Rickmer Kose

Energetics and kinetics of the interaction of acetylene and ethylene with Pd{100} and Ni{100}

Abstract The adsorption and reactions of C2H4and C2H2on Ni{100} and Pd{100} at room temperature have beeninvestigated by single-crystal adsorption calorimetry (SCAC). On Ni{100}, the initial heats of adsorption are 203and 264 kJ mol−1, respectively. From the correlation of the heat changes with the stable species formed (CH and/orCCH), a mean value of #204 kJ mol−1 is extracted for the NiMC bond energy. On Pd{100} acetylene adsorbsmolecularly in a rehybridized state with initial heat of 112 kJ mol−1, corresponding to a PdMC bond energy of#177 kJ mol−1. The kinetics of adsorption are similar for C2H2on Ni{100} and Pd{100} and C2H4on Ni{100},exhibiting precursor-mediated adsorption with high initial sticking probability, while C2H4adsorbs reversibly onPd{100} at room temperature. Nickel and palladium surfaces are known to favour the cleavage of CMC bonds,whereas platinum surfaces do not. This is not related to diVerences in metalMcarbon bond energies, and must insteadbe attributed to a kinetic origin.

Adsorption energetics and bonding from femtomole calorimetry and from first principles theory

Abstract The advent of an accurate, sensitive single-crystal adsorption calorimeter (SCAC) in 1991 meant that, for the first time, a general tool was available for determining the energetics of surface processes, both reversible and irreversible, under well-defined conditions. This is particularly valuable when the structure of the final state of the system, adsorbate and substrate, is well-characterized. Concurrently, first principles density functional theory (DFT) slab calculations were developed, particularly during the past six years, to the point where good comparisons can now be made with experimental results from the most reliable surface structure analyses. Results from the SCAC provide the most stringent benchmark currently available for these calculations: the total energy. Here, we provide a review of the current state of the art in both experimental and theoretical studies of the energetics of adsorption and surface reactivity, including an exhaustive comparison of data for which experimental SCAC data and DFT slab results are available for the same systems. We demonstrate how the current understanding of chemical bonding and reactivity at surfaces has been transformed through the use of these techniques.

Calorimetric measurements of the adsorption heat for ethene on Pt{211} and Pt{311}

The sticking probability and heat of adsorption for C2H4 on the stepped Pt(211) and Pt(311)-(1 x 2) surfaces have been measured for the first time. Adsorption on Pt(211) leads to the formation of several different species as a function of coverage, whereas adsorption on Pt(311)-(1 x 2) leads to the formation of only one species on the surface over the whole coverage range. The initial heats of adsorption for C-2,H-4 on Pt(211) and Pt(311)-(1 x 2) have been measured as 180 and 220 kJ mol(-1), respectively. The initial sticking probabilities are 0.84 for both surfaces. The most likely species to be formed on Pt(211) as a function of coverage are quad-a acetylene followed by ethylidyne whereas on Pt(311)-(1 x 2) ethylylidyne forms at all coverages

Energetics and CO-induced lifting of a (1×2) surface reconstruction observed on Pt{311}

Abstract Despite expectations from the literature, clean Pt{311} is shown to exhibit a (1×2) reconstruction, and CO adsorption leads to a lifting of this reconstruction to a (1×1) Pt structure. The reconstruction and its lifting are similar to those observed for CO on Pt{110}. A (1×1) island nucleation process occurs above a critical coverage of around 0.16 ML. Sticking probabilities and calorimetric adsorption heats are reported, and from the data the energy difference between Pt{311}-(1×1) and (1×2) is estimated to be in the order of ∼10 kJ mol −1 .

The role of adsorption heats and bond energies in the assignment of surface reaction products : ethyne and ethene on Ni{110}

In order to understand the complex dissociation processes that occur on adsorption of hydrocarbons on surfaces, it is necessary to understand the energetics involved. From investigations of the adsorption of various hydrocarbon species on surfaces, it has been possible to calculate M-C bond energies. These could then be used to calculate the expected adsorption heats for various possible adsorbed species on surfaces for the assignment of adsorbate states from measured heats in conjunction with spectroscopic data. Heats of adsorption and sticking probabilities for C2H2 and C2H4 on Ni{110} at 300 K have been measured. The initial sticking probability and heat of adsorption for C2H2 are 0.8 and 190 kJ mol(-1) respectively, while those for C2H4 are 0.78 and 120 kJ mol(-1). In both cases, CCH species are formed on the surface initially, and for the adsorption of C2H2, CH2 and CH are formed on the surface at higher exposures. Assuming the presence of these species on the surface, a value of the Ni-C bond strength of 191 kJ mol(-1) is found. This is in excellent agreement with the average value of 204 kJ mol(-1) calculated for hydrocarbon adsorption on the Ni{100} surface previously

Calorimetric heats of dissociative adsorption for O2 on Rh{100}

Using single crystal adsorption calorimetry (SCAC), the coverage dependent heats of adsorption and sticking coefficients for oxygen on Rh(100) were measured. This is the first time that SCAC experiments have been performed on a Rh crystal. In conjunction with results obtained by other techniques, it is possible to assign different regimes in the measured heat and sticking curves to different phases in the adsorption process. These are identified with the formation of particular adsorbate structures, namely the p(2 x 2) and the c(2 x 2) and (2 x 2)p4g structures, respectively. The initial heat of adsorption was found to be 386 kJ mol(-1). and the initial sticking coefficient 0.74

Microcalorimetric study of ethylene adsorption at 300 K on Pt{100}-hex and Pt{100}-(1×1)

Abstract Coverage-dependent heats of reaction and sticking probabilities have been measured for ethylene on Pt{100}-hex and Pt{100}-(1×1) at room temperature. The initial heats of reaction are 213 and 305 kJ/mol respectively, and the corresponding initial sticking probabilities are 0.75 and 0.71. Upon adsorption, C2H2, C2H3 and C2H4 species form on the surface, depending on the symmetry of the surface and the coverage. Average C–Pt bond strengths are extracted from the known structures of the species and the measured calorimetric heats. Within the accuracy of the measurement, these values are relatively insensitive to the surface structure, with a mean value of about 241 kJ/mol. There appears to be a systematic decrease in the bond dissociation energy as the number of Pt–C single bonds per adsorbate molecule increases from 2 to 4.

Determination of the Rh-C bond energy for C2H2 and C2H4 reactive adsorption on Rh{100}

Coverage dependent heats of adsorption and sticking probabilities for ethylene and acetylene on Rh{100} have been measured. For C2H2 and C2H4, the initial heats of adsorption are 210 +/- 10 and 175 +/- 10 kJ mol(-1), and the initial sticking probabilities are 0.86 +/- 0.01 and 0.88 +/- 0.01, respectively. No literature is available on the adsorption of acetylene on Rh{100}. However, it is known that ethylene adsorption on Rh{100} at 300 K produces sigma-bonded C=CH, and the corresponding Rh-C bond energy is therefore estimated as similar to 268 kJ mol(-1). This is used to determine the species most likely to be formed upon acetylene adsorption