JoséA. Rodriguez

High pressure catalytic reactions over single-crystal metal surfaces

Abstract Studies dealing with high-pressure catalytic reactions over single-crystal surfaces are reviewed. The coupling of an apparatus for the measurement of reaction kinetics at elevated pressures with an ultrahigh vacuum system for surface analysis allows detailed study of structure sensitivity, the effects of promoters and inhibitors on catalytic activity, and, in certain cases, identification of reaction intermediates by post-reaction surface analysis. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behaviour of high-surface-area supported catalysts. Studies of CO methanation and CO oxidation over single-crystal surfaces provide convincing evidence that these reactions are structure insensitive. For structure-sensitive reactions (ammonia synthesis, alkane hydrogenolysis, alkane isomerization, water-gas shift reaction, etc.) model single-crystal studies allow correlations to be established between surface structure and catalytic activity. The effects of both electronegative (S and P) and electropositive (alkali metals) impurities upon the catalytic activity of metal single crystals for ammonia synthesis, CO methanation, alkane hydrogenolysis, ethylene epoxidation and water-gas shift are discussed. The roles of “ensemble” and “ligand” effects in bimetallic catalysts are examined in light of data obtained using surfaces prepared by vapor-depositing one metal onto a crystal face of a dissimilar metal.

The interaction of ultrathin films of Ni and Pd with W(110): an XPS study

The interaction of ultrathin films of Ni and Pd with W(110) has been examined using X-ray photoelectron spectroscopy (XPS) and the effects of annealing temperature and adsorbate coverage (film thickness) are investigated. The XPS data show that the atoms in a monolayer of Pd or Ni supported on W(110) are electronically perturbed with respect to the surface atoms of Pd(100) and Ni(100). The magnitude of the electronic perturbations is larger for Pd than for Ni adatoms. Our results indicate that the difference in Pd(3d52) XPS binding energies between a pseudomorphic monolayer of Pd on W(110) and the surface atoms of Pd(100) correlates with the variations observed for the desorption temperature of CO (i.e., the strength of the Pdue5f8CO bond) on these surfaces. A similar correlation is seen for the Ni(2p32) XPS binding energies of Ni/W(110) and Ni(100) and the CO desorption temperatures from the surfaces. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of: (1) variations that occur in the Niue5f8Ni and Pdue5f8Pd interactions when Ni and Pd adopt the lattice parameters of W(110) in a pseudomorphic adlayer; and (2) transfer of electron density from the metal overlayer to the W(110) substrate upon adsorption. Measurements of the Pd(3d52) XP binding energy of Pd/W(110) as a function of film thickness indicate that the Pdue5f8W interaction affects the electronic properties of several layers of Pd atoms.

CO adsorption isotherms on Cu(100) at elevated pressures and temperatures using infrared reflection absorption spectroscopy

Abstract Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Cu(100) surface. Adsorption isotherms were determined at CO pressures from 10−6 to 10 Torr, and at temperatures from 115 to 340 K, and the isosteric heats of adsorption (δEads) evaluated as a function of CO coverage. For increasing CO coverages between 0-0.15 monolayers (ML), δEads decreases sharply from 16.7 to 12.7 kcal/mol. From 0.15 to 0.35 ML, δEads remains approximately 12.7 kcal/mol and exhibits little coverage dependence. These results are in excellent agreement with previously reported data for the CO/Cu(100) system acquired at much lower pressures ( 10−4, significant bathochromic shifts of the CO frequency to lower wavenumbers are observed.

Adsorption of CO, H2, O2 and CO2 on clean and Cu-covered Re(0001): an XPS study

Abstract The electronic and chemical properties of ultrathin films of Cu on Re(0001) have been investigated by means of X-ray photoelectron spectroscopy (XPS) and chemisorption of CO, H 2 , O 2 and CO 2 . A very similar Cu(2p 3 2 ) binding energy is obtained for a monolayer of Cu on Re(0001) and the surface atoms of Cu(100). Measurements of the Cu(2p 3 2 ) XPS peak position of Cu/Re(0001) as a function of film thickness show convergence to bulk properties for films with 4–5 layers of Cu atoms. Chemisorption of CO induces a large decrease in the electron density of the Cu adlayers. This is a consequence of: (1) charge transfer from the Cu overlayer to the Re(0001) substrate (induced by a repulsive interaction between the Cu σ charge and the electrons in the 5σ orbital of CO) and (2) transfer of electrons from the Cu adatoms into the unoccupied 2π orbitais of the CO molecules (π back-bonding). The Cuue5f8Re interaction enhances the electron donor capabilities of Cu atoms supported on Re(0001), making them more active for CO adsorption and CO 2 dissociation. Hydrogen adatoms shift the Cu(2p 3 2 ) XPS peak position of Cu/Re(0001) surfaces toward higher binding energies. The present results indicate that spillover of hydrogen from Re to Cu can occur in mixed Cu/Re catalysts.

A FT-IRAS study of ammonia adsorbed on Ru(0001)

Abstract The interaction between ammonia and Ru(0001) has been studied by means of Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS). Chemisorption of NH 3 on Ru(0001) enhances the IR cross section of the umbrella mode of the molecule. For the first adsorption layer, changes in the IR intensity of the umbrella mode correlate with variations in the orientation of the molecules observed in ESDIAD and work function measurements. Formation of hydrogen bonds between first- and second-layer NH 3 molecules reduces drastically the IR cross section of the umbrella mode of chemisorbed ammonia.

The effects of CO, H, NH3, CH3OH, H2O, and C2H4 on the electronic properties of ultrathin Cu films supported over Ru(0001): an XPS study

Abstract The interaction of CO, H, NH 3 , CH 3 OH, H 2 O, and C 2 H 4 with ultrathin Cu films supported on Ru(0001) has been studied by means of XPS and TPD. For films with θ Cu ⪕1, adsorption of CO, C 2 H 4 , and H induced shifts of +0.5, +0.3, and +0.25 eV, respectively, in the Cu(2p 1 2 ) peak position. Negligible shifts were observed upon adsorption of NH 3 , CH 3 OH, and H 2 O. The XPS results are consistent with a model in which the electron density transferred from a Cu monolayer to the adsorbates follows the trend: CO > C 2 H 4 , H>NH 3 , CH 3 OH, H 2 O.