Hydrogen loading of oxide powder particles: a transmission IR study for the case of zinc oxide.

Many hydrogenation reactions are catalyzed by metal oxides. A particularly important case is the synthesis of methanol from syngas (H2, CO, CO2). Although presently mainly ZnO-supported Cu particles are used in the technical process, ZnO alone is also active as a catalyst for this process. According to the standard model the sources for the hydrogen species in the hydrogenation reactions are hydroxide or hydride species at the surface of the oxide, which in turn are formed by heterolytic dissociation of H2. The dissociation of the H2-molecules is generally considered to be the rate-determining step. Recently, a number of unexpected observations as regards the interaction of hydrogen with oxides have been reported. First, in a paper by Freund and co-workers it has been found that hydrogen can actually be stored at the oxide metal interface in case of metal supported oxide particles. In addition, Yates and co-workers have reported the observation of bulk hydrogen after exposure of TiO2 powders to H2. [4, 5] Finally, in a recent paper by Yin et al. it has been found that heating of a hydroxylated single crystal TiO2(110) substrate does not lead to the expected desorption of H2 or H2O molecules, but instead to a diffusion of H atoms into the bulk of the crystal. In theoretical calculations this surprising behaviour was rationalized by the large activation energy of 2.52 eV for recombination of H atoms to form dihydrogen at the TiO2 surface. This value considerably exceeds that for diffusion into the bulk, for which an activation energy of 1.1 eV has been reported. Recently published STM images have provided direct evidence for the population of such subsurface sites by H atoms on TiO2 substrates. In case of ZnO single crystal surfaces it has been recently demonstrated that exposure of the mixed-terminated ZnO(10 10) to atomic hydrogen leads to the metallization of the surface, whereas exposure of the oxygen-terminated OZnO(000 1) surface leads to the occupation of bulk interstitial sites. In the latter case the quasi-elastic signal in electron energy loss spectroscopy (EELS) is significantly broadened due to the interstitial H atoms acting as shallow donors. From the temperature dependence of this broadening an apparent activation energy of 25 meV of the shallow donors was obtained. This observation is in full accord with the presence of interstitial H atoms as predicted by theoretical calculations. In addition, in this experimental work an activation barrier for the diffusion of H atoms into the bulk was determined to amount to 1.26 eV, a value which is in good agreement with earlier measurements of the temperature dependence of the electric conductivity. Although these experiments carried out with single crystal oxide surfaces are very interesting, the implications for real ZnO powder catalysts are to some extent unclear. One reason for the problems of transferring the information from single crystals to oxide nanoparticles is that of course these oxide nanoparticles expose a number of differently oriented crystallographic surfaces. In addition, it is known that the surfaces of oxide powder particles comprise a large number of defects such as steps, edges, kinks and vacancies, which are not present on the perfect single crystal surfaces. A second point which needs to be considered when transferring information from bulk samples to powder samples is the question of saturation. Whereas for macroscopic single crystal surfaces a saturation of the bulk by subsurface diffusion of surface species requires very long periods of time, for powder particles the bulkto-surface ratio is much smaller and therefore saturation effects have to be expected. Here, we report on a set of experiments where we have used infrared (IR) spectroscopy to monitor the occupation of hydrogen interstitial sites as a result of exposure of ZnO nanoparticles to H atoms and to H2 molecules. The density of interstitial H atoms is monitored by measuring the total absorption of IR light as a result of H-induced occupation of the conduction band. In the first set of experiments we used the same type of preparation as in the above quoted single crystal experiments, namely exposure of the powder particles to atomic hydrogen. It is demonstrated that the IR absorption results can be directly related to the broadening of the quasielastic EELS signal observed for the single crystalline surfaces. Then, in a second set of experiments we monitored the occupation of bulk interstitial sites by H atoms resulting from the exposure of the ZnO nanoparticles to H2 at room temperature. Also in this case an increase of the IR-absorption is observed, thus providing direct evidence that an exposure to H2 molecules at room temperature leads to the presence of bulk H atoms located at interstitial sites. These observations have im[a] Dr. H. Noei, Dr. Y. Wang, Prof. Dr. M. Muhler Laboratory of Industrial Chemistry Ruhr-University Bochum Universit ts str.150, 44780 Bochum (Germany) Fax: (+49) 234 321 4182 E-mail : [email protected] [b] Dr. H. Qiu, Dr. Y. Wang Department of Physical Chemistry I Ruhr-University Bochum Universit ts str.150, 44780 Bochum (Germany) [c] Prof. Dr. C. Wçll Institute of Functional Interfaces (IFG) Karlsruhe Institute of Technology (KIT) 76021 Karlsruhe (Germany) Fax: (+49) 7247 823478 E-mail : [email protected]