The effects of SO2 on the oxidation of CO and propane on supported Pt and Au catalysts

The catalytic activities of Pt and Au supported on TiO₂ were compared with respect to the oxidation of CO and propane. While the Au catalysts showed higher activities for CO oxidation, the Pt catalysts were more active for propane combustion. A strong de-activation of the CO oxidation activity by SO₂ was observed only over the TiO₂-supported Au catalyst, indicating that SO₂ can block the active sites for CO oxidation over Au catalysts. The results are consistent with a model in which the perimeter sites have a special role in the CO oxidation reaction over Au catalysts.     

A comparative study of TiO2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature

The TiO₂ supported noble metal (Au, Rh, Pd and Pt) catalysts were prepared by impregnation method and characterized by means of X-ray diffraction (XRD) and BET. These catalysts were tested for the catalytic oxidation of formaldehyde (HCHO). It was found that the order of activity was Pt/TiO₂  Rh/TiO₂ > Pd/TiO₂ > Au/TiO₂  TiO₂. HCHO could be completely oxidized into CO₂ and H₂O over Pt/TiO₂ in a gas hourly space velocity (GHSV) of 50,000 h⁻¹ even at room temperature. In contrast, the other catalysts were much less effective for HCHO oxidation at the same reaction conditions. HCHO conversion to CO₂ was only 20% over the Rh/TiO₂ at 20 °C. The Pd/TiO₂ and Au/TiO₂ showed no activities for HCHO oxidation at 20 °C. The different activities of the noble metals for HCHO oxidation were studied with respect to the behavior of adsorbed species on the catalysts surface at room temperature using in situ DRIFTS. The results show that the activities of the TiO₂ supported Pt, Rh, Pd and Au catalysts for HCHO oxidation are closely related to their capacities for the formation of formate species and the formate decomposition into CO species. Based on in situ DRIFTS studies, a simplified reaction scheme of HCHO oxidation was also proposed.     

In situ measurements under flow condition of the CO oxidation over supported gold nanoparticles

In situ FT-IR measurements for Au/TiO₂ and Au/Al₂O₃ have been carried out under the flow condition of CO oxidation at atmospheric pressure. It has been found that the Au particles remain neutral (Au⁰) in the presence of oxygen, while negatively charged particles (Au^δ−) is formed in the absence of oxygen, as a result of the charge transfer from the oxygen vacancies. Moisture did not significantly affect the adsorption states of CO over Au/TiO₂ and Au/Al₂O₃. Enhancement of the CO₂ production by moisture was observed over Au/Al₂O₃, which is accompanied by the decomposition of carbonate-like species by moisture.     

CO oxidation activity of gold catalysts supported on various oxides and their improvement by inclusion of an iron component

A number of oxide-supported gold catalysts have been prepared by deposition–precipitation, with variation of the pH over a wide range, the optimum pH for high activity being 9 for TiO₂, 7.5 for Fe₂O₃, and 7 for SnO₂ and CeO₂. Whereas the activity shown by Au/TiO₂ and Au/Fe₂O₃ decreased linearly with time, Au/CeO₂ and Au/SnO₂ underwent an initial major deactivation. Addition of iron in the preparation lowered the rate of deactivation when TiO₂, SnO₂ and CeO₂ were used as supports, and imparted activity when as with Bi₂O₃ it was previously lacking. XPS revealed the existence of a broad multi-state iron-containing region, and TEM and STEM/EDX indicated that small gold particles (1.5–4 nm) were partly in contact with it. Improved stability is therefore due to gold particles being in contact with an iron phase such as FeO(OH); calcination removed the stabilisation.     

Supported gold catalysis derived from the interaction of a Au–phosphine complex with as-precipitated titanium hydroxide and titanium oxide

Supported gold catalysts derived from interaction of a Au–phosphine complex Au(PPh₃)(NO₃) (1) with conventional titanium oxide TiO₂ and as-precipitated titanium hydroxide (*, as-precipitated) have been characterized by means of XRD, XPS, EXAFS, and CP/MAS–NMR. The Au complex 1 was supported on TiO₂ and without loss of Au–P bonding at room temperature. The Au complex 1 on TiO₂ was readily and completely decomposed to form metallic gold particles by calcination at 473 K, whereas only a small part of the complex 1 on was transformed to metallic gold particles. By calcination of 1/ at 573 K the formation of both metallic gold particles and crystalline titanium oxides became notable as evidenced by XRD, XPS and CP/MAS–NMR. The mean diameter of Au particles in 1/ calcined at 673 K was less than 30 Å as estimated from Au(2 0 0) diffraction, which was about one-tenth of that for the corresponding 1/TiO₂. Thus the as-precipitated titanium hydroxide was able to stabilize the Au complex 1 to lead to the simultaneous decomposition of Au complex and . The catalyst 1/ calcined at 673 K afforded remarkably high catalytic activity for low-temperature CO oxidation at 273–373 K as compared to the catalyst 1/TiO₂.     

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.     

Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts

Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH₃OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (-Fe₂O₃, TiO₂, etc.). While the supported gold catalysts are less active in H₂ oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO₂, the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol.     

Combinatorial investigation of Pt–Ru–M as anode electrocatalyst for direct methanol fuel cell

The addition of Au/TiO₂ and zeolites as active components to PtRu/C electrode in DMFC was investigated by using combinatorial high-throughput-screening test. Addition of Au/TiO₂ to PtRu/C electrode, especially in the ratio of PtRu/C: Au/TiO₂ 9:1, 8:2, 7:3, were effective to improve the performance of direct methanol fuel cell. The electrochemical properties of the prepared electrodes were compared using cyclic voltammetry, impedance spectroscopy and a single cell performance test of a direct methanol fuel cell (DMFC). The adsorbed CO on Pt might be easily oxidized on the surface of Au/TiO₂ by interaction between PtRu/C and Au/TiO₂. The addition of the solid acid proton conducting materials (ZSM-5) on PtRu/C anode leads to the high temperature operation. The cell performance was maintained over the cell temperature 120 °C (maximum current density was 200 mA/cm² at 160 °C) by the addition of ZSM-5 as proton conducting materials.     

Relationship between the formation of surface species and catalyst deactivation during the gas-phase photocatalytic oxidation of toluene

Various partial oxidation products were identified on the surface of TiO₂ and an 8% SiO₂–TiO₂ binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid.

The SiO₂–TiO₂ photocatalyst is more active and appears to deactivate slower than TiO₂. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO₂. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Brønsted acidity but when SiO₂ is cofumed with TiO₂, Brønsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it.     

Au对TiO2光电催化材料的改性策略研究进展

近年来,环境污染、能源枯竭问题日益严重,成为制约人类生存与发展的主要因素。光电催化技术能够同时实现污染物的降解与清洁能源的制备,有助于缓解环境污染与能源枯竭问题。作为典型的光（电）催化材料,TiO₂具有光活性强、性质稳定、廉价易得、环境友好等诸多优点,数十年来已成为光催化及相关领域的研究热点。然而,TiO₂存在的本征缺陷依然制约着其进一步推广应用,为此研究人员已提出多种方式对TiO₂进行改性。其中贵金属/TiO₂复合材料可显著提升TiO₂的光学活性并拓宽其吸收波长范围,尤其是Au/TiO₂材料体系已受到广泛关注和认可,表现出良好的应用前景。本文通过对目前Au/TiO₂复合材料的发展现状进行了总结,首先简单介绍了Au和TiO₂的化学性质及Au对TiO₂光学性能的增强原理;随后对Au/TiO₂复合材料的改性策略及相关作用机制展开讨论,包括Au对TiO₂光学性能的影响及调控、修饰方法的选择与影响等;最后总结出目前Au/TiO₂复合材料依然以克服TiO₂的两大本征缺陷为主,探讨各类新型Au/TiO₂复合材料有望使其得到逐步推广与实际应用。最后对目前Au/TiO₂复合材料的研究现状进行系统总结并探讨该材料未来的研究和发展方向。     

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.     

The CO–H2 and CO–H2O reactions over TiO2 nanotubes filled with Pt metal nanoparticles

We have investigated the catalytic behavior of Pt encapsulated TiO₂ nanotubes for the water gas shift reaction as well as the hydrogenation of CO. Pt–TiO₂ nanotube catalysts were prepared by employing fine fiber shaped crystals of [Pt(NH₃)₄](HCO₃)₂ complex as a structure determining template material. The turnover frequencies (TOF) of these nanotube catalysts were more than one order of magnitude larger than conventional impregnation Pt/TiO₂ catalysts, and the selectivity for methanol in CO–H₂ reaction was extraordinary high compared to the impregnation catalysts. The XPS and XRD analyses of the nanotubes revealed characteristic electronic state of reduced TiO₂ (Ti³⁺ in rutile structure) with zerovalent Pt even after the calcination at 773 K. In WGS reaction, electron rich Ti³⁺ on the nanotube wall may play an important role to activate water molecules for the oxidation of CO. In CO–H₂ reaction, similar promotion effect of Ti³⁺ species may be operating for selective methanol formation by supplying active OH(a).     

EPR spectroscopic characterization of DeNOx and SO2 oxidation catalysts and model systems

The industrial SO₂ oxidation catalyst VK69 deactivates at around 440°C in a 10% SO₂, 11% O₂, 79% N₂ gas mixture. In situ EPR measurements show that the deactivation is caused by the precipitation of V(IV) compounds. DeNO_x catalysts based on V₂O₅/TiO₂, the TiO₂ support, analytical grade anatase and transition metal-exchanged Al-PILCs (pillared clay) have been characterized by EPR spectroscopy and the catalytic activity of the catalysts monitored up to 500°C. Depending on the exchanged metal ion, a relatively large temperature range for the catalytic activity towards the SCR reaction was observed.     

Au/TiO2复合物的制备、表征及其增强光催化灭菌活性

为提升TiO₂光催化活性克服其可见光响应能力差的问题,采用沉积-沉淀法制备了Au/TiO₂复合物光催化剂,利用X射线衍射（XRD）、傅里叶红外光谱（FTIR）、X射线光电子能谱（XPS）、紫外-可见漫反射光谱（UV-vis DRS）、荧光发射光谱等对样品进行了表征。XRD、FTIR和XPS结果表明,Au/TiO₂中TiO₂为锐钛矿相且Au成功沉积至TiO₂。UV-vis DRS和荧光发射光谱结果表明,适量Au修饰不仅能提高TiO₂对可见光的吸收,还可促进TiO₂光生电子-空穴对分离,有利于增强其光催化活性。自由基捕获实验证实,形成?OH的数量与光照时间成正比且?OH生成量越多,光催化活性越高。对比考察了Au/TiO₂和TiO₂在氙灯光源照射下对大肠杆菌的光催化杀灭作用,并探讨了Au负载量、光照时间、光照强度、光催化剂浓度等因素对灭菌性能的影响。结果表明：Au/TiO₂的光催化灭菌活性优于TiO₂,且与光照时间和光照强度均成正比;Au的适宜负载量为3%（质量分数）;3%Au/TiO₂在光照时间60min、光照强度7mW/cm²、光催化剂浓度100μg/mL的条件下,对大肠杆菌的杀灭效率高达91.3%。     

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.     

Room temperature visible light oxidation of CO by high surface area rutile TiO2-supported metal photocatalyst

Visible light-active rutile TiO₂ with a high surface area of 200 m²/g was obtained by a low-temperature sol–gel synthesis, based on a long aging duration of a titania sol to stabilize the rutile phase. Decorated by an adequate amount of metallic nanoparticles, this non-doped TiO₂ displays high and stable performances for the on-stream room temperature oxidation of CO by visible light photocatalysis.     

Vanadium-containing catalysts for the selective oxidation of H2S to elemental sulfur in the presence of excess water

Various vanadium-containing catalysts were searched for the commercial application in the selective oxidation of H₂S to elemental sulfur at low temperatures (less than 250°C) in the presence of excess (more than 35 vol.%) water. In the test of binary oxides, it was found that TiVO_x was the only catalyst that could sustain its activity without deactivation at 230°C. The best catalytic activity (85–90% sulfur yield) was obtained when VO_x/TiO₂ was incorporated with other metals such as Fe, Cr and Mo. Reaction occurred via redox mechanism and the reoxidation of reduced vanadium was the rate-limiting step. A long-term deactivation observed during the reaction was due to slower reoxidation of reduced vanadium by oxygen than the reduction by H₂S. Catalytic activities of VO_x/SiO₂, VO_x/TiO₂ and V–Fe–Cr–Mo–O_x/TiO₂ were well correlated with their redox properties that were observed by TPR/TPO and XPS measurements.     

Preparation of Au/TiO2 catalysts from Au(I)–thiosulfate complex and study of their photocatalytic activity for the degradation of methyl orange

Gold loaded on TiO₂ (Au/TiO₂) catalysts were prepared using Au(I)–thiosulfate complex (Au(S₂O₃)₂³⁻) as the gold precursor for the first time. The samples were characterized by UV–vis diffuse reflectance spectra, X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption flame emission spectroscopy (AAS), and X-ray photoelectron spectroscopy (XPS) methods. Using Au(S₂O₃)₂³⁻ as gold precursor, ultra-fine gold nanoparticles with a highly disperse state can be successfully formed on the surface of TiO₂. The diameter of Au nanoparticles increases from 1.8 to 3.0 nm with increasing the nominal Au loading from 1% to 8%. The photocatalytic activity of Au/TiO₂ catalysts was evaluated from the analysis of the photodegradation of methyl orange (MO). With the similar Au loading, the catalysts prepared with Au(S₂O₃)₂³⁻ precursor exhibit higher photocatalytic activity for methyl orange degradation when compared with the Au/TiO₂ catalysts prepared with the methods of deposition–precipitation (DP) and impregnation (IMP). The preparation method has decisive influences on the morphology, size and number of Au nanoparticles loaded on the surface of TiO₂ and further affects the photocatalytic activity of the obtained catalysts.     

Solvent-free oxidation of benzyl alcohol using titania-supported gold–palladium catalysts: Effect of Au–Pd ratio on catalytic performance

The oxidation of benzyl alcohol with molecular oxygen under solvent-free conditions has been investigated using a range of titania-supported Au–Pd alloy catalysts to examine the effect of the Au–Pd ratio on the conversion and selectivity. The catalysts have been compared at high reaction temperature (160 °C) as well as at 100 °C, to determine the effect on selectivity since at lower reaction temperature the range of by-products that are formed are limited. Under these conditions the 2.5 wt.% Au–2.5 wt.% Pd/TiO₂ was found to be the most active catalyst, whereas the Au/TiO₂ catalyst demonstrated the highest selectivity to benzaldehyde. Toluene, formed via either a hydrogen transfer process or an oxygen transfer process, was observed as a major by-product under these forcing conditions.     

TiO2-photocatalysis as a tertiary treatment of naturally treated wastewater

Solar TiO₂-photocatalysis was applied to waters from a natural wastewater treatment plant located in the Universidad de Las Palmas de Gran Canaria. Degussa P-25 TiO₂ and its mixture with activated carbon (AC–TiO₂) were used as catalysts. The presence of ozone and certain ions such as phosphates on the photocatalytic degradation of organic matter was also studied.

Disinfection experiments have provided interesting results, particularly when using the catalyst AC–TiO₂ and ozone, since total disinfection was achieved in less than 60 min. No bacterial reappearance at 24 or 48 h was observed. Additionally, this catalyst gave important TOC and some ions concentrations reductions.

Studies in catalyst reuse revealed that the catalyst AC–TiO₂ showed almost no deactivation.