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Abstract:
The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as microkinetic models of atmospheric chemistry provided the predictive power that led to the Montreal Protocol reversing loss of stratospheric ozone, pursuing a microkinetic approach to heterogeneous catalysis has tremendous potential for societal impact. However, the development of this approach for catalysis faces great challenges. Methods for measuring rate constants are quite limited, and the present predictive theoretical methods remain largely unvalidated. Here, we present a short Perspective on recent experimental advances in the measurement of rates of elementary reactions at surfaces that rely on a stroboscopic pump-probe concept for neutral matter. We present the principles behind successful measurement methods and discuss a recent implementation of those principles. The topic is discussed within the context of a specific but highly typical surface reaction, CO oxidation on Pt, which, despite more than 40 years of study, was only clarified after experiments with velocity-resolved kinetics became possible. This deceptively simple reaction illustrates fundamental lessons concerning the coverage dependence of activation energies, the nature of reaction mechanisms involving multiple reaction sites, the validity of transition-state theory to describe reaction rates at surfaces and the dramatic changes in reaction mechanism that are possible when studying reactions at low temperatures.
