Deposition of tungsten hexacarbonyl on alumina: a diffuse reflectance infrared Fourier transform spectroscopy study

Guaiacol to Aromatics: Efficient Transformation over In Situ-Generated Molybdenum and Tungsten Oxides

The development of catalysts for the hydrodeoxygenation of bio-based feedstocks is an important step towards the production of fuels and chemicals from biomass. This paper describes in situ-generated bulk molybdenum and tungsten oxides in the hydrodeoxygenation of the lignin-derived compound guaiacol. The catalysts obtained were studied using powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transition electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, and Raman spectroscopy. The use of metal carbonyls as precursors was shown to promote the formation of amorphous molybdenum oxide and crystalline tungsten phosphide under hydrodeoxygenation conditions. The catalysts’ activity was investigated under various reaction conditions (temperature, H2 pressure, solvent). MoOx was more active in the partial and full hydrodeoxygenation of guaiacol at temperatures of 200–380 °C (5 MPa H2, 6 h). However, cyclohexane, which is an undesirable product, was formed in significant amounts using MoOx (5 MPa H2, 6 h), while WOx was more selective to aromatics. When using dodecane as a solvent (380 °C, 5 MPa H2, 6 h), the benzene-toluene-xylenes fraction was obtained with a 96% yield over the WOx catalyst.

Ethanol to Acetaldehyde Conversion under Thermal and Microwave Heating of ZnO-CuO-SiO2 Modified with WC Nanoparticles

The nonoxidative conversion of ethanol to acetaldehyde under thermal and microwave heating was studied on mixed oxide ZnO-CuO-SiO₂ catalysts modified with additives of tungsten carbide nanoparticles. The results revealed that the WC-modified catalyst exhibited superior activity and selectivity under microwave heating conditions. It is assumed that when microwave heating is used, hot zones can appear at the contact points of WC nanoparticles and active centers of the mixed oxide ZnO-CuO-SiO₂ catalyst, which intensively absorb microwave energy, allowing the more efficient formation of acetaldehyde at moderate temperatures. Thermodynamic calculations of equilibrium concentrations of reagents and products allowed us to identify the optimal conditions for effective acetaldehyde production. The initial catalyst and the catalyst prepared by the coprecipitation of the oxides with the addition of WC were characterized by physicochemical methods (TPR-H₂, XRD, DRIFTS of adsorbed CO). The active centers of the oxide catalyst can be Cu⁺ cations.

Dynamics of tungsten hexacarbonyl, dicobalt octacarbonyl, and their fragments adsorbed on silica surfaces

Tungsten and cobalt carbonyls adsorbed on a substrate are typical starting points for the electron beam induced deposition of tungsten or cobalt based metallic nanostructures. We employ first principles molecular dynamics simulations to investigate the dynamics and vibrational spectra of W(CO)6 and W(CO)5 as well as Co2(CO)8 and Co(CO)4 precursor molecules on fully and partially hydroxylated silica surfaces. Such surfaces resemble the initial conditions of electron beam induced growth processes. We find that both W(CO)6 and Co2(CO)8 are stable at room temperature and mobile on a silica surface saturated with hydroxyl groups (OH), moving up to half an Angström per picosecond. In contrast, chemisorbed W(CO)5 or Co(CO)4 ions at room temperature do not change their binding site. These results contribute to gaining fundamental insight into how the molecules behave in the simulated time window of 20 ps and our determined vibrational spectra of all species provide signatures for experimentally distinguishing the form in which precursors cover a substrate.

Electron induced reactions of surface adsorbed tungsten hexacarbonyl (W(CO)6)

Tungsten hexacarbonyl (W(CO)(6)) is frequently used as an organometallic precursor to create metal-containing nanostructures in electron beam induced deposition (EBID). However, the fundamental electron stimulated reactions responsible for both tungsten deposition and the incorporation of carbon and oxygen atom impurities remain unclear. To address this issue we have studied the effect of 500 eV incident electrons on nanometer thick films of W(CO)(6) under Ultra-High Vacuum (UHV) conditions. Results from X-ray Photoelectron Spectroscopy, Mass Spectrometry, and Infrared Spectroscopy reveal that the initial step involves electron stimulated desorption of multiple CO ligands from parent W(CO)(6) molecules and the formation of partially decarbonylated tungsten species (W(x)(CO)(y)). Subsequent electron interactions with these W(x)(CO)(y) species lead to ligand decomposition rather than further CO desorption, ultimately producing oxidized tungsten atoms incorporated in a carbonaceous matrix. The presence of co-adsorbed water during electron irradiation increased the extent of tungsten oxidation. The electron stimulated deposition cross-section of W(CO)(6) at an incident electron energy of 500 eV was calculated to be 6.50 × 10(-16) cm(-2). When considered collectively with findings from previous precursors (MeCpPtMe(3) and Pt(PF(3))(4)), results from the present study are consistent with the idea that the electron induced reactions in EBID are initiated by low energy secondary electrons generated by primary beam-substrate interactions, rather than by the primary beam itself.