John B. Hudson

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: I. Ni(100) surface

Abstract The initial stages of the interaction of oxygen gas with a clean Ni (100) surface have been studied by a combination of LEED, AES, work function change and ion bombardment sectioning techniques. The reaction could be divided into three reaction regions: a fast dissociative chemisorption leading to surface structures based on the initial nickel interatomic spacing and resulting in an oxygen coverage of approximately 0.4 monolayers; a rapid oxidation leading to epitaxial NiO, two layers thick ; and a final slow thickening of bulk NiO. The first two regions were dependent only upon oxygen exposure. The third region was observed only at high gas-phase oxygen pressures or very low surface temperatures. Kinetics analyses are developed to explain the rate of oxygen chemisorption and the rate of oxide nucleation and growth.

The kinetics of the reaction between oxygen and sulfur on a Ni(111) surface

Abstract We have observed the segregation of sulfur to the surface of a (111) oriented nickel single crystal, and the kinetics of the reaction of this sulfur with oxygen at temperatures near ambient. LEED studies indicate the formation of two sulfur rich surface structures, having symmetries (5 √3 × 2) and (8 √3 × 2). Exposure of these structures to oxygen gas results in removal of the sulfur as SO2 at a rate proportional to the gas-phase oxygen pressure. The kinetics of the removal process, for the case of the (8 √3 × 2) structure, can be described by a model assuming initial attack by physisorbed molecular oxygen at surface defect sites followed by growth around these sites of “holes” in the sulfur structure.

The adsorption and decomposition of ethylene on Ni(110)

Abstract The interaction of ethylene gas with a Ni(110) surface has been studied by a combination of modulated beam mass spectrometry and Auger electron spectroscopy techniques. At temperatures below 350°C, ethylene is chemisorbed with unity initial sticking coefficient up to a saturation coverage of one carbon atom per surface nickel atom. Above 150°C, hydrogen evolution is observed concurrent with the adsorption process, the amount of hydrogen liberated corresponding to complete dehydrogenation of the ethylene. Indirect evidence also suggests dehydrogenation down to 25°C. A physisorbed ethylene species is observed to coexist with the chemisorbed layer. Above 350°C formation of a graphitic surface layer is observed. The results obtained are consistent with earlier photoemission spectrometric measurements of Demuth and Eastman (Phys. Rev. Letters 32 (1974) 1123).

Characterization of Particulates from Power Plants

The emphasis on participate control from industrial processes has been shifted recently towards fine particulates, having diameters less than 3 microns. There exists an urgent need for more scientific information of fine particle characterization.1,2 Coal and oil fired power plants are among the largest anthropogenic point sources of particulate matter.3 Limited knowledge is available on particle size distribution and trace metal composition in power plant emissions.4-7 The morphological properties of particle emissions have been largely neglected. In this report we present some information on particle characteristics for an oil-fired and coal-fired power plant.

The adsorption and decomposition of N2O on nickel (100)

Abstract The adsorption and decomposition of N 2 O on clean and oxygen covered Ni(100) surfaces has been studied using a combination of Auger electron spectroscopy (AES) and molecular beam relaxation spectroscopy (MBRS) techniques. As observed in a previous study of this reaction on the Ni(110) surface, N 2 O decomposes to yield N 2 gas and adsorbed O at temperatures between 200 and 800 K. Measurements at temperatures below 200 K led to the identification of two weakly adsorbed precursor species, one on clean surfaces and the other on surfaces covered with 0.25 ML of adsorbed O. The adsorption rate constants measured for these two species are consistent with values inferred indirectly in the previous study.

The adsorption kinetics of molecular oxygen and the desorption kinetics of GeO on Ge( 100)

Abstract The reaction kinetics of molecular oxygen with the Ge(100) surface have been investigated by using molecular beam relaxation spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. The adsorption kinetics over a range of impingement conditions and surface temperatures are well described by a model involving dissociative Langmuir kinetics, with the initial sticking coefficient and the saturation coverage both increasing with surface temperature. The only species observed in desorption was GeO(g), which desorbed with a cosine spatial distribution. At low coverage, desorption followed first-order kinetics with an activation energy of 60 kcal/mol. At coverages greater than 0.07 ML, zero-order kinetics were observed, with the same activation energy. These results are explained in terms of the instantaneous coverage on active sites for desorption.

The adsorption of silver on tungsten (110)

Abstract Previous studies of the adsorption of metals on metals, by a mass spectrometric molecular beam technique, have been extended to the case of silver adsorbing on tungsten (110). The adsorption process has been studied by both the adsorption transient technique and the flash filament technique. Two adsorbed phases were observed, having heats of desorption of 66 and 41 kcal/mol. The more tightly bound phase saturated in coverage at 6 × 10 14 molecules/cm 2 . The other phase showed no fixed coverage limit. The equilibrium total coverage, n a , at any value of impingement rate, I , and substrate temperature, T , can be represented by the relation n a = I τ 1 1+ I τ 1 n 0 1+ I τ 2 (T) n 0 , in which τ 1 ( T ) and τ 2 ( T ) are the mean-stay-times for adsorption in the two adsorbed phases and no is the saturation coverage in the tightly bound phase. It was also observed that at impingement rates greater than the equilibrium evaporation rate of the bulk crystal phase of silver, this phase nucleated and grew without requiring an observable critical supersaturation, with a material accommodation coefficient of unity, and epitaxially with the substrate.

Condensation in CO2 free jet expansions. I. Dimer formation

A theoretical and experimental investigation of the kinetics of CO2 dimer formation in the adiabatically expanding core of a free jet expansion has been carried out. The theoretical analysis is based on dimer formation by an efficient termolecular process and dimer destruction by bimolecular collisions. Account is taken of the expected variation of the specific heat ratio γ during the expansion and of the effective increase in the termolecular collision rate at low temperatures due to the presence of loosely bound orbiting pairs. Experimental dimer concentrations were measured mass spectrometrically by forming a molecular beam from the expanded gas mixture after the onset of molecular flow. Observed dimer concentrations, for a range of pre‐expansion conditions of 400–900 torr pressure and room temperature, are in good agreement with the theoretical analysis, provided account is taken of the effect of loosely bound orbiting pairs at low temperatures.

The kinetics and mechanism of oxygen uptake on a polycrystalline iron surface

Abstract The initial uptake of oxygen has been studied on a clean, polycrystalline iron surface over the temperature range between 360 and 575 K, using a combination of Auger electron spectroscopy and mass spectrometry. An initial chemisorption period is followed in turn by a temperature dependent linear oxidation regime and a final period of decreasing oxidation rate. Chemisorption kinetics can be explained by an intrinsic molecular precursor mechanism. The linear oxidation period involves uptake into the near-surface bulk by a non-activated place-exchange mechanism.

The interaction of an O2 molecular beam with an Fe(110) surface

Abstract The initial interaction between an O 2 molecular beam and a cleaned Fe(110) surface has been studied by a combination of Auger electron spectrometric (AES) and mass spectrometric techniques. The incident molecular beam intensity was calibrated using a stagnation detector, and the initial sticking coefficient for chemisorption was determined by mass spectrometric measurement of the transient in molecular scattering behavior observed when the cleaned surface was exposed to the molecular beam. This permitted an absolute calibration of the AES system for oxygen, and allowed comparison of the kinetic measurements of the oxygen adsorption process by the two techniques. Results indicate that the initial sticking coefficient is 0.2 ± 0.01. Oxygen is initially chemisorbed up to a coverage of 1.6 ± 0.16 × 10 15 cm −2 , by a process following Langmuir kinetics. Beyond this point AES studies indicate a slower rate of oxygen uptake which is independent of gas-phase oxygen pressure. The mass spectrometric studies further indicate that for a cleaned, annealed surface those oxygen molecules which are not chemisorbed are scattered in a non-diffuse manner.