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In a combined photoelectron-vibrational spectroscopic study of the chemistry of formic acid interaction with A1(111) surfaces the influence of temperature, surface oxidation and coadsorption with water have been investigated. At the clean surface adsorption at low temperatures leads to deprotonation and on warming to room temperature the major species is formate. On the other hand direct interaction with A1(111) at room temperature leads to more extensive bond cleavage including the formation of chemisorbed oxygen, and carbidic carbon. At the preoxidized A1(111) surface deprotonation is facile at 80 K but there is no evidence for further reaction on warning to room temperature. When formic acid is coadsorbed with water at 295 K, HREEL and XP spectra confirm the formation of surface formate together with hydroxyl and chemisorbed oxygen species.

The major techniques presently used for characterising solid surfaces are summarized. These techniques include interactions (or release) of photons, electrons and neutral or charged atoms and molecules, and other miscellaneous methods which may be applied to single crystal surfaces or polycrystalline material

The interaction of nitric oxide at low pressures with iron surfaces in the temperature range 85-290 K has been studied by combined in situ ultraviolet and X-ray induced electron spectroscopy. By monitoring N(1s) and O(1s) binding energies, peak profiles and intensities during nitric oxide interaction regimes of dissociative and molecular adsorption have been recognized. At 85 K adsorption is largely molecular with an N(1s) peak at 400 eV, but some dissociative chemisorption with nitrogen desorption occurs at low coverage indicative of sites of high reactivity and possibly involving intermolecular elimination type reactions. On warming to 290 K further and extensive dissociation occurred, characterized by an N(1s) peak at 397 eV, followed by a small contribution from a pressure dependent weakly adsorbed molecular adsorption of nitric oxide and characterized by an N(1s) value of 401 eV. Information obtained from the O(1s) spectra was in complete accord with that obtained from the N(1s) data. The intensities of the N(1s) and O(1s) spectra enabled estimates to be made of the relative concentrations of nitrogen adatom species under various surface conditions. The interpretation of the molecular processes occurring was strengthened by also comparing peaks in the u. v. induced spectra with those known for NO(g).

The techniques of X-ray and u. v. -photoelectron spectroscopy have been used to investigate the chemisorption of the individual molecules nitric oxide, oxygen, and water, and also of coadsorbed nitric oxide and water, at Zn(0001) surfaces. The core and valence level spectra established that at 77 K, nitric oxide existed in both dissociative and molecular states, oxygen generated both surface O¯ (a) and subsurface O 2- (b) species, while water was only physically adsorbed. At 160 K and above, Zn(0001) surfaces were unreactive to water vapour, while at 295 K only the dissociative state of nitric oxide was present. The coadsorbed water-nitric oxide adlayer was reactive below 200 K. Surface hydroxyls were formed through the activation of molecularly adsorbed water by chemisorbed oxygen generated in the dissociative chemisorption of nitric oxide. These surface hydroxyls participated in a hydrogen transfer reaction leading to hydrogenation of the chemisorbed nitrogen at 273 K followed by desorption. In the absence of water, chemisorbed nitrogen adatoms are by comparison thermally stable. A detailed mechanism, with each surface species spectroscopically identified, is proposed for the hydrogenation of nitric oxide. Analysis of the X-ray induced core level spectra enabled the concentrations of the surface species to be calculated.

The interaction of NO with copper has been investigated by electron spectroscopy. Nitric oxide is shown to be molecularly adsorbed in a linearly bonded configuration at 85 K; this adlayer is unstable above 120 K, resulting in an exclusively oxygen adlayer at 295 K, the nitrogen being desorbed. At 85 K N(1s) peaks observed at 401 and 406 eV are associated with two different surface species. We suggest that the former dissociates while the latter desorbs at a binding energy of 406 eV on thermal activation of the adlayer. Interaction of nitric oxide with Cu at 295 K results in dissociative chemisorption with both oxygen and nitrogen retained at the surface. The surface concentration suggests that each nitrogen and oxygen adatom is bridge-bonded to two surface copper atoms. Estimates of absolute surface coverages are made by comparing the intensities of the N(1s) and O(1s) peaks with the Cu(2p) substrate intensity, while valence level spectroscopy (He I and II radiation) supplements core-level spectroscopy for discriminating between molecular and dissociated surface species.

The adsorption of nitric oxide by nickel and also nickel surfaces preexposed to oxygen has been studied by X-ray photoelectron spectroscopy in the temperature range 80 to 290 °K. The surface reactivity to NO interaction has been controlled by varying the temperature and conditions of preexposure to oxygen. This has enabled three distinct states of NO adsorption to be delineated, a dissociative state and two molecular states. One of the molecular states is weakly adsorbed while the other, more strongly chemisorbed, is the precursor to dissociation. We suggest the latter is adsorbed in the “bent” form while the former is linearly bonded to the surface. N2O is also observed within the adlayer at 80 °K particularly with nickel surfaces preoxidized at 290 °K. The N2O is weakly adsorbed. There are strong analogies between the present data and previous studies with copper, iron, and aluminum, metals with very different electronic structures. It should be noted, however, that although in all cases a new surface phase is generated, the inherent reactivities of the clean metals differ only in degree rather than in kind.

The activity of a surface oxygen transient, O−(s), for H-abstraction from an ammonia molecule undergoing surface diffusion at a Mg(0001) surface at 298 K has been simulated by computer modelling. The time dependence of the chemisorbed products, NH2(a), OH(a) and O2− (a), has been determined making reasonable assumptions of the activation energy for the surface diffusion of physisorbed ammonia, the surface life-time of O−(s) and the surface stoichiometry determined from XPS studies.

The factors relevant to the design of a flow-system for determining the sticking probabilities of gases on film surfaces are discussed critically. The sticking probability of nitrogen on tungsten films is 0.64 at 298°K and 0.94 at 77° and 130°K. These values, which are higher than those observed with tungsten ribbons, are in quantitative agreement with calculations based on a model for film surfaces suggested by Takaishi. The variation of the number of molecules adsorbed as a function of film weight has been analyzed in terms of equations based on a model for film growth and data determined previously with nickel films.

The possibility of analyzing the complex surfaces of reduced metals by comparing their adsorptive properties with the known behaviour of evaporated films has been explored by experiments on nickel powder. Nickel prepared by reduction of the oxide in hydrogen at 450°C for periods up to 12h has been shown by measurement of hydrogen and krypton adsorption at -183°C to have a surface containing up to 37% of clean nickel atoms. Traces of silica introduced during the preparation of the powder possibly account for the rest of the surface. Adsorption of hydrogen at -183°C is effectively complete within ½ min at about 10 -5 mm, and all gas is desorbed by evacuation at 450°C. Oxide is probably not a significant impurity, because the powder does not exhibit the slow adsorption at 20°C and above which is characteristic of incompletely reduced nickel. Such activated adsorption can be restored by the addition of oxygen and eliminated again by further reduction; its stoichiometry suggests that the process consists of hydroxyl formation. Reduction of the sulphide gives nickel of appreciable surface area but completely inert to hydrogen; presumably a stable layer of sulphide is retained. Some thermodynamic and kinetic problems involved in the preparation of metal powders with surfaces of known purity are briefly discussed in the light of these results.

The interaction of hydrogen sulfide and sulfur vapor with tungsten has been studied by field-emission microscopy. Adsorption of hydrogen sulfide at 22 °C begins along the [111] zones and at a pressure of about 5 × 10 −8 mm the adsorption appears to be complete within 4 min. An increase in the average work function of 0.3 volt has been estimated for the process. On heating a surface saturated with hydrogen sulfide, surface migration begins at about 1150 °C, leading to a concentration of sulfur along the [111] zones from which desorption takes place, i.e., the desorption processes are the reverse of the adsorption sequence. The activation energy for desorption is about 80 to 90 kcal/mole and complete removal of sulfur is observed at about 1600 °C. Sulfur adsorption at 10 −7 mm and 22 °C results in a large number of brightly emitting spots which move continually. These are mainly observed around the (111) and (011) planes. A field-induced process resulting in a sudden enlargement of the emitting area and consequent mechanical failure of the tip occurs at 22 °C. This is attributed to the growth of a sulfide whisker. Presorbed hydrogen has an inhibiting effect on this nucleation phenomenon.

Pyrex glass has been shown to be capable of developing during use a superactivity for the physical adsorption of the gases Xe, Kr, N 2 , and O 2 at −195 °C. This superactivity has been analyzed by applying the BET equation to the adsorption isotherms of krypton and xenon and has been shown to be comparable to the activity of some metal films in physical adsorption. The possible reason for and the significance of this superactivity in both adsorption measurements and the determination of the cross-sectional areas of adsorbed molecules are discussed briefly. The surface coverage of oxygen on Pyrex glass at 10 −3 mm pressure and −195 °C was about 10% of the xenon monolayer; this is in good agreement with Hobson's nitrogen data. The superactive glass was about 15 times more active under the same conditions.

As an essential preliminary to applying a number of different photoelectron spectroscopic methods to investigate the mechanism of solid/gas reactions their surface sensitivity has been explored. Data are reported for the sensitivity of e. s. c. a. for H 2 O, CO 2 and CO adsorbed on Au. Also the interaction of mercury vapour with Au has been studied, adsorption and lattice penetration of the mercury being observed by monitoring the Hg(4f) peaks. In general, less than 10% of a monolayer of H 2 O and CO 2 is detectable by X-ray photoelectron spectroscopy. Significant chemical shifts are observed between C(1s) present as an impurity in the Au lattice and C(1s) in adsorbed CO 2 , and also between O(1s) in H 2 O(ads) and CO 2 (ads). The surface sensitiveness of u. v. and X-ray photoelectron spectroscopy are compared.

It has been established1–3 that oxygen chemisorbed at a metal surface can activate an otherwise unreactive molecule, as in the activation of S–H, N–H, O–H and C–H bonds. We have now extended these studies to more complex systems involving coadsorbed molecules in the hope of throwing some light on the mechanisms of heterogeneous catalysts. We have previously established the mechanisms by which unreactive molecular water, adsorbed on a Zn(0001) surface, is activated when co-adsorbed with nitric oxide4, and have demonstrated N–H activation in ammonia co-adsorbed with nitric oxide at an atomically clean Mg(0001) surface at 295 K (Fig. la and ref. 5). Activation was attributed to surface oxygen generated in the dissociative chemisorption of nitric oxide. Here we establish the nature of the ‘oxygen’ species involved and reveal details of the molecular steps.

Nitric oxide is dissociatively chemisorbed at a Zn(0001)surface at 80 K, both the chemisorbed nitrogen and oxygen are stable up to 470K; in the presence of coadsorbed water, hydrogenation of the adsorbed nitrogen occurs at 200K by hydroxyl species formed as a result of the activation of molecularly adsorbed water by chemisorbed oxygen generated in the dissociative chemisorption of NO.

Adsorption, Surface Area and Porosity By S. J. Gregg and K. S. W. Sing. Pp. xi + 371. (London: Academic Press, Inc. (London), Ltd.; New York: Academic Press, Inc., 1967.) 95s.; $18.

The adsorption and subsequent decomposition of tetramethylsilane Si(CH3)4 on evaporated films of tungsten and iron have been investigated. The results are compared with results previously obtained with neopentane C(CH3)4. Si(CH3)4 is rapidly and irreversibly adsorbed on tungsten at 293 K, and gaseous hydrogen and methane are formed; further H2 and CH4 are desorbed on heating the surface to temperatures up to 420 K. Additional information regarding the adsorbed phase has been obtained from deuterium exchange experiments. Similar experiments carried out with iron show that much less dissociation of the Si(CH3)4 occurs on adsorption and on heating the adsorbed layer; no gaseous products are observed below 320 K. Comparison of both sets of results with those for C(CH3)4 indicates that Si(CH3)4 is more extensively dissociated under all conditions; this is attributed mainly to the weaker C—Si bond. The results suggest that the rate determining step in hydrocracking reactions of hydrocarbons is likely to be hydrocarbon breakdown rather than desorption of products.

Atomic hydrogen has been shown, in contrast to molecular hydrogen, to interact with lead films to an extent corresponding to about five atoms per surface lead atom at 78 K. At 273 K incorporation is substantially faster and more extensive. There is some evidence for the formation at 273 K of a hydride of fixed composition (PbH0·2) which is active in the adsorption of molecular hydrogen at 78 K. The kinetics of the formation and decomposition of this hydride have been investigated; gas-phase deactivation of thermally activated molecular species occurs at hydrogen pressures 10–2 mmHg.

Condensed multilayers of PbEt4 decompose when irradiated with u.v. light of wavelength 254 nm to give gaseous C2 and C4 hydrocarbons and a lead film; adsorbing PbEt4 on a photodeposit of Pb generates a more reactive species which is active in photolysis at longer wavelengths (280–290 nm).

The interaction of carbon monoxide, hydrogen, and oxygen with manganese films has been investigated. Hydrogen is weakly chemisorbed (ΔH≃ 50 kJ mol–1), but both carbon monoxide and oxygen are incorporated into the lattice. The oxide formed is likely to be MnO. Adsorption data suggest that oxygen incorporation occurs even at 78 K and this is confirmed by work function studies; the surface potential of oxygen on manganese at 78 K is ca.–1·6 V. Carbon monoxide is incorporated extensively above 400 K and the kinetics suggest a diffusion-controlled process involving weakly chemisorbed CO. The catalytic oxidation of CO over manganese was first order in CO and only occurred under conditions where oxygen incorporation was slow compared with the catalytic reaction. A surface which had been oxidised to its maximum extent was inactive in catalysis at the temperature of oxidation.

We have explored the possibility of acquiring information on the molecular nature of some novel polymer films by following Zisman's approach of determining the critical surface tension γC which is considered to reflect the molecular composition of the solid surface. This led us to obtain equilibrium contact angle data, using both a series of pure liquids and various alcohol solutions, not only with the polymer films but also with solid surfaces of polystyrene and polymethylmethacrylate.Lower values of γC were obtained with the solutions than with the pure liquids; these lower values are attributed to the preferential adsorption of the alcohol molecules at both the polymer-liquid and the polymer-vapour interfaces. The value of γC depends on the alcohol used, and is relatively independent of the solid : it is inferred that the alcohol is adsorbed with the hydroxyl group towards the polymer surface.It is concluded that in certain cases, the value of γC obtained using solutions cannot be used as being characteristic of the solid (as has been suggested by Zisman), and that changes at the solid-vapour interface cannot be neglected when interpreting contact angle data. Several sets of data reported in the literature are discussed from this viewpoint. The Gibbs adsorption isotherms is applied to the contact angle data and the results add further weight to the conclusions regarding the occurrence of adsorption at both interfaces.

The role of oxygen in determining the surface characteristics of nickel catalysts has been investigated by work function, photoelectric and chemical reactivity studies. The possible limitations of any one particular method are discussed. Surfaces which have been exposed to oxygen (10–4 mm min) at 23° exhibit the characteristics expected of a surface oxide although the chemical reactivity is not that associated with NiO. Oxide structures of low work function can form, and these were shown to have specific adsorption characteristics.The adsorption of carbon monoxide and oxygen and the catalytic oxidation of carbon monoxide by heavily oxidized nickel films were studied. Two distinct states of adsorbed oxygen were recognized. A substantial decrease in work function occurred on adsorbing carbon monoxide, the decrease being greater the higher the temperature. At 170° and 1.5 mm the decrease was ∼0.85 eV the major portion of this arising from a space charge term. The oxide work function remained virtually unchanged during catalysts at 23°. This was due to a low energy state of adsorbed oxygen inhibiting the adsorption of carbon monoxide thereby preventing the development of a spacecharge.

The adsorption of neopentane has been studied on tungsten and on palladium films; allied experiments were carried out with methane (W, Pd) and with propane (W). The evolution of hydrogen from each gas/metal system was studied as a function of time and temperature; reaction with gas phase deuterium was used to determine how many of the hydrogen atoms in the surface phase were exchangeable, either rapidly or by slower processes.Rapid dissociative chemisorption, involving the cleavage of an average of 3 or 4 C—H bonds occurred with neopentane or propane on tungsten. Neopentane was chemisorbed on palladium at room temperature but dissociation of methane was not observed below ∼373 K; the extent of the dissociation of both hydrocarbons was smaller than on tungsten. Further breakdown of the surface species, accompanied by evolution of hydrogen, occurred with all systems on raising the temperature. Evidence for C—C bond rupture was obtained from the hydrogenolysis of neopentane at higher temperatures.

The formation of thin films by electron bombardment has been studied and some of the important factors controlling the nature and surface characteristics of the films have been identified. A number of experimental techniques have been developed in order to obtain information on both the species likely to be involved in film growth and also the nature of the film surfaces. The surface characteristics have been monitored by contact angle studies.The possible importance of negative ions in film formation is illustrated with Si(CH3)4, where over 20 different negative ions were detected, the major ones being H–, C2H– and SiC2–. Such results also have an important bearing on electron affinity data obtained by the magnetron method.Thin films formed from perfluorobut-2-ene have surface characteristics which vary with the monomer pressure, the substrate temperature, and electron and ion energies. Information on the relative importance of the role of negative ions compared with electron and adsorbed monomer reaction is obtained. Some suggestions are discussed regarding possible surface structures inferred from wetting characteristics.