F.C.M.J.M. Van Delft

Determination of nucleation and growth mechanisms

Abstract A model based on that of Gallon is presented which describes the variations in the Auger electron spectroscopy intensities of the relevant elements for adsorbates growing on a substrate according to various growth mechanisms. The results are compared with experimental data from the literature. Our calculations predict that platinum deposited onto a gold substrate grows according to the Volmer-Weber mechanism, which is supported by experimental data. Exposure of a platinum surface to ethylene at 500 K yields a carbonaceous overlayer which grows according to the monolayer mechanism under the given experimental conditions, indicating that the adsorbate is rather mobile on the surface. It is demonstrated by means of a simple model that the growth mode (formation of crystallites or formation of a monolayer) is determined by the difference in binding energy between an adsorbate (atom) in a monolayer and an adsorbate (atom) in the bulk of a crystallite and not by the difference in surface free energy between the adsorbate and the substrate as has been suggested in the literature. This is in accordance with experimental data in the literature for adsorbates (metals, CO, xenon) on various substrates. It is proposed that a similar parameter may determine whether another monolayer (Frank-van der Merwe mechanism) or crystallites (Stranski-Krastanov mechanism) will form on top of one (or more) monolayer(s). These energy parameters can be correlated with the refined Gallon model.

The temperature dependence of the surface composition of Pt−Rh alloys

AES has been used to investigate the variation of the surface composition of a polycrystalline Pt 0.62 −Rh 0.38 foil with the equilibration temperature. The results show that an ultraclean surface exhibits a Pt enrichment that increases with increasing equilibration temperature from 80 to 89 at% Pt in the 800–1000 K range. In the range 1000–1500 K an almost constant surface concentration of 89 at% Pt was found. The influence of small amounts of impurities such as P, C and Sn is discussed. The surface enrichment in Pt is in agreement with a number of literature data, but it cannot be understood with the classical models of surface segregation. The Pt enrichment in the surface and its temperature dependence are qualitatively in line with a model based on vibrational entropy contributions in addition to the enthalpy contributions used in classical surface segregation models.

Reactivity of polycrystalline platinum and rhodium surfaces towards methane, ethene and carbon monoxide

Abstract The chemical characteristics and the reactivity of carbon on polycrystalline Rh and Pt surfaces towards hydrogen have been studied by Auger Electron Spectroscopy. Auger spectra of carbon on both metals have been subjected to spectrum subtraction techniques in order to reveal the chemical characteristics of it, as reflected by the fine structure of the carbon KVV Auger spectra. Two classes of carbon can be identified by AES: (A) amorphous and graphitic carbon; (B) molecular (or molecular fragments) and carbidic carbon, the first being almost unreactive in H 2 . Carbon B transforms into A and this transformation is faster on Pt than on Rh and is also faster in the absence than in the presence of H 2 .

Formation of a carbonaceous adsorbate layer on the surface of Pt-Cu alloys

Abstract Selective chemisorption of ethene and the chemisorption induced surface segregation of Pt upon the chemisorption of ethene (and its subsequent decomposition) on Pt-Cu alloys have been studied by Auger electron spectroscopy. The experimental results have also been evaluated by computer simulations of the various possible processes upon the chemisorption. In these computer simulations a modified Gallon model has been used in order to describe the variations in the intensities of the Pt and the Cu surface sensitive Auger transitions. Our present results indicate a preferential chemisorption of the carbonaceous species onto Pt sites, and a chemisorption induced surface segregation of Pt. Alloying of Pt with Cu results in a decreased coking of the catalyst.

The influence of ordering on the surface composition and surface site distribution of binary alloys as studied with the Monte Carlo method

Monte Carlo simulations of various binary alloy surfaces were performed in order to determine surface composition, composition depth profile and, in particular, the surface site distribution as a function of bulk composition, surface orientation and equilibration temperature. The influence of ordering was computed by varying the enthalpy of mixing of a hypothetical alloy with equal sublimation enthalpies and radii of the constituent metals. In addition, the Monte Carlo method has been applied to simulate the (111) and (100) surfaces of Pd−Ag, Pd−Cu, Ni−Cu and Pt−Rh. The results illustrate the large effect of exothermic enthalpy of mixing (tendency of ordering) on the surface composition and in particular, the surface site distribution. The formation of intermetallic compounds i.e. ordering effects can be observed with the computed surface composition versus bulk composition plots and the computed surface topography. The results of our calculations are compared with experimental data.

Co oxidation over silica supported Pt-Rh alloy catalysts

The CO oxidation over silica supported Pt-Rh alloys was investigated and compared with the reaction over silica supported Pt and Rh catalysts. The CO-O2 ratio was varied from CO-rich to CO-lean conditions. The activity expressed as turnover frequency changes gradually with increasing Pt bulk concentration: from Rh-like to Pt-like. The results can be understood on the basis of results obtained for Pt-Rh single crystal surfaces. Both the specific differences in properties of Pt and Rh and the surface composition determine the activity of the alloy catalysts.

Correlation of nucleation- and growth modes with wetting, alloy segregation, catalyst preparation and strong-metal support interaction

Abstract A simple thermodynamical model has been derived earlier, that enables to predict the growth mode (monolayer - or crystallite growth) for solid adsorbates on solid substrates. The influence of pseudomorphism on the model is discussed. It will be shown that the model can also be used for predicting the behaviour of liquids and gases on solid substrates. The terms cohesion and adhesion are valid for adsorbates of all states of aggregation. A scheme is presented, which correlates the thermodynamics of nucleation- and growth modes for metals on metallic substrates with parameters that determine surface segregation in binary alloys. This scheme allows a prediction of alloy segregation, when thermo-chemical data are lacking, by examination of the growth modes for both components of the alloy deposited onto each other. The behaviour of ionic solids deposited on ionic solids and the miscibility of organic compounds is discussed in relation to this scheme. Catalyst preparation methods, resulting in metal particles dispersed on a carrier, can be understood in terms of the behaviour of ionic solids and metals on ionic solid substrates. The recently found Strong Metal Support Interaction effects (S.M.S.I.) in heterogeneous catalysis may be expected to occur, within the framework of the present model, at a temperature at which an adhesive component becomes mobile.

Determination of surface debye temperatures by LEED

The method to determine the surface Debye temperature by LEED beam intensity versus temperature measurements is critically evaluated. Simulated in( I ) versus T plots obtained with the help of the Gallon-model using a kinematic approach for the specular beam at Bragg diffraction conditions are presented. It is shown that the assumed linearity in these plots is fundamentally incorrect. With our newly developed model good agreement is obtained with experimentally determined effective Debye temperature versus electron energy plots for Pt(111), Pt(100), Rh(111) and Rh(100). However, it is shown that the method of electron energy variation is not reliable for straightforward determination of the surface Debye temperature, whereas variation of angle of incidence is better suitable. In addition, plots measured on a Pt-Rh(100) surface with a Pt surface content varying between 25 and 60 at% are compared with simulated plots for disordered and ordered alloys.

Oxidation and dehydrogenation of alcohols by glauconite, a natural Fe(II)- and Fe(III)-containing sheet silicate, and ferri-sepiolite, its molecular sieve analogue

Abstract Glauconite, a natural three layer sheet silicate with Fe(II) and Fe(III) ions in octahedral positions, has been investigated by X-ray diffraction and Mossbauer spectroscopy after both hydrogen and oxygen treatments at ambient temperatures. The results show that in hydrogen up to 673 K Fe(III) ions are reduced to Fe(II) ions without any change in the silicate structure. At 873 K in hydrogen the silicate structure is unstable, and small amounts of α-Fe can be detected. In oxygen at 673 K the Fe(II) ions are oxidized without any change in the silicate structure. The same occurs in an argon atmosphere at 673 K, most likely due to decomposition of the hydroxyl groups and/or the crystal water attached to the Fe(II) ions. Preliminary investigations have been performed on the reactivity of alcohols on glauconite and on ferri-sepiolite (xylotile), which is a molecular sieve composed of ribbons of three-layer sheet silicate with a composition similar to that of glauconite. Both minerals dehydrate and dehydrogenate alcohols in an inert atmosphere and oxidize them as well in the presence of oxygen. The oxidation and dehydrogenation activities can be ascribed to the Fe(II)/Fe(III) redox couple. Ferri-sepiolite additionally shows selectivity towards small size products, which can be ascribed to its molecular sieve structure.

An Aes Investigation of the Reactivity of Pt, Rh and Various Pt-Rh Alloy Surfaces Towards O2, NO, CO and H2

The chemisorption and reactivity of oxygen, NO, CO and hydrogen on poly-crystalline Pt, Rh, Pt0.55-Rh0.45 and Pt0.12-Rh0.88 alloy surfaces and on a PtO.25-Rh0.75 (100) single crystal surface have been studied by AES. For Rh the results point to the presence of subsurface oxygen, following an exposure to 100 L 02 or NO at 290 K, under which conditions this species was not observed for Pt. The Pt-rich alloy behaved as pure Pt. The Rh-rich alloy showed occasionally a Rh-like or a Pt-like behaviour. The ambivalent behaviour of this Pt-Rh alloy appears to be determined by variations in the surface composition, which is strongly dependent on the equilibration temperature in vacuum. At low equilibration temperatures (below 1000 K) a Rh-rich surface is observed for Rh-rich alloys, whereas at high equilibration temperatures (above 1200 K) Pt surface segregation occurs. These effects are discussed in relation to surface segregation theories. The results show that the surface composition and the catalytic behaviour of Pt-Rh alloys are strongly dependent on the equilibration temperature, the presence of contaminants and the composition of the gas phase.