Principal Investigator Sylvia Ceyer
With a newly found capability to synthesize and detect hydrogen embedded in the bulk of Ni under low pressure, UHV conditions, we have been able to probe the chemistry of bulk hydrogen unambiguously for the first time and we find it to be unique. Specifically, we have shown that the species reactive with adsorbed CH3 is not a surface bound H atom but a bulk H atom. The reaction proceeds by the direct recombination of a bulk H atom with CH3 as the bulk H emerges onto the surface. Because the interstitial octahedral site in which the hydrogen atom is bound is directly beneath the threefold hollow site on which the CH3 is bound, the bulk H atom has the correct orientation relative to the CH3 required for sp3 hybridization, so that it reacts with CH3 and immediately desorbs as CH4. The reaction of CH3 with a surface bound H atom probably does not occur because access of the H atom to the Ni3-C bond is blocked. This result, published in Science 257, 223 (1992), documents a new mechanism for a surface reaction, a reaction between an adsorbed and a bulk species, and it unambiguously demonstrates the importance of bulk hydrogen as a reactant in a heterogeneous catalytic reaction.
We have also investigated the reactivity of bulk H with an unsaturated hydrocarbon, ethylene. Again we find that the species reactive in the hydrogenation of C2H4 is not a surface bound H atom but a bulk H atom. The surface bound hydrogen is simply unreactive for the hydrogenation of ethylene. However, a very intriguing result is that the surface bound H is reactive in exchange with C2H4. A coadsorbed layer of ethylene and D results in the formation and desorption of all the deuterated ethylenes. This observation indicates that it is not the mobility of the surface H that precludes its reaction with ethylene. Obviously, the surface H can get close enough to C2H4 to interact so strongly with the ethylene that it exchanges with the ethylene. Rather, these results imply that the barrier to hydrogenation via a side-on approach is too high for the reaction proceed, but if the hydrogen approaches from underneath the C2H4 molecule as it does in the case of bulk H then the barrier is significantly lower and the reaction proceeds readily. The lower barrier to hydrogenation for the underneath approach is physically realizable because the pi orbitals which participate in the reaction are positioned perpendicularly to the plane of the molecule. This result (J. Am. Chem. Soc. 116, 6001 (1994)) casts doubt on the generality of the Horiuti-Polanyi mechanism for catalytic hydrogenation reactions of unsaturated hydrocarbons.
Additional support for the direction of the hydrogen’s approach to the pi orbitals as a critical element comes from the observations of the hydrogenation of C2H2. Because C2H2 has two mutually perpendicular orbitals, one directed perpendicularly towards the Ni surface which is accessible to the bulk H and the other parallel to the surface which is accessible to the surface H, both surface H and bulk H hydrogenate C2H2 (J. Am. Chem. Soc. 120, 8885 (1998)). However, only bulk H hydrogenates C2H2 to gas phase C2H4 and C2H6. Surface H reacts with C2H2 to form ethylidyne, CCH3. Because this observation is the first of CCH3 adsorbed on Ni, a vibrational spectroscopic investigation and normal modes and bond dipole moment analysis has been carried out (J. Phys. Chem. B 102, 4952 (1998)). Studies of the rate of reaction between surface bound hydrogen and acetylene have strongly suggested that the different product distributions resulting from the reaction of acetylene with the two forms of hydrogen arise from the large energy difference between bulk and surface bound H. (J. Phys. Chem. B 105, 11480 (2001))
Reactivities of surface bound H and bulk H have now clearly been distinguished as a result of the capability to synthesize bulk H cleanly in a UHV environment. These results demonstrate that the reactivities of surface bound and bulk H atoms and their product distributions are distinct. Their distinctiveness arises both from their different directions of approach to the adsorbate and their large difference in potential energy. The fact that an emerging bulk H is a more energetic species than a surface bound H by 24 kcal/mol makes it likely that reaction channels readily accessible to bulk H will be closed to surface H. The emerging bulk H atom is, in a sense, a unique surface reactant that is present by virtue of the bulk-surface interface. It is an energetic species with a chemistry of its own. (Accts. Chem. Res. 34, 737 (2001))