Boreskov Institute of Catalysis of the Siberian Branch of Russian ...

PP-I-8After each adsorption or desorption attempt several trials of metal and/or adsorbate atomdiffusion has been executed. By simulation of the adsorption isotherms the supported particlewas preliminary established in equilibrium at given temperature. Then, step by step rising thepressure of adsorbate, we perform the simulations at each pressure value trying to achieve theequilibrium state both for the adsorbed layer and for the particle surface morphology itself.The adsorption isotherms has been simulated at different ratios of interaction (attraction)between adsorbed species and metal atoms at T = 500 K (J ma = 0.5×J mm , J mm and 2×J mm ,where J mm - the interaction energy between metal atoms, J ma – the interaction betweenadsorbed species and metal atoms).The increasing of J ma value leads to the transformation of the particle to the pyramidalshape, at that with increasing of the «metal-adsorbate» bond strength the height of thepyramid rises. But at strong interaction «metal-adsorbate» (J ma = 2J mm ) for bimoleculardissociative adsorption we have instead of the pyramidal shape of the particle thehemispherical one with very ulcerated surface in comparison with initial equilibrium particleand in the high-pressure interval the simulated coverages are lower in comparison withequilibrium values. In our opinion, it is due to the fact that the adsorbed on the surface B 2 -molecules attract immediately the neighboring metal atoms at that decreasing the probabilitiesof their own desorption and diffusion. As a consequence, at each next increasing of thepressure the coverage of the surface by B ads increased only slightly: the particle surfacemorphology is positioned on a metastable long-lived state (for more details see [6]).After the removing of the adsorbed layer the shape and the surface morphology of theparticle returned to the equilibrium characteristics peculiar to the given temperature. Thesimulations show that if the «adsorbate-metal» interaction energies become comparable withthe metal-metal ones the nanoparticle undergoes reversible adsorbate-induced reshaping andthat the particles size determines the degree of reshaping as in experiments [7].References:1. V.P. Zhdanov, B. Kasemo, Surf. Sci. Rep., 2000, 39, 25-104.2. V.I. Elokhin, A.V. Myshlyavtsev, In: Dekker Encycl. of Nanosci. and Nanotechn. J.A. Schwarz,C.I. Contescu and K. Putyera, Eds.; Marsel Dekker, Inc.: New York, 2004; pp.621-632.3. J. Lapujoulade, Surface Sci. Rep., 1994, 20, 191-249.4. G. Bracco, Phys. Low-Dim. Struct., 1994, 8, 1-22.5. N. Metropolis, A.V. Rosenbluth, M.N. Rosenbluth, A.H. Teller, E. Teller, J. Chem. Phys., 1953, 21, 1087-1092.6. E.V Kovalyov., V.I. Elokhin, A.V. Myshlyavtsev, J. Comp. Chem., 2008, 29, 79-86.7. P.L. Hansen, J.B. Wagner, S. Helveg, J.R. Rostrup-Nielsen, B.S. Clausen, H. Topsøe, Science, 2002, 295,2053-2055.226