D. Simon

Dependence of stretching frequency on surface coverage and adsorbate–adsorbate interactions: a density-functional theory approach of CO on Pd (111)

Abstract Total energy and stretching frequency calculations ν(C–O) are reported for the chemisorption of CO on a Pd (111) surface as a function of coverage with a periodic density-functional theory approach including generalized gradient approximation. At low [0.33 monolayer (ML)] and high (0.75xa0ML) coverages, the most stable structures always present CO in threefold fcc and hcp hollow sites, mixed with top sites for the high coverage structure, in agreement with the interpretation of the RAIRS or HREELS spectra. The calculated anharmonic frequencies for these sites (1828–1830xa0cm −1 at 0.33xa0ML) and (1893 and 2085xa0cm −1 at 0.75xa0ML) are coherent with the observed IR peaks. At medium coverage, 0.5xa0ML, the most stable model is the one with both fcc and hcp hollow sites. The observed blue shift of the frequency band between 0.33xa0ML and 0.5xa0ML is usually interpreted as a change of the chemisorption site from a hollow to a bridge adsorption. Although a direct comparison of the IR peak with the calculated anharmonic values at 0.5xa0ML for the structures with two hollow sites (1906xa0cm −1 ) or with two bridge sites (1937xa0cm −1 ) is this time more delicate, the calculations show that the observed frequency shift is fully compatible with the hollow site model and can be attributed to increased coverage and adsorbate–adsorbate interactions. The results show that two main phenomena occur when the coverage increases. First the shift caused by static interactions between adsorbates is ruled by the coverage-dependent backdonation from the surface metallic atoms towards the antibonding 2π molecular orbitals of the chemisorbed CO molecules. The second shift is due to dynamic interactions between adsorbates. The stability of a vibrational configuration taken from a particular mode is ruled by the electrostatic energy cost due to the competitive charge transfer between the adsorbates and the metallic surface.

Molecular and dissociative chemisorption of NO on palladium and rhodium (100) and (111) surfaces: A density-functional periodic study

The efforts to reduce NOx pollutants have stimulated a large interest in the understanding of the elementary processes for NO transformation on transition metal surfaces. Periodic density-functional calculations have been performed for the molecular and dissociative chemisorption of NO on Pd and Rh(100) and (111) surfaces, with generalized gradient approximation exchange-correlation functionals. The periodic systems are modeled by two-dimensional palladium or rhodium slabs with frozen geometry, on which a NO, N, O, or (N+O) adlayer is set. On Pd and Rh(100) at a coverage of 0.5 monolayer (ML), the bridge site is the most stable one with respective binding energies of −1.54 and −2.18u2009eV. On the (111) surfaces, at a coverage of 0.33 ML, the threefold hollow sites are favored with binding energies of −2.0u2009eV for Pd(111) and −2.18u2009eV for Rh(111). For the dissociated structures, the mixed coadsorption of N and O is favored in most cases compared to separated domains. The chemisorption of NO, N, or O is stronge...

VIBRATIONAL FREQUENCY AND CHEMISORPTION SITE : A DFT-PERIODIC STUDY OF NO ON PD (111) AND RH (111) SURFACES

Abstract Periodic density-functional calculations have been performed to evaluate NO stretching frequencies, for the chemisorption of NO on Pd and Rh (111) surfaces at three different coverages. The threefold fcc and hcp hollow sites are the most stable. The calculated frequency ranges for these sites (1544–1643 cm −1 ) agree with the HREELS and IR vibrational frequencies, in contrast with the usual assignment to a bridge adsorption. The NO stretching frequency shifts up with increase in coverage. The results suggest that the structural determination from the vibrational analysis using the frequency ranges of nitrosyl complexes could lead to incorrect assignments.

Structure sensitivity for NO dissociation on palladium and rhodium surfaces

Abstract We present a comparative density-functional theory study of the chemisorption and the dissociation of the NO molecule on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium. The energetic and kinetic properties of the reaction pathways are reported. The structure sensitivity is correlated to the catalytic activity, which can be linked to the calculated dissociation rate constants at 300 K: Rh (100)⩾terrace Rh (511)>step Rh (511)>step Pd (511)>Rh (111)>Pd (100)⩾terrace Pd (511)>Pd (111). The effect of the steps on the activity is found to be clearly favorable for the Pd (511) surface and unfavorable for the Rh (511) surface. The reaction barriers are correlated to the stability of the final states and the geometries of the molecular precursor states.

Breaking the NO bond on Rh, Pd, and Pd3Mn alloy (100) surfaces: A quantum chemical comparison of reaction paths

Total energy calculations have been performed within the periodic density-functional theory framework to study the dissociation of molecularly adsorbed nitrogen monoxide NO over three different catalytic surfaces: palladium, rhodium, and palladium-manganese (100). The potential energy surfaces for NO dissociation on these metallic surfaces have been calculated in order to determine the minimal energy paths. The accurate optimizations of the transition states and their characterization with a complete vibrational analysis, including the degrees of freedom of the surface, have been presented. The order of increasing activation energy barrier is Rh, Pd3Mn, and Pd. Two types of reaction paths have been found: one involving a horizontal molecular precursor state and a low activation energy barrier (Rh and Pd3Mn) and the other involving a vertical molecular state and a high activation energy (Pd). Hence the improvement of the catalytic activity for dissociating NO by alloying manganese to palladium has been exp...

Stress induced nanostructure in a Pd monolayer on Ni(1 1 0): a first principles theoretical study

The formation energy of an epitaxial Pd adlayer on Ni(1 1 0) was calculated as a function of the cell parameter in the [1 1 0] row direction. The optimal Pd–Pd distance is found to be 2.72 A, and the epitaxial stress energy of the monolayer at the Ni bulk parameter is estimated to be 90 meV/atom. A periodic reconstruction (Λ×1) with the creation of vacancies in the palladium layer is proposed to explain the experimental observations and to allow a partial relaxation of the stress. The stability of these structures as a function of the reconstruction period is studied by density functional theory plane wave calculations. The energy is minimum for structures with a typical length around 15 A. This particular stability is discussed as a competition between stress relaxation, bond breaking associated with a vacancy, and epitaxial misfit.

Analysis by the recursion method of the electronic transfers involved in the dihydrogen chemisorption on a platinum cluster

Abstract As a first example of a possible analysis of elementary steps of a heterogeneous catalytic reaction, the electronic transfers and the associated energy costs involved in the dihydrogen adsorption on various sites of a Pt13 cluster have been analysed in terms of the changes occurring in the local densities of states computed by the recursion method. The Hamiltonian defining the various metal-metal, hydrogen-hydrogen, and metal-hydrogen interactions is of extendedHuckel-type. This method gives local information on the behaviour of the surface orbitals along the reaction path. In particular, it shows the role of a late electron transfer from d-type to s or p-type metal orbitals, and it exhibits the role of electronic reservoir of the platinum atoms not directly bound to the adsorbate, in the electron transfer. Furthermore, it shows that the height of the activation barrier and the energy difference of the whole process are mainly related to the energy positions of the local HOMO and LUMO contributions of the metal atoms in direct interaction.