NO monomer and (NO)x polymeric chain chemisorption on Pt{110}: Structure and energetics

Theoretical Study of NO Dissociative Adsorption onto 3d Metal Particles M55 (M = Fe, Co, Ni, and Cu): Relation between the Reactivity and Position of the Metal Element in the Periodic Table

NO dissociative adsorption onto 3d metal particles M55 (M = Fe, Co, Ni, and Cu) was investigated theoretically using density functional theory computations. A transition state exists at higher energy in the Cu case but at lower energy in the Fe, Co, and Ni cases than the reactant (sum of M55 and NO), indicating that Cu55 is not reactive for NO dissociative adsorption because NO desorption occurs more easily than the N-O bond cleavage in this case, but Fe55, Co55, and Ni55 are reactive because NO desorption needs a larger destabilization energy than the N-O bond cleavage. This result agrees with the experimental findings. The energy of transition state E(TS) becomes higher in the order of Fe < Co < Ni ≪ Cu. Exothermicity E exo (relative energy to the reactant) decreases in the order of Fe > Co > Ni ≫ Cu. These results indicate that the reactivity for NO dissociative adsorption decreases kinetically and thermodynamically in this order. In addition, the E(TS) and E exo values show that 3d metal particles are more reactive than 4d metal particles when a comparison is made in the same group of the periodic table. Charge transfer (CT) from the metal particle to NO increases as the reaction proceeds. The CT quantity to NO at the TS increases in the order of Cu < Ni < Co < Fe, identical to the increasing order of reactivity. The negative charges of the N and O atoms of the product (N and O adsorbed M55) increase in the order of Ni < Co < Cu < Fe, identical to the increasing order of E exo except for the Cu case; in the Cu case, the discrepancy between the order of E exo and those of the N and O negative charges arises from the presence of valence 4s electron of Cu because it suppresses the CT from N and O to Cu55. From these results, one can infer that the d-valence band-top energy of M55 plays an important role in determining the reactivity for NO dissociative adsorption. Truly, the d valence orbital energy decreases in the order of Fe > Co > Ni ≫ Cu and the 3d metal > 4d metal in the same group of the periodic table, which reflects the dependence of reactivity on the metal element position in the periodic table.

Structural Study of NO Adsorbed on the Reconstructed Pt(110)-(1 × 2) Surface with X-ray Photoelectron Diffraction and Near-Edge X-ray Absorption Fine Structure Spectroscopy

The adsorption structure of NO on the reconstructed Pt(110)-(1 x 2) surface was studied with X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low-energy scanned-angle photoelectron diffraction (LESA-PD), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The experiments were performed at 180 K, where no surface lifting from (1 x 2) to (1 x 1) takes place after NO adsorption. XPS indicates that the (1 x 2) unit cell of the Pt(110) surface contains 1.5 NO molecules at the saturated coverage. XPD and LESA-PD analyses allow us to propose a structural model for the NO adlayer, where two-thirds of the NO molecules in the (1 x 2) unit cell are adsorbed on the atop site of the close-packed Pt rows (ridges) along the [10] direction with an inclined geometry and one-third of the NO molecules adsorb on the bridge site between the Pt ridges with an upright configuration. This model is supported by the N K-edge NEXAFS experiments and is consistent with the recently reported model based on the density functional theory (Orita, H.; Nakamura, I.; Fujitani, T. J. Phys. Chem. B 2005, 109, 10312).

Studies of NO Adsorption on Pt(110)-(1×2) and (1×1) Surfaces Using Density Functional Theory

Adsorption of NO on Pt(110)-(1 x 2) and (1 x 1) surfaces has been investigated by density functional theory (DFT) method (periodic DMol(3)) with full geometry optimization and without symmetry restriction. Adsorption energies, structures, and N-O stretching vibrational frequencies of NO are studied by considering multiple possible adsorption sites and comparing with the experimental data. Adsorption is strongly dependent on both coverage and surface phase. The assignment of adsorption sites has been carried out with precise calculation of vibrational frequencies for NO on various sites. We clearly show the NO site switching on both of the surfaces as found in the experiments: at low coverages, bridge species is formed on the surface, and at high coverages, NO switches to atop sites.

The mechanism of N[sub 2]O formation via the (NO)[sub 2] dimer: A density functional theory study

Catalytic formation of N(2)O via a (NO)(2) intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)(2) and cis-(NO)(2) configurations. A possible pathway involving (NO)(2) formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N(2)O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO + H(2) reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV.