Langmuir 1993,9, 2504-2508
2504
Catalytic Activity in Simulated Reactions with Surface Reconstruction F. Nieto Instituto de Investigaciones en Tecnologkr Qufmica, Universidad Nacional de San Luis, Casilla de Correo 290, 5700 San Luis, Argentina
J. L. Riccardo and G. Zgrablich' Instituto de Investigaciones en Tecnologkr Qufmica, Universidad Nacional de San Luis, Casilla de Correo 290,5700San Luis, Argentina, and Centro Regional de Estudios Avanzados (CREA), Gobierno de la Provincia de San Luis, Casilla de Correo 256, 5700 San Luis, Argentina Received October 20, 1992. I n Final Form: March 19,199P A simulation of the kinetics of dissociative chemisorption, recombinative reaction, and desorption of simple molecules concomitant with metal surface restructuring is presented. The simulation scheme is strictly based on the one-dimensionalmodel recently proposed by Levine and Somorjai.' Results for the behavior of surface coverage, aggregation, and catalytic activity are obtained and compared with former approximate theoretical calculations. An explanation of the strong differences between simulation and analytical results is given. 1. Introduction
Very interesting hypotheses have been recently stated, on the basis of strong experimental evidence, in order to explain some peculiar properties regarding surface phenomena such as chemisorption,catalyzedsurfacereactions like bond breaking, surface diffusion, and desorption taking place on metallic surfaces.1*2Three, among others, unsolved problems, that have been named "the puzzles of surface science",2have been emphasized: (1)the rougher the surface the higher its catalytic activity, as occurring for reactions involving H-H, C-H, or C-C bond breaking on stepped and kinked crystal faces of ~ l a t i n u m(2) ; ~bond breaking takes place in a narrow temperature range characteristicof the adsorbahubstrate ~ y s t e m ;(3) ~ *this ~ range shifts to lower temperatures in the presence of stronger adsorptive sites.6 In an attempt to give an explanation to these "enigmas" a chemisorption induced surfacerestructuring model (CISRM)has been proposed.2 This model assumes that, under chemisorption, surface metallicatoms do not remain in the same surfacelocations as for the bare surface but they move to new positions in response to the chemical changes in their environment. These displacements strengthen the chemical bond of the adsorbate to the surfaceand provoke a local surfacestrain. This strain can be removed by desorption (in the case of an isolaetd chemisorbed molecule) or it can be reduced by chemisorption of another molecule in the neighborhood of an already chemisorbed one, so that surface restructuring turns out to be a dynamic phenomenon. Preliminary analytical calculations using this model were made through a simple one-dimensional approximation for a chain of adsorptive sites where homonuclear diatomic moleculescan undergo dissociativechemisorption,reaction (Le. exchange of atoms),or desorptioneither in the original form or as reacted molecules.
* To whom all correspondence should be addressed.
* Abstractpublished in Advance ACSAbstracts, August 15,1993. ( 1 ) Levine, R. D.; Somorjai, G. A. Surf. Sci. 1990,232, 407.
(2) Somorjai, G. A. Surf. Sci. 1991,242, 481. (3) Somorjai, G. A. Chemistry in Two Dimensions Surfaces; Cornell University Press: Ithaca, NY, 1981; p 288. (4) Johnson, D. W.; Roberts, R. W. Surf. Sci. 1979,87, L255. (5) Salmeron, M.; Somorjai, G. A. J. Phys. Chem. 1982,86,341. ( 6 ) Lehwald, S.; Ibach, M. Surf. Sci. 1979, 89, 425.
T w o essential assumptionssupport this calculation: (a) the clean surface restructures simultaneously with the dissociative chemisorption (bond breaking)? this restructuring is rather local and is reversed when associative desorption proceeds (the leading proposition of the CISRM); (b)there existsa smallprobabilityfor dissociative chemisorptionof an isolated physisorbed molecule? The calculations predict that if local reconstruction is energetically important, then chemisorption and reaction behave like cooperative processes accompanied by substrate atoms cooperative relocation. Surprisingly, the results arising from the one-dimensional case seem to account for all three main features of surface processes cited above. However, the importance of the predictions and the fact that some approximations had to be usedto solvethe one-dimensionalproblem, make it worth to trying to reproduce such predictions through simulation techniques. In this work we perform a Monte Carlo simulation of the kinetic scheme proposed in ref 1. On one hand, the aim is to test the accuracy of the results derived from the model and discusshow severe are the approximationsmade to obtain the analyticalexpressionfor the catalyticactivity and, on the other hand, to briefly discuss further generalizations of the kinetic model to the more interesting two-dimensional case. h section 2 we define the kinetic model in terms of elementary steps, and section 3 is devoted to presenting the Monte Carlo procedure. Results for surfacecoverage, aggregation of chemisorbed molecules, and catalytic activity are compared with those obtained from the analytical approximation in section 4. In section 5 the results are discussed and physically explained and conclusions and future perspectives are outlined. 2. One-Dimensional Model of Surface Reaction
with Reconstruction Here, we briefly summarize the kinetic model to be simulated. The surface is to be considered as a chain of (7) Somorjai, G. A.; Van Hove, M. A. Prog. Surf. Sci. 1989,30, 201. (8)Poekema, B.; Coma, G.Scattering of Thermcrl Energy A t o m ; Springer: Berlin, 1989; p 90.
0743-7463/93/2409-2504$04.00/00 1993 American Chemical Society
Catalytic Actiuity and Surface Reconstruction adsorptive sites where homonuclear diatomic molecules For the sake of simplicity we call this step "the chemisorption of the reagents". Any individual atom can either associate with ita original companion (givinga nonreacted molecule)and desorb, or link to another atom belonging to a different dissociated molecule (giving a reacted molecule) and desorb. The dissociative chemisorptionof a singlemolecule leads to reconstruction of sites on which the atoms allocate. This process can be viewed as the simple reaction
Langmuir, Vol. 9, No. 10, 1993 2505
(n- 1, m - 1 )
A2 can be chemisorbed with bond breaking.
with equilibrium constant CO, where S and S', respectively, denote a bare surface site and a site after local reconstruction. POand Pa are the probabilities per unit time for the occurrence of the reaction from right to left and from left to right, respectively. The next process we consider is the chemisorption (desorption) of an A2 molecule on (from) a pair of sites which are next to an already chemisorbed molecule. This is represented by the reaction
with equilibrium constant s. Given that dissociation of an isolated molecule has a low probability to 0ccur',1*~ the ratio C O / S will be, in general, a small number. Following the nomenclature of ref 1, we related co to s through co = ys,with y
