P.W. Selwood

The chemisorptive bonding of hydrogen on nickel

Published experimental results plus a limited amount of new data show that chemisorbed hydrogen on nickel must consist of an atom carrying a negative charge. The nickel atom most affected retains a magnetic moment but loses its ability to participate in ferromagnetic exchange interaction. It retains most of its other properties as part of the metal particle. The stoichiometry is one hydrogen to one nickel with minor peripheral effects. There is no change of stoichiometry, or of bonding mode, over the whole range of surface coverage or, within limits, of temperature change. The bonding is quite different from that in nickel hydride.

The effect of a weak magnetic field on the rare earth catalyzed parahydrogen conversion rate

Abstract If the catalyst is placed in a magnetic field of a few oersteds there may be observed a decrease in the parahydrogen conversion rate at room temperature over the oxides, Pr 2 O 3 , Nd 2 O 3 , Sm 2 O 3 , and Yb 2 O 3 , but not over Eu 2 O 3 , Gd 2 O 3 , Tb 2 O 3 , Dy 2 O 3 , Ho 2 O 3 , Er 2 O 3 , or Tm 2 O 3 . At 40 Oe, the rate decreases range from 5 to 20%. If the rare earth catalysts are supported at relatively low surface concentration on the diamagnetic La 2 O 3 , then all except Gd 2 O 3 show the weak field magnetocatalytic effect. A plot of catalytic conversion rate versus extrinsic field shows an abrupt dependence of rate on field in the region of 3 to 10 Oe. It is possible, in this region, to observe the effect on the reaction rate of a change of field of less than 0.2 Oe.

Extrinsic field conversion of parahydrogen over the rare earths

Abstract Parahydrogen conversion rates have been measured at room temperature over most of the rare earths as catalysts. With the catalyst in zero extrinsic magnetic field the conversion rates are, within experimental error, proportional to the square of the magnetic moment of the paramagnetic species. With the catalyst in an extrinsic field of 18 kOe, large increases in conversion rates are observed for all rare earths. These increases bear no relation to the number of f electrons, or to the presence of even or odd numbers of unpaired electrons in the paramagnetic ions. It appears, therefore, that the electron-spin relaxation time cannot be rate-determining for either the zero field effect or the extrinsic field acceleration, over these catalysts.

Ferromagnetic resonance of supported nickel with adsorbed hydrogen, oxygen, and ethylene

Abstract Ferromagnetic resonance (FMR) spectra have been obtained for silica-supported nickel in several mean particle sizes, and both before and after the chemisorption of hydrogen, oxygen, and ethylene. The results are in general agreement with previously reported studies on these systems by classical methods. The FMR method is more sensitive than the classical by several orders of magnitude. It also gives more information concerning, especially, the magnetocrystalline anisotropy and the surface anisotropy, and the manner in which these are changed by adsorbed molecules.

Semiconductivity in Eu2O3 and Gd2O3, and exchange interaction in the system MnO-MgO, as possible criteria for the catalytic decomposition of ammonia

Abstract This work was part of an attempt to characterize more precisely the structural characteristics associated with catalytic activity in certain classes of solids. It was found that Eu 2 O 3 in H 2 atmosphere at 800 °C was an n -type semiconductor with a conductivity 19 times greater than that of Gd 2 O 3 under similar conditions. But no difference could be detected in the rate or activation energy for ammonia decomposition over these oxides. It was found that dilution with MgO did not alter the catalytic activity of MnO for the ammonia decomposition although, as was previously known, exchange interaction between Mn 2+ ions is diminished by such dilution. It appears from these results that neither semiconductivity nor exchange effects are directly related to catalytic activity in these solids, under the conditions described.

The activation of yttria, lutetia, and ytterbia for the ortho-parahydrogen conversion

The diamagnetic oxides Y2O3 and Lu2O3 may be activated for the p-H2 conversion by heating them in H2 between 823 and 973 K. This pretreatment appears to generate the ions Y2+ and Lu2+, respectively. From the quantity of water formed during the activation it is possible to estimate the surface concentration of paramagnetic sites. By combining this information with the specific conversion rates it is possible to obtain the net conversion rate per active site. The results are in agreement, to about one order, with recent theoretical calculations of the absolute rates. From conversion rate studies over Lu2O3 and Yb2O3 and from extrinsic field studies, it is concluded that heating Yb2O3 in H2, and possibly other rare earths in the C crystallographic form, causes surface reactions resulting in a 4fn5d0 → 4fn5d1 electronic change rather than 4fn5d0 → 4fn+15d0.

Magnetic effects on the ortho-parahydrogen conversion over α-Cr2O3, CoO, and MnO

Catalyzed ortho-parahydrogen conversion rates were measured on two antiferromagnetic oxides, α-Cr 2 O 3 and CoO over a range of temperature and both with and without an extrinsic magnetic field. A few measurements were made on MnO. The conversion mechanism was, in all cases, nondissociative. Results for the three catalysts, while qualitatively similar, differed in detail. A correlation between the zero field conversion rate and the extrinsic field rate suggests a relation to the paramagnetic-antiferromagnetic phase transition in catalysts of high surface site density. A positive extrinsic field effect was observed only at temperatures above the Neel point ( T N ). Negative effects were found, in some cases, both above and below T N . The latter results were characterized by a maximum rate decrease just below T N and by a rate saturation effect that was a function of applied field strength. The several effects observed suggest that, on high site density surfaces, two or more mechanisms are operative in an extrinsic field. These, and other related results, show that the current state of conversion theory over magnetic surfaces is still incomplete.

Parahydrogen conversion and magnetocatalytic effects over yttria and lutetia

Abstract Yttria (Y 2 O 3 ) and lutetia (Lu 2 O 3 ) pretreated in hydrogen above 550 °C become effective catalysts for the ortho-parahydrogen conversion by the magnetic mechanism. The source of activity in each case appears to be the paramagnetic 2+ ion, Y 2+ or Lu 2+ . This view is supported by electron spin resonance measurements. The rate of conversion is strongly influenced by a magnetic field of a few oersteds. Over lutetia a field of 40 Oe applied to the catalyst decreases the rate by 42%, and the presence of the Earths field may readily be demonstrated. Some recent theoretical work is consistent with these magnetocatalytic results.

The catalyzed o-pH2 conversion and magnetocatalytic effects on EuO and CrO2

Abstract Catalyzed ortho-parahydrogen conversion rates have been measured on both insulating and conducting types of europium monoxide over the temperature range 65–400 K. There was no rate anomaly in the neighborhood of the ferromagnetic-paramagnetic phase transition. A positive extrinsic field magnetocatalytic effect was observed in the paramagnetic phase but little effect was found below the Curie point. Chromium dioxide proved to be too reactive for measurements to be made near the Curie point. Catalytic activity was found, but no extrinsic field effect at room temperature. The theoretical implications of these results are discussed.

Parahydrogen conversion over chromiagel near the Néel point

Parahydrogen conversion rate measurements have been made over amorphous chromiagel, microcrystalline chromiagel, and macrocrystalline α-Cr2O3. The decrease in conversion rate that occurs over α-Cr2O3 as the temperature is raised through the Neel point is also observed over the microcrystalline gel, but not over the amorphous gel. Antiferromagnetic ordering has been shown to be associated with a large increase of conversion rate and this is independent of the increase in rate due to improved accessibility, as crystallinity develops. Magnetic ordering, when it occurs, is present in those catalyst ions accessible to molecular hydrogen and thus, of necessity, on the catalyst surface.