Preparation of supported gold catalysts from gold complexes and their catalytic activities for CO oxidation

A phosphine-stabilized mononuclear gold complex Au(PPh₃)(NO₃) (1) and a phosphine-stabilized gold cluster [Aug(PPh₃)₈](NO₃)₃ (2) were used as precursors for preparation of supported gold catalysts. Both complexes 1 and 2 supported on inorganic oxides such as -Fe₂O₃, TiO₂, and SiO₂ were inactive for CO oxidation, whereas the 1 or 2/ oxides treated under air or CO or 5% h₂/Ar atmosphere were found to be active for CO oxidation. The catalytic activity depended on not only the treatment conditions but also the kinds of the precursor and the supports used. The catalysts derived from 1 showed higher activity than those derived from 2. -Fe₂O₃ and TiO₂ were much more efficient supports than SiO₂ for the gold particles which were characterized by XRD and EXAFS.     

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.     

High activity supported gold catalysts by incipient wetness impregnation

It is generally thought that catalysts produced by incipient wetness impregnation (IW) are very poor for low temperature CO oxidation, and that it is necessary to use methods such as deposition–precipitation (DP) to make high activity materials. The former is true, indeed such IW catalysts are poor, and we present reactor data, XPS and TEM analysis which show that this is due to the very negative effect of the chloride anion involved in the preparation, which results in poisoning and excessive sintering of the Au particles. With the DP method, the chloride is largely removed during the preparation and so poisoning and sintering are avoided.

However, we show here that, contrary to previous considerations, high activity catalysts can indeed be prepared by the incipient wetness method, if care is taken to remove the chloride ion during the process. This is achieved by using the double impregnation method (DIM). In this a double impregnation of chloroauric acid and a base are made to precipitate out gold hydroxide within the pores of the catalyst, followed by limited washing. This results in a much more active catalyst, which is active for CO oxidation at ambient temperature. The results for DIM and DP are compared, and it is proposed that the DIM method may represent an environmentally and economically more favorable route to high activity gold catalyst production.     

超细TiO_2粉体光催化剂的制备

以TiCl3为原料制备了超细TiO2粉体,采用X射线衍射、热重分析和化学分析等方法,研究了制备条件对TiO2光催化活性的影响。结果表明,在500~600℃煅烧制得的TiO2粉体为金红石和锐钛矿混合相,其中的Ti3+的浓度较大,光催化活性较高。洗涤条件对光催化活性也有明显的影响,与用水洗涤制得的TiO2粉体相比,在同样煅烧条件下经乙醇洗涤后得到的TiO2粉体中Ti3+的浓度较大,具有较高的光催化活性。     

Synergism in methanol synthesis from carbon dioxide over gold catalysts supported on metal oxides

Gold deposited on various oxides with high dispersion was found to be active for the hydrogenation of CO₂ at temperatures between 150 and 400°C. Product selectivities greatly depended on the nature of support oxide. Acidic oxides like TiO₂ gave higher CO₂ conversions but low methanol yields. Zinc oxide component was indispensable for selective methanol synthesis. Significantly, large particle size effect of gold was observed and smaller gold particles gave higher methanol productivity per exposed surface area of gold. This can be explained by an increase in the perimeter area of gold particles with a decrease in particle size. Methanol yield was greatly enhanced in a Au/ZnO---TiO₂ catalyst probably due to an increase in gold-zinc oxide interface.     

Oxidation of mono- and polyalcohols with gold: Comparison of carbon and ceria supported catalysts

Aim of this work is the investigation of the gold catalysed liquid-phase oxidation of polyalcohols with both, primary and secondary alcohol groups, in order to evaluate the potential of gold catalysts to oxidise a secondary alcohol group in presence of a primary group. For that purpose we investigated the heterogeneously catalysed liquid-phase oxidations of n-propanol, as reference for the oxidation of the primary alcohol group only, of propylene glycol, where the competitive reaction can be examined and finally of glycerol, which is the target reaction due to known economic aspects. We report on the performance of ceria supported gold catalysts in these reactions and discuss the results in dependency of the specific surface area of the support, the catalyst and support preparation method and the catalyst activation conditions. Finally, in order to estimate the relative activity and selectivity of the ceria supported catalysts we compared the catalytic results with the performance of carbon and titania supported gold catalysts.     

Chemical vapor deposition of gold on Al2O3, SiO2, and TiO2 for the oxidation of CO and of H2

In order to clarify the effect of metal oxide support on the catalytic activity of gold for CO oxidation, gold has been deposited on SiO₂ with high dispersion by chemical vapor deposition (CVD) of an organo-gold complex. Comparison of Au/SiO₂ with Au/Al₂O₃ and Au/TiO₂, which were prepared by both CVD and liquid phase methods, showed that there were no appreciable differences in their catalytic activities as far as gold is deposited as nanoparticles with strong interaction. The perimeter interface around gold particles in contact with the metal oxide supports appears to be essential for the genesis of high catalytic activities at low temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidation state of oxide supported nanometric gold

We address the structure gap between surface science and catalysis studies of the activity of oxide supported Au clusters. Reviewing the recent literature we find that surface science investigations often deal with highly reduced systems that have anionic Au clusters and oxygen vacancies in the support. The catalysis studies on the other hand consistently report on oxidized samples with traces of cationic Au. Performing density functional theory calculations we show that the effect of oxidation of oxide supported Au clusters, Au₈/MgO, Au₇/TiO₂ and Au₁₀/TiO₂, is a strong increase in the Au/support adhesion energy and a great structural transformation of the clusters. Some of the Au atoms become positively charged (cationic) in the oxidation process as evidenced indirectly by calculated vibrational stretch frequency shifts of adsorbed CO.     

Selective liquid phase oxidation using gold catalysts

Au/C and Au/oxide (Al₂O₃, TiO₂) have been compared in the liquid phase oxidation of glycols and a different trend in reactivity revealed. On the oxides the activity of supported gold increases by decreasing particle size, whereas on carbon maximum activity is achieved with gold particle mean diameter around 7–8 nm. XPS revealed that in the latter case activity depends not only on the size of the gold particle but also on its surface concentration. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

纳米金催化剂在CO低温氧化和选择性氧化中的研究进展

介绍了纳米金催化剂在CO低温氧化和丙烯直接环氧化反应中的研究进展。在CO低温氧化反应中,催化剂的活性相和载体都具有明显的尺寸效应,纳米金颗粒和载体之间的相互作用主要表现载体不仅可以改变纳米金颗粒的大小和形状,而且也影响了氧的活化,从而提高反应活性;在丙烯直接环氧化反应中,由H2和O2在金颗粒表面反应生成的过氧化物种是反应中间体;在选择性氧化和选择性加氢反应中,金催化剂表现出优良的活性和稳定性。     

Research on unsupported nanoporous gold catalyst for CO oxidation

Nanoporous gold (NPG), a novel unsupported gold catalyst prepared by dealloying, exhibits exceptional catalytic activity for CO oxidation. Systematic studies were carried out on this new catalytic system, including the active sites of catalysts, the reaction kinetics, and activity dependence as functions of space velocity and temperature. Our results show strong evidence that metallic gold atoms on NPG are the intrinsic active sites at which the reaction of CO with O₂ occurs. The kinetic study found that the reaction rate of CO oxidation on unsupported NPG depends significantly on CO concentration but only slightly on O₂ concentration. We suggest that CO adsorption plays a decisive role in CO oxidation on NPG as the rate-limiting step. By completely ruling out the support influence, our findings provide considerable insight into the role of gold catalysts.     

熔盐法处理富钛渣制备金红石型TiO_2

工业生产钛白粉的方法主要有硫酸法和氯化法两种,但这两种方法都存在环境污染严重的问题。本文提出了一种常压、低温下钠碱熔盐分解富钛渣制备金红石型二氧化钛的新工艺,从生产源头消除了对环境的污染,实现钛资源的可持续发展。介绍了该反应的制备、水洗、固相离子交换-水解耦合和煅烧过程。实验结果表明,该工艺可以制得理想的金红石型二氧化钛,二氧化钛含量为96.66%。     

Temporal analysis of products (TAP) study on low temperature carbon monoxide oxidation on supported Au catalysts

TAP (temporal analysis of products) technique was used to clarify the controversial mechanism for low-temperature CO oxidation on supported Au catalysts involving unidentified moisture effects on the performances. The unique TAP transient technique, along the use of a specially prepared, highly active Au/Ti(OH) ₄ ^* catalyst, provided the information and characterization of each elementary step involving weak and reversible CO adsorption, strong and molecular O₂ adsorption, and their surface reaction, which are suppressed by the coexistence of water vapor.     

Catalytic oxidation of 1,2-diols to α-hydroxy-carboxylates with stabilized gold nanocolloids combined with a membrane-based catalyst separation

Gold (Au) nanosols stabilized with poly(vinylalcohol) can be used as homogeneous oxidation catalysts, both in water and in a variety of alcoholic solvents. Under 0.5 MPa O₂ pressure, 1,2-diols with varying chain length, such as 1,2-propanediol or 1,2-octanediol, are oxidized to the corresponding α-hydroxy-carboxylates with total chemoselectivity. The activity of the sol is of the same order as previously reported for stabilized nanoparticles deposited on carbon supports. The stabilized sols retain their activity over extended periods. The Au metal colloids can efficiently be recycled by means of a nanofiltration, either from an aqueous reaction medium, or from an organic solvent. The most suitable membranes for the Au sol recuperation are cellulose acetate membranes for the aqueous filtration, and poly(dimethyl)siloxane membranes for the solvent resistant filtration. Recycling tests after nanofiltration show that catalytic activity is largely preserved in consecutive runs.     

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.     

Titania supported Co-Mn-Al oxide catalysts in total oxidation of ethanol

Catalytic activity of the Co-Mn-Al mixed oxide catalysts (Co:Mn:Al molar ratio of 4:1:1) supported over titania was examined in total oxidation of ethanol. The prepared catalysts were characterized by chemical analysis (AAS), surface area measurements, and temperature programmed techniques (TPR, TPD). In ethanol oxidation, the catalysts activity gradually increased with increasing active phase content. Low concentration of Co-Mn-Al oxides in the catalyst negatively affected formation of reaction byproducts: carbon monoxide production steeply increased when Co + Mn metals concentration were lower than 5 wt.%. On the other hand, formation of the second main reaction intermediate, acetaldehyde was limited, when acidity of the catalyst was not high, i.e. concentration of Co-Mn metals over titania was low.     

Partial oxidation of methane to synthesis gas. Behaviour of different Ni supported catalysts

The behaviour of Ni supported catalysts, obtained using Ni(NO₃)₂ and Ni-acetylacetonate as precursor compounds, is analyzed. It is observed that initial activities and selectivities are similar for both systems, but the stability differs significantly. The systems show different carbon structures and sintering rates, depending on the precursor compound employed.     

Evidence of active species in CO oxidation catalyzed by highly dispersed supported gold

This paper is a brief review of suggested structures of catalytic sites and reaction mechanisms in CO oxidation catalyzed by highly dispersed supported gold. The review is focused on evidence of the sites, which has been determined primarily by infrared, X-ray absorption, and other spectroscopies, often combined with transmission electron microscopy; theory has also been applied to characterize supported gold catalysts. The literature gives strong evidence of the reaction of oxygen with CO adsorbed on zerovalent gold at low temperature, consistent with the role of such adsorbed CO in the catalytic reaction. There is also strong evidence of the involvement of cationic gold, even isolated single sites, in the catalysis at higher temperatures (e.g., room temperature). Because CO is only weakly adsorbed on zerovalent gold and more strongly adsorbed on cationic gold, it is suggested that the cationic gold sites may be kinetically more significant than the zerovalent gold sites under potentially practical reaction conditions.     

AuNPs composites of gelatin hydrogels crosslinked by ferrocene-containing polymer as recyclable supported catalysts

To fabricate a recyclable supported catalyst based on gold nanoparticles (AuNPs) and gelatin hydrogel (HG) composites, a ferrocene (Fc)-containing tetrablock copolymer [P(NCHO-b-NFc-b-NTEG-b-NCHO)] was used as both reducing and stabilizing agents for AuNPs and as a crosslinker for HG. First, the effects of Fc-containing polymers, including homopolymer PNFc and copolymer P(NCHO-b-NFc-b-NTEG-b-NCHO), and different solvent systems on the morphology and aggregation of AuNPs were examined by using ultraviolet–visible spectroscopy, transmission electron microscopy, and dynamic light scattering. Second, two strategies (blending and soaking) were applied to prepare different AuNPs/HG composites ( AuNPs-HG ), and their structure and properties were studied by using various techniques including scanning electron microscopy, X-ray diffraction, and thermogravimetry. Finally, the catalytic performance and reusability of AuNPs-HG-1 (via blending) and AuNPs-HG-2 (via soaking) were evaluated utilizing the model catalytic reduction of 4-nitrophenol to 4-aminophenol by NaBH₄. Results indicated that P(NCHO-b-NFc-b-NTEG-b-NCHO) dissolved in N,N-dimethylformamide was the optimal reductant and stabilizer to prepare AuNPs. The in situ reduction of Au^III ions to Au⁰ particles was very essential for the fabrication of AuNPs-HG in terms of hydrogel pore size, Au⁰ distribution and immobilization stability, and hydrogel thermal stability. Due to the stronger interactions among AuNPs, P(NCHO-b-NFc-b-NTEG-b-NCHO), and gelatin molecules in the blending strategy, AuNPs-HG-1 showed better mechanical stability and catalytic performance and more reusing cycles than AuNPs-HG-2 . This work highlights the design and fabrication of robust recyclable supported AuNP catalyst by using eco-friendly Fc-containing HGs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48653.