A highly effective Ni-modified MnO_x catalyst for total oxidation of propane: the promotional role of nickel oxide

Abstract

Nickel promoted manganese oxide (MnNiO_x) was prepared by the co-precipitation method and used as a catalyst for propane deep oxidation. The results show that the doping of Ni can effectively improve the catalytic performance of MnO_x for propane total oxidation, and when Ni/Mn is 0.2, the MnNi_0.2O_x catalyst exhibits the highest catalytic activity, for instance, the reaction temperature of 90% propane conversion (T₉₀) was only ∼240 °C, and it demonstrates good thermal stability in the operation at temperatures alternating between 220 °C and 350 °C. It was found that an Mn–Ni–O solid solution can be formed by adding a moderate Ni amount into MnO_x, resulting in changes in catalytically active sites by the synergetic interaction of Mn–Ni, such as a higher surface concentration of Mn⁴⁺ and oxygen vacancies, higher oxygen mobility and better reducibility. And the outstanding catalytic property of MnNi_0.2O_x can be also related to its high surface area. Furthermore, the in situ DRIFT technique was used to investigate the reaction process of propane oxidation over the MnO_x and MnNi_0.2O_x catalysts, and the results suggest that the reaction mechanism is hardly changed after Ni doping in MnO_x.

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1. Introduction

The release of volatile organic compounds (VOCs) to the atmosphere has resulted in serious environmental problems in recent years, and environmental legislation has imposed increasingly stringent standards for VOC emissions. The prevalent VOCs, linear short chain alkanes, one of the largest fractions of hydrocarbons (HCs) made from automobile exhausts, are some of the most difficult to eliminate, due to their use as transportation fuels as well as being essential feed stocks for chemical production.¹ Air pollution caused by these automotive exhausts is expected to worsen as the demand for privately owned vehicles increases.² Although physical adsorption is an appropriate and convenient way to remove them to some extent, in this case the pollution can easily be transferred onto a solid which must then be disposed of as chemical waste.³ To remove VOCs, catalytic oxidation represents a promising and low-cost technology as it provides the potential to eliminate pollutants totally to carbon dioxide and water.

Liquefied Petroleum Gas (LPG, composed of primarily propane and butane), as an alternative cleaner burning fuel, is gaining ground for application in fuel-powered vehicles because of its attractive features, such as clean combustion, high energy-density, simple storage and being ready for use in transportation.⁴ However, tailpipe emissions from LPG fueled vehicles and other combustors such as LPG engines and burners still contain high amounts of hazardous HCs. Up to 80% of HC emissions are produced in the first 60 to 90 s following a cold-start, because of the catalytic converter’s inability to oxidize HCs at low temperatures of 200–300 °C.⁵ As a result, it is quite essential to develop efficient catalysts that can completely remove propane emitted from LPG combustors, which is more difficult to eliminate than butane at low concentrations.⁶

Generally speaking, 75% of the catalysts used for VOC destruction are noble metal catalysts. Palladium and platinum based catalysts are strongly active even at relatively low temperature and have been studied thoroughly.⁷ For propane total oxidation, supported palladium and platinum catalysts are found to possess high activity and thermal stability.^8–12 Nevertheless, they have some disadvantages like high sintering rates, volatility, poisoning in the presence of water or sulfur compounds and high price.¹³ Hence many efforts have been directed towards transition metal oxides as effective and economical catalysts for the catalytic combustion of alkanes in recent years. These oxide materials are less costly, more resistant to poisoning and, in certain cases, they exhibit catalytic activity comparable to that of noble metal catalysts. Much attention has been paid to perovskites, spinels, hydrotalcites and some single-oxide catalysts.^14–18

Manganese oxides are reported to be one of the most efficient and environmentally friendly oxidation catalysts for catalytic combustion.¹⁹ Different polymorphs of MnO₂ ²⁰ and Mn₃O₄ ²¹ were reported to be active and stable catalysts for the oxidation of some organic compounds. Compared with single component oxide catalysts, MnO_x mixed oxides modified with other metal oxides exhibit higher catalytic activity for some combustion reactions, which might be attributed to the capability of manganese to form oxides with different oxidation states and its high oxygen storage capacity.²² It was reported that the single oxides of Ni, Co, Fe and Mn have high activity for the decomposition of ozone, and mixed 3d-transition metal oxides were proved to be more active than the single ones.²³ Hopcalite (CuMnO_x), one of the most widely used and long-standing mixed oxide catalysts discovered around 90 years ago,²⁴ has been mainly employed for low temperature CO oxidation²² and propane oxidation.²⁵ Solsona et al. observed that a co-precipitated CuMnO_x catalyst was more active than supported palladium catalysts, and its activity was further improved by the incorporation of gold nanoparticles into the CuMnO_x structure.²⁶ The preparation method can seriously affect the surface phase composition and physico-chemical properties of a catalyst, which may finally influence the catalytic activity. As a conventional preparation method of catalysts and inorganic materials, co-precipitation has been widely applied in the preparation of composite oxides, because of its simple synthesis process, short preparation period, low cost and good repeatability. The experimental variables can be easily controlled to fully optimize the preparation conditions, and the obtained materials are usually in a uniform chemical composition. Venkataswamy et al.²⁷ used the co-precipitation method to prepare a nanostructured CeMn mixed oxide catalyst for CO oxidation, and found that the presence of moderate Mn in CeO₂ led to the formation of solid solutions and the strong synergetic interaction of Mn–Ce, which is in favor of the improvement of reducibility and activation of surface oxygen, and the acceleration of reactant gas adsorption.²⁸

Herein, we want to develop low light-off oxidation catalysts for eliminating HC emissions, especially the emission of propane which is the main component in LPG. The conventional co-precipitation method was used to prepare a highly effective Ni-modified MnO_x catalyst for propane total oxidation, in order to realize the industrial application of this catalyst. The effect of Ni on the catalytic properties of MnO_x was investigated in detail. The results show that the MnNi_0.2O_x catalyst showed very high activity for propane deep oxidation compared with the single MnO_x. The reasons for MnNi_0.2O_x having the highest catalytic activity for propane combustion were illuminated and discussed.

2. Experimental section

2.1. Catalyst preparation

MnNiO_x catalysts were synthesized by the co-precipitation method in aqueous solutions, in which manganese nitrate (Mn(NO₃)₂, AR grade) and nickel nitrate (Ni(NO₃)₂·6H₂O, AR grade) were used as the precursors. The desired amounts of the precursors were solved and mixed in deionized water under stirring. An aqueous solution of Na₂CO₃ was added dropwise into the synthesis solution above until pH = 8.5. After it was aged for 14 h at 80 °C under stirring, the obtained precipitate was filtered and washed with deionized water to remove Na⁺ ions possibly left in the precipitate. Then the obtained solid was dried at 120 °C in air overnight, and calcined at 300–500 °C for 4 h in a muffle furnace. Pure MnO_x and NiO were prepared with the same procedures as the MnNiO_x mixed oxide catalysts.

2.2. Characterization techniques

The surface areas of the catalysts were measured by N₂ adsorption at −196 °C on a Micromeritics ASAP 2020 apparatus and calculated by the Brunauer–Emmett–Teller (BET) method. Powder X-ray diffraction (XRD) patterns were recorded on a Brook D8 Focus XRD diffraction spectrometer with Cu Kα radiation (λ = 0.154 nm, operated at 40 kV and 40 mA). Laser Raman spectra were measured on a Renishaw inVia-Reflex Micro-Raman spectroscopy system at ambient conditions and the 514 nm line of a Spectra Physics Ar⁺ laser was used for excitation. Scanning electron microscopy (SEM) images were taken on a Hitachi S-3400N scanning electron microscope operated at 15 kV, and the sample was covered with a thin layer of gold before testing. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100 microscope operated at 200 kV. The sample to be measured was dispersed in ethanol and then collected onto copper grids covered with carbon films. The grid was loaded into the microscope after evaporation of the liquid phase. X-ray photoelectron spectroscopy (XPS) spectra of samples were obtained on a Kratos Axis Ultra-DLD photoelectron spectrometer equipped with Al Kα (1486.6 eV) radiation as the excitation source. The sample to be tested was pre-treated in N₂ at 500 °C for 30 min. All the binding energies (BE) were determined with respect to the C 1s line (284.8 eV) originating from adventitious carbon.

Hydrogen temperature programmed reduction (H₂-TPR) was performed in a quartz U-tube with 100 mg of catalyst in a conventional flow system equipped with a thermal conductivity detector (TCD). The reducing gas consisted of 5% H₂/Ar (45 ml min⁻¹) and the heating rate was 10 °C min⁻¹.

Temperature programmed desorption of oxygen (O₂-TPD) was performed in a quartz U-tube reactor system equipped with an online Hiden Analytical HAL301 mass spectrometer. Prior to each test, 100 mg of the sample was pre-treated in vol 3% O₂/He mixture gas at 400 °C for 1 h. After cooling down to room temperature, pure He with a flow rate of 30 ml min⁻¹ instead of vol 3% O₂/He was purged into the reactor until stabilization of the MS baseline. The reactor was heated at 10 °C min⁻¹ from room temperature to 850 °C. The signal of desorbed oxygen (m/z = 32) was collected using an MS detector.

The in situ DRIFT spectra of propane were recorded on a Nicolet 6700 FT-IR spectrometer equipped with an MCT detector. In the DRIFT cell with ZnSe windows connected with a gas flow system, the catalyst was pretreated in Ar at 400 °C for 1 h, and then the catalyst was cooled down to 50 °C for propane adsorption. The background spectra were recorded at certain temperatures during the cooling. Then the Ar gas was replaced by the reaction mixture gas of 2000 ppm C₃H₈ and 5% O₂ balanced by Ar with a total flow rate of 50 ml min⁻¹. After the reaction in the cell was stabilized for 1 h at each temperature, the DRIFT spectra were obtained and background spectrum subtracted.

2.3. Catalytic performance testing

The catalytic activity of the sample for the total oxidation of propane was examined in a fixed bed reactor. 200 mg of catalyst was placed in a quartz tube reactor and the feed gas consisted of 0.2% C₃H₈ and 5% O₂ in Ar balance gas. The total flow rate was controlled to 100 ml min⁻¹ and the gas hourly space velocity (GHSV) was 30 000 h⁻¹. The reactants and products were analyzed online using a gas chromatograph (GC 2060 system) equipped with an FID. The catalytic activities were described as propane conversion (%), and T₁₀, T₅₀ and T₉₀, which are the reaction temperatures for the propane conversions of 10, 50, and 90%, respectively.

2.4. Reaction kinetics testing

The kinetics parameters for propane oxidation were measured in the fixed-bed reactor as mentioned above, and the catalytic reaction data were obtained after the reaction was stable for 1 h, and the propane conversion was controlled to <15%. The reaction rate (r, mol (g_cat s)⁻¹) of propane conversion was calculated according to the following eqn (1),where N_C3H8 was the C₃H₈ gas flow rate (mol s⁻¹), η_C3H8 was the conversion of propane and W_Cat. was the catalyst weight (g) used in the reaction.

When the conversion of propane is lower than 15%, the dependence of the reaction rate (r) on the products of CO₂ and H₂O can be ignored. Therefore, the empirical kinetic expression of the reaction rate equation can then be described by eqn (2),

where A was the pre-exponential factor and E_a was the apparent activation energy (kJ mol⁻¹). Take the logarithm of eqn (2) and (3) can be obtained,

ln r = ln A + αln P_C3H8 + βln P_O2 − E_a/RT (3)

As the conversion of propane is lower than 15% during the kinetics data testing, the change of feed gas may be approximately ignored. Hence, ln A, αln P_C3H8 and βln P_O2 are supposed to be approximately constant, and eqn (3) can be simplified to eqn (4).

ln r = −E_a/RT + C (4)

Thus the activation energy (E_a) of this catalytic reaction can be obtained from the slope of the resulting linear plot of ln r versus 1/T.

3. Results and discussion

3.1. Catalytic performance and kinetic parameters for propane oxidation

Since only CO₂ was detected in all the reaction processes, the MnNiO_x catalysts possessed excellent selectivity for propane deep oxidation. Firstly, the effect of the calcination temperature (300–500 °C) on the catalytic activity of the MnNi_0.2O_x sample was investigated, and the catalytic activities for propane combustion are shown in Fig. 1A. The results show that the calcination temperature obviously affected the catalytic activity of MnNi_0.2O_x and the samples calcined at lower temperature (300–400 °C) exhibited higher catalytic activity for propane total oxidation. Unless otherwise indicated, the calcination temperature of the catalyst was 400 °C hereinafter. For the MnNiO_x catalysts calcined at 400 °C, the effect of the Ni amount on the catalytic activities of the MnNiO_x catalysts for propane total oxidation are shown in Fig. 1B and their T₁₀ (the reaction temperature of 10% propane conversion), T₅₀, and T₉₀ are listed in Table 1. The results show that the catalytic activities of the MnNiO_x catalysts are affected obviously by the Ni/Mn molar ratio, and the activities of the MnNiO_x catalysts are ranked as follows: MnNi_0.2O_x > MnNi_0.1O_x ≥ MnNi_0.4O_x > MnO_x > NiO. Apparently, the presence of Ni can improve the catalytic activity of an MnO_x sample, compared with the single oxide of MnO_x and NiO. As shown in Fig. 1C, when the molar ratio of Ni/Mn is 0.2 the MnNi_0.2O_x sample possesses the best catalytic performance, and upon further increasing the Ni amount in the MnNiO_x catalyst, its catalytic activity would contrarily be reduced. The T₁₀, T₅₀ and T₉₀ values over the MnNi_0.2O_x catalyst are 186, 215 and 242 °C for propane deep oxidation, respectively, which are 67, 61 and 52 °C lower than those over pure MnO_x. The re-usability of the MnNi_0.2O_x was tested and the results are shown in Fig. 1D. The results show that after being repeatedly used ten times for propane oxidation, T₅₀ and T₉₀ on the catalyst still kept around 220 °C and 240 °C. No obvious deactivation was observed, which shows that the MnNi_0.2O_x catalyst exhibits favorable re-usability.

Fig. 1 The catalytic activities of MnNiO_x catalysts for propane total oxidation: (A) MnNi_0.2O_x calcined at different temperatures, (B) MnNiO_x with different Ni amounts calcined at 400 °C, (C) the effect of Ni/Mn molar ratio on T₅₀, and (D) the T₅₀ and T₉₀ values as functions of repeated use times over the MnNi_0.2O_x catalyst.

Table 1 Catalytic activities (T₁₀, T₅₀, T₉₀), BET surface areas (S_BET), reaction rates (r, at 270 °C) and activation energies (E_a) of MnNiO_x catalysts

Sample	T10 (°C)	T50 (°C)	T90 (°C)	SBET (m^2 g^−1)	r × 10^5 (mol (gcat s)^−1)	Ea (kJ mol^−1)
MnNi0.1Ox	221	242	258	82	20.7	106
MnNi0.2Ox	186	215	242	111	21.5	67
MnNi0.4Ox	214	243	277	97	19.5	114
MnOx	253	276	294	88	7.0	128
NiO	294	315	333	136	0.6	166

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It was reported that using a traditional CuMn catalyst propane could be totally oxidized at >300 °C, and the T₉₀ of propane oxidation over supported Co₃O₄ also reached as high as 260 °C.^24,26,29 Comparing with the typical noble metal catalysts (such as supported Pt and Pd catalysts), the activity of the MnNi_0.2O_x catalyst is quite close or even much better.^30–33 And the MnNi_0.2O_x catalyst exhibited much better operational stability than some of the catalysts reported. Therefore, this MnNi_0.2O_x catalyst is a potential catalyst for VOC oxidation.

The reaction rates of propane oxidation over the MnNiO_x catalysts at 270 °C were measured, and the results are shown in Table 1. The results show that the MnNi_0.2O_x catalyst has the highest rate of 20.7 × 10⁻⁵ mol (g_cat s)⁻¹, almost three times higher than that of the MnO_x (7.0 × 10⁻⁵ mol (g_cat s)⁻¹). Fig. 2 displays Arrhenius plots of ln r versus 1/T for all the samples, and the activation energy (E_a) was obtained from the slope of the linear plot and is listed in Table 1. Obviously, the E_a values of MnNiO_x samples are lower than that of MnO_x, in which the MnNi_0.2O_x sample exhibits the lowest E_a (67 kJ mol⁻¹).

Fig. 2 Arrhenius plots of ln r against 1000/T over the MnNiO_x catalysts for propane oxidation.

3.2. Thermal stability of MnNi_0.2O_x and MnO_x

Fig. 3A shows propane conversion as a function of the reaction time at different temperatures over the MnNi_0.2O_x and MnO_x catalysts. It can be found that the catalytic activities of the two samples are hardly changed after 50 h of the reaction, in which the reaction temperature for MnNi_0.2O_x was 240 °C and that for MnO_x was 290 °C, and the propane conversions can be kept at ∼90%. To further examine the thermal stability of the catalyst, an accelerated aging process by cycling the reaction temperature was carried out for each catalyst. As shown in Fig. 3B, the catalysts were tested under alternating temperature between 220 °C (270 °C) for 10 h and 350 °C for 10 h over MnNi_0.2O_x (or MnO_x), and the catalytic activities of both catalysts hardly varied for propane oxidation. These results suggest that MnO_x and MnNi_0.2O_x demonstrate favorable thermal stability, and the propane conversion over MnNi_0.2O_x is higher than that over MnO_x.

Fig. 3 Propane conversion as a function of the reaction time at different temperatures over the MnNi_0.2O_x and MnO_x catalysts.

3.3. BET surface area and XRD analyses

Table 1 lists the BET surface areas (S_BET) of MnNiO_x samples with different molar ratios of Ni/Mn obtained by low temperature N₂ adsorption. Among all samples NiO has the largest surface area (∼136 m² g⁻¹), and among the MnNi mixed oxides, MnNi_0.2O_x shows the largest specific surface area of ∼111 m² g⁻¹. In general, for a solid catalyst a larger specific surface area can support more active sites, thus the highest catalytic activity of MnNi_0.2O_x among the MnNiO_x samples is partially attributed to its largest surface area. However, the surface area of MnNi_0.1O_x is hardly changed compared with MnO_x, but the activity of MnNi_0.1O_x is obviously higher than that of MnO_x, which shows that the surface area is only one of the main factors affecting the catalytic activity rather than the sole factor. The presence of Ni in MnNiO_x not only affects the surface area of MnO_x, but also varies the nature of the active sites, the properties of surface oxygen species (including oxygen vacancies and oxygen mobility), reducibility of the catalyst, and surface concentration of Mn⁴⁺, etc. The combination of these factors leads to the improvement of the catalytic performance of the MnNi_0.2O_x catalyst.

The powder XRD patterns of the prepared catalysts are shown in Fig. 4. The nickel oxide sample showed primarily the diffraction peaks of the bunsenite phase of NiO (JCPDS 47-1049), and manganese oxide hardly exhibited the diffraction peaks, that is to say, the MnO_x sample is amorphous. After adding Ni into MnO_x, part of the Mn species still existed as amorphous MnO_x, and the diffraction peaks of NiO could be hardly observed. When doping a small amount of Ni, there are diffraction peaks of MnCO₃ in the XRD spectrum of the MnNi_0.1O_x catalyst, possibly owing to incomplete decomposition. With an increase in the Ni content, the diffraction peaks of new phases attributed to Ni₆MnO₈ (JCPDS 83-1186) and NiMnO₃ (JCPDS 75-2089) mixed oxides can be observed. Ni₆MnO₈ contains Mn⁴⁺ species which can supply oxygen vacancies.³⁴ Thus, MnNiO_x catalysts with moderate Ni amounts are expected to contain more oxygen vacancies, due to the formation of an Mn–Ni–O (as Ni₆MnO₈) solid solution.

Fig. 4 Powder XRD patterns of the MnNiO_x catalysts.

3.4. Raman spectroscopy

Raman spectra of MnNiO_x catalysts are shown in Fig. 5. For the MnO_x sample, the strong peak at 640 cm⁻¹ is assigned to the stretching vibration of the metal–oxygen chain of Mn–O–Mn,³⁵ and NiO shows a broad peak at ∼502 cm⁻¹ which is attributed to the Ni–O stretching mode.³⁶ In the Raman spectra of MnNiO_x, only a vibration peak of MnO_x can be observed when the Ni amount is less. With the increase in the Ni amount, the Raman peak of NiO became gradually visible and the peak of MnO_x slightly shifted to lower wave numbers, which means a sensitive indication of defective structures. Generally speaking, Raman frequency shift is dependent mainly on oxygen vacancies and lattice reconfiguration. Therefore, the addition of Ni can cause the formation of oxygen vacancies and deformation of the MnO_x structure through the synergistic effect of Mn and Ni species, which finally affects the nature of catalytically active sites.

Fig. 5 Raman spectra of MnNiO_x catalysts.

3.5. SEM and TEM images

Fig. 6 exhibits the SEM images of the MnNiO_x catalysts. For single oxides, MnO_x shows an agglomerated bulk of globular particles with blurred inter-particle boundaries³⁷ and NiO is composed of irregular polyhedra. After adding a small amount of Ni to MnO_x, the MnNi_0.1O_x sample consists of a few smaller globular particles besides some larger particles with a clear boundary like the MnO_x sample. Interestingly, when the Ni/Mn molar ratio is increased, in the particle surface of MnNi_0.2O_x there are many fiber-like nanowires or nano-leaves, and its particle size is 3–4 times larger than the size of the MnNi_0.1O_x sample. And these attached fibers on the MnNi_0.2O_x catalyst can be considered to be one of the reasons for its higher surface area. However, these fibers disappeared after adding a further Ni amount. And MnNi_0.4O_x has similar particles to MnO_x but these particles can be clearly separated. The results above suggest that doping Ni into MnO_x with an appropriate amount can bring visible morphological changes.

Fig. 6 SEM images of the MnNiO_x catalysts.

TEM images of MnO_x and MnNi_0.2O_x catalysts are shown in Fig. 7. In the TEM image of MnO_x, many crystallites with irregular diameters formed the assembling particles. Two different features can be detected for the MnNi_0.2O_x sample. As shown in MnNi_0.2O_x (A), crystallites with denser and smaller spherical particles than MnO_x were observed; when changing the scanning area, the image labeled as MnNi_0.2O_x (B) was obtained, in which ribbon-like nanowires with lengths of 20–140 nm were clearly observed, in accord with its surface fibers shown in the SEM image.

Fig. 7 TEM images of the MnO_x and MnNi_0.2O_x catalysts.

3.6. H₂-TPR

The reduction behavior of MnNiO_x catalysts was investigated and the results are shown in Fig. 8. Pure NiO has only one wide peak at ∼400 °C, assigned to the reduction of NiO to Ni.³⁸ The MnO_x sample exhibited two well-defined reduction peaks at ∼285 °C (peak α) and ∼415 °C (peak β), which can be attributed to the reduction of MnO_x to Mn₃O₄ and the reduction of Mn₃O₄ to MnO, respectively.^39,40 When doping a lesser amount of Ni in MnO_x, the α and β peaks of the MnNi_0.1O_x catalyst noticeably shifted to lower temperatures accompanying the decrease in their areas, and the reduction peak of NiO was hardly observed. With the increase in the Ni content, the top temperatures of peak α and β shifted slightly to lower temperatures and their areas further decreased, and the reduction peak of NiO gradually appeared. The results above show that the reduction peaks of both Mn and Ni species moved to lower temperatures for the MnNi mixed oxides with higher Ni content (as MnNi_0.2O_x and MnNi_0.4O_x). Generally, the redox cycles of Mn and Ni oxides are connected with gaseous oxygen activation. Since the sizes of metal ions (such as Mn⁴⁺, Mn³⁺, Mn²⁺, and Ni²⁺) are quite different, a strong interaction between them can be created to realize co-existence of these ions.⁴¹ After a suitable distribution of metal components was formed by a solid reaction, the Mn–Ni–O solid solutions would be formed, which would affect the activity of related bonded oxygen. The dropping of the reduction temperature should be ascribed to the synergistic effect between the Mn and Ni species, due to the formation of a solid solution, in which the mobility of oxygen species can be effectively promoted, resulting in the production of more active oxygen species for the reaction.⁴² Therefore, doping Ni into MnO_x can produce more active oxygen species in the MnNiO_x composite oxide through the strong synergistic effect between the Mn and Ni species, and improve the reducibility of this catalyst at low temperature.

Fig. 8 H₂-TPR profiles of MnNiO_x catalysts.

3.7. O₂-TPD

Fig. 9 shows the O₂-TPD curves of the MnNiO_x samples. The desorption peak was hardly observed in the O₂-TPD curve of pure NiO, suggesting little oxygen release. For pure MnO_x, the middle-temperature (MT) desorption peaks located at 350–650 °C can be attributed to the release of lattice oxygen, and the high-temperature (HT) peaks at >650 °C are related to the decomposition of bulk MnO_x.⁴³ But the desorption peaks of surface adsorbed oxygen (O₂, O₂⁻ and O⁻), which tend to be released easily, were hardly found at <350 °C on the MnO_x sample.^44,45 After adding Ni, the MT peaks of the MnNi_0.1O_x catalyst partially merged (three peaks merged to two peaks), indicating mobility of lattice oxygen by the aid of an interaction between Mn and Ni species. With an increase in the Ni content, the HT desorption peaks rapidly became invisible, from which we can infer that less bulk MnO_x existed in MnNi mixed oxides with higher Ni content and the lattice oxygen was mainly generated from Mn–Ni–O solid solutions. Compared with MnNi_0.4O_x, in addition, the area and intensity of the MT desorption peak of MnNi_0.2O_x are a bit higher, suggesting better desorption ability and mobility of lattice oxygen over the MnNi_0.2O_x catalyst.⁴⁶ Since the deep oxidation of propane over the MnO_x catalyst follows the Mars-van Krevelen mechanism,⁴⁷ the increase in the mobility and activity of lattice oxygen species of the MnNiO_x catalyst is beneficial to improve its catalytic activity.

Fig. 9 O₂-TPD profiles of MnNiO_x catalysts.

3.8. XPS analysis

Fig. 10 shows the XPS Mn 2p and O 1s spectra of MnO_x and MnNi_0.2O_x catalysts, and surface atom ratios of Mn³⁺/Mn²⁺, Mn⁴⁺/Mn³⁺ and O_ads/O_latt calculated from the XPS spectra using XPSPEAK 4.1 are listed in Table 2. The asymmetrical Mn 2p_3/2 XPS signal at binding energy (BE) of ∼641 eV can be decomposed to three Gaussian–Lorentzian peaks at ∼640.6, 641.8 and 643 eV, which are ascribable to the surface species of Mn²⁺, Mn³⁺ and Mn⁴⁺, respectively.^48,49 After adding Ni_0.2 into MnO_x, the atomic ratio of Mn³⁺/Mn²⁺ decreased from 1.85 (MnO_x) to 1.35 (MnNi_0.2O_x), and the atomic ratio of Mn⁴⁺/Mn³⁺ increased from 0.48 to 0.92. This is because the presence of Ni in the MnNi_0.2O_x solid solution can effectively restrain the diffusion of ions (such as Mnⁿ⁺ and O²⁺) and inhibit the transformation of Mn⁴⁺ to Mn³⁺, resulting in an increase in the Mn⁴⁺ concentration.⁵⁰

Fig. 10 XPS spectra of Mn 2p_3/2 and O 1s in MnO_x and MnNi_0.2O_x catalysts.

Table 2 XPS data of MnO_x and MnNi_0.2O_x catalysts

Sample	BE of Mn 2p (eV)					BE of O 1s (eV)
	Mn^2+	Mn^3+	Mn^4+	Mn^3+/Mn^2+	Mn^4+/Mn^3+	Olatt	Oad	Oad/Olatt
MnOx	640.6	641.8	643.2	1.85	0.48	529.7	531.2	0.72
MnNi0.2Ox	640.7	641.9	643.2	1.35	0.92	529.8	531.2	1.39

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The O 1s peaks can also be deconvoluted to three peaks: the lowest BE peak at ∼529.7 eV is assigned to the lattice oxygen (O_latt), the peak at BE of ∼531.2 eV is attributed to the surface adsorbed oxygen (O_ads), and the high BE peak at ∼532.5 eV is associated with adsorbed OH groups or adsorbed molecular water.^50–52 The results show that the O_ads/O_latt ratio of MnNi_0.2O_x is 1.39, and much higher than that of MnO_x (0.72). It was reported that when the Mn species existed as Mn⁴⁺–O_ads Lewis acid–base pairs, Mn⁴⁺ can be better dispersed on the surface, thus the catalyst with a higher Mn⁴⁺/Mn³⁺ ratio can contain more adsorbed oxygen species.^53,54 Therefore, compared with pure MnO_x, the MnNi_0.2O_x catalyst possesses a higher Mn⁴⁺/Mn³⁺ atomic ratio and contains more adsorbed oxygen species, which is beneficial to the improvement of its catalytic oxidation performance.

3.9. In situ DRIFT spectroscopy

Fig. 11 shows the in situ DRIFT spectra of MnO_x and MnNi_0.2O_x catalysts at 1000–4000 cm⁻¹ and 50–300 °C under an atmosphere of 2000 ppm C₃H₈ + 5 vol% O₂/Ar (50 ml min⁻¹). The IR spectra were recorded after 1 h of the reaction at each temperature. In these IR spectra, the band at 2969 cm⁻¹ and shoulder at 2900 cm⁻¹ correspond to C–H vibrations of gaseous propane, and the bands located at 1300–1700 cm⁻¹ were ascribed to various carboxylate species such as formate (1590 cm⁻¹), acetate (1575, 1430 and 1433 cm⁻¹) and mono- or poly-dentate carbonate (1350 cm⁻¹).^55,56 Besides, the band at ∼3500 cm⁻¹ on MnNi_0.2O_x was owing to combined OH groups on the surface.⁵⁷ When the reaction temperature was increased from 50 to 200 °C, the carboxylate peaks gradually became weaker and finally disappeared, implying the partial oxidation of propane over the MnO_x and MnNi_0.2O_x catalysts, in which labile carboxylates should be the oxygenated intermediates. At 300 °C, the IR spectra of the surfaces of both catalysts became very weak, reflecting the deep oxidation of propane. It is interesting to find that the Ni-modified sample possessed obvious carboxylate peaks at a lower wavenumber (1575 cm⁻¹) compared with the IR spectra of MnO_x (1590 cm⁻¹), possibly resulting from the enhanced adsorption of reactants.⁵⁸ Therefore, it can be speculated that Ni modification hardly changes the reaction mechanism of propane deep oxidation, and only the nature of surface carbonate species is varied by the presence of Ni, which improved the property of the active sites for propane adsorption during the preliminary oxidation process, accelerating the activation of propane gas.

Fig. 11 In situ DRIFT spectra of MnO_x and MnNi_0.2O_x catalysts under an atmosphere of 2000 ppm C₃H₈ + 5 vol% O₂/Ar (50 ml min⁻¹) at 50–300 °C.

4. Conclusions

In summary, the MnNiO_x catalysts synthesized by the co-precipitation method exhibited excellent catalytic performance for the combustion of propane. The doping of Ni into MnO_x apparently enhanced the catalytic activity, and the best molar ratio of Ni/Mn was 0.2. Using the MnNi_0.2O_x catalyst, T₉₀ (the reaction temperature of 90% propane conversion) and T₅₀ were ∼240 °C and 215 °C respectively. At 270 °C, the reaction rate on the MnNi_0.2O_x catalyst reached 20.7 × 10⁻⁵ mol (g_cat s)⁻¹, which was three times higher than that on MnO_x. The MnNi_0.2O_x also demonstrated good thermal stability for propane total oxidation, and no deactivation was observed after online testing as long as 50 h, at temperatures alternating between 220 °C for 10 h and 350 °C for 10 h.

The addition of Ni can promote the formation of oxygen vacancies and deformation of the MnO_x structure through the synergistic effect of Mn and Ni species, which affects the nature of catalytically active sites, but a lesser Ni doping amount was not able to change the nature of the active sites. When the introduction of Ni into MnO_x reaches Ni/Mn > 0.2, an Mn–Ni–O (as Ni₆MnO₈) solid solution can form in the MnNiO_x sample, resulting in an increase in the Mn⁴⁺ content, amount of oxygen vacancies, oxygen mobility and reduction property, which can effectively improve its catalytic performance for propane total oxidation. Contrarily, the presence of excess Ni content would weaken its promotional role for the MnO_x catalyst. The presence of Ni in the MnNiO_x sample hardly changes the reaction mechanism of propane deep oxidation, and only the nature of surface carbonate species is varied on the surface, which improves the property of the active sites for propane adsorption during the preliminary oxidation process, accelerating the activation of propane gas.

Acknowledgements

This work was supported financially by the National High Technology Research and Development Program of China (2012AA111717), the Commission of Science and Technology of Shanghai Municipality (15DZ1205305) and Fundamental Research Funds for the Central Universities (WJ1514020).

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