Dispersed PdO Research Articles

Novel Nanostructured Pd/Co-Alumina Materials for the Catalytic Oxidation of Atmospheric Pollutants.

Cobalt-doped alumina catalysts were prepared using different methods, either conventional wet impregnation (WI) and/or advanced spray impregnation (SI), and they were evaluated as novel oxidation catalysts for CO and MeOH oxidation. The spray impregnation technique was used with the aim of achieving the synthesis of core-shell catalytic nanostructures to secure the chemical/thermal stability of active sites on the catalyst carrier. The catalysts were further promoted with a low Pd content (0.5 wt.%) incorporated via either incipient wetness impregnation (DI) or spray impregnation. The results revealed the superior performance of the spray-impregnated catalysts (Co/γ-Al2O3-SI) for both reactions. The deposition of Co oxide on the outer surface of the alumina particle (SEM images) and the availability of the active Co phase resulted in the enhancement of the Co/γ-Al2O3 catalysts' oxidation activity. Pd incorporation increased the catalysts' reducibility (TPR-H2) and improved the catalysts' performance for both reactions. However, the Pd incorporation method affected the catalytic performance; with the SI method, the active phase of Co3O4 was probably covered with PdO and was not available for the oxidation reactions. On the contrary, the incorporation of Pd with the DI method resulted in a better dispersion of PdO all over the Co/Al catalyst surface, maintaining available Co active sites and a better Pd-Co interaction. MeOH desorption studies revealed the methanol oxidation mechanism: the Co/Al catalysts promoted the partial oxidation of MeOH to formaldehyde (HCHO) and dehydration to dimethyl ether (DME), while the Pd-based Co/Al catalysts enhanced the complete oxidation of methanol to CO2 and H2O.

Lean methane combustion over zeolite-supported Pd catalysts: Structure-performance relationship and deactivation mechanism

Zeolites are a promising support for Pd catalysts in lean methane (CH4) combustion. Herein, three types of zeolites (H-MOR, H-ZSM-5 and H-Y) were selected to estimate their structural effects and deactivation mechanisms in CH4 combustion. We show that variations in zeolite structure and surface acidity led to distinct changes in Pd states. Pd/H-MOR with external high-dispersing Pd nanoparticles exhibited the best apparent activity, with activation energy (Ea) at 73 kJ/mol, while Pd/H-ZSM-5 displayed the highest turnover frequency (TOF) at 19.6 × 10−3 sec−1, presumably owing to its large particles with more step sites providing active sites in one particle for CH4 activation. Pd/H-Y with dispersed PdO within pore channels and/or Pd2+ ions on ion-exchange sites yielded the lowest apparent activity and TOF. Furthermore, Pd/H-MOR and Pd/H-ZSM-5 were both stable under a dry condition, but introducing 3 vol.% H2O caused the CH4 conversion rate on Pd/H-MOR drop from 100% to 63% and that on Pd/H-ZSM-5 decreased remarkably from 82% to 36%. The former was shown to originate from zeolite structural dealumination, and the latter principally owed to Pd aggregation and the loss of active PdO.

Surface graft for effective PdO nanoparticles anchoring on Beta zeolite with outstanding lean methane combustion performance

A post-synthetic modification strategy for anchoring PdO nanoparticles on the Beta zeolite surface and obtaining stable and highly dispersed PdO sites with abundant active Pdn+ (0 < n < 2) species was reported. The modified Beta supported Pd catalyst with a 0.94 wt% Pd loading amount shows (>) 99% methane conversion at 375 °C with a space velocity of 30,000 mL h−1 g−1 and a satisfied long-term (>30 h) CH4 oxidation stability at 340 °C. Such a post-synthetic strategy provides a new way to disperse and stabilize noble metal particles on zeolite support for harsh catalytic process.

Comparative Study of Pd-Mayenite Catalysts Prepared via Aerogel Approaches.

Pd-containing catalysts based on highly dispersed aerogel-derived mayenite were prepared via two approaches. The Pd@C12A7 sample was obtained through the addition of Pd nitrate solution to a fresh Ca(OH)2-Al(OH)3 gel. Pd/C12A7 was synthesized through conventional wet impregnation of the aerogel-derived mayenite. The evolution of the textural characteristics of the support (C12A7) depending on the calcination temperature was investigated. Pd-containing samples were explored using transmission electron microscopy and spin probe EPR spectroscopy. Using the latter method, the presence of active oxygen species capable of producing nitroxyl radicals from diphenylamine was observed. The activity of these species and the reproducibility of their redox behavior were studied in three cycles of temperature-programmed reduction in both hydrogen and CO atmospheres. A prompt thermal aging technique was used to access and compare the activity of the samples towards CO oxidation. The state of Pd species before and after the aging procedure was studied via UV-Vis spectroscopy. It was found that the dispersion of PdO was higher in the case of the Pd/C12A7 catalysts compared to the Pd@C12A7 sample. This is why the Pd/C12A7 catalyst demonstrated higher activity in CO oxidation and better reducibility in TPR cycles.

Insight into the roles of Pd state and CeO2 property in C3H8 catalytic oxidation on Pd/CeO2

Propane oxidation is a key reaction in the three-way catalytic reaction. The dispersion state of Pd species and property of CeO2 have significant impacts on the C3H8 oxidation over Pd/CeO2 catalyst via altering the activation pathways of C3H8. Herein, Pd/CeO2 catalysts with highly dispersed Pd species (Pd/CeO2-IWI) and uniform PdO nanoparticles (4.3 ± 0.3 nm) (Pd/CeO2-EG) were prepared, respectively. The highly dispersed Pd species on Pd/CeO2-IWI promoted Pd entering the lattice to form PdxCe1-xO2-σ solid solution through strong Pd-CeO2 interaction, while PdO nanoparticles were the main existing form on Pd/CeO2-EG. The results showed that C3H8 was adsorbed and activated to form acrylate on PdxCe1-xO2-σ, while C3H8 preferred to generate acetone on PdO. However, the main surface intermediates were converted to acetate and formate at the elevated reaction temperature. Compared with PdxCe1-xO2-σ, PdO was more favorable for the rapid oxidative decomposition of acetate to formate, which facilitated the C3H8 oxidation activity. Meanwhile, the property of CeO2 could also influence the C3H8 oxidation activity through the accumulation of formate and carbonate. In which, Pd/CeO2(1000)-EG with the most PdO species and the lowest accumulation of surface formate/carbonate performed the highest C3H8 oxidation activity.

Precisely deposited Pd on ZnO (002) facets derived from complex reduction strategy for methanol steam reforming

Pd/ZnO catalysts are promising for onboard hydrogen production through methanol steam reforming (MSR), but their selectivity still requires improving. In this research, a flower-like spherical ZnO material with mainly exposed (002) facet was prepared through surfactant-assisted hydrothermal method. The surfactant tartaric groups also serve as coordinating groups to adsorb Pd2+ to form a complex, which is further reduced to metallic Pd. By virtue of preferential adsorption of the surfactant on (002) facet, Pd particles are confined to (002) facet. The calcined PdO-ZnO-T-X catalyst shows highly dispersed PdO nanoparticles. Characterization results suggest that strong interaction between ZnO (002) facet and small PdO particles can facilitate complete PdO reduction and PdZn formation, and can effectively retard PdZn decomposition under oxidizing atmosphere. This research discloses structure-performance relationship of Pd/ZnO catalysts and provides a novel strategy for further improving their MSR performance.

A hyperbranched polymer synthetic strategy for the efficient fixation of metal species within nanoporous structures: Application in automotive catalysis

In the present study, a novel synthetic approach, called assisted impregnation technique, is described for the production of highly active Pd monometallic three-way catalysts supported on SBA-15. The assistance in this method originates from a hyperbranched polymer (carboxy-methylated polyethyleneimine, Trilon-P), able to entrap metal ions and therefore act as a metal binding agent due to its unique chemical and chelating properties. A series of Pd / SBA-15 samples with different Pd loadings, varying from 0.13 to 3 wt%, were produced. All the as-developed materials, evaluated under simulated exhaust conditions at the stoichiometric point, exhibit an excellent CO and HC oxidation performance, providing even very high CH4 conversions at low temperatures. However, material with 3 wt% Pd loading stands out affording complete NO reduction at 300 °C. The positive effect of the employed synthetic procedure is clearly demonstrated by comparing the latter sample with its counterpart derived from conventional dry impregnation technique. As deduced by a combination of techniques, assisted impregnation technique succeeds to deliver very small highly dispersed PdO nanoparticles uniformly distributed within SBA-15 porous network, through metal-polymer complex formation. The derived PdO nanoparticles demonstrate an average size ranging between 2.1 and 2.4 nm with tight distribution and pronounced stability considering the negligible aggregation and particle growth phenomena after the catalytic reaction. On the contrary when dry impregnation is employed the size of PdO nanoparticles undergoes a two-fold increase from approximately 3 to 6 nm upon exposure at automotive exhaust conditions.

Combination of reduction-deposition Pd loading and zeolite dealumination as an effective route for promoting methane combustion over Pd/Beta

Catalytic removal of trace methane emission over supported Pd catalysts has drawn much attention over the years. For practical application, high activity, excellent thermal stability and outstanding water resistance are always desirable. Herein, we report a facile route for simultaneously improving the activity, stability and water resistance of the zeolite supported Pd catalyst through the combination of reduction-deposition Pd loading and support dealumination. This combination strategy leads to highly dispersed and stable PdO nanoparticles with strong interaction with the Beta support, rich active Pdn+ (0 < n < 2), and plentiful surface chemisorbed oxygen. The as-designed Pd/Beta shows a high methane conversion above 99 % at 375 °C with a space velocity of 30,000 mL h−1 g−1 and 1 vol.% CH4 feed concentration, enhanced long-term stability, and greatly improved water resistance with a low 0.5 wt.% Pd loading amount. Such a synthetic strategy is expected to provide a useful approach towards designing efficient zeolite supported noble metal catalysts.

Superior oxygen transfer ability of Pd/MnOx-CeO2 for enhanced low temperature CO oxidation activity

The low temperature CO oxidation activity of Pd doped MnOx-CeO2 (MC) solid solution catalyst is evaluated to determine the role of Pd speciation and oxygen transfer. Dynamic reduction behavior of Pd2+ species in the presence of CO at low temperatures is characterized by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and X-ray absorption spectroscopy (XAS). H2 temperature-programmed reduction (TPR) and CO/O2 transient pulse experiments are conducted to evaluate the lattice oxygen reducibility of Pd/CeO2 and Pd/MC.Our results show that highly dispersed PdO species form on the freshly calcined CeO2 and MC supports. Despite the fact that similar Pd species form on the CeO2 and MC supports, PdO can be reduced immediately on CeO2 in the presence of CO, while the MC support can preserve the oxidized Pd species during CO reduction. In the case of CO oxidation, Pd2+ species are maintained through lattice oxygen transfer facilitated by the MC support. CO/O2 transient pulse experiments confirm the higher reducibility of the MC support, which can favor CO oxidation at 50°C.

Effect of Pt addition on sulfur dioxide and water vapor tolerance of Pd-Mn-hexaaluminate catalysts for high-temperature oxidation of methane

An effect of Pt addition to improve the catalytic performance of Pd-modified Mn-hexaaluminate in the high-temperature oxidation of methane, especially in SO2 and water presence, has been studied. X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry and differential thermal analysis were used for the characterization of fresh and spent catalysts. Temperature-programmed reduction by hydrogen was employed to study the redox properties of the catalysts. Four high-temperature tests (at 670°C) of the methane oxidation were applied to reveal the water and sulfur resistance of the catalysts: 1) dry methane-air feed; 2) wet methane-air feed containing 3wt% of water vapor; 3) methane-air feed containing 1000ppm of SO2; and 4) wet methane-air feed with 1000ppm of SO2. The Pt-doped Pd-Mn-hexaaluminate catalyst with the atomic ratio of Pt/Pd<0.3 has been shown to possess the highest catalytic activity in the oxidation of methane, high water and sulfur tolerance, and reducibility by hydrogen as compared to the monometallic Pd-Mn-hexaaluminate catalyst and the bimetallic catalysts with Pt/Pd>0.5. From the formal kinetic data of the oxidation of methane on the fresh and deactivated catalysts, we assumed that deactivation of the catalysts is due to a decrease in the amount of catalytically active sites in the bimetallic 0.33Pt-0.67Pd(0.27)/MnLaAl11O19 catalyst and a change in the state of active component in the other catalysts. Their oxidation ability and redox behavior were shown to be associated with the highly dispersed PdO particles, a PdO layer (3–5nm) covering the metal palladium particles as well as the Mn3+-enriched MnLaAl11O19. The improved water and sulfur resistance is found to correlate with the presence of particles of PtPd alloy, with its fraction not exceeding 50%.

P-Type PdO nanoparticles supported on n-type WO3 nanoneedles for hydrogen sensing

We report the synthesis of palladium nanoparticle (NP) decorated WO3 nanoneedles (NNs) employing a single-step, aerosol assisted chemical vapor deposition approach. Two different Pd precursors were investigated in view of optimizing the morphology and the gas sensing performance of the resulting nanostructured films. In particular, palladium acetylacetonate was found to be more suitable than ammonium hexachloropalladate for obtaining n-type WO3 NNs uniformly decorated with well dispersed p-type PdO NPs. The active films could be directly deposited on the electrode area of microelectromechanical system-based resistive transducers. The morphology and chemical composition of the films was investigated by scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy analysis. PdO-decorated WO3 NNs show a response toward hydrogen that is about 680 times higher than that of bare WO3 NNs. Finally, PdO-loaded sensors display extremely low-cross sensitivity to water vapor, which makes them remarkably immune to changes in the background humidity.

Surfactant-Mediated One-Pot Method To Prepare Pd-CeO2 Colloidal Assembled Spheres and Their Enhanced Catalytic Performance for CO Oxidation.

A simple, one-pot method to fabricate ordered, monodispersed Pd–CeO2 colloidal assembled spheres (CASs) was developed using the surfactant-mediated solvothermal approach, which involves a tunable self-assembled process by carefully controlling different chemical reactions. The evolution process and formation mechanism of the CASs were thoroughly investigated by time-controlled and component-controlled experiments. For CO oxidation, this CAS nanocatalyst exhibited much higher catalytic activity and thermal stability than Pd/CeO2 prepared by an impregnation method, and its complete CO conversion temperature is ∼120 °C. The enhanced catalytic performance for CO oxidation could be attributed to the synergistic effect of highly dispersed PdO species and Pd2+ ions incorporated into the CeO2 lattice. For this CAS catalyst, each sphere can be viewed as a single reactor, and its catalytic performance can be further improved after being supported on alumina, which is obviously higher than results previously reported. Furthermore, this method was used to successfully prepare M–CeO2 CASs (M = Pt, Cu, Mn, Co), showing further that this is a new and ideal approach for fabricating active and stable ceria-based materials.

A facile synthesis of Pd-doped SnO2 hollow microcubes with enhanced sensing performance

A novel and convenient synthesis of Pd-doped SnO2 hollow microcubes (PdO–SnO2 HCs) have been achieved via successively annealing and acid etching cube-like hollow zinc hydroxystannate (ZnSn(OH)6) decorated with Pd(OH)2. The resulting PdO–SnO2 HCs possess special cube-like hollow interiors with void architectures together with a porous surface composed of nano-sized primary SnO2 nanoparticles. Utilized as the composite materials in sensors, the PdO–SnO2 HCs doping of Pd with 1.0% molar ratio (1.0 mol% PdO–SnO2) demonstrated an excellent selectivity towards ethanol in various gases at an operating temperature of 300°C, performing with a high response value of 90 along with a short response and recovery time (3s, 22s) to 200ppm ethanol. Such a superior sensing performance of the PdO–SnO2 HCs could be attributed to the synergistic interactions between the uniformly dispersed PdO and the SnO2-based hollow architectures, in which a hollow framework of SnO2 provides a large active surface area and the catalytic effect of the PdO enhances the chemisorption and dissociation of the target gas. With a simple fabrication procedure and a desirable sensing performance, this method offers a highly promising candidate for the development of special morphologies of materials for sensors.

Preparation and characterization of Pd loaded Sr-deficient K2NiF4-type (La, Sr)2MnO4 catalysts for NO–CO reaction

Pd-loaded K2NiF4-type perovskite oxides, Pd/La0.2Sr1.8−xMnO4 were prepared by liquid-phase adsorption process, and their structural and catalytic properties were investigated. Pd deposition was carried out by mixing La0.2Sr1.8MnO4 with Pd(NO3)2 in aqueous solution, followed by filtration, drying, and calcination at 800°C. During the Pd deposition process, 5% of Sr in La0.2Sr1.8MnO4 oxides was dissolved into the aqueous phase, whereas the perovskite structure retained. The content of Pd loaded on La0.2Sr1.8MnO4 was 0.24wt%. X-ray absorption spectroscopy studies showed that highly dispersed PdO nanoparticles were supported on Pd/La0.2Sr1.8MnO4 prepared by the liquid-phase adsorption process, whereas aggregated PdO particles were formed on Pd/La0.2Sr1.8MnO4 prepared by an impregnation method. The Pd/La0.2Sr1.8MnO4 catalyst prepared by the liquid-phase adsorption process exhibited higher catalytic activity for NO–CO reaction than the 0.24wt% Pd/La0.2Sr1.8MnO4 catalyst prepared by the impregnation method.

Preparation and characterization of a novel washcoat material and supported Pd catalysts

A nanosized Ce0.5Mn0.5O1.5 powder was successfully synthesized by macromolecule surfactant modified method and then it was used as the cordierite honeycomb washcoat of VOCs combustion catalysts. The effect of Ce-Mn-O powder with different Ce/Mn molar ratio on the catalytic activity and the cohesive ability of washcoat on the cordierite honeycomb substrate was also investigated. The structure and catalytic properties of Ce0.5Mn0.5O1.5 washcoat were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), differential thermal analysis and thermo-gravimetric analysis (TG/DTA), and N2 adsorption and desorption techniques. The results indicated that the Ce0.5Mn0.5O1.5 powder belongs to a novel washcoat with the advantages of strong cohesiveness, high surface area and without agglomeration even after 1000°C calcination. The total oxidation temperature for toluene, acetone and ethyl acetate over the 0.1 wt % Pd/Ce0.5Mn0.5O1.5/cordierite honeycomb catalysts was at 200, 220 and 220°C, respectively. Compaired with the catalysts without Ce0.5Mn0.5O1.5 washcoat, the catalyst with Ce0.5Mn0.5O1.5 as washcoat on the cordierite honeycomb could improve the dispersion of PdO and shows a good catalytic activity for toluene, acetone and ethyl acetate.

Short channeled Zr-Ce-SBA-15 supported palladium catalysts for toluene catalytic oxidation

Short channeled Zr-Ce-SBA-15 (ZCS) mesoporous materials were synthesized through hydrothermal routes without addition of mineral acids. 0.5 wt.% palladium was loaded on ZCS and SBA-15 via an ethanol reduction approach. Pd/ZCS possesses unique hexagonal platelet morphologies with short channels running parallel to the thickness in the range of 400–500 nm, while Pd/SBA-15 has a fibrous macrostructure with channels at the micrometer scale. Palladium species present in supports as well dispersed PdO nanocrystals with diameter of ca. 5–6 nm. Comparing with Pd/SBA-15, Pd/ZCS shows enhanced catalytic activity for toluene oxidation, which is ascribed to short channeled supports facilitating the molecular diffusion. ► Short channeled Zr-Ce-SBA-15(ZCS) was synthesized without addition of mineral acids. ► ZCS possesses unique hexagonal platelet morphologies. ► Pd/ZCS catalysts were prepared by ethanol reduction approach. ► Palladium species well dispersed in supports as PdO nanocrystals of ca. 5–6 nm. ► Pd/ZCS catalysts show enhanced catalytic activity for toluene oxidation.

Au–Pd alloying-promoted thermal decomposition of PdO supported on SiO 2 and its effect on the catalytic performance in CO oxidation

We have prepared a series of Au–Pd/SiO 2 catalysts by the routine deposition–precipitation method followed by calcination at 200 °C in air for 4 h. The structures of these catalysts have been characterized with powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and H 2-temperature-programmed reduction. Their catalytic activity in CO oxidation has also been studied. We found that the preparation procedure leads to the formation of highly dispersed PdO supported on SiO 2 in Pd/SiO 2, but to the formation of both highly dispersed PdO and Au x Pd alloy nanoparticles in Au–Pd/SiO 2 catalysts. The fraction of metallic Pd in Pd species increases with the increase of Au:Pd molar ratio in Au–Pd/SiO 2 catalysts. We proposed that the alloying of Au and Pd can promote the thermal decomposition of PdO supported on SiO 2. Because of the existence of metallic Pd, Au–Pd/SiO 2 catalysts are more active in CO oxidation than Pd/SiO 2. Au–Pd/SiO 2-0.5 exhibits the same activity as Pd/SiO 2-H 2 in CO oxidation. Our results provide some novel information on the fabrication of supported Au x Pd alloy nanoparticles.

Catalytic oxidation of toluene over Pd/Co3AlO catalysts derived from hydrotalcite-like compounds: Effects of preparation methods

A series of novel Pd/Co3AlO catalysts derived from hydrotalcite-like compounds (HTlcs) have been prepared and investigated for total oxidation of toluene. The HTlcs phase Co–Al precursors were prepared by coprecipitation method and Pd active species were introduced by different approaches, i.e., impregnation (IMP), wet ion exchange (WIE) or directly at coprecipitation stage (COP). It is found that all hydrotalcite-derived Pd/Co3AlO catalysts are much more active than the Pd/Co3AlO catalyst prepared via traditional thermal combustion method (TCB) in toluene elimination. The activities of all synthesized catalysts obey the following sequence: Pd/Co3AlO (COP)>Pd/Co3AlO (WIE)≥Pd/Co3AlO (IMP)>Pd/Co3AlO (TCB). Excellent catalytic activities of the novel hydrotalicite-derived Pd/Co3AlO catalysts could be contributed to their high surface area, small mean crystallized size of support and highly dispersed PdO particles. Besides, they are well positively associated with catalyst reducibilities and the amounts of oxygen vacancies. According to the XPS, TEM and TPR results, it was found that the strong synergistic effect between Co3O4 and PdO, but not the amount of the surface palladium species which are generally believed to be the active sites, is the main factors determining the catalytic activity in this work. The stability tests indicate that all Pd/Co3AlO catalysts have no obvious deactivation, and some catalysts even show improved activities during the reaction process due to the emergence of metal Pd.

Methane Combustion over Pd Catalysts Loaded on Medium and Large Pore Zeolites

Methane combustion was conducted over Pd catalysts supported on medium (ZSM-5, ferrierite, TNU-9, TNU-10) and large (USY, mordenite, beta) pore zeolites. Among the catalysts studied here, Pd/H-TNU-10 exhibited the highest combustion activity, together with excellent durability in the presence of water vapor. A combination of TEM and Pd K-edge XAFS measurements reveals the formation of highly dispersed PdO particles within the TNU-10 pores. This is also the case of Pd/H-ferrierite, while frequently there are large particles (60–100 A) on its outer surface. In contrast, most of PdO on zeolites other than these two medium pore materials were found to exist as aggregated particles (>50 A) on their outer crystallite surface. It appears that zeolite structures with intersecting 10- and 8-ring pores may be better supports for stabilizing nanometric PdO particles than those with the uniform 10-ring size or with the 12-ring pore system.

Effects of CeO 2-ZrO 2 present in Pd/Al 2O 3 catalysts on the redox behavior of PdO x and their combustion activity

The ceria-zirconium-modified alumina-supported palladium catalysts are prepared using impregnation method with H 2PdCl 4 as Pd source, hydrazine hydrate as reducing agent. The physicochemical properties of these catalysts are characterized by BET surface area (BET), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (H 2-TPR) and temperature programmed oxidation (O 2-TPO) techniques, and their catalytic activities for the combustion of methane are examined. The results show that the palladium mainly exist in a highly dispersed PdO species on Ce-Zr-rich grains as well as Al 2O 3-rich grains surfaces, and a stable PdO species due to the strong interaction between PdO and CeO 2-ZrO 2 on the Ce-Zr/Al 2O 3 surfaces. The catalytic activity is strongly related to the redox behavior of PdO species highly dispersed on Ce-Zr-rich grains and Al 2O 3-rich grains surfaces, and the higher the reducibility of the PdO species, the higher the catalytic activity. The presence of Ce-Zr in Pd/Al 2O 3 catalyst would inhibit the site growth of PdO x particles and decomposition of PdO to Pd 0, and the reoxidation property of Pd 0 to PdO x is significantly improved, which obviously increases thermal stability and catalytic activity of Pd/Ce-Zr/Al 2O 3 catalyst for the methane combustion.