Effect of preparation conditions and additives on the activity of Au- and Pt- containing ceria and silica catalysts for the water-gas-shift reaction.

摘要

Reforming of liquid fuels such as gasoline or diesel oil has been considered as an attractive and viable solution to the issue of hydrogen generation for the PEM fuel cells to be used in residential/commercial generators and in future transportation applications. However, the reforming process will produce a carbon monoxide by-product, which is a poison to the anode of the fuel cell. To alleviate this problem, fuel reforming will be coupled with two additional reaction processes. First, a water-gas-shift (WGS) reactor will reduce the CO concentration from 10--15 to ~1%, then in a second reactor preferential oxidation of CO (PROX) will reduce the CO concentration to below 20 ppm, which can be tolerated by today's PEM fuel cells. Highly active catalysts are under rapid development for these applications. Recently, noble metals supported on reducible oxide catalysts, such as ceria, have been shown to be very active for the WGS reaction and suitable for the fuel cell operation conditions.; In this thesis, to investigate the role of the reducible oxide, platinum-based catalysts were supported on a non-reducible oxide: silicon dioxide. These Pt-SiO2 materials were ineffective as catalysts of the WGS reaction processes. Interestingly, addition of small amounts of sodium (1.5at%) yielded a catalyst with activity and reducibility characteristics comparable to that of platinum supported on CeO2. New synthesis procedures were examined in this work for this type of catalyst. Accordingly, silica-based materials in this work were produced by a reverse microemulsion procedure. These materials showed platinum nanoparticles encapsulated within SiO 2 and SiO2-CeO2 shells. These unique materials exhibited excellent dispersion, high surface area, reducibility, and WGS catalytic activity.; Also, an investigation of the preparation parameters on Au and Pt supported cerium oxide catalysts was performed. For Au-CeO2 catalysts, the novel method of direct anionic exchange (DAE) proved to be the best method of preparation of this type catalyst, which exhibited excellent recyclability in redox experiments. The WGS activity of the DAE-prepared catalyst was twice as high as that of other Au-ceria catalysts. A novel method was also developed for Pt-CeO2 catalysts---deposition precipitation. Results from this method and the impregnation method suggest that residual chlorine may not be detrimental to the development of platinum-based catalysts.; In this thesis, several different preparation processes were examined: depositionprecipitation, direct anionic exchange, impregnation, and reverse microemulsion. The catalysts were characterized by scanning transmission electron microscopy (STEM), Xray diffraction (XRD), BET surface area measurements, and by H2- and COtemperature- programmed reduction (TPR). Also, catalyst activities were evaluated for the WGS reaction under various conditions that included product-free and full reformate gas mixtur