Horst Winkler

Investigations on hydrogen spillover. Part 1.—Electrical conductivity studies on titanium dioxide

The influence of hydrogen spillover on the conductivity of polycrystalline platinum-containing (in comparison to platinum-free) titania samples has been investigated. A special two-component sample geometry, consisting of a Pt/TiO2 part and a TiO2 part, was used to separate the conductivity effects after exposure to hydrogen which could arise owing to changes of the platinum/titania interface and to the action of the spilt-over hydrogen species.It was found that the electrical resistance of Pt/TiO2 samples was decreased by treatment with hydrogen at low temperatures (room temperature to 80 °C) and that, vice versa, the evacuation of hydrogen led to an increase in sample resistance. The electrical resistance of pure TiO2 was not affected by hydrogen.Using two-component (Pt/TiO2)–TiO2 samples the conductivity changed in both sample components after hydrogen adsorption. The decrease of resistance in the TiO2 component of the (Pt/TiO2)–TiO2 sample occurred with a delay due to the diffusion of the spilt-over hydrogen species from the Pt/TiO2 into the TiO2 component. The conductivity change in the TiO2 component can be correlated with the action of activated hydrogen species formed on platinum and having diffused onto the titania support in a spillover form. The spilt-over hydrogen species can be described as surface electron donors, i.e. as coexisting H atoms and H+ ions. Their chemisorption on titania corresponds to an electron transfer to the support and, thus, to an increase of the n-type conductivity.

Studies of the influence of a magnetic field on the diffusion of spilt-over deuterium on Y zeolites

After deuterium spillover from Pt supported on NaY zeolite onto NaY, the spilt-over particles diffuse into the HNaY part of a two-component Pt/NaY–HNaY sample where an H–D exchange of the OH groups takes place, as shown by diffuse-reflectance near-infrared spectroscopy, and where a reservoir of such activated spilt-over deuterium species is formed, remaining adsorbed after pumping off the D2 gas atmosphere at room temperature. The chief result is that on loading the sample with D2 in the presence of a magnetic field B perpendicular to the main direction of diffusion of the spilt-over particles, H–D exchange is less than without B, which leads to the conclusion that the diffusing spilt-over particles are electrically charged.

Exchange processes in nmr

Abstract The exchange of particles and the (noncorporeal) exchange of magnetization in systems are considered where nuclei exist in various regions which differ in their magnetic resonance properties. We start with the stochastic theory of nuclear magnetic resonance and relaxation in such systems. Two-region systems are treated in some detail; here, also, the modified Bloch equations are introduced. Experimental examples concerning adsorbent-adsorbate systems are given. Finally, the influence of cross-relaxation on the relaxation of molecules adsorbed on interfaces is taken into account, the corresponding theory being treated. NMR methods which allow to distinguish between particle and spin exchange are discussed.

I.r. and n.m.r. 1H longitudinal relaxation studies of the cation distribution in rare-earth and calcium exchanged Y zeolites

Abstract I.r. spectroscopy of OD vibration, lattice vibration and CO adsorption, together with n.m.r. longitudinal relaxation measurements of adsorbed water, were used to study the cation distribution in dehydrated calcium and rare-earth (RE3+) Y zeolites. In this Paper it is shown that in CaNaY zeolites the Ca2+ ions are selectively located in SI positions up to an exchange degree of 60%. The RE3+ ions introduced consecutively into CaNaY are directed to sites in the supercages. This directing effect depends on the order of ion exchange. In dehydrated RENaY the RE3+ ions preferentially occupy SI′ sites. Depending on the exchange and pretreatment conditions, there are different REO species in RENaY and RECaNaY zeolites. The unexpected high activity of 0.28RE0.60CaNaY in the isomerization of light gasoline is explained in terms of the observed cation distribution.

Proton spin relaxation in the system NaPtY zeolite/water; the state of platinum

Abstract In commercial zeolites the longitudinal proton spin relaxation time T 1 of adsorbed molecules is predominantly caused by paramagnetic impurities (0.07 wt% of iron in the starting NaY zeolite). Only 1 wt% of platinum, introduced as [Pt(NH 3 )4] 2+ , enhances T 1 of water up to one order of magnitude, depending on the thermal pretreatment of the exchanged zeolite. In connection with the behaviour of the transverse relaxation time T 2 and with the mass spectrum of the gases escaping during thermal pretreatment, it is concluded from the strong effect on T 1 that the Pt 2+ ions are reduced by the ammonia to atomic platinum and that the Pt atoms migrate preferentially to Fe 3+ ions. Thus, Fe 3+ ions, controlling T 1 , are covered and up to 90% of paramagnetic sites for water are occupied by Pt atoms, whereas in the case of a statistical distribution of the Pt atoms, the probability for a paramagnetic site to be occupied by platinum amounts to 0.1% to 1%. Presumably, the iron species covered are Fe 3+ ions in cationic sites of the zeolite, as in this case besides dispersion energy (E dis ≈ kcal mol ) polarization energy of the pair Pt-Fe 3+ comes into play, surmounting E dis by two orders of magnitude.

On two-component NMR relaxation

Abstract For two-component relaxation behaviour the time constant (apparent relaxation time) and the preexponential factor (apparent probability) of each of the components are complicated functions of the real relaxation times, the population fractions (real probabilities) and the lifetimes referring to two regions, between which an exchange of molecules, nuclei or spins occurs. The theoretical treatment of Zimmerman and Brittin is sketched and the apparent quantities as functions of the real quantities and vice versa are plotted in detail.

On the position of rare earth cations in zeolite Y

The number Ns′ of rare earth (RE) ions located in the supercage can be derived from the longitudinal proton n.m.r. relaxation time T1 of water adsorbed in 0.64 RENaY zeolite. This is because about 70% of these RE ions are paramagnetic and thus dominate the proton T1. The results do not agree completely with X-ray diffraction studies, but Ns′, is in good agreement with recent results of Lee and Rees.

Fourier transform infrared (DRIFT) investigation of glass-covered samples. Study of hydrogen spillover on zeolites

Abstract Glass-covered samples can be investigated in the mid-IR region by DRIFT spectroscopy down to 2200 cm −1 . These samples are leak-proof and permit investigations over long periods. HD exchange due to hydrogen spillover was investigated on Pt-NaY/HNaY couples. The HD exchange on OH groups was found to take place at room temperature. All types of OH groups of the zeolite are involved. Water is formed as a by-product of the HD exchange owing to hydrogen spillover. Charged particles seem to take part in the spillover process. The application of a magnetic field reduces the diffusion of the spillover hydrogen inside the supercage of the Y zeolite.

On the positions of rare earth cations in Y zeolites as studied by NMR relaxation of adsorbed molecules

Abstract NMR relaxation measurements of adsorbed molecules yield information about the occupation of cationic sites in Y zeolites by paramagnetic RE3+ ions. In RECaNaY zeolites Ca2+ ions, introduced before the RE3+ ions, direct the latter to sites in the supercages. If the order of succession of exchange is inverted, no such effect is observed. In a RENaY zeolite the RE3+ ions preferentially occupy sites outside the supercages. Dehydration at 450°C in high vacuum or in air makes part of the RE3+ ions leave the supercages, partly irreversibly. With increasing water coverage an increasing number of RE3+ ions migrate back into the supercages.

Proton NMR relaxation studies of changes in the vicinity of impurity Fe3+ ions in zeolite/adsorbate systems

Abstract In commercial zeolites the proton relaxation time T 1 of adsorbed molecules is controlled by impurity Fe 3+ ions. A considerable number of various zeolite/adsorbate systems show an enhancement of T 1 by about one order of magnitude, corresponding to a decrease in the number of accessible Fe 3+ ions. This indicates changes in the vicinity of the Fe 3+ ions. In these zeolites bridging hydroxyl groups were produced and water was present during or after this process.