Heterogeneity of the vanadia phase dispersed on titania. Co-existence of distinct mono-oxo VOx sites

The structural and configurational characteristics of the species comprising the (VO_x)_n phase dispersed on TiO₂(P25) are studied under oxidative dehydration conditions by in situ molecular vibrational spectroscopy (Raman, FTIR) complemented by in situ Raman/¹⁸O isotope exchange and Raman spectroscopy under static equilibrium at temperatures of 175-430 °C and coverages in the 0.40-5.5 V nm^-2 range. It is found that the dispersed (VO_x)_n phase consists of distinct species with different configurations. At low coverages of 0.40 and 0.74 V nm^-2, isolated (monomeric) species prevail. Two distinct mono-oxo species are found: (i) a majority Species-I, presumably of distorted tetrahedral OV(-O-)₃ configuration with VO mode at 1022-1024 cm^-1 and (ii) a minority Species-II, presumably of distorted octahedral-like OV(-O-)₄ configuration with VO mode at 1013-1014 cm^-1. Cycling the catalysts in the 430 → 250 → 175 → 430 °C sequence results in temperature-dependent structural transformations. With decreasing temperature, a Species-II → Species-I transformation with concomitant surface hydroxylation takes place by means of a hydrolysis mechanism mediated by water molecules retained by the surface. A third species (Species-III, presumably of di-oxo configuration with ν_s/ν_as at ∼995/985 cm^-1) occurs in minority and its presence is increased when further lowering the temperature according to a Species-I → Species-III hydrolysis step. Species-II (OV(-O-)₄) shows the highest reactivity to water. For coverages above 1 V nm^-2, an association of VO_x units takes place leading to gradually larger polymeric domains when the coverage is increased in the 1.1-5.5 V nm^-2 range. Polymeric (VO_x)_n domains comprise building units that maintain the structural characteristics (termination configuration and V coordination number) of Species-I, Species-II, and Species-III. The terminal VO stretching modes are blue-shifted with increasing (VO_x)_n domain size. A lower extent of hydroxylation is evidenced under static equilibrium forced dehydrated conditions, thereby limiting the temperature dependent structural transformations and excluding the possibility of incoming water vapors as the cause for the temperature dependent effects observed in the in situ Raman/FTIR spectra. The results address open issues and offer new insight in the structural studies of VO_x/TiO₂ catalysts.

High-throughput operando Raman-quadrupole mass spectrometer (QMS) system to screen catalytic systems

This paper describes the design and setup of a high-throughput Raman system for an array of eight parallel catalytic reactors during reaction conditions. The "operando" methodology combines in situ spectroscopy during catalytic reaction with a simultaneous activity measurement. The high-throughput operando Raman system, multi-operando, is a device that automates this operando methodology for several catalyst samples at the same time, all samples being in the same reaction conditions. We describe how the system is made, how Raman system positions and acquires spectra, and how each reactor outlet gas is selected and analyzed.

Transient and steady state investigation of selective and non-selective reaction pathways in the oxidative dehydrogenation of propane over supported vanadia catalysts

Mechanistic aspects of the formation of C3H6, CO and CO2 in the oxidative dehydrogenation of propane over VOx/gamma-Al2O3 materials have been investigated by means of steady state and transient isotopic tests. The materials possessed highly dispersed and polymerised VOx species as well as bulk-like V2O5. Propene was primarily formed via oxidative dehydrogenation of propane by lattice oxygen of VOx species. It was suggested that non-selective consecutive propene oxidation is initiated by the breaking of the C-C bond in the molecule by the lattice oxygen, forming formaldehyde as a side product, which is further oxidised to CO and CO2. The following order of initial steady state propene selectivity (at a zero degree of propane conversion) as a function of the nature of VOx species was established: a mixture of bulk-like V2O5 and polymerised VOx>polymerised VOx>highly dispersed VOx species. The low propene selectivity over highly dispersed VOx species was explained by the fact that these species do not fully cover the bare acidic surface of gamma-Al2O3 where propene adsorption and further oxidation take place. Thus, two different locations of COx formation were considered: (i) in the vicinity of acidic sites of the support and (ii) on VOx species. The propene selectivity over samples possessing polymerised VOx species and bulk-like V2O5 strongly decreased with an increasing degree of propane conversion. Contrarily, highly dispersed VOx species showed the lowest ability for consecutive propene oxidation.

Off-line direct deposition gas chromatography/surface-enhanced Raman scattering and the ramifications for on-line measurements

Surface-enhanced Raman spectra (SERS) of molecules separated by gas chromatography (GC) were measured off-line by condensing the analyte on a moving, liquid-nitrogen-cooled ZnSe window on which a 5 nm layer of silver had been formed by physical vapor deposition. After the components that eluted from the chromatograph had been deposited, the substrate was allowed to warm up to room temperature and transferred to the focus of a Raman microspectrometer where the spectrum of each component was measured. Band intensities in the spectrum of 3 ng of caffeine prepared in this way were approximately the same as in the spectrum of bulk caffeine. By making some logical assumptions, it was shown that identifiable GC/SERS spectra of 30 pg of many molecules could be measured over a 300 cm(-1) region in real-time and that if an optimized substrate were used the minimum identifiable quantity could be reduced to 1 pg or less.