It is seen that the evaluation of the gold crystallite size requires proper background estimation and some notable differences in the previously reported data could be attributed to the background assessment. In the large excess of O 2, the redox transfer Ce 4+↔Ce 3+ is a fast process and we do not see a correlation between TPD and the catalytic activity data for the catalysts with a lower Y doping.Ī close-up of the (1 1 1) peaks of gold is presented in the lower section of Figure 4. That is why the oxygen supplying could start to be a limiting step for the gold catalysts with a 7.5% dopant amount. The surface microstructures of oxygen vacancies around clustered Y 3+ cations with a stable valence state will not participate in the redox process, hindering the provision of oxygen. However, clustering of Y 3+ cations at the ceria surface cannot be excluded. % Y 2O 3) to avoid vacancy ordering in the case of heavy Y-doping above 10–15 at. It was already mention that in the present study, the variation of the Y amount was limited to 11 at. The step of active oxygen supplying via the Mars-van Krevelen mechanism has to be related to the redox transfer Ce 4+↔Ce 3+, accompanied by the filling and emptying of the adjacent oxygen vacancies. A correlation between the catalytic activity and the amount of released oxygen, measured as weight loss (%) versus temperature, is seen only for 7.5% Y 2O 3 doping. The TGA curves, corresponding to step 5 (see the Experimental Section) are shown in Figure 1B for those catalysts for which CBO activity is respectively displayed in section A. During the TGA measurements with air being introduced over the catalysts at 200 ☌ and 350 ☌, the O 2 consumption peaks were detectable and the subsequent TPD experiments showed different amounts of oxygen desorption. Īnother reason for better activity after the higher-temperature treatment in air could be sought in the active oxygen supply. In this regard, ceria is a very appropriate support for gold catalysts, because of its ability to maintain a high gold dispersion and to take part in the oxidation reaction providing active oxygen species. A high activity could be expected for the readily reducible metal oxide supports. The authors summarized that the predominant opinion for the total oxidation of VOCs over gold supported on reducible oxides is that the reaction takes place thorough the Mars-van Krevelen mechanism with the participation of active oxygen supplied by the surface of the oxide support followed by reoxidation with the oxygen from the gas phase. The influence of different factors, such as the nature and the properties of the support, the gold particle size and shape, the electronic state of the gold, the preparation method and the pretreatment conditions of catalysts, have been discussed in details. In a recent review by Scire and Liotta ( and references therein), gold-based catalysts were suggested as promising candidates for the total oxidation of VOCs. A strong economic impact is related to the selection of catalysts with high activity and selectivity at relatively low temperatures. Gold and ceria particle agglomeration or coke formation should be excluded as a possible reason, and the most probable explanation could be associated with the importance of the benzene activation stage.Ĭatalytic oxidation is considered one of the most promising technologies for the abatement of volatile organic compounds (VOCs) as hazardous and toxic air pollutants. A 100% benzene conversion was achieved only over this catalyst and Au/ceria, while it was not reached even at 300 ☌ over all other studied catalysts. The gold catalyst on an IM support with 1% Y-doping exhibited the best performance. On the basis of XPS results of fresh and used in CBO catalysts, the presence of cationic gold species does not seem important for high CBO activity. The oxygen supply cannot be considered as a limiting step for benzene oxidation except for the high 7.5%-doped samples, as suggested by TGA and TPR data. A positive role of air pretreatment at 350 ☌ as compared to 200 ☌ was established for all Y-containing catalysts and it was explained by cleaning the active sites from carbonates. The samples were characterized by XRD, TGA, XPS and TPR techniques. A low-extent Y modification was chosen to avoid ordering of oxygen vacancies. % Y 2O 3) ceria supports prepared by coprecipitation (CP) or impregnation (IM) were studied in complete benzene oxidation (CBO).
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