Revealing Structural Dynamics in Catalytic Reactions Using Ultrafast Transient X-ray Absorption Spectroscopy

Progression of a typical heterogeneous catalytic process as a function of a reaction parameter such as temperature can often be segmented, according to Fig. 1. Reactants first bind to active sites to form reaction intermediates with increases in temperature and kinetic energy (Region I). Further temperature increase leads to a full “light-off” of the catalytic conversion and the reaction is dominated by the intra-particulate diffusion (Region II). The characteristic “light-off” temperature at the boundary of region I and II often defines the activity of the catalyst. At even higher temperature enters the bulk diffusion region (III), where the catalytic reaction is limited mainly by mass transport between different phases. Significant efforts in practical catalyst design involve improving catalytic activities in the kinetic region (I) and reducing the “light-off” temperature. Reaction rates at the kinetic region are defined by potential saddle points on top of which a series of “transitional state” complexes are formed between the active sites and the adsorbed reactants (Fig. 2). Capturing structures of the “transition state complexes” from the active center's perspective will provide ultimate understanding of catalytic mechanisms and insight into new catalyst design. Experimentally, however, it is a very challenging proposition.