Articles you may be interested inHighresolution electron energyloss spectroscopy studies of hydrogen adsorption on the GaP(110) surface J. Vac. Sci. Technol. A 6, 752 (1988); 10.1116/1.575100 Dynamics of the activated dissociative chemisorption of N2 on W(110): A molecular beam study J. Chem. Phys. 85, 7452 (1986); 10.1063/1.451334 Activated dissociative chemisorption of CH4 on Ni(111): Observation of a methyl radical and implication for the pressure gap in catalysis Abstract: The dynamics of dissociative chemisorption: CH4 on rhodium

Studies in heterogeneous catalysis have long speculated on or have provided indirect evidence for the role of hydrogen embedded in the catalyst bulk as a primary reactant. This report describes experiments carried out under single-collision conditions that document the distinctive reactivity of hydrogen embedded in the bulk of the metal catalyst. Specifically, the bulk H atom is shown to be the reactive species in the hydrogenation of CH(3) adsorbed on Ni(111) to form CH(4), while the H atoms bound to the surface were unreactive. These results unambiguously demonstrate the importance of bulk species to heterogeneous catalytic chemistry.

The stability and chemistry of methyl radicals adsorbed on Ni (111) A detailed analysis of the vibrational spectra of CH 3 , CH 2 D, and CD 3 adsorbed on Ni͑111͒ and the products of their reactions is presented. The synthesis of adsorbed methyl radicals from CH 4 , CH 3 D, or CD 4 is effected by molecular beam techniques. The ability to measure these spectra by high-resolution electron energy loss spectroscopy ͑HREELS͒ at higher resolution ͑35 cm Ϫ1 ͒ and higher sensitivity ͑5ϫ10 6 counts/s͒ has allowed new features to be observed and a symmetry analysis to be carried out. It is concluded that the CH 3 radical is adsorbed with C 3v symmetry on a threefold hollow site. The symmetric C-H stretch mode of CH 3 and the overtone of the antisymmetric deformation mode are observed to be in Fermi resonance. At temperatures above 150 K, CH 3 dissociates to form adsorbed CH. Confirmation for the assignment to a CH species is found in the observation that the spectrum measured after thermal decomposition of CH 2 D is a superposition of those from the decomposition of CH 3 and CD 3 . The adsorption site of the CH species is concluded to be a threefold hollow site and the geometry of the Ni 3 -C-H is concluded to be pyramidal. At temperatures above 250 K, carbon-carbon bond formation between the CH species is observed to yield C 2 H 2 . Low coverages of C 2 H 2 are shown to dehydrogenate at 400 K. High coverages of C 2 H 2 are shown conclusively to trimerize to form adsorbed benzene in contrast to a literature report of C 2 H 2 dissociation to adsorbed CH at these temperatures. The relative stabilities of the hydrocarbon species on Ni͑111͒ are determined to be CH 3 ϽCHϩ2H Ͻ1/2C 2 H 2 ϩ2HϽ1/6C 6 H 6 ϩH 2͑g͒ .

The vibrational modes of hydrogen embedded in a Ni crystal are shown to be detectable by highresolution electron-energy-loss spectroscopy and to be unambiguously distinguishable from the vibrational modes of adsorbed hydrogen on the basis of the dependence of the inelastic electron intensity on electron impact energy. The embedded hydrogen has a vibrational frequency of 800-850 cm"' and is observed to recombine and desorb as H2 between 180 and 220 K. The absorption of hydrogen into Ni(l 11) is achieved under UHV conditions by exposure to atomic hydrogen. As much as an equivalent of 8 monolayers has been absorbed.

Hydrogen atoms emerging from the bulk of Ni metal to the surface are observed to be the reactive species in the hydrogenation of adsorbed methyl radical, ethylene, and acetylene to gas-phase products. Surface-bound H atoms are unreactive. The distinctive chemistry of a bulk H atom arises largely from its significantly higher energy as compared to that of a surface-bound H atom. These results demonstrate that bulk H is not solely a source of surface-bound H in catalytic hydrogenation as proposed 50 years ago, but rather, a reactant with a chemistry of its own.