(429a) Two Pulses Are Better Than One: Using Pump/Probe Dynamics to Deconvolve the Complexity of Properties and Pathways on Heterogeneous Catalysts

The time dependence of a probe molecule pulse response encodes the complex reaction network orchestrated by the catalyst surface. With careful design of different reactant/product pulsing regimes, the interworking parts of the reaction network can be deconvolved. A key question in development of high performing heterogeneous catalysts is understanding what components of the industrial catalyst composition control the selective vs. nonselective reaction pathways. This talk presents how the isotopic pump/probe experiment addresses this question in diverse catalytic systems such as ammonia synthesis/decomposition, propane dehydrogenation, and oxidative coupling of methane (OCM). For example, by separating the timing of NH₃ and D₂ pulses, the lifetime of surface intermediates was quantified on iron, cobalt and CoFe bimetallic catalysts to understand why the bimetallic composition offers better decomposition performance. By controlling the time delay between ¹⁶O₂ and ¹⁸O₂, a short-lived (<3 s) surface oxygen species on the MnO_x-Na₂WO₄/SiO₂ OCM catalyst was identified and then through ¹⁶O₂/¹³CH₄ pump/probe experiments, this selective surface oxygen was attributed to methyl radical formation while more stable lattice oxygen was found to generate CO₂. The pump/probe methodology for precise control of surface coverage, quantification of different surface populations, measurement of surface lifetimes, discrimination of forward and reverse surface reactions as well as the timing of hidden surface processes, will be discussed.