Mechanistic Study of Low-Temperature CO₂ Hydrogenation over Modified Rh/Al₂O₃ Catalysts

The hydrogenation of CO₂ on Rh/Al₂O₃ catalysts modified with Ni and K was studied by in situ and operando DRIFTS spectroscopy comprising transient and isotopic exchange experiments to study the influence of this modification on the catalytic performance in CO and methane formation at 250–350 °C and to gain mechanistic insight. Catalytic testing and spectroscopic studies revealed that the modification with particularly K promotes the formation of CO being the highest over Rh, K, Ni/Al₂O₃, whereas methane formation is preferred over the unmodified catalyst. It was found that CO₂ does not dissociatively adsorb but is adsorbed at the support, forming mainly hydrogen carbonate, and in the presence of K, also carbonate species. The dissociative adsorption of H₂ proceeds on Rh. The activated H₂ reacts mainly with the hydrogen carbonate species forming CO adsorbed on Rh and formate (F1) species stably adsorbed on the support. On the K-containing catalysts, an additional formate species (F2) was identified as more reactive than F1 formate and can act as a reaction intermediate in the CO formation pathway. Furthermore, adsorbed formyl species were detected, which are assumed to be intermediates in the methanation reaction. The modifying additives change the surroundings of the Rh particles. This influences the strength of CO adsorption and the activation ability of Rh for H₂ dissociation. Thus, desorption of the formed CO from the catalyst surface is favored, and the methanation of CO is hindered. The modification with K enhances the ability for CO₂ fixation by formation of additional carbonate species which cover adsorption sites for unreactive F1 formate species and favors the formation of reactive F2 formate species

Ternary VZrAlON Oxynitrides - Efficient Catalysts for the Ammoxidation of 3‑Picoline

Starting from previous binary VZrON (VAlON) oxynitrides with high (low) activity and low (high) selectivity, a new class of ternary VZrAlON catalysts has been developed for the ammoxidation of 3-picoline to 3-cyanopyridine (3-CP), which combine the beneficial properties of the binary oxynitrides, leading to improved selectivity at retained high activity and to the highest space-time yield of 3-CP ever measured (488 g L^–1 h^–1). This is attributed to the formation of a special −⊡–V⁵⁺(O)–N–Al­(Zr)– surface moiety consisting of a V⁵⁺O species in the vicinity of a surface nitrogen and an anion vacancy occupied by an electron, which is supposed to provide optimum conditions for a double Mars–van Krevelen mechanism comprising activation of gas-phase oxygen and ammonia via reversible incorporation into the catalyst surface as well as an efficient electron transport

Effects of Imidazole-Type Ligands in Cu^I/TEMPO-Mediated Aerobic Alcohol Oxidation

Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)­Cu^I(IM)/TEMPO catalyst (IM represents differently substituted imidazoles) has been studied by simultaneous operando electron paramagnetic resonance/UV–vis/attentuated total reflectance infrared spectroscopy in combination with cyclic voltammetry to explore the particular role of imidazole in terms of ligand and/or base as well as of its substitution pattern on the catalytic performance. For molar ratios of IM/Cu ≥ 2, a (bpy)­Cu^I/II(IM)_a(IM)_b complex is formed, in which the Cu–N distances and/or angles for the two IM ligands a and b are different. The coordination of a second IM molecule boosts the oxidation of Cu^I to Cu^II and, thus, helps to activate O₂ by electron transfer from Cu^I to O₂. The rates of Cu^I oxidation and Cu^II reduction and, thus, the rates of benzaldehyde formation depend on R of the R–N moiety in the IM ligand. Oxidation is fastest for R = H and alkyl, while reduction is slowest for R = H. The Cu^I/Cu^II interplay leads to decreasing total benzaldehyde formation rates in the order R (I+ effect) > R (conjugated system) > R = H

Heterostructured Copper–Ceria and Iron–Ceria Nanorods: Role of Morphology, Redox, and Acid Properties in Catalytic Diesel Soot Combustion

This work reports the synthesis of heterostructured copper–ceria and iron–ceria nanorods and the role of their morphology, redox, and acid properties in catalytic diesel soot combustion. Microscopy images show the presence of nanocrystalline CuO (9.5 ± 0.5 nm) and Fe₂O₃ (7.3 ± 0.5 nm) particles on the surface of CeO₂ nanorods (diameter is 8.5 ± 2 nm and length within 16–89 nm). In addition to diffraction peaks of CuO and Fe₂O₃ nanocrystallites, X-ray diffraction (XRD) studies reveal doping of Cu²⁺ and Fe³⁺ ions into the fluorite lattice of CeO₂, hence abundant oxygen vacancies in the Cu/CeO₂ and Fe/CeO₂ nanorods, as evidenced by Raman spectroscopy studies. XRD and Raman spectroscopy studies further show substantial perturbations in Cu/CeO₂ rods, resulting in an improved reducibility of bulk cerium oxide and formation of abundant Lewis acid sites, as investigated by H₂-temperature-programmed reduction and pyridine-adsorbed Fourier transform infrared studies, respectively. The Cu/CeO₂ rods catalyze the soot oxidation reaction at the lowest temperatures under both tight contact (Cu/CeO₂; T50 = 358 °C, temperature at which 50% soot conversion is achieved, followed by Fe/CeO₂; T50 = 368 °C and CeO₂; T50 = 433 °C) and loose contact conditions (Cu/CeO₂; T50 = 419 °C and Fe/CeO₂; T50 = 435 °C). A possible mechanism based on the synergetic effect of redox and acid properties of Cu/CeO₂ nanorods was proposed: acid sites can activate soot particles to form reactive carbon species, which are oxidized by gaseous oxygen/lattice oxygen activated in the oxygen vacancies (redox sites) of ceria rods

Efficient VO_<i>x</i>/Ce_1–<i>x</i>Ti_<i>x</i>O₂ Catalysts for Low-Temperature NH₃‑SCR: Reaction Mechanism and Active Sites Assessed by in Situ/Operando Spectroscopy

Supported V₂O₅/Ce_1–<i>x</i>Ti_<i>x</i>O₂ (3, 5, and 7 wt % V; <i>x</i> = 0, 0.1, 0.3, 0.5, 1) and bare supports have been tested in the selective catalytic reduction (SCR) of NO by NH₃ at different gas hourly space velocities (GHSVs) and were comprehensively characterized using XRD, pseudo in situ XPS, and UV–vis DRS as well as EPR and DRIFTS in in situ and operando mode. The best V/Ce_1–<i>x</i>Ti_<i>x</i>O₂ (<i>x</i> = 0.3, 0.5) catalysts showed almost 100% NO conversion and N₂ selectivity already at 190 °C with a GHSV value of 70000 h^–1, which belongs to the best performances observed so far in low-temperature NH₃-SCR of NO. The corresponding bare supports still converted around 80% to N₂ under the same conditions. On bare supports, SCR proceeds via a Langmuir–Hinshelwood mechanism comprising the reaction of adsorbed surface nitrates with adsorbed NH₃. On V/Ce_1–<i>x</i>Ti_<i>x</i>O₂, nitrate formation is not possible, and an Eley–Rideal mechanism is working in which gaseous NO reacts with adsorbed NH₃ and NH₄⁺. Lewis and Brønsted acid sites, though adsorption of NH₃, do not scale with the catalytic activity, which is governed rather by the redox ability of the materials. This is boosted in the supports by replacing Ce with the more redox active Ti and in catalysts by tight connection of vanadyl species via O bridges to the support surface forming −Ce–O–V­(O)–O–Ti– units in which the equilibrium valence state of V under reaction conditions is close to +5

Control of Bridging Ligands in [(V₂O₃)₂(RXO₃)₄⊂F]⁻ Cage Complexes: A Unique Way To Tune Their Chemical Properties

In this work, the new organic–inorganic hybrid compound Ph₄P­[(V₂O₃)₂(PhAsO₃)₄⊂F] (<b>VAsF</b>) has been prepared and characterized by single-crystal XRD and multinuclear magnetic resonance (¹H, ¹⁹F, ³¹P, and ⁵¹V). Redox properties and thermal stability have been investigated by EPR, cyclic voltammetry, and thermal analysis in comparison to its Ph₄P­[(V₂O₃)₂(PhPO₃)₄⊂F] (<b>VPF</b>) analogue. The <b>VAsF</b> cluster has a lower redox potential and higher electrochemical stability in solution, while it is thermally less stable in the solid state. Density functional theory (DFT) calculations showed that the difference in the redox potential is due to the different electron affinities of <b>VPF</b> and <b>VAsF</b>. With this approach of modifying the type of the ligand of the molecular vanadium cage, we hope to enhance the utility of such compounds as building blocks for the design of new hybrid materials with desirable properties

Hierarchical ZSM‑5 Materials for an Enhanced Formation of Gasoline-Range Hydrocarbons and Light Olefins in Catalytic Cracking of Triglyceride-Rich Biomass

A hierarchical ZSM-5 material with a high fraction of mesoporosity coupled to well-preserved intrinsic zeolite characteristics has been successfully prepared by postsynthesis modifications involving optimization of base treatment and subsequent strong acid washing of commercial Al-rich ZSM-5 (parent ZSM-5). The resulting hierarchical ZSM-5 material was thoroughly characterized before being tested in the cracking of triglyceride-rich biomass, i.e., model feedstock triolein and real feedstock waste cooking oil under fluid catalytic cracking conditions. The results show that the introduction of intracrystalline mesoporosity enhances the utilization of zeolite acid sites by the enlarged external surface, leading to an increased conversion. At the same time, it partially suppresses the undesired secondary reactions by shortening micropore diffusion path lengths. With such a hierarchical ZSM-5 material, higher selectivities toward the desired products, i.e., gasoline-range hydrocarbons and light olefins, than with commercial ZSM-5 have been achieved

Solar Hydrogen Production by Plasmonic Au–TiO₂ Catalysts: Impact of Synthesis Protocol and TiO₂ Phase on Charge Transfer Efficiency and H₂ Evolution Rates

The activity of plasmonic Au–TiO₂ catalysts for solar hydrogen production from H₂O/MeOH mixtures was found to depend strongly on the support phase (anatase, rutile, brookite, or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition, or deposition–precipitation). Structural and electronic rationale have been identified for this behavior. Using a combination of spectroscopic in situ techniques (EPR, XANES, and UV–vis spectroscopy), the formation of plasmonic Au particles from precursor species was monitored, and the charge-carrier separation and stabilization under photocatalytic conditions was explored in relation to H₂ evolution rates. By in situ EPR spectroscopy, it was directly shown that abundant surface vacancies and surface OH groups enhance the stabilization of separated electrons and holes, whereas the enrichment of Ti³⁺ in the support lattice hampers an efficient electron transport. Under the given experimental conditions, these properties were most efficiently generated by depositing gold particles on anatase/rutile composites using the deposition–precipitation technique

Structure–Activity Relationships in Bulk Polymeric and Sol–Gel-Derived Carbon Nitrides during Photocatalytic Hydrogen Production

Photocatalytic hydrogen evolution rates and structural properties as well as charge separation, electron transfer, and stabilization have been analyzed in advanced sol–gel-derived carbon nitrides (SG-CN) pyrolyzed at different temperatures (350–600 °C) and in bulk polymeric carbon nitride reference samples (CN) by XRD, XPS, FTIR, UV–vis, Raman, and photoluminescence as well as by in situ EPR spectroscopy. SG-CN samples show about 20 times higher H₂ production rates than bulk CN. This is due to their porous structure, partial disorder, and high surface area which favor short travel distances and fast trapping of separated electrons on the surface where they are available for reaction with protons. In contrast, most of the excited electrons in bulk polymeric CN return quickly to the valence band upon undesired emission of light, which is responsible for their low catalytic activity