Daniel D. Eley

Adsorption and Oxide Formation on Aluminium Films

Type I aluminium films deposited at 0 °C were of relatively low area, ca. 1000 cm2, stable and with a certain porosity since CO uptake was proportional to film weight. Thus a film area could be determined from its weight, since at saturation CO molecules occupy 0·75 of the total surface area given by the B.E.T. method (established for higher area type II films). It was found that H2 and N2 were not chemisorbed, N2O and NO reacted giving surface oxide, C2H4 gave a slow reversible adsorption, and CO gave a slow irreversible adsorption, rate proportional to p/g, where p is the gas pressure, g the quantity of gas taken up by the film. The kinetics of oxygen uptake were followed in the range 10-2 to 10-3 mm Hg pressure. The first two monolayers were taken up too rapidly for kinetic measurement, and thereafter gas uptake followed the direct logarithmic law, until about 7 Å of film had been formed dg/dt = a e-γg/RT p0·6, where a and γ are constants. The rate-determining step is considered to be a place exchange of surface oxygen with underlying aluminium atoms, stimulated by reversibly chemisorbed oxygen atoms. As the film thickens it changes from covalent amorphous to ionic spinel character, and the activation energy for place exchange increases. The process of film growth in the ionic film (thickness >ca. 10 Å) involves electron tunnelling and cation migration.

Emulsions of water in asphaltene-containing oils 1. Droplet size distribution and emulsification rates

Abstract Emulsions of simulated sea water in three crude oils and in a model oil consisting of n-heptane, m-xylene and asphaltenes were examined by optical microscopy to obtain droplet diameters which were found to fit the log-normal distribution function. The rate of formation of interfacial area with respect to stirring time in the crude oil emulsions was first order. Values of Sm, the limiting area stabilised, increased with increasing asphaltene content while k, the rate constant, decreased. Addition of surfactants to either the oil phase or to the aqueous phase before mixing slowed the formation of Kuwait crude emulsions and increased Sm. This effect could result from a decrease in the oil/water interfacial tension causing a change in the equilibrium contact angle between asphaltene particles and the interface.

The polymerisation of ethylene on chromium oxide catalysts: III. An infrared study of the adsorption of carbon monoxide on active catalyst

Abstract Supported chromium (VI) oxide which has been pretreated with either ethylene or carbon monoxide at 300 °C is active for both the polymerisation of ethylene and the adsorption of carbon monoxide. The reversible adsorption of carbon monoxide at ca. 50 °C has been studied using infrared spectroscopy. Correlations between the capacity of the catalyst for the adsorption of carbon monoxide and the rate of polymerisation have shown that carbon monoxide is adsorbed on sites which are catalytically active. Catalyst which has been reduced in carbon monoxide develops a higher capacity for adsorption of carbon monoxide and gives higher rates of polymerisation than catalyst reduced in ethylene. Adsorbed carbon monoxide poisons the polymerisation reaction, but evacuation at 50 °C results in a complete recovery of catalyst activity. Partial restoration of the polymerisation activity is also effected by displacement of carbon monoxide from some of the active sites when ethylene is contacted with the poisoned catalyst at 50 °C. The modes of adsorption of carbon monoxide on the catalysts and the nature of the polymerisation sites are discussed.

Spin centers and catalysis on γ-alumina

The pH2 → oH2 and oD2 → pD2 (77–293 °K) and H2 + D2 → 2HD (195–293 °K) reactions have been studied over the temperature ranges indicated on γ-Al2O3 previously outgassed in vacuo at 550 °C. The reaction rates are similar over the ranges indicated and at 293 °K decrease in the order pH2 > oD2 > H2 + D2. In the outgassed state and before irradiation, the solid showed 4 × 1016 free spins g−1, g = 2.003, line width 6 gauss, and it was concluded that surface free valencies were catalyzing all three reactions by chemical mechanisms. The conversions probably involve 2 atom recombination, and the H2 + D2 molecule-atom exchange, on a small fraction of active sites. A 2 Mrad dose of γ-irradiation gave a 100-fold increase in free spins in the γ-Al2O3, which now showed an anisotropic signal, g (average) = 2.01, line width 40 gauss and a 2–3-fold increase in catalyzed rate for the pH2 conversion, but no change in HD equilibration rate. Presumably, therefore, the pH2 increase arises from onset of a paramagnetic mechanism. Admission of oxygen to the irradiated γ-Al2O3 gave first an intensification of signal associated with a chemisorbed monolayer and at higher pressure a “pressure broadening” of part of the signal associated with an interaction with physically adsorbed oxygen. The broadening left a residual signal attributable to bulk spins, and by difference, a surface spin concentration was calculated and could be associated with a rate of pH2 conversion. This rate checks to a factor 1.9 with that calculated using Wigners equation, confirming the view that the conversion on the radiation-induced surface free spins of γ-Al2O3 occurs mainly by the paramagnetic mechanism. A close similarity exists with α-Al2O3, and we revise our earlier view about this catalyst, concluding now that low temperature conversion on the solid before irradiation involves the chemical mechanism, as for γ-Al2O3.

Hydrogen chemisorption and exchange on platinum

Abstract The pH 2 , oD 2 and H 2 + D 2 reactions have been studied from 10 −1 to 10 Torr and 77 to 500K, on a polycrystalline Pt wire cleaned in ultrahigh vacuum. Sticking coefficient studies have also been made from 5 × 10 −8 to 5 × 10 −5 Torr and 77 to 350K. Three temperature ranges are distinguished in terms of the following molecular mechanisms, 1. (a) 77–110K; D 2 + MH → MD + HD involving molecular adsorption on defect sites induced by the oxygen activation procedure. There is also a paramagnetic contribution to the spin isomerizations, since both oD 2 and pH 2 reactions go faster than H 2 + D 2 2. (b) 110–200K; similar to (a) but involving molecules in the whole van der Waals layer. 3. (c) >200K; 1/2H 2 + 1/2D 2 + 2M → MH MD → 2M + HD.

γ Irradiation of alumina catalysts

Abstract Thermal activation of α-alumina in vacuo creates active centers in the surface which catalyze the parahydrogen conversion (examined over 77 ° to 823 °K) and the H2D2 equilibration (193 ° to 823 °K). The conversion mechanism is mainly paramagnetic at low temperatures, changing to mainly chemical at high temperatures, while the equilibration can only involve the chemical mechanism. The apparent activation energy E of the conversion is 290 cal mole−1 over 77 ° to 383 °K, and above 383 °K, where the rate is a maximum, E = −490 cal mole−1. The activation energy for the H2D2 equilibration is 1400 cal mole−1, the rate being nearly equal to that of the conversion at 823 °K. These active centers, which probably arise in a dehydrated surface possessing mechanical strain, are thermally stable, and show a negligible ESR signal. Irradiation of the thermally activated catalyst in vacuo at 77 °K with γ rays from Co60 gives an ESR signal with an absorption line 40 gauss wide, which is stable up to 193 °K but anneals out at 273 °K. There is a proportional increase in activity of irradiated catalysts for the parahydrogen conversion, but no increase in the rate of H2D2 equilibration. These new thermolabile active centers give a purely paramagnetic conversion. The ratio of observed rate to that calculated using the Wigner transition probability is 4030 at 77 °K, and in line with data for rare earth oxides and transition metals.

Spin centers and catalysis on magnesia

The rates of pH2 → oH2, oD2 → pD2 (77–273 °K) and H2 + D2 → 2HD (195–273 °K) have been studied on MgO (outgassed at 900 °C). These rates are similar at 293 °K, decreasing over the series oD2 > pH2 > H2 + D2 and all have positive temperature coefficients. The reaction orders are zero for pH2 and oD2, and 0.6 for H2 + D2, not much affected by temperature. No surface free electrons were detectable by ESR before irradiation. A 2 Mrad dose of γ-radiation from 60Co enhanced the rate of all three reactions 3-fold, and raised the order of the pH2 reaction to 0.3. The irradiated oxide showed an ESR signal indicative of three surface centers, designated F3, V3, and X. Of these, the V3, center signal decreased on exposure to H2 gas and is probably, therefore, the chemisorption center which catalyzes the hydrogen isomerization and equilibration reactions. It is concluded that all 3 reactions, both before and after irradiation, go by chemical mechanisms. The conversions probably involve 2 atom recombination and the H2 + D2 reaction molecule-atom exchange, on a small fraction of active sites. This may be compared with α and γ-Al2O3 in the previous paper, where before irradiation the oxides catalyze all three reactions by the chemical mechanism, and subsequent irradiation of the solid appears to promote a paramagnetic mechanism for the two conversion reactions.

The catalytic activity of the rare earth oxides for parahydrogen conversion and hydrogen-deuterium equilibration

Abstract Parahydrogen conversion and hydrogen-deuterium equilibration have been investigated on neodymium, gadolinium, dysprosium, and erbium oxides over the temperature range 77 ° to 657 °K. At low temperatures, where the equilibration reaction is absent, the conversion proceeds by a paramagnetic mechanism in which the rate is dependent upon the 4 f electronic structure of the oxides. There is evidence for a similar mechanism for conversion at intermediate temperatures (140–400 °K). At high temperatures both the conversion and equilibration reaction proceed with comparable rates on all four oxides studied. Both reactions are considered to proceed by the same chemical mechanism, in which the similar outer electronic states, 5 s and 5 p , control catalytic activity.

Parahydrogen conversion on the first transition series

Abstract The effect of pressure on the reaction velocity has revealed a new phenomenon, in which zero, or negative reaction order changes over to a fractional or first order reaction at higher pressures. This was found for Mn, Fe, Co, Ni at 90 °K, and Co at 293 °K. The mechanisms suggested are hydrogen atom recombination in a saturated layer, probably accompanied by hydrogen poisoning at low pressures and hydrogen molecule-atom exchange in a partly filled “second layer” at higher pressures. The reactions on Cr at 90 °K, and Cr, Fe, and Ni at 293 ° K are simple zero order, and Mn, 293 °K, Zn, 368 °K, fractional and first order, respectively. Hydrogen-deuterium equilibration goes at comparable rates with the conversion, for Ti, V, Cr, and Ni; there is evidence for Ni that this holds down to 77 °K. Attempts to estimate the rate of the paramagnetic conversion at 90 °K using the Wigner equation confirm the dominance of the chemical mechanisms for all metals except Zn, where the mechanism is probably paramagnetic. A correlation between reaction velocity and sublimation energy for the range of metals at 293 °K is associated with a mechanism of recombination of H atoms held on sites of minimum bond energy. The low frequency of these sites for body-centered cubic metals gives a frequency factor of 10 16 molecules cm −2 sec −1 compared with 10 19 for close-packed metals. A lower activation energy is found for bcc metals and is associated with a greater interaction energy between adsorbed atoms, in its turn associated with the broad character of the d band in this group of metals.

Microwave Hall mobility measurements on rat liver mitochondria and spinach chloroplasts

The effect of known respiratory inhibitors on the charge carrier hall mobility of rat liver mitochondria has been investigated using a microwave technique. Potassium cyanide and rotenone were found to reduce the Hall mobility, but no effect was observed for antimycin-A. The marked effect of potassium cyanide and the low mobility value obtained for the lipid extract of the mitochondria, suggest that electronic conduction through the electron transport chain is being observed. Volume-corrected values of between 50 and 80 cm2/V sec are found for the electron Hall mobility.Measurements on spinach chloroplasts give P-type Hall mobility values of 0·5 and 0·8 cm2/V sec.