Ab initio study of surface chemistry for fuel processing: Adsorptive desulfurization and carbon resistant steam reforming

This dissertation studies the surface chemistry of adsorptive desulfurization and steam reforming for on-board or on-site hydrogen production for fuel cell systems. Ab initio calculations examined the structural-functional relationship of the adsorbents and catalysts. Calculated results were correlated with and used to explain experimental observations.;For adsorptive desulfurization, a TixCe1-xO 2 adsorbent was considered for thiophene adsorption. Surfaces of TiO 2, CeO2, and Ce-doped TiO2 were examined to model the mixed oxide system. Since the concentration of TiO2 is much higher than CeO2 in the adsorbent, calculations were first conducted on the adsorption of thiophene over the TiO2 anatase surfaces. The stoichiometric, O-poor (with oxygen vacancies), and Orich (with activated O2 on the surface) anatase surfaces were built, and the interaction of thiophene molecule with these surfaces was examined. The results showed that thiophene is only adsorbed strongly on the O-rich surfaces through an interaction with activated O atoms on the surface with the S atom in thiophene to form a sulfone-like species. Adsorption interactions were also found to provide for selective adsorption of thiophene over benzene. Though the results indicate that the O-rich sites play an important role in the adsorption desulfurization, the formations of O-rich sites on anatase surfaces are endothermic in most cases. Therefore, the addition of CeO2 might facilitate the formation of O-rich sites. Our calculations results show that CeO2 does not directly contribute to the increase of adsorption capacity because the energetics of thiophene adsorption on CeO2 surfaces and Ce-doped anatase surfaces are not thermodynamically more favored than those on anatase surfaces. However, formation of a CexTi1-xO2 mixed surface decreases the relative surface formation energy of anatase (001), thus a greater fraction of the (001) facet could be exposed on the adsorbent. Ceria could also act as an oxygen storage reservoir, providing oxygen for formation of O-rich sites that strongly adsorb thiophenic species.;For steam reforming catalytic reaction, the carbon-resistance properties of the Ni/Rh catalyst were examined. Experimental results show that the carbon deposition on steam reforming catalyst is significantly reduced with inclusion of Rh. Ab initio calculations show that a Ni/Rh alloy is thermodynamically more stable than Ni and Rh alone at the synthesis and reaction temperatures (550 oC), in agreement with the Extended X-ray Absorption Fine Structure (EXAFS) analysis results indicating close interactions between Ni and Rh atoms. Adsorption, diffusion and oxidation of carbon species over Ni and Ni/Rh alloy surfaces were examined. Alloying with Rh reduces the stability of deposited carbon atoms and clusters over both terraces and steps of the catalyst. Moreover, Rh addition enhances the competition of oxidation reactions against carbon deposition by altering the relative diffusion and bond formation rates of the two processes. The reforming activity of Ni/Rh catalyst was also studied by examining the energy barrier to methane dissociation.