Static SIMS studies of catalyst structure and activity

Label free biochemical 2D and 3D imaging using secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) provides a method for the detection of native and exogenous compounds in biological samples on a cellular scale. Through the development of novel ion beams the amount of molecular signal available from the sample surface has been increased. Through the introduction of polyatomic ion beams, particularly C(60), ToF-SIMS can now be used to monitor molecular signals as a function of depth as the sample is eroded thus proving the ability to generate 3D molecular images. Here we describe how this new capability has led to the development of novel instrumentation for 3D molecular imaging while also highlighting the importance of sample preparation and discuss the challenges that still need to be overcome to maximise the impact of the technique.

Kinetic measurements of hydrocarbon conversion reactions on model metal surfaces

Examples from recent studies in our laboratory are presented to illustrate the main tools available to surface scientists for the determination of the kinetics of surface reactions. Emphasis is given here to hydrocarbon conversions and studies that rely on the use of model systems, typically single crystals and controlled (ultrahigh vacuum) environments. A detailed discussion is provided on the use of temperature-programmed desorption for the determination of activation energies as well as for product identification and yield estimations. Isothermal kinetic measurements are addressed next by focusing on studies under vacuum using molecular beams and surface-sensitive spectroscopies. That is followed by a review of the usefulness of high-pressure cells and other reactor designs for the emulation of realistic catalytic conditions. Finally, an analysis of the power of isotope labeling and chemical substitutions in mechanistic research on surface reactions is presented.

Spectroscopic and Structural Characterization of Chlorine Loading Effects on Mo/Si:Ti Catalysts in Oxidative Dehydrogenation of Ethane

The structural changes induced in a silica-titania mixed-oxide support (1:1 molar ratio) by chlorine addition at different loading levels, their relation to the structural characteristics of supported MoOx species over the support, and their correlation with ethane oxidative dehydrogenation (ODH) activity have been examined. The molybdenum and chlorine precursors are incorporated into the Si/Ti support network as it forms during gelation by using a "one-pot" modified sol-gel/coprecipitation technique. In situ X-ray diffraction during calcination shows the Si/Ti 1:1 mixed-oxide support is in a state of nanodispersed anatase titania over amorphous silica. With the addition of molybdenum and chlorine modifier, this anatase feature becomes more pronounced, indicating a decreased dispersion of titania. The effective titania surface area on the chlorine-doped Si:Ti support obtained from 2-propanol temperature-programmed reaction supports this observation. Raman spectra of dehydrated samples point to an enhanced interaction of MoOx species with silica at the expense of titania. X-ray photoelectron spectroscopic results show that, without forming a molybdenum chloride, the presence of chlorine significantly alters the relative surface concentration of Si vs Ti, the electronic structure of the surface MoOx species, and the oxygen environment around supported MoOx species in the Si/Ti network. Secondary ion mass spectrometry detected the existence of SiCl fragments from the mass spectra, which provides molecular insight into the location of chlorine in Mo/Si:Ti catalysts. The observed increase in ethane ODH selectivity with chlorine modification may be ascribed to the MoOx species sharing more complex ligands with silica and titania with the indirect participation of chlorine. Steady-state isotopic transient kinetic analysis (SSITKA) is used to to examine the oxygen insertion and exchange mechanisms. The catalysts show very little oxygen exchange with the gas phase in the absence of a reaction medium. During the steady-state ODH reaction, lattice oxygen appears to be the primary source of oxygen in the formation of water and CO2.