Active and super active oxygen on metals in comparison with metal oxides

Abstract

ABSTRACT In the context of heterogeneous catalysis, oxygen on metals and oxygen on metal oxides are often considered as two different phenomena that are little related to each other. The former is mainly investigated in the field of Surface Science and the calculation chemistry, while the latter in the field of conventional catalysis. In this review we performed a joint analysis of the literature data obtained in both fields, which showed that in many aspects these two types of oxygen are chemically similar. It is usually assumed that, as in the case of oxides, the reactivity of oxygen on metals is determined by its binding energy to the surface. Trying to confirm this idea by comparing the catalytic activity of metals with the heat of oxygen adsorption (QO2), we came across a paradox of excessive metal activity, which was associated with the presence of “hot” or super active (SA) oxygen. This oxygen is capable of oxidizing methane and other substances at cryogenic temperatures but does not make a significant contribution to QO2. A comparison of SA oxygen on metals with the O•– radicals on oxides revealed a clear similarity of these species. This allows one to conclude that the oxygen on metals is a radical, which well explains its super high reactivity without using the idea of an energetically excited state. A hypothesis is proposed that not only O•–, but also the surface O2- species have a certain degree of the radical nature. Results of low-temperature reactions of SA oxygen offer a more precise interpretation of the selectivity rule. Not the high or low reactivity of surface oxygen in itself is essential for the selective catalyst. A consent between the rates of the product formation and its desorption from the surface is of vital importance. This concept opens the possibility for new approaches in the development of selective catalysts, including those based on metals.