Eley-Rideal surface chemistry: Direct reactivity of gas phase atomic hydrogen with adsorbed species

We have discussed the use of gas-phase atomic hydrogen to effect surface reactions via Eley-Rideal kinetics under UHV conditions and at low surface temperatures. The Eley-Rideal mechanism describes a reaction between a reactant which has chemisorbed and one which has not; i.e., the impinging hydrogen may or may not have been dynamically trapped at the surface, but it has not chemisorbed locally and accommodated to the surface temperature. This protocol realizes that the rate of many hydrogenation reactions which obey Langmuir-Hinshelwood kinetics is limited by the strength of the metal - H bond, which is on the order of 65 kcal/mol in the low-coverage limit. The reactant atoms being produced in the gas phase, and not by dissociative chemisorption, the metal - H bond does not form and, hence, does not need to be activated for reaction to occur. We have illustrated these ideas with four specific examples, taken from our laboratory, involving the Ru(001) surface, namely, the abstraction of chemisorbed hydrogen, forming dihydrogen, which desorbs at least 150 K below the threshold temperature for recombinative desorption of two hydrogen adatoms; the hydrogenation of chemisorbed CO, forming η1- and η2-formyls and η2-formaldehyde; the selective hydrogenation of chemisorbed formate, forming a formyl and a hydroxyl; and the hydrogenation of oxygen adatoms to form water. We anticipate that this powerful experimental technique will continue to provide deep insights into the mechanisms of surface reactions that have not heretofore been possible to examine. We anticipate also that other radical beams will be employed in the future, such as atomic oxygen, atomic nitrogen, and molecular radicals. This is certainly one of a number of extremely exciting current research activities in the broad and rich field of surface chemistry.