Gold catalysts deposition precipitation

As for ZrO -based modified catalysts, Idakiev and coworkers prepared Au/CeOJ mesoporous 7x0 catalysts by depositing CeO onto mesoporous ZrO followed by loading gold via deposition-precipitation [64], The resulting catalysts were more active than Au/mesoporous ZrO in WGS. 

Figure 1. TEM image of a titania supported gold catalyst (1.7wt.% Au) prepared by deposition-precipitation (gold particle size = 5.3+ 0.3 nm, dispersion = 36%). (Reprinted from Reference [84], 2000, with permission from American Chemical Society).

Tyrpical experimental procedures are as follows for DP method, which is used for the industrial production of gold catalysts. To an aqueous solution of H AuCU, the pH of which is 2-3, an aqueous solution of NaOH is added to adjust the pH at a fix point in the range of 6-10. In neutral or weakly basic solution AuClj" ion is transformed into AuCl (OH)4l ( = 4 0). To this solution support materials are immersed or dispersed. When the pH, concentration, and temperature are carefully adjusted, solid Au(OH)3 precipitate is deposited exclusively on the... 

One solution-based approach that works for gold catalysts, in that it produces highly active catalysts, is the deposition-precipitation (DP) method [8]. The DP method entails adjusting the pH, temperature, and gold concentration of an HAUCI4 solution to form a gold hydroxide species which is then deposited onto the support material [8]. This catalyst precursor is washed, dried, and annealed to form small (<5nm) catalyst particles [9]. The DP method has a number of limitations for example, DP cannot produce Au particles with diameters less than 5 nm on support materials with low-isoelectric points (lEPs) like SiOz and WO3 [5,10,11]. 

The activity of gold catalyst is normally strongly size dependent and the control as well as the narrowest possible distribution of particle size represent the main goal for the production of an active gold catalyst. From a catalytic point of view, several preparation methods have been proposed for obtaining highly dispersed gold catalyst, most of them derived from deposition-precipitation method proposed by Haruta et al. [3]. 

For the study of the electrocatalytic reduction of oxygen and oxidation of methanol, our approach to the preparation of catalysts by two-phase protocol " provides a better controllability over size, composition or surface properties in comparison with traditional approaches such as coprecipitation, deposition-precipitation, and impregnation. " The electrocatalytic activities were studied in both acidic and alkaline electrolytes. This chapter summarizes some of these recent results, which have provided us with further information for assessing gold-based alloy catalysts for fuel cell reactions. 

Au/TiC>2 catalysts made by deposition-precipitation have been examined for the PROX reaction 27 they show some differences from Au/Fe203 catalysts. Hydrogen now interferes with the oxidation of carbon monoxide, perhaps by competing for the same adsorption sites this was shown by a marked increase in the order of reaction when hydrogen was present, but not in the oxygen order (see Table 7.2). However, there is no reason to expect that gold particles on these two supports differ in any fundamental... 

Most of the published methods for preparing gold catalysts in small research quantities are unlikely to prove suitable for commercial applications.1 Complete removal of precious metal from the liquid phase is desirable when using solution methods deposition-precipitation (DP) techniques, whilst producing highly active catalysts, also consume large quantities of water and the cost of treatment of wastewater is an expensive additional process. Other preparation methods such as appropriate modifications of impregnation via incipient wetness techniques are more likely to be suitable for commercial production if they lead to reproducible, stable... 

Centeno, M. A., Carrizosa, I. and Odriozola, J. A. (2003). Deposition-precipitation method to obtain supported gold catalysts dependence of the acid-base properties of the support exemplified in the system Ti02—TiOxNy—TiN. Appl. Catal. A-Gen. 246(2), 365-372. 

The air pollutants of volatile organic compoimds emitted from many industrial processes and transportation activities could be abated by catalytic combustion processes. Scire et al. reported the catalytic combustion of 2-propanol, methanol, and toluene on ceria-gold catalysts. The catalysts were prepared with coprecipitation and deposition-precipitation methods. The gold significantly enhanced the catalytic activity of ceria for the oxidation of these volatile organic compounds. The supposed reason is that the gold NFs weakened the mobility/reactivity of surface lattice oxygen (Scire et al., 2003). 

Since supported gold catalysts prepared by coprecipitation were found to be active for CO oxidation even at temperatures far below room temperature, attempts are increasing to prepare other noble metal catalysts by coprecipitation, deposition-precipitation, and grafting methods, which were used for the preparation of active supported gold catalysts. Although the affinity to CO is markedly different between Pt-group metals and Au supported on selected metal oxides, the contribution of metal-support interactions to the enhancement of low-temperature catalytic activity for CO oxidation appears to be similar, namely, the enhancement of oxygen activation at the perimeter interface. This line of approach may be valid to seek for a new type of catalysts active at lower temperatures for reactions other than CO oxidation [82,83]. 

E. E. Stangland, B. Taylor, R. P. Andres, W. N. Delgass, Direct vapor phase propylene epoxidation over deposition-precipitation gold-titania catalysts m the presence of H2/O2 Effects of support, neutralizing agent, and pretreatment,/. Phys. Chem. B 109 (2005) 2321. 

Key Words Direct propylene epoxidation. Propylene oxide, Gold, Titanium, Propene, Au/Ti catalysts. Catalysis by gold. Titanium silicalite, TS-1, Gold/TS-1, Hydrogen peroxide, Kinetics, Design of experiments, Deposition-precipitation, Ammonium nitrate, Selective oxidation, Alkene epoxidation, Density functional theory, DFT calculations, QM/MM calculations. 2008 Elsevier B.v. 

Vapor-phase epoxidation of propylene using H2 and O2 was carried out over gold catalysts supported on mesoporous ordered (MCM-41) and disordered titanosilicates prepared hydrothermally or by modified sol-gel method. Gold nanoparticles were homogeneously dispersed on the titanosilicate supports by deposition-precipitation (DP) method. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. NaOH was found to be the best precipitant to prepare Au catalysts with optimum propylene oxide yields and H2 efficiency. The extent of catalysts washing during preparation was found to affect the activity of the catalyst. The activity and hydrogen efficiency was found to depend on the type of mesoporous support used. 

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