Showing papers on "Chemisorption published in 1995"
TL;DR: In this paper, a simple surface reaction, the dissociation of H2 on the surface of gold and of three other metals (copper, nickel and platinum) that lie close to it in the periodic table, was studied.
Abstract: THE unique role that gold plays in society is to a large extent related to the fact that it is the most noble of all metals: it is the least reactive metal towards atoms or molecules at the interface with a gas or a liquid. The inertness of gold does not reflect a general inability to form chemical bonds, however—gold forms very stable alloys with many other metals. To understand the nobleness of gold, we have studied a simple surface reaction, the dissociation of H2 on the surface of gold and of three other metals (copper, nickel and platinum) that lie close to it in the periodic table. We present self-consistent density-functional calculations of the activation barriers and chemisorption energies which clearly illustrate that nobleness is related to two factors: the degree of filling of the antibonding states on adsorption, and the degree of orbital overlap with the adsorbate. These two factors, which determine both the strength of the adsorbate-metal interaction and the energy barrier for dissociation, operate together to the maxima] detriment of adsorbate binding and subsequent reactivity on gold.
2,721 citations
TL;DR: In this article, the authors present a consistent picture of some key physical properties determining the reactivity of metal and alloy surfaces, and suggest that trends in reactivities can be understood in terms of the hybridization energy between the bonding and anti-bonding adsorbate states and the metal d-bands (when present).
2,008 citations
TL;DR: In this paper, the performance of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C and PtFe/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated.
Abstract: The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a two- to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte. The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano plot) of different compositions. The latter investigation showed no activity enhancement for the ORR in phosphoric acid. This study therefore suggested the possibility of differences in electrochemical properties of bulk vs. supported alloy electrocatalysts (small particles of 35-85 A). A recent study on supported PtCo electrocatalysts ~ revealed the possibility that particle termination, primarily at the vicinal planes in the supported alloy electrocatalyst, is the reason for the enhanced ORR electrocatalysis (i.e., vicinal planes are more active than ). Paffett et al., 3 attributed higher activities for the ORR on bulk PtCr alloys in phosphoric acid to surface roughening, and hence increased Pt surface area, caused by the dissolution of the more oxidizable alloying component Cr. In contrast to these findings on bulk alloys, the supported alloy electrocatalysts have been reported to retain their nonnoble alloying element in the electrode during long periods (6000-9000 h) of operation in phosphoric acid fuel cells (PAFCs) 6 and proton exchange membrane fuel ceils (PEMFCs). 7 Based on these previous investigations and in the context of the ORR mechanisms, the principle explanations for the
1,063 citations
TL;DR: In this article, the formation of the oxide films can be examined in detail by recording the distinguishable stages in the film's electrochemical reduction in linear-sweep voltammetry which is sensitive down to OH O fractional coverages as low as 0.5% and over time-scales down to 50μs in experiments on time-evolution and transformation of oxide films.
548 citations
TL;DR: Copper-based catalysts are active in a wide range reactions of transformation of nitrogen oxides and represent an useful model system to better understand the fundamental aspects of the chemistry and mechanism of reaction of catalytic transformation of these pollutants.
Abstract: Copper-based catalysts are active in a wide range reactions of transformation of nitrogen oxides and represent an useful model system to better understand the fundamental aspects of the chemistry and mechanism of reaction of catalytic transformation of these pollutants. After an introduction on the reactivity of copper-based catalysts (supported and unsupported copper oxide, Cu-zeolites, cuprates and other copper compounds) in various reactions of conversion of nitrogen oxides, four main sub-topics are discussed in detail: (i) nature of copper species, (ii) chemisorption and surface transformations of NO, (iii) relationship between copper species and activity in the conversion of nitrogen oxides and (iv) mechanism of reduction of nitrogen oxides to N 2 . Five reactions of transformation of nitrogen oxides are discussed in detail: (i) decomposition of NO, (ii) reduction of NO with ammonia in the presence or not of oxygen, (iii) reduction of NO with hydrocarbons in the presence of oxygen, (iv) reduction of NO with CO and (v) decomposition of N 2 O. The mechanism of reduction of nitrite and N 2 O by copper enzymes is also discussed, with a view to provide some useful insights on the chemistry of transformation. In this review particular attention is directed towards controversial points in the literature, underestimated questions, and hypothesis and theories which do not allow interpretation of all sets of experimental data. Discussion is also focused on the presence of multiple and competitive pathways of transformation, the relative roles of which depend on reaction conditions.
420 citations
TL;DR: Carbon dioxide adsorbs reversibly on a silica gel containing 3-aminopropyl groups bonded to surface atoms of silicon, which act as the active sites for the chemisorption of CO 2 at room temperature which is liberated by temperature programmed desorption at about 100 °C as mentioned in this paper.
351 citations
TL;DR: In this paper, the authors show that at about 25 K the oxygen molecules physisorb on the surface of O 2 Pt and two chemisorbed phases are observed at 90 and 135 K, respectively.
251 citations
TL;DR: In this paper, a series of characterizations of monolayers formed from butanethiol and octanetiol at annealed mica-supported gold (Au/mica) and at gold supported by both chromium-primed silicon (au/Si) and glass (au/glass).
241 citations
TL;DR: Per-formed ab-initio total energy calculations show that a chemisorption reaction involving areconstruction to form a (111) hydroxyl surface is strongly preferred with ∆E = −90.2 kJ mol.
Abstract: The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab initio total-energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with \ensuremath{\Delta}E=-90.2 kJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$. We conclude that protonation stabilizes the otherwise unstable (111) surface and that this, not the bare (001), is the most stable surface of MgO under ambient conditions.
228 citations
TL;DR: In this article, a Fourier transform-ion cyclotron resonance mass spectrometer was used to investigate the gas phase reactivities of Niobium clusters Nb+n with molecular hydrogen, water, methane, ethane, n−propane, cyclohexane, acetylene, ethylene, allene, benzene, propene, toluene, xylene, and acetonitrile.
Abstract: In a Fourier transform‐ion cyclotron resonance mass spectrometer the gas phase reactivities of niobium clusters Nb+n (n=1–28) with molecular hydrogen, water, methane, ethane, n‐propane, n‐heptane, cyclohexane, acetylene, ethylene, allene, benzene, propene, toluene, xylene, and acetonitrile were investigated under single collision conditions as well as the reactivities of oxidized niobium cluster cations with ethylene and benzene. The reactions of larger clusters with a variety of unsaturated hydrocarbons are believed to proceed via long lived ‘‘physisorbed’’ addition intermediate complexes, which subsequently rearrange to form ‘‘chemisorbed,’’ extensively dehydrogenated final products. The overall reaction seems to proceed with near collision rates, almost independent of cluster size. In some cases also the physisorbed primary products are stabilized and detected. Their yields depend sensitively on the specific nature of the reactant, and on the niobium cluster size n. Fully saturated hydrocarbons unable ...
197 citations
TL;DR: In this paper, the influence of surface chemistry and metal precursor species on the properties of Pt/C catalysts has been analyzed and the effect of these parameters in the final metal dispersion has also been investigated.
TL;DR: In this paper, the authors investigated the adsorption and thermal conversion of molecular oxygen (O2) states on the TiO2(110) surface by making use of the distinct photodesorption behavior of each adaption state.
Abstract: We have investigated the adsorption and thermal conversion of molecular oxygen (O2) states on the TiO2(110) surface by making use of the distinct photodesorption behavior of each adsorption state. Oxygen chemisorbs at the oxygen vacancy defect sites on the annealed TiO2(110) surface at 105 K to a saturation coverage of less than 0.12 monolayers (ML), producing mostly the α‐O2 species which is observed to undergo slow photodesorption. Upon heating this surface to above 250 K, the α‐O2 is converted to the β‐O2 state which can photodesorb at a significantly higher rate. The β‐O2 species dissociates above 400 K to produce atomic oxygen, eliminating the oxygen anionic vacancies. Both the α‐ and β‐photodesorption processes have a threshold energy at the TiO2 band gap (3.1 eV), indicating a substrate excitation mediated process. The photodesorption time‐profile is fitted with an exponential decay function with a cross section of ∼8×10−17 cm2 for the α‐O2 and ∼1.5×10−15 cm2 for the β‐O2 species at a photon energy...
TL;DR: In this article, the semi-empirical MO method SINDO1 was used for the investigation of molecular and dissociative adsorption of water at titanium dioxide surfaces rutile (110) and anatase (001).
TL;DR: In this paper, the binding energy of CO on the non-defective TiO2(110) surface is 9.9 kcal/mole in the limit of zero coverage, which suggests that CO adsorbs more strongly on lattice Ti sites in the vicinity of anion vacancy sites.
Abstract: CO chemisorption has been studied on TiO2(110) under surface conditions where oxygen anion vacancy sites are not present (oxidized surface), compared to conditions where the vacancy sites are present (annealed surface). The binding energy of CO on the nondefective TiO2(110) surface is 9.9 kcal/mole in the limit of zero coverage. CO...CO repulsive interactions have been observed at higher coverages. When anion vacancy sites are produced under controlled annealing conditions in vacuum at 900 K, a significant increase in the desorption temperature of a portion of the chemisorbed CO is observed. This observation, coupled with measurements showing that defective TiO2(110) does not have enhanced CO chemisorption capacity, suggests that CO adsorbs more strongly on lattice Ti sites in the vicinity of anion vacancy sites. It is postulated that enhanced CO bonding occurs via the interaction of the O moiety of CO with the anion vacancy site while primary adsorbate bonding occurs via the C moiety to Ti lattice sites. Neither CO2 production nor oxygen exchange in CO occurs when CO desorbs from defective TiO2(110).
TL;DR: In this article, a buckled Si-Si dimer on the bare reconstructed surface was shown to form a four-member disilacyclobutene ring without breaking the si-Si bond, and upon acetylene chemisorption, the buckled dimers become symmetric.
Abstract: Both cluster and extended slab models for the bare Si(OO1) surface and for acetylene chemisorbed on that surface were studied by various theoretical methods. Our calculations point to a buckled Si-Si dimer on the bare reconstructed surface. Acetylene adsorbs onto the dimer to form a four-member disilacyclobutene ring without breaking the Si-Si bond. Upon acetylene chemisorption, the buckled dimers become symmetric. The surface reaction is orbital symmetry-forbidden. A likely pathway for the cycloaddition, via a n-complex precursor and a biradical intermediate, is suggested. Introduction Silicon is at the heart of our information age. The most common form of this element has the beautiful bulk structure of diamond (1, a conventional unit cell), in which every Si atom is covalently bonded to four Si’s. Both the bulk material and silicon surfaces are of technological importance. If a silicon crystal is cut, one of the simplest surfaces one gets, and one very well studied, is Si(OO1). The unreconstructed surface would expose a two-dimensional square lattice (2; a conventional unit cell is outlined), with each surface
TL;DR: In this article, the role of step sites on metal surfaces in promoting chemical reactions is discussed showing experimental data from the author's laboratory, and the adsorption of CO on two stepped Pt surfaces, Pt (112) and Pt (335), is compared using various methods that are sensitive to adsorptive site character.
Abstract: The role of step sites on metal surfaces in promoting chemical reactions is discussed showing experimental data from the author’s laboratory. The adsorption of CO on two stepped Pt surfaces, Pt (112) and Pt (335), is compared using various methods that are sensitive to adsorption site character. The existence of one‐dimensional arrays of chemisorbed CO molecules along the step sites is demonstrated, and CO...CO repulsive steric interactions are observed to cause orthogonal tilting of the step‐bound CO species as the coverage is changed on the step sites. For Pt (335), the most active site for the oxidation of CO is found to involve a chemisorbed oxygen atom at the step site, reacting with a CO molecule chemisorbed on a terrace site. It is also shown that a new surface phenomenon, electron stimulated migration (ESM), may be spectroscopically observed for CO on Pt (335). The atomic steps act as trap sites for CO molecules that have been excited by 150 eV electrons. It is likely that this ESM phenomenon is a property of thermally nonaccommodated species in the electronic ground state of the chemisorbed CO that have been produced by quenching from an excited electronic state.
TL;DR: In this article, a high surface area carbon black was pre-treated at 1223 K under H 2 or Ar to remove sulfur contaminants and oxygen-containing functional groups on the surface.
TL;DR: In this paper, a complete description of the composition of the mixed copper-anion overlayers is given, and it is shown that the coadsorption of a copper adatom and the anion has a cooperative character and that the ion affects the kinetics and energetics of the copper deposition reaction.
TL;DR: In this article, the same defect creation and healing processes were also observed on a nearly defect-free thermally annealed single-crystal surface using XPS, which was used to confirm the defect type and to quantify the density of Ti3+ defects created.
TL;DR: In this paper, the first experimental observation of two chemisorption states for molecular oxygen on a TiO2(110) surface containing anion vacancy sites was reported, where the first molecular species can be photoactivated to oxidize coadsorbed CO to CO2 (α channel) and undergoes slow photodesorption (β channel).
Abstract: We report the first experimental observation of two chemisorption states for molecular oxygen on a TiO2(110) surface containing anion vacancy sites. The first molecular species can be photoactivated to oxidize coadsorbed CO to CO2 (α channel) and undergoes slow photodesorption. The second molecular oxygen species only undergoes fast photodesorption (β channel). Conversion from α‐O2, to β‐O2 occurs upon heating the surface to above 200 K.
TL;DR: In this paper, total energy calculations using density functional theory with gradient corrections for CO chemisorption on an extended Pd{110} slab yield good agreement with experimental adsorption energies.
TL;DR: In this article, the authors propose that the strong interaction arises according to a traditional SMSI model, by the migration of partially reduced ceria over the surface of the platinum particles (at ⩾ 600°C).
Abstract: The ability of Pt/CeO 2 to exhibit three-way catalytic activity at low temperatures results from a strong metal-support interaction, which is induced by activating the catalyst under a reducing atmosphere. Carbon monoxide chemisorption measurements show that a loss of exposed metal area occurs during the activation procedure, even though the conditions do not favour platinum sintering. Following activation (and subsequent exposure to air at ambient temperature), the surface ceria is left in a highly reducible state, which is characterised by a subambient peak during temperature-programmed reduction. We envisage that the strong interaction arises according to a traditional SMSI model, by the migration of partially reduced ceria over the surface of the platinum particles (at ⩾ 600°C). The resultant high degree of contact between the metal (with high work-function) and the metal oxide (with high band gap) promotes the formation of oxygen vacancies on the ceria surface. Although the presence of this highly reducible ceria-covering blocks conventional three-way sites on the platinum, it provides new sites that are active even at low temperatures. Therefore, unlike most previous explanations of promotion caused by a strong interaction, we propose that the ‘support’ becomes the active phase. Re-oxidation at elevated temperatures causes the ceria covering to coalesce, leading initially to its partial thinning, and subsequently to the re-exposure of the platinum particles.
TL;DR: In this article, the interaction of carbon with Ni(111) has been studied by scanning tunnelling microscopy and the surface atoms are found to rearrange into an almost square surface mesh with a clock reconstruction which is quite similar to that observed for carbon on the Ni(100) surface.
TL;DR: In this article, the first steps of H 2 S adsorption have been studied on polycrystalline copper samples, which are more relevant to industrial copper samples than to single-crystal surfaces.
Abstract: The first steps of H 2 S adsorption have been studied on polycrystalline copper samples, which are more relevant to industrial copper samples than to single-crystal surfaces. The tools for characterization are ultrahigh vacuum spectroscopic methods, such as x-ray photoelectron and ion scattering spectroscopies. The H 2 S adsorption on metallic copper and on the corresponding Cu 2 O and CuO oxides is dissociative and highly dependent on the chemical nature of the exposed surface rather than on the temperature, from room temperature up to 623 K. In our conditions, Cu° sulphidation deals with the top Cu° layer whereas two to three oxide surface monolayers are modified at H 2 S saturation. The results on oxides are interpreted by an oxygen/sulphur replacement mechanism following the dissociative adsorption. Contaminated copper surfaces by phenanthrene or H 2 O treatments are hardly sulphided. As far as surface reactivity is concerned, the adsorbed sulphide species have been shown to be quite strongly bound to the different surface states tested in this work: only oxidative treatments induce desorbing products at 623 K
TL;DR: In this paper, the results of an investigation concerning the NO reduction activity of different carbons loaded by ion exchange with varying amounts of a potassium catalyst were reported, and the reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined.
Abstract: The results of an investigation are reported concerning the NO reduction activity of different carbons loaded by ion exchange with varying amounts of a potassium catalyst. The samples were characterized by physical adsorption of CO 2 (at 0 o C) and N 2 (at -196 o C) and by chemisorption of CO 2 at 250 o C. The reactivity of the pure carbons was determined in both NO and CO 2 . The catalytic effect of potassium in NO reduction was evaluated in a fixed-bed flow reactor at atmospheric pressure using two types of experiments: (i) temperature-programmed reaction in a NO/He mixture; and (ii) isothermal reaction at 300-600 o C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. Potassium was found to be an excellent catalyst for the NO reduction by carbon. At temperatures below 600 o C, the products of the reaction were primarily N 2 and CO 2 . The catalytic activity, and thus the concentration of catalytically active sites, is dependent on both catalyst dispersion and carbon reactivity
TL;DR: In this article, the pK a of the free base porphyrin monolayer is ca. 2.5 and the lower limit of the two-electron catalysis rate constant is 6 x 10 4 M -1 s -1.
Abstract: Visible spectroscopy and electrochemistry have been employed to study monolayer films of 5,10,15,-20-tetrakis[o-(2-mercaptoethoxy)phenyl]porphyrin in free base, H 2 (o-TMEPP), and cobalt-metalated Co-(o-TMEPP) forms chemisorbed onto transparent gold electrodes. Transmission spectra of these films are consistent with approximately monolayer ((4.8-6.5) x 10 11 mol/cm 2 ) coverages and edge-edge porphyrin-porphyrin interactions within the monolayers as revealed by red shifts and broadening in the Soret bands compared to solution species. Chemical modifications of H 2 (o-TMEPP) monolayers, namely, cobalt metalation and protonation/deprotonation, are described. The pK a of the free base porphyrin monolayer is ca. 2.5. Spectroelectrochemical experiments of Co(o-TMEPP) monolayers allow optical tracking of the redox state of the porphyrin, which was applied in electrocatalytic dioxygen reduction to show that the onset of dioxygen reduction is coincident with the Co(III) → Co(II) process in the monolayer and that the lower limit of the two-electron catalysis rate constant is 6 x 10 4 M -1 s -1 .
TL;DR: In this paper, a microkinetic model for CO methanation on nickel based on CO dissociation and stepwise hydrogenation of surface carbon is presented, which is very good agreement with the measured on Ni(100) by Goodman et al. and on nickel foils by Polizzotti and Schwarz (J. Catal. 77, 1 (1982)).
TL;DR: In this paper, three different coadsorption systems were used to study the reactivity of different oxygen species; (1) CO on the O2 preadsorbed Pt(111) surface, (2) CO in the nearly perfect Pt(1) p(2×2) O surface, and (3) CO at the disordered atomic oxygen-preaded Pt(2) surface.
Abstract: Thermal excitation of adsorbed oxygen species is found to initiate the CO oxidation on Pt(111). We have prepared three different coadsorption systems to study the reactivity of different oxygen species; (1) CO on the O2 preadsorbed Pt(111) surface, (2) CO on the nearly perfect Pt(111) p(2×2)‐O surface, and (3) CO on the disordered atomic oxygen‐preadsorbed Pt(111) surface. Four CO2 desorption peaks (α‐CO2 at 125 K, β3‐CO2 at ∼225 K, β2‐CO2 at ∼260 K, and β1‐CO2 at 320 K) are observed. The desorption temperatures of CO2 strongly depend on the adsorbed states of oxygen species. We have shown that the α‐CO2 state, β2,3‐CO2 states, and β1‐CO2 state are correlated with adsorbed O2, disordered oxygen atoms, and p(2×2) oxygen atoms, respectively. The difference in CO2 desorption temperature is related to thermal excitation of each oxygen species, which is derived from the structural information of coadsorbed states during thermal evolution by means of low‐energy electron diffraction and infrared reflection absor...
TL;DR: In this paper, a Ni(100) single crystal has been studied under thermal conditions as a function of pressure and temperature, and the initial sticking coefficient was measured in the pressure range of 0.010 −7.0 mbar at temperatures ranging from 375 to 500 K. A strong pressure dependence was observed, consistent with a direct dissociation mechanism under these thermal conditions.
Abstract: The dissociative chemisorption of methane on a Ni(100) single crystal has been studied under thermal conditions as a function of pressure and temperature. The initial sticking coefficient was measured in the pressure range of 0.010–7.0 mbar at temperatures ranging from 375 to 500 K. A strong pressure dependence was observed, consistent with a direct dissociation mechanism under these thermal conditions. This was further confirmed by experiments where the gas at a low pressure was heated by a thermal finger facing the crystal surface. With the thermal finger at the same temperature as the surface, it was possible to ensure that the methane was fully equilibrated to the crystal and an activation energy of 59±1.5 kJ/mol was determined under isothermal conditions.