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Neutron spectroscopic studies of the adsorption and decomposition of C2H2 and C2H4 on raney nickel
A271 E.J. V A N L O E N E N
FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ .4msterdam, The Netherlands Received 26 November 1984; accepted for publication 28 January 1985 We have studied the geometric structure of the clean Si(111)-(7×7) surface using Medium Energy Ion Scattering combined with the effects of channeling and blocking. Experimental surface blocking minima have been compared with surface blocking minima calculated with a Monte Carlo simulation program for a number of structural models proposed recently. The trimer model proposed by Himpsel and the stacking fault models proposed by Bennett and by McRae give good agreement with experimental results. Models involving small displacements from the bulklike structure along the surface normal, like Chadi's buckled ring model and the pyramidal cluster model are not compatible with experimental results. We show that the Low Energy Ion Scattering results obtained by Aono et al., which have been interpreted as unique evidence for the pyramidal cluster model, are consistent with various other models as well. We have also studied various versions of the adatom model and find that it does not agree with the experiments, unless the surface contains a stacking fault. We conclude that the triangle-dimer stacking fault is most likely a key ingredient of the structure. We speculate that an additional reconstruction in a hexagonal (V~ × Vr~) unit cell is needed to fully explain the structure.
480
Surface Science 155 (1985) 480-498 North-Holland, Amsterdam
NEUTRON SPECTROSCOPIC STUDIES OF THE ADSORPTION DECOMPOSITION OF C2H 2 AND C2H 4 ON RANEY NICKEL R.D. KELLEY,
R.R. CAVANAGH,
J.J. R U S H
AND
and T.E. MADEY
National Bureau of Standards, Gaithersburg, Maryland 20899, USA Received 3 October 1984; accepted for publication 14 December 1984 Incoherent neutron inelastic scattering has been applied to the study of the chemisorption of C2H 2 and C2H 4 on Raney nickel (a high surface area nickel powder) as a function of temperature. Surface vibrational spectra of the adsorbed layer obtained through neutron scattering demonstrate that at 150 K C2H 2 adsorbs molecularly on the nickel surface. Ethylene, however, is more reactive. Even at 120 K there is some C2H 4 decomposition. In addition, perdeuteroethylene coadsorbed with atomic hydrogen undergoes isotopic exchange at 120 K. As the temperature is raised to 275 K both hydrocarbons decompose. The vibrational spectra of the molecularly adsorbed species are similar to those observed on flat N i ( l l l ) single crystal surfaces with EELS. However, the decomposition process at higher temperature is not simply related to results reported on either flat or stepped N i ( l l l ) surfaces. We conclude that the dominant influence of steps (or other defect structures) on high surface area nickel powders is to lower thd activation energy for dissociation.
FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ .4msterdam, The Netherlands Received 26 November 1984; accepted for publication 28 January 1985 We have studied the geometric structure of the clean Si(111)-(7×7) surface using Medium Energy Ion Scattering combined with the effects of channeling and blocking. Experimental surface blocking minima have been compared with surface blocking minima calculated with a Monte Carlo simulation program for a number of structural models proposed recently. The trimer model proposed by Himpsel and the stacking fault models proposed by Bennett and by McRae give good agreement with experimental results. Models involving small displacements from the bulklike structure along the surface normal, like Chadi's buckled ring model and the pyramidal cluster model are not compatible with experimental results. We show that the Low Energy Ion Scattering results obtained by Aono et al., which have been interpreted as unique evidence for the pyramidal cluster model, are consistent with various other models as well. We have also studied various versions of the adatom model and find that it does not agree with the experiments, unless the surface contains a stacking fault. We conclude that the triangle-dimer stacking fault is most likely a key ingredient of the structure. We speculate that an additional reconstruction in a hexagonal (V~ × Vr~) unit cell is needed to fully explain the structure.
480
Surface Science 155 (1985) 480-498 North-Holland, Amsterdam
NEUTRON SPECTROSCOPIC STUDIES OF THE ADSORPTION DECOMPOSITION OF C2H 2 AND C2H 4 ON RANEY NICKEL R.D. KELLEY,
R.R. CAVANAGH,
J.J. R U S H
AND
and T.E. MADEY
National Bureau of Standards, Gaithersburg, Maryland 20899, USA Received 3 October 1984; accepted for publication 14 December 1984 Incoherent neutron inelastic scattering has been applied to the study of the chemisorption of C2H 2 and C2H 4 on Raney nickel (a high surface area nickel powder) as a function of temperature. Surface vibrational spectra of the adsorbed layer obtained through neutron scattering demonstrate that at 150 K C2H 2 adsorbs molecularly on the nickel surface. Ethylene, however, is more reactive. Even at 120 K there is some C2H 4 decomposition. In addition, perdeuteroethylene coadsorbed with atomic hydrogen undergoes isotopic exchange at 120 K. As the temperature is raised to 275 K both hydrocarbons decompose. The vibrational spectra of the molecularly adsorbed species are similar to those observed on flat N i ( l l l ) single crystal surfaces with EELS. However, the decomposition process at higher temperature is not simply related to results reported on either flat or stepped N i ( l l l ) surfaces. We conclude that the dominant influence of steps (or other defect structures) on high surface area nickel powders is to lower thd activation energy for dissociation.