Coadsorption Of Hydrogen Research Articles

Ab Initio Surface Phase Diagrams for Coadsorption of Aromatics and Hydrogen on the Pt(111) Surface

Supported metal catalysts are commonly used for the hydrogenation and deoxygenation of biomass-derived aromatic compounds in catalytic fast pyrolysis. To date, the substrate–adsorbate interactions under reaction conditions crucial to these processes remain poorly understood, yet understanding this is critical to constructing detailed mechanistic models of the reactions important to catalytic fast pyrolysis. Density functional theory (DFT) has been used in identifying mechanistic details, but many of these works assume surface models that are not representative of realistic conditions, for example, under which the surface is covered with some concentration of hydrogen and aromatic compounds. In this study, we investigate hydrogen-guaiacol coadsorption on Pt(111) using van der Waals-corrected DFT and ab initio thermodynamics over a range of temperatures and pressures relevant to bio-oil upgrading. We find that relative coverage of hydrogen and guaiacol is strongly dependent on the temperature and pressure o...

Vibrational sum-frequency generation study of the CO2 electrochemical reduction at Pt/EMIM-BF4 solid/liquid interfaces

Electrochemical reduction of CO2 on polycrystalline Pt catalysts in a 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM]BF4 room-temperature ionic liquid (RTIL) is not impaired by well-known effects such as high ohmic losses or vigorous hydrogen evolution reactions. Vibrational broadband sum-frequency generation (SFG) combined with cyclic voltammetry has been applied to identify the formation of a [CO2-EMIM] complex at the interface which gives rise to a SFG active vibrational mode at 2355cm−1. The [CO2-EMIM] complex serves as a stable precursor to CO2 reduction and is linked to a remarkable small potential barrier for CO2 reduction and subsequent CO production. Vibrational SFG spectra reveal CO linearly bonded to Pt atop sites as the predominant surface-adsorbed reduction product. Extended electrochemical reduction of CO2 results in an intense CO band and an electrochemical Stark tuning rate of 33cm−1/V at potentials >−0.2V vs. SHE which is indicative of a close-packed CO adlayer. Anomalous Stark tuning for potentials <−0.2V is observed and attributed to hydrogen co-adsorption from residual H2O into the CO adlayer. CO modification of Pt leads to considerable reduction of the observed reduction current density that is caused by a poisoning effect of the CO adlayer that persists at the interface for +0.5 to −0.8V.

Ionic Adsorbate Structures on Metal Electrodes Calculated from First-Principles

The equilibrium coverage of metal electrodes in contact with an electrolyte has been addressed by periodic quantum-chemical calculations based on density functional theory. The electrolyte has been treated in a grand-canonical approach using the concept of the computational hydrogen electrode. After briefly reviewing the theory and illustrating it using the coadsorption of bromide and hydrogen as an example, the interaction of nitrate with a Pb(111) electrode is addressed. A spontaneous reconstruction of the uppermost Pb layer is observed leading to the formation of a salt-like layer.

Hydrogen and halide co-adsorption on Pt(111) in an electrochemical environment: a computational perspective

The adsorbate structures on electrode surfaces in an electrochemical environment are controlled by thermodynamic parameters such as temperature, concentration, pH and electrode potential. Knowledge of these structures is important as specifically-adsorbed ions on an electrode impact catalytic reactions that take place at the electrode-electrolyte interface. From a theoretical point of view, the equilibrium structures of adsorbates can be conveniently estimated using the concept of the computational hydrogen electrode. Here we extend this concept to determine equilibrium co-adsorption structures of halides with hydrogen on Pt(111) as a function of the corresponding electrochemical potentials. We find that hydrogen-halide co-adsorption is of a competitive character, which means that mainly dense-packed structures of either halides or hydrogens are stable on the surface, in good agreement with experiment.

H3PO3 electrochemical behaviour on a bulk Pt electrode: adsorption and oxidation kinetics

Polybenzimidazole-type polymer doped with H3PO4 is commonly used as the proton-conductive phase in high-temperature proton-exchange membrane fuel cells. However, H3PO4 is not stable during fuel cell operation and undergoes reduction by hydrogen on a Pt surface to phosphorus compounds in a lower oxidation state, such as H3PO3. In this work the kinetics of H3PO3 oxidation on Pt electrode was studied, including an investigation of H4P2O6 as a possible oxidation intermediate. H3PO3 adsorption in hydrogen underpotential deposition region was described by a triple Langmuir isotherm corresponding to adsorption on specific Pt crystalline planes. Co-adsorption of hydrogen as well as SO42−, HSO4− ions decreased the total amount of adsorbed H3PO3. The determined apparent charge transfer coefficients of H3PO3 anodic oxidation on a metallic Pt surface were found to be concentration and temperature-dependent, indicating that the nature of the anodic process is complex. From chronopotentiometric measurements of H3PO3 and H4P2O6 oxidation on a preoxidised Pt surface it was concluded that, while H3PO3 is oxidised by means of a chemical reaction with PtOx, H4P2O6 undegoes anodic oxidation on the PtOx surface. According to voltammetry and bulk electrolysis experiments H4P2O6 is not formed as an intermediate product during electrochemical oxidation of H3PO3 on a metallic Pt surface.

Understanding the Effects of pH and Alkali Cations on Cyclic Voltammograms and the Hydrogen Oxidation Reaction on Transition Metal Catalyst Surfaces

The electrochemical interface between a catalytic electrode surface and electrolyte is complex and its structure and composition have a direct effect on the rates and mechanisms of electrocatalytic reactions. For example, it has been found that the rate of the hydrogen oxidation reaction on a platinum electrode is strongly dependent on electrolyte pH, and this effect had tentatively been correlated to a shift in hydrogen binding strength with pH (beyond the expected Nernstian shift) as measured by cyclic voltammetry [1]. We used density functional theory (DFT) to calculate the thermodynamics of adsorption of hydrogen, hydroxide, water, and alkali cations on the low index facets of platinum to support prior work which has shown that the low potential peaks measured by cyclic voltammetry on Pt(100), Pt(110), and higher order facets containing 100 and 110 steps, actually correspond to the competitive and co-adsorption of hydrogen and hydroxide [2]. We further show that increasing pH leads to an increasing amount of specifically adsorbed alkali metal cation which weakens the binding of hydroxide, leading to the experimentally observed shift in the low potential peaks measured by cyclic voltammetry, as opposed to a strengthening of hydrogen binding [2]. Now that we understand the mechanism for these experimentally measured effects of pH and alkali cations, we use experiment and DFT to examine their effect on the rate of the hydrogen oxidation reaction, which we find, on a platinum electrode, to depend on cation identity. We also discuss this effect of pH and alkali metal cation on cyclic voltammograms and the hydrogen oxidation reaction across transition metal surfaces (Ru, Ir, Pt, and Au) as measured by Strmcnik et al. [3] and Durst et al. [4]. [1] Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Correlating Hydrogen Oxidation and Evolution Activity on Platinum at Different pH with Measured Hydrogen Binding Energy. Nat. Commun. 2015, 6:5848. [2] McCrum, I.T.; Janik, M. J. pH and alkali cation effects on the Pt cyclic voltammogram explained using density funtional theory. J. Phys. Chem. C. 2016, In Press. DOI: 10.1021/acs.jpcc.5b10979 [3] Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der, V.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Improving the Hydrogen Oxidation Reaction Rate by Promotion of Hydroxyl Adsorption. Nat. Chem. 2013, 5, 300-306. [4] Durst, J.; Siebel, A.; Simon, C., Hasché, F.; Herranz, J.; Gasteiger, H.A. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 2014, 7, 2255-2260.

Mechanism of the Surface Hydrogen Induced Conversion of CO2 to Methanol at Cu(111) Step Sites

Cu/ZnO/Al2O3 is an industrially important heterogeneous catalyst for the conversion of CO2 to methanol, which is in worldwide demand, and for the solution of the activation mechanism of catalytically inactive CO2. Recent studies have achieved numerous improvements in active sites of catalysts for this process, which can be described as “active copper with step sites” decorated with ZnOx. In spite of these improvements, the mechanism of this process is still unknown, and even its initial stage remains unclear. In this study, we simplified the catalytic system to bare Cu(111) and Cu(775) surfaces in order to systematically determine the mechanistic effects of step sites. The reaction was conducted by using a CO2/H2 gas mixture at 1 Torr at various temperatures and characterized with infrared reflection absorption spectroscopy (IRRAS). The initial activation of CO2 was found to occur only with the coadsorption of hydrogen; it cannot on its own be converted into other activated species. This coadsorbed hydrog...

The role of surface Pt on the coadsorption of hydrogen and CO on Pt monolayer film modified Ru(0001) surfaces

We have investigated the impact and role of the Pt surface modification on the coadsorption of hydrogen and CO on structurally well defined bimetallic Pt monolayer island/film modified Ru(0001) surfaces with Pt contents up to a complete Pt layer, employing temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS). Kinetic limitations in the surface diffusion are shown to play an important role for adsorption at 90K, and lead to profound effects of the dosing sequence on the adsorption and desorption characteristics. Furthermore, they are responsible for spill-over effects during the TPD measurements, where COad becomes mobile and can spill-over from weakly bonding Pt monolayer areas to strongly bonding Pt-free Ru(0001) areas, which displaces Dad from these surface areas. The present findings are discussed in comparison with previous results on related metallic and bimetallic adsorption and coadsorption systems.

Kinetics of gas phase formic acid decomposition on platinum single crystal and polycrystalline surfaces

Formic acid dehydrogenation turnover rates (TORs) were measured on Pt(111), Pt(100), and polycrystalline Pt foil surfaces at a total pressure of 800Torr between 413 and 513K in a batch reactor connected to an ultra-high vacuum (UHV) system. The TORs, apparent activation energies, and reaction orders are not sensitive to the structure of the Pt surface, within the precision of the measurements. CO introduced into the batch reactor depressed the formic acid dehydrogenation TOR and increased the reaction's apparent activation energies on Pt(111) and Pt(100), consistent with behavior predicted by the Temkin equation. Two reaction mechanisms were explored which explain the formic acid decomposition mechanism on Pt, both of which include dissociative adsorption of formic acid, rate limiting formate decomposition, and quasi-equilibrated hydrogen recombination and CO adsorption. No evidence was found that catalytic supports used in previous studies altered the reaction kinetics or mechanism.

Hydrogen and methoxy coadsorption in the computation of the catalytic conversion of methanol on the ceria (111) surface

Methanol decomposition to formaldehyde catalyzed by the ceria (111) surface was investigated using the DFT+U method. Our results rationalize experimental temperature programmed desorption experiments on the fully oxidized surface. Particular attention was paid to the effect of coadsorption of methoxy and hydrogen on various aspects of the conversion process. This issue had been raised by the experimental observation of water desorption at low temperature removing hydrogen from the system. Within this context, we also investigated hydrogen diffusion on the ceria surface. The hydrogen/methoxy interaction on ceria was shown to be ionic regardless of separation distance. The barrier for dehydrogenation of methoxy using the ionic model system, where hydrogen is coadsorbed, is above 1eV. This barrier becomes negligible if an incorrect neutral model without coadsorbed hydrogen is employed. While water formation from isolated surface hydrogen is unlikely at low temperature, the presence of coadsorbed methoxy reduces the reaction energy for water formation considerably. For the dehydrated surface, we observed that the preference of the electron to locate at the methoxy oxygen instead of the cerium atom results in a surface that does not contain Ce3+ ions, despite the existence of a vacancy.

Chiral modification of platinum: ab initio study of the effect of hydrogen coadsorption on stability and geometry of adsorbed cinchona alkaloids.

The cinchona alkaloids cinchonidine and cinchonine belong to the most efficient chiral modifiers for the noble metal-catalyzed enantioselective hydrogenation of C=O and C=C bonds. Under reaction conditions these modifiers are coadsorbed on the noble metal surface with hydrogen. Using density functional theory, we studied the effect of coadsorbed hydrogen on the adsorption mode of cinchonidine and cinchonine on a Pt(111) surface at different hydrogen coverages. The theoretical study indicates that the presence of coadsorbed hydrogen affects both the adsorption geometry as well as the stability of the adsorbed cinchona alkaloids. At all hydrogen coverages the cinchona alkaloids are found to be adsorbed via anchoring of the quinoline moiety. In the absence of hydrogen as well as at low hydrogen coverage the quinoline moiety adsorbs nearly parallel to the surface, whereas at higher hydrogen coverage it becomes tilted. Higher hydrogen coverage as well as partial hydrogenation of the quinoline part of the cinchona alkaloid and hydrogen transfer to the C[double bond, length as m-dash]C double bond at 10, 11 position of the quinuclidine moiety destabilize the adsorbed cinchona alkaloid, whereas hydrogen transfer to the nitrogen atom of the quinoline and the quinuclidine moiety stabilizes the adsorbed molecule. The stability as well as the adsorption geometry of the cinchona alkaloids are affected by the coadsorbed hydrogen and are proposed to influence the efficiency of the enantiodifferentiating ability of the chirally modified platinum surface.

Thermodynamic study of benzene and hydrogen coadsorption on Pd(111)

Periodic density functional theory (DFT) has been used to study the coadsorption of hydrogen and benzene on Pd(111). The most stable coverages are predicted by constructing the thermodynamic phase diagram as a function of gas-phase temperature and pressure. The common approximation that neglects vibrational contributions to the surface Gibbs free energy, using the PW91 functional, is compared to the one that includes vibrational contributions. Higher coverages are predicted to be thermodynamically the most stable including vibrational frequencies, mainly due to the different entropy contributions. The first approach is also compared to the one using a (optPBE-vdW) vdW-DF functional without vibrational contributions, which predicts higher benzene coverages for benzene adsorption, and lower hydrogen coverages for hydrogen adsorption and coadsorption with a fixed benzene coverage. Inclusion of vibrational contributions using the vdW-DF method has not been implemented due to computational constraints. However, an estimate of the expected result is proposed by adding PW91 vibrational contributions to the optPBE-vdW electronic energies, and under typical hydrogenation conditions high coverages of about θH = 0.89 are expected. Inclusion of vibrational contributions to the surface Gibbs free energy and a proper description of van der Waals interaction are recommended to predict the thermodynamically most stable surface coverage.

The co-adsorption of hydrogen and carbon dioxide on catalysis

In the elucidation of the reaction mechanism of a catalytic process it is important to establish the reaction intermediates and their possible role in the reaction. In most cases, however, this is not an easy task as a real reaction intermediate exists only transitorily and in a very low concentration on the catalysts. The adsorption of CO2 and the co-adsorption of H2 + CO2 on Re supported by Al2O3, TiO2, MgO and SiO2 have been investigated by FT-IR spectroscopy. The dissociation of CO2 was not experienced on the Re/Al2O3 reduced at 673 K, it occurred, however, on the sample reduced at 1073 K. Addition of H2 to CO2, initiated the dissociation on all catalysts as indicated by CO bands at 2022-2053 cm-1 . Besides, new spectral features were developed at 1600-1550, 1395 and 1365 cm-1 attributed to format species. No bands due to format were detected on Re/SiO2 and no format was detected following the co-adsorption of CO2-containing gas mixture on the supporting oxides alone. It was assumed that the format species identified in the surface interactions is located on the support, where it is stabilized. The possible pathways of the occurrence of format complex on the oxides are described.

Adsorption and dissociation of H2S on Mo(1 0 0) surface by first-principles study

Density-functional theory calculations had been used to investigate the adsorption and dissociation of H2S on Mo(100) surface. Adsorption mechanisms of H2S, HS, S and H on the Mo(100) surface were analyzed. H2S was found to be adsorbed at bridge, hollow and top sites with adsorption energies of −1.25, −1.03 and −0.92eV, respectively. HS was strongly chemically absorbed at hollow, bridge and top sites with adsorption energies of −4.51, −4.08 and −3.45eV, respectively, and sulfur and hydrogen preferred to be absorbed at hollow and bridge sites, respectively. In addition, potential energy profiles of H2S dissociation on Mo(100) had been constructed by a climbing image nudged elastic band method. Four possible dissociation pathways of the first H2S dehydrogenation were examined with reaction barriers of 0.28, 0.37, 0.075, and 0.21eV, respectively, while the energy barrier to break the SH bond of HS with or without hydrogen co-adsorption was almost the same low. This work showed that the decomposition of H2S on the molybdenum surface was kinetically and thermodynamically facile. Local densities of electronic states were further used to characterize the interaction between H2S and substrate.

Co-adsorption of hydrogen and CO on Pt film: An in-situ ATR-IR study combined with DFT calculations

The co-adsorption of hydrogen and CO on Pt film was studied by in-situ attenuated total reflection Infrared (ATR-IR) Spectroscopy. It was found that the adsorption of CO on Pt is much stronger than that of hydrogen, and the co-adsorbed hydrogen species lead to dramatic red shifts of both linear and bridged-adsorbed CO peaks. The more hydrogen species adsorbed on Pt surface, the more red shifts of CO frequencies were observed. Based on the results obtained from the well-designed experiments, it is deduced that H may act as an electron donor on Pt surface and negative charge is transferred from H to Pt. Thus, the surface charge properties of Pt film were changed, which enhances the d–π∗ backdonation effect between Pt and CO and consequently leads to dramatic red shifts of CO peaks. The density functional theory (DFT) calculations based on simple models confirmed the charge transfer effect between hydrogen and Pt. This study also shows that ATR-IR technique is a powerful tool to investigate the gas–solid interface.

Nanoparticle Shape Selectivity in Catalysis: Butene Isomerization and Hydrogenation on Platinum

New advances in colloidal and other self-assembly synthetic methods have afforded the controlled growth of nanoparticles with well-defined sizes and shapes. Recently, the catalysis community has been trying to capitalize on this knowledge for the design of new catalytic processes. In particular, the use of metal nanoparticles with specific shapes has been explored in several instances as a way to control reaction selectivity. Here we review the results from our efforts to use platinum nanoparticles dispersed on high-surface-area supports to perform selective olefin conversions. Emphasis is given to the surface-science experiments and quantum-mechanics calculations that led us to identify potential variations in selectivity in carbon–carbon double-bond isomerization and hydrogenation reactions with the structure of the metal surface. Temperature programmed desorption (TPD) and reflection–absorption infrared spectroscopy data for 2-butenes adsorbed on Pt(111) single-crystal surfaces highlighted the relative higher stability of adsorbed cis-2-butene compared to trans-2-butene and the preference for the promotion of trans-to-cis conversions on that surface. It was also determined that coadsorbed hydrogen plays a key role in defining the relative stabilities of the adsorbates, favoring pi rather than di-sigma bonding and reversing the higher stability of the trans adsorbates seen on clean Pt(111). DFT calculations suggested that such unique results may be accounted for by the need for extensive surface reconstruction to accommodate the adsorbates on such flat planes, a requirement that appears to be less severe with the cis isomer. TPD experiments on stepped Pt(557) surfaces pointed to the minimal importance of steps in promoting these isomerization reactions, although they do seem to help with the full hydrogenation to the alkanes. More extensive olefin adsorption destabilization with hydrogen coadsorption and faster alkane production was seen on Pt(100), but selectivity towards the cis isomer was still identified. On the more open (2 × 1)-reconstructed Pt(110) surface, on the other hand, trans-2-butene is the most stable of the two isomers. It was finally shown that these surface-science results translate into changes in selectivity in real catalysts with platinum nanoparticle shape. Catalysts were prepared by using colloidal Pt nanoparticles with tetrahedral, cubic, and rounded shapes, and unique selectivity toward cis-2-butene formation was measured on the first of those samples. It appears that the (111) facets exposed by the tetrahedral Pt nanoparticles do show the same trans-to-cis conversion preference in catalysis seen in the surface-science studies carried out with single-crystal surfaces and under ultrahigh vacuum conditions.

Density functional theory (DFT) investigation of the adsorption of the CH3OH/Au(100) system

The adsorption of methanol on flat Au (100) surface with different coverages (θ = 1.0, 0.5 and 0.25 monolayer (ML)) is studied using density functional theory. Among the three sites (top, bridge and hollow) and coverages investigated in the present work, no adsorption is stable for θ = 1.0 ML. The most energetically preferred site of adsorption for CH3OH is found to be the hollow site for coverages of 0.25 ML and 0.50 ML. We also find that for all adsorption sites, an increase in CH3OH coverage triggers a decrease in the adsorption energy. The geometric parameters, local density of states and work function changes are analysed in detail. The coadsorption of methoxy and hydrogen has also investigated. In addition, the dissociation of methanol on Au(100) has been studied, and an activation energy was found to be 1.72 eV. This result compare with existing data in the literature for Au(111) surface. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Interaction of magnetic transition metal dimers with spin-polarized hydrogenated graphene

The coadsorption of hydrogen and transition metal dimers Fe2, Co2, Ni2, and FeCo on graphene is investigated using density functional theory calculations. Our work is motivated by observations that the magnetic moments of these transition metal dimers are large and that hydrogen adsorption partitions the graphene lattice into magnetic subdomains. Thus, we expect the magnetic dimers to interact strongly with the lattice. Our results show that the majority-spin direction of the lattice electronic states depends upon the dimer identity, the lattice spin polarization being in the same direction as the dimer spin polarization for Fe2 and FeCo, but opposite for Co2 and Ni2. We can understand this by examining the electronic density of states of the dimer and the lattice. We also show that coadsorption significantly increases the adsorption energies of both dimer and hydrogen leading to a more strongly-adsorbed dimer, while the bond length and magnetic moment of the upper dimer atom, the latter important for potential magnetic storage applications, are negligibly changed. Our work shows that the coadsorbed hydrogen and metal dimer interact over a long-range, this interaction being mediated by the hydrogen-induced spin-polarization of the graphene lattice. We obtain general insight into how the elemental identity of these magnetic dimers determines the spin-polarized states on the hydrogenated graphene lattice. These results could be important for potential applications of magnetic properties of decorated graphene lattices.

The Effect of Hydrogen and Fluorine Coadsorption on the Piezoelectric Properties of Graphene

Motivated by a search for electromechanical coupling in monolayer materials, we study graphene chemically modified by hydrogen adsorbed on one side and fluorine adsorbed on the other side. Such adsorption under experimental conditions can potentially lead to a variety of configurations of atoms on the surface. We perform an exhaustive evaluation of candidate configurations for two stoichiometries, C2HF and C4HF, and examine their electromechanical properties using density functional theory. While all configurations exhibit an e31 piezoelectric effect, the lowest energy configuration additionally exhibits an e11 effect. Therefore, both e31 and e11 piezoelectricity can potentially be engineered into nonpiezoelectric monolayer graphene, providing an avenue for monolithic integration of electronic and electromechanical devices in graphene monolayers for resonators, sensors, and nanoelectromechanical systems (NEMS).

K-doping of Co/Al2O3 low temperature Fischer–Tropsch catalysts

The effect of potassium doping on Co/Al2O3 Fischer–Tropsch catalyst has been investigated using in situ IR spectroscopic measurements and catalytic activity tests in flow reactor. The data confirm the inhibiting effect of K doping on reducibility and catalytic activity, together with a stabilization of the catalytic activity, a reduction of methane selectivity and a slight increase of CO2 selectivity.CO adsorbs over the Co/Al2O3 catalyst forming on-top surface carbonyls, responsible for a Infrared CO stretching band at a position more typical of fcc 100 face than of hcp 0001 face. In the presence of hydrogen CO adsorption shifts in part from terminal to bridging sites. Potassium doping significantly increases the electron donating ability of cobalt metal particles, as evidenced by the shift down of 15cm−1 of the CO stretching frequency of adsorbed carbon monoxide. CO adsorbed on surface defects is also evident. After CO adsorption at 250°C even lower frequency carbonyls are observed only on the K-doped catalyst (2000 and 1825cm−1) providing evidence of a stronger interaction of CO with the surface. At 250°C in the presence low pressure of CO+H2, methane, ethylene and other lower olefins form in the IR cell. The ethylene/methane molar ratio produced over the K-doped catalyst is definitely higher than over the K-free sample, suggesting that potassium favours the first chain growth step in the Fischer–Tropsch process thus allowing decreased selectivity to methane. Reasons for the increased catalyst stability are briefly discussed.