Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

Methanol and formaldehyde molecular states on copper surfaces at 300 K?

In a recent paper, Kojima, Sugihara, Miyazaki and Yasumori concluded that methanol and formaldehyde adsorb molecularly (non-dissociatively) on polycrystalline copper at 300 K. Methanol and methyl formate were also found to produce adsorbed formaldehyde. We demonstrate that the “ formaldehyde” UPS spectrum in their study was incorrectly assigned, and is identical to that of adsorbed formate generated during dissociative exposure of formaldehyde to a Cu(110) surface. We have measured the He II spectra of formaldehyde (120 K) and formate (300 K) on clean Cu(110) and show that they are distinctly different. No evidence is found in the present work for stabilization of molecular formaldehyde, methanol or methyl formate on the Cu(110) surface at 300 K.     

XPS,UPS and thermal desorption studies of alcohol adsorption on Cu(110): I. Methanol

The adsorption of methanol on clean and oxygen dosed Cu(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Methanol was adsorbed on the clean surface at 140 K in monolayer quantities and subsequently desorbed over a broad range of temperature from 140 to 400 K. The UPS He (II) spectra showed the 5 highest lying emissions seen in the gas phase spectrum of methanol with a chemisorption bonding shift of the two highest lying orbitais due to bonding to the surface via the oxygen atom with which these orbitals are primarily associated. A species of quite a different nature was produced by heating this layer to 270 K. Most noticeably the UPS spectrum showed only 3 emissions and the maximum coverage of this state was approximately $\text{1}\text{2}$ monolayer. The data are indicative of the formation of a methoxy species, thus showing that methanol is dissociated on the clean Cu(110) surface at 270 K. The same dissociated species was observed on the oxygen dosed surface, the main difference in this ease being the production of large amounts of H₂CO observed in TPRS at 370 K.     

Adsorption of water and methanol on GaAs(110) surfaces studied by ultraviolet photoemission

UV photoemission spectroscopy (UPS) with He 1 radiation (hν = 21.2 eV) has been used to study the interaction of H₂O and CH₃OH with GaAs(110) surfaces prepared by cleavage in ultrahigh vacuum (UHV). For H₂O two molecularly adsorbed phases can be distinguished at 300 K: at low coverage H₂O is chemisorbed by its oxygen lone-pair orbital to the surface whereas for higher exposures a less perturbed species which resembles more a “physisorbed” or condensed H₂O layer is found. At 180 K only the less perturbed species can be identified. Also CH₃OH is chemisorbed molecularly at lower coverage with its oxygen end to the GaAs surface. For higher exposures two additional emission bands are observed which are interpreted as due to the methoxy radical CH₃O resulting from a partial decomposition of CH₃OH.     

Structural studies of formate and methoxy groups on the Cu(100) surface using NEXAFS and SEXAFS

Local chemisorption geometries of formate (HCO₂) and methoxy (CH₃O) groups on Cu(100) were examined by means of surface extended (SEXAFS) and near-edge X-ray absorption fine structure (NEXAFS) measurements above the O K edge. At 300 K the oxygen of the formate group are equivalent and asymmetrically located near the four-fold hollow site yielding two CuO distances between 2.31 and 2.45 Å. These distances are at least 0.3 Å longer than typical CuO distances of surface and bulk compounds due to a Cu-C steric interaction. The CO bonds and the OCO angle of the formate are estimated to be $\text{1.27 ± 0.04}\text{A}\text{?}$ and 127 ± 7°, respectively. At 200 K the methoxy group has a CuO distance of $\text{1.97 ± 0.05}\text{A}\text{?}$ and the CO axis is tilted with respect to the surface normal. The exact chemisorption site of the methoxy goup could not be determined, but the atop site is ruled-out.     

Adsorption and reaction of formaldehyde on thin-film cerium oxide

Formaldehyde adsorption and reaction have been studied on cerium oxide thin films that were vapor deposited on Ru(0 0 0 1). The formaldehyde behavior was examined as a function of temperature, exposure and Ce oxidation state. Formaldehyde chemisorbs on fully oxidized CeO₂ as dioxymethylene, CH₂O₂. The dioxymethylene decomposes and desorbs as formaldehyde between 200 K and 400 K. No other products are formed. On reduced ceria, formaldehyde also adsorbs as dioxymethylene. In addition to the formaldehyde desorption between 200 K and 400 K, a more strongly bound form of dioxymethylene is formed that produces formaldehyde at 440 K. Above 400 K, some of the dioxymethylene reacts to form formate and methoxy on the surface. These species decompose to produce H₂, CO and CH₂O above 500 K.     

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.     

Characterization of the adsorption and decomposition of methanol on Ni(110)

The chemisorption, condensation, desorption, and decomposition of methanol, both CH₃OH and CH₃OD, on a clean Ni(110) surface have been characterized using high resolution electron energy loss spectroscopy, temperature programmed reaction spectroscopy, and low energy electron diffraction. The vibrational spectrum of the saturated chemisorbed layer, 7.4 × 10¹⁴ molecules cm^?2, is almost identical to the infrared spectrum of liquid or solid methanol. Condensation of multilayers of methanol is facile at 80 K. The only quasi-stable intermediate isolated during the decomposition is a methoxy species, CH₃O, which decomposes thermally to CO and H. The evolution of both CO and H₂ occurs in desorption limited processes.     

Photoemission studies of H2S,H2 and S adsorbed on Ru(110): Evidence for an adsorbed SH species

H₂S, H₂ and S adsorbed on Ru(110) have been studied by angle-integrated ultraviolet photoemission (UPS) as part of a study of the effect of adsorbed sulfur, a common catalytic poison, on this Ru surface. For low exposures of H₂S at 80 K, the work function rises to a value 0.16 eV above that of clean Ru(110) while the associated UPS spectra (hν = 21.2 eV) exhibit features similar to those of H(ads) and S(ads) and different from those of molecular H₂S. We conclude that H₂S dissociates completely at low coverages on Ru(110) at 80 K. At intermediate exposures the work function drops and the UPS spectra show new features which are attributed to the presence of an adsorbed SH species. This appears to be the first direct observation of this surface complex. At higher exposures the work function saturates at a value 0.36 eV below the clean value; the UPS spectra change markedly and indicate the adsorption of molecular H₂S. Heating adsorbed H₂S leaves a stable layer of S(ads) on Ru(110). The surface with adsorbed sulfur strongly modifies the adsorption at 80 K of a number of molecules relative to the clean Ru(110) surface.     

The oxidation of H2CO on a copper(110) surface

The oxidation of H₂C¹⁶O by adsorbed ¹⁸O was studied on an Cu(110) sample by temperature programmed reaction spectroscopy. Formaldehyde exchanged its oxygen with surface ¹⁸O upon adsorption to yield H₂C¹⁸O_(a) and ¹⁶O_(a). Formaldehyde was also oxidized by surface ¹⁶O and ¹⁸O atoms to H₂COO which subsequently released one of the hydrogen atoms to form HCOO. The evolution of H₂ from the Cu(110) surface was desorption limited, and the low pre-exponential factor for the recombination of the surface hydrogen atoms suggested stringent requirement on the trajectories of the colliding partners. The formate was very stable and dissociated at elevated temperatures to simultaneously yield H₂ and CO₂. The surface concentration of ¹⁸O exerted a pronounced affect on the activity of the oxidation of formaldehyde on Cu(110).     

Identification of adsorbed species at metal surfaces by electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy (EELS) is a surface analysis method for measuring vibrational spectra of adsorbed species on metal surfaces. This paper summarizes recent work on the study of bonding of simple adsorbates on metal surfaces, and the identification of new chemical intermediates in reactions between two or more species in the adsorbed monolayer. The spectra of atomic oxygen, di-oxygen, water and ammonia adsorbed on platinum, copper and silver are discussed with emphasis on identification of the adsorbed species and their orientations relative to the surface plane. Surface reactions between atomic oxygen and water, methanol and formic acid yield the new surface intermediates hydroxyl (OH), methoxy (CH₃O) and formate (HCOO), respectively, on copper and silver surfaces. Each species was identified by comparison of surface spectra with known infrared spectra and through the use of deuterium isotopic shifts. The ability to identify and distinguish between chemical species at surfaces with high sensitivity will allow direct correlation of low pressure UHV surface experiments with high pressure surface reactions on catalysts and liquid-solid interfaces.     

A study of the relative brönsted acidities of surface complexes on Ag(110)

The relative acidities of a number of Brönsted acids have been established on the Ag(110) surface under UHV conditions. For acids which react completely with adsorbed oxygen atoms on this surface to form H₂O, relative acidities were determined by means of acid-base titration reactions. Adsorbed species such as carboxylates, alkoxides, etc., were formed by reaction of the parent acids with O(a) and then displaced from the surface by titration with stronger acids. Relative acidities of the acids which did not react to completion with O(a) were established on the basis of their relative extents of reaction. The relative acidity scale on Ag(110), according to the reaction BH(g) + B'(a) B'H(g) + B(a) was found to be HCOOH ≈ CH₃COOH>C₂H₅OH> C₂H₂>CH₃OH>C₃H₆, H₂O>C₂H₄, C₂H₆, H₂. This order is in excellent agreement with the acidity scale for these species in the gas phase according to BH(g)B^?(g) + H⁺ (g); it cannot be explained by aqueous dissociation constants or homolytic bond dissociation energies. This result is in accord with the appreciable anionic character of the adsorbed species, since the electron affinity of the base, B, is a strong thermodynamic factor in the acidity in the gas phase. Both XPS and UPS results for adsorbed species on the Ag(110) surface are consistent with this interpretation.     

A spectroscopic study of the coordination of dimethyl sulfoxide to a platinum (111) surface

Spectroscopic studies of the adsorption of dimethyl sulfoxide, (CH₃)₂S = O, on Pt(111) have shown that the molecule is bound to the surface via the sulfur atom in an inverted pyramid configuration. A comparison of XPS and EELS data for the adsorbed multilayer and monolayer with XPS and infrared data on the complex PtCl₂(DMSO)₂ is consistent with sulfur bonding. In addition, we detect a considerable increase of the v(S=O) frequency in the DMSO monolayer with decreasing coverage, indicating a coverage dependent heat of adsorption. UPS data show that on adsorption to form a monolayer, the highest occupied molecular orbital of DMSO, presumably the sulfur “lone pair” orbital, shifts to a higher binding energy. These results show a remarkable similarity between DMSO bonding to a metal surface and bonding to a single Pt²⁺ species.     

Adsorption of ethylene on clean and oxygen precovered Cu(110) surfaces

UV photoemission spectroscopy (UPS) with He I and He II radiation is used to study the interaction of C₂H₄ with clean and oxygen precovered Cu(110) surfaces at 90 K. On the clean surface only-bonding of the C₂H₄ molecules is observed whereas preadsorbed oxygen causes a second molecular orbital to be involved in the chemisorption. This result is consistent with the differing behaviour of the work function change during thermal desorption of C₂H₄.     

Core hole screening and lifetimes in adsorbates on metallic surfaces

The C 1s photoemission lineshape and width has been studied for methoxy (CH₃O), formate (HCO₂) and CO on Cu(100). We find the Doniach-Sunjic asymmetry of the lineshape to be different for the three adsorbates and correlate these differences with the adsorption geometry. The observed shake up satellites exhibit a greatly increased width compared to the main lines, which is attributed to the reduced lifetime of the excited state of the C 1s core hole.     

Vibrational spectra of ethylene and acetylene on metal surfaces - an electron energy loss study of ethylene adsorbed on Ni(110) and its carbided surface,and the use of metal-cluster analogies

An analysis has been made of on- and off-specular electron energy loss spectra (EELS) from C₂H₄ and C₂D₄ adsorbed on a clean Ni(110) and also a carbided Ni(110) surface. The carbided surface was prepared by heating the clean Ni surface in ethylene to 573 K or above. EELS spectra were obtained using a Leybold-Heraeus spectrometer at a beam energy of 3.0 eV and with a resolution of ca. 6.5 meV (ca. 50 cm^?1).The loss spectrum from ethylene at low temperatures (110 K) showed principal features at 3000 (w), 1468 (w), 1162 (s), 879 (w) and 403 cm^?1 (s) (C₂D₄ adsorption) and 2186 (w), 1258 (ms), 944 (ms), 645 (w) and 400 cm^?1 (s) (C₂D₄ adsorption). The overall pattern of wavenumbers and intensifies of the C₂H₄/C₂D₄ loss peaks is very similar in form (although systematically different in positions) to those previously observed on Ni(111) (ref.1) and Pt(111) (ref.2) surfaces at low temperatures. Like these earlier spectra,the EELS results for C₂H₄/C₂D₄ adsorbed on clean Ni(110) can be well interpreted in terms of a MCH₂CH₂M/MCD₂CD₂M species (M = metal) with the CC bond parallel to the surface.After adsorption on the carbided Ni(110) surfaces at 125 K,the main loss features occur at 3065 (m), 2992 (m), 1524 (ms), 1250 (s), 895 (s), and 314 cm^?1 (vs) (C₂H₄ adsorption) and 2339 (m), 2242 (m), 1395 (s), 968 (s), 661 (m) and 314 cm^?1 (vs). With the exceptions of reduced intensities of the bands at 895 cm^?1 (C₂H₄) and 661 cm^?1 (C₂D₄) this pattern of losses - particularly the 1550-1200 cm^?1 features which can be assigned to coupled νCC and δCH₂/δCD₂ modes - is well related to similar results on Cu(100) (ref.3) and Pd(111) (ref.4) which have been interpreted convincingly in terms of the presence of π-bonded species, (C₂H₄)M or (C₂D₄)M on the surface. This structural assignment is supported by comparison with the vibrational spectra of Zeise's salt, K[PtCl₃(C₂H₄)].H₂O (refs.5&6).Spectral changes occur on warming C₂H₄ on the clean Ni(110) surface with a growth of a feature near 895 cm^?1 at 200 K. At 300 K a rather poorly-defined spectrum occurs, which differs substantially from those found on (111) surfaces of Pt (ref.2), Rh (ref.7) or Pd (ref.8) at room temperature. These latter have been attributed to the ethylidyne, CH₃.CM₃, surface species (ref.9). For adsorption on Ni(110) there is clearly a mixture of species at room temperature.The analysis of the vibrational spectra of selected metal-cluster compounds of known structure with selected hydrocarbon ligands has helped substantially to assign the spectra of surface species in terms of bonding structures of the adsorbed species, as in the cases of the identification of (C₂H₄)M π-adsorbed (refs.5&6) and the ethylidyne CH₃.CM₃ species (ref.9). We have recently analysed the infrared and Raman spectra of the cluster compound (C₂H₂)Os₃(CO)₁₀ and its deuterium-containing analogue. The infrared frequency and intensity pattern for the A′ modes (C_S symmetry) of the two isotopomers bears a remarkable resemblance to EELS spectra previously obtained at low temperature for C₂H₂/C₂D₂ adsorbed on Pt(111) (ref.2) and (after taking into account systematic frequency shifts) for Pd(111) (ref.4). There is good evidence for believing that the structure of the hydrocarbon ligand interacting with the osmium complex takes the form

where the arrow denotes a π-bond to the third metal atom. This strongly confirms the structure for the low-temperature acetylene species on Pt(111) as proposed by Ibach and Lehwald (ref.2).Finally the room-temperature spectra for ethylene adsorbed on finely-divided silica-supported Pt and Pd catalysts have previously been interpreted in terms of the presence of MCH₂CH₂M (ref.10) and π-bonded (C₂H₄)M species (ref.11). However comparisons with the more recent EELS spectra from ethylene on Pt(111) at room temperature (ref.2) now leads to a reassignment of the 2880 cm^?1 band, on Pt, previously assigned to MCH₂CH₂M, together with a new, related,band at 1340 cm^?1 (ref.12), to the ethylidyne species CH₃CPt₃ found on the single crystal surface.More detailed analyses of the spectra reported here will be published later. Acknowledgement is given to substantial assistance for this programme of research from the Science and Engineering Research Council.     

An XPS study of the water adsorption on Cu(110)

The adsorption of H₂O on clean and oxygen precovered Cu(110) is studied at temperatures between 90 and 300 K by XPS. On the oxygen precovered surface three O(1s) emission lines are detected at lower temperature. They originate from adsorbed atomic oxygen, from OH groups, and from H₂O molecules. For an oxygen coverage of half a monolayer, XPS indicates that during H₂O decomposition the preadsorbed oxygen does not directly participate in the OH formation. After water adsorption on the clean surface three O(1s) emission lines are found, which are due to H₂O molecules, “disturbed” H₂O molecules, and OH groups. The XPS results are directly correlated with information about the adsorbates obtained by UPS. Coverages are determined from the XPS spectra for the detected species.     

A photoelectron spectroscopic investigation of the interaction between H2O and oxygen on Ag(110)

The adsorption and reaction of H₂O with adsorbed oxygen atoms on Ag(110) was examined by UPS. In agreement with previous EELS results, H₂O formed multilayers of ice upon adsorption at 140 K. The ice layers could be easily distinguished from monolayer coverages of chemisorbed H₂O (present above 160 K) by UPS. The ice layers produced (1) strong attenuation of the emission from the Ag d-bands, (2) a nearly 2 eV shift of H₂O valence levels to higher binding energy and (3) strong attenuation of emission from the H₂O 3a₁ orbital. H₂O was observed to react stoichiometrically with O(a) above 250 K to produce a pure layer of adsorbed hydroxyl species. The UPS spectra for these species exhibited features at ?5.8 and ?8.7 eV, as well as strong features above the d-bands. These spectra were compared with those for OH(a) on other surfaces, and the difficulties of identifying OH by UPS due to contamination by excess H₂O are discussed.     

TPD study of the adsorption and reaction of nitromethane and methyl nitrite on ordered Pt–Sn surface alloys

The adsorption and reaction of the isomers nitromethane (CH₃NO₂) and methyl nitrite (CH₃ONO) on two ordered Sn/Pt(111) surface alloys were studied using TPD, AES, and LEED. Even though the Sn–O bond is stronger than the Pt–O bond and Sn is more easily oxidized than Pt, alloying with Sn reduces the reactivity of the Pt(111) surface for both of these oxygen-containing molecules. This is because of kinetic limitations due to a weaker chemisorption bond and an increased activation energy for dissociation for these molecules on the alloys compared to Pt(111). Nitromethane only weakly adsorbs on the Sn/Pt(111) surface alloys, shows no thermal reaction during TPD, and undergoes completely reversible adsorption under UHV conditions. Methyl nitrite is a much more reactive molecule due to the weak CH₃O–NO bond, and most of the chemisorbed methyl nitrite decomposes below 240 K on the alloy surfaces to produce NO and a methoxy species. Surface methoxy is a stable intermediate until 300 K on the alloys, and then it dehydrogenates to evolve gas phase formaldehyde with high selectivity against complete dehydrogenation to form CO on both alloy surfaces.     

Thermal stability of adsorbed CH3NCO on Cu{110} AND Pt{110} from vibrational and photoelectron spectroscopies

The thermal stability of CH₃NCO adsorbed on Cu{110} and Pt{110} is investigated using HREELS, TPD, and ARUPS. CH₃NCO desorbs largely without fragmentation from Cu{110}, but on Pt{110} only about 20% of the adsorbed CH₃NCO desorbs intact, with 80% decomposing on the surface at T > 200 K into CO(a), H(a), CH_x(a), N(a) and NH_y(a) fragments. The kinetics of the surface decomposition were characterised for 220 < T < 300 K by HREELS and the activation energy for CH₃NCO decomposition was found to vary strongly as a function of coverage.