Catalysts For Selective Oxidation Research Articles

One-Pot Synthesis of Mixed-Phase MoVTeNbOx Catalysts for Selective Oxidation of Propane to Acrylic Acid.

One-pot hydrothermal synthesis assisted by sodium citrate and citric acid is developed to fabricate mixed-phased MoVTeNbOx catalysts with different phase compositions for selective oxidation of propane to acrylic acid. Structures of various catalysts are comprehensively characterized. Interfacial interactions occur among the M1 and M2 phases of mixed-phase MoVTeNbOx catalysts to exert synergistic effects on the selective production of acrylic acid. Various mixed-phase MoVTeNbOx catalysts exhibit the same type of active site on the M1-phase component, with a significantly enhanced ability to adsorb and activate propane. The propane conversion increases linearly with the density of surface site for propane adsorption, while the propene selectivity increases linearly with the density of surface site for propene adsorption. Among the synthesized catalysts, a MoVTeNbOx catalyst composed of 35.3 wt % M1 phase, 19.2 wt % M2 phase, and 45.7 wt % amorphous phase exhibits the largest density of active site and consequently the best catalytic performance with 38.9% propane conversion and 71.2% acrylic acid selectivity at 380 °C.

Study on the effect and mechanism of support structure and characteristic on Ag-based catalysts for low-temperature selective catalytic oxidation of ammonia

Herein, the influence of support type and properties on the catalytic activity and selectivity of Ag-based NH3-SCO catalyst was studied through the evaluation of NH3-SCO performance, the physicochemical characterization of H2-TPR, NH3-TPD, N2 isothermal adsorption–desorption, XPR and XPS, and the mechanism study of 1H MAS NMR, In-situ DRIFTS and DFT calculations. Ag/nano-TiO2, Ag/Al2O3, and Ag/MnO2 all showed better NH3-SCO activity because of more abundant Brønsted and Lewis acid sites of Al2O3 and TiO2 and more actual Ag loading amount due to the larger specific surface area and suitable surface chemistry. The N2 selectivity of Ag/nano-TiO2 is significantly better, which may be due to the highly dispersed Ag species and the abundant Brønsted acid sites with higher N2 selectivity. The mechanism study showed that the terminal hydroxyl was the preferred consumption target of Ag species, and this consumption promoted the effective dispersion and stable anchoring of Ag on the TiO2 surface.

Cerium(III) phosphate as a promising support for gold catalysts addressed to selective oxidation of glucose under base-free conditions

Supported gold nanoparticles are widely used active oxidation catalysts, however, their efficiency and stability vastly depend on the type of supports utilized. This study aims to unravel the potential application of cerium(III) phosphate as a new support for gold catalysts for selective oxidation of glucose under base-free conditions. Particular interest was directed to: i) the identification of the key factors that promote high activity of supported gold species, ii) the optimization of reaction conditions to assess the impact of catalyst loading and reaction time/temperature on the efficiency of glucose oxidation, and iii) the elucidation of the stability of Au/CePO4 catalyst in several reaction cycles. The results show that Au/CePO4 exhibits unique surface properties that enable obtaining of much higher activity of gold species in glucose oxidation compared to other well-known metal oxide-based systems. Precisely, it was found that the TOF value calculated for Au/CePO4 was ca. 2.5 times higher than in the case of Au/CeO2 (TOF = 5846 vs. 2309 h-1, respectively), and ca. 1.5 times higher than that observed for Au/TiO2 (Mintek, TOF = 3839 h-1). The presented results facilitate the development of more active and highly selective supported gold catalysts for oxidation of biomass-derived platform molecules (e.g., glucose).

Ru/MgO-catalysed selective aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

Biomass valorisation through the selective oxidation of carbohydrate and lipid derivatives offers access to an array of platform chemicals through energy- and atom-efficient catalytic processes. Supported metal nanoparticles are promising catalysts for the aerobic selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), but typically require strong liquid base to achieve high selectivity. Here, we explore the utility of MgO as a solid base support for the Ru-catalysed aerobic oxidation of HMF, obtaining 68% FDCA yield at 160°C and 1.5 MPa of O2 using <1 mol-% metal.

Atomically Permeated MoP‐WOx Core‐Shell Catalysts for Efficient and Selective Oxidation of Thiophenic Sulfides in Fuels via Direct Electron Transfer Mechanism

AbstractHeterogeneous interfaces designed rationally in catalysts can induce geometrical, compositional, and electronic effects that can be applied to modulate catalytically active sites and consequently accelerate reaction kinetics. Here, a core‐shell heterostructure catalyst consisting of crystalline molybdenum phosphide (MoP) cores and amorphous tungsten oxide (WOx) shells on porous carbon nanosheets (PCN) is developed for oxidative desulfurization (ODS) of fuels. The strongly coupled core‐shell heterogeneous interface triggers a distinctive atomic permeation effect that induces substantial electron transfer from MoP to WOx and subsequent generation of electron‐rich W sites, consequently optimizing the adsorption of intermediates and substrates. The resulting catalyst (WOx@MoP/PCN) demonstrates a turnover frequency as high as 768.1 h−1 for ODS at 60 °C, exceeding that of almost all the state‐of‐the‐art ODS catalysts by 1–2 orders of magnitude. Moreover, a novel surface oxidation pathway based on direct electron transfer is identified in the WOx@MoP/PCN‐H2O2 system, which bypasses the formation of reactive oxygen species, consequently endowing the system with a moderate redox potential. Thus, WOx@MoP/PCN exhibits unprecedented selectivity, achieving 100% removal of thiophenic sulfides from real diesel with minimal consumption of oxidant.

Impact of M (M = Co, Cu, Fe, Zr) Doping on CeO2-Based Catalysts for Ammonia Selective Catalytic Oxidation at Low Temperatures

Impact of M (M = Co, Cu, Fe, Zr) Doping on CeO2-Based Catalysts for Ammonia Selective Catalytic Oxidation at Low Temperatures

Enhancing acidity of Ru/Ce/ZrO2 bifunctional catalysts for promoting N2 selectivity in selective catalytic oxidation of NH3

To solve the problem of low N2 selectivity of Ru/Ce/ZrO2 (R/C/Z) catalysts in NH3 selective catalytic oxidation (NH3-SCO) reaction, herein ZrO2 supports were treated with sulfuric acid and its effects on NH3-SCO performance were investigated systematically. The results showed that Ru/Ce/ZrO2-10S (R/C/Z-10S) catalyst, in which the mass ratio of sulfuric acid to ZrO2 was 10 %, exhibited N2 selectivity of 95.7 % at 350 °C, that was significantly higher than R/C/Z catalyst (57.8 %). NH3-TPD tests indicated that sulfate treatment of ZrO2 supports in R/C/Z catalysts led to more abundant acidic sites on catalyst surface, which was in favor of enhancing NH3 adsorption capacity remarkably. But when mass ratio of sulfuric acid to ZrO2 increased up to 20 %, an aggregation of metal oxides on the surface of R/C/Z-20S catalyst could be observed clearly, which might impose an adverse impact on NH3-SCO performance. In-situ DRIFTS results revealed that, compared to R/C/Z catalyst, there were much more Brönsted acid sites on the surface of R/C/Z-10S catalyst, and the corresponding NH3 species on acidic sites could be consumed quickly in intermediate reactions. More importantly, amide (-NH2) species rather than imide (-NH) and nitrosyl (-HNO) species, were the majority on the surface of R/C/Z-10S catalyst, implying that there might be a suppressing effect on further dehydrogenation of -NH2 species into -NH species owning to sulfate treatment of ZrO2 supports in R/C/Z catalysts. Therefore, internal selective catalytic reduction (i-SCR) mechanism rather than -NH mechanism played a vital role in NH3-SCO reaction over R/C/Z-10S catalyst.

Comprehensive mechanism and microkinetic model-driven rational screening of 4N-modulated single-atom catalysts for selective oxidation of benzene to phenol

Comprehensive mechanism and microkinetic model-driven rational screening of 4N-modulated single-atom catalysts for selective oxidation of benzene to phenol

Surface chlorination of IrO2(110) by HCl.

The ability to controllably chlorinate metal-oxide surfaces can provide opportunities for designing selective oxidation catalysts. In the present study, we investigated the surface chlorination of IrO2(110) by HCl using temperature programmed reaction spectroscopy (TPRS), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We find that exposing IrO2(110) to HCl, followed by heating to 650K in ultrahigh vacuum, produces nearly equal quantities of on-top and bridging Cl atoms on the surface, Clt and Clbr, where the Clbr atoms replace O-atoms that are removed from the surface by H2O formation. After HCl adsorption at 85K, only H2O desorbs at low Cl coverages during TPRS, but HCl begins to desorb in increasing yields as the Cl coverage is increased above about 0.5 monolayer (ML). The desorption of Cl2 was not observed under any conditions, in good agreement with the high barrier for this reaction predicted by DFT. A maximum Cl coverage of 1 ML, with nearly equal coverages of Clt and Clbr atoms, could be generated by reacting HCl with IrO2(110) in UHV. Our results suggest that a kinetic competition between recombinative HCl and H2O desorption under the conditions studied limits the saturation Cl coverage to a value less than the 2 ML maximum predicted by thermodynamics. XPS further shows that the partitioning of Cl between the Clt and Clbr states can be altered by subjecting partially chlorinated IrO2(110) to reductive or oxidative treatments, demonstrating that the Cl site population can change dynamically in response to the gas environment. Our results provide insights for understanding the chlorination of IrO2(110) by HCl and can enable future experimental studies to determine how Cl-modification alters the surface chemical reactivity of IrO2(110) and potentially enhances selectivity toward partial oxidation chemistry.

Nitrogen-Tungsten Oxide Nanostructures on Nickel Foam as High Efficient Electrocatalysts for Benzyl Alcohol Oxidation.

Electrocatalytic alcohol oxidation (EAO) is an attractive alternative to the sluggish oxygen evolution reaction in electrochemical hydrogen evolution cells. However, the development of high-performance bifunctional electrocatalysts is a major challenge. Herein, we developed a nitrogen-doped bimetallic oxide electrocatalyst (WO-N/NF) by a one-step hydrothermal method for the selective electrooxidation of benzyl alcohol to benzoic acid in alkaline electrolytes. The WO-N/NF electrode features block-shaped particles on a rough, inhomogeneous surface with cracks and lumpy nodules, increasing active sites and enhancing electrolyte diffusion. The electrode demonstrates exceptional activity, stability, and selectivity, achieving efficient benzoic acid production while reducing the electrolysis voltage. A low onset potential of 1.38 V (vs. RHE) is achieved to reach a current density of 100 mA cm-2 in 1.0 M KOH electrolyte with only 0.2 mmol of metal precursors, which is 396 mV lower than that of water oxidation. The analysis reveals a yield, conversion, and selectivity of 98.41%, 99.66%, and 99.74%, respectively, with a Faradaic efficiency of 98.77%. This work provides insight into the rational design of a highly active and selective catalyst for electrocatalytic alcohol oxidation.

Electrons redistribution of palladium-copper nanoclusters boosting the direct oxidation of methane to methanol

The direct oxidation of methane to methanol (DOMM) at mild conditions is an enormous attractive yet highly challenging, which is known as the “Holy Grail” reaction. In this work, we prepare the PdxCuy NCs supported on mesoporous sulfur-carbon materials and investigate their catalytic DOMM performance. The Pd14Cu1 NCs exhibits the excellent DOMM performance with the productivity of 34.3 mmol g−1 h−1 and the selectivity of 92.3 % at 150 °C when H2/O2 as active oxygen species, which was obviously superior to other PdCu catalysts. The enhanced performance of Pd14Cu1 NCs in DOMM is attributed to the non-crystalline atomic arrangement of tiny cluster structures and the synergistic electronic effect between Pd and Cu atoms, which can reduce the desorption temperature for methane, improve the in-situ generation of H2O2, and provide more active oxygen. The time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) demonstrated that a radical-involved reaction pathway on Pd14Cu1 NC. This work provides the understanding for synergistic effect dual metallic and helps to design highly efficient and selective catalysts for the direct oxidation of CH4.

S-scheme TiO2/Bi2WO6 heterojunction for enhanced photocatalytic selective oxidation of toluene

Selective activation of saturated C–H bonds in hydrocarbons, such as the activation of the C–H bond in toluene, is an effective method for the production of high value-added chemicals. However, improving the C–H bond activation of toluene remains a challenge. In this study, S-scheme TiO2/Bi2WO6 composites were synthesized by loading TiO2 nanoparticles on Bi2WO6 flakes using a simple hydrothermal method. The physical and chemical properties of the prepared materials were characterized using X-Ray Diffraction (XRD), Fourier Transform Infrared Spectoscopy (FT-IR), Scanning Electronic Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and other characterization methods. Selective oxidation of toluene to benzaldehyde under visible light irradiation using air as an oxidizing agent. When the molar ratio of TiO2 and Bi2WO6 was 0.20, the composites exhibited excellent photocatalytic performance with a benzaldehyde production rate of 1725.41 μmol g−1 h−1, which was 2.31 and 1.91 times higher than that of TiO2 and Bi2WO6, respectively. This excellent performance is attributed to the formation of heterojunction structure by the loaded TiO2 nanoparticles, which improves the separation efficiency of photogenerated carriers and thus enhances its photocatalytic performance. In addition, the active substances involved in the photocatalytic reaction were identified by free radical trapping experiments as ·O2− and h+. Finally, the photocatalytic mechanism of S-scheme TiO2/Bi2WO6 composites was proposed by energy band structure analysis and radical trapping experiments. This study provides a simple design pathway for the construction of visible-light responsive bismuth tungstate-based heterojunction photocatalysts and some ideas for the development of new and efficient photocatalytic toluene selective oxidation catalysts.

Synergistic catalytic effect of titanium and iron incorporated to spherical MCM-41 in selective catalytic oxidation of diphenyl sulphide with H2O2

Spherical mesoporous silicas of MCM-41 type modified with titanium, iron or both these metals were prepared by co-condensation method and tested in the role of catalysts for selective oxidation of diphenyl sulphide with H2O2 to diphenyl sulphoxide and diphenyl sulphone. The monometallic catalysts containing titanium or iron were found to be active catalysts of diphenyl sulphide mainly to diphenyl sulphoxide. A very significant increase in catalytic activity was observed for the bimetallic samples, containing both titanium and iron. Moreover, in the case of bimetallic catalysts with higher titanium loading, diphenyl sulphone is a dominating reaction product. The increased activity of the bimetallic catalysts has been related to the synergistic operation of titanium and iron in conversion of hydrogen peroxide into reactive species, which in the next step oxidise diphenyl sulphoxide.

A broad-spectrum oxidation capability Ru-CeO2 catalyst for efficient synergistic selective oxidation of benzyl alcohol

Selective oxidation of alcohols to aldehydes by molecular oxygen is one of the most important transformations in multiphase catalysis. In this paper, a catalyst with selective oxidation ability was designed and synthesized for the selective catalytic oxidation of benzyl alcohol. N-doped carbon materials were prepared by high-temperature calcination using hydrothermal reaction products of citric acid and melamine as a carbon source, and a bimetallic catalyst supported by Ru-CeO2 was constructed using the mixture as the carrier for the selective oxidation of benzyl alcohol to benzaldehyde. The results showed that the introduction of CeO2 could effectively improve the oxidizing ability and catalytic activity of the catalyst. The Ru-Ce0.05/NC170–600 catalyst was able to convert 85.5 % benzyl alcohol into benzaldehyde at 140 °C, 6 h, and 2 MPa, and the selectivity for benzaldehyde was 100 %. It was demonstrated by DFT that it was theoretically feasible to generate benzaldehyde by extracting hydrogen atoms from benzyl alcohol molecules. Further, the selective oxidation experiments on other alcohols showed that the catalyst has high activity for the oxidation of various alcohols to generate the corresponding carbonyl products, and it is a catalyst with broad-spectrum selective oxidation ability. In this study, a green synthesis route of benzaldehyde was proposed to solve the problems in the traditional process. Meanwhile, it provides a new idea for the synthesis of carbonyl products by selective oxidation of alcohols.

Nitrogen Vacancy-Rich C3Nx-Confined Fe–Cu Diatomic Catalysts for the Direct Selective Oxidation of Methane at Low Temperature

Nitrogen Vacancy-Rich C₃N_<i>x</i>-Confined Fe–Cu Diatomic Catalysts for the Direct Selective Oxidation of Methane at Low Temperature

[Ti]-YNU-5: a large-pore titanosilicate as an efficient catalyst for selective oxidations

[Ti]-YNU-5: a large-pore titanosilicate as an efficient catalyst for selective oxidations

Iron-Based Catalysts Derived from Iron-Containing Sludge for Enhanced Catalytic Performance of H2S Selective Catalytic Oxidation.

In this work, iron-containing sludge is used to prepare iron-based catalysts for efficient H2S selective catalytic oxidation. First, the effect of calcination temperatures on the catalytic activities of H2S selective oxidation is carried out and it can be found that S-500 calcined at 500 °C performs excellent catalytic activity. Then, the catalytic performance of the S-500 catalyst is further optimized using alkaline treatment with different concentrations of NaOH solution. The results indicate that S-500(2.0) treated with 2 M NaOH solution has the highest catalytic activity of H2S selective oxidation. Next, various characterization methods are used to analyze the structure and physical-chemical of the sludge-based catalysts. N2-Brunauer-Emmett-Teller (N2-BET) and X-ray photoelectron spectroscopy analyses show that the S-500(2.0) catalyst has the smallest average particle (11.17 nm), the biggest ratio of S ext/S micro(17.98) with bigger external specific surface area (49.09 m2·g-1), a higher proportion of Fe3+ species (50.88%), and surface adsorbed oxygen species (48.07%). Meanwhile, O2-TPD and CO2-TPD analysis indicates that the S-500(2.0) catalyst has a bigger value of the Oads/OTotal ratio (50.56%) and (CO2)(weak+moderate) /(CO2)Total ratio of (31.41%), indicating that there are much more oxygen vacancies and weak alkaline sites. As a result, the excellent catalytic performance of H2S selective oxidation can be attributed to its outstanding physical-chemical properties.

Catalytic performance and mechanism of PtxVW catalyst for selective catalytic oxidation of high-concentration NH3

It is foreseeable that a high-concentration NH3 may exist in exhaust gas of NH3-fuel engines, which may result in some negative damages to ecological environment. Herein, a series of PtxVW catalysts with extremely low Pt doping amounts were synthesized with the aim to remove high-concentration NH3 efficiently via selective catalytic oxidation (SCO) method. The results showed that, Pt0.04VW catalyst (0.04 wt% Pt) achieved a complete NH3 conversion at 250 ºC. Both catalytic activity and N2 selectivity of Pt0.04VW catalyst were much superior over Pt/Al2O3 catalyst (1 wt% Pt) in a wide temperature range of 250–450 ºC. XPS, Raman and H2-TPR results indicated that Pt existed in the lattice of VOx and formed Pt-V bimetallic oxides. The interaction between Pt and V played a key role in the NH3-SCO reactions. DFT calculation demonstrated that dual electron transfer pathways between Pt and V atoms in PtxVW catalysts were beneficial for NH3 coordination on Lewis acid sites, then enhancing NH3-SCO reactions. In-situ DRIFTS and NH3-TPD product analysis suggested that NH3-SCO reactions on Pt0.04VW catalyst surface abide by internal selective catalytic reduction (i-SCR) and hydrazine mechanisms. Moreover, coordinated NH3 species on Lewis acid sites, bidentate nitrate species were the main intermediates in NH3-SCO reactions over Pt0.04VW catalyst.

Insight into the performance-boosting mechanism of Ag/MgAlOx for NH3 selective catalytic oxidation: Perspective from the support crystalline phase structures

Arranging an ammonia oxidation process after the selective catalytic reduction of NOx by NH3 (NH3-SCR) system is an effective way for ensuring the DeNOx efficiency and controlling the slip ammonia. To improve operability and reduce operating costs in ammonia selective catalytic oxidation (NH3-SCO), developing feasible and efficient catalysts becomes meaningful. Herein, focusing on the dilemma of Ag/Al2O3 catalysts in NH3-SCO applications, Ag/MgAlOx catalysts with different support different crystalline phase structures (5 %Ag/MgAl-LDO and 5 %Ag/MgAl2O4) were innovatively synthesized and systematically evaluated application potential in NH3-SCO. Results indicated that the introduction of Mg apparently broadened reaction temperature window and catalytic performance of 5 %Ag/MgAl2O4 was higher than 5 %Ag/MgAl-LDO, while the N2 selectivity was completely opposite. The combination of series characterization and density functional theory (DFT) calculations revealed that 5 %Ag/MgAl2O4 surface has more Ag0 species with small particle size, which is favorable for ammonia adsorption and activated dehydrogenation to further promote reaction of NOx species, resulting in a higher NH3-SCO activity. In contrast, 5 %Ag/MgAl-LDO readily induces oxidized Ag species and Ag0 is more inclined to be present in large particle sizes, which is more conducive to dissociation of O2 to produce reactive O species (O*) leading to the peroxidation of ammonia to nitrate and thus, positively affects the improvement of N2 selectivity. Model catalyst 5 %Ag/MgAl-650 (mixed-phase) was prepared by controlling the roasting conditions and exhibited excellent catalytic performance in NH3-SCO with almost 93 % of NH3 oxidized at 250 °C (N2 selectivity up to 71 %). This work could afford theoretical support for the optimization and application of catalysts with high performance at low temperatures in selective catalytic oxidation of slip ammonia.

Impact of Ceria Support Morphology on Au Single‐Atom Catalysts for Benzyl Alcohol Selective Oxidation

AbstractAlcohol oxidations are a key industrial chemical transformation, with aldehydes and ketones finding use in an array of applications. Nobel metals are known for their activity towards this chemoselective transformation, however, sustainable catalyst synthesis requires optimal utilisation of these scarce elements. Here, we report Au catalytic systems based on the deposition of isolated Au sites on different morphologies of ceria in which different surface facets of the support are exposed. Through tailoring the support morphology and from extensive catalyst characterisation, it is shown that the exposed facet is critical for controlling the formation (or not) of isolated Au sites. Both the 110 and 111 facets are capable of this feat, yielding single‐atom sites for rod, octahedron, and polyhedron morphologies. In contrast, the 100 facet is not, resulting in Au nanoparticles on cubic ceria. This dictation over Au species is critical to benzyl alcohol oxidation capacity at mild conditions and in the absence of a soluble base, with only single‐atom catalyst (SAC) systems demonstrating activity. Furthermore, the exposed surface facet also governs the degree of surface oxygen vacancies, which is critical to catalyst activity due to their control over substrate adsorption strength, as revealed through T1/T2 NMR relaxation measurements.