COMBINED THEORETICAL AND EXPERIMENTAL STUDY OF CO ADSORPTION AND REACTIVITY OVER PLATINUM AND COBALT

Master'sMASTER OF ENGINEERIN

Analysis of the limitations in the oxygen reduction activity of transition metal oxide surfaces

The oxygen reduction reaction (ORR) is the key bottleneck in the performance of fuel cells. So far, the most active and stable electrocatalysts for the reaction are based on Pt group metals. Transition metal oxides (TMOs) constitute an alternative class of materials for achieving operational stability under oxidizing conditions. Unfortunately, TMOs are generally found to be less active than Pt. Here, we identify two reasons why it is difficult to find TMOs with a high ORR activity. The first is that TMO surfaces consistently bind oxygen atoms more weakly than transition metals do. This makes the breaking of the O–O bond rate-determining for the broad range of TMO surfaces investigated here. The second is that electric field effects are stronger at TMO surfaces, which further makes O–O bond breaking difficult. To validate the predictions and ascertain their generalizability for TMOs, we report experimental ORR catalyst screening for 7,798 unique TMO compositions that generally exhibit activity well below that of Pt

CO Adsorption Site Preference on Platinum: Charge Is the Essence

The adsorption of CO on transition-metal surfaces is a key step in catalysis and a reference system for surface science and computational catalysis. Here, the change in CO site preference with coverage, from top to bridge and back to top, is analyzed using charge transfer and chemical bonding. The relative stability of top and bridge sites is related to the variation in the surface platinum charge with CO coverage. Both the Pt–C σ* (Pauli repulsion) and the C–O π* (back-donation) occupancies increase with platinum charge; however, destabilizing Pauli repulsion dominates over stabilizing back-donation, and adsorption weakens with increasing surface charge. CO at the top sites is more sensitive to Pauli repulsion, leading to a change in site preference from top to bridge with increasing platinum charge and, consequently, with increasing CO coverage. The higher back-donation at the bridge sites eventually switches the site preference back to top near monolayer coverage

Ethylene Hydrogenation over Pt/TiO₂: A Charge-Sensitive Reaction

Controlled charge transfer between a support and small metal particles provides unique opportunities to tune the activity of supported metal catalysts, as first proposed by Schwab [G. M. Schwab et al., <i>Angew. Chem</i>. <b>1959</b>, <i>71</i>, 101–104]. By controlling the thickness of polycrystalline anatase TiO₂ films, the TiO₂ carrier concentration can be manipulated by an order of magnitude. When 1 nm Pt particles are deposited on these TiO₂ films, the variation in the charge transfer between the TiO₂ support and the Pt particles is found to dramatically increase the ethylene hydrogenation activity. The sensitivity of ethylene hydrogenation to charge transfer was anticipated from the large effect of the Pt charge on the ethylene and ethylidyne adsorption energy, e.g., compared to CO and H. Our results demonstrate that the controllable Schwab effect provides a powerful tool to tune catalytic activity. An even larger effect can be expected for supported sub-nanometer clusters, and for the selectivity of hydrogenation reactions