Neutron investigations of rotational motions in monolayer and multilayer films at the interface of MgO and graphite surfaces

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.

The Characterisation of Hydrogen on Nickel and Cobalt Catalysts

We have investigated a series of supported and unsupported nickel and cobalt catalysts, principally using neutron vibrational spectroscopy (inelastic neutron scattering, INS). For an alumina supported Ni catalyst we are able to detect hydrogen on the metal for the first time, all previous work has used Raney Ni. For an unsupported Ni foam catalyst, which has similar behaviour to Raney Ni but with a much lower density, the spectra show that there are approximately equal numbers of (100) and (111) sites, in contrast to Raney Ni that shows largely (111) sites. The observation of hydrogen on cobalt catalysts proved to be extremely challenging. In order to generate a cobalt metal surface, reduction in hydrogen at 250–300 °C is required. Lower temperatures result in a largely hydroxylated surface. The spectra show that on Raney Co (and probably also on a Co foam catalyst), hydrogen occupies a threefold hollow site, similar to that found on Co($$10\bar{1}0$$ 10 1 ¯ 0 ). The reduced surface is highly reactive: transfers between cells in a high quality glovebox were sufficient to re-hydroxylate the surface.

Methyl tunnelling of adsorbed methoxy on alumina catalysts

The aim of this paper is to investigate whether the methyl group of the adsorbed methoxy intermediate on industrial grade alumina catalysts undergoes rotational tunnelling on the wavenumber energy scale. The data show that this is clearly the case for a fraction of the methyl groups and potentially allows the subtle intermolecular interactions between adsorbed species and catalyst to be probed through the exponential dependence of the tunnel frequency on the rotational potential.

A theoretical study of rotational and translational diffusion dynamics of molecules with a six-fold point symmetry adsorbed on a hexagonal lattice by neutron scattering

A complete analytical model for the rotational and translational diffusion of molecules with a six-fold point symmetry on a hexagonal lattice is presented. It can be applied, in particular, to the diffusion of benzene molecules adsorbed flat on the basal plane of graphite in the case of incoherent scattering. Under the weak hindered approximation, the classical mechanics framework and making use of the van Hove formalism of correlation functions, the intermediate scattering function and its Fourier transform, the scattering law, are both obtained. They can be expressed as sums of exponential decays or Lorentzian functions, respectively, containing the contribution of each of the dynamical processes taking place. In the case of benzene lying flat on the substrate we expect translational diffusion, continuous rotations of isolated molecules and hindered rotations of molecules within clusters. Each particular diffusive mechanism can be recognized owing to its particular signature in the dependence of the quasi-elastic broadening on the momentum transfer.

Phase behavior of heptanamide adsorbed on a graphite substrate

In this letter, the phase behavior of a saturated alkylamide, heptanamide (C(7)), adsorbed on the surface of graphite using synchrotron X-ray diffraction is presented. The diffraction patterns indicate that heptanamide undergoes a solid-solid phase transition in the monolayer at 330 K from pgg symmetry at lower temperatures to p2 symmetry at high temperatures. Other alkylamides with similar carbon chain lengths do not show this phase change, making the C(7) homologue unusual.