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.     

XPS and DRS of Au/TiO2 catalysts: effect of pretreatment

TiO₂ impregnated with AuCl₃ was subjected to different pretreatments and then characterized by XPS and DRS. After drying at 298 K under vacuum, the catalyst contains highly dispersed, nonmetallic Au species; whereas drying at 393 K in an oven caused the Au to be partially reduced and agglomerate. Further treatments of the oven-dried sample at higher temperatures resulted in the disappearance of Au signals in XPS except the one after a HTR/C/LTR (high-temperature reduction/calcination/low-temperature reduction) sequence. The high-temperature reduction at 773 K shifted the plasmon resonance peak in DRS to higher wavelength, and the following C and LTR treatment did not change the peak position. This peak shifting is interpreted as a change in the electronic status of the Au. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared Study of the NO + CO Interaction over Au/TiO2 Catalyst

It was demonstrated that, in contrast to previous results, an NCO surface complex does form on Au/TiO₂ catalysts in the high-temperature reaction of NO + CO. By means of Fourier transform infrared spectroscopy two new absorption bands at 2285 and 2105-2210 cm^-1, not observed in separate adsorption of reacting gases, were detected during the catalytic reaction. The former is attributed to Au-NCO and the latter to Ti-NCO species. It was assumed that, similar to the case of supported Pt metals, NCO is primarily formed on Au crystallites and then spills over onto titania. This idea was strengthened by the results obtained following the adsorption of HNCO on the catalyst and on the support.     

Aerobic oxidation of alcohols over Au/TiO2: An insight on the promotion effect of water on the catalytic activity of Au/TiO2

The unique and significant promotion effect of water has been evidenced by the selective oxidation of benzyl alcohol to benzaldehyde over Au/TiO₂ catalysts. Water has dual promotional functions in the reaction system: to help form unique microdroplets in a multiphase reaction system and to assist the oxygen adsorption and activation. The conversion of benzyl alcohol at a molar ratio of water to solvent (p-xylene) of 7 is 7 times higher than in the absence of water. The present work has highlighted the potential of Au/TiO₂ catalysts in aerobic oxidation of alcohols in the unique multiphase reaction system with water as promoting solvent.     

Methanol oxidation on Au/TiO2 catalysts

We have investigated the adsorption and reaction of methanol with Au/TiO₂ catalysts using a pulsed flow reactor, DRIFTS and TPD. The TiO₂ (P25) surface adsorbed a full monolayer of methanol, much of it in a dissociative manner, forming methoxy groups associated with the cationic sites, and hydroxyl groups at the anions. The methoxy is relatively stable until 250 °C, at which point decomposition occurs, producing mainly dimethyl ether by a bimolecular surface reaction. As the concentration of methoxy on the surface diminishes, so the mechanism reverts to a de-oxygenation pathway, producing mainly methane and water (at ~330 °C in TPD), but also with some coincident CO and hydrogen. Au catalysts were prepared by the deposition-precipitation method to give Au loadings between 0.5–3 wt %. The effect of low levels of Au on the reactivity is marked. The pathway which gives methane, which is characteristic of titania, remains, but a new feature of the reaction is the evolution of CO₂ and H₂ at lower temperature (a peak is seen in TPD at 220 °C), and the elimination of the DME-producing state. Clearly this is associated with the presence of Au and appears to be due to the production of a formate species on the surface of the Au component. This formate species is mainly involved in the reaction of methanol with the Au/TiO₂ catalysts which results in a combustion pathway being followed, with complete conversion occurring by ~130 °C.     

Low-temperature preferential oxidation of CO in a hydrogen rich stream (PROX) over Au/TiO2: Thermodynamic study and effect of gold-colloid pH adjustment time on catalytic activity

By simulating CO and H₂ oxidations at thermodynamic equilibrium and studying the catalytic oxidations over Au/TiO₂, preferential oxidation of CO in a H₂ rich stream (PROX) was investigated. During the simulation, at least two cases under different gaseous feeds, H₂/CO/O₂/N₂ = 50/1/0.5/48.5 or 50/1/1/48 (vol.%) were examined under the assumption of an ideal gas and one atmosphere pressure in the reactor. It was found that the addition of 1% O₂ (the latter case) effectively reduced CO concentration to less than 100 ppm in the temperature range between 0 and 90 °C. This range narrowed to between 0 and 50 °C with the addition of 3% H₂O and 15% CO₂ in the feed. The thermodynamic study suggests that 1% CO in a H₂ rich system can be decreased to below 100 ppm within those low temperature ranges, if there is no substantial adsorptions onto the catalyst surface and the reactions rapidly reach equilibrium. During the catalysis reaction study, a well-pH adjusted Au/TiO₂ catalyst was found very active for PROX. CO conversions at the reactor outlet were close to those at equilibrium. Au/TiO₂ used in this work was prepared via deposition-precipitation (DP) method. The influence of gold colloid pH (at 6) adjustment time on gold loading, gold particle size and chloride residue on TiO₂ surface was detected by atomic absorption (AA), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A pH adjustment time of at least 6 h for the preparation of gold colloids at room temperature was demonstrated to be essential for the high catalytic activity of Au/TiO₂. This was attributed to the smaller gold particle and the less chloride residue on the catalyst surface.     

Adsorption and reactivity during low temperature oxidation of carbon monoxide on Au/TiO2 catalysts studied by rapid response mass spectrometry

The adsorption and reaction of CO, CO₂ and O₂ on TiO₂ and Au/TiO₂ have been studied using a mass spectrometric method which can detect processes occurring on a time scale of seconds. Adsorption of CO on TiO₂ at 300 K is rapidly reversible and less on reduced samples than oxidised ones indicating that the adsorption sites are oxide ions. The amount adsorbed reversibly on reduced Au/TiO₂ is less still, consistent with enhanced reduction, but additional amounts adsorb irreversibly at a slower rate. The amount of CO₂ adsorbed under similar conditions is also greater on TiO₂ than reduced Au/TiO₂ and approximately one order of magnitude greater than that of CO. However, adsorption of O₂ is undetectable on the time scale of the measurement. Exposure of Au/TiO₂ to mixtures of CO and O₂ results in near instantaneous generation of CO₂ although its appearance is attenuated by adsorption. Adsorption of CO occurs concurrently in a way similar to that seen with CO alone except that the amount of the more slowly adsorbed form seems less. This suggests that it is the form utilised in catalysis. Oxygen uptake beyond that generating CO₂ is appreciable during the initial stages of exposure to reaction mixtures and this capacity is enhanced if one or other reactant is removed and then reintroduced, possibly due to the generation of reducible interface sites. It is concluded that the remarkable activity of Au/TiO₂ for CO oxidation at ambient temperature resides in a very high turnover frequency on sites at the interface between the metal and oxide. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reproducibility of highly active Au/TiO2 catalyst preparation and conditioning

We present results of a comparative study on the reproducibility of Au/TiO₂ catalyst preparation and conditioning. Comparison of their physical characteristics and their catalytic activity for CO oxidation shows that highly active catalysts can indeed be prepared in a very well-defined and reproducible way, with particle sizes of 2 nm (conditioning via reductive or redox treatment) or 3 nm (conditioning via calcination).     

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.     

Photocatalytic oxidation of toluene over TiO2 catalysts supported on glass fiber

Titania-impregnated photocatalysts supported on glass fiber (GF) were prepared and employed in the photocatalytic oxidation of toluene. Amorphous titania sol (ATS) prepared from titanium isopropoxide was useful to impregnate titania on glass fiber in the shape of a thin film. TiO₂/GF catalysts prepared by using the ATS showed better stability against water and vibration, but lower catalytic activity in the photo-oxidation of toluene compared to crystalline anatase powder (CAP) impregnated catalyst. Photocatalytic activity of the ATS catalyst was considerably enhanced by the addition of hydrogen peroxide, resulting in high activity comparable to that of the CAP catalyst.     

On-line deactivation of Au/TiO2 for CO oxidation in H2-rich gas streams

Au-based catalysts, known for ambient temperature CO oxidation, have to provide stable performance of up to 5000 h in order to be commercially applicable in automotive fuel cells. In this report, the on-line deactivation characteristics of Au/TiO₂ in unconventional PROX conditions are discussed. As opposed to CO removal from air, results in this report suggests that carbonates have a minor effect on deactivation of Au/TiO₂ in dry H₂-rich conditions. Also, no conclusive correlation between surface hydration and deactivation was observed. Rather, deactivation appeared to have occurred as a result of an intrinsic transformation in the oxidation state of the active species in the reducing operating conditions; a process which was reversible in an oxidizing atmosphere.     

制备条件对Au/TiO2液相氧化反应活性的影响

采用沉积-沉淀法制备纳米级Au/TiO_2催化剂,以葡萄糖液相催化氧化制葡萄糖酸为探针反应,考察了催化剂制备条件对Au/TiO_2活性的影响,并利用TEM、XRD和XPS等方法对催化剂进行了表征。结果表明,Au/TiO_2对葡萄糖液相氧化反应的催化活性与催化剂的制备条件密切相关,纳米金的颗粒尺寸不是决定催化活性的惟一因素,金在催化剂中的价态对催化活性有重要影响。     

SO2 oxidation over the V2O5/TiO2 SCR catalyst

The effects of V₂O₅ loading of the V₂O₅/TiO₂ SCR catalyst on SO₂ oxidation activity were examined by infrared spectroscopy (DRIFT) and SO₂ oxidation measurement. Vanadium oxide added to the catalyst was found to be well dispersed over the TiO₂ carrier until covered with monolayer V₂O₅. The rate of SO₂ oxidation increased almost linearly with V₂O₅ loading below the monolayer capacity and attained saturation with further increase. The hydroxyl groups bonded to vanadium atoms, V–OH, might be altered by SO₂ oxidation. Both V=O and V–OH groups are likely involved in the adsorption and desorption of SO₂ and SO₃.     

Mesoporous silica supports for improved thermal stability in supported Au catalysts

In this work, we present a comprehensive review of our research on the role of mesoporous silica pore architecture, composition of the pore walls (addition of Co or Al), and silica surface chemistry (surface modification by TiO₂) to improve the hydrothermal stability of Au particles. We have found that mesoporous silica architecture plays an important role in improving Au stability, with three dimensional mesoporous architectures being less effective than one dimensional (1-D) pores. The tortuous 1-D pores in aerosol silica were found to be most effective at controlling Au particle size. Since Au particles continue to grow larger than the pore diameter, we conclude that Ostwald ripening must be the dominant sintering pathway for these Au catalysts. These catalysts are active for CO oxidation even after the Au particles have grown large enough to block the pores, suggesting that the thin walls of mesoporous silica provide easy access to gas phase molecules. Further improvements in Au stability and reactivity were obtained by surface modification of the aerosol and MCM-41 silica with TiO₂. After TiO₂ modification of the silica, the Au particles remained smaller than the pore size (< 3 nm) even after three cycles of CO oxidation at temperatures up to 400 °C.     

Reactivity and sintering kinetics of Au/TiO2(110) model catalysts: particle size effects

We review here our studies of the reactivity and sintering kinetics of model catalysts consisting of gold nanoparticles dispersed on TiO₂(110). First, the nucleation and growth of vapor-deposited gold on this surface was experimentally examined using x-ray photoelectron spectroscopy and low energy ion scattering. Gold initially grows as two-dimensional islands up to a critical coverage, θ _cr, after which 3D gold nanoparticles grow. The results at different temperatures are fitted well with a kinetic model, which includes various energetic parameters for Au adatom migration. Oxygen was dosed onto the resulting gold nanoparticles using a hot filament technique. The desorption energy of O_a was examined using temperature programmed desorption (TPD). The O_a is bonded ~40% more strongly to smaller (thinner) Au islands. Gaseous CO reacts rapidly with this O_a to make CO₂, probably via adsorbed CO. The reactivity of O_a with CO increases with increasing particle size, as expected based on Br?nsted relations. Propene adsorption leads to TPD peaks for three different molecularly adsorbed states on Au/TiO₂(110), corresponding to propene adsorbed on gold islands, to Ti sites on the substrate, and to the perimeter of gold islands, with adsorption energies of 40, 52 and 73 kJ/mol, respectively. Thermal sintering of the gold nanoparticles was explored using temperature-programmed low-energy ion scattering. These sintering rates for a range of Au loadings at temperatures from 200 to 700 K were well fitted by a theoretical model which takes into consideration the dramatic effect of particle size on metal chemical potential using a modified bond additivity model. When extrapolated to simulate isothermal sintering at 700 K for 1 year, the resulting particle size distribution becomes very narrow. These results question claims that the shape of particle size distributions reveal their sintering mechanisms. They also suggest why the growth of colloidal nanoparticles in liquid solutions can result in very narrow particle size distributions.     

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.     

The preparation of Au/CeO2 catalysts and their activities for low-temperature CO oxidation

Au/CeO₂ catalysts prepared by co-precipitation (CP) and deposition-precipitation (DP) methods were tested for low temperature CO oxidation reaction. The structural characters and redox features of the catalysts were investigated by XRD, XPS and H₂-TPR. Their catalytic performances for low temperature CO oxidation were studied by means of a microreactor -GC system. It showed that the catalytic activities of Au/CeO₂ catalysts greatly depended on the preparation method. The catalysts prepared by DP method exhibited a surprisingly higher activity towards CO oxidation than that prepared by CP method. This may arise from the differences in the particle sizes of Au and redox properties of the catalysts. The low Au loading and the resistance to high temperature of DP-prepared catalyst made it more applicable.     

Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Nanosized solid superacids SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂, as well as MCM-41-supported SO₄ ²⁻/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄ ²⁻/ZrO₂/MCM-41, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO₄ ²⁻/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈ ²⁻/TiO₂ even stronger than those on SO₄ ²⁻/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄ ²⁻/ZrO₂/MCM-41. It can be concluded that the existence of more Br?nsted acid sites was favorable for proton participation in the cyclization reaction. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(2): 239–244 [译自: 高校化学工程学报]     

Activity and selectivity of PtNi/TiO2 catalysts for hydrogenation of crotonaldehyde

The hydrogenation of crotonaldehyde over a series of titania supported PtNi catalysts was investigated. Pt/TiO₂ had the highest activity to hydrogenate the bond of crotonaldehyde. With the addition of Ni to Pt the activity for carbonyl group hydrogenation decreased, while that for the bond hydrogenation increased. Two different types of sites were established to be active for bond hydrogenation, located at (i) the Pt-TiO₂ interface and (ii) the PtNi bimetallic phase. For the Pt-TiO₂ interface the presence of accessible Ti ^x+ cations in close distance to Pt and for the PtNi phase a slightly positively charged Ni on the surface of these alloy particles were concluded to be responsible for the enhanced selectivity to crotylalcohol. The hydrogenation of the bond occurs mainly on the clean metal surfaces of Ni and Pt.