Structure sensitivity of CO oxidation over model Au/TiO2 2 catalysts

Model catalysts of Au clusters supported on TiO₂ thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO₂ catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)^-1 × s^-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO₂(001)/Mo(100) is structure sensitive. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction of Au with titania: the role of reduced Ti

The interaction of Au with oxide supports has been found to play a vital role in determining the unique properties of Au catalysts. In this study, the binding of Au with titania was investigated using scanning tunneling microscopy (STM) and ultra-violet photoemission spectroscopy (UPS). Two support systems, rutile TiO₂(110) and an ordered TiO _x /Mo(112) thin film were used. On a highly defective TiO₂(110) surface, Au particles were found to bind first on the oxygen vacancy sites. Complete wetting of the oxide by Au was found on an ordered and reduced Ti³⁺O _x /Mo(112) film to form two ordered structures, a (1 × 1)-monolayer and a (1 × 3)-Au/TiO _x /Mo(112) bi-layer. Detailed STM images confirm the proposed structural models. Reduced titania was found to enhance the binding of Au with Ti sites and promote electron donation from Ti^δ+ to Au, leading to electron-rich Au and to enhanced CO bonding.     

The Effect of Coating TiO<Subscript>2</Subscript> on the CO Oxidation of the Pt/γ-Alumina Catalysts

Abstract  

The effect of coating TiO₂ on the CO oxidation of the Pt/γ-alumina catalysts was observed through activity tests and surface characterization spectroscopy by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) experiments. XPS results evidenced the occurrence of different Pt²⁺ species and metallic Pt⁰ at the surface which suggest electron transfer of titanium (cation) to the platinum atom and the reduction of titanium (Ti⁴⁺ → Ti³⁺). FTIR analyses suggested oxygen spillover mechanism at the interface between titanium dioxide and platinum that may explain the catalytic activity of the platinum titania-supported catalysts. The apparent activation energy for the CO oxidation was 52.5 kJ/mol and similar for all catalysts. However, the frequency factor changed significantly, indicating interfacial phenomena caused by CO and oxygen adsorptions over TiO _x species and Al₂O₃ support with similar dispersions.     

Mesoporous Au/TiO2 Catalysts for Low Temperature CO Oxidation

The activity and stability of structurally well defined mesoporous Au/TiO₂ catalysts with different support morphologies and pore sizes for low temperature CO oxidation was investigated by kinetic measurements and in-situ IR spectroscopy. The resulting catalysts with Au particle sizes of ∼3 nm exhibit a high activity for CO oxidation, similar to or exceeding that of highly active standard Au/TiO₂ catalysts with similar size Au nanoparticles and loading, and a significantly lower tendency for deactivation. Possible reasons for the improved performance of these catalysts are discussed.     

Gold dispersed on TiO2 and SiO2: adsorption properties and catalytic behavior in hydrogenation reactions

Titania-supported Au catalysts were given both low temperature reduction and high temperature reduction at 473 and 773 K, respectively, and their adsorption and catalytic properties were compared to identically pretreated Pt/TiO₂ catalysts and pure TiO₂ samples as well as Au/SiO₂ catalysts. This was done to determine whether a reaction model proposed for methanol synthesis over metals dispersed on Zn, Sr and Th oxides could also explain the high activities observed in hydrogenation reactions over MSI (Metal-Support Interaction) catalysts such as Pt/TiO₂. This model invokes O vacancies on the oxide support surface, formed by electron transfer from the oxide to the metal across Schottky junctions established at the metal-support interface, as the active sites in this reaction. The similar work functions of Pt and Au should establish similar vacancy concentrations, and O₂ chemisorption indicated their presence. However, these Au catalysts were completely inactive for CO and acetone hydrogenation, and ethylene hydrogenation rates were lower on the supported Au catalysts than on the supports alone. Consequently, this model cannot explain the high rate of the two former reactions over TiO₂-supported Pt although it does not contradict models invoking specialinterfacial sites.     

Identification of valence shifts in Au during the water-gas shift reaction

In situ X-ray absorption spectroscopy (XAS) has been performed to investigate the active site on Au-based catalysts in the water-gas shift (WGS) reaction. The surface area and hence the WGS activity is higher for AuTiO₂ catalysts supported on carbon nanofibres (CNF) than TiO₂. The WGS reaction rate depends on the Au coordination number with an apparent maximum close to eight which corresponds to a particle size of approximately 2.5–3.0 nm. A likely cause for the changes in the electronic structure of Au is the adsorption of CO on the surface, which also creates a small positive charge in the Au atoms. The catalytic activity significantly improves when titania is present compared to Au deposited directly on CNF.     

TiO2 Supported Nano—Au Catalysts Prepared via Solvated Metal Atom Impregnation for Low-Temperature CO Oxidation

TiO₂ supported nano-Au catalysts were prepared by solvated metal atom impregnation (SMAI) method. The catalysts were characterized by means of AAS, TPD, H₂ reduction desorption (H₂-RD), XRD, TEM, XPS and tested for low-temperature CO oxidation. XRD and TEM results showed that the pretreatment temperature had an influence on the particle size of Au/TiO₂catalysts. The average particle size increased with the increase in pretreatment temperature. XPS indicated that gold in the catalysts was presented in the form of metallic state clusters. Catalytic studies showed these catalysts were very active and stable in low-temperature CO oxidation. The CO oxidation activity of the catalysts increased as the Au particle size decreased. The measurement results of AAS, TPD and H₂-RD revealed that there were some organic fragments on the surface of Au particles which might be responsible for the high stability of the Au/TiO₂ catalysts.     

Characterization of Au/MnO x /TiO2 for Photocatalytic Oxidation of Carbon Monoxide

The Au/MnO _x /TiO₂ catalyst was used for the photocatalytic oxidation of carbon monoxide. The catalytic activity of Au/MnO _x /TiO₂ with low concentration of manganese (3–7 mol%) was much higher than that of Au/TiO₂. The surface of Au/MnO _x /TiO₂ was characterized by XPS and Raman spectroscopy. While the main state of manganese in 13.8 mol% MnO _x /TiO₂ was Mn⁴⁺ species, Mn³⁺ was the dominant species in the samples with below 6.5 mol% manganese. Raman spectroscopy revealed that the interaction between the MnO _x and TiO₂ form Mn–O–Ti species in which the state of manganese was Mn³⁺. The Au particles also interacted with both MnO _x and TiO₂ to modify the surface of them. In the case of the Au species, low loading of manganese produced the metallic Au⁰ and perimeter interfacial Au^δ+, whereas high loading showed the coexistence of three components which were metallic Au⁰, perimeter interfacial Au^δ+, and oxidic Au³⁺. The catalytic active component was the metallic Au⁰ and perimeter interfacial Au^δ+ species, which were dispersed on TiO₂ and Mn³⁺/TiO₂.     

A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au-TiO2 catalysts

Titania-supported gold catalysts are extremely active for room temperature CO oxidation; however, deactivation is observed over long periods of time under our reaction conditions Impregnated AuTiO₂ is most active after a sequential pretreatment consisting of high temperature reduction at 773 K, calcination at 673 K and low temperature reduction at 473 K (HTR/C/LTR); the activity after either only low temperature reduction or calcination is much lower. A catalyst prepared by coprecipitation had much smaller Au particles than impregnated AuTiO₂ and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiO_x overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the activity of AuTiO₂ catalysts and argues for the formation of active sites at the AuTiO_x interface produced by the mobility of TiO_x species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated AuTiO₂ reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO₂; however, a band for adsorbed CO is observed on the pure TiO₂. Kinetic studies with a 1.0 wt.-% impregnated AuTiO₂ sample showed a near half-order rate dependence on CO and a near zero-order rate dependence on O₂ between 273 and 313 K with an activation energy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and O₂ adsorbing on TiO₂, is consistent with Langmuir-Hinselwood kinetics for noncompetitive adsorption, fits partial pressure data well and shows consistent enthalpies and entropies of adsorption. The formation of carbonate and car☐ylate species on the titania surface was detected but it appears that these are spectator species. DRIFTS experiments under reaction conditions also show the presence of weak, reversible adsorption of CO₂ (near 2340 cm⁻¹) which may be competing with CO for adsorption sites.     

Au/MnO2–TiO2 catalyst for preferential oxidation of carbon monoxide in hydrogen stream

The catalytic activity of nanosize gold catalysts supported on MnO₂–TiO₂ and prepared by deposition–precipitation method has been investigated for preferential oxidation of carbon monoxide in H₂ stream. The catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction, nitrogen sorption, transmission electron microscopy, and X-ray photoelectron spectroscopy. The influence of pH in the preparation process and the amount of MnO₂ loading on the catalytic properties of the Au/MnO₂–TiO₂ catalysts were also studied. Fine dispersion of gold nanoparticles on all the supports was obtained. Especially, Au/MnO₂–TiO₂ with MnO₂/TiO₂ mol ratio of 2:98, showed a mean Au particle size of 2.37 nm. The nanosized support constrained the size of gold. The addition of MnO₂ on Au/TiO₂ catalyst improved the selectivity of CO oxidation without sacrificing CO conversion in hydrogen stream between 50 and 100 °C. This could be attributed to the interactions of gold metal with MnO₂–TiO₂ support and the optimum combination of metallic and electron-deficient gold on the catalyst surface.     

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.     

Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO₂, Au/Al₂O₃, Au/TiO₂) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (~1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O₂/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO₂ and Au/CeO₂ developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al₂O₃ remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina ~ 0. The EXAFS of samples pretreated in air at 773 K and in H₂ at 573 K reveals the presence of only metallic particles for Au/TiO₂ and Au/Al₂O₃. Au(III) supported on CeO₂ remains unreduced after calcination, but reduces during the treatment with H₂. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO₂ is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.     

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.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

CO oxidation over Au/TiO2 prepared from metal-organic gold complexes

A series of Au/TiO₂ catalysts has been prepared from precursors of various metal-organic gold complexes (Au _n , n = 2–4) and their catalytic activity for CO oxidation studied. The Au/TiO₂ catalyst synthesized from a tetranuclear gold complex shows the best performance for CO oxidation with transmission electron microscopy of this catalyst indicating an average gold particle size of 3.1 nm.     

Deposition of Gold Particles on Mesoporous Catalyst Supports by Sonochemical Method, and their Catalytic Performance for CO Oxidation

Supported gold catalysts on the mesoporous (MSP) metal oxides were prepared by a one-step, ultrasound-assisted reduction method, and characterized by XRD, HRTEM, EDX, BET, and XPS analysis. Their catalytic activities were examined in the oxidation of CO. Compared to the Au/Fe₂O₃(MSP) catalyst, the Au/TiO₂(MSP) and Au/Fe₂O₃-TiO₂(MSP) catalysts exhibited higher catalytic activity in the oxidation of CO at low temperatures. The high catalytic activity of Au/TiO₂(MSP) was attributed to the metallic state of the gold nanoparticles, their small size (2–2.5 nm), and their high dispersion on the catalyst support.     

How Different Characterization Techniques Elucidate the Nature of the Gold Species in a Polycrystalline Au/TiO2 Catalyst

TiO₂‐supported gold species were prepared via the deposition‐precipitation route, with conservation of the initial speciation by freeze‐drying. The structural and electronic properties of the Au species were investigated by X‐ray absorption spectroscopy, electron microscopy, and IR spectroscopy of adsorbed CO in four states. Exclusively Au^III was deposited on the TiO₂ surface in patches ranging from isolated Au ions to three‐dimensional clusters. This paper illustrates in detail the unique contributions of all characterization techniques to this structural model.     

Novel Au/TiO2/Al2O3 · xH2O catalysts for CO oxidation

Au/Al₂O₃ · xH₂O and Au/TiO₂/Al₂O₃ · xH₂O (x = 0–3) catalysts were prepared by assembling gold nanoparticles on neat and TiO₂-modified Al₂O₃, AlOOH, and Al(OH)₃ supports, and their catalytic activity in CO oxidation was tested either as synthesized or after on-line pretreatment in O₂–He at 500 °C. A promotional effect of TiO₂ on the activity of gold catalysts was observed upon 500 °C-pretreatment. The catalyst stability as a function of time on stream was tested in the absence or presence of H₂, and physiochemical characterization applying BET, ICP-OES, XRD, TEM, and ²⁷Al MAS NMR was conducted.     

Promotional effect of H2 on CO oxidation over Au/TiO2 studied by operando infrared spectroscopy

The oxidation of carbon monoxide in the presence of various concentrations of molecular hydrogen has been studied over a Au/TiO₂ reference catalyst by combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. It is shown for the first time that H₂ enhances the CO oxidation rate on Au/TiO₂ without leading to any major loss of selectivity. Increasing the H₂ pressure induces higher CO and H₂ oxidation rates. Under H₂-free conditions, the surface species detected are Au^δ+–CO, Ti⁴⁺–CO, carbon dioxide and carbonates. Upon the addition of H₂, Au⁰–CO, water and hydroxyl groups become the main surface species. The occurrence of a preferential CO oxidation mechanism involving H_xO_y species under the present experimental conditions is proposed.     

Rational design of gold catalysts with enhanced thermal stability: post modification of Au/TiO2 by amorphous SiO2 decoration

Au/TiO₂ is highly active for CO oxidation, but it often suffers from sintering in high-temperature environments. In this work, we report on a novel design of gold catalysts, in which pre-formed Au/TiO₂ catalysts were post decorated by amorphous SiO₂ to suppress the agglomeration of gold particles. Even after being aged in O₂–He at 700 °C, the SiO₂-decorated Au/TiO₂ was still active for CO oxidation at ambient temperature.