Investigation of Structure-Function Relationships of Supported Metal Catalysts for the Aldehyde-Water-Shift Reaction

heterogeneous catalysts;hydrogen production;aldehyde water shift reaction;bifunctional mechanism;Chemical Engineering;Engineering;Chemical Engineering

Wen, Wei-ChungSchwank, Johannes W ;

University of Michigan

关键词: heterogeneous catalysts;    hydrogen production;    aldehyde water shift reaction;    bifunctional mechanism;    Chemical Engineering;    Engineering;    Chemical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/149853/weichung_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The conversion of biomass into fuels and chemicals is considered as sustainable alternatives of non-renewable fossil fuel and petroleum. Given the high water content in biomass, biomass conversion with water has also drawn increasing attention. The present research investigates the aldehyde water shift (AWS) reaction as a model reaction, in which acetaldehyde is oxidized by water and converted into acetic acid and hydrogen. Inspired by cascade catalytic systems reported for homogeneous catalysts and the bifunctional mechanism reported for the water gas shift reaction, we proposed that high AWS activities could be achieved for catalysts with highly dispersed sites for water dissociation and aldehyde oxidation that are in close proximity. With the hypothesis, we designed a series of oxide- and molybdenum carbide-supported metal catalysts and examined their physical/chemical properties and reactivity, aiming to understand the structure-function relationship. The respective roles of the support and the admetal in AWS, characters of active sites, and kinetic models are also elucidated. In the research, catalysts were prepared via incipient wetness impregnation and wet impregnation. Physisorption/chemisorption, x-ray diffraction, temperature programmed desorption, and other experiments were implemented to access the structural and surface characteristics. For oxide-supported metal catalysts, we highlight the importance of support reducibility and admetal selection. Supported Cu catalysts had ~4 times higher areal rate than those for supported Pt and Au catalysts. The combination of Cu and ceria yielded the highest AWS activity, and had a turnover frequency that was 8-fold higher than that for the bulk Cu. For the mix-phase molybdenum carbide-based catalysts, the bare carbide outperformed oxide supported Cu catalysts with a 2-fold or higher AWS rates, and its activity was enhanced by 100% upon 1.0ML Cu deposition. When increasing the Cu loading on the carbide, AWS rates of Cu predicted by the perimeter model agreed well with the experimental results. This suggests that Cu-carbide interfacial sites play a key role in catalyzing the reaction. These results of ceria and carbide supported Cu catalysts are consistent with the bifunctional mechanism hypothesis, in which water dissociation on the reducible oxide and carbide support is coupled with aldehyde oxidation on Cu admetal. The structure of carbide was also determined to be important to its reactivity. The hexagonal carbide showed a ~130% higher AWS rate than that of the cubic carbide. For all carbide based catalysts, the AWS reaction appeared to be limited by the surface reaction between the adsorbed water and adsorbed acetaldehyde. As metal oxide and carbide-based catalysts showed significant differences in selectivity, characteristics driving the selectivity were also investigated. For oxide-based catalysts, crotonaldehyde was produced as the major side product via aldol condensation. The rates of aldol condensation were found to correlate well with the weak acid site densities, implying that acid sites can be responsible for the crotonaldehyde formation. For carbide-based catalysts, ethanol produced via Cannizzaro reaction was found to be the major side product. The rates of Cannizzaro reaction were a strong function of acid site densities of carbide catalysts, indicating that the Cannizzaro reaction could be catalyzed by acid sites. This research establishes a groundwork for using supported metal catalyst and will help guide the development of future AWS catalysts.