A53 Surface Science 103 (1981) 353-360 0 North-Holland Publishing Company THE MELTING OF A MONATOMIC AMORPHOUS SURFACE: A MOLECULAR DYNAMICS STUDY M.R. MRUZIK, S.H. GAROFALINI and G.M. POUND Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA Received 19 June 1980 A monatomic amorphous surface has been simulated above and below the gradual melting transition using molecular dynamics for atoms interacting through a Lennard-Jones 12-6 potential energy function. Detailed atomic trajectories are presented and averaged to yield dif- fusion constants and activation energies. Surface melting, which occurs at lower temperatures than for the amorphous bulk and most crystalline surfaces, is described in terms of radial distri- bution functions, velocity correlations, and vibrational spectra. Surface Science 103 (1981) 361-396 Q North-Holland Publishing Company SULFUR INDUCED SELECTIVITY CHANGES FOR METHANOL DECOMPOSITION ON Ni( 100) S. JOHNSON and R.J. MADIX Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA Received 27 May 1980;accepted for publication 23 September 1980 The adsorption and reactions of methanol on Ni(lOO), Ni(lOO)p(2 X 2)s and Ni(100) c(2 X 2)s were studied by temperature programmed reaction spectroscopy. On the clean surface mixed isotopes of dihydrogen and CO were formed in desorption-limited processes above 300 K. Evidence suggestive of the existence of an adsorbed [COD] intermediate was obtained. Similar results were found on the p(2 X 2)s surface. The sulfur decreased the amount of adsorbed methanol which reacted, and it interacted significantly with the high temperature binding state of deuterium produced by decomposition of the [COD] intermediate. On the c(2 X 2)s structure the primary reaction intermediate was methoxy, CHsO. Formaldehyde was formed with an activation energy of 26 kcal/gmol. No desorption-limited CO was observed. These results clearly demonstrate the role of structural overlayers in selective poisoning. Surface Science 103 (1981) 397-403 0 North-Holland Publishing Company EFFECT OF IMPURITIES ON SURFACE SEGREGATION IN BINARY ALLOYS Raju P. GUPTA and B. PERRAILLON Centre d’Etudes Nuclbires de Saclay, Section de Recherches de MPtallurgie Physique, Boite Postale No. 2, F-91 190 Gif-sur- Yvette, France Received 9 July 1980 It is shown that the trace amount of an impurity with a high enthalpy of segregation, can have a strong influence on the segregation characteristics of the main segregating component in a binary alloy. The effect of weakly segregating impurities is unimportant, especially in the temperature range of interest.

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Page 1: Effect of impurities on surface segregation in binary alloys

A53

Surface Science 103 (1981) 353-360 0 North-Holland Publishing Company

THE MELTING OF A MONATOMIC AMORPHOUS SURFACE:

A MOLECULAR DYNAMICS STUDY

M.R. MRUZIK, S.H. GAROFALINI and G.M. POUND Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA

Received 19 June 1980

A monatomic amorphous surface has been simulated above and below the gradual melting transition using molecular dynamics for atoms interacting through a Lennard-Jones 12-6 potential energy function. Detailed atomic trajectories are presented and averaged to yield dif- fusion constants and activation energies. Surface melting, which occurs at lower temperatures than for the amorphous bulk and most crystalline surfaces, is described in terms of radial distri- bution functions, velocity correlations, and vibrational spectra.

Surface Science 103 (1981) 361-396 Q North-Holland Publishing Company

SULFUR INDUCED SELECTIVITY CHANGES FOR METHANOL

DECOMPOSITION ON Ni( 100)

S. JOHNSON and R.J. MADIX Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA

Received 27 May 1980;accepted for publication 23 September 1980

The adsorption and reactions of methanol on Ni(lOO), Ni(lOO)p(2 X 2)s and Ni(100) c(2 X 2)s were studied by temperature programmed reaction spectroscopy. On the clean surface mixed isotopes of dihydrogen and CO were formed in desorption-limited processes above 300 K. Evidence suggestive of the existence of an adsorbed [COD] intermediate was obtained. Similar results were found on the p(2 X 2)s surface. The sulfur decreased the amount of adsorbed methanol which reacted, and it interacted significantly with the high temperature binding state of deuterium produced by decomposition of the [COD] intermediate. On the c(2 X 2)s structure the primary reaction intermediate was methoxy, CHsO. Formaldehyde was formed with an activation energy of 26 kcal/gmol. No desorption-limited CO was observed. These results clearly demonstrate the role of structural overlayers in selective poisoning.

Surface Science 103 (1981) 397-403 0 North-Holland Publishing Company

EFFECT OF IMPURITIES ON SURFACE SEGREGATION

IN BINARY ALLOYS

Raju P. GUPTA and B. PERRAILLON Centre d’Etudes Nuclbires de Saclay, Section de Recherches de MPtallurgie Physique, Boite Postale No. 2, F-91 190 Gif-sur- Yvette, France

Received 9 July 1980

It is shown that the trace amount of an impurity with a high enthalpy of segregation, can have a strong influence on the segregation characteristics of the main segregating component in a binary alloy. The effect of weakly segregating impurities is unimportant, especially in the temperature range of interest.