Adsorption and desorption kinetics with no precursor trapping: Hydrogen and deuterium on W{100}

A285 Surface Science 215 (1989) l-28 North-Holland, Amsterdam

1

DISSOCIATIVE CHEMISORPTION OF METHANE ON Pt( 111) Gregory R. SCHOOFS ‘, Christopher R. ARUMAINAYAGAM 2, Mark C. McMASTER ’ and Robert J. MADIX l-2 * Department of Chemical Engineering Stanford CA 94305, USA Received

21 June 1988; accepted

’ and Department

for publication

of Chemistv

27 December

‘, Stanford

University,

1988

Dissociative chemisorption of methane on clean Pt(ll1) was studied with a supersonic molecular beam. Initial dissociative sticking probabilities from 0.01 to 0.19 were measured directly with incident total beam energies from 68 to 95 kJ/mol, surface temperatures from 500 to 1250 K, and angles of incidence from O” to 45O measured from the surface normal. The nozzle temperature and stagnation pressure were both fixed so that the effect of translational energy at a fixed incident vibrational energy could be probed. The initial dissociative sticking probability of methane on clean Pt(ll1) equalled 0.06f0.02 and was independent of surface temperature between 500 and 1250 K for a fixed normal incident kinetic energy of 68 kJ/mol, implying that dissociation proceeded via direct collisional activation rather than via trapping or precursor-mediated processes in this energy range. The initial dissociative sticking probability of methane on clean Pt(ll1) increased exponentially with increasing normal kinetic energy. The barrier height for C-H bond rupture by kinetic energy is 121 kJ/mol. The exponential dependence is consistent with a model for dissociative methane adsorption that involves quantum mechanical tunneling of a hydrogen atom through a one-dimensional, parabolic barrier of this height with a thickness at half height of 0.13 kO.01 A. Differences in the initial dissociative sticking probabilities observed on Pt(ll1) versus Ni(ll1) and W(110) in studies from different laboratories can be reconciled on the basis of the different vibrational energies employed, but this explanation does not account for the high reactivity on Ir(llO>(l x2). The activation barriers predicted by the bond order conservation theory of Shustorovich agree closely with the barrier heights estimated from the tunneling model for Pt(lll), W(llO), and Ir(ll0) if no correction for energy dissipation to the lattice is made. The activation barriers predicted by the molecular orbital analysis of Anderson and Maloney are much lower than the barrier heights estimated from the tunneling model. The barrier thicknesses show qualitative agreement with the C-H bond elongations in the transition state predicted by their molecular orbital theory, but do not correlate with the crystalline radii of the metal atoms or the stiffness of the metal lattices. Furthermore, the values of the tunneling parameters may be dependent on the vibrational and translational energies employed in the studies.

Surface Science 215 (1989) 29-46 North-Holland, Amsterdam

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ADSORPTION AND DESORPTION KINETICS WITH NO PRECURSOR TRAPPING: HYDROGEN AND DEUTERIUM ON W{ 100) P. ALNOT *, A. CASSUTO * * and D.A. KING * * * The Donnan Laboratories, Received

30 March

University of Liverpool, Liverpool L69 3BX, UK

1988; accepted

for publication

22 December

1988

Using a molecular beam technique, accurate dissociative adsorption sticking probabilities are reported for both H, and Ds interacting with W{lOO), over a wide range of surface and beam temperatures, and of surface coverage. By following the scattering of HD and Da from the crystal surface produced from mixed beams of H, and Ds, isothermal desorption data have also been

A286 obtained at crystal temperatures between 380 and 480 K which yield model-independent variations of desorption activation energy and desorption pre-exponential factors with surface coverage. Within an experimental accuracy of 51, no isotope effect was observed in adsorption or desorption kinetics for H, and 4. The slicking probability s falls linearly with coverage, according lo the expression s = (0.72 - 5.4 X 10e4Ta) (1 - 8/2) where Ta is the gas temperature; s is independent of substrate temperature over the range 200 to 1150 K. The data indicate no influence of trapping into a precursor state. At low coverages the desorption energy Ed is 159 kJ mol-‘, falling precipitately at 8 = 0.46 to 88 kJ mol-‘. At the same coverage, the desorption pre-exponential term Y falls by a factor of 10’. The results are discussed in terms of the known adsorbate-induced, coverage-dependent displacive phase transition which occurs for this system. It is proposed that at low coverages desorption occurs with a normal pre-exponential (of 7 x 10” s-l) from the local&d pinched dimer structure; above B = 0.5, however, desorption occurs from a delocalised ad-layer, with a low pre-exponential factor ( - 4X lo8 s-l). Since the adsorbate is localised at all coverages al 300 K, it is further proposed that a localised-delocalised phase transition, with AS = 60 J mol-’ K-’ and AH = 21 kJ mol-‘, occurs between 300 K and the desorption temperature.

47

Surface Science 215 (1989) 47-54 North-Holland, Amsterdam

THEORY OF CHEMISORPTION INDUCED RECONSTRUCTION OF SOLID SURFACES V.E. ZUBKUS, A.A. VLASOVA and E.E. TORNAU Institute of Semiconductor Physics, Vilnius 232600, Lithuania, USSR

Academy

of Sciences

Received 4 August 1988; accepted for publication

of Lithuanian

SSR

K. Poielos

52,

14 December 1988

A theory of solid surface reconstruction induced by chemisorbed atoms is proposed. The interactions between the adatom and the substrate atom displacements are taken into account. The case with the adatoms disordered on the reconstructed surface is investigated. Thermodynamic characteristic calculations are carried out by the mean field approximation. The dependence of the reconstruction temperature on the coverage of chemisorbed atoms is obtained in the H/W(OOl) system. The model parameters are estimated.

Surface Science 215 (1989) 55-64 North-Holland, Amsterdam

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COADSORPTION OF OXYGEN AND HYDROGEN ON A STEPPED NICKEL SURFACE A.-S. MARTENSSON Department

of Physics, Chalmers University of Technology S-VIZ 96 Giiteborg, Sweden

Received 28 June 1988; accepted for publication 28 December 1988 Electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED) have been used to study the adsorption of oxygen on the stepped Ni(510) face at a surface temperature around 100 K. As the coverage increases three different adsorption states for the oxygen atoms are observed; initially a low coverage (8 -C0.25) layer of oxygen adsorbed in the hollow sites on the