Role of La2O3 in Pd-Supported Catalysts for Methanol Decomposition

Systems of Pd supported on various La₂O₃-modified -Al₂O₃ and CeO₂–Al₂O₃ catalysts were tested for catalytic methanol decomposition and characterized by means of XRD, BET, TPR, H₂-chemisorption and CO–FTIR. The addition of lanthanum significantly improved the selectivity of CO and H₂ for all the catalysts but showed a different influence on the catalytic activity in two systems. Methanol conversion decreased on La₂O₃-modified Pd/-Al₂O₃ catalysts, in line with the reduction of Pd dispersion, while the addition of La₂O₃ improved the dispersion of Pd and reinforced Pd–CeO₂ interaction for La₂O₃-modified Pd/CeO₂–Al₂O₃ catalysts, which resulted in a high production rate of CO and H₂. Thus, a synergistic effect between CeO₂ and La₂O₃ was observed on -Al₂O₃-supported Pd catalyst for methanol decomposition.     

Sol–Gel Pd Exhaust Catalysts and N2O Production

Al₂O₃, CeO₂–Al₂O₃, CeO₂–Tb₄O₇–Al₂O₃ and ZrO₂–Al₂O₃ supported Pd samples have been prepared by sol–gel methods. Extents and mechanisms of N₂O production in CO–NO and CO–NO–O₂ reactions on these have been considered. This occurs most selectively under oxidising (lean-burn) conditions or in the presence of CeO₂ and CeO₂–Tb₄O₇ promoters near the CO–NO light off temperature. Over Pd/ZrO₂–Al₂O₃ the CO–NO reaction at 573 K has CO and NO conversions that are second order with respect to p _CO and p _NO. Over this catalyst NO conversion is faster than that of CO until O_2(g) is added, causing CO conversion and N₂O production at 573 K to rise simultaneously. CeO₂ or CeO₂–Tb₄O₇ incorporation into a Pd/Al₂O₃ catalyst enhances N₂O production near the CO–NO light-off temperature in the absence of added O₂ without CO conversion being raised. There is current attention on pollution control opportunities through lean-burn conditions, Pd catalysts and oxygen storage capacity enhancement. The present work suggests that their role in N₂O production may need to be better understood and controlled. For the moment N₂O formation provides a window on mechanisms of TWC operation.     

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.     

Decomposition and oxidation of CH3 13CH2OH on A12O3, Pd/Al2O3, and PdO/Al2O3 catalysts

Temperature-programmed desorption (TPD) and oxidation (TPO) were used to investigate the decomposition and oxidation of ethanol on Al₂O₃, Pd/Al₂O₃, and PdO/Al₂O₃. Ethyl--¹³C alcohol (CH₃ ¹³CH₂OH) was adsorbed on the catalysts so that reaction pathways of the two carbons could be distinguished. Alumina was mainly a dehydration catalyst, but dehydrogenation was also observed and some carbon remained on the surface. In the presence of O₂, A1₂O₃ oxidized the decomposition products and the-carbon was oxidized faster. Ethanol, which was adsorbed on A1₂O₃, decomposed much faster on Pd/A1₂O₃ by diffusing to Pd and undergoing CO elimination to form CH₄,¹³CO, H₂, and surface carbon. On PdO/A1₂O₃, the decomposition was slower than on Pd/A1₂O₃ until lattice oxygen was extracted above 450 K; the decomposition products were oxidized by lattice oxygen. In the presence of gas phase O₂, Pd/Al₂O₃ was an active oxidation catalyst at low temperature, but lattice oxygen had to be extracted from PdO/A1₂O₃ before it had significant oxidation activity.     

Catalytic activity of Pd/Al2O3 toward the combustion of methane

This work focuses on the improving of the activities and stabilities of Pd/Al₂O₃ catalysts for lean methane catalytic combustion. The influence of preparation conditions on performance of Pd/Al₂O₃ catalyst has been studied. Results showed that excellent performance of the catalyst was attributed to high hydrothermal stability at the support calcination temperature of 1100 °C. In addition, the catalytic activity was enhanced due to high dispersion of active species at lower catalyst calcination temperature. The catalysts were studied by XPS analysis. Results showed that the active phase of Pd/Al₂O₃ was Pd or Pd/PdO mixture. And the state transformation of Pd species resulted in the deactivation of Pd/Al₂O₃.     

The effect of ceria content on the properties of Pd/CeO2/Al2O3 catalysts for steam reforming of methane

The effect of CeO₂ loading on the surface properties and catalytic behaviors of CeO₂–Al₂O₃-supported Pd catalysts was studied in the process of steam reforming of methane. The catalysts were characterized by S_BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), UV–vis diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). The XRD measurements indicated that palladium particles on the surface of fresh and reduced catalysts are well dispersed. TPR experiments revealed a heterogeneous distribution of PdO species over CeO₂–Al₂O₃ supports; one fraction of large particles, reducible at room temperature, another fraction interacting with CeO₂ and Al₂O₃, reducible at higher temperatures of 347 and 423 K, respectively. The PdO species reducible at room temperature showed lower CO adsorption relative to the PdO species reducible at high temperature. In contrast to Pd/Al₂O₃, the FTIR results revealed that CeO₂-containing catalyst with CeO₂ loading ≥12 wt.% show lower ratio (LF/HF) between the intensity of the CO bands in the bridging mode at low frequency (LF) and the linear mode at high frequency (HF). This ratio was constant with increasing the temperature of reduction. The FTIR spectra and the measurement of Pd dispersion suggested that Pd surface becomes partially covered with ceria at all temperature of reduction and with increasing ceria loading in Pd/CeO₂–Al₂O₃ catalysts. Although the PdO/Al₂O₃ showed higher Pd dispersion compared to that of CeO₂-containing catalysts, the addition of ceria resulted in an increase of the turnover rate and specific rate to steam reforming of methane. The CH₄ turnover rate of Pd/CeO₂–Al₂O₃ catalysts with ceria loading ≥12 wt.% was around four orders of magnitude higher compared to that of Pd/Al₂O₃ catalyst. The increase of the activity of the catalysts was attributed to various effects of CeO₂ such as: (i) change of superficial Pd structure with blocking of Pd sites; (ii) the jumping of oxygen (O^*) from ceria to Pd surface, which can decrease the carbon formation on Pd surface. Considering that these effects of CeO₂ are opposite to changes of the reaction rate, the increase of specific reaction rate with enhancing the ceria loading suggests that net effect results in the increase of the accessibility of CH₄ to metal active sites.     

A calorimetric study of oxygen-storage in Pd/ceria and Pd/ceria–zirconia catalysts

Heats of adsorption were measured calorimetrically for O₂ adsorption on reduced Pd/alumina, Pd/ceria, and Pd/ceria–zirconia catalysts, all with 1 wt% Pd. Significantly more O₂ adsorbed on the ceria-containing catalysts due to oxidation of the support. For Pd/alumina, the heats were found to be between 180 and 220 kJ/mol, only slightly higher in magnitude than the heat of reaction for bulk oxidation of Pd. However, the heats of adsorption for both ceria and ceria–zirconia were also 200 kJ/mol, much lower than the heat of reaction for Ce₂O₃ oxidation to CeO₂, but in reasonable agreement with estimates from O₂ desorption studies on model ceria films. The implications of these results for understanding oxygen-storage properties on ceria-based catalysts are discussed.     

High-temperature close coupled catalysts Pd/Ce-Zr-M/Al2O3 (M = Y, Ca or Ba) used for the total oxidation of propane

A series of high-temperature close coupled catalysts Pd/Ce-Zr-M/Al₂O₃ (M = Y, Ca or Ba) were prepared by ultrasonic-assisted successive impregnation. The catalysts were subjected to a series of characterization measurements. The results of activity evaluation show that Y is the best promoter for propane total oxidation, especially at the calcination temperature of 1100 °C. It is interesting that although the BET specific surface areas and the dispersion of Pd species decrease, the Y-promoted catalyst calcined at 1100 °C shows higher catalytic activity than the corresponding one calcined at 900 °C and better sulfur-resisting performance. The results of TEM, TPHD and CO chemisorption indicate that Y can remarkably increase the dispersion of Pd species. However, the dispersion is hard to be connected with the activity increase as the calcination temperature is elevated from 900 to 1100 °C. The change of active phases and the interaction between Pd species and the supports may account for the activity enhancement. Combined with XRD, H₂-TPR and O₂-TPD results, it is deduced that the coexistence of metallic Pd and PdO species in the catalysts calcined at 1100 °C may be also favorable to C₃H₈ oxidation. In a word, Pd/Ce-Zr-Y/Al₂O₃ is indeed a promising high-temperature close coupled catalyst applicable to high temperature.     

Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Promotional effect of Ca on the Pd/Ce–Zr/Al2O3 catalyst for low-temperature catalytic combustion of methane

Promotional effect of Ca on the catalytic property of Pd/Ce–Zr/Al₂O₃ catalyst towards methane combustion is examined. The surface properties and the oxidation/reduction behavior of these catalysts are investigated by BET, TEM, XPS, TPR, TPO and TPSR techniques. Activity tests in methane combustion show that addition of Ca to Pd/Ce–Zr/Al₂O₃ can promote remarkably its low-temperature activity. The thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst to the exposure at high temperature is also enhanced by Ca loading. XPS and TEM results show that the addition of Ca to Pd/Ce–Zr/Al₂O₃ catalyst generates well-dispersed PdO particles on support. H₂–TPR, O₂–TPO and CH₄/O₂–TPSR experiments show that the addition of Ca improves the reduction/reoxidation properties and thermal stability of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst.     

Comparative Study on Partial Oxidation of Methane over Ni/ZrO2, Ni/CeO2 and Ni/Ce–ZrO2 Catalysts

The partial oxidation of methane has been studied by sequential pulse experiments with CH₄ O₂ CH₄ and simultaneous pulse reaction of CH₄/O₂ (2/1) over Ni/CeO₂, Ni/ZrO₂ and Ni/Ce–ZrO₂ catalysts. Over Ni/CeO₂, CH₄ dissociates on Ni and the resultant carbon species quickly migrate to the interface of Ni–CeO₂, and then react with lattice oxygen of CeO₂ to form CO. A synergistic effect between Ni and CeO₂ support contributes to CH₄ conversion. Over Ni/ZrO₂, CH₄ and O₂ are activated on the surface of metallic Ni, and then adsorbed carbon reacts with adsorbed oxygen to produce CO, which is composed of the main path for the partial oxidation of methane. The addition of ceria to zirconia enhances CH₄ dissociation and improves the carbon storage capacity. Moreover, it increases the storage capacity and mobility of oxygen in the catalyst, thus promoting carbon elimination.     

Investigation of CH4 Reforming with CO2 on Meso-Porous Al2O3-Supported Ni Catalyst

Meso-porous Al₂O₃-supported Ni catalysts exhibited the highest activity, stability and excellent coke-resistance ability for CH₄ reforming with CO₂ among several oxide-supported Ni catalysts (meso-porous Al₂O₃ (Yas1-2, Yas3-8), -Al₂O₃, -Al₂O₃, SiO₂, MgO, La₂O₃, CeO₂ and ZrO₂). The properties of deposited carbons depended on the properties of the supports, and on the meso-porous Al₂O₃-supported Ni catalyst, only the intermediate carbon of the reforming reaction formed. XRD and H₂-TPR analysis found that mainly spinel NiAl₂O₄ formed in meso-porous Al₂O₃ and -Al₂O₃-supported catalysts, while only NiO was detected in -Al₂O₃, SiO₂, CeO₂, La₂O₃ and ZrO₂ supports. The strong interaction between Ni and meso-porous Al₂O₃ improved the dispersion of Ni, retarded its sintering and improved the activated adsorption of CO₂. The coking reaction via CH₄ temperature-programed decomposition indicated that meso-porous Al₂O₃-supported Ni catalysts were less active for carbon formation by CH₄ decomposition than Ni/-Al₂O₃ and Ni/-Al₂O₃.     

Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

Investigation of the Intrinsic Activity of ZrxCe1−xO2 Mixed Oxides in the CO + NO Reactions: Influence of Pd Incorporation

The intrinsic activity of various Zr _x Ce_1–x O₂ mixed oxides and after a Pd deposition has been investigated in the CO + NO reactions from temperature-programmed experiments performed under stoichiometric conditions. It has been found that the activity of Zr _x Ce_1–x O₂ depends on either the specific surface area or the number of Ce cations and their intrinsic activity, Zr_0.5Ce_0.5O₂ being the most active support. The addition of palladium strongly enhances the catalytic activity of the supports probably due to a synergistic effect between CeO₂ and the metal since the initial activity of palladium-based catalysts is directly related to their Ce content. Such a catalytic enhancement has been explained by a bifunctional mechanism involving active sites probably composed of Pd and ceria. A strong deactivation operates leading to the disappearance of the beneficial effect of ceria. Such a deactivation seems to be dependent on the support composition, Pd supported Zr_0.25Ce_0.75O₂ being the most resistant to deactivation.     

Development of Monolithic Catalysts with Low Noble Metal Content for Diesel Vehicle Emission Control

Monolith washcoated catalysts with potential for diesel emission control have been developed. Two types of catalysts have been prepared for further study: (1) MnO _x supported on granulated -Al₂O₃, (2) MnO _x supported on cordierite monolith washcoated with -Al₂O₃. Both catalysts have been calcined at 500 and 900 °C and subsequently modified by doping with 0.1–1.0 wt% of Pt or Pd. The influence of the concentration of both manganese oxide (0–10 wt%) and noble metals Pt and Pd in the range 0–1.0 wt% on the catalytic activity in methane oxidation has been studied. Comparison of the catalytic activity of MnO _x /Al₂O₃ and MnO _x + Pt(Pd)/Al₂O₃ with that of a standard 1 wt%Pt/Al₂O₃ catalyst shows the existence of a synergetic effect. This effect is more pronounced for the samples calcined at 900 °C. The developed monolithic catalysts MnO _x + Pt(Pd)/Al₂O₃ demonstrate higher activity and thermal stability (up to 900 °C) compared to the commercial monolithic catalyst (TWC's).     

Influence of calcination temperature on the structure and properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> as support for Pd catalyst

We investigated the influence of the calcination temperature on the structural properties of Al₂O₃ and how the resultant Al₂O₃ support affects the characteristics of Pd/Al₂O₃ catalysts. Al₂O₃ pretreated at different calcination temperatures ranging from 500 °C to 1,150 °C, was used as catalyst supports. The Pd/Al₂O₃ catalysts were prepared by a deposition-precipitation method using a pH 7.5 precursor solution. Characterization of the prepared Pd/Al₂O₃ catalysts was performed by X-ray diffraction (XRD), N₂-physisorption, CO₂-temperature programmed desorption (TPD), CO-chemisorption, and field emission-transmission electron microscopic (FE-TEM) analyses. The CO-chemisorption results showed that the Pd catalyst with the Al₂O₃ support calcined at 900 °C, Pd/Al₂O₃ (900), had the highest and most uniformly dispersed Pd particles, with a Pd dispersion of 29.8%. The results suggest that the particle size and distribution of Pd are related to the phase transition of Al₂O₃ and the ratio of isolated tetrahedral to condensed octahedral coordination sites (i.e., functional groups), where the tetrahedral sites coordinate more favorably with Pd.     

Characterization and Catalytic Properties of Combustion Synthesized Au/CeO2 Catalyst

Ceria-supported Au catalyst has been synthesized by the solution combustion method for the first time and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Au is dispersed as Au⁰ as well as Au³⁺ states on CeO₂ surface of 20-30 nm crystallites. On heating the as-prepared 1% Au/CeO₂ in air, the concentration of Au³⁺ ions on CeO₂ increases at the expense of Au⁰. Catalytic activities for CO and hydrocarbon oxidation and NO reduction over the as-prepared and the heat-treated 1% Au/CeO₂ have been carried out using a temperature-programmed reaction technique in a packed bed tubular reactor. The results are compared with nano-sized Au metal particles dispersed on -Al₂O₂ substrate prepared by the same method. All the reactions over heat-treated Au/CeO₂ occur at lower temperature in comparison with the as-prepared Au/CeO₂ and Au/Al₂O₂. The rate of NO + CO reaction over as-prepared and heat-treated 1% Au/CeO₂ are 28.3 and 54.0 mol g^-1 s^-1 at 250 and 300 °C respectively. Activation energy (E _a) values are 106 and 90 kJ mol^-1 for CO + O₂ reaction respectively over as-prepared and heat-treated 1% Au/CeO₂ respectively.     

Oxygen isotope equilibration and exchange as probes for differing dioxygen interactions with pure and rhodia-promoted CeO2 and Al2O3

Comparisons are made between the catalytic activities of CeO₂, Al₂O₃ and Rh₂O₃ when pure, or in the case of CeO₂ and Al₂O₃ when promoted by rhodia dispersed thereon, in respect of: (a) activity at 290 K for homomolecular oxygen isotope equilibration of an equimolar (¹⁸O₂ + ¹⁶O₂) probe gas, the so-called R₀ process, (b) activity under T-ramp for heterophase oxygen isotope exchange between surface lattice and O₂ highly enriched in ¹⁸O, which is shown to occur predominantly by single-stay exchange of both oxygen atoms of dioxygen (the so-called R₂ process). This revised version was published online in June 2006 with corrections to the Cover Date.     

SSITKA studies of the catalytic flameless combustion of methane

This paper presents results which were obtained for the flameless combustion of methane over the Pd(PdO)/Al₂O₃ catalyst by using the steady state isotopic transient kinetic analysis method. During the reaction switches between ¹⁶O₂/Ar/CH₄/He and ¹⁸O₂/CH₄/He were carried out. The obtained results indicate the presence of large amounts of oxygen as well as of intermediates leading to the formation of carbon dioxide on the surface of the palladium catalyst. Additionally, information was obtained proving that the complete oxidation of methane over Pd/Al₂O₃ catalyst proceeds according to the Mars and van Krevelen redox mechanism. With the increase of the reaction temperature there is an increase in the number of active centres on the Pd(PdO)/Al₂O₃ catalyst surface—a larger amount of oxygen from the lattice of the catalyst is accessible for the reaction of methane oxidation.     

Incorporation of La3+ into a Pt/CeO2/Al2O3 catalyst

The effects of La³⁺ incorporation into a Pt/CeO₂/Al₂O₃ catalyst were investigated by a combination of activity, temperature-programmed reduction (TPR), oxygen storage capacity (OSC), noble-metal surface area, and X-ray diffraction (XRD) measurements. Incorporation of La³⁺ ions into the Al₂O₃, before CeO₂ is added, promoted higher Pt and CeO₂ dispersions. The oxygen storage capacity was also higher in the presence of La³⁺. This is attributed to a combination of Pt and CeO₂ particle-size effects and possible blockage of the reaction between Al₂O₃ and CeO₂. The XRD data show that La³⁺ forms LaAlO₃ with Al₂O₃ and prevents -Al₂O₃ formation after various heat treatments.