Close-packed Transition Metal Surfaces Research Articles

Partial Electrooxidation of Glycerol on Close-Packed Transition Metal Surfaces: Insights from First-Principles Calculations

Glycerol is a byproduct of biodiesel production and an abundant feedstock for the synthesis of high-value chemicals. One promising approach for valorization of glycerol is electrooxidation yielding hydrogen and value-added products. However, due to the vast amount of intermediary steps and possible products, the process is not fully understood. Here, the first two deprotonations of glycerol on close-packed transition metals (Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) are investigated using density functional theory calculations together with the computational hydrogen electrode. We find that the theoretical limiting potential for the studied reaction is close to 0 V vs the reversible hydrogen electrode, ranging from −0.12 V for ruthenium to +0.35 V for gold. Furthermore, the results show that Ru, Rh, Ir, Ag, and Au are selective toward dihydroxyacetone and its derivatives, while Pd and Pt are selective toward either dihydroxyacetone or glyceraldehyde and their derivatives, and that Cu, Co, and Ni are selective toward hydropyruvic acid. The results can be rationalized in terms of the relative bond strengths of carbon and oxygen on the metal. In addition, we find that solvent effects are generally small, the exceptions being the limiting potential on copper and the mechanism on rhodium. These results can be used to steer the selectivity toward more valuable products and thereby increase the economic yield of biodiesel production.

Signatures of multiple jumps in surface diffusion on honeycomb surfaces

The jump distribution, a property of the motion of adsorbates on a corrugated surface, contains crucial information on adsorbate-substrate energy dissipation processes. To provide a means to study jump distributions in a honeycomb array of adsorption sites, we derive analytical expressions for the intermediate scattering function (ISF) describing jump diffusion taking into account jumps up to fourth-nearest neighbor in length. To enable testing the analytical expressions against experimental or simulated data, we develop a global fitting routine that can be applied to experimental or simulated ISFs to infer multiple jumps. We demonstrate the analysis method by studying the jump distribution arising from classical Langevin molecular dynamics simulations of two model systems, cyclopentadienyl (Cp) on Cu(111), and deuterium (D) on Pd(111). The simulations and analysis confirm that diffusion of Cp/Cu(111) at a surface temperature ${T}_{s}=135$ K takes place in a regime of predominantly single jumps. Classical simulations of D/Pd(111) at ${T}_{s}=350$ K, with a realistic Langevin friction, suggest that the diffusion of D/Pd(111) involves a high proportion of multiple jumps. The parameters that apply to D/Pd(111) are typical of the interaction of hydrogen atoms with close-packed transition metal surfaces, suggesting that long jumps are a general feature of the high temperature surface diffusion of hydrogen.

The influence of hydroxy groups on the adsorption of three-carbon alcohols on Ni(111), Pd(111) and Pt(111) surfaces: a density functional theory study within the D3 dispersion correction.

Experimentally, steric and inductive effects have been suggested as key parameters in the adsorption and reactivity of alcohols on transition-metal (TM) surfaces, however, our atomistic understanding of the behavior of alcohols in catalysis is far from satisfactory, in particular, due to the role of hydroxy groups in the adsorption properties of C3 alcohols on TM surfaces. In this study, we investigated those effects through ab initio calculations based on density functional theory employing a semilocal exchange-correlation functional within van der Waals corrections (the D3 framework) for the adsorption of C3 alcohols with different numbers and positions of OH groups, namely, propane, 1-propanol, 2-propanol, 1,2-propanediol, 1,3-propanediol and glycerol, on the compact Ni(111), Pd(111) and Pt(111) surfaces. As expected, we found that the adsorption energy is affected by the number of hydroxy groups with similar values for each pair of regioisomers, which clearly indicates the effect of the number of OH groups. Based on Bader charge analysis, we found an effective charge transfer from the C3 molecules to the substrates, which can explain the reduction in the work function due to adsorption. Upon adsorption, the alpha carbon to the OH group closest to the surface and the central carbon are the most positively charged atoms, which increases the lability of their bonded H atoms. In addition, the depletion of electron density between the C-H and O-H bonds closer to the surfaces corroborated their stretching, suggesting that the proximity of the adsorbates to the surfaces affects the acidity of these H atoms, as well as inductive effects within the molecules.

Using Brønsted-Evans-Polanyi relations to predict electrode potential-dependent activation energies

Various elementary (de-)hydrogenation reactions at transition metal surfaces in heterogeneous catalysis have shown a linear Brønsted-Evans-Polanyi relation (BEP) between the activation energy and the reaction energy across metal surfaces. Using Density Functional Theory (DFT), we investigate if these BEP relations can be extended to elementary electrochemical reduction reactions involving transfer of a proton-electron pair. We examine the effect of the applied electrode potential on the BEP relations. We focus in particular on elementary electrochemical CH, OH and NH bond formation reactions on 7 close-packed transition metal surfaces, including Ag (111), Au (111), Cu (111), Ni (111), Pt (111), Pd (111) and Rh (111). The potential-dependent activation energies used to construct the BEP relations are calculated using a Marcus theory based approach. The role of interfacial water in the kinetics and the reaction mechanism of CH, OH and NH bond formation is explored. Specifically, two mechanisms are considered, a Tafel-like scheme involving direct surface hydrogenation and a Heyrovsky-like mechanism in which the proton is shuttled via an explicit water molecule. The potential-dependent elementary kinetics across three reduction reaction series, C* → CH4, O* → H2O and N* → NH3, are also discussed.The Heyrovsky-like scheme is the preferred mechanism for CH, OH and NH bond formation on all but two metals. BEP relations hold at the same applied electrode potential for CH, OH and NH formation for both examined mechanisms. However, BEP parameters differ among CH, OH, and NH reactions, indicating that elementary reaction energies alone are not sufficient for comparing the activity of catalysts when electrocatalytic reactions involve such different elementary steps.

Dehydrogenation Selectivity of Ethanol on Close-Packed Transition Metal Surfaces: A Computational Study of Monometallic, Pd/Au, and Rh/Au Catalysts

Ethanol (EtOH) decomposition has been widely studied in recent years. However, the initial dehydrogenation selectivity on catalytic surfaces, which plays a crucial role in EtOH partial oxidation and steam reforming, is not well understood. Here, density functional theory (DFT) was used to calculate the initial dehydrogenation selectivities of EtOH on monometallic and X/Au (X = Pd and Rh) close-packed surfaces. The energy for the initial bond scissions of O–H and α- and β-C–H were calculated on each surface. The binding energy of EtOH is found to be a good reactivity descriptor for the scission of O–H and β-C–H bonds, while the binding energy of CH3CHOH is a good reaction descriptor for α-C–H bond scission. The scaling relationships between the activation energy barriers and binding energies on Pd/Au and Rh/Au surface alloys are significantly different from those of monometallic surfaces. Additionally, the specific atomic ensembles on the Pd/Au and Rh/Au surfaces have different initial dehydrogenation sele...

Density-functional theory with screened van der Waals interactions applied to atomic and molecular adsorbates on close-packed and non-close-packed surfaces

Modeling the adsorption of atoms and molecules on surfaces requires efficient electronic-structure methods that are able to capture both covalent and noncovalent interactions in a reliable manner. In order to tackle this problem, we have developed a method within density-functional theory (DFT) to model screened van der Waals interactions (vdW) for atoms and molecules on surfaces (the so-called DFT+${\text{vdW}}^{\mathrm{surf}}$ method). The relatively high accuracy of the DFT+${\text{vdW}}^{\mathrm{surf}}$ method in the calculation of both adsorption distances and energies, as well as the high degree of its reliability across a wide range of adsorbates, indicates the importance of the collective electronic effects within the extended substrate for the calculation of the vdW energy tail. We examine in detail the theoretical background of the method and assess its performance for adsorption phenomena including the physisorption of Xe on selected close-packed transition metal surfaces and 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on Au(111). We also address the performance of DFT+${\text{vdW}}^{\mathrm{surf}}$ in the case of non-close-packed surfaces by studying the adsorption of Xe on Cu(110) and the interfaces formed by the adsorption of a PTCDA monolayer on the Ag(111), Ag(100), and Ag(110) surfaces. We conclude by discussing outstanding challenges in the modeling of vdW interactions for studying atomic and molecular adsorbates on inorganic substrates.

Structure and energetics of hydrogen-bonded networks of methanol on close packed transition metal surfaces.

Methanol is a versatile chemical feedstock, fuel source, and energy storage material. Many reactions involving methanol are catalyzed by transition metal surfaces, on which hydrogen-bonded methanol overlayers form. As with water, the structure of these overlayers is expected to depend on a delicate balance of hydrogen bonding and adsorbate-substrate bonding. In contrast to water, however, relatively little is known about the structures methanol overlayers form and how these vary from one substrate to another. To address this issue, herein we analyze the hydrogen bonded networks that methanol forms as a function of coverage on three catalytically important surfaces, Au(111), Cu(111), and Pt(111), using a combination of scanning tunneling microscopy and density functional theory. We investigate the effect of intermolecular interactions, surface coverage, and adsorption energies on molecular assembly and compare the results to more widely studied water networks on the same surfaces. Two main factors are shown to direct the structure of methanol on the surfaces studied: the surface coverage and the competition between the methanol-methanol and methanol-surface interactions. Additionally, we report a new chiral form of buckled hexamer formed by surface bound methanol that maximizes the interactions between methanol monomers by sacrificing interactions with the surface. These results serve as a direct comparison of interaction strength, assembly, and chirality of methanol networks on Au(111), Cu(111), and Pt(111) which are catalytically relevant for methanol oxidation, steam reforming, and direct methanol fuel cells.

Effects of van der Waals density functional corrections on trends in furfural adsorption and hydrogenation on close-packed transition metal surfaces

The hydrogenation of furfural to furfuryl alcohol on Pd(111), Cu(111) and Pt(111) is studied with both standard Density Functional Theory (DFT)-GGA functionals and with van der Waals-corrected density functionals. VdW-DF functionals, including optPBE, optB88, optB86b, and Grimme's method, are used to optimize the adsorption configurations of furfural, furfuryl alcohol, and related intermediates resulting from hydrogenation of furfural, and the results are compared to corresponding values determined with GGA functionals, including PW91 and PBE. On Pd(111) and Pt(111), the adsorption geometries of the intermediates are not noticeably different between the two classes of functionals, while on Cu(111), modest changes are seen in both the perpendicular distance and the orientation of the aromatic ring with respect to the planar surface. In general, the binding energies increase substantially in magnitude as a result of van der Waals contributions on all metals. In contrast, however, dispersion effects on the kinetics of hydrogenation are relatively small. It is found that activation barriers are not significantly affected by the inclusion of dispersion effects, and a Brønsted–Evans–Polanyi relationship developed solely from PW91 calculations on Pd(111) is capable of describing corresponding results on Cu(111) and Pt(111), even when the dispersion effects are included. Finally, the reaction energies and barriers derived from the dispersion-corrected and pure GGA calculations are used to plot simple potential energy profiles for furfural hydrogenation to furfuryl alcohol on the three considered metals, and an approximately constant downshift of the energetics due to the dispersion corrections is observed.

Interaction of Hydrogen with a Cobalt(0001) Surface

Abstract The interaction of hydrogen and deuterium with the Co(0001) surface has been studied in UHV between 90 and 500 K by means of LEED, temperature-programmed thermal desorption (TPD) and work function change (ΔΦ) measurements. Hydrogen adsorbs spontaneously and dissociatively in two atomic binding states denoted as β 1 and β 2 with a high initial sticking probability. The adsorption energies E(β 1) and E(β 2) are 80 and 100 kJ/mol, respectively, if a first-order desorption kinetics is assumed (a second-order kinetic analysis yields unreasonably low values for both the desorption energies and frequency factors). At sufficiently low temperatures, the adsorbed H atoms form a faint (2 × 2) LEED superstructure, which is best developed after an exposure of ∼20 L. From the temperature dependence of the fractional-order beam intensity at different exposures we determine the critical temperature of the (2 × 2) phase as ∼ 243 ( ± 10) K. Similar to the H-on-Ni(111) system the existence range of the (2 × 2) phase in the temperature–coverage plane is asymmetric; i.e., below the critical coverage Θ crit the respective long-range order has a higher range of stability than above Θ crit. By analogy with H/Ni(111), we assume a similar honeycomb H structure also for the H-on-Co(0001) system and suggest the critical coverage Θ crit to be 0.5, i.e., half a monolayer of H atoms. The H-induced work function change is surprisingly small; it decreases, forms a shallow minimum of −18 meV after ∼20 L exposure around the optimum coverage of the (2 × 2) phase and reaches a saturation value of −10 meV. Our data are discussed and compared with previous work on H/Co(0001) and other close-packed transition metal surfaces, especially with the H-on-Ni(111) system.

A density functional theory analysis of trends in glycerol decomposition on close-packed transition metal surfaces

We describe an accelerated density functional theory (DFT)-based computational strategy to determine trends in the decomposition of glycerol via elementary dehydrogenation, C-C, and C-O bond scission reactions on close-packed transition metal surfaces. Beginning with periodic DFT calculations on Pt(111), the thermochemistry of glycerol dehydrogenation on Pd(111), Rh(111), Cu(111) and Ni(111) is determined using a parameter-free, bond order-based scaling relationship. By combining the results with Brønsted-Evans-Polanyi (BEP) relationships to estimate elementary reaction barriers, free energy diagrams are developed on the respective metal surfaces, and trends concerning the relative selectivity and activity for C-C and C-O bond scission in glycerol on the various metals are obtained. The results are consistent with available theoretical and experimental literature and demonstrate that scaling relationships are capable of providing powerful insights into the catalytic chemistry of complex biomolecules.

Reactivity Descriptors for Borohydride Interaction with Metal Surfaces

First-principles density functional theory calculations were performed to study the adsorption of borohydride (BH4–) on close-packed transition-metal surfaces, M(111) (M = Au, Pt, Ir, Os, Ag, Pd, Rh, Ru). A correlation between the relative adsorption energies of BH4ad and the d-band center of the metals is established. In terms of the adsorbate configuration, both molecular (BH4ad) and dissociated (BH4-y,ad + yHad, y = 1, ...,3) structures are possible regardless of the adsorption energy value. On Os, Rh, and Ru surfaces, molecular (i.e., undissociative) adsorption is preferred despite the strong surface binding energy of BH4ad. Orbital-specific analysis of the bonding, points to the role of the dzz and dyz states of the surface metal atoms in determining the final BH4ad configuration on all metals. However, in the presence of H2O molecules, the preference for strong molecular adsorption may be lost because of BH4ad–H2Oad interaction. Using the coadsorption on Os(111) of BH4ad and H2Oad with and without the presence of OHad (generated either by electrosorption of OH– or dissociative water adsorption), the origins of the adsorbate–adsorbate and adsorbate–metal interactions are discussed. Electronic factors to predict the BH4ad conformation on metal catalysts in water environment are proposed.

Trends of Water Gas Shift Reaction on Close-Packed Transition Metal Surfaces

The mechanism of the water gas shift reaction (WGSR) on the close-packed transition metal surfaces of Co, Ni, Cu (from the 3d row), Rh, Pd, Ag (from the 4d row), Ir, Pt, and Au (from the 5d row) has been systematically examined by periodic density functional theory (DFT) calculations. The comparison of potential energy surface (PES) concludes that WGSR activity is influenced by two kinds of elementary steps: O−H bond dissociation and C−O bond formation. Activation barriers (Ea) and reaction energies (ΔH) on a series of metal surfaces show good BEP relationship; however, their energetic trends are opposite in these two kinds of steps. In O−H bond dissociation steps, trends of Ea and ΔH are groups 9 < 10 < 11 and 3d < 4d < 5d. On the other hand, C−O bond formation steps on the surfaces of the lower-right metals in the d block (Cu, Ag, Pt, Au) have relatively lower Ea and ΔH, which is responsible for their high WGSR activity of metal/oxide catalysts. In addition, the fundamental of energetic trends has been examined from the analyses of adsorption energy, density of state (DOS), and charge density. The result shows that the surfaces of upper-left d-block metals (Co, Ni, Rh) with higher energy and smaller delocalization of their d orbitals yield a stronger adsorption energy with higher induced charges that will stabilize dissociating fragments to lower the barrier and retard desorptions to lift the barrier in O−H bond dissociation and C−O bond formation steps, respectively. The prediction of energetic trends in the present work is also appropriate for other catalytic reactions, such as ethanol decomposition and CO oxidation, and can help us scientifically design a better catalyst for the desired reaction.

Insight from first principles into the nature of the bonding between water molecules and 4d metal surfaces

We address the nature of the bond between water molecules and metal surfaces through a systematic density-functional theory (DFT) study of H(2)O monomer adsorption on a series of close-packed transition metal surfaces: Ru(0001), Rh(111), Pd(111), and Ag(111). Aiming to understand the origin behind energetic and structural trends along the 4d series we employ a range of analysis tools such as the electron reactivity function, decomposition of densities of states, electron density differences, and inspection of individual Kohn-Sham orbitals. The results obtained from our DFT calculations allow us to rationalize the bonding between water and transition metal surfaces as a balance of covalent and electrostatic interactions. A frontier orbital scheme based on so-called two-center four-electron interactions between the molecular orbitals of H(2)O--mainly the 1b(1)--and d-band states of the surface proves incisive in understanding these systems.

Reactivity descriptors for direct methanol fuel cell anode catalysts

We have investigated the anode reaction in direct methanol fuel cells using a database of adsorption free energies for 16 intermediates on 12 close-packed transition metal surfaces calculated with periodic, self-consistent, density functional theory (DFT–GGA). This database, combined with a simple electrokinetic model of the methanol electrooxidation reaction, yields mechanistic insights that are consistent with previous experimental and theoretical studies on Pt, and extends these insights to a broad spectrum of other transition metals. In addition, by using linear scaling relations between the adsorption free energies of various intermediates in the reaction network, we find that the results determined with the full database of adsorption energies can be estimated by knowing only two key descriptors for each metal surface: the free energies of OH and CO on the surface. Two mechanisms for methanol oxidation to CO 2 are investigated: an indirect mechanism that goes through a CO intermediate and a direct mechanism where methanol is oxidized to CO 2 without the formation of a CO intermediate. For the direct mechanism, we find that, because of CO poisoning, only a small current will result on all non-group 11 transition metals; of these metals, Pt is predicted to be the most active. For methanol decomposition via the indirect mechanism, we find that the onset potential is limited either by the ability to activate methanol, by the ability to activate water, or by surface poisoning by CO ∗ or OH ∗/O ∗. Among pure metals, there is no obvious candidate for a good anode catalyst, and in order to design a better catalyst, one has to look for bi-functional surfaces such as the well-studied PtRu alloy.

Improved oxygen reduction reactivity of platinum monolayers on transition metal surfaces

The catalytic activity of platinum monolayers supported on close-packed transition metal surfaces (Au(1 1 1), Pt(1 1 1), Pd(1 1 1) and Ir(1 1 1)) is investigated for the oxygen reduction reaction (ORR) by generating free energy diagrams and performing Sabatier analysis based on periodic, self-consistent density functional theory (DFT) calculations. Three different ORR mechanisms, involving direct or hydrogen-assisted activation of O 2, are considered. At the ORR equilibrium potential of 1.23 V, the reactivity of all surfaces is shown to be limited by the rate of OH removal from the surface. At a cell potential of 0.80 V, the ORR reactivity of different surfaces is dictated by the strength of oxygen adsorption, with OH removal via hydrogenation and O–O bond scission in either O 2, O 2H or H 2O 2 being the rate-limiting steps for surfaces with stronger and weaker oxygen binding, respectively. Among the surfaces studied, Pt monolayer on a Pd(1 1 1) substrate shows the highest reactivity and is more active than Pt(1 1 1). These results are in excellent agreement with our earlier experimental and theoretical work, which was based on a simpler model for the ORR.

Bridging the temperature and pressure gaps: close-packed transition metal surfaces in anoxygen environment

An understanding of the interaction of atoms and molecules with solid surfaces on themicroscopic level is of crucial importance to many, if not most, modern high-tech materialsapplications. Obtaining such accurate, quantitative information has traditionally beenthe realm of surface science experiments, carried out under ultra-high vacuumconditions. Over recent years scientists have realized the importance of obtaining suchknowledge also under the high pressure and temperature conditions under which manyindustrial processes take place, e.g. heterogeneous catalysis, since the materialunder these conditions may be quite different to that under the conditions oftypical surface science experiments. Theoretical studies too have been aimedat bridging the so-called pressure and temperature gaps, and great strides havebeen made in recent years, often in conjunction with experiment. Here we reviewrecent progress in the understanding of the hexagonal close-packed surfaces oflate transition and noble metals in an oxygen environment, which is of relevanceto many heterogeneous catalytic reactions. In many cases it is found that, onexposure to high oxygen pressures and elevated temperatures, thin oxide-likestructures form which may or may not be stable, and which may have little similarityto the bulk oxides, and thus possess unique chemical and physical properties.