NEXAFS investigation of benzaldehyde reductive coupling to form stilbene on reduced surfaces of TiO2(001)
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abstract
Near-edge X-ray absorption fine structure (NEXAFS) was used to investigate the reaction of benzaldehyde on reduced surfaces of TiO2(0 0 1) to form stilbene by reductive coupling of the carbonyl groups. Reductive coupling of carbonyls (commonly referred to as the `McMurry reaction') has been extensively studied in both liquid slurries containing reduced metals and as a gas-solid reaction on reduced surfaces of titania. The reactive intermediate proposed for both slurry and surface chemistry is a metal pinacolate. Previous investigations of this chemistry on reduced TiO2(0 0 1) have yielded circumstantial evidence supporting such an intermediate on the surface. However, spectroscopic evidence has proven difficult to obtain. In the current study, we employ NEXAFS to probe the chemical identity and orientation of reactive intermediates in the reductive coupling reaction further. By heating benzaldehyde-covered surfaces of TiO2(0 0 1) to progressively higher temperatures and recording polarization-dependent NEXAFS spectra at the Ti L-edge and the O and C K-edges, one can track the progression of the reaction from benzaldehyde to the product, stilbene. These results support an adsorbate structure consistent with a pinacolate intermediate.
