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 gaseous CO reacts with preadsorbed oxygen adatoms, the CO2 produced has an excess energy of 16±8 kJ/mol.

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons-higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.

The nanoscale description of the reaction pathways and of the role of the intermediate species involved in a chemical process is a crucial milestone for tailoring more active, stable, and cheaper catalysts, thus providing “reaction engineering” capabilities. This level of insight has not been achieved yet for the catalytic hydrogenation of CO2 on Ni catalysts, a reaction of enormous environmental relevance. We present a thorough atomic-scale description of the mechanisms of this reaction, studied under controlled conditions on a model Ni catalyst, thus clarifying the long-standing debate on the actual reaction path followed by the reactants. Remarkably, formate, which is always observed under standard conditions, is found to be just a “dead-end” spectator molecule, formed via a Langmuir−Hinshelwood process, whereas the reaction proceeds through parallel Eley−Rideal channels, where hydrogen-assisted C−O bond cleavage in CO2 yields CO already at liquid nitrogen temperature.

We demonstrate that the key step for the reaction of CO 2 with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO 2 is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H-CO 2 complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO 2 hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.

We show that dissociative oxygen adsorption on Ag͑001͒ induces below room temperature a missing row 2ͱ2ϫͱ2 reconstruction of the substrate. As demonstrated by the analysis of the photoelectron diffraction patterns, the oxygen atoms sit thereby in a c(2ϫ2) arrangement in the previous fourfold hollow sites nearly coplanar with the Ag atoms, while rows of substrate atoms are removed along the ͓100͔ directions. Annealing the crystal above 350 K restores the p(1ϫ1) symmetry and the oxygen moves to 0.6 Å above the fourfold hollow site. It becomes then more oxidic in nature, as demonstrated by the shift of the O 1s level from 530.3 eV to 528.3 eV. The phase transition affects also the O 2s and O 2p levels as well as the surface component of Ag 3d 5/2 . The vibrational frequency of the oxygen adatoms against the surface decreases at the phase transition, in accord with the larger adsorption distance. The higher temperature phase is active towards CO and C 2 H 4 oxidation, while the low-temperature phase is not. When cooling the sample below room temperature the reconstructed phase is restored. The time constant of this process as well as the chemical reactivity of the high-temperature phase are weakly reproducible since they depend on the previous history, i.e., presumably on the subsurface oxygen content of the sample.

Calorimetric heats of adsorption and sticking probabilities are reported for NO and CO on both the reconstructed hex and the unreconstructed ͑1ϫ1͒ surfaces of Pt͕100͖ by single crystal adsorption calorimetry ͑SCAC͒, at room temperature. The hex surface reverts to the ͑1ϫ1͒ structure during adsorption of both gases, as previously reported. The initial heat of adsorption on the ͑1ϫ1͒ surface is 215 kJ/mol for CO and 200 kJ/mol for NO. Adsorbate-adsorbate interactions determine not only the dependence of the heat of adsorption on coverage but also the formation of different ordered structures. A model is suggested to explain the observed dependence of the differential heat on coverage and the LEED patterns, and a Monte Carlo simulation is performed to derive the corresponding differential heat, thus allowing estimates to be made of the magnitude of adsorbateadsorbate interactions. For CO adsorption, the critical contribution is the pairwise interaction energy ⑀ d between molecules in nnn sites while for NO triplet formation is suggested with significant repulsive interaction between molecules in the same triplet ͑⑀ t ͒ and an even stronger repulsion between triplet pairs ͑⑀ tt ͒. NO-NO repulsive interactions ͑⑀ t ϭ20 kJ/mol, ⑀ tt ϭ80 kJ/mol͒ are considerably stronger than CO-CO interactions ͑⑀ d ϭ5 kJ/mol͒; thus, at half monolayer coverage CO gives rise to a c͑2ϫ2͒ pattern while NO gives a c͑2ϫ4͒ pattern. Moreover, with CO the coverage can be increased to 0.75 ML, with the formation of compressed structures, while for NO the saturation coverage is just 0.5 ML. The differential heat on the hex surface is also discussed showing the possible role of adsorption at defect sites in the energetics of the system. The surface energy difference between the clean ͑1ϫ1͒ and hex surfaces is obtained as 20 kJ͑mol Pt s ͒ Ϫ1 by comparing the integral heats of adsorption of CO on both surfaces at ϭ0.5, when the final states of the two systems are identical.

We present a combined experimental and theoretical study of the CO(2) interaction with the Ni(110) surface. Photoelectron spectroscopy, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy measurements are performed at different coverages and for increasing surface temperature after adsorption at 90 K with the aim to study the competing processes of CO(2) dissociation and desorption. Simulations are performed within the framework of density functional theory using ab initio pseudopotentials, focusing on selected chemisorption geometries, determining the energetics and the structural and vibrational properties. Both experimental and theoretical vibrational frequencies yield consistent indications about two inequivalent adsorption sites that can be simultaneously populated at low temperature: short-bridge site with the molecular plane perpendicular to the surface and hollow site with the molecular plane inclined with respect to the surface. In both sites, the molecule has pure carbon or mixed oxygen-carbon coordination with the metal and is negatively charged and bent. Predicted energy barriers for adsorption and diffusion on the surface suggest a preferential adsorption path through the short-bridge site to the hollow site, which is compatible with the experimental findings. Theoretical results qualitatively support literature data concerning the increase of the work function upon chemisorption

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