The main classes of heterogeneous catalysts are: (i) metals and alloys (supported or not), (ii) metallic oxides (including mixed oxides, heteropolyacids, superacids),

(iii) zeolites and molecular sieves in general, and (iv) sulfides. It will take some more years before deciding if metal-organic frameworks (MOFs) become a new member of the catalysts family.

Several neutron techniques are used to study catalytic systems: neutron dif­fraction (ND), small-angle neutron scattering (SANS), inelastic neutron scattering (INS), and quasi-elastic neutron scattering (QENS). We will limit ourselves here to INS and QENS of hydrogen species and dihydrogen molecules adsorbed on the surface of catalyst particles or inside porous materials.

H. Jobic (H)

Centre National de la Recherche Scientifique, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Lyon, France e-mail: herve. jobic@ircelyon. univ-lyon1.fr

© Springer International Publishing Switzerland 2015 17

G. J. Kearley and V. K. Peterson (eds.), Neutron Applications in Materials for Energy, Neutron Scattering Applications and Techniques,

DOI 10.1007/978-3-319-06656-1_2

Hydrogen has recently been associated with the words fuel cells and energy storage, but it is also an essential component in catalytic reactions and the hydrogen produced is mainly used in petroleum refining and ammonia production for fer­tilizer. Nowadays, about 90 % of the H2 production comes from catalytic steam reforming of natural gas at high temperatures (subsequent reactions of water-gas shift and preferential oxidation are required to decrease the CO level of the gas mixture to a few ppm before it can be used in a fuel cell). At the time being, one is facing a huge increase of H2 needs (with related CO2 emissions), so that until Jules Verne’s predictions are realized (water: the coal of the future), we may reach an H2 deficit, as predicted by some experts.

The applications of INS to catalysis have been mainly focused to systems which are either difficult or impossible to study by other spectroscopies such as trans­mission or reflection-absorption infrared, and Raman. The kind of catalyst which is studied in INS has generally an inhomogeneous surface, e. g. oxides, sulfides, and metals, although zeolites, which are well-crystallized materials, are well suited. These substrates can be almost transparent to neutrons if they contain a small quantity of hydrogen, in which case the neutron spectrum will be fairly flat and it will be possible to observe all the vibrational modes of the adsorbate.

QENS has been mainly used to measure the diffusion of H or hydrogenated molecules, although the transport of deuterated molecules and of molecules which do not contain hydrogen atoms can now be followed. The dimensionality of dif­fusion has been studied, even if the samples are in powder form. On zeolites [1], MOFs [2], or clays [3], anisotropic diffusion (one or two dimensional) has been evidenced. The technique allows us to probe diffusion over length scales ranging from an A to hundreds of A. The mechanism of diffusion can thus be followed from the elementary jumps between adsorption sites to Fickian diffusion.