H2 D2 Exchange Research Articles

Studies of the hydrogen held by solids. Part 17.—The ortho-para H2Conversion and H2–D2exchange reactions over a transition alumina

The rates of the H2–D2 exchange, the o-p H2 and p-o D2 conversions were measured on the surfaces of a very pure alumina, with and without a portion of the sites selectively poisoned with CO2. These data were supplemented with results obtained from surface dehydroxylated to different extents. When the alumina was dehydroxylated below 525°, the rate of the ortho-para H2 conversion at 0° was only a little higher than the rate of the H2–D2 exchange. Both rates fell in a commensurate manner as the surface was poisoned with CO2(in a way which eliminated the strongest sites first) to less than 1 % of their initial values at about 16 × 1012 CO2/cm2. At –195°, the exchange reaction was immeasurably slow, while both conversion reactions (H2 and D2) had comparable and easily measurable rates, the D2 conversion being a little faster. These conversion rates fell as the surface was poisoned with CO2 to less than 1 % of their initial values at 18 × 1012 CO2/cm2. Both reactions were also poisoned by lowering the dehydroxylation temperature below 300°. After pretreatment at 800°, the site density for the conversion reactions increased to about 30 × 1012 CO2/cm2; that for the exchange reaction fell to about 9 × 1012CO2/cm2. Arrhenius plots for the H2–D2 exchange were linear on poisoned and unpoisoned surfaces and indicated an activation energy of about 2.3 kcal/mol. The Arrhenius plots for the conversion reactions were non-linear and varied with the extent of poisoning with CO2. In all cases, however, they converged with the plots for the exchange reaction above –80°, but curved to lower slopes (lower activation energies) at lower temperatures. Evidently, the conversion and exchange reactions both proceeded by the same dissociative mechanism at high temperatures but only the paramagnetic component of the conversion reaction was significant near the temperature of liquid nitrogen. An extensive search was made, but no e.p.r. signal could be found even though the site density determined by CO2 poisoning was ∼1019/g. It is suggested, therefore, that the conversion reactions are catalyzed by interaction with exposed 27Al nuclei.

Reaction of H2 + D2⇆2HD Catalyzed by Molybdenum and Observed by Mass Spectrometer

Papers I, II, and III of this series emphasize chemical reactions of the same designation. I describes H2–D2 exchange, II describes 28N2–30N2 exchange, and III reports some measurements on ammonia synthesis. All reactions were catalyzed by the same Mo ribbon. I and II are simpler than III and their study facilitates understanding III. Publications for I show a trend toward the expected equilibrium, but evaluation of catalytic effectiveness usually involved many sorptions of each reactant molecule with the heated catalyst. Our technique permits evaluation resulting from a single sorption interaction and shows for I that sorbed molecules leave our catalyst S at relative abundances agreeing with the equilibrium constant K of the exchange reaction. This equilibration during a single interaction occurs throughout the experimentally accessible temperature range 335°K to above 1000°K and for covers between 0.05 and approximately a complete monolayer. Contamination by nitrogen and by CO does not affect equilibration. Results imply that the catalyst splits the reactant molecules at least intermittently and may suggest atomic, rather than molecular sorption. However, published experiments warn against complete acceptance of atomic sorption. If exchange reactions involve only the catalyst surface, as generally thought, relative departure rates for H2, D2 and HD molecules would satisfy the usual conservation relation calculated from incidence rates from the gas phase onto the catalyst. This relation is independent of K but, with K, would completely specify relative desorption rates of the three molecular forms of hydrogen. Experimentally, only K is satisfied. Desorption rates are not directly related to incidence rates and are only understood if much of the hydrogen and deuterium participating in exchange comes from a phase dissolved in the catalyst, rather than from an adsorbed layer. This and other results suggest that H2–D2 exchange is related more critically to the volume of the catalyst than to its surface. If such results occur for other catalysts and other reactions, widely accepted models for catalysis must be reconsidered.

Activated Adsorption of Hydrogens on Aromatic—Alkali-Metal Charge-Transfer Complexes

The ortho—parahydrogen-conversion reaction (p-H2⇌o-H2) and the hydrogen—deuterium exchange reaction (H2+D2⇌2 HD) were used to investigate the activated adsorption of hydrogen on aromatic—alkali-metal charge-transfer complexes. When deuterium gas was introduced onto these complexes, it was found that hydrogen exchange takes place between deuterium and the complexes (denoted as H2⇌D2). The kinetics of the parahydrogen conversion and of the H2—D2 exchange were studied as a function of the temperature and it was found that the rate of reactions was first order at a constant pressure. On the basis of the effect of the temperature on the rate constant, an apparent activation energy E for p-H2⇌o-H2 was 4.4 kcal/mole in the presence of phthalonitrile—Na (I), 3.7 kcal/mole with tetracyanopyrene—Cs (II), 10.0 kcal/mole with violanthrene B–Cs (III) and 0.6 kcal/mole with graphite—Cs (IV). Further, E for H2+D2⇌2 HD was 6.6 kcal/mole in the presence of (I) and ∼0.6 kcal/mole with (IV). These reaction suggests that the strong activated adsorption of hydrogen on the complexes takes place with the excess charge transferred from alkali metal to the organic molecule. Further, it was found that the hydrogen exchange reaction between gaseous hydrogen and phthalonitrile takes place reversibly.

Catalysis by Electron Donor-Acceptor Complexes.

Publisher Summary This chapter provides an overview of the mechanism of hydrogen exchange or hydrogenation over the electron donor–acceptor (EDA) complexes of alkali metals, and of organic electron donor molecules, such as phenothiazine. The hydrogen exchange reaction takes place between D2 or C2D2 and various EDA complexes, and that the H2–D2 exchange reaction to form HD proceeds at a considerable rate over the complexes, while no reaction takes place in the absence of alkali metals even at 200°. A closed circulating system was employed to follow the reaction rate. The reactor was a U-shaped glass tube equipped with two side-arms, each containing electron acceptor (phthalocyanine) and donor (sodium), respectively. When, C2D2 (15 cm Hg) was admitted to the EDA complexes of various phthalocyanines with organic electron acceptors such as 2,3-dicyanoquinone at temperatures between 25 and 90°, the hydrogen exchange reactions of acetylene proceeded at a negligible rate and the components of the complexes were separated from each other above 100° as a result of sublimation.

Reactivity of electron donor-acceptor complexes. Part 9.—Isomerization of butene

The isomerization of butene was studied over various electron donor-acceptor (EDA) complexes for which the behaviour of hydrogen had been examined. The relative catalytic activity of the EDA complexes for cis-2-butene isomerization to 1-butene and trans-2-butene was closely connected with that for the H2—D2 exchange reaction, the activities for both reactions depending upon the electron affinity of the acceptor molecules. The activity of the EDA complexes of sodium has a maximum in the region of 0–0.6 eV for the electron affinity of the acceptor molecules and is small or negligible for either stronger or weaker acceptor molecules. The EDA complexes of magnesium, on the other hand, showed a maximum in the region of 1.2 and 1.6 eV for the electron affinity. Accordingly, a weaker electron donor gives optimal activity with a stronger electron acceptor. The cis-trans isomerization of butene was also studied over various EDA complexes of aromatic compounds with SbCl5, an electron acceptor; an optimal region of ionization potential (about 7.0 eV) of electron donors was demonstrated.

Reactivity of electron donor-acceptor complexes. Part 6.—Hydrogen exchange reaction between acetylene, or molecular hydrogen, and EDA complexes of aromatic cyanosubstituted compounds

The exchange reaction of hydrogen between acetylene, or molecular hydrogen, and the EDA complexes of sodium with aromatic hydrocarbons and cyanosubstituted compounds has been studied. The hydrogen exchange reaction with acetylene proceeds via the bonded hydrogen in the acceptor molecules and the rate is correlated with densities of unpaired electron in the anion radicals of acceptors in the EDA complexes. The H2—D2 exchange reaction over the EDA complexes proceeds via two different mechanisms; one involves bonded hydrogen, the other involves hydrogen chemisorbed on the EDA complexes. The rate and the activation energy for the two mechanisms were separately measured. The isomerization of butene over the EDA complexes was also studied. The isomerization of cis-2-butene proceeded rapidly to trans-2-butene and 1-butene over the complexes where the chemisorption of hydrogen takes place. Over the complexes where no H2—D2 exchange reaction takes place, the isomerization of cis-2-butene proceeded only to trans-2-butene and not to 1-butene.

Mass Spectrometry of Ions in Glow Discharges: Application to H2—D2 Exchange Reactions

The negative glow of the dc glow discharge was used to study H2—D2 exchange by an ionic chain reaction. The ions present were observed directly by mass spectrometry. The chain carrier was H3+ and the chain length about 15 at 0.3 torr. The inhibition by added rare gases was examined and detailed comparisons are made with radiolysis experiments. Qualitatively the agreement is excellent. Several possible inhibition mechanisms are considered.

Hydrogen–Deuterium Exchange on Iso-structural Sesquioxides of the First Long Period

A study was made on the H2-D2 exchange on sintered specimens of the sesquioxides Ti2O3, V2O3, and Cr2 O3. The results show that V2O3 is the most active oxide at high temperatures and the least active at low temperatures, and that Cr2O3 is the most active oxide at low temperatures. The results are discussed with respect to the crystal structure and the d orbitals of the metals, and it is concluded that the formation of a covalent d bend between the metal and adsorbed H2 is the critical step in the catalytic exchange.

The Hydrogen–Deuterium Exchange Activity and Radiation. Behavior of Some Silica Catalysts

Catalytic activity for H2-D2 exchange has been found in all of the types of silica cxamined, gel, quartz, alpha cristobalite, vitreous and amorphous. The activity per unit surface differs widely, being highest for quartz and vitreous silica. Since these were prepared by grinding, the high activity may have heen introduced by that process. The activity observed immediately after activation declines by a factor of ten or more over ten days, and more slowly for several weeks. Irradiation with gamma rays or neutrons produces a dramatic increase in the activity of these silicas. The activation energy with the gel is reduced by irradiation from about 9 to about 2 kcal/mole. Quartz and vitreous silica show low activation energies even before irradiation. The enhanced activity declines with time, but at a rate indistinguishable from thnt of the unirradiated materials. The activity obtained by irradiation has not been equalled in an unirradiated catalyst by any activation procedure. With the low activation energies observed, only a small number of sites is needed to explain the increases in activity. Since the yields of sites are likewise small, a primary process of low probability, such as atom displacement, is suggested. The activity per displaced atom ismore » even lower for the neutron bombardments. Within rather large uncertainties, the activity increase per unit dose is nearly equal for the different types of silica. Three silica-alumina hydrocarbon cracking catalysts behaved much like the silicas. The similarities shown by the different silicas (and silica-alumina) may mean that the local arrangement of silicon and oxygen atoms is responsible for the catslytic activity, for the only common feature of the structures is the siliconoxygen tetrahedron.« less

Adsorption and Diffusion of Hydrogen on Nickel

The surface diffusion and adsorption of hydrogen on Ni field emitters has been investigated by a technique previously used for similar studies of H and O diffusion on W. It was found that diffusion occurs with an activation energy of 7±1 kcal in the chemisorbed layer, without the precipitation boundaries observed on W. This is believed to result from the inherent homogeneity of f.c.c. substrates, on which most adsorption sites seem equivalent. This is supported by the marked lack of emission anisotropy of hydrogen-covered Ni field emitters. A field dependent transformation in the adsorbed film was found between 2° and 4°K and interpreted as a shift in the equilibrium between adsorbed H2 molecules and the terminal fraction of adsorbed H atoms. The implications of this result for the mechanism of H2–D2 exchange are discussed. The activation energy of desorption at very low coverage is determined to be 46±4 kcal from the temperature dependence of desorption rates.

The catalysis of H2—D2exchange by oxides

H2—D2exchange has been investigated a t the surfaces of 13 oxides of the first transition period, at temperatures between —195 and 20° C and pressures between 1 and 5 cm. There appears to be a simple correlation between the catalytic activity per unit surface area and the electronic configuration of the metal ion. The condition for high activity is that the oxide should possess some but not too many unpairedd-electrons. Thus, high activity occurs just after the beginning of the period (Cr2O3) and towards the end (Co3O4and NiO). If there are no unpairedd-electrons (TiO2, V2O5, Cu2O, ZnO, Ga2O3, GeO2) or very few (V2O3, CuO) or very many (MnO, Fe2O3) the activity is low. A tentative explanation of the results is offered, mainly in terms of the influence of electronic configuration on the velocity of hydrogen chemisorption. The results suggest that hydrogen behaves similarly at metal and at oxide surfaces, giving catalysis by these solids a common motif. There is no simple correlation with oxides between exchange activity and semi-conductivity. Pressure dependences of the exchange velocity have been measured for Cr2O3, NiO, ZnO and Ga2O3; these throw some light on the question of the exchange mechanism.