Structure and reaction pathways of methyl lactate on Pd(1 1 1)

The adsorption and reaction of methyl lactate (CH₃CH(OH)COOCH₃) is studied in ultrahigh vacuum on a Pd(1 1 1) surface using temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). Methyl lactate reacts at relatively low temperatures (220 K) by O–H bond scission. This intermediate can either react with hydrogen to reform methyl lactate at 280–300 K or undergo β-hydride elimination to form flat-lying methyl pyruvate. This decomposes to form acetyl and methoxy carbonyl species as found previously following methyl pyruvate adsorption on Pd(1 1 1). These species predominantly react to form carbon monoxide, methane and hydrogen.     

Adsorption and thermal decomposition of N-methylaniline on Pt(1 1 1)

The adsorption and thermal decomposition of N-methylaniline (NMA) on the Pt(1 1 1) surface has been studied with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). NMA adsorbs molecularly at 85 K through the nitrogen lone pair and is stable up to 300 K. At temperatures of 300–350 K it converts to two or more surface intermediates including the N-methyleneaniline (NMEA) species. This NMEA intermediate dissociates upon annealing to 450 K, and further annealing leads to the desorption of HCN and H₂, leaving only C on the surface at 800 K.     

Acetaldehyde photochemistry on TiO2(1 1 0)

The ultraviolet (UV) photon induced decomposition of acetaldehyde adsorbed on the oxidized rutile TiO₂(1 1 0) surface was studied with photon stimulated desorption (PSD) and thermal programmed desorption (TPD). Acetaldehyde desorbs molecularly from TiO₂(1 1 0) with minor decomposition channels yielding butene on the reduced TiO₂ surface and acetate on the oxidized TiO₂ surface. Acetaldehyde adsorbed on oxidized TiO₂(1 1 0) undergoes a facile thermal reaction to form a photoactive acetaldehyde–oxygen complex. UV irradiation of the acetaldehyde–oxygen complex initiated photofragmentation of the complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate.     

Comparative dynamical study of the HD and isotopomers interacting with Pd(1 1 1) and Cu(1 1 0) surfaces

We study the dynamics of HD and H₂ molecules interacting with Pd(1 1 1) and Cu(1 1 0) using the classical trajectory method based on potential energy surfaces obtained from Density Functional Theory calculations. Our results predict a negligible isotopic effect on the dissociative adsorption probability on Pd(1 1 1) whereas on Cu(1 1 0), the adsorption probability for HD(ν_i=0) is slightly lower than for H₂(ν_i=0), mainly due to its lower initial vibrational zero point energy. The final rotational energy distribution of scattered HD and H₂ molecules are very similar. This shows that the asymmetric mass distribution of HD, barely affects the fraction of initial translational energy transferred to rotation during the scattering process. Our calculations point to the larger number of open rotational excitation channels for HD, as the main cause of rotational excitation probabilities larger than for H₂. The theoretical apparent rotational temperature, T_rot, of HD molecules scattered from Pd(1 1 1) at impact energy , is in good agreement with the experimental value. In contrast, for Cu(1 1 0) the theoretical T_rot is much lower than the value measured for Cu(1 0 0). Possible reasons for such a discrepancy between theory and experiments are discussed.     

Structural properties of Cu clusters on Si(1 1 1):Cu2Si magic family

Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 × 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)‘5.5 × 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates 20 Cu atoms with 20 Si atoms being expelled from the corresponding 7 × 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu₂Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 × 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 × 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images.     

Model NSR catalysts: Fabrication and reactivity of barium oxide layers on Cu(1 1 1)

The growth of barium oxide on a Cu(1 1 1) substrate, formed by the deposition of barium and its subsequent oxidation, yields stable BaO films which expose predominantly the BaO(1 0 0) surface. The interaction of the oxide films with common components of motor-vehicle exhaust gases (CO₂, H₂O, NO_x) has been studied using surface analytical techniques, including X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and reflection IR spectroscopy (RAIRS). The spectroscopic identification of Ba(OH)₂, BaCO₃ and Ba(NO₂)₂ phases is discussed, and the relative stabilities and decomposition mechanisms of these materials when supported on Cu(1 1 1) is revealed by a combination of TPD and XPS. BaO is shown to be resistant to reaction with pure NO and NO/O₂ mixtures, but exposure to NO₂ leads to the rapid formation of barium nitrite. The formation of the nitrite is proposed to be the first-step in the production of barium nitrate, which has previously been shown to be the main phase involved in NO_x storage and reduction (NSR) catalysis.     

Theoretical study of hydrogenation process of formate on clean and Zn deposited Cu(1 1 1) surfaces

We have studied the effect of Zn on hydrogenation of formate to dioxomethylene on the Cu(1 1 1) surface by using a density functional theory–generalized gradient approximation (DFT–GGA)-pseudopotential method. We show that substitutionally adsorbed Zn changes the stability of intermediate states and the activation barrier of the hydrogenation process only slightly. On the other hand, the Zn atom adsorbed on the Cu surface stabilizes all formate, transition state, and dioxomethylene relative to the gas-phase molecules. Our results support a previously proposed reaction scheme that the adsorption state of Zn changes from substitutional to on-surface adsorption during the methanol synthesis.     

The electrodeposition of copper onto UHV-prepared GaAs(0 0 1) surfaces

Synchrotron surface X-ray diffraction has been used to investigate in situ the morphology and epitaxy of monolayer amounts of copper electrodeposited from aqueous electrolyte onto ultra-high vacuum prepared, smooth, Ga- or As-terminated GaAs(0 0 1) surfaces. The fcc lattice of the epitaxial Cu islands is rotated by 5° and tilted by about 9° with respect to the GaAs substrate lattice, leading to eight symmetry equivalent domains of Cu islands terminated by {1 1 1} facets.     

Free radical reactions at the Ru(0 0 0 1) surface: Atomic oxygen and dissociated NH3

X-ray photoelectron spectroscopy was used to study the effect of atomic oxygen on Ru(0 0 0 1), and the effect of dissociated ammonia on RuO₂/Ru(0 0 0 1), in UHV conditions at ambient temperature. The Ru(0 0 0 1) surface was exposed, at ambient temperature, to a mixed flux of atomic and molecular oxygen generated by dissociation of O₂ in a thermal catalytic cracker, with 45% dissociation efficiency. The detailed study of the XPS spectra shows the formation of a disordered multilayer oxide (RuO₂). No formation of higher oxides of Ru was observed. The formation of RuO₂ proceeded without saturation for total oxygen exposures of up to 10⁵ Langmuir, at which point an average oxide thickness of 68 Å was observed. RuO₂ formed by the reaction with atomic oxygen was exposed to a flux of NH_x (x = 1, 2) + H generated by the cracker. The reduction of RuO₂ to Ru metal was observed by XPS. An exposure of 3.6 × 10² L of NH_x + H, resulted in the observation of adsorbed H₂O and OH, but no evidence of lattice oxide. The chemisorbed species were removed by additional NH_x + H exposure. No nitrogen adsorption was observed.     

Characterization of methylidyne on Pt(1 1 1) with infrared spectroscopy

Methylidyne (CH) was prepared on Pt(1 1 1) by three methods: thermal decomposition of diiodomethane (CH₂I₂), ethylene decomposition at temperatures above 450 K, and surface carbon hydrogenation. Methylidyne and its precursors are characterized by reflection absorption infrared spectroscopy (RAIRS). The C-I bond of diiodomethane breaks upon adsorption to produce methylene (CH₂), which decomposes to methylidyne at temperatures above 130 K. Above 200 K, methylidyne is the only hydrocarbon species observed with RAIRS, although reaction channels for the formation of methane (CH₄) and ethylene (C₂H₄) are indicated by temperature programmed desorption (TPD). As is well known from numerous previous studies, ethylene decomposes to ethylidyne (CCH₃) upon exposure to Pt(1 1 1) at 410 K. Upon annealing to 450 K, ethylidyne dissociates through two reaction pathways, dehydrogenation to ethynyl (CCH) and C-C bond scission to methylidyne. Ethylene dehydrogenation on the surface at 750 K and under low ethylene exposures produces surface carbon that can be hydrogenated to methylidyne with C-H and C-D stretch frequencies of 2956 and 2206 cm⁻¹, respectively. Hydrogen co-adsorption on the surface causes these frequencies to shift to higher values. Methylidyne is stable on Pt(1 1 1) to temperatures up to 500 K.     

Adsorption and decomposition of formic acid on the NiO(111)-p(2 × 2) surface: TPD and steady state kinetics studies

The adsorption and decomposition of formic acid on NiO(111)-p(2 × 2) films grown on Ni(111) single crystal surface were studied by temperature-programmed desorption (TPD) spectroscopy. Exposure of formic acid at 163 K resulted in both molecular adsorption and dissociation to formate. The adsorbed formate underwent further dissociation to H₂, CO₂ and CO. H₂ and CO₂ desorbed at the same temperatures of 340, 390 and 520 K, while CO desorbed at 415 and 520 K. The desorption features varied with the formic acid exposure. Two reaction channels were identified for the decomposition of formate under equilibrium with gas-phase formic acid with a pressure of 2.5 × 10⁻⁴Pa, one preferentially producing H₂ and CO₂ with an activation energy of 22 ± 2 kJ mol⁻¹ and the other preferentially producing CO and H₂O with an activation energy of 16 ± 2 kJ mol⁻¹. The order of both reaction paths was 0.5 with respect to the pressure of formic acid.     

Site-blocking effects of preadsorbed H on Pt(1 1 1) probed by 1,3-butadiene adsorption and reaction

The influence of hydrogen coadsorption on hydrocarbon chemistry on transition metal surfaces is a key aspect to an improved understanding of catalytic selective hydrogenation. We have investigated the effects of H preadsorption on adsorption and reaction of 1,3-butadiene (H₂CCHCHCH₂, C₄H₆) on Pt(1 1 1) surfaces by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed hydrogen adatoms decrease the amount of 1,3-butadiene chemisorbed on the surface and chemisorption is completely blocked by the hydrogen monolayer (saturation) coverage (θ_H = 0.92 ML). No hydrogenation products of reactions between coadsorbed H adatoms and 1,3-butadiene were observed to desorb in TPD experiments over the range of θ_H investigated (θ_H = 0.6-0.9 ML). This is in strong contrast to the copious evolution of ethane (CH₃CH₃, C₂H₆) from coadsorbed hydrogen and ethylene (CH₂CH₂, C₂H₄) on Pt(1 1 1). Hydrogen adatoms effectively (in a 1:1 stoichiometry) remove sites from interaction with chemisorbed 1,3-butadiene, but do not affect adjacent sites. The adsorption energy of coadsorbed 1,3-butadiene is not affected by the presence of hydrogen on Pt(1 1 1). The chemisorbed 1,3-butadiene on hydrogen preadsorbed Pt(1 1 1) completely dehydrogenates to H₂ and surface carbon upon heating without any molecular desorption detected, which is identical to that observed on clean Pt(1 1 1). In addition to revealing aspects of site blocking that should have broad implications for hydrogen coadsorption with hydrocarbon molecules on transition metal surfaces in general, these results also provide additional basic information on the surface science of selective catalytic hydrogenation of butadiene in butadiene-butene mixtures.     

CO2 dissociation on Ni(2 1 1)

The direct and H-mediated dissociation of CO₂ on Ni(2 1 1) were investigated at the level of density functional theory. Although formate (HCOO) formation via CO₂ hydrogenation was widely reported for CO₂ adsorption on metal surfaces, it is found that on Ni(2 1 1) HCOO dissociation into CHO and O is much difficult, while direct dissociation of adsorbed CO₂ into CO and O is more favorable. It is also found that the degree of electron transfer from surface to adsorbed CO₂ correlates with the elongation of C-O bond lengths and the reduction of the CO₂ dissociation barrier.     

Shock tube/laser absorption measurement of the rate constant of the reaction: H2O2 + CO2 2OH + CO2

We address the role of the linear mixing rule in the kinetics of the H₂O₂ decomposition system by reporting the rate constant for H₂O₂ + M = 2OH + M (M = Ar and CO₂) in the temperature range of 1087–1234 K at low pressures in a mixture of 20% CO₂ in Argon. The reaction rate constant was inferred from H₂O concentrations monitored by using a laser-absorption spectroscopy-based water diagnostic. To the best of our knowledge, this is the first measurement of the rate constant of this reaction in a mixture to be reported in literature. A significant discrepancy was found between the rate constants derived using the traditional linear mixing rule and the reduced pressure linear mixing rule. This discrepancy can have serious implications on the predictive accuracy of these kinetic models, especially under conditions relevant to the operation of supercritical CO₂ (sCO₂) power cycles that rely on oxy-fuel combustion in a working fluid comprised almost entirely of CO₂.     

Nanoscale anglesite growth on the celestite (0 0 1) face

In situ atomic force microscopy (AFM) was used to study the growth behaviour of anglesite (PbSO₄) monolayers on the celestite (0 0 1) face. Growth was promoted by exposing the celestite cleavage surfaces to aqueous solutions that were supersaturated with respect to anglesite. The solution supersaturation, β_ang, was varied from 1.05 to 3.09 (where β_ang = a(Pb²⁺) · a(SO₄²⁻)/K_sp,ang). In this range of supersaturation, two single anglesite monolayers (3.5 Å in height each) from pre-existent celestite steps were grown. However, for solution supersaturation β_ang < 1.89 ± 0.06, subsequent multilayer growth is strongly inhibited. AFM observations indicate that the inhibition of a continuous layer-by-layer growth of anglesite on the celestite (0 0 1) face is due to the in-plane strain generated by the slight difference between the anglesite and celestite lattice parameters (i.e. the linear misfits are lower than 1.1%). The minimum supersaturation required to overcome the energy barrier for multilayer growth gave an estimate of the in-plane strain energy: 11.4 ± 0.6 mJ/m²_. Once this energy barrier is overcome, a multilayer Frank–Van Der Merwe epitaxial growth was observed.     

Thermo‐Raman spectroscopy of selected layered double hydroxides of formula Cu6Al2(OH)16CO3 and Zn6Al2(OH)16CO3

Raman spectroscopy using a hot stage was used to characterise layered double hydroxides (LDHs) of the formula (Cu,Zn)₆Al₂(OH)₁₆(CO₃)^·4H₂O. The spectra were used to assess the molecular assembly of the cations in the LDH structure. The sharp band at 1058 cm⁻¹for the Zn₆Al₂(OH)₁₆(CO₃)^·4H₂O is assigned to the ν₁CO₃²⁻ symmetric stretching mode. This band shifts to higher wavenumbers and is observed at 1103 cm⁻¹at 600 °C. It is proposed that metal carbonate species formed during the decomposition of the hydrotalcite structure is responsible for the increase in the band position. The Cu–Al hydrotalcite did not show the same trend. The symmetric stretching mode of carbonate is observed at around 1110 cm⁻¹, and at temperatures above 200 °C a shoulder appears at around 1210 cm⁻¹, suggested to be due to CuCO₃. Copyright © 2008 John Wiley & Sons, Ltd.     

Mechanism of CO2 hydrogenation over Cu/ZrO2(2̅12) interface from first-principles kinetics Monte Carlo simulations

It has been a goal consistently pursued by chemists to understand and control the catalytic process over composite materials. In order to provide deeper insight on complex interfacial catalysis at the experimental conditions, we performed an extensive analysis on CO₂ hydrogenation over a Cu/ZrO₂ model catalyst by employing density functional theory (DFT) calculations and kinetic Monte Carlo (kMC) simulations based on the continuous stirred tank model. The free energy profiles are determined for the reaction at the oxygen-rich Cu/m-ZrO₂ (2?12) interface, where all interfacial Zr are six-coordinated since the interface accumulates oxidative species at the reaction conditions. We show that not only methanol but also CO are produced through the formate pathway dominantly, whilst the reverse-water-gas-shift (RWGS) channel has only a minor contribution. H₂CO is a key intermediate species in the reaction pathway, the hydrogenation of which dictates the high temperature of CO₂ hydrogenation. The kinetics simulation shows that the CO₂ conversion is 1.20%, the selectivity towards methanol is 68% at 500 K and the activation energies for methanol and CO formation are 0.79 and 1.79 eV, respectively. The secondary reactions due to the product readsorption lower the overall turnover frequency (TOF) but increase the selectivity towards methanol by 16%. We also show that kMC is a more reliable tool for simulating heterogeneous catalytic processes compared to the microkinetics approach.     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

Adsorption of methanol on Ni3Al(1 1 1) and NiAl(1 1 0): A high resolution PES study

The adsorption of methanol on Ni₃Al(1 1 1) and NiAl(1 1 0) has been studied using high resolution photoemission spectroscopy (HR-PES) and density functional theory (DFT). Both methanol and methoxy are formed on these surfaces after the initial methanol exposure at low temperatures. Heating to 200 K leads to further formation of methoxy. On NiAl(1 1 0) two different methoxy species are observed where the first is formed upon methanol adsorption, and the other results from methanol decomposition during heating. The DFT calculations show that methanol and methoxy interacts with the Al atoms on both surfaces. Methanol is found to bond through the oxygen atom to the Al on-top site on Ni₃Al(1 1 1) and NiAl(1 1 0) with the C–O axis tilted with respect to the surface normal. On Ni₃Al(1 1 1) methoxy is situated in a 2Ni+Al hollow site, whereas on NiAl(1 1 0) the Al–Al bridge site is preferred.     

Nucleation, growth and morphology of Co thin films on Ag(1 1 0)

Scanning tunneling microscopy (STM) experiments reveal that Co growth on Ag(1 1 0), at coverages of Co < 1 ML and low substrate temperatures (150 K), involves a concomitant insertion of Co into the top Ag layer and exchange of Ag out onto the surface. At 300 K, coverages of Co > 1 ML gives rise to a 3D nanocluster growth on the surface, with the clusters covered by Ag. Depending slightly on coverage, the clusters have a typical diameter of 3 nm and a height of 0.4 nm. Upon annealing to 500 K, major changes are observed in the morphology of the surface. STM and AES show that there is a reduction of the number of Co islands on the surface, partly due to subsurface Co cluster migration and partly due to sintering into larger clusters.