Erik Vesselli

Carbon Dioxide Hydrogenation on Ni(110)

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.

Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template

The chemical and physical properties of nanoclusters largely depend on their sizes and shapes. This is partly due to finite size effects influencing the local electronic structure of the nanocluster atoms which are located on the nanofacets and on their edges. Here we present a thorough study on graphene-supported Rh nanocluster assemblies and their geometry-dependent electronic structure obtained by combining high-energy resolution core level photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. We demonstrate the possibility to finely control the morphology and the degree of structural order of Rh clusters grown in register with the template surface of graphene/Ir(111). By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of the nanoclusters. We describe how small interatomic distance changes occur while varying the nanocluster size, substantially modifying the properties of surface atoms. The properties of under-coordinated Rh atoms are discussed in view of their importance in heterogeneous catalysis and magnetism.

Core level shifts of undercoordinated Pt atoms

We present the results of high-energy resolution core level photoelectron spectroscopy experiments paralleled by density functional theory calculations to investigate the electronic structure of highly undercoordinated Pt atoms adsorbed on Pt(111) and its correlation with chemical activity. Pt4f(7/2) core level binding energies corresponding to atoms in different configurations are shown to be very sensitive not only to the local atomic coordination number but also to the interatomic bond lengths. Our results are rationalized by introducing an indicator, the effective coordination, which includes both contributions. The calculated energy center of the valence 5d-band density of states, which is a well known depicter of the surface chemical reactivity, shows a noteworthy correlation with the Pt4f(7/2) core level shifts and with the effective coordination.

Highly under-coordinated atoms at Rh surfaces: interplay of strain and coordination effects on core level shift

The electronic structure of highly under-coordinated Rh atoms, namely adatoms and ad-dimers, on homo-metallic surfaces has been probed by combining high-energy resolution core level photoelectron spectroscopy and density functional theory calculations. The Rh3d5/2 core level shifts are shown to be proportional to the number of Rh nearest-neighbours (n = 3, 4 and 5). A more refined analysis shows that the energy position of the different core level components is correlated with the calculated changes of the individual inter-atomic bond length and to the energy changes of the d-band centre, which is known to be a reliable descriptor of local chemical reactivity.

Reverse Water–Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure

The interaction of CO, CO2, CO + H2, CO2 + H2, and CO + CO2 + H2 with the nickel (110) single crystal termination has been investigated at 10(-1) mbar in situ as a function of the surface temperature in the 300-525 K range by means of infrared-visible sum frequency generation (IR-vis SFG) vibrational spectroscopy and by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Several stable surface species have been observed and identified. Besides atomic carbon and precursors for graphenic C phases, five nonequivalent CO species have been distinguished, evidencing the role of coadsorption effects with H and C atoms, of H-induced activation of CO, and of surface reconstruction. At low temperature, carbonate species produced by the interaction of CO2 with atomic oxygen, which stems from the dissociation of CO2 into CO + O, are found on the surface. A metastable activated CO2(-) species is also detected, being at the same time a precursor state toward dissociation into CO and O in the reverse water-gas shift mechanism and a reactive species that undergoes direct conversion in the Sabatier methanation process. Finally, the stability of ethylidyne is deduced on the basis of our spectroscopic observations.

Modelling of ethanol decomposition on Pt(111): a comparison with experiment and density functional theory

Ethanol decomposition on the clean Pt(111) surface has been studied in the zero-coverage limit within the framework of the unity bond index-quadratic exponent potential (UBI-QEP) model. Previous work, both experimental and theoretical, was already available in the literature on this reaction. The system has therefore been used as a benchmark for evaluating the accuracy of the simple phenomenological UBI-QEP model. The latter allows the estimation of key reaction parameters such as adsorption energies and reaction barriers. The stability of possible dissociation intermediates has been investigated and the most probable decomposition pathway has been simulated by integration of the related rate equations. We find that the model provides good estimates for adsorption energies of mono-coordinated molecules with long bond distances and gives realistic values for dehydrogenation barriers. Poor agreement with density functional theory (DFT) is found in the estimates of C–C and C–O bond cleavage barriers, even though the results obtained are in line with the experiments. It is found that transition and final state energies obtained from the model satisfy the linear Bronsted–Evans–Polanyi relation. Temperature programmed desorption spectra and surface coverage of the adspecies as a function of the temperature have been simulated in order to provide a direct comparison with previous experimental data. A possible pathway for ethanol decomposition on Pt(111) is finally proposed on the basis of the present calculations, conciliating previous DFT and experimental results.

Nanostructured Fe–Ag electrocatalysts for the oxygen reduction reaction in alkaline media

The impregnation of Ketjen Black (C) with iron(II) and silver(II) phthalocyanines (MPc) individually or as a 1 : 1 stoichiometric mixture, followed by heat treatment at 600 °C under inert atmosphere, gave a series of novel nanostructured electrocatalysts AgPc/C(600), FePc/C(600) and FeAgPc/C(600) (ca. 3 wt% metal loadings) for the oxygen reduction reaction (ORR) in alkaline media. The catalysts were structurally characterized by XRPD, XPS, HR-TEM/STEM and chemisorption measurements. During the synthetic heat treatment of AgPc/C(600) at temperatures above 250 °C, the AgPc decomposed to form small finely dispersed carbon supported Ag nanoparticles (mean diameter 8.5 nm). This process was delayed for FeAgPc/C(600) to above 300 °C and the resulting Ag nanoparticles were much smaller (mean diameter 2.3 nm). As expected, at 600 °C the FePc/C(600) forms highly dispersed arrays of single Fe ions coordinated by four nitrogen atoms (Fe–N4 units). Electrodes coated with AgPc/C(600), FePc/C(600) and FeAgPc/C(600) were investigated for ORR in alkaline media by linear sweep voltammetry and the RRDE system was used to probe the production of HO2−. The electrochemical activity of all materials was analyzed by Tafel and Koutecky–Levich plots and the stability of each catalyst was followed using chronopotentiometry. Both Fe-containing electrocatalysts, FeAgPc/C(600) and FePc/C(600), were highly active for the ORR promoting exclusively the four electron pathway also during chronopotentiometry, while AgPc/C(600) was found to produce up to 35 mol% HO2−. Compared to FePc/C(600), the binary FeAgPc/C(600) catalyst displayed remarkably higher activity and stability. This experimental evidence could be explained in terms of a synergistic Ag–Fe interaction which results from the unique nanostructure that forms during heat treatment which consists of very finely dispersed Ag nanoparticles and Fe–N4 moieties.

Reactivity of Carbon Dioxide on Nickel: Role of CO in the Competing Interplay between Oxygen and Graphene

The catalytic conversion of carbon dioxide to synthetic fuels and other valuable chemicals is an issue of global environmental and economic impact. In this report we show by means of X-ray photoelectron spectroscopy in the millibar range that, on a Ni surface, the reduction of carbon dioxide is indirectly governed by the CO chemistry. While the growth of graphene and the carbide-graphene conversion can be controlled by selecting the reaction temperature, oxygen is mainly removed by CO, since oxygen reduction by hydrogen is a slow process on Ni. Even though there is still a consistent pressure gap with respect to industrial reaction conditions, the observed phenomena provide a plausible interpretation of the behavior of Ni/Cu based catalysts for CO2 conversion and account for a possible role of CO in the methanol synthesis process.

Structural and kinetic effects on a simple catalytic reaction: Oxygen reduction on Ni(110)

Oxygen hydrogenation at 100 K by gas phase atomic hydrogen on Ni(110) has been studied under ultrahigh vacuum conditions by temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Formation of adsorbed water and hydroxyl species was observed and characterized. The coverage of the reaction products was monitored as a function of both temperature and initial oxygen precoverage. On the contrary, when high coverage oxygen overlayers were exposed to gas phase molecular hydrogen, no hydrogenation reaction took place. The results are compared to the inverse process, exposing the hydrogen covered surface to molecular oxygen. In this case, at 100 K, simple Langmuir-Hinshelwood modeling yields an initial sticking coefficient for oxygen adsorption equal to 0.26, considerably lower than for the clean surface. Moreover, formation of hydroxyl groups is found to be twice as fast as the final hydrogenation of OH groups to water. Assuming a preexponential factor of 10(13) s(-1), an activation barrier of 6.7 kcal/mol is obtained for OH formation, thus confirming the high hydrogenating activity of nickel with respect to other transition metals, for which higher activation energies are reported. However, oxygen is hardly removed by hydrogen on nickel: this is explained on the basis of the strong Ni-O chemical bond. The hydrogen residual coverage is well described including a contribution from the adsorption-induced H desorption process which takes place during the oxygen uptake and which is clearly visible from the TPD data.

NO adsorption on Rh(100). I. Structural characterization of the adlayers

A detailed experimental and theoretical investigation of the structure of nitric oxide adsorption layers formed at 140 K on Rh(100) has been carried out by means of x-ray photoelectron spectroscopy, x-ray photoelectron diffraction, near-edge x-ray absorption fine structure, and ab initio calculations. At saturation, a single NO species is present. Both theory and experimental results indicate a bridge adsorption site for NO in this phase, with the molecules standing upright on the surface. At low NO coverage, the presence of a different molecular species is experimentally revealed and it is fully characterized by ab initio calculations. This species has been identified with a NO molecule with the molecular axis almost parallel to the surface, lying above a Rh(100) hollow site with the two atoms in asymmetric bridge sites.