Carbon-Supported Platinum Catalysts for Hydrogen Production and Butyric Acid Decarboxylation

This work deals with an effort to evaluate carbon as a potential support for water-gas shift catalysts and to better characterize the active site on these materials. It is of particular interest to study the ability of sodium to stabilize such a complex in the absence of surface and bulk support oxygen. Multi-walled carbon nanotubes were used as the support for platinum catalysts. Various surface modifications of the carbon were carried out in an effort to understand the effects on the water-gas shift activity of the corresponding catalysts.;The oxidation of the carbon nanotubes by nitric acid was used to introduce surface oxygen groups of various acidity and function onto the surface. In order to selectively remove groups, a heat treatment was applied. The ability of carboxylic anhydride groups to stabilize ionic Pt and activate water was thus demonstrated. While this provides active sites for the water-gas shift reaction, the rates are considerably lower than those obtained by the ion exchange of sodium onto the surface prior to platinum addition. This is due to the ability of sodium to activate water to a greater degree. Interestingly, the presence of highly electronegative groups, such as carboxylic acids, on the surface while useful for dispersing Pt were not highly active for the WGS reaction. The ability of sodium to stabilize platinum was also shown by a simple co-impregnation technique.;Various characterization approaches were taken to understand these effects in detail such as XPS, XANES, EXAFS, and HAADF-STEM. The preparation of a highly annealed, oxygen-free carbon nanotube support allowed for the comparison of Pt and PtNa systems under reaction conditions using atmospheric pressure X-ray photoelectron spectroscopy. These findings indicate that while platinum can be finely dispersed on a carbon nanotube support, it is not active for the WGS reaction due to the absence of water activation sites. When sodium is added to the surface with platinum, a highly active catalyst is prepared. The AP-XPS study indicates that Pt-OHx is present on the Na-promoted sample to a high degree. Furthermore, this surface was found to be extremely responsive to changes in the reaction conditions. Removal of water from a product-free reaction gas results in the increase of bridge CO and CO bound to low-coordinated Pt sites, in line with previous studies. The introduction of hydrogen to this environment resulted in an apparent local reduction, causing a decrease in --OH content and an increase in CO binding.;Determinations of reaction orders on the MWNT-supported Na-promoted catalyst showed that water activation seems to be somewhat suppressed on the carbon support compared to metal oxides. This is thought to be related to the ability of carbon nanotubes to adsorb hydrogen by spillover from activation sites on the surface. Therefore a competition for these sites is established in the presence of hydrogen and water.;A process by which butyric acid is derived from biomass through a fermentation step is also proposed as a means of the sustainable production of propane through decarboxylation. Platinum and palladium catalysts were evaluated for the decarboxylation of butyric acid and it was found that both suffer from severe deactivation due to coking. Efforts to control the extent of deactivation by modifying surface functionality of carbons were unsuccessful.;The use of bimetallic formulations appeared to show some promise for improved stability and selectivity to propylene, a commodity chemical. Furthermore, the use of zeolites was found to be of interest for this reaction, although energetically unfavorable conditions are necessary to achieve high yields. (Abstract shortened by UMI.).