H₂ addition on the NO reduction on various catalysts^ab

Catalyst	Temperature/K	NOx conversion to N2 (%)
		With H2	Without H2
a Reprinted from Appl. Catal., B, 42, S. Satokawa, J. Shibata, K. Shimizu, A. Satsuma and T. Hattori, Promotion effect of H2 on the low temperature activity of the selective reduction of NO by light hydrocarbons over Ag/Al2O3, pp. 179–184, © 2003, with permission from Elsevier.^88b Feed: 1000 ppm NO, 1000 ppm C3H8, 10% O2, 0 or 5000 ppm H2, and He as balance. Flow rate: 100 cm^3 min^−1, catalyst: 0.1 g.
Ag/Al2O3	573	36	0
Ag/TiO2	773	3	3
Ag/ZrO2	773	4	2
Ag/SiO2	773	0	0
Ag/Ga2O3	773	12	34
Co/Al2O3	623	10	12
Pt/Al2O3	598	6	8
Al2O3	773	35	15

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Since the temperature of diesel exhausts ranges from 473–623 K, a sufficiently high SCR activity at lower temperatures is required for HC-SCR catalysts. The presence of H₂ in the exhaust is a very attractive method for boosting the low temperature SCR activity. Burch et al. reported the best SCR performance of Ag/Al₂O₃ by the use of octane as a reductant of NO.^5,90,91Fig. 8 shows NO_x conversion of HC-SCR with various hydrocarbons over Ag/Al₂O₃ in the presence and absence of H₂.⁵ In the absence of H₂, the conversion of NO increased from 450 °C for octane-SCR and from 250 °C for butanol- and butanone-SCR. Surprisingly, in the presence of H₂, a sufficient NO conversion was observed from 200 °C for these SCR reactions. In particular, in the case of octane-SCR, the NO conversion started from 250 °C, a lower temperature than in the absence of H₂, and nearly 100% conversion of NO was achieved in the wide range of temperatures of 200–500 °C.

Fig. 8 Reduction of NO by various reductants for Ag/Al₂O₃ catalyst under lean burn conditions with and without the addition of H₂. (▲)1-butanol; (Δ) 1-butanol + H₂; (●) 2-butanone; (○) 2-butanone + H₂; (■) octane; (□) octane + H₂.⁵

Furthermore, the “hydrogen effect” is also applicable to NH₃-SCR over Ag/Al₂O₃. Richter et al. discovered that above 90% conversion of NO can be achieved in the range of 200–500 °C by the addition of 1 vol% of H₂ into NH₃-SCR of NO over 5% Ag/Al₂O_3.^96–98 The phenomena is very interesting and may be applied to the exhausts from small sized power plants, rubbish disposal, and so on.

The discovery of a “hydrogen effect” on supported Ag catalysts may provide a novel approach to the design of diesel SCR catalysts. From a scientific point of view, the roles of H₂ are still under debate, and various roles of H₂ addition have been proposed, i.e., the promotion of hydrocarbon activation to surface oxygenates,^80,92 formation of partially charged Ag_n^δ+ cluster,^81–84,99 contribution to gas phase radical reactions,¹⁰⁰ and generation of peroxide species.¹⁰¹ The details are presented in the section 3.3 and 3.5.

3. Mechanistic investigation of HC-SCR over Ag catalysts

3.1 Role of adsorbed NO_x on Ag/Al₂O₃

There has been much attention on the reaction mechanism of the HC-SCR and active silver species from scientific points of view. The mechanistic investigations have been carried out mainly by means of in situ and operando analysis, especially in situ FTIR. These are mainly focused on: (1) roles of adsorbed NO_x species, (2) roles of hydrocarbon-derived surface species in the initial stage of the reaction, and (3) intermediates in the final stage of the reaction.

For various types of SCR catalysts, it is generally accepted that gaseous or adsorbed NO₂ is the initial intermediate for the selective reduction of NO to N₂.^102–104 The key roles of nitrates, which are formed from adsorption of gaseous NO₂ onto the catalyst surface, have been pointed out in the case of alumina-based catalysts including Ag/Al₂O₃. We have clarified that the surface nitrates on alumina-based catalysts play a crucial role in the initial stage of the HC-SCR reaction on catalysts such as Al₂O₃ and Cu–Al₂O₃ by means of in situ FTIR spectra which are measured under real or pseudo-reaction conditions.^104–108 Kinetic analysis of the surface adsorbed species revealed that the selective reaction of NO to N₂ initially proceeds by the parallel reactions of the oxidation of NO to surface nitrates (1) and the oxidation of hydrocarbons to surface oxygenates (2), followed by the selective reaction between these species (3). The surface reaction between nitrates and oxygenates leads to the formation of NCO and CN species, and finally leads to the formation of N₂ through oxidation or hydrolysis of these nitrogen-containing species, as shown in Fig. 9.

Fig. 9 Proposed reaction scheme of the HC-SCR reaction on alumina-based catalysts.

From the in situ FTIR spectra, basically the same reaction pathways were proposed on HC-SCR over Ag/Al₂O₃.^48,49Fig. 10 (0 min) shows the IR spectra of adsorbed species on Ag/Al₂O₃ prepared by a sol-gel method after exposing the catalyst in a flow of NO + O₂ at 623 K. The bands assignable to unidentate nitrate (1554, 1292 cm⁻¹) and bidentate nitrate (1580, 1246 cm⁻¹) and a small band due to weakly adsorbed NO₂ (1624 cm⁻¹) are observed. Since the nitrates were not observed in a flow of NO, the formation of nitrates proceeds by NO oxidation to NO₂ and the subsequent adsorption of NO₂ on basic oxygen sites. Switching to a flow of n-hexane + O₂, the nitrate bands gradually disappeared while new bands appeared at around 1570–1580 and 1460 cm⁻¹ assigned to ν_as(COO) and ν_s(COO) of adsorbed acetate. In addition, shoulder bands assignable to formate (1378 and 1390 cm⁻¹) and carbonate (1630, 1410, 1300–1336 cm⁻¹) were also observed. Fig. 11 shows a time course of the nitrate concentration in a flow of He, n-hexane or n-hexane + O₂. The band intensity was not affected much by a flow of He, indicating thermal desorption of nitrates can be neglected in such a time scale. However, in a flow of n-hexane + O₂, the nitrates immediately disappeared within 12 min, indicating that the nitrates reacted with hexane in the presence of O₂. The reaction rate of the nitrates can be estimated using the slope of the curve in the figure. The reaction rate of nitrate in a flow of n-hexane + O₂ (357 nmol g⁻¹ s⁻¹) was 10 times higher than that of only n-hexane (34 nmol g⁻¹ s⁻¹).

Fig. 10 Dynamic changes in the IR spectra as a function of time in flowing n-hexane + O₂ on Ag/Al₂O₃ at 623 K. Before the measurements, the catalyst was pre-exposed to a flow of NO + O₂ for 120 min at 623 K. Reprinted from Appl. Catal., B, 30, K. Shimizu, J. Shibata, H. Yoshida and T. Hattori, Silver–alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism, pp. 151–162, © 2001, with permission from Elsevier.⁴⁹

Fig. 11 Time dependence of nitrate concentration in flowing He (open square), n-hexane (closed circle) and n-hexane + O₂ (open circle) on Ag/Al₂O₃. Before the measurements, the catalyst was pre-exposed to a flow of NO + O₂ for 120 min at 623 K. Reprinted from Appl. Catal., B, 30, K. Shimizu, J. Shibata, H. Yoshida and T. Hattori, Silver–alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism, pp. 151–162, © 2001, with permission from Elsevier.⁴⁹

These reaction rates of the surface species can be compared with the rates of gas phase products in n-hexane-SCR over Ag/Al₂O₃, as shown in Fig. 12. In a temperature range of 573–648 K, the initial rates of nitrate consumption were close to the steady state rates of NO reduction, and the apparent activation energy was 61 kJ mol⁻¹, which is comparable to the apparent activation energy for NO reduction (67 kJ mol⁻¹). The good agreement of these rates clearly indicates that N₂ is formed via surface nitrates as intermediates. It should be noted that the apparent activation energy for NO reduction increased to 156 kJ mol⁻¹ below 573 K. The apparent activation energy for n-hexane conversion to CO_x also increased below 573 K. The higher activation energy at lower temperature suggests self-poisoning of nitrate on Ag/Al₂O₃. Furusawa et al. reported that the ability of Ag/Al₂O₃ for the oxidation of NO without hydrocarbon reductants was rather lower than that of Ag–MFI.¹⁰⁹ They explained that the reaction is retarded on Ag/Al₂O₃ by the formation of nitrate species, which were produced by the reaction of NO₂ with Ag₂O phases. Iglesias-Juez et al. carefully investigated the reactivity of nitrates by in situ DRIFTS.¹¹⁰ They elucidated that type II bidentate nitrates (1305 and 1580–1590 cm⁻¹) on Ag/Al₂O₃ react with C-containing intermediates, forming the potential intermediate for HC-SCR.

Fig. 12 Arrhenius plots of the initial rate of NO₃⁻ consumption in n-hexane + O₂ (●), and the steady state rates of NO reduction to N₂(○) and n-hexane conversion to CO (Δ), for the NO + n-hexane + O₂ reaction over Ag/Al₂O₃. Conditions: NO = 1000 ppm, HC = 6000 ppm C, O₂ = 10%. Reprinted from Appl. Catal., B, 30, K. Shimizu, J. Shibata, H. Yoshida and T. Hattori, Silver–alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism, pp. 151–162, © 2001, with permission from Elsevier.⁴⁹

The crucial role of nitrate species is also clarified by other methods. By means of NO-TPD and NO-TPR spectra of Ag–MFI catalysts, Shi et al.¹¹¹ proposed that surface nitrate species are formed in NO/O₂ and could be effectively reduced by methane to N₂. The nitrate species were found to be more reactive to CH₄ than O₂, being preferentially and selectively reduced to N₂. Furthermore, from the temperature-programmed decomposition of AgNO₃, they attributed the high activity of Ag–MFI to the formation of strong adsorption centers for surface NO₃⁻ species which could be effectively reduced by activated CH_4.¹¹²

3.2 Role of oxygenated hydrocarbons

As for the hydrocarbon-derived species, a key role of the surface oxygenated hydrocarbons has been pointed out. Our research group has demonstrated that the surface adsorbed acetate species reacts with surface nitrate and leads to the formation of N₂ over alumina-based catalysts^105–108 including Ag/Al₂O₃.⁴⁹Fig. 13 shows the transient behaviour of IR spectra of surface adsorbed species after the NO + n-hexane + O₂ reaction followed by treatment in a flow of NO + O₂. During the NO + n-hexane + O₂ reaction, at 0 min in the figure, bands due to acetate (1460 cm⁻¹), formate (1378 and 1390 cm⁻¹) and unidentate and bidentate nitrates (1292 and 1246 cm⁻¹, respectively) were observed. Shoulder bands are assigned to carbonyl (1720 cm⁻¹) and carbonate species (1630, 1410, 1300–1336 cm⁻¹). In the region of 2300–2000 cm⁻¹, the bands assigned to –NCO bound to Ag⁺ ions (2232 cm⁻¹) and –CN species (2162 and 2130 cm⁻¹) are observed. After the flowing gas was switched to NO + O₂, the bands due to acetate (1460 cm⁻¹) decreased, while nitrate bands progressively appeared. The result can be interpreted as showing that acetate is a reactive species toward nitrates formed by the NO + O₂ reaction. The reaction of nitrate with oxygenated hydrocarbon species, possibly acetate, is proposed to be a crucial step in the initial stage of HC-SCR reaction. Furthermore, the band due to Ag⁺–NCO and CN decreased in a flow of both NO + O₂ and O₂, indicating the reaction between these species. From the time course of these adspecies, the main reaction pathways of over Ag/Al₂O₃ catalyst can be depicted as shown in Fig. 9.

Fig. 13 Dynamic changes in the IR spectra as a function of time in a flow of NO + O₂ on Ag/Al₂O₃ at 623 K. Before measurement, the catalyst was pre-exposed to a flow of NO + n-hexane + O₂ for 120 min at 623 K. Reprinted from Appl. Catal., B, 30, K. Shimizu, J. Shibata, H. Yoshida and T. Hattori, Silver–alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism, pp. 151–162, © 2001, with permission from Elsevier.⁴⁹

The strong dependence of the rates of NO reduction on the type of hydrocarbon reductants can be rationalized based on the reactivity of the surface oxygenated species.⁴⁸Fig. 14 shows the dynamic changes in the acetate band formed by the reaction of gaseous hydrocarbons with surface pre-adsorbed nitrate. The response of acetate, i.e., the apparent rate of acetate formation, was in the order of n-hexane > propene > 2,2-dimethylbutane (2,2-DMB) > n-butane > propane. The activity pattern is same as in the reaction rate of the HC-SCR, i.e., the dependence of the rate of NO reduction on the hydrocarbon reductants was as follows: higher alkane (hexane) > lower alkane (propane), alkene (propene) > alkane (propane), linear alkane (hexane) > branched alkane (2,2-DMB). The proposed mechanism in Fig. 9 explains the hydrocarbon effect on the HC-SCR activity: the NO reduction activity depends on the reactivity of the hydrocarbons, which affects the rate of hydrocarbon oxidation to oxygenates and hence the rate of nitrate reaction with oxygenated hydrocarbons.

Fig. 14 Dynamic changes in the IR spectra as a function of time in flowing HC + O₂ on Ag/Al₂O₃ at 623 K. Conditions: HC = 6000 ppc C, O₂ = 10%. K. Shimizu, J. Shibata, A. Satsuma and T. Hattori, Phys. Chem. Chem. Phys., 2001, 3, 800. Reproduced by permission of the PCCP Ownership Board.⁴⁸

Recently, Yeom et al. identified the intermediates in ethanol-SCR over Ag/Al₂O₃ by means of FTIR of both gas phase and surface species.¹¹³ They observed the formation of ethyl nitrite in the gas phase as an initial intermediate for ethanol-SCR. Fig. 15 shows gas phase FTIR spectra taken after exposing Ag/Al₃O₃ to various mixed gases. The bands at 1745 and 1671 cm⁻¹ after exposure of Ag/Al₃O₃ to NO₂ + ethanol + O₂ were assigned to ethyl nitrite. The shift of the band at 1671 cm⁻¹ (Fig. 15b) to 1642 cm⁻¹ (Fig. 15c) by the feed of ¹⁵NO clearly elucidates the formation of N-containing molecules. They confirmed very fast decomposition of ethyl nitrite to acetaldehyde and then acetate from dynamic behaviour. As shown in Fig. 16c, the bands at 1606, 1687, and 1466 cm⁻¹ were observed under ethanol-SCR conditions. The band at 1687 and 1466 cm⁻¹ can be assigned to surface nitrate and acetate, respectively. From the comparison with the spectra of adsorbed nitromethane (Figs. 16f–16g) the shoulder band at 1606 cm⁻¹ under ethanol-SCR conditions (Fig. 16c) was attributed to surface nitromethane. From these detailed results and discussions, they proposed the following reaction scheme for ethanol-SCR

ethanol → ethyl nitrite → acetaldehyde → acetate → nitromethane → NCO →→ N₂. (2)

They successfully demonstrated the detailed consecutive reaction pathway to N₂ formation with reasonable identification of the adsorbed and gas phase intermediates using isotopes. Since NO₂ is essential for the formation of ethyl nitrite and nitromethane, this reaction scheme is in accordance with the fact that the oxidation of hydrocarbons to acetate is faster in the presence of gaseous NO₂ or adsorbed NO₃⁻ than in O₂. The behaviour of HC-SCR reactions can be rationalized with this multi-step reaction mechanism including oxygenated and N-containing hydrocarbons.

Fig. 15 Gas phase FTIR spectra: (a) 2 Torr of ethyl nitrite at 25 °C, (b) spectrum of the gas phase over Ag/γ-Al₂O₃ after exposure of Ag/γ-Al₂O₃ to 2 Torr NO₂ + 3.8 Torr 2-¹³C-ethanol + 60 Torr O₂, (c) spectrum of the gas phase over Ag/γ-Al₂O₃ after exposure of Ag/γ-Al₂O₃ to 2 Torr ¹⁵NO + 3.8 Torr ethanol + 60 Torr O₂. Prior to admission of the premixture of 3.8 Torr ethanol + 60 Torr O₂, the Ag/γ-Al₂O₃ was pre-exposed to 3.8 Torr of NO₂ (or ¹⁵NO), © 2006, with permission from Elsevier.¹¹³

Fig. 16 IR spectra of Ag/γ-Al₂O₃ exposed to: (a) acetic acid, (b) ethanol + O₂, (c) ethanol + NO₂ + O₂, (d) 2-¹³C ethanol + NO₂ + O₂, (e) ethanol + O₂ + ¹⁵NO₂, (f) nitromethane and (g) ¹³C-nitromethane. In each case, the cell was evacuated after 1 min of exposure to the indicated sample. The acetic acid pressure was 0.8 Torr, ethanol partial pressure was 3.8 Torr, NO₂ partial pressure was 3.8 Torr, O₂ partial pressure was 60 Torr, and nitromethane pressure was 2 Torr. The temperature was 200 °C for traces (a)–(e) and 37 °C for traces (f) and (g), © 2006, with permission from Elsevier.¹¹³

The key role of surface oxygenated hydrocarbons as intermediates for HC-SCR is generally accepted, but the structure of oxygenated hydrocarbons is still a subject of debate, i.e., other molecules are proposed to be possible key species. Iglesias-Juez et al.¹¹⁰ also emphasized the importance of C₃H₆ activation in C₃H₆-SCR over Ag/Al₂O₃, which involves generation of surface carboxylate-type species as a partially oxidized intermediate. In their in situ DRIFTS experiment of Ag/Al₂O₃ under stepwise treatment by NO, O₂, and propene, they observed a group of bands at 1642–1575, 1454–1458, 1378–1392, and 1296–1298 cm⁻¹. They assigned the bands to surface acrylate and postulated that it was of importance in catalytic performance on C₃H₆-SCR over Ag/Al₂O₃.

In the case of ethanol-SCR reaction, He et al. proposed that surface enolic species were more reactive adspecies than acetate.^31,51,114Fig. 17 shows their in situ DRIFTS spectra of adsorbed species on Ag/Al₂O₃ during the partial oxidation of EtOH. A series of bands assignable to enolic species were observed at 1633, 1416 and 1336 cm⁻¹ together with the bands due to acetate (ν_as(OCO) at 1579 cm⁻¹ and ν_s(OCO) at 1466 cm⁻¹). They tentatively assigned the band at 1633 cm⁻¹ to ν_as(CH₂=CH–O⁻), that at 1416 cm⁻¹ to ν_s(CH₂=CH–O⁻), and that at 1336 cm⁻¹ to C–H deformation mode. As shown in Fig. 18, a sharp decrease in the bands due to enolic species was observed in a flow of NO + O₂ on Ag/Al₂O₃ after the exposure to C₂H₅OH + O₂. The band assignable to enolic species sharply disappeared after 5 min with simultaneous formation of an NCO band. On the other hand, the band assignable to acetate at 1464 cm⁻¹ slowly decreased after 5 min, showing the lower reactivity of acetate than enolic species. From the dynamic changes of these bands in a flow of NO + O₂, they concluded that enolic species on Ag/Al₂O₃ are highly active towards NO₂ and NO₃⁻, resulting in the formation of the NCO species which is the key reaction intermediate in the SCR. Although the importance of partial oxidation for HC-SCR on Ag/Al₂O₃ is commonly indicated in these papers, different types of hydrocarbon-derived intermediates have been proposed. The difference in the assignment of the hydrocarbon-derived intermediates may come from the difference in the reaction conditions, such as the concentration and properties of reductant, catalyst preparation and pretreatment, acidic property of support, and so on.

Fig. 17 In situ DRIFTS spectra of adsorbed species in the steady states on 5 wt% Ag/Al₃O₃ at different temperatures in a flow of C₂H₅OH + O₂. Conditions: C₂H₅OH 1565 ppm, O₂ 10%. Reprinted from Appl. Catal., B, 49, Y. Yu, H. He, Q. Feng, H. Gao and X. Yang, Mechanism of the selective catalytic reduction of NO_x by C₂H₅OH over Ag/Al₂O₃, pp. 159–171, © 2004, with permission from Elsevier.¹¹⁴

Fig. 18 Time dependence of the integrated area of the bands in a flow of NO + O₂ at 673 K. Before the measurement, the catalysts was pre-exposed to a flow of C₂H₅OH + O₂ for 60 min at 673 K. Conditions: NO 800 ppm, C₂H₅OH 1565 ppm, O₂ 10%. Reprinted from Appl. Catal., B, 49, Y. Yu, H. He, Q. Feng, H. Gao and X. Yang, Mechanism of the selective catalytic reduction of NO_x by C₂H₅OH over Ag/Al₂O₃, pp. 159–171, © 2004, with permission from Elsevier.¹¹⁴

In the latter part of the HC-SCR reaction, nitrogen-containing species are thought to play a key role in the formation of N₂. As already shown in Fig. 13, the bands assignable to cyanide (–CN) and isocyanate (–NCO) can be observed on Ag/Al₂O₃ under the HC-SCR reaction conditions. Kameoka et al. studied the reactivity of surface isocyanate species with NO, O₂ and NO + O₂ by pulse reaction of adsorbed NCO species with gaseous O₂ or NO + O₂.¹¹⁵ After the formation of adsorbed NCO species by exposing Ag/Al₂O₃ or Al₂O₃ under ethanol-SCR conditions, they injected several pulses of the oxidants. The adsorbed NCO species on Ag/Al₂O₃ reacted with O₂ or NO + O₂ gas to produce N₂ effectively at 250 °C. The reactivity of NCO species with the reactant gases was in good agreement with the changes in NCO bands measured by in situ DRIFTS. The results clearly indicate that NCO species is the important intermediate in HC-SCR for the formation of N₂. In the case of Al₂O₃ alone, less N₂ was detected in the reaction with NO + O₂, indicating that Ag plays an important role in the formation of N₂ from NCO.

Bion et al. demonstrated the dynamics of the surface –CN and –NCO species on Ag/Al₂O₃ under ethanol-SCR by coupling of in situ DRIFT spectroscopy and mass spectrometry.¹¹⁶ In the in situ FTIR spectra, just after the injection of ethanol onto Ag/Al₂O₃, the characteristic bands of cyanide (–CN) increase in intensity, followed by isocyanates (–NCO). They assigned these bands to silver cyanide (Ag–CN) and its transformation into Al–NCO, respectively. Fig. 19 shows the dynamic changes in the amount of adsorbed NCO species and the gas phase CO₂ and NH₃ produced during the introduction of H₂O pulses on Ag/Al₂O₃. The behaviour of these species indicates hydrolysis of isocyanate with water vapour into CO₂ and NH₃. Then, the NH₃ thus formed gives rise to N₂ through the reaction with NO in excess oxygen.

Fig. 19 Dynamic changes of NCO consumption and CO₂ and NH₃ gas species produced during the introduction of H₂O pulses on Ag/Al₂O₃ previously treated under CO + NO at 873 K. Reprinted from J. Catal., 217, N. Bion, J. Saussey, M. Haneda and M. Daturi, Study by in situ FTIR spectroscopy of the SCR of NO_x by ethanol on Ag/Al₂O₃—evidence of the role of isocyanate species, pp. 47–58, © 2003, with permission from Elsevier.¹¹⁶

The important roles of gas phase reactions are also proposed.^99,117,118 Li and Villani emphasized the importance of ammonia formation during NO_x trap operation.¹¹⁷ Arve et al. proposed a combination of heterogeneous and homogeneous (radical) reactions over HC-SCR over Ag/Al₂O₃.¹⁰⁰ Radicals of low molecular weight were detected in the gas phase. Cyanogen isocyanate, NC–NCO, was detected and suggested to be a key intermediate in the formation of amines and ammonia via the hydrolysis of isocyanate species. Since these gaseous intermediates significantly influences the HC-SCR performance depending on the reactor design, further clarification of the role of the gaseous active species should be necessary.

3.3 Reaction mechanism of the H₂ HC-SCR reaction

The boosting of the HC-SCR activity in the presence of H₂ is interpreted as an acceleration of a specific reaction pathway. Satokawa et al. observed the significant promotion of the oxidation reactions of NO to NO₂ and propane to CO_x in the presence of H₂.⁸⁸ The reduction of NO₂ by propane, however, was not promoted by the addition of hydrogen. From these results, the promotion of SCR activity by hydrogen was attributed mainly to the promotion of activation of hydrocarbons.

Burch et al.⁹⁰ and Sazama et al.⁹² also observed the promotion of the activation of octane or decane at lower temperatures. They ruled out the contribution of the enhancement of the oxidation NO to NO₂ to the increased rate of the SCR reaction. From transient kinetics and in situ DRIFTS measurements, Burch et al. ⁹⁰ indicated that hydrogen has a direct role in the reaction mechanism by either promoting the formation and storage of an organic C=N species which can then readily reduce NO_x and/or removing a species which acts as a poison to the SCR reaction at low temperatures. Sazama et al.⁹² showed that the reaction steps most enhanced by the addition of hydrogen to the SCR-NO_x reaction are the transformations of the intermediate CN species into NCO and oxidation of the hydrocarbon to formates or acrylates.

Shibata et al. clearly demonstrated that the “hydrogen effect” is mainly attributed to the promotion of oxidation of hydrocarbons to surface oxygenates.⁸⁰ The addition of H₂ significantly affected the reaction rates of the surface adsorbed species. Fig. 20 shows the time dependence of the surface concentrations of nitrates and acetate on Ag/Al₂O₃ during the reaction of surface nitrates with C₃H₈ + O₂ in the absence and presence of H₂. From the slope of the transient responses, both the consumption rate of nitrates and the accumulation rate of acetate were greatly enhanced by the addition of H₂. The initial rates of accumulation and consumption of surface acetate and nitrates were summarized in Table 2 showing corresponding reaction pathways in Fig. 9. The initial rate of nitrate consumption (run 4) was 89 nmol g⁻¹ s⁻¹ in the presence of H₂, while it was below 0.1 nmol g⁻¹ s⁻¹ in the absence of H₂. It should be noted that these initial rates are measured at the surface nitrate concentration of 340 μmol g⁻¹ (time = 0 min in the figure), which is higher than that under the steady state (280 μmol g⁻¹) in a flow of NO + C₃H₈ + O₂ + H₂. From the estimation of the transient curve at 10 min, the consumption rate of surface nitrates can be determined as 75 nmol g⁻¹ s⁻¹, which corresponds well to the consumption rate of gaseous NO (run 4′, 68 nmol g⁻¹ s⁻¹) in the steady state. The comparison of the surface reaction rates (Table 2) should reflect the “hydrogen effects” observed in the gas phase NO conversion. The formation of acetate by C₃H₈ oxidation (step 2 and 2′) and nitrates by NO oxidation (step 1) were remarkably promoted by the addition of H₂. The acceleration of step 3 by the addition of H₂ is caused by increases in the concentration terms of adsorbed species, which is due to the promotion of step 1 and 2′. Since the NO oxidation to nitrates (step 1) is excluded from the rate-determining step in the C₃H₈-SCR, the promotion of NO oxidation to nitrates by the addition of H₂ does not contribute to the promotion of C₃H₈-SCR. The partial oxidation of C₃H₈ to mainly acetate at step 2′ is remarkably promoted by H₂. This promotion leads to the increase in the production rate of N₂via the acceleration of step 3 through an increase in acetate concentration. The appearance of the acetate band by the addition of H₂ during the C₃H₈-SCR is explained by the remarkable promotion of C₃H₈ oxidation to acetate. From these kinetic analyses, the promotion effect of the addition of H₂ on the C₃H₈-SCR is attributed to the remarkable promotion of partial oxidation of C₃H₈ to mainly surface acetate.

Fig. 20 Time dependence of (○, ●) nitrate and (□, ■) acetate concentration in (○, □) C₃H₈ + O₂ and (●,■) C₃H₈ + O₂ + H₂ on Ag/Al₂O₃ at 473 K. Before the measurement, the catalyst was pre-exposed to a flow of NO + O₂ for 3 h at 473 K. J. Shibata, K. Shimizu, S. Satokawa, A. Satsuma and T. Hattori, Phys. Chem. Chem. Phys., 2003, 5, 2154. Reproduced by permission of the PCCP Ownership Board.⁸⁰

Table 2 Rates of the steady state and transient reaction at 473 K over Ag/Al₂O₃^a

Run	Gas phase adspecies	Observed change	Step	Rate/nmol g^−1 s^−1
				Without H2	With H2
a J. Shibata, K. Shimizu, S. Satokawa, A. Satsuma and T. Hattori, Phys. Chem. Chem. Phys., 2003, 5, 2154 – Reproduced by permission of the PCCP Ownership Board.^80b C1 basis.c Estimated from slopes at 10 min in Fig. 20.d Not measured.
1	NO, O2	Nitrates	1	66	560
	None	Accumulation
2	C3H8, O2	Acetate	2	<0.1^b	19^b
	None	Accumulation
3	NO, O2	Acetate consumption	−1, −3	17^b	350^b
	Acetate	Nitrates accumulation	1, −3	28	630
4	C3H8, O2	Nitrates consumption	−2′, −3	<0.1	89
	Nitrates	Acetate accumulation	2′, −3	<0.1^b	40^b
4′	C3H8, O2	Nitrates consumption	−2′, −3	—^d	75
	Nitrates^c	Acetate accumulation	2′, −3	—^d	35^b
5	NO, C3H8, O2	NO consumption		<0.1	68
	Flow reaction	C3H8 consumption		<0.5^b	180^b
5′	NO, O2, H2	NO consumption			<0.1
	Flow reaction

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3.4 Active Ag species

The importance of the state of Ag species is widely recognized. So far, the HC-SCR activity has been correlated to the specific types of Ag species, such as nano-Ag particles,¹¹¹ Ag clusters, ^{50,81–84,99,124} nano-sized Ag₂O clusters,^119,120 the presence of both Ag⁺ and Ag particles,^111,121,122 and a β silver aluminate-like phase.^110,123 The proposed active species on supported Ag catalysts are summarized in Table 3 together with the reaction conditions.

Table 3 Proposed active Ag species and reaction conditions

Active Ag Species	Temp/K	Hydrocarbon	Ref.
HC-SCR reaction
Both Ag^+ and metallic Ag	523–673	C3H6	122
Ag2O cluster	573–773	C3H6	119, 120
Ag^+ compound	600–750	C3H6, C2H5OH	45
Silver aluminate	600–750	C3H6	110, 123
Ag^+ in AgCl	600–773	C3H6, C4H8	68
Ag^+ and Ag nano particle	673–773	CH4	111, 121
Agn^+ cluster	573–673	C10H22	50, 124
H2-HC-SCR reaction
Agn^+ cluster	573–723	C3H8	81–84, 99
Ag^+ or small Ag2O	473–673	C10H22	92
Chemical effect	473–623	C8H18	125
Ag–hydride/peroxide	473–673	C10H22	101

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Due to the strong dependence of catalytic performance on Ag loading, Miyadera et al.⁴⁵ pointed out that the state of the Ag species is the important factor for the HC-SCR activity of Ag/Al₂O₃. Usually, the highest HC-SCR activity can be observed at Ag loadings of 2–4 wt%. Since these catalysts are white and show no diffraction lines attributable to Ag-derived species in XRD patterns (Ag₂O or Ag metal), well-dispersed Ag species are thought to be the active species for the HC-SCR. This idea was also supported by the effect of pre-treatment of Ag/Al₂O₃.¹²⁶ Ag/Al₂O₃ after oxidation in 10% O₂ at 873 K showed higher ethanol-SCR and propene-SCR activity than after reduction in H₂ at 573 K with metallic Ag particles. From the characterizations by SEM, XRD, and XPS, they concluded that oxidized Ag species interacting with the alumina support were the active phase for the HC-SCR reactions.

Furusawa et al.^109,119 examined propylene-SCR in the presence of 5% oxygen over Ag supported on Al₂O₃, H–MFI and H–Y. Several characterizations revealed that silver was present in the form of Ag₂O clusters, but not in metallic form for these catalysts. Comparing the amounts and the size of the Ag₂O clusters, the difference in catalytic activity (Ag/Al₂O₃ > Ag–MFI > Ag–HY) was correlated to Ag₂O clusters.

Iglesias-Juez et al.¹¹⁰ suggested that Ag⁺ species in an alumina matrix are the active species for C₃H₈-SCR reaction, and correlated the high non-selective hydrocarbon oxidation activity to the formation of metallic Ag. The Ag K-edge XANES and EXAFS data revealed the presence of Ag⁺ ions in a β-AgAlO₂-like phase in 4.5 wt% Ag/Al₂O₃ that showed the highest activity on C₃H₈-SCR. They also emphasized the importance of the oxygen lability of the β-AgAlO₂-like phase for the formation of surface acrylate from the reduction–oxidation of Ag species.

On the other hand, the formation of Ag clusters is also correlated to the good performance of the SCR reaction in several cases. Shi et al. investigated the effect of silver loading on the structure of silver species as well as on the activity for SCR with methane over Ag/MFI.¹¹¹ They reported a sigmoid relationship between NO conversion and Ag loading; the NO conversion was increased slightly by the introduction of 3–5 wt% Ag to the zeolite supports but was greatly enhanced on the 7 and 9 wt% Ag/H–MFI catalysts. This result suggests that aggregated silver species are more effective sites for SCR than isolated Ag⁺ ions, and this was supported by the UV-Vis spectra which showed that the band assignable to nano-sized Ag clusters (410 nm) were observed on the higher Ag loading samples (≥7 wt%). Ag clusters are also identified to be the active species for the propane-SCR in the presence of H₂.^81–84

Sato et al. reported the strong correlation between the formation of Ag clusters by the addition of promoters and the improvements in the SCR activity.^50,124 As shown in Table 4, a considerable promoting effect by Rh addition was found on the catalytic performance of Ag/Al₂O₃ for decane-SCR of NO at low temperatures. On the other hand, the addition of Ir, Ru, Pd, and Pt decreased the SCR activity of Ag/Al₂O₃. Fig. 21 shows diffuse reflectance UV-Vis spectra of Ag/Al₂O₃ and promoted Ag/Al₂O₃ after subtracting the Al₂O₃ spectrum. Ag/Al₂O₃ showed broad absorption bands around 223 and 352 nm, while Ag/Rh/Al₂O₃ gave an additional broad band around 246 nm, and Ag/Ir/Al₂O₃ and Ag/Ru/Al₂O₃ had a broad band around 290 nm. Note that these bands are not attributed to the added Rh, Ir, and Ru. According to their assignments, the bands at 238–272 nm, 275–326 nm, and 330–385 nm are attributed to Ag_n^δ+ clusters, Ag_n1 small metallic clusters, and Ag_n2 large metallic clusters, respectively. The band at 223 nm is assigned to the electronic transition from 4d¹⁰ to 4d⁹5s¹ of highly dispersed Ag⁺ ions. It is clear from these spectra that Ag⁺ and Ag_n2 clusters are the major Ag species on Ag/Al₂O₃, the band characteristic of Ag_n^δ+ species is observed only on Ag/Rh/Al₂O₃, while metallic Ag clusters were the main species on Ag/Ir/Al₂O₃ and Ag/Ru/Al₂O₃. In situ DRIFT spectra revealed that the formation rate of surface –NCO species, which is known to be a reaction intermediate for the formation of N₂, was significantly increased by the addition of Rh to Ag/Al₂O₃. They concluded that the role of added Rh is to form Ag_n^δ+ clusters, which are more active for the formation of NCO species as a reaction intermediate.

Fig. 21 Diffuse reflectance UV-Vis spectra of (a) 4% Ag/Al₂O₃, (b) 4% Ag/0.05% Rh/Al₂O₃, (c) 4% Ag/0.05% Ir/Al₂O₃ and (d) 4% Ag/0.05% Ru/Al₂O₃. Reprinted from Appl. Catal., B, 44, K. Sato, T. Yoshinari, Y. Kintaichi, M. Haneda and H. Hamada, Remarkable promoting effect of rhodium on the catalytic performance of Ag/Al₂O₃ for the selective reduction of NO with decane, pp. 67–78, © 2003, with permission from Elsevier.⁵⁰

Table 4 Catalytic activity of Ag/Al₂O₃ and Ag/noble metal/Al₂O₃ for the selective reduction of NO with decane^ab

	NO to N2 conversion (%)
Catalyst	250/°C	300/°C	350/°C	400/°C	450/°C	500/°C	550/°C	600/°C
a Reprinted from Appl. Catal., B, 44, K. Sato, T. Yoshinari, Y. Kintaichi, M. Haneda and H. Hamada, Remarkable promoting effect of rhodium on the catalytic performance of Ag/Al2O3 for the selective reduction of NO with decane, pp. 67–78, © 2003, with permission from Elsevier.^50b Reaction conditions: 1000 ppm NO, 10% O2, 333 ppm n-C10H22, 10% H2O, He balance, W/F = 0.05 g s cm^−3.
4% Ag/Al2O3	13	30	22	12	10	15	13	12
4% Ag/0.05% Rh/Al2O3	22	47	40	29	12	6	4	3
4% Ag/0.05% Ir/Al2O3	6	3	5	10	6	4	3	3
4% Ag/0.05% Ru/Al2O3	27	10	6	3	2	2	4	3
4% Ag/0.05% Pd/Al2O3	1	4	4	3	3	2	2	4
4% Ag/0.05% Pt/Al2O3	2	4	4	3	1	1	0	0

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It can be seen that the proposed active Ag species summarized in Table 3 depends on the reaction conditions, especially on the type of reductant and the reaction temperatures. In the case of SCR by alkene, Ag⁺ ions or Ag⁺ containing compounds have been proposed as active species. In the case of the SCR with alkane, which is carried out at higher temperatures because of the lower reactivity of alkane than alkene, Ag clusters are mainly proposed as the active species. These correlations can be roughly rationalized by assuming (1) the activation of hydrocarbon reductants is the critical step for HC-SCR and (2) agglomerated Ag species such as Ag clusters are more active for hydrocarbon activation than dispersed Ag species such as Ag⁺ ion and silver aluminate. However, we should take high mobility of supported Ag species ^127–129 into account when determining the active Ag species. For example, the reversible agglomeration–dispersion behaviour of silica supported Ag species can be observed during CO oxidation. Qu et al.¹³⁰ investigated restructuring and re-dispersion of Ag on SiO₂ under the oxygen–hydrogen pre-treatment. Oxygen pre-treatment at 773 K results in the formation of subsurface oxygen and also activates the catalyst for CO oxidation. The subsequent He treatment at lower temperatures (373–573 K) causes surface faceting and re-dispersion of the surface Ag particles. Interestingly, the re-dispersion occurs without destroying the silver subsurface species. Prolonged H₂ treatment at above 573 K includes the aggregation of silver particles. The effect of the oxidation/reduction cycle is reversible. Similar phenomena are expected on supported Ag catalysts under the HC-SCR reaction conditions. Therefore, careful in situ characterizations should be necessary for the determination of active Ag species for the HC-SCR.

3.5 Active species in presence of hydrogen

The “hydrogen effect” on HC-SCR over supported Ag catalysts has been confirmed by several research groups, although the reason for the enhancement of the SCR activity is still unclear. Modification of surface Ag species by H₂ is one possible reason. Our research group has found that the promotion effect by H₂ on HC-SCR activity can be attributed to the formation of Ag clusters by means of a in situ UV-Vis spectrophotometer equipped with an in situ flow cell (JASCO UHR-630).⁹⁹Fig. 22 shows in situ UV-Vis spectra of 2 wt% Ag/Al₂O₃ when flowing various model feed gases at 523 K. In a flow of O₂, a band assignable to the 4d¹⁰ to 4d⁹s¹ transition band of the Ag⁺ ion ^50,131–136 was observed below 300 nm. In the presence of H₂, the spectra showed other broad bands at 300–400 nm, which can be assigned to Ag clusters including ionic Ag_n^δ+ clusters and small metallic Ag_n clusters.^50,137–141 In a flow of NO + O₂, these bands disappeared again and the bands due to Ag⁺ ions was observed, indicating Ag clusters reversibly dispersed in the oxidative atmosphere. The formation of Ag clusters is coincident with the C₃H₈-SCR activity shown in Fig. 7. The NO conversion was negligible in the absence of H₂, while the NO conversion was boosted up to 40% in the presence of H₂, and the SCR activity again became negligible after an evacuation of H₂. The reversible changes in the Ag species on alumina indicate that a possible reason for the promotion effect by H₂is the formation of Ag clusters. Interestingly, it was found from the time course of the band of Ag clusters at 306 nm that the reversible formation of Ag clusters is a very rapid process, as shown in Fig. 23. The formation of Ag clusters in H₂ + O₂ was complete within 30 s, while re-dispersion into isolated Ag⁺ ions in NO + O₂ takes approximately 1 min. The rates of formation and dispersion of Ag clusters are strongly dependent on the Ag content. Since Ag clusters and Ag metal particles are reported to be highly active for oxidation of hydrocarbons and NO,^{44,131,132,142} the transformation of Ag species can reasonably explain the changes in the activity in surface reaction steps. The reversible transformation of Ag species between Ag⁺ ions to Ag clusters and Ag metal by H₂ at relatively low temperatures is a well-known phenomenon on Ag–zeolites.^127–129 However, it is interesting that the formation of Ag clusters was observed under highly oxidized atmospheres, i.e., in the presence of NO and excess oxygen. The results of in situ UV-Vis spectra indicate the contribution of the following reversible changes of Ag species to the “hydrogen effect” on HC-SCR .

Fig. 22 In situ UV-Vis spectra of 2 wt% Ag/Al₂O₃ at 523 K in a flow of (a) 10% O₂, (b) 10% O₂ + 0.5% H₂, (c) difference spectrum of (b) and (a), and (d) 0.1% NO + 10% O₂. Reprinted from Stud. Surf. Sci. Catal., 145, A. Satsuma, J. Shibata, A. Wada, Y. Shinozaki and T. Hattori, In-situ UV-visible spectroscopic study for dynamic analysis of silver catalyst, pp. 235-238, © 2003, with permission from Elsevier.⁹⁹

Fig. 23 Time course of the band at 306 nm at 523 K under a flow of (a) 10% O₂ to 10% O₂ + 0.5% H₂ and (b) O₂ + H₂ mixture to 0.1% NO + 10% O₂ mixture. Reprinted from Stud. Surf. Sci. Catal., 145, A. Satsuma, J. Shibata, A. Wada, Y. Shinozaki and T. Hattori, In-situ UV-visible Spectroscopic Study for Dynamic Analysis of Silver Catalyst, pp. 235-238, © 2003, with permission from Elsevier.⁹⁹

Richter et al. also confirmed the reversible reduction/re-oxidation of nano-sized Ag₂O clusters on Ag/Al₂O₃ to intermediate Ag⁰ on a short-term scale by means of TEM, TPR, and DTA analysis under the periodic treatment by H₂ and O₂.⁹⁵ Since more oxidized adspecies (nitrite/nitrate) are observed in the presence of H₂, they concluded that the hydrogen effect can be attributed to a dissociative activation of gas phase oxygen on Ag⁰ which leads to conversions of gaseous NO to adsorbed nitrate and propane to acetates and other oxygenates. It should be noted that the hydrogen effect was not limited to Ag/Al₂O₃ prepared by impregnation but also to that prepared by sol-gel processes starting from colloidal alumina sources.

The formation of Ag clusters in the presence of H₂ is more clearly observed in Ag–zeolites. Ag–MFI and Ag–BEA also show boosted SCR activity in the presence of H₂, as reported by our group.⁸⁴ In the absence of H₂, the reaction rate of NO for C₃H₈-SCR at 573 K was less than 50 nmol g⁻¹ s⁻¹ for Ag–MFI and Ag–BEA at an Ag/Al ratio of approximately 0.6. In the presence of H₂, the reaction rate of NO significantly increased to 360 nmol g⁻¹ s⁻¹ for Ag–MFI (Ag/Al = 0.57) and 130 nmol g⁻¹ s⁻¹ for Ag–BEA (Ag/Al = 0.62). On the other hand, the “hydrogen effect” was not observed for Ag–Y and Ag–MOR. It should be noted that, in the case of Ag–MFI, the reaction rate of NO significantly increased with an increase in the Ag : Al ratio, indicating that the ratio of Ag and amount of acid sites is one of the controlling factors for the promotion effect of H₂. The strong dependence of the “hydrogen effect” on the type of acid sites suggests another property of acid sites, i.e., acid strength, is also the controlling factor. Furthermore, the sigmoid relationship was observed between the Ag : Al ratio and the reaction rate of NO of Ag–MFI. Richter et al. also found the similar relation in H₂ C₃H₈-SCR over Ag/Al₂O₃.⁹⁵ The “hydrogen effect” is very significant on 1–2 wt% Ag/Al₂O₃ but not in 0.5 wt% Ag/Al₂O₃. These results suggests that the addition of hydrogen is not effective for highly dispersed Ag species but effective for moderately dispersed Ag species, i.e., the structure of Ag species is the important factor for the “hydrogen effect”.

Fig. 24 shows UV-Vis spectra of Ag–zeolites in the absence and presence of 0.5% H₂. In the presence of H₂, the bands above 260 nm indicate the formation of Ag clusters due to partial reduction and agglomeration of Ag species, however, the type of Ag clusters is strongly affected by the zeolite supports. An adsorption band due to the Ag⁺ ion was observed below 230 nm after C₃H₈-SCR in the absence of H₂. After C₃H₈-SCR in the presence of 0.5% H₂, the spectrum of Ag-MOR was hardly changed, while new bands appeared at 260 and 285 nm due to the Ag_n^δ+ cluster for Ag–MFI. The longer wavelength of the bands of Ag–BEA indicates the formation of larger sized Ag clusters than on Ag–MFI. The extent of the increase in NO conversion by the “hydrogen effect”, inserted in the figure, is in accordance with the formation of Ag_n^δ+ clusters, i.e., the “hydrogen effect” was most significant on Ag–MFI and was not observed on Ag–MOR. Therefore, it is concluded that moderately agglomerated Ag_n^δ+ clusters are highly active species for the C₃H₈-SCR in the presence of H₂, and that the role of H₂ is to reduce Ag⁺ ions to Ag_n^δ+ clusters. Interestingly, the stability of charged Ag species (Ag⁺ and Ag_n^δ+ clusters) under the H₂ HC-SCR (Ag–MOR > Ag–MFI > Ag–BEA) was well correlated to the acid strength of the zeolites, i.e., the heat of adsorption of NH₃ on H-form zeolites is in the order MOR (145 kJ mol⁻¹) > MFI (130 kJ mol⁻¹) > BEA (120 kJ mol⁻¹).^143–145 It can be rationalized that the stronger acid sites, i.e., the ion-exchange sites of zeolite, stabilize the positively charged Ag⁺ ions or Ag_n^δ+ clusters, while weaker acid sites easily release Ag⁺ to form partially reduced Ag species by reduction with H₂.

Fig. 24 UV-Vis spectra of Ag–MFI, Ag–BEA, Ag–MOR and Ag–Y after (dotted line) C₃H₈-SCR and (solid line) C₃H₈-SCR with 0.5% H₂ at 573 K. Conditions: NO = 0.1%, C₃H₈ = 0.1% and O₂ = 10%.

Comparing Fig. 22 and 24, Ag–zeolites give sharper UV-Vis bands after H₂ HC-SCR than Ag/Al₂O₃. The broader UV-Vis spectra of Ag/Al₂O₃ reflect a wide distribution of size or charge of Ag species, which may be due to heterogeneous surface properties of alumina. The ordered structure and uniform chemical profiles of zeolites may result in the formation of uniform Ag species, i.e., Ag–zeolite catalysts would be good models for the supported Ag catalysts for HC-SCR. Detailed structures were characterized for the Ag_n^δ+ clusters on Ag–MFI.⁸² The H₂-TPR result suggests that the average charge of the supported Ag species is +0.5 in the temperature range 373–573 K. Combined with the UV-Vis bands at 255 and 305 nm and the Ag K-edge EXAFS result, the average structure of the clusters is identified as Ag₄²⁺. UV-Vis and Ag K-edge XANES/EXAFS results reveals that, during C₃H₈-SCR with H₂ at 573 K, part of the Ag⁺ ions are converted to Ag_n^δ+ clusters, whose average structure can be close to Ag₄²⁺.

Recently, Sazama et al. investigated the dynamic analysis of the formation and re-dispersion of Ag clusters on alumina under realistic reaction conditions by in situ UV-Vis spectra using a spectrometer built in house.⁹² The bands at 41 600 cm⁻¹ (240 nm) and 46 600 cm⁻¹ (215 nm) attributable to isolated Ag⁺ ions were observed in in situ UV-Vis spectra under reaction condition of decane-SCR-NO without H₂, and the additional band at around 31 000 cm⁻¹ (323 nm) assignable to Ag_n^δ+ clusters was also observed in the presence of H₂. The absorption band observed for Ag/Al₂O₃ was attributed to Ag₈^δ+ clusters having cubic symmetry.¹³⁹ The preferable formation of rather unstable Ag₄^δ+ clusters in MFI may be due to the steric hindrance of the zeolite channel. Fig. 25 shows the response of NO conversion and the intensity of the UV-Vis band at 31 000 cm⁻¹ (323 nm) assignable to the Ag cluster during time-resolved experiments with hydrogen switching on/off in the decane-SCR at 523 K. The NO_x consumption increased sharply within seconds after the addition of H₂, with a sharp increase in NO_x conversion. After hydrogen was switched off, NO_x conversion decreased with a slightly longer response (within 4 min). Comparing the responses of the Ag_n^δ+ cluster and NO conversion, the dispersion of Ag clusters to Ag ions was slower than the decrease in NO conversion. Their group also examined the effect of co-feeding H₂ and CO into the HC-SCR by using in situ UV-Vis.¹⁴⁶ Although only the addition of H₂ enhanced decane-SCR, the formation of Ag_n^δ+ clusters was observed in the treatment with both H₂ and CO. From this, they claimed that the formation of Ag clusters is not essential for the enhancement of the SCR activity in the presence of H₂, and that another effect, such as contributions from hydroperoxy and hydroxy radical species, is more important for the HC-SCR activity in the presence of H₂.

Fig. 25 Effect of hydrogen switching on and off on decane-SCR-NO over Ag/Al₂O₃ at 523 K, 0.1% NO, 0.06% decane, 6% O₂ and 0 or 0.2% H₂. GHSV = 60 000 h⁻¹. NO_x consumption and intensity of the UV-Vis band at 31 000 cm⁻¹ (323 nm) as a function of time and hydrogen switching (A) on and (B) off. Reprinted from J. Catal., 232, P. Sazama, L. Čapek, H. Drobná, Z. Sobalík, J. Dědeček, K Arve and B. Wichterlová, Enhancement of decane-SCR-NO_x over Ag/alumina by hydrogen. Reaction kinetics and in situ FTIR and UV–vis study, pp. 302–317, © 2005, with permission from Elsevier.⁹²

Breen et al.¹²⁵ applied in situ EXAFS for the determination of Ag species of Ag/Al₂O₃ under the H₂–HC-SCR conditions. In their EXAFS spectra, the change of Ag structure by the presence of H₂ was not significant. The Ag_n^δ+ (n = ca. 3 from the coordination number) cluster was already present on the alumina support under SCR conditions without H₂. Furthermore, the Ag_n^δ+ cluster was also observed in the presence of CO, although a promoting effect was not observed in the CO–HC-SCR. They concluded that the enhanced activity in the presence of hydrogen is thought to be due to a chemical effect and not the result of a change in the structure of the active sites.

One may wonder what the chemical effect on HC-SCR in the presence of H₂ is. Recently, Sazama et al. published very interesting communication.¹⁰¹Table 5 shows the effect of hydrogen and hydrogen peroxide on the C₁₀H₂₂-SCR over Ag/Al₂O₃ at 473 K.¹⁰¹ The conversion of NO is negligible without co-feeding hydrogen or hydrogen peroxide. The NO conversion significantly increased to 60% in the presence of 0.2% H₂, and very interestingly, the promotion effect was also observed in decane-SCR with 0.2% of hydrogen peroxide as well as the “hydrogen effect”. At the same time, decane conversion also increased from nearly zero to 12%. They attributed this promotion effect of SCR by hydrogen peroxide to highly reactive hydroxy and hydroperoxy radicals formed from hydrogen peroxide. It should be noted that in the decane-SCR with hydrogen peroxide, the NO₂ yield was higher than that with H₂, which may be due to the reaction between hydroxy radicals, HO₂ + NO → NO₂ + OH. The similarity of the effect of H₂ and hydrogen peroxide on decane-SCR performance strongly suggests the role of hydroperoxy-like species during HC-SCR in the presence of H₂.

Table 5 Comparison of the effect of hydrogen and hydrogen peroxide on the C₁₀H₂₂-SCR-NO reaction over Ag/Al₂O₃ at 473 K^a

	Reducing agent
	Decane	Decane + H2O2	Decane + H2
a P. Sazama and B. Wichterlova, Chem. Commun., 2005, 4810 – Reproduced by permission of The Royal Society of Chemistry.^101b Conversion of NO to nitrogen and nitrogen dioxide.
X NO^b	2.5	60.0	49.5
Yield of N2	0	11.8	21.0
Yield of NO2	2.5	48.2	28.5

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Very recently, Kondratenko et al.¹⁴⁷ reported an advanced understanding of the promotion effect of H₂ on the NH₃-SCR reaction based on the results of a transient low-pressure technique, the temporal analysis of products (TAP) reactor, in combination with isotopic traces. They found that the NH₃-SCR activity increased tremendously after ex situ reduction of Ag/Al₂O₃ in a hydrogen flow at 373 K for 30 min. This observation was related to the creation of reduced Ag species, which catalyze O₂ and NO dissociation. The obtained adsorbed oxygen species plays a key role in NH₃ dehydrogenation, yielding reactive intermediates for N₂ formation. They clearly demonstrated that the reduction of Ag surface is an essential requirement for the promotion effect of H₂.

Given the fact that the presence of H₂ significantly promotes the activation of hydrocarbons, it is reasonable to consider that the most important role of hydrogen is the formation of reactive oxygen species involved in the oxidative activation of hydrocarbons. It is known that the addition of hydrogen also promotes the partial oxidation of hydrocarbons such as CH₄,^148–150 ethane,¹⁵¹ C₃H₆,¹⁵² and benzene.¹⁵³ Wang et al.^149,151 investigated the promotion effect of hydrogen on the selective oxidation of methane or ethane by oxygen over an iron phosphate catalyst, and found the formation of peroxide (O₂²⁻) on the catalyst. They proposed that hydrogen, acting as the electron donor as well as the proton donor, reductively activates molecular oxygen. It is reasonable to extend these roles of active oxygen to the H₂-HC-SCR over supported Ag catalysts. Furthermore, in the case of the H₂-HC-SCR over supported Ag catalysts, the dependence of NO conversion on Ag content and UV-Vis spectra suggested an essential contribution of Ag cluster.^81–84 The lesser “hydrogen effect” on the Ag catalysts of low Ag content,^{81,82,112,154} indicates the isolated Ag⁺ is not effective even if H₂ is present in reaction atmospheres. Summarizing all the related data, it can be concluded that both the formation of Ag clusters and hydroperoxy-like species are essential for the “hydrogen effect”. The contribution of these factors on the “hydrogen effect” can be rationalized by considering the following close relationship between Ag clusters and Ag–hydride.^155–157 Spectroscopic evidence on the interaction of hydrogen with the partially reduced Ag₃⁺ cluster in Ag-A zeolite has been well investigated using ¹H MAS NMR.¹⁵⁷ Hydrogen dissociates over Ag₃⁺ sites to generate an acidic proton and Ag₃–H. It is reasonable to assume that hydrogen addition results in the formation of hydride on silver clusters, which may react with oxygen to form a reactive oxidant such as hydroperoxy radicals (HO₂), peroxide (O₂²⁻) or superoxide ions (O₂⁻).

Conclusions

Ag/Al₂O₃ is a promising candidate for practical uses of selective catalytic reduction of NO by hydrocarbons (HC-SCR). Several engine bench tests demonstrated the high catalytic performance of Ag/Al₂O₃ and its tolerance to SO_x and water vapour. Due to the high sensitivity of HC-SCR activity of supported Ag catalysts to the reaction conditions, the control of the reaction conditions by engine operation and secondary fuel injection would be the key factor for creating effective deNO_x in practical automotive engines. The investigations on the reaction mechanism clarified the crucial role of nitrate formed from NO₂ and oxygenated hydrocarbons formed from the partial oxidation of hydrocarbon reductants. The SCR reaction proceeds by the surface reaction between nitrate and oxygenated hydrocarbons, such as acetate, and N₂ is produced via nitrogen-containing intermediates, i.e., CN, NCO and NH₃. The oxidative activation of hydrocarbons is the key step for obtaining high SCR activity at lower temperatures. The addition of hydrogen is effective for enhancing the low temperature SCR activity. Although the mechanism of the “hydrogen effect” is under debate, the key roles of both the formation of Ag clusters and hydroperoxy-like species have been highlighted.

Acknowledgements

A part of this study was supported by the Grant-in-Aid from the Ministry of Education, Science, Sports and Culture in Japan. The authors are deeply indebted to co-authors of the papers cited in this Invited Article.

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