A. Cassuto

Rate expressions for adsorption and desorption kinetics with precursor states and lateral interactions

Abstract Experimental and theoretical evidence for the existence of mobile, transitory precursor states and for the importance of lateral interactions between chemisorbed species is briefly surveyed. Generalised rate expressions for adsorption and desorption are derived which specifically take into account the existence of intrinsic and extrinsic precursor states, and which can be readily adapted to account for dissociative or non-dissociative adsorption, with and without interactions, and which also account for the possibility of direct transfer to or from the chemisorbed state, without trapping in the intrinsic precursor state. These expressions, derived using kinetic schemes, are shown to be identical with rate expressions obtained from a successive site statistical model. Thermodynamic conditions are used to restrict the number of parameters in these expressions.

A study of the adsorption of several oxygen-containing molecules (O2, CO, NO, H2O) on Re(0001) by XPS, UPS and temperature programmed desorption

Abstract We have studied the adsorption of oxygen, carbon monoxide, nitric oxide, and water vapour on Re(0001), using X-ray and ultra-violet photo electron spectroscopies (XPS and UPS) and temperature-programmed desorption. As on polycrystalline rhenium, adsorbed oxygen is completely dissociated, even at room temperature. Furthermore, the formation of a superficial oxide at room temperature seems probable. Carbon monoxide is almost completely molecularly adsorbed, only a very small fraction being dissociatively adsorbed in a single β- state. However, an attractive interaction still exists between the adsorbed atoms in this β- state. Nitric oxide is adsorbed in a dissociated β 2 state and a molecular β 1 state. The population is smaller than on polycrystalline rhenium, corresponding to half a monolayer. Mathematical treatment of the desorption spectra allowed us to determine the activation energy for desorption of nitrogen resulting from the decomposition of adsorbed species. These quantities were found to be similar to those measured for polycrystalline rhenium.

Molecular beam study of the mechanism of carbon monoxide oxidation on platinum and isolation of elementary steps

Abstract Carbon monoxide oxidation is catalyzed by platinum even at room temperature. Attack of adsorbed oxygen proceeds through two mechanisms: an Eley-Rideal reaction involving carbon monoxide in the gas phase (or loosely adsorbed molecules) and a Langmuir-Hinshelwood reaction between strongly adsorbed species. Use of the molecular beam technique enabled us to isolate these two reactions and to determine their rate constants. The first reaction is not activated while the activation energy for the second is around 22 kcal/mol.

A comparative study of 1,3-butadiene and 1-butene chemisorbed on Pt(111), and Pd(111)

Abstract The adsorption of 1,3-butadiene and 1-butene on Pt(111), and Pd(111) was studied by near edge X-ray absorption fine structure (NEXAFS). It is found that at 95 K, 1-butene is π-bonded to Pd(111), while it is di-σ bonded to Pt(111). At 95 K, 1,3-butadiene is loosely bonded on both Pd(111) and Pt(111). However, at 300 K, 1,3-butadiene adsorption differs, depending on the substrate. A di-σ bonding is observed on Pd(111) while on Pt(111), a di-σ interaction, keeping one central carbon-carbon double bond, is proposed. In the butadiene molecule half of the carbon atoms undergo a large rehybridization from sp 2 to sp 3 , as evidenced by high resolution electron energy loss spectroscopy (HREELS). These differences between 1,3-butadiene adsorption on Pt(111) and Pd(111) are discussed in terms of differences in catalytic activities and selectivities in the 1,3-butadiene hydrogenation reaction.

Kinetics and mechanism of low-pressure, high-temperature oxidation of silicon-II

The reaction of oxygen at low pressures with silicon layers on tungsten ribbons was studied. An abrupt transition was observed between a condition of “passivation,” in which a thin film of SiO2 formed at low temperatures, and a steady-state “combustion” condition at high temperatures. The latter state is characterized by the formation of volatile SiO. The boundary between these two states has been defined in terms of the pressure-temperature relation. Oxygen consumption in the “combustion” state is represented by first-order reaction kinetics with an activation energy of 13 ± 1 kcal/mole. The stability of the two states has been defined by a thermodynamic analysis of the SiO2 layer stability. The oxygen consumption dependence on temperature has been described by a kinetic model which involves a consideration of the various elementary steps in the reaction.

Buta-1,3-diene and but-1-ene chemisorption on Pt(111), Pd(111), Pd(110) and Pd50Cu50(111) as studied by UPS, NEXAFS and HREELS in relation to catalysis

The chemisorption of both buta-1,3-diene and but-1-ene on Pt(111), Pd(111), Pd(110) and Pd50Cu50(111) samples has been characterized by near-edge X-ray absorption fine structure (NEXAFS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS). The buta-1,3-diene hydrogenation reaction, investigated on the same surfaces of Pd(111), Pd(110) and Pd50Cu50(111), displays a very good selectivity in butenes and a higher activity compared to Pt(111). Furthermore, the Pd activity greatly depends on the surface crystalline orientation and is also influenced by alloying effects. The origin of these effects has been sought in differences of chemisorption modes of buta-1,3-diene and but-1-ene on the various surfaces: at 95 K, buta-1,3-diene and but-1-ene are physisorbed on the different Pd-based single crystals. On Pt(111), buta-1,3-diene is π-bonded when but-1-ene is di-σ-bonded. At 300 K, buta-1-diene either dehydrogenates into butadiene on Pd(110) and Pd50Cu50(111) or very probably transforms into butylidyne on Pt(111) and on Pd(111) by analogy to ethene adsorbed on these (111) surfaces. Buta-1,3-diene leads to a di-σ mode on Pt(111) and to a di-π one on the different Pd surfaces. Moreover, the NEXAFS esperiments reveal that the π*1–π*2 splitting variations [2.0 eV on Pd(111), ca. 2.4 eV on Pd(110) and Pd50Cu50(111), cf. 2.7 eV on the condensed multilayer] agree with a decrease of the hydrocarbon–substrate interaction according to the sequence: Pd(111) > Pd(110) Pd50Cu50(111). The activity would therefore obey the reverse sequence in agreement with the reactivity results.

The adsorption of carbon monoxide on rhenium: Basal (0001) and stepped 14 (0001) × (101̄1) planes

Abstract The influence of surface defects on the adsorption of CO by rhenium is investigated using LEED, AES and linear temperature programmed desorption. On both surfaces, thermal desorption reveals two adsorption states, the lower temperature α state being resolved into two substates, and one β state, all desorbing with first order kinetics. The α state is unaffected by the surface texture, its maximum population being the same on both surfaces, around 4 × 1014 molecules cm−2, similar to the value found for poly crystalline rhenium. On the other hand, the β state is strongly dependent on surface structure. On Re(0001) a maximum of 4 × 1013 molecules cm−2 was found, and 2 × 1014 molecules cm−2 on the stepped surface. The adsorption is activated and can be increased, by heating to 550 K, to 2 × 1014 molecules cm−2 on the basal plane and 3.5 × 1014 molecules cm−2 on the stepped surface. Ordered structures are now seen in LEED. Comparison of these results with previous results from polycrystalline rhenium indicate that the dissociation of β-CO on the latter surface must occur at defects other than steps.

Etude par spectroscopies d'electrons, emission ionique secondaire et reactivite chimique des modes d'adsorption du monoxyde d'azote et de la vapeur d'eau sur rhenium

Abstract The adsorption of nitric oxide as well as water is characterized, on rhenium (strongly oriented parallel to (0001) plane) by both dissociative and molecular adsorption. For nitric oxide, two types of molecular states have been detected (stable till around 500 and 750 K), while for water, decomposition into oxygen (or OH) and hydrogen atoms occurs at much lower temperatures (below 300 K). Analysis of the adsorption states, using spectroscopic methods (XPS, AES, SIMS) leads to an interpretation of temperature programmed desorption spectra, based on repulsive interaction models and competitive or independent reactions, and to an extension of our previous model of the attack of transition metals by oxygen to the gasification of rhenium by these two oxygenated molecules.

États d'adsorption, vitesses d'adsorption, de désorption de l'oxygène sur rhénium et mécanismes d'atomisatton, d'oxydation à haute température et basse pression

Abstract We have studied interactions of oxygen with rhenium at low pressures (10 −4 to 10 −6 torr) in an ultra high vacuum apparatus where all desorption products, including instable and condensible species are analyzed by mass spectrometry. The chemical reactions depend on the surface coverage which is fixed by an equilibrium between the rate of adsorption and the rates of desorption steps, varying with temperature and pressure. We have measured these rates, independently, or directly (pressure variations after a flash, thermal desorption). When the coverage is lower than 0.7, the main reaction is oxygen atomization. For higher values, desorption of atomic oxygen is progressively replaced by desorption of rhenium trioxide, ReO 3 , main product of reaction. The following mechanism, based on the kinetics of the elementary steps, experimentally isolated, gives a quantitative picture of all the phenomena, for pressures between 10 −4 to 10 −6 torr, and sample temperatures varying from 1500 to 2300°K: (a) dissociative adsorption of oxygen; (b) desorption of atomic oxygen; (c) desorption of trioxide, only formed by surface reaction . The different rates depend on coverage, frequency factors and binding or activation energies. Optimization of the results in stationary conditions give the values of these parameters. The agreement with the values obtained directly is excellent.

Ethylene monolayer and multilayer on Pt(111) below 52 K: determination of bond lengths by near-edge X-ray fine structure

Abstract At temperatures lower than 52 K, ethylene condenses on Pt(111). Valence-band analysis shows that the first layer and the condensed phase exhibit almost the same positions of the molecular orbitals. Carbon K-shell excitation leads to the appearance of π∗ and σ∗ resonances from which it is deduced that the low temperature ethylene monolayer is π-bonded to the substrate with minor C-C elongation, namely about 0.004 nm. A comparison is made with C-C bond lengths of ethylene monolayers and multilayers measured using NEXAFS on various substrates and given in the literature. It shows that they correlate nicely with the values of the πσ parameter deduced from HREELS results for the same monolayers.