Au–TiO2 catalysts on carbon nanofibres prepared by deposition-precipitation and from colloid solutions

Au catalysts have been prepared (i) on TiO₂, (ii) on carbon nanofibres (CNF) and (iii) on TiO₂ deposited onto CNF. Catalysts prepared from deposition-precipitation (DP) and from colloid solutions have been characterised using XRD, TEM, TGA and XAS and tested in the water–gas shift (WGS) reaction. DP yields large Au particles (>50 nm) on CNF-containing supports. High Au dispersion on carbon nanofibres requires preparation via other methods such as colloid formation. Au particle growth is more pronounced during the synthesis steps than during thermal treatments. This increase is not observed for the Au particles on TiO₂ but only when CNF is present, indicating that the surface properties of TiO₂ are altered by the CNF. TiO₂ XANES analyses show that distortions in the lattice symmetry of TiO₂ are introduced when the oxide is deposited on CNF. The distortion of the TiO₂ structure by the CNF may also introduce changes that promote the turnover frequencies. The WGS activity significantly improves when titania is present. This shows that coexistence of Au and TiO₂ is needed to obtain high catalytic activity in the WGS reaction, indicating that the active sites are either on the Au–TiO₂ interface or that the reaction follows a bifunctional mechanism.     

The role of cationic Au and nonionic Au species in the low-temperature water–gas shift reaction on Au/CeO2 catalysts

The roles of cationic and nonionic Au species in the water–gas shift (WGS) reaction on Au/CeO₂ catalysts were studied by comparing the reaction behavior of a cyanide leached catalyst, after removal of the Au nanoparticles by cyanide leaching, with that of non-leached catalysts, following the technique introduced by Q. Fu et al. [Science 301 (2003) 935]. Using rate measurements as well as in situ spectroscopic and structure-sensitive techniques, we found that based on the Au mass balance, cyanide leaching removed all Au except for ionic Au³⁺ species, and that leaching resulted in a pronounced decay of the catalyst mass normalized activity to 1–25% of that of a non-leached catalyst. The extent of the activity loss strongly depended on the post-treatment of the leached catalyst. Both the catalyst treatment after leaching and, in particular, the WGS reaction resulted in considerable reformation of Au⁰ species by thermal decomposition of Au oxides (Au³⁺) and subsequent nucleation and growth of very small Au⁰ aggregates and metallic Au⁰ nanoparticles, as indicated by Au(4f) signals at 85.9 eV (Au³⁺), 84.0–84.6 eV (up-shifted signal of small Au⁰ aggregates), and 84.0 eV (metallic Au⁰). In this work, correlations between ionic and nonionic Au species and between total WGS activity and activity for the formation/decomposition of bidentate formate species are evaluated, and the role of the respective Au species in the WGS reaction on Au/CeO₂ catalysts is discussed.     

Ceria-zirconia supported Au as highly active low temperature Water-gas shift catalysts

Au/CeZrO₄ catalysts have shown very low temperature activity for the WGS reaction. Characterisation of the as-prepared catalysts shows Au is present primarily as isolated Au³⁺ atoms. It has been found that a higher proportion of Au³⁺ present in the as-prepared catalyst leads to a higher WGS activity, although under reaction conditions reduction to Au⁰ is observed. Use of TPR and iso-thermal H₂O re-oxidation has shown that Au reacts with H₂O at lower temperatures than an equivalent Pt/CeZrO₄ catalyst, indicating that H₂O activation is key in the onset of low temperature WGS activity.     

Gold catalysts supported on ceria-modified mesoporous zirconia for low-temperature water–gas shift reaction

New gold catalytic system prepared on ceria-modified mesoporous zirconia used as water–gas shift (WGS) catalyst is reported. Mesoporous zirconia was synthesized using surfactant templating method through a neutral [C₁₃(EO)₆-Zr(OC₃H₇)₄] assembly pathway. Ceria modifying additive was deposited on mesoporous zirconia by deposition–precipitation method. Gold-based catalysts with different gold content (1–3 wt. %) were synthesized by deposition–precipitation of gold hydroxide on mixed metal oxide support. The supports and the catalysts were characterized by powder X-ray diffraction, high-resolution transmission electron microscopy, N₂ adsorption analysis and temperature programmed reduction. The catalytic behavior of the gold-based catalysts was evaluated in WGS reaction in a wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new Au/ceria-modified mesoporous zirconia catalysts was compared with that of gold catalysts supported on simple oxides CeO₂ and mesoporous ZrO₂, revealing significantly higher catalytic activity of Au/ceria-modified mesoporous zirconia. A high degree of synergistic interaction between ceria and mesoporous zirconia and a positive modification of structural and catalytic properties by ceria have been achieved. It is clearly revealed that the ceria-modified mesoporous zirconia is of much interest as potential support for gold-based catalyst. The Au/ceria-modified mesoporous zirconia catalytic system is found to be effective catalyst for WGS reaction.     

Gold nanoparticles on ceria: importance of O vacancies in the activation of gold

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO₂ and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO₂ was investigated on Au/CeO₂(111) and AuO _x /CeO₂ surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO₂(111) was 4–7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO₂ on the Au/CeO₂(111) surfaces. The full decomposition of SO₂ was observed only after introducing O vacancies in the ceria support. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−x (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO₂(111) single crystal corroborate that cationic Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.     

Gold Catalysts Supported on Cerium–Gallium Mixed Oxide for the Carbon Monoxide Oxidation and Water Gas Shift Reaction

The synthesis, characterization and catalytic properties of gold supported on ceria, gallia and a cerium–gallium mixed oxide were investigated. The nanostructural characterization of the cerium–gallium support (nominal atomic composition Ce80Ga20) showed that gallium(III) cations are homogenously distributed into the ceria matrix by substituting cerium(IV) cations of the fluorite-type structure of ceria. Au was added to the supports by the deposition–precipitation method using urea. High Au dispersions were achieved for all the fresh materials (D > 60%). The CO oxidation and the water gas shift (WGS) reaction were tested on the whole set of catalysts. All the supported-gold catalysts showed high activity for the CO oxidation reaction. However, those containing gallium in their formulation deactivated due to gold particle sinterization. Au(2%)/CeO₂ was the most active material for the WGS reaction, and the Au(2%)/Ce80Ga20 was as active as a Au(3%)/Ce68Zr32 catalyst for CO oxidation, and even more active than the reference catalyst of the World Gold Council, Au(2%)/TiO₂.     

Influence of the crystalline structure of ZrO2 on the activity of Cu/ZrO2 catalysts on the water gas shift reaction

The structure sensitivity of copper catalysts supported on monoclinic and tetragonal ZrO₂ was studied on the water gas shift (WGS) reaction. The results show that the activity of copper supported on tetragonal ZrO₂ is much higher than when supported on monoclinic ZrO₂. The obtained results favour an associative mechanism, which considers the formation of formate species on the surface of ZrO₂. The differences on the observed WGS activity are attributed to the stability of adsorbed formates on the different ZrO₂ polymorphs. It was found that the active Cu species for the WGS reaction is the bulk CuO.     

Effect of Aging Time and Calcination on the Preferential Oxidation of CO Over Au Supported on Doped Ceria

Au/CeLaO_x mixed oxide catalysts containing 0.6–1.0 wt% Au were prepared by co-precipitation with Na₂CO₃. BET surface areas ranged from 15 to 45 m²/g depending on aging time (precipitation time) and calcination conditions. The differences in the activity of the catalysts for preferential oxidation (PROX) of CO are ascribed to the differences in the metal loading, Ce/La ratio and support crystallinity, chloride content, and the resultant effect on the reduction properties of the catalysts. The catalysts did not require activation in H₂ prior to reaction. The temperature at which the catalysts exhibit significant activity correlates with the temperature of reduction, indicating that reduction of the metal and support is important for high activity.     

Water–gas shift activity of ceria supported Au–Re catalysts

Au–Re/ceria, Au/ceria and Re/ceria catalysts were prepared using deposition precipitation and impregnation techniques for Au and Re addition, respectively, except the sample prepared by sequential impregnation. Catalysts were characterized by HRTEM-EDS, SEM-EDS, XPS and XRD. WGS activity tests on the samples were performed in the temperature range 200–450 °C. The effects of Re incorporation, metal addition sequence, space velocity and H₂O/CO ratio on the catalytic performance were investigated. The novel Au–Re/ceria catalysts showed high activity in WGS reaction, especially at high H₂O/CO ratios, led by the presence of catalytically active and steam tolerant sites formed on the bimetallic catalysts.     

The Water-Gas Shift Reaction Over Au-Based, Bimetallic Catalysts. The Au-M (M=Ag, Bi, Co, Cu, Mn, Ni, Pb, Ru, Sn, Tl) on Iron(III) Oxide System

The bimetallic Au-M/Fe₂O₃ catalysts were prepared by deposition coprecipitation method with Au/M atomic ratio of 1. All the catalysts were measured for WGS reaction and characterized by TPR/TPO studies. Ruthenium- and nickel-modified catalysts showed higher WGS activities compared to the other systems including unmodified Au/Fe₂O₃ at low temperature (100 °C). At higher temperature (240 °C), ruthenium-, nickel-, bismuth-, lead-, copper-, silver-, thallium- and tin-modified catalysts were more active than unmodified Au/Fe₂O₃. Manganese- and cobalt-modified catalysts were less active than unmodified Au/Fe₂O₃. TPR analyses indicated a shift in reduction temperature in the bimetallic catalysts, suggesting a degree of interaction between gold and the second metal.     

Au–TiO2 catalysts stabilised by carbon nanofibres

The catalytic activity and stability in the water–gas shift reaction have been tested for Au-based catalysts prepared by deposition of Au from colloid solutions. The supports that have been used are TiO₂, TiO₂ supported on carbon nanofibres (CNF) and CNF. Thermal treatments of the samples show that the Au particle size depends on the support material and hence the interaction between the Au particles and the support. In situ X-ray absorption spectroscopic (XAS) measurements during the water–gas shift reaction show no changes in the first Au–Au coordination number for the catalysts containing CNF. Furthermore, improved short-time stability is obtained compared to the AuTiO₂ catalysts. The improved stability is achieved by the CNF stabilising small TiO₂ particles and hence prevent subsequent sintering of the Au particles.     

Water-gas shift reaction over supported Pt and Pt-CeOx catalysts

A comparison study was performed of the water-gas shift (WGS) reaction over Pt and ceria-promoted Pt catalysts supported on CeO₂, ZrO₂, and TiO₂ under rather severe reaction conditions: 6.7 mol% CO, 6.7 mol% CO₂, and 33.2 mol% H₂O in H₂. Several techniques—CO chemisorption, temperature-programmed reduction (TPR), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES)—were employed to characterize the catalysts. The WGS reaction rate increased with increasing amount of chemisorbed CO over Pt/ZrO₂, Pt/TiO₂, and Pt-CeO _x /ZrO₂, whereas no such correlation was found over Pt/CeO₂, Pt-CeO _x /CeO₂, and Pt-CeO _x /TiO₂. For these catalysts in the absence of any impurities such as Na⁺, the WGS activity increased with increasing surface area of the support, showed a maximum value, and then decreased as the surface area of the support was further increased. An adverse effect of Na⁺ on the amount of chemisorbed CO and the WGS activity was observed over Pt/CeO₂. Pt-CeO _x /TiO₂ (51) showed the highest WGS activity among the tested supported Pt and Pt-CeO_x catalysts. The close contact between Pt and the support or between Pt and CeO _x , as monitored by H₂-TPR, is closely related to the WGS activity. The catalytic stability at 583K improved with increasing surface area of the support over the CeO₂- and ZrO₂-supported Pt and Pt-CeO _x catalysts.     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

Low-temperature activity for CO oxidation over diesel oxidation catalysts studied by High Throughput Screening and DRIFT spectroscopy

We have investigated the low-temperature activity for CO oxidation for a series of platinum catalysts supported on Al₂O₃, TiO₂, ZSM-5, CeO₂ and ZrO₂-CeO₂. The results show major differences in activity, due to the support for Pt, especially in the presence of water. Improved activity over ceria containing samples in presence of water is likely due to the water-gas shift (WGS) reaction. Studies with in situ IR spectroscopy suggest a surface formate mechanism for the WGS reaction on Pt/CeO₂.     

Palladium and platinum catalysts supported on carbon nanofiber coated monoliths for low-temperature combustion of BTX

In this work carbon nanofiber (CNF)-coated monoliths with a very thin, homogeneous, consistent and good adhered CNF layer were obtained by means of catalytic decomposition of ethylene on Ni particles.The catalytic behaviour of Pt and Pd supported on the CNF-coated monoliths was studied in the low-temperature catalytic combustion of benzene, toluene and m-xylene (BTX) and compared with the performance of Pt and Pd supported on γ-Al₂O₃ coated monoliths.The catalysts supported on CNF-coated monoliths were the most active, independent of the metal catalyst or the type of the tested aromatic compound. TPD experiments showed that the γ-Al₂O₃ phase retained important amounts of the water molecules produced during the reaction. When water vapour was supplied to the reactant flow, the activity of Pd catalysts decreased much stronger than the Pt ones, and the activity of the Pt catalysts supported on the γ-Al₂O₃ was more affected than that of the catalysts supported on CNF.BTX combustion reactions seem to be catalyzed by Pt and Pd through different kinetic mechanisms, explaining why Pt catalysts always were more active than the Pd ones deposited on the same type of support. Pd catalyzed combustion of benzene is strongly inhibited by oxygen and by water.Catalysts supported on CNF-coated monoliths showed a selectivity to burn benzene better than toluene or m-xylene, attributed to a better aromatic-CNF surface interaction.     

WGS activity of a novel Cu–ZrO2 catalyst prepared by a reflux method. Comparison with a conventional impregnation method

The activity in the WGS reaction of Cu/ZrO₂ catalysts prepared by a method of refluxing in an aqueous NH₄OH solution is studied. It is shown that at 3% Cu load the methods of impregnation over monoclinic or tetragonal ZrO₂ do not produce active catalysts for the WGS reaction. However, the method of refluxing generates highly active catalysts with Cu loads of 3% (w/w) or higher. The activity of the catalysts prepared by refluxing is associated with the formation of small Cu clusters, which would allow the regrouping of the H atoms to generate molecular H₂ in the presence of the crystalline tetragonal ZrO₂.     

Cu/Ni‐Loaded CeO2‐ZrO2 Catalyst for the Water‐Gas Shift Reaction: Effects of Loaded Metals and CeO2 Addition

Two different types of metals (Cu and Ni) and the effect of CeO₂ addition to produce a CeO₂‐ZrO₂ co‐supporter were investigated through the water‐gas shift (WGS) reaction. It was found that the WGS activity could be enhanced with CeO₂ addition. At relatively high temperature, Ni‐loaded catalysts exhibited higher CO conversion while Cu‐loaded catalysts demonstrated better performance at low temperatures. The stability and yield of the CO₂ and H₂ products of the Cu catalysts were higher than those of the Ni catalysts. These results may be caused by an irreversible adsorption of CO on Ni and the reverse WGS reaction occurring on the Ni catalysts. In situ diffuse‐reflection infrared Fourier transform spectroscopy data suggests that the WGS mechanism likely proceeded via formate species.     

Gold catalysts supported on mesoporous zirconia for low-temperature water–gas shift reaction

Mesoporous ZrO₂ with high surface area and uniform pore size distribution, synthesized by surfactant templating through a neutral [C₁₃(EO)₆–Zr(OC₃H₇)₄] assembly pathway, was used as a support of gold catalysts prepared by deposition–precipitation method. The supports and the catalysts were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, N₂ adsorption analysis, temperature programmed reduction and desorption. The catalytic activity of gold supported on mesoporous zirconia was evaluated in water–gas shift (WGS) reaction at wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. The catalytic behaviour and the reasons for а reversible deactivation of Au/mesoporous zirconia catalysts were studied. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new Au/mesoporous zirconia catalyst was compared to the reference Au/TiO₂ type A (World Gold Council), revealing significantly higher catalytic activity of Au/mesoporous zirconia catalyst. It is found that the mesoporous zirconia is a very efficient support of gold-based catalyst for the WGS reaction.     

Cu–ZrO2 Catalysts for Water-gas-shift Reaction at Low Temperatures

A series of Cu–ZrO₂ catalysts with Cu content in the range of 10–70 at.% Cu (=100×Cu/(Cu+Zr)) were prepared by coprecipitation, and their performances were tested for the water-gas-shift (WGS) reaction. The activity of the catalyst increased with Cu loading and, depending on the loading, the activity was comparable to or better than the activity of a conventional Cu–ZnO–Al₂O₃ catalyst at low temperatures below 473 K. Characterization of the catalysts revealed that the amount of Cu⁺ present on the catalyst surface, after being reduced by a H₂ mixture at 573 K, was well correlated with the activity of the catalyst, indicating that the Cu⁺ species were the active sites of the WGS reaction. The easy redox between Cu²⁺ and Cu⁺ during the WGS reaction was considered to be responsible for the high activity of Cu–ZrO₂ at low temperatures. A reaction mechanism based on the redox was proposed.     

Performance of Au/M x Oy/TiO2 Catalysts in Water-Gas Shift Reaction

Our group recently developed a series of Au/M _x O _y /TiO₂ catalysts for CO oxidation, and demonstrated that some of these catalysts are still active after high-temperature treatment whereas Au/TiO₂ deactivates significantly due to the sintering of gold nanoparticles at elevated temperatures (Ma Z, Overbury SH, Dai S (2007) J Mol Catal A 273:97). In the current work, the performance of Au/M _x O _y /TiO₂ (M = Al, Ca, Fe, Zn, Ga, Y, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) catalysts in water-gas shift (WGS) reaction was evaluated. The influences of different metal oxide (M _x O _y ) additives and pretreatment temperatures were investigated, and the catalyst stability as a function of reaction time on stream was tested. Some of these novel gold catalysts, with high activity and stability in water-gas shift, furnish new possibilities for further fundamental research and industrial development.