E.I. Ko

The adsorption of CO, O2, and H2 on Pt(100)-(5×20)

Abstract The adsorption/desorption characteristics of CO, O 2 , and H 2 on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol −1 . These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O 2 fluxes up to coverages of 2 3 of a monolayer with a net sticking probability to ssaturation of ⩾ 10 −3 . Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O 2 , and H 2 , the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.

Reactions of methanol on W(100) and W(100)-(5 × 1)C surfaces

The decomposition of methanol-OD was studied on W(100) and W(100)-(5 × 1)C surfaces by temperature-programmed reaction spectroscopy. Initial adsorption of methanol-OD on the clean W(100) surface resulted in the complete dissociation of the molecule into hydrogen, carbon, and oxygen (β-CO). Methane, methanol-OH, and formaldehyde were observed as additional products after the CO(β) states had been saturated. The W(100)-(5 × 1)C surface produced the same products with the addition of carbon dioxide, water, and methyl formate. Moreover, the carbide surface enhanced the selectivity for hydrocarbon formation by an order of magnitude compared to the clean surface due to the suppression of the dissociation of methanol to β-CO and H2 on the carbon chemilayer. The reaction mechanism was explained in terms of three intermediates: methoxy, formate, and a surface complex comprised of methoxy radicals and trapped hydrogen atoms. The “trapped” hydrogen atoms were apparently stabilized by the methoxy intermediates.

Effects of adsorbed carbon and oxygen on the chemisorption of H2 and Co on Mo(100)

Abstract The deposit of carbon and oxygen adatoms on Mo(100) was characterized by AES and LEED. Carbon was introduced by the thermal cracking of ethylene; several ordered structures were observed as a function of coverage with carbon atoms residing on four-fold sites. The Mo(100)—O system exhibited two different sequences of LEED patterns depending on the adsorption temperature of oxygen. The effects of adsorbed carbon and oxygen on the chemisorption properties of Mo(100) was investigated by FDS. The presence of either carbon or oxygen severely hindered the ability of Mo(100) to dissociatively adsorb hydrogen or carbon monoxide. The amount of CO dissociated was directly related to the available four-fold sites on the carbide surfaces. The molecular adsorption of CO was not significantly affected by the adlayers. It was found that one monolayer of adsorbed oxygen reduced the binding energy of molecular CO considerably more than the same amount of adsorbed carbon. A continuous shift in the binding energy of CO with the C/O ratio on the surface was observed.

The characterization of surface carbides of tungsten

The properties of surface carbides of tungsten single crystals were studied using LEED, AES, and thermal desorption spectroscopy. Carburization of W (100) produced a series of surface structures; at θ = 0.5 a c(2 × 2) structure was observed which compressed into a double row (3−) structure at θ = 0.67. This surface was inert to further carburization below 1000 K. Heating the crystal to 1500 K in ethylene resulted in the formation of an interstitial carbon layer having the structure of ditungsten carbide at the surface. The dissociative adsorption of hydrogen and CO were inhibited by adsorbed carbon or oxygen occupying fourfold symmetry interstitial sites on the W (100) surface. Dissociative adsorption of CO was directly related to the number of pairs of unoccupied fourfold sites. Adsorption of molecular CO correlated with the number of surface sites passivated by adsorbed carbon or oxygen. A surface passivated by dissociatively adsorbed CO was found to behave qualitatively similarly to a carbide surface.

The oxidation of CO on the Pt(100)-(5 × 20) surface

Abstract The kinetics of the CO oxidation reaction were examined on the Pt(100)-(5 × 20) surface under UHV conditions. The transient isothermal rate of CO2 production was examined both for exposure of an oxygen-dosed surface to a beam of CO and for exposure of a CO-dosed surface to a beam of O2. Langmuir-Hinshelwood kinetics were found to apply in both cases. For the reaction of CO with preadsorbed oxygen atoms, the reaction rate was dependent upon the square-root of the oxygen atom coverage, suggesting that oxygen atoms were adsorbed in islands on this surface. The oxidation of preadsorbed CO was observed only when the initial CO concentrations were less than 0.5 monolayer (c(2 × 2) structure), suggesting that the dissociative adsorption of oxygen required adjacent four-fold surface sites. The activation energy calculated for the reaction of CO with preadsorbed oxygen was 31.4 kcal/mol. This value was 30 kcal/mol greater than the activation energy measured for the reaction of O2 with preadsorbed CO. Strong attractive interactions within the oxygen islands were at least partially responsible for this difference. The reaction kinetics in both cases changed dramatically below 300 K; this change is believed to be due to phase separation at the lower temperature.

Reactions of formaldehyde on W(100) and W(100)-(5 × 1)C

Abstract The reactions of formaldehyde on W(100) and W(100)-(5 × 1)C surfaces were studied by temperature-programmed reaction spectroscopy (TPRS). The clean W(100) surface was very reactive, completely decomposing formaldehyde upon adsorption at 300 K. The W(100) surface passivated by either adsorbed CO(β) or adsorbed carbon adsorbed formaldehyde nondissociately which reacted to form methane and methanol. Methane was formed from adsorbed methoxy groups complexed with hydrogen. The reactions of formaldehyde on W(100) and W(100)-(5 × 1)C were found to be very similar to the reactions of methanol. Two differences that were observed were (i) methyl formate was a major reaction product from the reactions of H 2 CO on a carbide surface, but only a minor product from CH 3 OH, and (ii) there was a second reaction pathway leading to methane formation from H 2 CO that was not observed for CH 3 OH. These differences have been attributed to lateral interactions among adsorbed species on the surface.

Reactions of acetaldehyde and ethanol on W(100) and W(100)−(5 × 1)C surfaces

Abstract The reactions of acetaldehyde and ethanol were studied on clean and carburized W(100) surfaces using temperature-programmed reaction spectroscopy (TPRS). On a clean tungsten surface the room-temperature adsorption of either molecule resulted in complete dissociation into adsorbed hydrogen, carbon, and oxygen atoms at low exposures. With increasing exposures acetaldehyde, ethanol, ethylene, and methane were observed to form as additional reaction products via an ethoxy surface intermediate. Passivation of the tungsten surface, either by the initial breakup of the parent molecule or by the deposit of carbon from cracking ethylene, resulted in markedly different product distributions. In particular, methane formation from Cue5f8C bond scission was eliminated by carbide formation, Cue5f8O bond rupture was reduced somewhat, and the reduction of surface oxygen to form water was enhanced. In the case of ethanol the dehydrogenation/dehydration selectivity was increased by a factor of 5 on the carbide surface. Formation of surface oxygen or surface alkoxide groups promoted a lower energy pathway for decomposition to all products.