Low-temperature catalytic oxidation of CO over highly active mesoporous Pd/CeO₂–ZrO₂–Al₂O₃ catalyst

Abstract

Mesoporous nano Pd/CeO₂, Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ catalysts were prepared via a soft template-assisted hydrothermal method and employed in low-temperature catalytic CO oxidation. Almost 100% of CO conversion could be obtained by the optimal Pd/CeO₂–ZrO₂–Al₂O₃ catalyst at 60 °C. Addition of Zr and Al into Pd/CeO₂ lead to an increase of the pore volume, average size of mesopores, the surface Ce³⁺/Ce⁴⁺ atomic ratios and O_adsorbed/O_lattice, while the activation energy (E_a) decreased in the order Pd/CeO₂ (50.4 kJ mol⁻¹) > Pd/CeO₂–ZrO₂ (40.7 kJ mol⁻¹) > Pd/CeO₂–ZrO₂–Al₂O₃ (32.4 kJ mol⁻¹). The low-temperature CO catalytic conversion increased with the decreased activation energy (E_a).

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1. Introduction

Catalytic oxidation of CO has been widely employed in indoor air cleaning, industrial catalytic CO combustion, automotive exhaust abatement and academic research.^1,2 Various transition metal oxides,³ Ag,⁴ Au,⁵ and platinum-group metals (PGMs)⁶ components have been widely explored in CO catalytic oxidation. Spinal Co₃O₄,⁷ MnO_x,⁸ supported Ag, Au and Pd catalysts exhibit excellent low-temperature CO oxidation performance. Among them, supported Pd catalysts have been employed in industrial applications for decades because of their long life cycles and moisture and thermal stability.⁹ To date, researchers continue focusing on light-off temperature, durability, thermal stability and the cost of the Pd-containing catalysts for their catalytic combustion of low-caloric-value industrial CO-containing waste gases. Light-off temperature is closely related to low-temperature catalytic activity.¹⁰ In addition, to promote surface palladium species dispersion is rather critical to enhance the catalytic activity and reduce the content and cost of palladium over the supported palladium catalysts.^10,11 Great amounts of study have been carried out to promote Pd dispersion and enhance the supports durability, thermal stability and redox properties.¹¹ A CeO₂ component, with excellent oxygen-storing capacity (OSC), has been employed in various catalytic oxidation processes,¹² while its remaining challenges are to improve the CeO₂ specific surface area (SSA) and thermal stability. Researchers continue to focus on mixing metal and silicon components with CeO₂ to improve the durability, thermal stability, OSC and SSA.¹³ Although OSC and CeO₂ thermal stability could be enhanced with the CeO₂–ZrO₂ (CZ) solid solution formation by introducing Zr into the CeO₂ lattice,¹⁴ promoting SSA on the CZ support remains a great challenge. While an increase of SSA is critical to improve Pd dispersion of CZ supported Pd catalysts. It is reported that CeO₂ mixed with Zr and Al oxides (CeO₂–ZrO₂–Al₂O₃) are regarded as a new generation of oxygen-storing and thermal-stable materials for the catalytic process in the presence of oxygen.¹⁵ Improved thermal stability, durability, high SSA of Al₂O₃ and good OSC of CZ could be simultaneously maintained by introducing Al₂O₃ into CeO₂–ZrO₂ solid solution on the nanometer scale.^16,17

In general, mesoporous materials with uniform pore-size distribution and large surface area are preferably employed as a catalyst support¹⁸ because of their promotion effect on noble metal dispersion. Compared with ordered silica materials (MCM-41),¹⁹ non-silica mesoporous CeO₂–ZrO₂–Al₂O₃ catalysts support exhibited superior thermal stability and OSC. Soft and hard templates are employed to synthesize mesoporous metal oxides catalysts.²⁰ Hard templates are used to synthesize shape-controlled mesoporous materials,^21,22 but time and energy consumption, a high thermal treating temperature (more than 650 °C²²), or the relatively low yield²³ of the product are great disadvantages for the industrial application of this method. Compared with hard templates, a facile hydrothermal method reported²⁴ using commercially-available surfactants as a soft template is industrially achievable. In addition, it was found in our previous study²⁵ that the hydrothermal method has the advantage of improving the specific surface area (SSA), pore volume and catalytic activity.

The present study has attempted to employ a soft template-assisted hydrothermal method to synthesize mesoporous Pd/CeO₂, Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ catalysts for CO catalytic oxidation. Further explorations of the porous properties, kinetic behavior, as well as the active species surface abundance were conducted.

2. Experimental

2.1 Catalyst preparation

2.1.1 Preparation of CeO₂–ZrO₂–Al₂O₃. 3.5 g of Pluronic F127 (Ma 12, 600, Sigma-Aldrich), 0.009 mol of cerium nitrate, 0.009 mol of zirconium nitrate and 0.002 mol of aluminum nitrate (molar ratios of Ce : Zr : Al = 0.45 : 0.45 : 0.1) and urea (0.08 mol) were sequentially dissolved into 200 mL of deionized water at 60 °C. The resulting solution was then transferred to a stainless steel Teflon-lined autoclave and heated at 140 °C for 12 h. The final products were washed with distilled water, dried at 100 °C and calcined at 425 °C for 3 h in static air with a 3 °C min⁻¹ heating rate.

2.1.2 Preparation of CeO₂–ZrO₂. 3.5 g of Pluronic F127 (Ma 12600, Sigma-Aldrich), 0.01 mol of cerium nitrate, 0.01 mol of zirconium nitrate (molar ratios of Ce : Zr = 1 : 1) and urea (0.08 mol) were sequentially dissolved into 200 mL of deionized water at 60 °C. The resulting solution was then transferred to a stainless steel Teflon-lined autoclave and heated at 140 °C for 12 h. The final products were washed, dried and calcined, as described in 2.1.1.

2.1.3 Preparation of CeO₂. CeO₂ was prepared as described in 2.1.1 and 2.1.2, using 0.02 mol of cerium nitrate as the precursor and 3.5 g of Pluronic F127 (Ma 12600, Sigma-Aldrich) as soft template.

2.1.4 Preparation of Pd/CeO₂, Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃. CeO₂, CeO₂–ZrO₂ and CeO₂–ZrO₂–Al₂O₃ supported Pd catalysts were prepared by the wet impregnation method using an aqueous palladium nitrate solution as the precursor with 1% (wt%) Pd content. The impregnated samples were dried at 60 °C and calcined at 350 °C for 2 h. The catalysts are denoted as Pd/CeO₂, Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃, respectively.

2.2 Catalyst characterization

The X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance diffractometer system at 40 kV and 40 mA using Cu-Kα radiation (λ = 1.5406 Å). N₂ adsorption–desorption tests were carried out on a Tristar II 3020 specific surface area and porosity analyzer. The samples were evacuated at 300 °C for 4 h and then tested at liquid N₂ temperature (−196 °C). Transmission electron microscopy (TEM) was carried out on a FEI Tecnai G220 apparatus operated at 200 kV. Gatan Digital Micrograph (Version 3.9) software was employed to analyze the catalysts crystal structure and morphology. Low-temperature H₂-temperature programmed reduction (H₂-TPR) analyses were carried out in a U-shaped tube reactor (i.d. 3.0 mm). 50 mg of the sample (60–80 mesh) was pre-treated under N₂ flow at 300 °C for 1 h and then cooled to 0 °C in an ice-water bath followed by the shifted flow of H₂/Ar (5 vol% of H₂, 30 mL min⁻¹) and a linear heating rate of 2 °C min⁻¹. A thermal conductivity detector (TCD) was used to record the consumption of H₂ signals. XPS experiments were carried out on an ULVAC PHI 5000 Versa Probe-II equipped with an Al Kα UHV. The C 1s peak (284.8 eV) was used for calibrating the binding energy values.

2.3 Catalytic performance of CO oxidation

The catalytic activity of CO oxidation was carried out in a down-flow quartz-tube fixed bed reactor (i.d. 4 mm) filled with 200 mg of samples (60–80 mesh). Prior to the reaction, the catalysts were pretreated in an anhydrous air flow at 250 °C for 40 min. The reaction gas components were 1.0 vol% CO, 10.5 vol% O₂, and N₂ balanced, at a 50 mL min⁻¹ total flow rate. The effluent gas was measured by an online gas chromatograph (GC) equipped with a flame ionization detector (FID) (equipped with a methanation conversion furnace) under steady-state conditions. The CO conversion was calculated as follows:
where [CO]_in represents the CO concentration in the feed gas, [CO]_out represents the CO concentrations in the effluent gases. T₁₀₀, T₉₀ and T₅₀ were introduced to record the reaction temperature when CO conversion was 100%, 90% and 50%, respectively.

3. Results and discussion

3.1 Catalytic activity

It can be observed from Fig. 1 that T₁₀₀, T₉₀ and T₅₀ of the various catalysts increased in the order Pd/CeO₂–ZrO₂–Al₂O₃ < Pd/CeO₂–ZrO₂ < Pd/CeO₂. Therefore, it can be concluded that the addition of zirconia and aluminum oxide into Pd/CeO₂ can obviously enhance the low-temperature CO catalytic activity. Over 99% of the CO conversion could be obtained by the optimal Pd/CeO₂–ZrO₂–Al₂O₃ catalyst at 60 °C. In addition, over 99% of CO conversion could be maintained at 70 °C for over 48 h (Fig. 2), indicating superior durability and industrial application potential of the Pd/CeO₂–ZrO₂–Al₂O₃ catalyst. Further research indicated that catalytic activity (ESI Fig. S1†) of the CeO₂–ZrO₂–Al₂O₃ supported palladium catalysts could be adjusted when different soft templates were used to synthesize Pd/CeO₂–ZrO₂–Al₂O₃ (ESI S1.1 and S1.2 and Fig. S1†).

Fig. 1 Conversion of low-temperature CO catalytic oxidation over various catalysts.

Fig. 2 Stability test for the Pd/CeO₂–ZrO₂–Al₂O₃ catalyst at 70 °C (reaction gas components: 1.0 vol% CO, 10.5 vol% O₂, N₂ balanced).

3.2 XRD analysis

It can be observed from Fig. 3 that no obvious diffraction peaks of aluminum and palladium species could be observed, indicating that aluminum and palladium species existed in the forms of highly dispersed or amorphous phase with low aluminum and palladium species content. Pd/CeO₂ showed the highest CeO₂ diffraction peaks, while the addition of zirconia and aluminum oxide into Pd/CeO₂ obviously decreased the CeO₂ crystallinity. Shoulders of asymmetric peaks between peak of CeO₂ (111) at 2θ = 28.549° and ZrO₂ (111) at 2θ = 30.119° indicated the formation of Ce_xZr_1−xO₂ (0 < x < 1) solid solutions, which was reported to be critical for enhancing the OSC and catalytic activity.²⁶ However, the present XRD analysis contained inadequate structure and texture information of Ce_xZr_1−xO₂ (0 < x < 1) solid solutions. Further XPS analysis contained inadequate structure and texture information for the Ce_xZr_1−xO₂ (0 < x < 1) solid solutions. The addition of aluminum into Pd/CeO₂–ZrO₂ further enhanced catalytic conversion of CO oxidation (Fig. 1) despite the increased CeO₂ crystallinity and ZrO₂ over the Pd/CeO₂–ZrO₂–Al₂O₃ catalyst. Therefore, it could be inferred that the CeO₂ and ZrO₂ crystallinity were not the determining factors in the experimental conditions.

Fig. 3 XRD patterns of various catalysts.

3.3 N₂ adsorption–desorption tests

N₂ adsorption–desorption isotherms and NL-DFT pore size distribution curves of the various catalysts are shown in Fig. 4. All the catalysts exhibited typical desorption curves of IUPAC IV and the hysteresis of H2, indicating the characteristics of a mesoporous structure with a wide pore size distribution. All samples showed uptakes of 16–19 cm³ (g STP)⁻¹, indicating the presence of a minority of micro pores. It was notable that the N₂ adsorption–desorption isotherms of Pd/CeO₂–ZrO₂–Al₂O₃ exhibited the lowest uptake around P/P⁰ = 0, indicating the highest ratio of mesorpores over the Pd/CeO₂–ZrO₂–Al₂O₃ catalyst (ESI Table S2†).

Fig. 4 N₂ adsorption–desorption isotherms and pore size distribution of various catalysts.

It can be observed from Table 1 that addition of Zr and Al oxides into CeO₂ can slightly improve the SSA of the catalysts. However, comparing with Pd/CeO₂–ZrO₂–Al₂O₃, Pd/CeO₂–ZrO₂ exhibited no significant SSA within systematic errors. Thus, it may be concluded that SSA are not the determining factors influencing catalytic activity (Fig. 1). Adding Zr oxide and further adding Al oxides could gradually increase the pore volume and average pore size, as well as the ratios and mesopore sizes over these cerium-based palladium catalysts. Pore-size distributions over the Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ catalysts were gradually extended as well (Fig. 4). Further studies indicated that a porous structure (ESI Fig. S2 and Table S3†) and SSA (ESI Table S3) of CeO₂–ZrO₂–Al₂O₃ supported palladium catalysts could be adjusted when different soft templates were used to synthesize Pd/CeO₂–ZrO₂–Al₂O₃ (ESI S1.1, S1.2, Fig. S2 and Table S3†).

Table 1 Specific surface area (SSA), pore volume and pore size of the various catalysts

Samples	SSA (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore size (nm)
Pd/CeO2	98	0.084	3.9
Pd/CeO2–ZrO2	106	0.115	5.0
Pd/CeO2–ZrO2–Al2O3	105	0.127	5.5

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3.4 TEM analysis

TEM images of the various catalysts are shown in Fig. 5. No obvious characteristic lattice fringes of aluminum oxides and aggregated palladium particles could be observed from the TEM analysis, indicating high dispersion and low crystallinity of aluminum oxides and palladium species, which is consistent with the XRD analysis. Projected images of Pd/CeO₂ (Fig. 5(b)) exhibited compactly accumulated non-uniform particles with 15 to 80 nm particle size. Projected images of Pd/CeO₂–ZrO₂ (Fig. 5(d)) exhibited accumulated nanoparticles with single particles sized 5–25 nm, while Pd/CeO₂–ZrO₂–Al₂O₃ (Fig. 5(f)) exhibited uniform dispersed nanoparticles with particles sized 5–10 nm. Therefore, it can be inferred that addition of Zr into the Pd/CeO₂ can decrease supports particle size. Further addition of Al into the Pd/CeO₂–ZrO₂ could lead to formation of uniformly dispersed nanoparticles and the support particle size could be further decreased as well. In terms of particle size over these cerium-based catalysts supports, the particle size decrease and uniform dispersed nanoparticles are preferred to enhance the low-temperature catalytic CO conversion.^22,27

Fig. 5 TEM images of (a) and (b) Pd/CeO₂; (c) and (d) Pd/Ce–ZrO₂; (e) and (f) Pd/CeO₂–ZrO₂–Al₂O₃ catalysts.

In addition, all catalysts listed (Fig. 5(b), (d) and (f)) possessed characteristics of interparticle space and sponge-like pore (with a pore size below 10 nm) networks with mesoporous size homogeneously distributed within the particles, indicating a mesoporous catalyst structure, which is consistent with our XRD and N₂ adsorption desorption analysis. The lattice fringes of all the catalysts listed (Fig. 5(a), (c) and (e)) with a 0.314 nm interplanar distance revealed the (111) lattice plane of the cubic CeO₂ structure. Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ with 0.295 nm interplanar distances (Fig. 5(c) and (e)) could be attributed to lattice planes of ZrO₂ (111), indicating the existence of the ZrO₂ structure.

3.5 Low-temperature H₂-TPR analysis

Low-temperature H₂-TPR profiles of various catalysts are shown in Fig. 6. The peaks denoted with α for Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ could be attributed to reduction of the surface exposed oxygen-containing PdO species.²⁸ A shoulder in Pd/CeO₂–ZrO₂–Al₂O₃, denoted with β indicates the surface dissociated trans-state hydrogen (H–Pd–H) assisted reduction of surface exposed Pd_xM_1−xO_2−δ (M = Ce, Zr) structure;^28,29 Pd/CeO₂–ZrO₂–Al₂O₃ showed overall higher H₂ consumption than Pd/CeO₂–ZrO₂ from 0 to 80 °C, indicating a higher surface active Pd site dispersion over the Pd/CeO₂–ZrO₂–Al₂O₃ catalyst, while no obvious uptake of H₂ consumption could be observed from the low-temperature H₂-TPR profile of Pd/CeO₂, indicating that palladium species over Pd/CeO₂ catalyst might be incompletely exposed or the exposed palladium species over Pd/CeO₂ are harder to be activated than that of Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ below 80 °C. Further study of H₂-TPR (provided in the ESI Fig. S5†) indicated uptake of H₂ consumption could be observed above 100 °C. Thus, it could be concluded that different abundances of surface exposed active palladium species and surface activation behavior of oxygen and CO may exist among these catalysts. Therefore, to determine the ratios of the surface exposed palladium species and activation energy over these palladium-containing catalysts, further surface XPS analysis, as well as a kinetic study was performed to explore the potential influencing factors and kinetic behaviors of the Pd/CeO₂, Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ catalysts.

Fig. 6 Low-temperature H₂-TPR profiles of the various catalysts.

3.6 XPS analysis

The Pd electronic state is critical for catalytic activity for many catalytic process over Pd-based catalysts^30,31 and X-ray photoelectron spectroscopy (XPS) analysis was employed to explore the Pd electronic state. XPS spectra of Pd 3d and Zr 3p³² are shown in Fig. 7. It can also be observed in Fig. 7 that all palladium existed in the Pd(II) form (surface PdO and strong metal-support interacted Pd(II) (Pd_xM_1−xO_2−δ)),^33,34 which is consistent with the low-temperature H₂-TPR analysis (Fig. 6). Previous studies on positively charged Pd(II) species indicated that surface PdO exhibited a higher catalytic activity of CO oxidation than Pd_IMSI.^27,35 In the present study, however, it was hard to obtain a quantitative analysis for surface PdO and Pd_xM_1−xO_2−δ from the XPS analysis as the influence of the Zr 3p peaks at 331.7 eV and 345.3 eV were less than 1.5 eV from the Pd_IMSI Pd 3d peak. Despite the lack of quantitive analysis of the different Pd(II) species, the low-temperature H₂-TPR analysis (Fig. 6) indicated the existence of hydrogen-activated surface PdO species over both Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃ below 80 °C and in the absence of surface hydrogen-activated PdO species over Pd/CeO₂ at 0 to 80 °C.

Fig. 7 Pd 3d and Zr 3p XPS spectra.

XPS spectra of the Ce 3d are shown in Fig. 8. The BE peaks marked as v′₀ (902.2 eV), v′₁ (908.3 eV) and v′₂ (917.5 eV) arise from Ce⁴⁺ 3d_3/2, while the BE peaks labeled as v₀ (882.5 eV), v₁ (889.1 eV) and v₂ (898.4 eV) arise from Ce⁴⁺ 3d_5/2. The u′₀ (904.5 eV) and u₀ (885.4 eV), refer to the 3d_3/2 and 3d_5/2 orbitals, respectively, suggesting the presence of a Ce³⁺ final state.^36–38 The O 1s XPS spectra of the catalysts can be resolved into two peaks, as shown in Fig. 9. The peaks at around 529.5 eV (denoted with O_lattice) are the characteristic peak of metal–oxygen (the lattice oxygen O²⁻) and the peak at 531.5 eV (denoted with O_adsorbed) is usually associated with surface adsorbed oxygen (chemisorbed oxygen OH⁻ or CO₃²⁻).^39,40 It has been reported^26,41,42 that surface molar ratios of Ce³⁺/Ce⁴⁺ and O_adsorbed/O_lattice have an important influence on the oxygen storing and releasing capacity in the presence of oxygen due to the dynamic balance of between the losses of electron from CO and gains of electrons from gaseous, adsorbed and lattice oxygen.^{9,26,29,41,42} In the present study, the surface molar ratios of Ce³⁺/Ce⁴⁺ and O_adsorbed/O_lattice were analyzed over various catalysts using ULVAC-PHI MultiPak™ Software Manual (Version 9, ULVAC-PHI Inc.) and XPS PeakFit (Version 4.0, AISN Software Inc.) software.

Fig. 8 Ce 3d XPS spectra.

Fig. 9 O 2s XPS spectra.

The surface atomic ratios of Ce³⁺/Ce⁴⁺ were determined according to the following equation:

where S₁, S₂, S₃, S₄, S₅, S₆, S₇ and S₈ represent the peak areas of the corresponding v₀, u₀, v₁, v₂, v′₀, u′₀, v′₁ and v′₂ lines, respectively.

The surface atomic ratios of O_adsorbed/O_lattice were calculated according to the following formula:

O_adsorbed/O_lattice = S_adsorbed/S_lattice,

where S_adsorbed and S_lattice are the peak areas corresponding to O²⁻ and OH⁻/CO₃²⁻, respectively.

Surface atomic ratios of Ce³⁺/Ce⁴⁺ and O_adsorbed/O_lattice are listed in Table 2. With the addition of zirconium and aluminum into the Pd/CeO₂, surface Ce³⁺/Ce⁴⁺ and O_adsorbed/O_lattice, as well as the low-temperature catalytic activity (Fig. 1) increased in the order Pd/CeO₂ < Pd/CeO₂–ZrO₂ < Pd/CeO₂–ZrO₂–Al₂O₃ (Table 2).

Table 2 Surface atomic ratios, activation energy (E_a) and T₅₀ of the various catalysts

Samples	Surface cerium^a	Surface oxygen^a	Ea^b (kJ mol^−1)	T50 (°C)
	Ce^3+/Ce^4+	Oadsorbed/Olattice
a Derived from XPS analysis, calculated by ULVAC-PHI MultiPak™ Software Manual (Version 9, ULVAC-PHI Inc.) and XPS PeakFit (Version 4.0, AISN Software Inc.).b Calculated from the CO oxidation Arrhenius dependencies (Fig. 11).
Pd/CeO2–ZrO2–Al2O3	0.215	1.398	32.4	42
Pd/CeO2–ZrO2	0.212	0.923	40.7	55
Pd/CeO2	0.196	0.862	50.4	68

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3.7 Kinetic study of CO oxidation over supported Pd catalysts

Fig. 10 shows the effect of CO concentration on the reaction rate at different temperatures. In these kinetic experiments, temperature was varied in the range of 313–323 K to keep CO conversion below 20%, i.e., a differential reactor approach could be assumed.⁴³

Fig. 10 Effects of CO concentration and temperature on the CO oxidation rate, with Pd/CeO₂, Pd/CeO₂–ZrO₂ and Pd/CeO₂–ZrO₂–Al₂O₃, in the 313–323 K range. (The concentration of O₂ was maintained constant).

The effects of CO concentration and temperature on the CO oxidation rate at 313–323 K indicated that the reaction rate increased monotonically with CO concentration (Fig. 10). Therefore, it may inferred that the reaction was not inhibited by the CO concentration, despite the low-temperature catalytic oxidation (LTO) of CO over pure noble-metal catalysts under vacuum conditions generally occurring by the Langmuir–Hinshelwood (L–H) or Eley–Rideal (E–R) mechanisms^44,45 based on a single site for CO and O₂ adsorption. However, it has been reported⁴⁶ that the reaction order with respect to oxygen should approach zero, while a positive order with respect to CO is expected. Because CO could be first adsorbed on the surface active sites of the supported noble-metal catalysts and react on the surface (set to control steps) to form CO₂,^3,9,29 followed by CO₂ desorption, the reaction can be simplified to zero order with respect to oxygen and +1 with respect to CO for low CO concentrations.⁴⁰ The reaction rate was calculated using the formula:^43,47 W(molecules per cm² per s) = (C⁰_CO × X × V_RM)/(m × S_sp), where C⁰_CO is the initial concentration of CO (molecules per cm³), X is the CO conversion, V_RM is the reaction mixture flow rate (cm³ s⁻¹), m is the catalyst weight (g), and S_sp is the specific surface area (cm² g⁻¹).

Arrhenius dependences of the CO oxidation rate for various catalysts are shown in Fig. 11. The calculated activation energies⁴⁸ of the various catalysts decreased in the order Pd/CeO₂ (E_a = 50.4 kJ mol⁻¹) > Pd/CeZrO₂ (E_a = 40.7 kJ mol⁻¹) > Pd/CeO₂–ZrO₂–Al₂O₃ (E_a = 32.4 kJ mol⁻¹) (Table 2). Taking the significant differences of E_a (Table 2), H₂-TPR (Fig. 6) results and previous reports^3,9,29,47 into consideration, the dramatic catalytic activity differences over these catalysts may be caused by different adsorption and (or) activation behaviors of CO. However, further mechanistic study into this is expected.

Fig. 11 Arrhenius dependences of CO oxidation rate over various catalysts.

4. Conclusions

Mesoporous nano CeO₂, CeO₂–ZrO₂ and CeO₂–ZrO₂–Al₂O₃ supported palladium catalysts were prepared and employed in low-temperature CO catalytic oxidation. The optimal Pd/CeO₂–ZrO₂–Al₂O₃ catalyst exhibited over 99% CO conversion at 70 °C for more than 48 hours. The surface atomic ratios of Ce³⁺/Ce⁴⁺ and O_adsorbed/O_lattice, pore volume, average pore size and low-temperature CO catalytic conversion gradually increased with the addition of Zr into the Pd/CeO₂ and with further addition of Al into Pd/CeO₂–ZrO₂, while particle size of these cerium-based support decreased with the addition of Zr into Pd/CeO₂ and further addition of Al into Pd/CeO₂–ZrO₂. In addition, the activation energy (E_a) decreased in the order Pd/CeO₂ (50.4 kJ mol⁻¹) > Pd/CeZrO₂ (40.7 kJ mol⁻¹) > Pd/CeO₂–ZrO₂–Al₂O₃ (32.4 kJ mol⁻¹), which was critical to the increased low-temperature CO catalytic conversion.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (No. 21307047).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05193f

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