The co-adsorption of oxygen and hydrogen on Rh(111)

The co-adsorption of oxygen and hydrogen on Rh(111) at temperatures below 140 K has been studied by thermal desorption mass spectrometry, Auger electron spectroscopy, and lowenergy electron diffraction. The co-adsorption phenomena observed were dependent upon the sequence of adsorption in preparing the co-adsorbed overlayer. It has been found that oxygen extensively blocks sites for subsequent hydrogen adsorption and that the interaction splits the hydrogen thermal desorption into two states. The capacity of the oxygenated Rh(111) surface for hydrogen adsorption is very sensitive to the structure of the oxygen overlayer, with a disordered oxygen layer exhibiting the lowest capacity for hydrogen chemisorption. Studies with hydrogen pre-adsorption indicate that a hydrogen layer suppresses completely the formation of ordered oxygen superstructures as well as O₂ desorption above 800 K. This occurs with only a 20% reduction in total oxygen coverage as measured by Auger spectroscopy.