A Critical View on the Quantification of Model Catalyst Activity

The conversion of reactants, reaction rate referred to catalyst mass, and turnover frequency (TOF) are values typically employed to compare the activity of different catalysts. However, experimental parameters have to be chosen carefully when systems of different complexity are compared. In order to characterize UHV-based model systems, we use a highly sensitive sniffer setup which allows us to investigate the catalytic activity by combining three different measurement modes: temperature-programmed desorption, continuous flow, and pulsed-reactivity experiments. In this article, we explore the caveats of quantifying catalytic activity in UHV on the well-studied and highly defined reference system of CO oxidation on Pt(111), which we later compare to the same reaction on Pt₁₉ clusters deposited on Fe₃O₄(001). We demonstrate that we can apply fast heating ramps for TOF quantification, thus inducing as little sintering as possible in the metastable clusters. By changing the reactant ratio, we find transient reactivity effects that influence the TOF, which should be kept in mind when comparing catalysts. In addition, the TOF also depends on the surface coverage that itself is a function of temperature and pressure. At a constant reactant ratio, in the absence of transient effects, however, the TOF scales linearly with total pressure over the entire measured temperature range from 200 to 700 K since the reaction rate is dependent on both reactant partial pressures with temperature-dependent reaction order. When comparing the maximum TOF at this particular reactant ratio, we find a 1.6 times higher maximum TOF for Pt₁₉/Fe₃O₄(001) than for Pt(111). In addition, pulsed-reactivity measurements help identify purely reaction-limited regimes and allow for a more detailed investigation of limiting reactants over the whole temperature range.