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The interaction between metal particles and the support in heterogeneous catalysis has been the subject of a large number of studies. While strong metal–support interactions can lead to deleterious...

CO2 hydrogenation to methanol and to other alcohols constitutes an appealing route to recycle the large amount accumulated in the atmosphere through fossil-derived fuels burning. However, CO2 high chemical stability makes the overall process difficult and appropriate catalysts are needed. Transition metal carbides, either as active phase or as a support for noble metal clusters, have been shown to be able to activate CO2. Here, the mechanism involved in the decomposition of H2 and CO2 on many early transition metal carbides (TMC) surfaces is analyzed with the help of density functional theory (DFT) based calculations complemented by key experiments. Results show that H2 dissociation on VC and δ-MoC is unlikely, that TiC and ZrC are more reactive leading to an exothermic but activated process and that the C:Mo ratio is determinant factor since H2 dissociation on β-Mo2C(001) surface is even more exothermic. The DFT based calculations also show that CO2 adsorption on TMC results in an activated species with TMC → CO2 charge transfer, C–O bond elongations and OCO bending. Supporting Cu4 and Au4 clusters on TMCs(001) surfaces leads to more active catalysts due to the induced charge polarization. For H2 dissociation, TiC appears to be the best support, enhancing both H2 thermodynamics and kinetics. CO2 is strongly adsorbed on supported Cu4 and Au4 clusters, and the adsorption energy strength correlates with the methanol formation rate: Cu4/TiC(001) > Au4/TiC(001) > Cu/ZnO(001) ≫ Cu(111), thus providing potential alternative catalysts for methanol synthesis, in principle dozens of times better than commercial Cu/ZnO based catalysts.

Recent studies dealing with the structural, electronic, chemical and catalytic properties of well-defined bimetallic surfaces are reviewed. LEED and STM show that two metals interacting on a surface can form compounds with structures not seen in bulk alloys. Many novel phenomena related to the kinetics of growth of metals on metals have been discovered. The knowledge gathered in this area provides a solid basis for the synthesis of new materials with applications in areas of catalysis, electro-chemistry and microelectronics. In many cases, the formation of a surface bimetallic bond induces large changes in the band structure of the metals. For surfaces that contain transition or s,p metals, the strongest metal-metal interactions occur in systems that combine a metal with a valence band almost fully occupied and a metal in which the valence band is almost empty. A very good correlation is found between the electronic perturbations in a bimetallic system and its cohesive energy. Bimetallic bonds that display a large stability usually involve a significant redistribution of charge around the metal centers. The electronic perturbations affect the reactivity of the bonded metals toward small molecules (CO, NO, H2, O2, S2, C2H4, CH3OH, etc.). For supported monolayers of Ni, Pd, Pt and Cu a correlation is observed between the shifts in surface core-level binding energies and changes in the desorption temperature of CO from the metal adlayers. Examples are provided which demonstrate the utility of single-crystal studies for understanding the role of “ensemble” and “ligand” effects in bimetallic catalysts.

The nature of the electronic and chemical properties of ultrathin Pd films on Re(0001) and Ru(0001) has been studied using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and CO chemisorption. The results indicate that the Pd(3${\mathit{d}}_{5/2}$) binding energy for a monolayer (ML) of Pd on Re(0001) and Ru(0001) is perturbed by +0.60 and +0.30 eV, respectively, from that of the surface atoms of Pd(100). These electronic perturbations induce large changes in the chemical properties of the Pd films. TPD results indicate that the desorption temperature of CO from 1 ML of Pd on Re(0001) and Ru(0001) is \ensuremath{\sim}120 K lower than the corresponding desorption temperature from Pd(100). The XPS and CO-TPD data indicate that Pd transfers charge to the Re and Ru substrates, becoming electron deficient and less efficient at \ensuremath{\pi} backdonation toward CO. By comparison of these results with those reported previously for Pd, Ni, and Cu adlayers, a correlation is observed among the electronic perturbations of the adlayers, the cohesive metal-substrate bond strength, the ability of the film to chemisorb CO, and the CO-induced shift in the metal core-level binding energy. In general, the results indicate that the formation of a metal-metal bond at a surface leads to a gain of electron density by the element initially having the greater fraction of empty states in its valence band. This behavior is completely contrary to that seen in bulk alloys, likely a consequence of the anisotropic character of a surface that changes the relative electronegativities of the metal atoms. On the basis of these results, a qualitative scale of surface electronegativities is developed, showing trends that are very different from those found in three-dimensional bulk alloys.

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 atomic structure and electronic properties of Cun nanoclusters (n = 4, 6, 7, and 10) supported on cubic nonpolar δ-MoC(001) and orthorhombic C- or Mo-terminated polar β-Mo2 C(001) surfaces have been investigated by means of periodic density functional theory based calculations. The electronic properties have been analyzed by means of the density of states, Bader charges, and electron localization function plots. The Cu nanoparticles supported on β-Mo2 C(001), either Mo- or C-terminated, tend to present a two-dimensional structure whereas a three-dimensional geometry is preferred when supported on δ-MoC(001), indicating that the Mo:C ratio and the surface polarity play a key role determining the structure of supported clusters. Nevertheless, calculations also reveal important differences between the C- and Mo-terminated β-Mo2 C(001) supports to the point that supported Cu particles exhibit different charge states, which opens a way to control the reactivity of these potential catalysts.

The chemisorption and dehydrogenation of cyclohexene on clean and bismuth-covered Pt(111) was studied using thermal desorption mass spectrometry (TDS), X-ray photoelectron spectroscopy (XRS), and Auger electron spectroscopy. Four different molecular cyclohexene desorption states appear in the thermal desorption spectra of cyclohexene on clean Pt(111). The temperatures and activation energies for desorption of these states are 158 K (9.1 kcal/mole), 190 K (11.0 kcal/mole), 255 K (15.0 kcal/mole), and 300 K (17.9 kcal/mole). The XPS data indicate that about 4.5 Pt(111) surface atoms are required on average to accommodate each adsorbed cyclohexene molecule. At low coverages (ΘC6H10 < 0.05 molecule per Pt atom) all the adsorbed cyclohexene dehydrogenates upon heating to produce absorbed benzene (at temperatures below 350 K), which further decomposes on the surface by 800 K to give graphitic carbon and H2. At high coverages of either cyclohexene or coadsorbed bismuth, the adsorbed cyclohexene and the product benzene molecularly desorb from the surface. A different intermediate in cyclohexene dehydrogenation is stabilized at these high coverages (perhaps π-allyl c-C6H9,a). During the TDS experiments on clean or Bi-dosed Pt(111), neither cyclohexadiene nor products of CC bond breaking were detected with the mass spectrometer. The blocking of Pt surface sites with coadsorbed Bi adatoms, which have only minor electronic influences on the Pt sites, showed that the rate constants for diffusion and dehydrogenation of cyclohexene on Pt(111) are considerably larger than that for desorption. As a consequence, the effective ensemble requirement for cyclohexene dehydrogenation is relatively small, especially compared to benzene and cyclohexane.

Molybdenum carbides are increasingly used in heterogeneously catalyzed hydrogenation reactions, which imply the adsorption and dissociation of molecular hydrogen. Here a systematic density functional theory based study, including or excluding dispersion terms, concerning the interaction and stability of H2 with cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces, is presented. In the latter case the two possible C or Mo terminations are considered. In addition, different situations for the H covered surfaces are examined. Computational results including dispersive forces predict an essentially spontaneous dissociation of H2 on β-Mo2C(001) independently of the surface termination, whereas on δ-MoC(001) molecular hydrogen dissociation implies a small but noticeable energy barrier. Furthermore, the ab initio thermodynamics formalism has been used to compare the stability of different H coverages. Finally, core level binding energies and vibrational frequencies are presented with the aim to assist the interpretation of yet unavailable data from X-ray photoelectron and infrared spectroscopies.

The ever growing increase of CO2 concentration in the atmosphere is one of the main causes of global warming. Thus, CO2 activation and conversion towards valuable added compounds is a major scientific challenge. A new set of Au/δ-MoC and Cu/δ-MoC catalysts exhibits high activity, selectivity, and stability for the reduction of CO2 to CO with some subsequent selective hydrogenation towards methanol. Sophisticated experiments under controlled conditions and calculations based on density functional theory have been used to study the unique behavior of these systems. A detailed comparison of the behavior of Au/β-Mo2C and Au/δ-MoC catalysts provides evidence of the impact of the metal/carbon ratio in the carbide on the performance of the catalysts. The present results show that this ratio governs the chemical behavior of the carbide and the properties of the admetal, up to the point of being able to switch the rate and mechanism of the process for CO2 conversion. A control of the metal/carbon ratio paves the road for an efficient reutilization of this environmental harmful greenhouse gas.