| AVS 61st International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS-TuP |
| Session: | Surface Science Poster Session |
| Presenter: | Luan Nguyen, University of Notre Dame |
| Authors: | L.T. Nguyen, University of Notre Dame L. Liu, University of Notre Dame S. Zeng, University of Notre Dame F. Tao, University of Notre Dame |
| Correspondent: | Click to Email |
Heterogeneous catalysis is a chemical process performed at a solid/gas or solid/liquid interface. An elevated pressure of a reactive environment could cause materials surface likely to restructure geometrically and electronically to adapt to the surroundings. Since catalytic performance (activity, selectivity, promotion effect, deactivation, etc.) depends on the surface structure of a catalyst during the reaction, it is necessary to study the surface under reaction condition (a reactant) and during catalysis (a mixture of all reactants).
Since a high pressure of a reactant gas typically results in high adsorbate coverage, one alternative approach to mimic the gas environment of a high pressure is to increase coverage of absorbate by lowering the catalyst temperature. However, decreasing of catalyst temperature in low pressure environment is only valid when entropy effects are negligible and the relevant adsorption structure is not kinetically hindered. In fact, the magnitudes of entropy contribution to the surface free energy in ultrahigh vacuum (UHV) and under an ambient pressure of a reactant gas are actually different by about 0.3 eV or even larger. Therefore, pressure-dependent entropy can lead to large restructuring of catalyst surfaces. It is difficult to predict how a system will respond to the change of pressures of gases and temperatures of catalysts. Here we will present an in-situ investigation of a model catalyst surface Rh(110) which revealed interesting restructuring of Rh(110) surface in pure CO and a mixture of CO and O2 in the temperature range of 25oC-130oC.
In UHV environment clean Rh(110) surface exhibits a (1x1) structure. However, upon O2 adsorption at elevated temperature (approximately 500 °C), the surface reconstructs to an O-covered (1x2) missing row structure, namely Rh(110) (2x2)p2mg-O. High pressure STM and ambient pressure XPS were used in in-situ studies of surface chemistry and structure of Rh(110) in the mixture of CO and O2. . Initially, the Rh surface exhibits a (1x2) missing row structure. During CO (0.08 Torr) oxidation with O2 (0.02 Torr) at room temperature, the (1x2) structure reconstructs to form (1x1) islands. Interestingly, no surface reconstruction was observed when Rh(110)-(2x2)p2mg-O surface was exposed to only one reactant gas, CO or O2 at room temperature, or in the mixture of CO and O2 at in the pressure range of 10-7 Torr. The surface chemistry and structure of Rh(110) in pure CO, O2 and mixture of CO and O2 at different temperatures and pressures will be presented. A correlation between surface chemistry and structure of this catalyst during CO oxidation and its catalytic activity will be discussed.