The mechanism of methanol formation and other reactions following formaldehyde adsorption on Ni(110)

The adsorption/desorption and reactive behavior of formaldehyde was studied on clean single-crystal Ni(110) at adsorption temperatures down to 200 °K. For low exposures of the surface to formaldehyde, hydrogen and CO binding states were populated due to decomposition of the molecule upon adsorption. Higher exposures gave rise to a decomposition-limited hydrogen peak exhibiting an activation energy of 20 kcal/gmol and an apparent frequency factor of 10¹⁴ sec^?1. At initial coverages of H₂CO exceeding about 0.5, monolayer methanol was observed to form. The formation of methanol involved a hydrogen atom transfer between two adsorbed H₂CO molecules and did not occur totally via surface hydrogen. Self-oxidation to form CO₂ was also observed. The surface exhibited reaction heterogeneity, and the surface reactivity was observed to depend on the temperature of adsorption of reactants, suggesting strong adsorbate-induced surface “reconstruction.”     

The interaction of hydrogen and carbon monoxide on Cu-Ni (110) single-crystal surfaces

The co-adsorption of H₂ and CO on a Cu-Ni (110) surface was studied by thermal desorption spectroscopy (TDS) and ultraviolet photoemission spectroscopy (UPS). Strong interactions between adsorbed CO and hydrogen observed in the CO desorption spectrum and CO valence emissions were attributed to a blockage of certain CO adsorption sites by hydrogen.     

CO and methanol adsorption on (2 × 1)Pt(110) and ion‐eroded Pt(111) model catalysts

Methanol adsorption on ion‐sputtered Pt(111) surface exhibiting high concentration of vacancy islands and on (2 × 1)Pt(110) single crystal were investigated by means of photoelectron spectroscopy (PES) and thermal desorption spectroscopy. The measurements showed that methanol adsorbed at low temperature on sputtered Pt(111) and on (2 × 1)Pt(110) surfaces decomposed upon heating. The PES data of methanol adsorption were compared to the data of CO adsorbed on the same Pt single crystal surfaces. In the case of the sputtered Pt(111) surface, the dehydrogenation of H_xCO intermediates is followed by the CO bond breakage. On the (2 × 1)Pt(110) surface, carbon monoxide, as product of methanol decomposition, desorbed molecularly without appearance of any traces of atomic carbon. By comparing both platinum surfaces we conclude that methanol decomposition occurs at higher temperature on sputtered Pt(111) than on (2 × 1)Pt(110). Copyright © 2010 John Wiley & Sons, Ltd.     

Electron spectroscopic studies of the adsorption and decomposition of methanol on transition metals. A review

Results of investigations of the adsorption and decomposition of methanol on the surface of transition metals such as Fe, Ni, Cu, Pd, Ag, Mo, W and Pt byuv and x-ray photoelectron spectroscopy, electron energy loss spectroscopy, Auger electron spectroscopy and thermal desorption spectroscopy have been reviewed. The first step in the decomposition of CH₃OH on these metal surfaces is the formation of the methoxy species, OCH₃ radical. In the case of Fe, Mo and W, complete decomposition of CH₃OH occurs leaving CO(β), H₂ and CH₄ on the surface. Dissociation proceeds upto CO(α) and H₂ on the surface of Ni, Pd and Pt whereas on Ag and Cu, selective oxidation of CH₃OH to H₂CO is preferred. The difference in the reactivity of metals towards CH₃OH is rationalised from the heats of adsorption of O₂, CO and H₂ on these metals. Contribution No. 253 from the Solid State and Structural Chemistry Unit.     

Cryogenic Chemistry and Quantitative Non-Thermal Desorption from Pure Methanol Ices: High-Energy Electron versus X-Ray Induced Processes

X-Ray irradiation of interstellar ice analogues has recently been proven to induce desorption of molecules, thus being a potential source for the still-unexplained presence of gaseous organics in the coldest regions of the interstellar medium, especially in protoplanetary disks. The proposed desorption mechanism involves the Auger decay of excited molecules following soft X-ray absorption, known as X-ray induced electron-stimulated desorption (XESD). Aiming to quantify electron induced desorption in XESD, we irradiated pure methanol (CH₃OH) ices at 23 K with 505 eV electrons, to simulate the Auger electrons originating from the O 1s core absorption. Desorption yields of neutral fragments and the effective methanol depletion cross-section were quantitatively determined by mass spectrometry. We derived desorption yields in molecules per incident electron for CO, CO₂, CH₃OH, CH₄/O, H₂O, H₂CO, C₂H₆ and other less abundant but more complex organic products. We obtained desorption yields remarkably similar to XESD values.     

Decomposition of methane on polycrystalline thick films of Ga2O3 investigated by thermal desorption spectroscopy with a mass spectrometer

Methane in air can be detected by the conductivity increase of Ga₂O₃ films. Films (200 μm) of β-Ga₂O₃ were prepared by depositing a suspension of β-Ga₂O₃ powder (Johnson Matthey; 32102; 99,99%) on alumina substrates. The films were exposed to 20 kPa O₂ for 15 min at 934 K. In thermal desorption spectroscopy (TDS, β = 4,6 K/s, UHV conditions) only O₂ occured at temperatures above 934 K. On reduction in 100 Pa H₂ for 5 min at 800 K, only a suboxide, Ga₂O (above 880 K), indicating a destabilisation of the lattice [1], a broad hydrogen peak (440–930 K) and the formation of water (700–900 K) were observed. No Ga₂O₃ and O₂ were found in desorption. At temperatures between 260 K and 934 K the film was exposed to methane (100 Pa, 5 min). For exposure temperatures between 630 K and 934 K, CO, CO₂, H₂, and small amounts of CH₄ and the suboxide Ga₂O appeared in desorption. A reaction scheme for the decomposition of methane is proposed. It includes the adsorption of CH₄, the dissociation of CH₄, the desorption of H₂O and the formation of oxygen vacancies. These vacancies and the adsorbed hydrogen both acting as donors may explain the conductance increase on exposure to methane observed by other authors.     

The character of interactions of CO,H<Subscript>2</Subscript>, and CH<Subscript>3</Subscript>OH with the surface of γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Cu-Zn oxide catalyst

The thermal desorption of CO, H₂, and CH₃OH from the surface of Katalco-58 industrial catalyst for the synthesis of methanol and γ-Al₂O₃ was studied. Weak interaction of the gases with the surface of samples was observed over the temperature range 75–400°C. The desorption of the gases obeyed the second-order Wigner-Polyani equation. The desorption energies of the gases were calculated. The mechanism of dimethyl ether synthesis was studied.     

Effect of hydrogen gas impurities on the hydrogen dissociation on iron surface

A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O₂ would successfully compete with H₂ for surface adsorption sites due to the grater attractive force acting on the O₂ molecule compared to H₂. The H₂ dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H₂ + O₂, other practical systems were considered, that is, H₂ + CO and CH₄. Calculations were performed for the CO chemisorption and H₂ dissociation on iron surface with predissociated CO, as well as, CH₄ surface dissociation. The results indicate that CO inhibition of H₂ dissociation proceeds via similar mechanism to O₂ induced inhibition, whereas CH₄ traces in the H₂ gas have no effect on H₂ dissociation. © 2014 Wiley Periodicals, Inc.     

Adsorption and desorption of different gases on Ru film in the dark and under UV irradiation

The adsorption and desorption of some vapors and gases (water, hydrogen, dinitrogen oxide, carbon monoxide, oxygen) on Ru film has been investigated at 305 K using a Tian-Calvet microcalorimeter. The initial heat of adsorption, the irreversible coverage and the temperature of maximum desorption rate indicate the following binding energy order: H₂>N₂O>H₂O>CO>O₂. The amount of photoadsorption (for H₂O, N₂O, H₂) and photodesorption (for CO and O₂) has also been determined by recording the pressure changes induced by UV irradiation of the Ru film.     

Desolvation and Dehydrogenation of Solvated Magnesium Salts of Dodecahydrododecaborate: Relationship between Structure and Thermal Decomposition

Attempts to synthesize solvent‐free MgB₁₂H₁₂ by heating various solvated forms (H₂O, NH₃, and CH₃OH) of the salt failed because of the competition between desolvation and dehydrogenation. This competition has been studied by thermogravimetric analysis (TGA) and temperature‐programmed desorption (TPD). Products were characterized by IR, solution‐ and solid‐state NMR spectroscopy, elemental analysis, and single‐crystal or powder X‐ray diffraction analysis. For hydrated salts, thermal decomposition proceeded in three stages, loss of water to form first hexahydrated then trihydrated, and finally loss of water and hydrogen to form polyhydroxylated complexes. For partially ammoniated salts, two stages of thermal decomposition were observed as ammonia and hydrogen were released with weight loss first of 14 % and then 5.5 %. Thermal decomposition of methanolated salts proceeded through a single step with a total weight loss of 32 % with the release of methanol, methane, and hydrogen. All the gaseous products of thermal decomposition were characterized by using mass spectrometry. Residual solid materials were characterized by solid‐state ¹¹B magic ‐ angle spinning (MAS) NMR spectroscopy and X‐ray powder diffraction analysis by which the molecular structures of hexahydrated and trihydrated complexes were solved. Both hydrogen and dihydrogen bonds were observed in structures of [Mg(H₂O)₆B₁₂H₁₂] ? 6 H₂O and [Mg(CH₃OH)₆B₁₂H₁₂] ? 6 CH₃OH, which were determined by single‐crystal X‐ray diffraction analysis. The structural factors influencing thermal decomposition behavior are identified and discussed. The dependence of dehydrogenation on the formation of dihydrogen bonds may be an important consideration in the design of solid‐state hydrogen storage materials.     

The Adsorption and Thermal Decomposition of CH2CO (CD2CO) on Si(111)-7 × 7

The adsorption and thermal decomposition of ketene on Si(l 11)-7 × 7 were investigated using various surface analysis techniques. When the surface was exposed to ketene at 120 K, two CO stretching modes at 220 and 273 meV appeared in HREELS, corresponding to two adsorbed ketene states. After the sample was annealed at ?250 K, the 273 and the 80 meV peaks vanished, indicating the disappearance of one of the adsorption states by partial desorption of the adsorbate. In a corresponding TPD measurement, a desorption peak for ketene species was noted at 220 K. Annealing the sample at 450 K caused the decomposition of the adsorbate, producing CH_x and O adspecies. Further annealing of the surface at higher temperatures resulted in the breaking of the CH bond, the desorption of H and O species and the formation of Si carbide. The desorption of H at 800 K was confirmed by the appearance of the D₂ (m/e = 4) TPD peak at that temperature when CD₂CO was used instead of CH₂CO.     

Photocatalytic Dissociation of CH3OH on ZnO(0001) Surface

Photocatalysis of CH₃OH on the ZnO(0001) surface has been investigated by using temperature-programmed desorption (TPD) method with a 266 nm laser light. TPD results show that part of the CH₃OH adsorbed on ZnO(0001) surface are in molecular form, while others are dissociated. The thermal reaction products of H₂, CH₃^·, H₂O, CO, CH₂O, CO₂ and CH₃OH have been detected. Experiments with the UV laser light indicate that the irradiation can promote the dissociation of CH₃OH/CH₃O^· to form CH₂O, which can be future converted to HCOO^- during heating or illumination. The reaction between CH₃OH_Znand OH_ad can form the H₂O molecule at the Zn site. Both temperature and illumination promote the desorption of CH₃^· from CH₃O^·. The research provides a new insight into the photocatalytic reaction mechanism of CH3OH on ZnO(0001).     

Kinetics and equilibrium of adsorption on 4A zeolite

Adsorption isotherms for Ar, 0₂, N₂, CO, CO₂, CH₄, and C₂H₆ on 4A zeolite at three or more temperatures were determined. An adsorption equation based on a 2-dimensional virial equation in terms of integer powers of the reciprocal of (A - σ) was shown to fit the equilibrium data accurately with three constants for C₂H₆ and two constants for other gases. Here A is the area per molecule and σ is the area of the molecule in a close-packed situation.Rates of adsorption and desorption of Ar, N₂, CO, CH₄, and C₂H₆ on 4A zeolite were determined over ranges of temperature in which the rate was moderately fast. Electron microscopy showed that the particles were cubes, and their size-distribution was determined. The conventional Fick's law rate equation for cubes was used to produce a generalized rate curve for the particle size distribution of the adsorbent. This curve was applied to the last 20% of the rate curve to obtain a diffusivity that could be related to the final amount adsorbed. This procedure also avoids the initial rapid portion of the adsorption, in which large variations of adsorbent temperature from that of the bath often occur.The diffusivities increased with amount adsorbed by a small extent for Ar and CH₄ and by larger amounts for N₂, CO, and C₂H₆. The activation energy for diffusion, as well as the heat of adsorption, were nearly independent of amount adsorbed for Ar and CH₄, but these quantities decreased substantially with coverage for N₂, CO, and C₂H₆. The dependence upon amount adsorbed of diffusivity and activation energy seemed related to the shape of the adsorption isotherm; those for Ar and CH₄ were nearly linear, whereas isotherms for the other gases had large curvatures. The activation energy for diffusion varied with coverage in the same way as heat of adsorption.     

Reaction model for methane oxidation on reduced SnO2 (110) surface

A reaction model for methane oxidation on a reduced SnO₂ (110) crystal surface has been proposed theoretically using a point‐charge model. The geometric and electronic structures for all the molecules along the four reaction channels have been calculated by means of the MP2/6‐311++G(2d, p) level of theory. On the basis of the optimized geometries in the gas phase, the single‐point calculations of the energies on the point‐charge model are carried out. The results indicate that the energetically favorable reaction paths to yield methanol and formaldehyde on the reduced SnO₂ surface are via the reactant complex CH₃O H₂O and via the secondary production of methanol oxidation, respectively. It is also found that CH₃O⁻ is a stable anion on the surface due to having the high barriers of about 70 kcal/mol in both hydrogen abstraction with O⁻ and thermal decomposition, which is favorable to yield methanol and also is consistent with X‐ray photoelectron spectroscopy (XPS) experiments. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 423–433, 1999     

Different hydrogen-bonding patterns in two [Zn(ENTPP)] complexes with water or methanol as ligands

We have synthesized zinc complexes of H₂ENTPP (5-(8-ethoxycarbonyl-1-naphthyl)-10,15,20-triphenyl porphyrin) as a model to study hydrogen-bonding interactions. When water or methanol is a ligand, crystals of [Zn(ENTPP)(CH₃OH)] or [Zn(ENTPP)(H₂O)]?·?C₆H₅CH₃ were obtained. In both structures, the ligand has hydrogen-bonding interactions, but in different patterns. In [Zn(ENTPP)(CH₃OH)], the methanol oxygen and carboxylate oxygen in the naphthyl group form an intermolecular hydrogen bond. In [Zn(ENTPP)(H₂O)]?·?C₆H₅CH₃, there are two independent molecules A and B. In molecule B, there is an intramolecular hydrogen bond between the water oxygen and the carboxylate oxygen, while in molecule A, besides the intramolecular hydrogen bond, there is an intermolecular hydrogen bond between the water oxygen and the carboxylate oxygen. ¹H NMR spectra suggest the binding of methanol or water to zinc are equilibrium processes in solution. Equilibrium constant has been determined by UV-Vis measurements, and it suggests the binding affinity of zinc to methanol has been moderately increased.     

Surface Chemistry of Ga(CH3)3 on Pd(111) and Effect of Pre-covered H and O

The adsorption and decomposition of trimethylgallium (Ga(CH₃)₃, TMG) on Pd(111) and the effect of pre-covered H and O were studied by temperature programmed desorption spectroscopy and X-ray photoelectron spectroscopy. TMG adsorbs dissociatively at 140 K and the surface is covered by a mixture of Ga(CH₃)_x (x=1, 2 or 3) and CHx(a) (x=1, 2 or 3) species. During the heating process, the decomposition of Ga(CH₃)₃ on clean Pd(111) follows a progressive Ga-C bond cleavage process with CH₄ and H₂ as the desorption products. The desorption of Ga-containing molecules (probably GaCH₃) is also identi ed in the temperature range of 275-325 K. At higher annealing temperature, carbon deposits and metallic Ga are left on the surface and start to di use into the bulk of the substrate. The presence of precovered H(a) and O(a) has a signi cant effect on the adsorption and decomposition behavior of TMG. When the surface is pre-covered by saturated H₂, CH₄, and H₂ desorptions are mainly observed at 315 K, which is ascribed to the dissociation of GaCH₃ intermediate. In the case of O-precovered surface, the dissociation mostly occurs at 258 K, of which a Pd-O-Ga(CH₃)₂ structure is assumed to be the precusor. The presented results may provide some insights into the mechanism of surface reaction during the lm deposition by using trimethylgallium as precursor.     

Catalytic decomposition of azomethane and reactions of adsorbed methyl radicals on the molybdenum surface

The methods of temperature-programmed reaction/desorption (TPR/TPD) are used to study azomethane (CH³N=NCH³) decomposition and the reactions of the products of its pyrolysis (CH ₃ ^* radicals and N₂) on the polycrystalline molybdenum surface. A TPR spectrum of adsorbed azomethane decomposition shows mainly N₂, H₂, and unreacted azomethane. Upon preliminary adsorption of azomethane pyrolysis products on a catalyst sample, a TPR spectrum shows N₂, H₂, and CH₄ in comparable amounts. The difference in the composition of desorption products found for these two types of experiments shows that, in the decomposition of adsorbed azomethane, surface methyl moieties are not formed. The rate constants were calculated for the dissociation of adsorbed CH₃, CH₂, and CH, recombination of hydrogen atoms with each other and with CH₃ and CH₂, and the recombinative desorption of nitrogen atoms. Deceased.     

Supported catalysts based on Pd–In nanoparticles for the liquid-phase hydrogenation of terminal and internal alkynes: 1. formation and structure

The formation of Pd–In catalysts synthesized from the heteronuclear acetate complex PdIn(CH₃COO)₅ was studied by temperature-programmed reduction, electron microscopy, IR spectroscopy of adsorbed CO and hydrogen temperature-programmed desorption (H₂-TPD). IR spectroscopy of adsorbed CO and H₂-TPD confirmed the formation of bimetallic Pd–In nanoparticles. It was found that the Pd–In nanoparticle surface contains predominantly Pd atoms separated from one another by indium atoms, which is evidenced by the disappearance of the CO band shift resulting from the lateral dipole–dipole interaction between adsorbed CO molecules and by a significant decrease in the band intensity of CO adsorbed in bridged form. Almost complete inhibition of palladium hydride (PdH_x) provides additional evidence of the formation of Pd–In bimetallic particles.     

The Adsorption,Thermal Desorption and Photochemistry of Methyl Iodide on an Ag‐Covered TiO2(110) Surface

The thermal reactions and photochemistry of monolayer methyl iodide (CH₃I) on a silver covered TiO₂(110) surface have been studied using combinative techniques of temperature programmed desorption (TPD) and x‐ray photoelectron spectroscopy (XPS). About ? 60% of CH₃I at monolayer coverage on Ag/TiO₂(110) dissociates between 130 and 200 K yield adsorbed CH₃ and I, with the rest desorbing molecularly at a peak temperature of 200 K in a TPD study. Photochemistry of CH₃I on Ag/TiO₂(110) is wavelength dependent. Irradiation of monolayer CH₃I by 404 nm photon causes C‐I bond dissociation and CH₃ desorption. Upon 290 nm, UV irradiation, the depletion of CH₃I_(a) is dominated by photodesorption of molecular CH₃I.     

Temperature-programmed desorption/pulse surface reaction (tpd/tpsr) studies of CH4, C2H6, C2H4, and CO over a cobalt/MWNTs catalyst

Summary Temperature-programmed desorption (TPD) of CH₄, C₂H₆, C₂H₄, and CO and temperature-programmed pulse surface reactions (TPSR) of CH₄, C₂H₆, C₂H₄, CO, and CO/H₂ over a Co/MWNTs catalyst have been investigated. The TPD results indicated that CH₄ and C₂H₆ mainly exist as physisorbed species on the Co/MWNTs catalyst surface, whilst C₂H₄ and CO exist as both physisorbed and chemisorbed species. The TPSR results indicated that CH₄ and C₂H₆ do not undergo reaction between room temperature and 450^oC. Pulsed C₂H₄ can be transformed into CH₄ at 400 ^oC whilst pulsed CO can be transformed into CO₂ at 100 or 150^oC. In gaseous mixtures of CO and H₂ containing excess CO, the products of pulsed reaction were CH₃CHO and CH₃OH. When the ratio of CO and H₂ was 1:2, pulsed CO and H₂ were transformed into CH₃CHO, CH₃OH and CH₄. In H₂ gas flow, pulsed CO was transformed into a mixture of CH₃CHO and CH₄ between 200 and 250^oC and was transformed into CH₄ only above 250^oC.