Chemisorbed hydrogen and hydrogenous molecules

Neutron Scattering Studies of Heterogeneous Catalysis

Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure-catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron-nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis.

Investigations of Hydrocarbon Species on Solid Catalysts by Inelastic Neutron Scattering

The status of surface species on solid catalysts during heterogeneous catalysis is often mysterious. Investigations of these surface species are crucial to deconvolute the reaction network and design more efficient catalysts. Vibrational spectroscopy is a powerful technique to study the interactions between surface species and the catalysts and infrared (IR) and Raman spectroscopies have been widely applied to study reaction mechanisms in heterogeneous catalysis. However, IR/Raman spectra are difficult to model computationally and important vibrational modes may be IR-, Raman- (or both) inactive due to restrictions by optical selection rules. Inelastic neutron scattering (INS) is another form of vibrational spectroscopy and relies on the scattering of neutrons by the atomic nucleus. A consequence of this is that INS is not subject to any optical selection rules and all vibrations are measurable in principle. INS spectroscopy has been used to investigate surface species on catalysts in a wide range of heterogeneous catalytic reactions. In this mini-review, we focus on applications of INS in two important fields: petrochemical reactions and C1 chemistry. We introduce the basic principles of the INS technique, followed by a discussion of its application in investigating two key catalytic systems: (i) the behaviour of hydrocarbons on metal-oxide and zeolite catalysts and (ii) the formation of hydrocarbonaceous species on methane reforming and Fischer–Tropsch catalysts. The power of INS in studying these important catalytic systems is demonstrated.

Experimental arrangements suitable for the acquisition of inelastic neutron scattering spectra of heterogeneous catalysts

Inelastic neutron scattering (INS) is increasingly being used for the characterization of heterogeneous catalysts. As the technique is uniquely sensitive to hydrogen atoms, vibrational spectra can be obtained that emphasize a hydrogenous component or hydrogen-containing moieties adsorbed on to an inorganic support. However, due to sensitivity constraints, the technique typically requires large sample masses (∼10 g catalyst). A reaction system is hereby described that enables suitable quantities of heterogeneous catalysts to be appropriately activated and operated under steady-state conditions for extended periods of time prior to acquisition of the INS spectrum. In addition to ex situ studies, a cell is described which negates the need for a sample transfer stage between reaction testing and INS measurement. This cell can operate up to temperatures of 823 K and pressures up to 20 bar. The apparatus is also amenable to adsorption experiments at the gas-solid interface.