In situ UV-vis-NIR diffuse reflectance and Raman spectroscopy and catalytic activity studies of propane oxidative dehydrogenation over supported CrO3/ZrO2 catalysts

Non-Oxidative Propane Dehydrogenation on CrOx-ZrO2-SiO2 Catalyst Prepared by One-Pot Template-Assisted Method

A series of CrO_x-ZrO₂-SiO₂ (CrZrSi) catalysts was prepared by a "one-pot" template-assisted evaporation-induced self-assembly process. The chromium content varied from 4 to 9 wt.% assuming Cr₂O₃ stoichiometry. The catalysts were characterized by XRD, SEM-EDX, temperature-programmed reduction (TPR-H₂), Raman spectroscopy, and X-ray photoelectron spectroscopy. The catalysts were tested in non-oxidative propane dehydrogenation at 500-600 °C. The evolution of active sites under the reaction conditions was investigated by reductive treatment of the catalysts with H₂. The catalyst with the lowest Cr loading initially contained amorphous Cr³⁺ and dispersed Cr⁶⁺ species. The latter reduced under reaction conditions forming Cr³⁺ oxide species with low activity in propane dehydrogenation. The catalysts with higher Cr loadings initially contained highly dispersed Cr³⁺ species stable under the reaction conditions and responsible for high catalyst activity. Silica acted both as a textural promoter that increased the specific surface area of the catalysts and as a stabilizer that inhibited crystallization of Cr₂O₃ and ZrO₂ and provided the formation of coordinatively unsaturated Zr⁴⁺ centers. The optimal combination of Cr³⁺ species and coordinatively unsaturated Zr⁴⁺ centers was achieved in the catalyst with the highest Cr loading. This catalyst showed the highest efficiency.

An operando surface enhanced Raman spectroscopy (SERS) study of carbon deposition on SOFC anodes

Thermally robust SERS probes enable the study of coking kinetics on the nickel surface at early stages and at the Ni–YSZ interface.

Identifying active functionalities on few-layered graphene catalysts for oxidative dehydrogenation of isobutane

The general consensus in the studies of nanostructured carbon catalysts for oxidative dehydrogenation (ODH) of alkanes to olefins is that the oxygen functionalities generated during synthesis and reaction are responsible for the catalytic activity of these nanostructured carbons. Identification of the highly active oxygen functionalities would enable engineering of nanocarbons for ODH of alkanes. Few-layered graphenes were used as model catalysts in experiments to synthesize reduced graphene oxide samples with varying oxygen concentrations, to characterize oxygen functionalities, and to measure the activation energies for ODH of isobutane. Periodic density functional theory calculations were performed on graphene nanoribbon models with a variety of oxygen functionalities at the edges to calculate their thermal stability and to model reaction mechanisms for ODH of isobutane. Comparing measured and calculated thermal stability and activation energies leads to the conclusion that dicarbonyls at the zigzag edges and quinones at armchair edges are appropriately balanced for high activity, relative to other model functionalities considered herein. In the ODH of isobutane, both dehydrogenation and regeneration of catalytic sites are relevant at the dicarbonyls, whereas regeneration is facile compared with dehydrogenation at quinones. The catalytic mechanism involves weakly adsorbed isobutane reducing functional oxygen and leaving as isobutene, and O2 in the feed, weakly adsorbed on the hydrogenated functionality, reacting with that hydrogen and regenerating the catalytic sites.

Visible-light-driven water oxidation at a polychromium-oxo-electrodeposited TiO2 electrode as a new type of earth-abundant photoanode

A polychromium-oxo-deposited TiO₂ electrode was fabricated as an earth-abundant photoanode for visible-light-driven water oxidation.

Oxidative Dehydrogenation of Propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: Structural Characterization and Catalytic Function

The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VO(x) and MoO(x) or VO(x) and CrO(x) on Al(2)O(3) were examined using Raman and UV-visible spectroscopies, the dynamics of stoichiometric reduction in H(2), and the oxidative dehydrogenation of propane. VO(x) domains on Al(2)O(3) modified by an equivalent MoO(x) monolayer led to dispersed binary structures at all surface densities. MoO(x) layers led to higher reactivity for VO(x) domains present at low VO(x) surface densities by replacing V-O-Al structures with more reactive V-O-Mo species. At higher surface densities, V-O-V structures in prevalent polyvanadates were replaced with less reactive V-O-Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV-visible data indicate that polyvanadates predominant on Al(2)O(3) convert to dispersed binary oxide structures when MoO(x) is deposited before or after VO(x) deposition; these structures are less reducible and show higher UV-visible absorption energies than polyvanadate structures on Al(2)O(3). The deposition sequence in binary Mo-V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO(x) on Al(2)O(3) led to more reactive VO(x) domains than those deposited on pure Al(2)O(3) at similar VO(x) surface densities. At all surface densities, the replacement of V-O-Al or V-O-V structures with V-O-Cr increased the reducibility and catalytic reactivity of VO(x) domains; it also led to higher propene selectivities via the selective inhibition of secondary C(3)H(6) combustion pathways, prevalent in VO(x)-Al(2)O(3), and of C(3)H(8) combustion routes that lead to low alkene selectivities on CrO(x)-Al(2)O(3). VO(x) and CrO(x) mix significantly during synthesis or thermal treatment to form CrVO(4) domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO(x) domains can be modified significantly by the formation of binary dispersed structures. VO(x)-CrO(x) structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VO(x) domains present at similar surface densities on pure Al(2)O(3) supports.