NEXAFS investigation of benzaldehyde reductive coupling to form stilbene on reduced surfaces of TiO2(001)

Near-edge X-ray absorption fine structure (NEXAFS) was used to investigate the reaction of benzaldehyde on reduced surfaces of TiO2(001) to form stilb ene by reductive coupling of the carbonyl groups. Reductive coupling of car bonyls (commonly referred to as the "McMurry reaction") has been extensivel y studied in both liquid slurries containing reduced metals and as a gas-so lid reaction on reduced surfaces of titania. The reactive intermediate prop osed for both slurry and surface chemistry is a metal pinacolate. Previous investigations of this chemistry on reduced TiO2(001) have yielded circumst antial evidence supporting such an intermediate on the surface. However, sp ectroscopic 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 benza ldehyde-covered surfaces of TiO2(001) to progressively higher temperatures and recording polarization-dependent NEXAFS spectra at the Ti L-edge and th e O and CK-edges, one can track the progression of the reaction from benzal dehyde to the product, stilbene, These results support an adsorbate structu re consistent with a pinacolate intermediate. (C) 2000 Elsevier Science B.V . All rights reserved.