Probing electron transfer in catalytic reactions: A Catalysis Golden Rule and the role of non-innocent supports

Catalytic reactions in which the rate-determining step (rds) is electron transfer often display activation energies that vary little with composition or supports. The present study presents an experimental means to confirm that electron transfer is involved in the rds. The concepts presented here have applicability in heterogeneous catalysis. electrocatalysis. and battery systems. Ozone decomposition serves an example of a catalytic reaction in which electron transfer is rate-limiting. Use is made of in situ X-ray absorption near-edge structure (XANES) to probe the electron transfer and extended X-ray absorption fine structure to characterize catalyst structure. The catalysts were 3.0 and 10 wt% manganese oxide supported on SiO₂ and Al₂O_3. The alumina-supported catalysts were more active for the decomposition of ozone with higher turnover frequencies (TOFs) than the silica-supported catalysts. In both catalysts kinetic studies indicated that the rate-determining step was the decomposition of an adsorbed peroxo species on a Mn center, a step that involves electron transfer. XANES measurements at reaction conditions showed that alumina-supported catalysts had larger post-edge areas, namely higher unoccupied density of states ρ(E), which is associated with higher electron-accepting capability. The rate constant for the peroxide decomposition depends on ρ(E), so has the same form as Fermi's Golden Rule governing intra-atomic electron transitions. As such, the catalytic electron transfer is described as following a Catalysis Golden Rule. The usage is applicable to systems where loading is varied with a single support or where different supports are involved, in which case the supports are designated as non-innocent supports if they affect $\rho \left(ϵ\right)$. The in situ XANES method provides a means of probing an electronic effect that differs from the conventional ligand effect.