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The oxidation of CO on Pt(100): Mechanism and structure
A441 E.W. PLUMMER
Department of Physics and the Laborato(v for Research on the Structure of Matter. University of Penn3Th;ania, Philadelphia, Pennsylvania 19104. USA Received 11 April 1984; accepted for publication 16 July 1984 Valence band photoemission has been used to investigate the surface configuration and thermal decomposition of acetylene and ethylene chemisorbed on the Co(0001 ) surface. Although acetylene chemisorbs on Co(0001) in a manner common to acetylene on many other transition metal surfaces, ethylene bonds to Co(0001) in a manner that causes it to decompose completely below room temperature. The saturated ethylene overlayer at low temperature is unusual since it contains enough vacant sites to allow the coadsorption of 25% of a monolayer of CO. Literature precedents are utilized to propose a bonding mode and decomposition mechanism consistent with this data.
Surface Science 147 (1984) 143 161 North-Holland, Amsterdam
143
THE OXIDATION OF CO ON Pt(ID0): M E C H A N I S M A N D R.J. BEHM,
P.A. THIEL
*, P . R . N O R T O N
STRUCTURE
** a n d P . E . B I N D N E R
**
lnstitut J'ur Physikalische Chemie der UniversitSt M~inchen, Sopkienstrasse 11, D- 8000 Miinchen 2, Fed. Rep. of Germany Received 29 August 1983; accepted for publication 3 July 1984
Using dynamic LEED measurements of spot intensities and profiles, together with thermal desorption data, we have investigated the oxidation of CO on Pt(100)-(1 x1). At T = 355 K, either CO or O was preadsorbed and reacted off with the other species. Results from both titration sequences point to the following conclusions: Titration of preadsorbed oxygen with COg leads to rapid reaction, with a reaction probability of unity for each chemisorbed CO. Adsorbed CO does not accumulate on the surface until 0o 4 0.05, i.e. an intermediate, rather clean (1 x 1) Pt surface is obtained. Further evidence for this clean intermediate is provided by the fact that characteristics of the diffraction spots of the c(2 x 2) of CO develop identically during this reaction sequence and during adsorption of CO on a clean (1 :x: 1) Pt surface. In the reverse case, titration of preadsorbed CO with 02. g, the reaction rate is slower than the oxygen adsorption rate, leading to a pressure-dependent development of coexisting O~a and COad domains, which we observe directly with LEED. The stable phases coexisting are the c(2X2) of CO and the oxygen-related ( 3 x l ) . Thermal desorption peak shapes, together with L E E D observations, indicate that the CO in this case is held in c(2 x 2) islands by a matrix of surrounding oxygen atoms. In no case do mixed structures form, nor is an existing structure compressed by subsequent adsorption of the second species. Starting from a Langmu ir-Hi nshelwood mechanism, the differences between the two reaction sequences are discussed in terms of different activation barriers for reaction and different sticking coefficients of the adsorbing species. Special attention is given to the mobilities of the adsorbed reactants.
Department of Physics and the Laborato(v for Research on the Structure of Matter. University of Penn3Th;ania, Philadelphia, Pennsylvania 19104. USA Received 11 April 1984; accepted for publication 16 July 1984 Valence band photoemission has been used to investigate the surface configuration and thermal decomposition of acetylene and ethylene chemisorbed on the Co(0001 ) surface. Although acetylene chemisorbs on Co(0001) in a manner common to acetylene on many other transition metal surfaces, ethylene bonds to Co(0001) in a manner that causes it to decompose completely below room temperature. The saturated ethylene overlayer at low temperature is unusual since it contains enough vacant sites to allow the coadsorption of 25% of a monolayer of CO. Literature precedents are utilized to propose a bonding mode and decomposition mechanism consistent with this data.
Surface Science 147 (1984) 143 161 North-Holland, Amsterdam
143
THE OXIDATION OF CO ON Pt(ID0): M E C H A N I S M A N D R.J. BEHM,
P.A. THIEL
*, P . R . N O R T O N
STRUCTURE
** a n d P . E . B I N D N E R
**
lnstitut J'ur Physikalische Chemie der UniversitSt M~inchen, Sopkienstrasse 11, D- 8000 Miinchen 2, Fed. Rep. of Germany Received 29 August 1983; accepted for publication 3 July 1984
Using dynamic LEED measurements of spot intensities and profiles, together with thermal desorption data, we have investigated the oxidation of CO on Pt(100)-(1 x1). At T = 355 K, either CO or O was preadsorbed and reacted off with the other species. Results from both titration sequences point to the following conclusions: Titration of preadsorbed oxygen with COg leads to rapid reaction, with a reaction probability of unity for each chemisorbed CO. Adsorbed CO does not accumulate on the surface until 0o 4 0.05, i.e. an intermediate, rather clean (1 x 1) Pt surface is obtained. Further evidence for this clean intermediate is provided by the fact that characteristics of the diffraction spots of the c(2 x 2) of CO develop identically during this reaction sequence and during adsorption of CO on a clean (1 :x: 1) Pt surface. In the reverse case, titration of preadsorbed CO with 02. g, the reaction rate is slower than the oxygen adsorption rate, leading to a pressure-dependent development of coexisting O~a and COad domains, which we observe directly with LEED. The stable phases coexisting are the c(2X2) of CO and the oxygen-related ( 3 x l ) . Thermal desorption peak shapes, together with L E E D observations, indicate that the CO in this case is held in c(2 x 2) islands by a matrix of surrounding oxygen atoms. In no case do mixed structures form, nor is an existing structure compressed by subsequent adsorption of the second species. Starting from a Langmu ir-Hi nshelwood mechanism, the differences between the two reaction sequences are discussed in terms of different activation barriers for reaction and different sticking coefficients of the adsorbing species. Special attention is given to the mobilities of the adsorbed reactants.