In Situ Methods for Identifying Reactive Surface Intermediates during Hydrogenolysis Reactions: C-O Bond Cleavage on Nanoparticles of Nickel and Nickel Phosphides.

Identifying individual reactive intermediates within the "zoo" of organometallic species that form on catalytic surfaces during reactions is a long-standing challenge in heterogeneous catalysis. Here, we identify distinct reactive intermediates, all of which exist at low coverages, that lead to distinguishable reaction pathways during the hydrogenolysis of 2-methyltetrahydrofuran (MTHF) on Ni, NiP, and NiP catalysts by combining advanced spectroscopic methods with quantum chemical calculations. Each of these reactive complexes cleaves specific C-O bonds, gives rise to unique products, and exhibits different apparent activation barriers for ring opening. The spectral features of the reactive intermediates are extracted by collecting in situ infrared spectra while sinusoidally modulating the H pressure during MTHF hydrogenolysis and applying phase-sensitive detection (PSD), which suppresses the features of inactive surface species. The combined spectra of all reactive species are deconvoluted using singular-value decomposition techniques that yield spectra and changes in surface coverage for each set of kinetically differentiable species. These deconvoluted spectra are consistent with predicted spectral features for the reactive surface intermediates implicated by detailed kinetic measurements and DFT calculations. Notably, these methods give direct evidence for several anticipated differences in the coordination and composition of reactive MTHF-derived species. The compositions of the most abundant reactive intermediate (MARI) on Ni, NiP, and NiP nanoparticles during the C-O bond rupture of MTHF are identical; however, MARI changes orientation from Ni(μ-CHO) to Ni(η-CHO) (i.e., lies more parallel with the catalyst surface) with increasing phosphorus content. The shift in binding configuration with phosphorus content suggests that the decrease in steric hindrance to rupture the C-O bond is the fundamental cause of increased selectivity toward C-O bond rupture. Previous kinetic measurements and DFT calculations indicate that C-O bond rupture occurs on Ni ensembles on Ni, NiP, and NiP catalysts; however, the addition of more electronegative phosphorus atoms that withdraw a small charge from Ni ensembles results in differences in the binding configuration, activation enthalpy, and selectivity. The results from this in situ spectroscopic methodology support previous proposals that the manipulation of the electronic structure of metal ensembles by the introduction of phosphorus provides strategies for designing catalysts for the selective cleavage of hindered C-X bonds and demonstrate the utility of this approach in identifying individual reactive species within the zoo.