D.A. King

Calorimetric heats for CO and oxygen adsorption and for the catalytic CO oxidation reaction on Pt{111}

Single crystal adsorption calorimetry was applied to investigate the heats of adsorption of CO and oxygen and the reaction heats for the CO oxidation process on Pt{111} at room temperature. Both sticking probabilities and heats of adsorption for CO and oxygen are presented as a function of coverage. These results are used to interpret the subsequent measurements taken for the CO oxidation process on the same surface. The initial heats of adsorption of CO and oxygen on Pt{111} are 180±8 and 339±32 kJ/mol, respectively. In addition the pairwise lateral repulsive interaction between CO molecules in a (√3×√3)R30° ordered layer at θ=1/3 is found to be 4 kJ/mol. A detailed Monte Carlo modeling of the dissociative adsorption and sticking probability of oxygen on Pt{111} is performed. The initial rapid fall in heat is attributed to adsorption on defect sites, and subsequent adsorption on the planar {111} surface proceeds with a third neighbor interaction energy between the oxygen adatoms ω3∼22 kJ/mol. When gaseou...

Molecular adsorbate-induced surface reconstruction: CO/Pd{110}

Abstract CO adsorption on Pd{110} has been studied using reflection-absorption infrared spectroscopy (RAIRS), low energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS) over the temperature range 180 to 300 K. A unique adsorption sequence occurs involving a CO-induced reconstruction of the Pd{110} surface, transforming the surface from the bulk truncation Pd{110}(1×1) structure to a missing row Pd{110}(1×2) structure over the coverage range 0.3 to 0.75 of a monolayer, driven by the higher heat of adsorption of CO on the (1×2) Pd surface. Increasing coverage to saturation results in the lifting of the missing row structure and regeneration of the bulk truncation Pd{110}(1×1) surface. This is the first conclusive report of a substantial surface reconstruction induced by a molecular adsorbate.

Calorimetric heats of adsorption for CO on nickel single crystal surfaces

An adsorption calorimeter for studies on well‐defined single crystal surfaces under ultrahigh vacuum conditions is now available, based on supersonic molecular beam dosing onto ultrathin metal single crystals. Here we discuss the relationship between the calorimetric heat of adsorption as measured in this system and the related parameters: the differential heat of adsorption, the isosteric heat, and the Arrhenius desorption energy. Coverage‐dependent calorimetric heats of adsorption and sticking probabilities for CO on Ni{111}, {110}, and {100} are presented, and comparisons made with literature values for isosteric heats and Arrhenius desorption energies. At intermediate coverages some significant discrepancies occur which are attributed to a temperature‐dependent adlayer structure. By combining sticking probability with heat measurements at high coverage, at 300 K, where significant desorption occurs, the desorption preexponential has been accurately determined; differential entropies of adsorption are ...

Oxygen chemisorption and oxide film growth on Ni{100}, {110}, and {111}: Sticking probabilities and microcalorimetric adsorption heats

Using single-crystal adsorption calorimetry, heat data have been measured for the adsorption of oxygen on the three low-index planes of Ni at 300 K along with corresponding sticking probabilities. New data are presented with coadsorbed potassium on each plane, and temperature-dependent data for O2/Ni{100}. The initial heats of adsorption of oxygen on Ni{100}, {110}, and {111} are 550, 475, and 440 kJ (mol O2)−1, respectively, at 300 K, and the heat is found to drop rapidly with coverage in the chemisorption regime, indicating strong interadsorbate interactions. However, this rapid decline is not seen with coadsorbed potassium, a difference discussed both in terms of electron availability and coadsorbate attractions. The integral heats of adsorption for oxide film formation are 220, 290, and 320 kJ mol−1, respectively. Corresponding sticking probability measurements show initial values, all less than unity, of 0.63, 0.78, and just 0.23, again for the {100}, {110}, and {111} surfaces in that order. The cove...

Calorimetric Measurement of the Energy Difference Between Two Solid Surface Phases

A recently designed single-crystal surface calorimeter has been deployed to measure the energy difference between two solid surface structures. The clean Pt{100} surface is reconstructed to a stable phase in which the surface layer of platinum atoms has a quasi-hexagonal structure. By comparison of the heats of adsorption of CO and of C2H4 on this stable Pt{100}-hex phase with those on a metastable Pt{100}-(1x1) surface, the energy difference between the two clean phases was measured as 20 � 3 and 25 � 3 kilojoules per mole of surface platinum atoms.

Energetics, geometry and spin density of NO chemisorbed on Pt{111}

Abstract Ab initio total energy calculations using spin density functional theory with the generalised gradient approximation have been performed for NO chemisorption on Pt{111}. Chemisorbed NO loses its spin identity but induces some weak magnetic changes in the slab. At 0.25 ML coverage, NO is adsorbed upright with its molecular axis normal and with N close to the surface at all the high symmetry sites. The strength of the chemisorption bond is in the following order: fcc > hcp > bridge >> atop. The diffusion barriers from hcp to fcc sites are 0.22 and 1.02 eV across the bridge and the atop sites, respectively.

An automated tensor LEED analysis of the Ni{111}-c(4×2)-2CO structure

Abstract The Ni{111}-c(4×2)-2CO system has been studied using fully dynamic low-energy electron diffraction. CO is found to occupy both hcp and fcc threefold hollow sites. This is in contrast to earlier conclusions from vibrational spectroscopy where assignment was made to bridge sites. This study also represents an improvement over earlier SEXAFS and photoelectron diffraction studies in that the full surface geometry is obtained revealing significant bucklings of the nickel first and second surface layers and points to possible bends and tilts of the CO molecules.

Single crystal adsorption microcalorimetry

Abstract An ultrahigh vacuum microcalorimeter has been developed which enables calorimetric heats of adsorption to be obtained on single-crystal surfaces, as a detiled function of coverage. The system comprises a pulsed supersonic molecular beam source, an ultrathin metal single crystal, and remote infrared temperature sensing. Sticking probabilities and coverages are determined pulse-wise by the King and Wells method, and heat capacity calibrations are conducted in situ by laser beam pulsing. Results for oxygen on Ni{110} demonstrate excellent sensitivity to 0.01 monolayer pulses.

Breakdown of adsorbate site assignment from vibrational frequencies. NO on Ni(111) re-visited by tensor LEED

Abstract The c(4×2)NO-Ni(111) system has been studied using fully dynamic low-energy electron diffraction. NO is found to occupy both hcp and fcc threefold hollow sites on the surface as proposed in an earlier SEXAFS study. This is in contrast to earlier conclusions from vibrational spectroscopy where assignment was made to the bridge sites, and to a recent X-ray photoelectron diffraction study which led to an assignment to all fcc like sites. Both of these structures can be ruled out on symmetry grounds alone. The current assignment underlines a dramatic failure of site assignment by vibrational spectroscopy, indicating that adsorbate site assignment for NO cannot be simply based on group frequencies obtained from organometallic compounds.

Characterization and orientation of adsorbed NO dimers on Ag{111} at low temperatures

Reflection‐absorption infrared spectroscopy (RAIRS) and near‐edge x‐ray absorption fine structure(NEXAFS) have been used, with isotopic 14NO/15NO mixtures, to determine the structure and orientation of the monolayer species formed by NO adsorption on Ag{111} at 40 to 60 K. The adlayer is composed of NO dimers bonded with the N–N axis in the surface plane and with the molecular plane tilted away from the surface normal by about 30°. This structure provides a simple basis for understanding the facile reaction to adsorbed N2O and O which occurs on heating to 70 to 90 K.