Kinetic models of CO oxidation on gold nanoparticles

Despite 27 years of experimental and theoretical studies, the mechanism and kinetics of CO oxidation on gold nanoparticles are still open for debate. One of the key features of this reaction is that the reaction turnover frequency rapidly drops with an increasing particle size presumably due to a crucial role of a small number of sites located at the perimeter of nanoparticles. This factor limits the applicability of the conventional mean-field kinetic models implying that the reaction steps occur in a Langmuir overlayer. To clarify this aspect, the conventional kinetics are herein compared with those calculated in the opposite limit implying the reaction to occur on kinetically independent pairs of sites. The results predicted by the models of these two categories are found to differ if the reaction itself is rapid compared to other steps. In the practically interesting case when the reaction is slow, the results are similar. The analysis of different reaction schemes indicates that for the low-temperature reaction regime the apparent reaction orders can be explained assuming cooperative CO and O2 adsorption at different sites. In addition, the scale of the apparent pre-exponential factor for this reaction has been rationalized on the basis of the conventional transition-state theory.