Study On Preparation And Catalytic Performance Of Supported Nano-Gold Catalysts For Low-temperature CO Oxidation

Supported gold catalysts are a new kind of catalytic materials. The low-temperature CO oxidation performance over the gold catalysts foreshows a wide application prospect in a large number of industrial and environmental protection fields, such as closed-cycle CO₂ lasers, sealed-spaceships and submarines, CO gas sensors, CO safety masks and hydrogen fuel cells etc. In recent years, more attentions have been paid to the new methods for the preparation of the gold catalysts with low gold loadings and high activity, especially the high stability.In this paper, Au/Al₂O₃ and Au/MO_x/Al₂O₃ catalysts were prepared bymany methods. The catalytic activities for low-temperature CO oxidation ofthese catalysts were investigated. The effects of preparation factors such asgold loadings, pH value of solution and pretreatment conditions etc on thecatalytic activities were studied in detail. The preparation conditions wereoptimized. In addition, the long-time stability of the catalysts reacting in dry reactant gas and reactant gas saturated with H₂O vapor was compared, the activity changes of the catalysts treated by air saturated with H₂O vapor or by air heated above 100℃were examined, and the reasons for the deactivation of the catalysts were discussed. The characterization of the catalysts were carried out by XRD,BET,XPS,AAS,TEM,TPD,FTIR measurements, and the results were related to the catalytic performance.The main results obtained are as follows:1．Study of the preparation of Au/Al₂O₃ by DP methodThe preparation conditions such as gold loadings, the pH value of reaction solution, reaction temperature and pretreatment conditions have great influence on the catalytic activities. The size of gold particles in the catalysts pretreated by reduction or calcination can be controlled below 10 nm. The catalysts pretreated by reduction show higher activity than those obtained by calcination, which may be resulted from that reduction pretreatment gives smaller gold particles with higher dispersion, while calcination pretreatment tends to decrease the catalytic activity by giving bigger gold particles.2．Study of the preparation of Au/Al₂O₃ by anionic IMP methodThe preparation conditions such as the pH value of impregnation solution, the steeping way, impregnation time and pretreatment conditions have great influence on the catalytic activities. Compared with the DP method, the anionic impregnation method has the following merits: simplicity, no need to be heated, higher gold loading ratio and good reproduction. The effect of steeping the samples in NH₃ ? H₂O is to eliminate the Clˉions completely and improve the catalytic activity. The appropriate range of pH value is wider, even under the condition of strong acidity, the high activity of Au/Al₂O₃ catalyst can be obtained.3．Study of the deactivation and stability of Au/Al₂O₃(1) The high initial activity of the Au/Al₂O₃ catalyst is followed by a gradual decrease during CO oxidation; while the catalyst exhibits longer life-time and better stability reacting in reactant gas saturated with H₂O vapor than in dry reactant gas. Treatment either by air saturated with H₂O vapor or by air heated above 100℃may deactivate the catalysts. (2) There are three main deactivations of Au/Al₂O₃ catalysts for CO oxidation: i) CO oxidation deactivation, ii) thermal treatment deactivation, and iii) H₂O vapor treatment deactivation. The first two deactivations are reversible, which may be caused by the accumulation of carbonate-like species during CO oxidation or the removal of-OH ligands in active site. The growing of gold particles may be responsible for the last deactivation, which is irreversible.4．Study of the preparation of Au/FeO_x/Al₂O₃ by two-step methodThe preparation conditions such as the amount and the nature of FeO_x, pH value of impregnation solution have influence on the catalytic activities to the different degree. Au/Fe₂O₃/Al₂O₃ catalyst pretreated by calcination shows higher activity, the Fe₂O₃ loadings have little influence on the activity; while Au/Fe₃O₄/Al₂O₃ catalyst pretreated by reduction exhibits lower activity, the higher the FeO loadings, the lower the catalytic activity. Au/Fe₂O₃/Al₂O₃ catalyst with high activity can be obtained in a wider pH range.5．Study of the preparation of Au/MO_x/Al₂O₃ by one-step methodThe preparation conditions such as the gold loadings, pH value of solution, calcination temperature, the kind of MO_x have great influence on the catalytic activities. This one-step method has the following merits: simplicity, efficiency, higher gold loading ratio, high gold dispersion and good reproduction. Compared with Au/Al₂O₃, all of Au/MO_x/Al₂O₃ ( M = Fe, Fe+Cu, Co, Zn ) catalysts prepared by this method show the improved activity.6．Study of stability of Au/FeO_x/Al₂O₃(1) By comparison with Au/Al₂O₃, Au/Fe₂O₃/Al₂O₃ catalyst shows the improved stability for the CO oxidation and the longer life-time. (2) Au/Fe₂O₃/Al₂O₃ catalyst pretreated by calcination exhibits higher stability and the longer life-time than Au/Fe₃O₄/Al₂O₃ catalyst pretreated by reduction. (3) The reason why the Au/Fe₂O₃/Al₂O₃ is more active than Au/Al₂O₃ may be that the addition of Fe₂O₃ can supply more active O species needed for CO oxidation.