A first-principles study of methanol decomposition on Pt(111)

A periodic, self-consistent, Density Functional Theory study of methanol decomposition on Pt(111) is presented. The thermochemistry and activation energy barriers for all the elementary steps, starting with O[bond]H scission and proceeding via sequential hydrogen abstraction from the resulting methoxy intermediate, are presented here. The minimum energy path is represented by a one-dimensional potential energy surface connecting methanol with its final decomposition products, CO and hydrogen gas. It is found that the rate-limiting step for this decomposition pathway is the abstraction of hydroxyl hydrogen from methanol. CO is clearly identified as a strong thermodynamic sink in the reaction pathway while the methoxy, formaldehyde, and formyl intermediates are found to have low barriers to decomposition, leading to very short lifetimes for these intermediates. Stable intermediates and transition states are found to obey gas-phase coordination and bond order rules on the Pt(111) surface.     

Kinetic mechanism of methanol decomposition on Ni(111) surface: a theoretical study

The decomposition of methanol on the Ni(111) surface has been studied with the pseudopotential method of density functional theory-generalized gradient approximation (DFT-GGA) and with the repeated slab models. The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained in the present work. The major reaction path on Ni surfaces involves the O-H bond breaking in CH(3)OH and the further decomposition of the resulting methoxy species to CO and H via stepwise hydrogen abstractions from CH(3)O. The abstraction of hydrogen from methoxy itself is the rate-limiting step. We also confirm that the C-O and C-H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and hydroxymethyl and atom hydrogen, respectively, have higher energy barriers. Therefore, the final products are the adsorbed CO and H atom.     

Methane dehydrogenation on Rh@Cu(111): a first-principles study of a model catalyst

The issue of tuning the relative height of the first two dehydrogenation barriers of methane (CH(4) --> CH(3) + H and CH(3) --> CH(2) + H) is addressed using density-functional theory. It is shown that the combination of a very active reaction center-such as Rh-with a more inert substrate-such as Cu(111)-may hinder the second dehydrogenation step with respect to the first, thus resulting in the reverse of the natural order of the two barriers' heights.     

Catalytic activity of Pd ensembles over Au(111) surface for CO oxidation: a first-principles study

Employing the first-principles pseudopotential plane-wave methods and nudged-elastic-band simulations, we studied the reaction of CO oxidation on Pd-decorated Au(111) surface. We found that the contiguous Pd ensembles are required for the CO + O(2) reaction. Interestingly, Pd dimer is an active site for the two-step reaction of CO+O(2)→OOCO→CO(2)+O, and a low energy barrier (0.29 eV) is found for the formation of the intermediate metastable state (OOCO) compared to the barrier of 0.69 eV on Pd trimer. Furthermore, the residual atomic O in the CO + O(2) reaction can be removed by another CO on Pd dimer with the barrier of 0.56 eV close to the value of 0.52 eV on Pd monomer via Langmuir-Hinshelwood mechanism. The higher energy barriers (0.96 and 0.64 eV) are also found for the CO + O reaction on Pd trimers. The calculated results indicate Pd dimer is highly reactive for CO oxidation by O(2) via association mechanism on Pd-decorated Au(111) surface.     

Experimental and theoretical study of reactivity trends for methanol on Co/Pt(111) and Ni/Pt(111) bimetallic surfaces

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on NiPt(111) and CoPt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various NiPt(111) and CoPt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni-Pt-Pt(111), as compared to the subsurface monolayer Pt-Ni-Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on NiPt(111) and CoPt(111) bimetallic surfaces. On Ni-Pt-Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300 K; this species reacted at approximately 300 K to form CO and H2. On Co-Pt-Pt(111), methoxy was stable up to approximately 350 K and decomposed to form CO and H2. Overall, trends in methanol reactivity on NiPt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.     

1,3,5-trinitro-1,3,5-triazine decomposition and chemisorption on Al(111) surface: first-principles molecular dynamics study

We have investigated the decomposition and chemisorption of a 1,3,5-trinitro-1,3,5-triazine (RDX) molecule on Al(111) surface using molecular dynamics simulations, in which interatomic forces are computed quantum mechanically in the framework of the density functional theory (DFT). The real-space DFT calculations are based on higher-order finite difference and norm-conserving pseudopotential methods. Strong attractive forces between oxygen and aluminum atoms break N-O and N-N bonds in the RDX and, subsequently, the dissociated oxygen atoms and NO molecules oxidize the Al surface. In addition to these Al surface-assisted decompositions, ring cleavage of the RDX molecule is also observed. These reactions occur spontaneously without potential barriers and result in the attachment of the rest of the RDX molecule to the surface. This opens up the possibility of coating Al nanoparticles with RDX molecules to avoid the detrimental effect of oxidation in high energy density material applications.     

A first-principles study of K adsorption on Pb(111)

Ab initio total-energy density functional theory calculations with supercell models have been employed to investigate the R30 degrees and (2 x 2) structures of K on the Pb(111) surface. Four "on-surface" sites and a substitutional site were considered. The calculations showed that the substitutional site is more stable than all the on-surface sites, due to its low vacancy formation energy. The calculated R30 degrees geometry agrees well with the LEED results. The density-of-states analysis indicates that the K atom loses part of its loosely bound valence s electron. From the electron density distributions, it was found that the lowering of the work function after the substitutional adsorption can be attributed to the dipole moment, associated with the positively polarized adsorbate atom that is characterized by charge depletion from the K vacuum sides and charge accumulation in the region between K and Pb atoms. Our results indicate that the bonding of K with the Pb(111) surface has a mixed ionic and metallic bond character.     

The interaction of CO with PdAg/Pd(111) surface alloys--a case study of ensemble effects on a bimetallic surface

The interaction of CO with structurally well-defined PdAg/Pd(111) surface alloys was investigated by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to unravel and understand contributions from electronic strain, electronic ligand and geometric ensemble effects. TPD measurements indicate that CO adsorption is not possible on the Ag sites of the surface alloys (at 120 K) and that the CO binding strength on Pd sites decreases significantly with increasing Ag concentration. Comparison with previous scanning tunneling microscopy (STM) data on the distribution of Pd and Ag atoms in the surface alloy shows that this modification is mainly due to geometric ensemble effects, since Pd(3) ensembles, which are the preferred ensembles for CO adsorption on non-modified Pd(111), are no longer available on Ag-rich surfaces. Consequently, the preferred CO adsorption site changes with increasing Ag content from a Pd(3) trimer via a Pd(2) dimer to a Pd monomer, going along with a successive weakening of CO adsorption. Additionally, the CO adsorption properties of the surface alloys are also influenced by electronic ligand and strain effects, but on a lower scale. The results are discussed in comparison with previous findings on PdAg bulk alloys, supported PdAg catalysts and PdAu/Pd(111) model systems.     

EHT study of hydrogen chemisorption on Pt-Au(111) bimetallic clusters

The results of an EHT study of the chemisorption of hydrogen on Pt, Au and mixed Pt-Au clusters (up to 19 atoms) are presented. On pure Pt clusters the adsorption sites have equal stability, while on pure Au the top site is the less favoured one. When Au atoms substitute Pt, a destabilization of the metal-H bond is observed, while the insertion of Pt into an Au cluster leads to bond stabilization.     

Adsorption and dissociation of methanol on Au(1 1 1) surface: A first-principles periodic density functional study

The adsorption and dissociation of methanol on Au(111) surface were studied using the first-principles calculations based on density functional theory (DFT) with the generalized gradient approximation (GGA). Adsorption energies, geometric structures, Mulliken charges population, and vibrational frequencies of the various intermediates were computed from full-geometry optimization with a three-layer slab model. The most stable adsorption modes of the species, i.e. CH₃OH, CH₃O, and HCHO were considered in calculation. The possible decomposition pathways were investigated with transition state search methods. The results indicate that methoxyl radical is likely the decomposition intermediate.     

Adsorption of thiols on the Pd(111) surface: a first principles study

Using density functional theory formalism, we have investigated the adsorption behavior of thiols on the Pd(111) surface. Two different thiol molecules, viz. (a) methane thiol and (b) thiophene 2-thiol (TSH), were used as model adsorbates for this purpose. The results revealed that whereas the methane thiol molecule undergoes spontaneous dissociative chemisorption onto the palladium surface, the adsorption of the thiophene 2-thiol molecule does not involve cleavage of the S-H bond, leading to weak interaction energy or physisorption. The variation in the adsorption behavior has been explained based on the difference in the electronic environment of the terminal sulfur atom. The nature of binding at the interface has been analyzed through calculation of the partial density of states of the sulfur atom at the interface.     

A first-principles study of (R)- and (S)-PPA Molecules on Cu(111)

The adsorption of (R)- and (S)-2-phenylpropionamide (PPA, C(9)H(11)ON) molecules on a Cu(111) surface has been investigated using the density functional method with supercell models. The adsorption orientations of both (R)- and (S)-PPA molecules on the surface are the same: the phenyl rings are approximately parallel to the Cu(111) surface and positioned in the hollow sites, the amino and methyl groups occupy two-bridge sites, and the carbonyl occupies the top site. After the adsorption, the bond lengths in the two enantiomers are almost unchanged, but the changes for two dihedral angles show differences, especially for (R)-PPA molecule. The first angles between the (N,C9,C7) plane and the (C9,C7,C6) plane are 19.4 and 0.7 degrees for (R)- and (S)-PPA molecules, respectively, and the second angles between the (C8,C7,C6) plane and the (C7,C6,C5) plane are 74.8 and 0.4 degrees for (R)- and (S)-PPA molecules, respectively. The adsorption energies of (R)- and (S)-PPA molecules are calculated to be -34 and -26 kJ mol(-1), respectively. The simulated scanning tunneling microscopy (STM) images of (R)- and (S)-PPA molecules on the Cu(111) surface display different features and are coincident with the experimental ones. The interaction between the adsorption molecule and the metal surface is found to be responsible for the discrimination of (R)- and (S)-PPA molecules on the surface.     

Ethanol adsorption on the Si (111) surface: first principles study

Equilibrium atomic configurations and electron energy structure of ethanol adsorbed on the Si (111) surface are studied by the first principles density functional theory. Geometry optimization is performed by the total energy minimization method. Equilibrium atomic geometries of ethanol, both undissociated and dissociated, on the Si (111) surface are found and analysed. Reaction pathways and predicted transition states are discussed in comparison with available experimental data in terms of the feasibility of the reactions occurring. Analysis of atom and orbital resolved projected density of states indicates substantial modifications of the Si surface valence and conduction electron bands due to the adsorption of ethanol affecting the electronic properties of the surface.     

Hydrogen-bonded assembly of methanol on Cu(111)

Investigation of methanol's surface chemistry on metals is a crucial step towards understanding the reactivity of this important chemical feedstock. Cu is a relevant metal for methanol synthesis and reforming, but due to the weak interaction of methanol with Cu, an atomic scale view of methanol's coverage-dependent ordering and self-assembly on Cu(111), the most abundant facet of most nanoparticles, has not yet been possible. Low and variable temperature scanning tunneling microscopy coupled with density functional theory reveal a coverage-dependent range of highly ordered structures stabilized by two hydrogen bonds per molecule. While extended chains that resemble the hydrogen-bonded zigzag structures reported for solid methanol are an efficient way to pack methanol at higher coverages, lower surface coverages yield isolated hexamer units. These hexamers form the same number of hydrogen bonds as the chains but appear to repel one another on the surface. Annealing treatments lead to the desorption of methanol with almost no decomposition. This data serves as a useful guide to both the preferred adsorption geometries and energies of a variety of methanol structures on Cu(111) surfaces as a function of surface coverage.     

Mixed aromatic-alkyne system on a Pd surface: a first-principles study

The chemistry of mixed aromatic-alkyne systems on a metal surface is of general interest in many industrial processes. We use density functional theory (DFT) to investigate the chemistry of one such system (i.e., 1,4-diphenyl-butadiyne (DPB) in contact with Pd(110) and Pd(111) surfaces). Reaction pathways and the energetics of important processes are explored, including H2 adsorption, dissociation and migration on the metal surface, the DPB-metal interaction, the energetics of H uptake, and the effects of impurities such as CO and CO2 on H chemistry. We find that (i) strong aromatic-metal interaction leads to significant binding strength of the DPB molecule to both Pd surfaces, especially the (110); (ii) H2 molecules readily dissociate on the Pd surface into H-radicals, which get taken up by alkyne triple bonds; (iii) CO has strong binding to the metal surface, but interacts weakly with H radicals; and (iv) CO2 binds weakly to the metal surface, but could potentially lead to interesting chemical reactions with H.     

Theoretical study of the diffusion of alkali metals on a Cu(111) surface

We present a theoretical study of the diffusion of Li, Na and K on a Cu(111) surface. Various diffusional paths are identified and characterized in terms of kinetic parameters such as diffusion constants and activation energies. We use a model potential parametrized from DFT calculations to determine adsorption energies, surface corrugation and diffusional behaviors. Two representations of the copper surface (2D and 3D) are used to investigate its effect on the adsorption patterns, diffusion constants and activation energies. An interesting result is that the adsorption pattern for Na and K changes when adding layers of substrate (2D → 3D) favouring unusual adsorption sites, which is in agreement with recent theoretical evidence.     

Decomposition of 1,1-dichloroethene on Pd(111)

The decomposition of 1,1-dichloroethene on Pd(111) is investigated using conventional thermal desorption, laser-induced thermal desorption (LITD), and FT reflection absorption infrared spectroscopy (FT-RAIRS). The decomposition mechanism produces at least three hydrocarbon surface intermediates, including ethylidyne. Thermal desorption results differ between high and low coverages because of relative surface concentrations of Cl and H in combination with kinetic effects.     

Self-assembly of manganese phthalocyanine on Pb(111) surface: a scanning tunneling microscopy study

The self-assembled structure of submonolayer manganese phthalocyanine (MnPc) on Pb(111) surface is investigated by using low-temperature scanning tunneling microscopy (STM). A "holelike" superlattice, which is superimposed on the self-assembled nearly quadratic network, is observed. High resolution STM images reveal that there are two distinct azimuthal orientations of MnPc molecules. It is found that by taking the two different orientations the self-assembly can further be optimized energetically by maximizing intermolecular orbital overlapping. It is this intralayer energy minimization process that leads to the characteristic holelike superlattice.     

Similarities and differences on the molecular mechanism of CO oxidation on Rh(111) and bimetallic RhCu(111) surfaces

The reaction between adsorbed CO and atomic O on various sites of Rh(111) and on the bimetallic RhCu(111) surface has been investigated by first principles density functional theory using slab models. The most likely reaction pathway for CO oxidation on Rh(111) involves probably migration of atomic oxygen from fcc to hcp sites. On the bimetallic surface the mechanism is similar, although depending on the type of bimetallic site a reduction of the energy barrier is predicted. Consequences for the NO reduction by CO reaction are analyzed.     

Decomposition of methylamine on Mo(100) surface: A DFT study

The initial decomposition of methylamine on Mo(100) surface has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. The adsorption energies of possible species and the activation energies for possible elementary reactions involved are obtained in the present work. Our results indicate that the barriers decreased with the order of C-N>N-H>C-H. In addition, metastable adsorption of the abstracted hydrogen atom on the hollow site in the final state is also considered for the N-H and C-H bond breaking. For the C-H bond cleavage, the reaction barrier that the abstracted hydrogen located on the hollow site in the final state is lower than that on the bridge site. However, for the N H bond breaking, the barriers are alike for the abstracted hydrogen on both hollow and bridge sites in the final state.