Supported Transition Metal Oxide Catalysts Research Articles

Elusive supported surface M2Ox dimer active site (M = Re, W, Mo, Cr, V, Nb, and Ta)

Supported transition metal oxide catalysts are extensively used as heterogeneous catalysts for various energy, chemical, and environmental applications. The molecular structures of dehydrated surface metal oxide phases are crucial for understanding structure-activity/selectivity relationships that guide the design of enhanced catalysts. Some early studies suggested that dimeric (aka binuclear) surface metal oxide sites were more active/selective than monomeric (aka mononuclear) sites, prompting interest in synthesizing catalysts with supported dimeric metal oxide structures. This review examines the literature on dehydrated silica-based supported group 7-5 MOx catalysts (ReOx, WOx, MoOx, CrOx, VOx, NbOx, and TaOx on SiO2, MCM-41, AlOx/SiO2, and H-ZSM-5) for their surface metal oxide structures. In situ Raman, extended x-ray absorption fine structure, and ultraviolet-visible spectroscopy indicate that monomeric surface MOx structures predominate in all such catalysts. Therefore, the cursory use of dimeric surface M2Ox sites in catalytic mechanisms and reaction models in heterogeneous catalysis by supported metal oxides is questionable, and moving forward, the invoking of supporting dimeric surface M2Ox sites should be critically examined and backed up with direct spectroscopic methods.

Conversion of 5‐Hydroxymethylfurfural and Carbohydrates to 2,5‐Diformylfuran Over MoOx/Biochar Catalysts

AbstractHere, a series of nitrogen‐doped carbon (NC) supported transition metal oxide catalysts were prepared by pyrolysis of chitosan‐metal salt mixtures for the efficient synthesis 2,5‐diformylfuran (DFF) from 5‐hydroxymethylfurfural (HMF) and fructose. Among them, supported molybdenum‐based species on nitrogen‐doped carbon (Mo/NC) exhibited excellent activity and selectivity for selective oxidation of HMF to DFF. The presence of nitrogen species reduces the electron cloud density around the Mo species, thereby improving the catalytic activity. Moreover, the interaction between active MoO2 species on the catalyst surface and NC ensured the stability of the catalyst, resulting in no significant loss of activity after four catalytic cycles. 99.5 % and 55.1 % yields of DFF with full conversion were obtained from HMF and fructose respectively over Mo/NC‐chitosan‐600 in DMSO under ambient air. In order to enhance DFF yield from fructose, sulfonate groups were introduced into Mo/NC‐chitosan‐600 catalyst, leading to the highest DFF yield of 70.2 %. In addition, this catalyst also allowed 10.0%, 22.9% and 33.3% DFF yields from glucose, sucrose and inulin, respectively.

In-situ DRIFTS study of chemically etched CeO2 nanorods supported transition metal oxide catalysts

• Effect of NaBH 4 chemical etching on the reducibility of CeO 2 nanorods. • CO adsorption band for polydentate carbonate is found from in situ DRITFS as a result of NaBH 4 etching. • Surface defects obtained by NaHB 4 chemical etching are critical to strong TM (TM-transition metals: Cu, Co, Ni, Fe, Mn)-CeO 2 interactions, therefore resulting in the improved low temperature catalytic activity. In this work, hydrothermally synthesized CeO 2 nanorods (CeO 2 NR) were chemically etched by strong reducing agent NaBH 4 with 0.6~30 wt% addition, and further transition metal (TM) oxides (TM: Cu, Co, Ni, Fe and Mn) were loaded on the surface modified 6 wt% NaBH 4 CeO 2 NR powder (mCeO 2 NR) to prepare mCeO 2 NR supported TM oxide catalysts. Both mCeO 2 NR supports (treated by 0.6–30 wt% NaBH 4 ) and mCeO 2 NR supported TM oxide catalysts were employed to investigate the effect of chemical etching on their surface structure, CO adsorption, CO 2 desorption and catalytic performance. Compared with pristine CeO 2 NR, one strong CO adsorption band for polydentate carbonate is found from in situ DRITFS as a result of NaBH 4 etching, which can explain the enhanced low temperature reducibility and catalytic performance of mCeO 2 NR supports and mCeO 2 NR supported TM oxide catalysts. The vibrational band signals of bicarbonate, monodentate/bidentate/polydentate carbonate and bridged carbonate are detected in all mCeO 2 NR supported TM catalysts and the effect of CO adsorption mode on CO oxidation activity is discussed.

Selective catalytic reduction of NOx in marine engine exhaust gas over supported transition metal oxide catalysts

The selective catalytic reduction (SCR) of nitrogen oxides (NOx) in the presence of methanol (methanol-SCR) was investigated over commercial oxide (γ-Al2O3 and TiO2) supported transition-metal oxide catalysts in lab scale. Of all the prepared catalysts, CuO/γ-Al2O3 catalyst exhibited the highest reduction efficiency in the methanol-SCR process. The practical test results in a marine engine further showed that the 2 wt% CuO/γ-Al2O3 catalyst can remove 93.9% of NOx without catalyst deactivation in several hours. Evidenced by relevant characterization results, the fast-redox properties of copper and rich acidic sites of γ-Al2O3 support were responsible for the excellent catalytic activity of the CuO/γ-Al2O3 catalyst. Revealed by In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), formate-like species derived from methanol dehydrogenation act as the reaction intermediates for NOx reduction. Moreover, this work provides a novel process to reduce NOx and make use of adverse hydrocarbons in the flue gas simultaneously, opening a new research direction in NOx reduction technologies.

Activated carbon-supported catalyst loading of CH4N2O for selective reduction of NO from flue gas at low temperatures

Selective catalytic reduction of nitrogen oxides with loaded CH4N2O (low-temperature urea-SCR) is a novel and promising technology to remove nitrogen oxides from low-temperature oxygen-containing flue gas, which can avoid the problem of NH3 escape. In the present study, a series of industrial-grade biomass-based activated carbon (AC)-supported transition metal oxide catalysts with urea loading were prepared by ultrasound-assisted impregnation, and the physicochemical properties of the catalysts were observed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), graphite furnace atomic absorption spectroscopy (GFAAS), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The influences of the AC type, reaction temperature, AC particle size, metal oxide loading, urea load level and loaded active element type on the catalytic activity were studied through experiments. Moreover, the NO adsorption capacities of the AC carrier at different temperatures were also tested and calculated. The results of NO adsorption tests show that the adsorption capacity of AC decreased with increasing temperature. The results of the catalytic performance tests indicate that the copper- and manganese-based catalysts with 6 wt% urea exhibited better activity than the other catalysts. The copper-based catalyst, in particular, yielded better than 93% NO conversion at low temperatures (50–100 °C). Finally, on the basis of the combined characterization results and thermodynamics analysis, a NO removal mechanism of the copper- and manganese-based catalysts was proposed and discussed; the electron transfers of Mn4+ ⇌ Mn2+ and Cu2+ ⇌ Cu0 promoted the low-temperature urea-SCR method.

Tuning the configuration of dispersed oxometallic sites in supported transition metal oxide catalysts: A temperature dependent Raman study

Abstract The speciation of the deposited MOx phase (M = ReVII, MoVI, WVI) in titania-supported transition metal oxide catalysts is studied by in situ Raman spectroscopy under dehydrated conditions at temperatures up to 430 °C. An astonishing temperature-dependent heterogeneity is established for MOx phases at coverages roughly less than half the monolayer. Unlimited cycling between the majority high temperature mono-oxo termination configuration (Ti–O)nM=O and the di-oxo termination configuration (Ti–O)n-1M(=O)2 that predominates at intermediate temperatures (e.g. 100–150 °C) can be achieved by appropriate temperature control and adequate exposure. Support surface hydroxyls and retained water molecules mediate the structural transformation following a proposed molecular level mechanism. Monolayer catalysts (e.g. 5.9 Mo/nm2) do not exhibit the reported effect due to the complete titration of support surface hydroxyls during the deposition stage. The inherent extent of hydroxylation of the support affects the extent of the reversible transformation. As a paradigm, a higher degree of reversible transformation is observed for TiO2(P25)–supported catalysts compared to TiO2(anatase)–supported counterparts due to the well-established higher hydroxylation of the former TiO2(P25) support material. TGA analysis and Raman measurements under static equilibrium ( p O 2 = 0.23 atm ) in evacuated and sealed cells extending also in the hydroxyl stretching region further complement the in situ Raman experiments.

Structural Dynamics of Dispersed Titania During Dehydration and Oxidative Dehydrogenation Studied by In Situ UV Raman Spectroscopy

The structural dynamics of dispersed titania, i.e., silica supported titania, is investigated during dehydration and oxidative dehydrogenation (ODH) of ethanol using optical spectroscopy. UV Raman spectroscopy enabling resonance enhancements proves to be a valuable tool to identify Ti–OH, Ti–O–Si, and Ti–O–Ti groups. Upon dehydration, a transformation of Ti–OH into Ti–O–Si and Ti–O–Ti groups is observed. Two types of Ti–OH vibrations (isolated, geminal) are identified at around 700 and 800 cm− 1 in agreement with theoretical models. Dispersed titania is catalytically active in ethanol ODH with a performance comparable to dispersed vanadia. In situ UV Raman spectra reveal a consumption of Ti–O–Ti, Ti–O–Si, and Ti–OH groups during ethanol adsorption to the titania surface. The presented results are consistent with an ODH reaction mechanism involving a structural transformation of oligomerized or closely neighbored monomeric TiOX structures. The relevance of the proposed mechanism is discussed in the context of other supported transition metal oxide catalysts.

Heterogeneity of deposited phases in supported transition metal oxide catalysts: reversible temperature-dependent evolution of molecular structures and configurations.

In situ high-temperature Raman spectroscopy under steady state oxidative dehydrated conditions was used for determining the temperature dependence of the molecular structures and configurations of (MOx)n (M = Re, Mo, W) sites supported at low submonolayer loadings on TiO2(P25). Prior to the Raman analysis, the studied catalyst samples underwent calcination at 450-480 °C for 4-5 h. Regularly repeated random sequences of heating and cooling under flowing 20%O2/He (in the absence of incoming water vapor) in the 35-430 °C temperature range were shown to cause drastic changes in the vibrational properties of the M-O stretching modes and in the molecular structures and configurations of the deposited ReOx, MoOx, and WOx sites in a reversible and reproducible manner. A heterogeneity of the deposited oxometallic phase was evidenced with three distinctly different species (i.e., MOx-I, MOx-II, and MOx-III) present in each system, each one prevailing in a particular temperature range. It was shown that the temperature could tune the molecular structure of the deposited oxometallic phase presumably on account of minima in the surface free energy. In the direction of temperature lowering, a mechanism leading to a hydrolysis-like of the anchoring bonds by activation of the surface hydroxyls and/or water molecules extant on the uncovered TiO2(P25) surface took place. In situ FTIR spectroscopy under identical conditions and similar temperature sequence protocols complemented the Raman results and corroborated the proposed prevailing configurations and pertinent band assignments.

Plasma-induced adsorption of elemental mercury on TiO2 supported metal oxide catalyst at low temperatures

A plasma–catalyst reactor was used for the adsorption of elemental mercury at low temperatures. SiO2, TiO2 and SiO2 or TiO2 supported transition metal oxide catalysts were packed in the plasma discharge zone for adsorption enhancement. The results indicated that non-thermal plasma could induce the adsorption of elemental mercury effectively by catalysts. TiO2 displays much higher adsorption efficiency than SiO2 does. The adsorption efficiency is greatly improved by loading transition metal oxides on TiO2. Mn/TiO2 is the most efficient catalyst, and ~99% of Hg0 adsorption efficiency can be obtained under a SED of 2.3±0.3JL−1. The interaction between the plasma and catalyst was also investigated. The chemical properties of the catalyst, such as the active sites and electron transfer capacity, are key factors that affect adsorption efficiency. The mercury adsorption in plasma–catalyst reactor results primarily from the interaction between weakly adsorbed mercury and activated oxygen on catalyst surface induced by plasma. In addition, the effect of the O2 concentration and HCl on mercury adsorption efficiency was investigated. The adsorption efficiency has a logarithmic correlation with O2 concentration in the gas flow.

Three-dimensionally ordered macroporous SiO2-supported transition metal oxide catalysts: facile synthesis and high catalytic activity for diesel soot combustion

Three-dimensionally ordered macroporous (3DOM) SiO2-supported transition metal oxide catalysts were successfully synthesized, and they exhibit high catalytic activities for soot combustion.

Catalytic ozonation of benzotriazole over alumina supported transition metal oxide catalysts in water

Mineralization of benzotriazole (BTZ) by heterogeneous catalytic ozonation was studied in aqueous phase. Performance of three catalysts (Mn/Al2O3, Cu/Al2O3 and Mn–Cu/Al2O3) were compared at various pH levels and operating conditions. Compared to noncatalytic ozonation, a higher level of mineralization of BTZ was achieved over a wide range of pH values. Adsorption of BTZ on alumina support and catalysts was negligible. At neutral pH, Cu/Al2O3 was the most effective catalyst with close to 90% mineralization in 15min reaction time. At pH 2, Mn/Al2O3 had the fastest ozone decomposition and the fastest reaction with 75% mineralization of BTZ in 10min. Under an alkaline pH environment, the mineralization of BTZ by using Cu/Al2O3 showed a weak catalytic activity (22%) whereas mineralization with Mn–Cu/Al2O3 was 77% complete in 30min of reaction. It was observed that the presence of molecular ozone is essential for catalytic oxidation of organic compounds and ozone decomposition has a direct relationship with catalyst activity. The results indicate that not only the pH of solution but also the type of buffer used can influence the catalytic activity. The chloride and sulfate ions in aqueous solutions act as hydroxyl radical scavengers. All the catalysts showed high stability and reusability.

Supported transition metal-oxide catalysts for HC-SCR DeNO x with propene

The selective catalytic reduction of nitrogen oxides by hydrocarbons (HC-SCR DeNO x ) on supported mixed-oxide catalysts is a promising technology for the treatment of exhaust gases from spark-ignition engines. In this work, the SCR of NO with propene on catalysts with Fe-, Cu-, V- and Ce-oxides supported on Al 2O 3, phosphated Al 2O 3 or SAPO-11, was investigated. As shown by their temperature-programmed reduction-profiles, the catalysts’ behaviour for NO conversion depends mainly on the redox properties of the supported metal oxide. The acidic properties of these catalysts assessed by temperature-programmed reduction of ammonia can be neglected for NO x conversion. Catalysts prepared by sol–gel synthesis are more active for NO x conversion than catalysts prepared by wet impregnation. Probably, this is due to smaller metal oxide particles on the support surface in catalysts from sol–gel synthesis compared to those prepared by excess solution impregnation. Catalysts containing CuO x show highest NO conversion (at 350 °C), CeO x - and FeO x -containing catalysts show moderate NO conversion (at 350–450 °C), whereas VO x - and FeO x -containing catalysts favor total oxidation of propene already at low temperature.

Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts

A series of transition metal oxides promoted titania catalysts (MO x /TiO2; M = Cr, Mn, Fe, Ni, Cu) were prepared by wet impregnation method using dilute solutions of metal nitrate precursors. The catalytic activity of these materials was evaluated for the selective catalytic reduction (SCR) of NO with CO as reductant in the presence of excess oxygen (2 vol.%). Among various promoted oxides, the MnO x /TiO2 system showed very promising catalytic activity for NO + CO reaction, giving higher than 90% NO conversion over a wide temperature window and at high space velocity (GHSV) of 50,000 h−1. It is remarkable to note that the catalytic activity increased with oxygen, up to 4 vol.%, under these conditions leading primarily to nitrogen. Our TPR studies revealed the presence of mixed oxidation states of manganese on the catalyst surface. Characterization results indicated that the surface manganese oxide phase and the redox properties of the catalyst play an important role in final catalytic activity.

Nature of carbonaceous deposits on the alumina supported transition metal oxide catalysts in the wet air oxidation of phenol

Al2O3 supported transition metal (Mn, Fe, Co, Ni, and Cu) oxide catalysts were prepared and tested for the wet oxidation of phenol. The supported copper oxide catalysts showed the highest catalytic activity due to their highest surface reducibility. There was carbonaceous deposits on the used catalysts for the wet oxidation of phenol and the supported manganese oxide catalysts showed the highest amount of carbonaceous deposits. These carbonaceous deposits must have their own micropores which resulted in the decrease of the pore volume and the increase of the surface area. The NMR and FTIR spectroscopy showed that the carbonaceous deposits were mostly of aromatic nature and contained some oxygen-bearing groups such as carboxylic acids and alcohols.

Study on Catalytic Incineration of Methane Using Cr2O3/γ-Al2O3 as the Catalyst

A fixed bed reactor was employed to investigate the catalytic incineration of CH4 by various supported transition metal oxide catalysts, with a view of finding the optimal one. Results indicated that the active species, the support, the metal content, the weight hourly space velocity (WHSV), and the inlet CH4 concentration were all important factors affecting CH4 oxidation. Cr2O3/γ-Al2O3 was found to be the most active catalyst among the seven γ-Al2O3-supported metal oxide catalysts tested. With Cr2O3 as the active species, γ-Al2O3 was the most suitable of six supports tested. Furthermore, the optimal Cr content of Cr2O3/γ-Al2O3 was 9 wt.%. X-ray diffraction (XRD) patterns showed that it was formation of Cr2O3 crystals that caused a decline in catalyst activity at Cr content above 9 wt.%. Using the optimal Cr2O3/γ-Al2O3 catalyst, CH4 was completely oxidized at about 390°C, much lower than the temperature required by noble metal catalysts for the same outcome. The stability of Cr2O3/γ-Al2O3 was good and was not affected by the reaction temperature, demonstrated by a nearly constant conversion rate of CH4 of 57% at 350°C and 97% at 380°C during a 20 h on-stream test. However, WHSV and inlet concentration of CH4 did affect CH4 conversion noticeably. For complete oxidation of CH4, the reaction temperature required increased with WHSV and inlet CH4 concentration.

Catalytic destruction of NH 3 and SO 2 over alumina supported transition metal oxides

The catalytic destruction of NH 3 and SO 2 has been examined over a series of alumina supported transition metal oxide catalysts in the laboratory. It was found that alumina supported vanadium oxide is the best candidate for this reaction. These studies show that employing a feed containing 4% NH 3 + 3% SO 2 and a residence time of 0.75 s at 700 °C, conversions of 100 and 87.7% for NH 3 and SO 2, respectively, can be achieved over fresh alumina supported vanadium oxide with the main products being only N 2, water and sulfur. The effect of pretreatment of the catalyst by sulfidation was also examined in this study. Based on these results it was shown that feeding a simulated industrial sour water stripper gas (SWSG) plus oxygen over alumina supported vanadium oxide at 750 °C with a residence time of 4 sec, resulted in an NH 3 conversion of 100%, while no SO 2 was detected in the products.

Low-temperature destruction of chlorinated hydrocarbons over lanthanide oxide based catalysts.

Chlorinated hydrocarbons (CHCs) have widespread industrial applications as, for example, lubricants, cleaning solvents, heat-transfer fluids, and intermediates of pharmaceuticals, herbicides, and fungicides.[1] The increasing volume of CHCs released into the environment, together with the suspected toxicity and carcinogenic properties, have prompted researchers world-wide to find clean and effective routes for destroying CHCs. The method currently used most frequently is thermal incineration at temperatures higher than 1300 8C, so as to avoid formation of dioxins and polychlorinated biphenyls (PCBs).[2] The high incineration temperatures and related costs forced researchers to look for other solutions. As a result, a number of methods have been developed for the degradation of CHCs. These include for example, sonolysis,[3] electrochemical dehalogenation,[4] microbial systems,[5] destructive adsorption,[6] and catalytic transformation over supported metals, noble metals, and transition metal oxides.[7±10] Two main catalytic routes are reported in the literature. The first one is the (total) oxidation of CHCs at temperatures between 300 and 550 8C over supported noble metal catalysts (for example, Pt, Pd, and Au).[7] The essential drawback of this method is the deactivation of the catalyst by the decomposition products, such as Cl2 and HCl. This problem can be (partially) solved by using supported transition metal oxide catalysts (for example, V2O5 and Cr2O3), although the formation of volatile metal oxychlorides can still be a problem. Another possibility to avoid catalyst deactivation is the use of steam.[9] A second catalytic route is hydrodechlorination, in which CHCs are transformed in the presence of hydrogen into alkanes and HCl. Commonly used catalysts are supported Ni, Pd, and Pt.[8,10] Although this method has clear economic and environmental advantages, including the re-use of reaction products and the elimination of hazardous by-products (for example, Cl2 and COCl2), it is not very often used. The main reason is the very fast deactivation of the catalyst material as a result of chlorine poisoning and coke formation. Here, we report on a new and stable catalyst material, which destroys different CHCs, such as CCl4, in the presence of steam at temperatures between 250 and 350 8C. This destruction process can be written as CCl4 þ 2H2O!4HCl þ CO2, and no other products than HCl and CO2 are found in the effluent gas and condensate. The reaction is also not equilibrium limited. The catalyst itself is based on the use of lanthanide oxides, such as La2O3, Nd2O3, Pr2O3, and Ce2O3, which are supported on a high surface area support, preferably Al2O3. Figure 1 illustrates the catalytic performance of

Synthesis of Supported Transition Metal Oxide Catalysts by the Designed Deposition of Acetylacetonate Complexes

The molecular designed dispersion method is used to prepare supported transition metal oxide catalysts (Mo, Cu, V). The corresponding transition metal acetylacetonate complexes were reacted with silica or alumina, and subsequently thermally converted into the supported metal oxide. The deposition of these acetylacetonate complexes depends on the geometry and stability of the complex, the support properties and the synthesis procedure. By thorough control of the reaction parameters, this novel synthesis method enables the creation of uniform and highly dispersed supported transition metal oxides. Therefore, the molecular designed dispersion method plays an important role in the development of a new generation of catalysts. This paper is forwarded to the special issue of the Langmuir journal, devoted to the ISSHAC-3 symposium.

Oxidative dehydrogenation of isobutane to isobutylene over supported transition metal oxide catalysts

Various supported transition metal oxides have been tested for the oxidative dehydrogenation of isobutane to isobutylene. Chromium oxide supported on lanthanum carbonate has been found to be active and selective for this reaction.

Temperature programmed reduction of chromium impregnation catalysts: Mathematical treatment of complex reduction profiles.

A quite general mathematical model for analysing nonisothermal reduction processes is proposed, allowing the estimation of kinetic parameters for several overlapping single steps from one temperature programmed reduction experiment. The computer program includes the mathematical deconvolution of incompletely resolved individual peaks thus revealing the mode in which they contribute to normally observed complex TPR profiles. The application is demonstrated for a representative system of supported transition metal oxide catalysts (CrO 3/SiO 2). Questions of parametric sensitivity are discussed.