J.N. Russell

Reaction of methanol with Cu(111) and Cu(111) + O(ads)

Abstract The reactive chemistry of methanol on the Cu(111) surface, both with and without preadsorbed oxygen atoms, is investigated between 190 and 700 K. The clean Cu(111) surface is inert, and molecularly absorbed methanol, the only stable surface species identified on this surface, desorbs at about 210 K. Various trends are examined as a function of oxygen coverage, from the clean surface to the saturation oxygen coverage (approximately 0.45 O atom/Cu atom). The capacity of the surface to adsorb methanol (190 K), and the formaldehyde yield (∼ 400–450 K) are both maximized when the oxygen coverage is about 0.26 O atom/Cu atom. Trends in the yield of other products, and the temperature for decomposition of the stable methoxy intermediate are examined. Also, the rate of methoxy decomposition is limited by CH bond breaking as evidenced by a deuterium kinetic isotope effect (CH versus CD). A minor decomposition path for methanol on O + Cu(111) involves CO2 formation, probably via a formate surface intermediate. Preadsorbed oxygen serves as an acceptor of the methanol hydroxyl hydrogen, enabling facile methanol conversion to methoxy at low oxygen coverage for T ⩾ 190K. However. at high oxygen coverage ( θ ⪆ 0.26 O atom/Cu atom ) oxygen inhibits surface reactivitv. A two-dimensional model which defines three types of surface sites is used to explain the general trend of methanol reactivity as a function of oxygen coverage.

Bond activation sequence observed in the chemisorption and surface reaction of ethanol on Ni(111)

Abstract The mechanism of ethanol decomposition on the Ni(111) surface has been investigated between 155 and 500 K. The sequence of bond scission steps which occur as ethanol undergoes dissociative reactions on this surface has been deduced using deuterium and 13 C isotopic labels. Bond activation occurs in the order (1) OH, (2) CH 2 (methylene CH), (3) CC, (4) CH 3 (methyl CH). The products observed are CH 3 CHO(g), CH 4 (g), CO(g), H 2 (g) and surface carbon, C(a). The latter species exhibits a carbidic AES lineshape in the temperature range 450 to 670 K, at which temperature it dissolves into the Ni bulk. Acetaldehyde, CH 3 CHO, and methane, CH 4 , desorb with the same threshold temperature (260–265 K), and the formation of both of these products is controlled by scission of the methylene CH bond (CH 2 group). The CH 3 group is cleaved from the intermediate surface CH 3 CHO species to form CH 3 (ads). H 2 exhibits a broad, doublet desorption peak from 300 to 450 K. The carbonoxygen bond in ethanol remains intact and CO ultimately desorbs in a single desorption limited process ( T p = 430 K). A small fraction of CO(a) species undergo exchange with the carbidic surface carbon in a minor process observed above 440 K.

Scanning kinetic spectroscopy (SKS): A new method for investigation of surface reaction processes

Abstract Multiple reaction pathways are available to a polyatomic molecule interacting with a solid surface. Delineation of exact temperature regions in which the various pathways are either active or inactive is accomplished using a new method, Scanning Kinetic Spectroscopy (SKS). SKS uses a calibrated and collimated beam of reactant molecules incident upon a clean single crystal surface in UHV. A multiplexed quadrupole mass spectrometer (QMS) is enclosed inside a differentially pumped random flux shield, in line of sight to the crystal surface. The crystal temperature is programmed with a linear ramp ( dT / dt = 2 K / s .) and reactant consumption, product evolution, and desorption of stable surface species are simultaneously measured in one experiment. SKS data are presented here which characterize the reactions of methanol with the single crystal surfaces Ni(111), Cu(111), and Cu(111) plus preadsorbed oxygen. Application of the SKS method as an efficient probe of surface reaction pathways is illustrated by the contrasting chemistry of these surfaces. The methanol plus Ni(111) system is examined in detail in order to relate the observed SKS features to specific molecular reaction pathways on the Ni(111) surface.

Isotope effects in hydrogen adsorption on Ni(111): Direct observation of a molecular precursor state

The nature of the interaction of hydrogen with Ni(111) is probed by comparing the adsorption and desorption kinetic behavior of H2 and D2. Pure H2 and pure D2 adsorbed on Ni(111) at 140 K exhibit identical desorption behavior. The adsorption rate law depends upon (1‐θ) rather than (1‐θ)2, suggesting that the rate determining step involves the interaction of an H2 molecule with a single Ni site. The temperature programmed desorption spectra show two desorption features, β1 and β2, which have maxima in desorption rate at 290 and 370 K. When either pure H2 and pure D2 or a mixture of H2 and D2 interact with the Ni(111) surface at 87 K, evidence for kinetic retardation of D2 adsorption is observed. When the H2:D2 mixture is exposed to the Ni(111) surface, held at 87 K, an additional desorption feature, α, is uniquely observed at 100 K for D2. α‐D2 desorbs with first order kinetics exhibiting Ed =11.1 kJ/mol and a preexponential factor of 1.2×105 s−1. It may be due to desorption from an intrinsic molecular pre...

Observation of a deuterium kinetic isotope effect in the chemisorption and reaction of methanol on Ni(111)

Abstract Measurements of the reactive sticking coefficient, S R 0 , under zero coverage (irreversible) conditions for the reaction of methanol with a clean Ni(111) surface have revealed a significant deuterium kinetic isotope effect. S R 0 measured for CH 3 OH and CD 3 OH are equal, and are about 1.61 to 1.35 times greater than 5 R 0 for CH 3 OD, in the temperature range 290–400 K. The ratio S R 0 ( OH ) S R 0 ( OD ) decreases monotonically from 1.35 to 1.10 in the temperature range 400 to 500 K. The measurements support the rate-controlling formation of adsorbed methoxy, CH 3 O (ads) , as an unstable surface intermediate in the decomposition of methanol to CO and H 2 . These observations indicate that the deuterium kinetic isotope effect may be a generally useful tool for determining the reaction site location on a molecule interacting with a surface.

Surface reaction pathways of methylamine on the Ni(111) surface

The interaction and bond scission sequence of methylamine, CH3NH2, on Ni(111) have been investigated by means of Auger electron spectroscopy and temperature programmed desorption under UHV conditions in the temperature range 87–800 K. Comparisons have been made to the NH3/Ni(111) and N/Ni(111) systems. Methylamine is found to absorb molecularly through its lone pair up to ∼330 K after which a dehydrogenation channel opens which competes with the desorption channel. The sequence of the initial bond breaking was investigated by measuring the initial reactive sticking coefficient of deuterium labeled molecules at 363 K. Methylamine decomposition was found to take place through both ends of the molecule, initially with a slight rate preference at the C end. The CN residue left on the surface by the dehydrogenation process resulted in self‐poisoning of the active sites. This residual species was found to decompose at ∼530 K, leading to N2 desorption and the diffusion of carbon into the crystal at 700–800 K.

Angular distributions of H2 thermal desorption: Coverage dependence on Ni(111)

Temperature programmed desorption measurements for H2/Ni(111) yield two desorption states, β1(T=290 K) and β2(T=370 K) for saturation H coverage. The two states are found to have distinctly different angular distributions. β2‐H2 desorption is strongly focused along the surface normal, while β1‐H2 desorbs diffusely. The angular distribution of desorbing β1‐H2 is very close to that for the CO/Ni(111) system which has a cos θ angular distribution. The different angular distributions can be explained by a model involving a coverage‐dependent location of an activation energy barrier on the potential energy surface which describes the interaction of hydrogen with Ni(111).Temperature programmed desorption measurements for H2/Ni(111) yield two desorption states, β1(T=290 K) and β2(T=370 K) for saturation H coverage. The two states are found to have distinctly different angular distributions. β2‐H2 desorption is strongly focused along the surface normal, while β1‐H2 desorbs diffusely. The angular distribution of desorbing β1‐H2 is very close to that for the CO/Ni(111) system which has a cos θ angular distribution. The different angular distributions can be explained by a model involving a coverage‐dependent location of an activation energy barrier on the potential energy surface which describes the interaction of hydrogen with Ni(111).

Methanol decomposition on Ni(111): Investigation of the C-O bond scission mechanism

The possibility of a C-O bond scission mechanism in the decomposition of CH3OH on Ni was examined by mole fraction analysis of the desorbing CO isotopes produced by the decomposition of an adsorbed mixture of 13CH316OH and 12CH318OH on a Ni(111) surface. No isotopic scrambling was observed on the clean, H-precovered, or sputter roughened Ni(111) surfaces. However, isotopic scrambling could be induced in the desorbing CO by Ar+ sputtering the adsorbed mixture of methanol isotopes. Whether the C-O bond scission mechanism could be methanol-flux dependent was also explored. Carbon deposition at trace levels, a proposed consequence of C-O bond scission, was observed for high methanol fluxes (F>1×1016 cm−2 s−1). However, the small amount of carbon deposited was exposure rather than flux dependent and was due to low impurity levels in the methanol. We conclude that C-O bond scission for methanol on Ni(111) is not a viable elementary process under any of the methanol flux conditions explored, and that its maximum efficiency at 180 K is -2×1013 cm−2 s−1), two H2 desorption features are observed. The low temperature feature (370 K) results from desorptionlimited H2 desorption and corresponds to the β2-H2 desorption feature from H/Ni. The high temperature feature (430 K) can be explained by accelerated methanol surface reactivity during exposure to methanol after blocked surface sites open by thermal desorption of CO. This differs from the proposed CHx(a) decomposition proposed by others as an explanation.

Hydrogen implantation in Ni(111) — A study of H2 desorption dynamics from the bulk

Abstract Hydrogen implanted in Ni(111) has been investigated by the use of temperature programmed desorption (TPD) as a function of implantation energy (0.08–5.0 keV) and dose (0−2.6 × 1017 H+2cm−2). The amount of hydrogen implanted was calibrated by comparison to the amount chemisorbed on the surface since separation of the desorption features for the two types of hydrogen was possible. It is shown that trap formation plays an important role in the observed behavior of the implanted hydrogen desorption features for implantation energies above 1.0 keV. The desorption of implanted hydrogen is found to be strongly forward-peaked along the crystal surface normal. Since the surface was saturated with hydrogen when the implanted hydrogen was released, this effect is explained by recombination of an implanted and a chemisorbed hydrogen atom. Here, a translationally excited H2 molecule desorbing from the surface is produced by a hydrogen atom originating from an elevated potential region inside the solid.

Isotopic Effects in the Adsorption and Desorption of Hydrogen by Ni(111)

The dynamics of the interaction of hydrogen with the Ni(111) surface have been investigated using several methods involving comparisons between H2 and D2 adsorbates. In addition, the desorption of hydrogen has been studied by methods which yield information about the angular distribution of the desorbing species. It has been found that on Ni(111), hydrogen molecules adsorb via an interaction with a single site on the Ni(111) surface. The adsorption process produces an activated complex which is located near the entrance channel (for adsorption at low coverages) on the potential energy surface describing the interaction. The adsorption process occurs through the formation of an intrinsic molecular precursor state. This state may be observed by TPD above one-half monolayer coverages of atomic deuterium in the case of molecular D2. Adsorbed molecular H2 is not observed. The D2 molecular state desorbs via first order kinetics with an activation energy of 11.1 kJ/mole and a preexponential factor of 1.2 × 105 sec−1. These conclusions are confirmed by angular distribution measurements of H2 desorption which indicate that adsorbed H recombines over an exit channel (for desorption) barrier when the coverage is below one-half monolayer. For atomic hydrogen coverages above one-half monolayer, recombination occurs over an entrance channel (for desorption) barrier. These results suggest that the location of the activation barrier for H + H recombination is dependent on the coverage of adsorbed atomic hydrogen.