(268f) The Influence of the Metal and Coadsorbates On the Activation of C-H and O-H Bonds Over Transition Metal Surfaces
The activation of O-H and C-H bonds is an important fundamental step in many classes of chemical reactions including dehydrogenation, dehydration and oxidation of various organic chemicals. In order to understand the nature and strength of these bonds in a catalytic environment, a wide range of chemicals were examined and each unique C-H or O-H bond was activated over a range of catalyst surfaces. Among these chemicals were C1?C3 alkanes, propene and propyne, as well as C1?C4 alcohols; glycerol and dimethyl ether were also included. The examination of this set of species can lead to an understanding of chain length, substituent effects and carbon hybridization on the relative acidity of hydrogens. The activation of these bonds was done across a bare metal surface, as well as across a surface with an atomic oxygen and a hydroxyl group present to abstract the hydrogen directly from the bond: a) RC-H → RC* + H b) RC-H + O* → RC* + OH* c) RC-H + OH* → RC* + H2O* Understanding these reactions on different metal surfaces as well as the influence of O* and OH* on these steps is important in the development of structure-reactivity relationships and elucidating the principles that govern the conversion a wide range of hydrocarbon sources. The metals surfaces studied include Co (0001), Ni (111), Cu (111), Mo (110), Ru (0001), Rh (111), Pd (111), Ag (111), W (110), Re (0001), Os (0001), Ir (111), Pt (111) and Au (111). We outline the periodic trends over different transition metals and the factors that control these reactions and examine the role of O* and OH* in assisting these reactions. The results show that O* more readily activates C-H bonds with more acidic character. The more strongly bound to the surface the oxygen is, the more difficult it becomes to shift it from an inactive 3-fold state to an active bridge site. This trend is essentially true for hydroxyl as well; however, the penalty to shift hydroxyl from its most stable position to an active one is much smaller across most metals than the similar penalty for atomic oxygen. Thus, for most reactions involving C-H cleavage, the reaction is more favorable over the bare surface than with oxygen for metals left of group IB on the periodic table. The group IB metals (Cu, Ag, Au) have sufficiently weak binding to oxygen and each bind hydroxyl relatively strongly compared to oxygen. Using hydroxyl as an hydrogen-accepting group is much easier for most metals, with the activation barriers either decreasing slightly or remaining constant for most sp3 hybridized C-H bonds; both sp2 and sp hybridized C-H bonds benefitted from the direct abstraction of hydroxyl due to their increased acidity compared to sp3 bonds. This benefit of acidity was also observed in O-H bonds, which became easier when either O* or OH* was present for many of the metals and reactions studied.