Mixed Oxide Catalysts Research Articles

Insights into the Contribution of Oxidation-Reduction Pretreatment for Mn0.2Zr0.8O2-δ Catalyst of CO Oxidation Reaction.

A Mn0.2Zr0.8O2-δ mixed oxide catalyst was synthesized via the co-precipitation method and studied in a CO oxidation reaction after different redox pretreatments. The surface and structural properties of the catalyst were studied before and after the pretreatment using XRD, XANES, XPS, and TEM techniques. Operando XRD was used to monitor the changes in the crystal structure under pretreatment and reaction conditions. The catalytic properties were found to depend on the activation procedure: reducing the CO atmosphere at 400-600 °C and the reaction mixture (O2 excess) or oxidative O2 atmosphere at 250-400 °C. A maximum catalytic effect characterized by decreasing T50 from 193 to 171 °C was observed after a reduction at 400 °C and further oxidation in the CO/O2 reaction mixture was observed at 250 °C. Operando XRD showed a reversible reduction-oxidation of Mn cations in the volume of Mn0.2Zr0.8O2-δ solid solution. XPS and TEM detected the segregation of manganese cations on the surface of the mixed oxide. TEM showed that Mn-rich regions have a structure of MnO2. The pretreatment caused the partial decomposition of the Mn0.2Zr0.8O2-δ solid solution and the formation of surface Mn-rich areas that are active in catalytic CO oxidation. In this work it was shown that the introduction of oxidation-reduction pretreatment cycles leads to an increase in catalytic activity due to changes in the origin of active states.

Low-temperature catalytic performance of toluene oxidation over Cu-Mn oxide catalysts derived from LDH precursor

A series of CuMn mixed oxides catalysts (CMO-T) were prepared by calcination of CuMn layered double hydroxides (CuMn-LDH) for catalytic oxidation of toluene. The obtained CMO-400 catalyst exhibited a high catalytic activity with T50 and T90 of 210 and 231 °C (WHSV = 30000 mL·g−1·h−1). Meanwhile, the CMO-400 exhibited high stability and durability even at 20 vol% water vapor. The effects of calcination temperature on the texture and structural properties were investigated systematically, which proved the formation of copper-manganese solid solution with abundant multiple-phase interfaces. In addition, the strong interaction between Cu and Mn could lead to more surface adsorbed oxygen due to oxidation reduction cycle of Mn3++Cu2+=Mn4++Cu+. Accordingly, for CMO-500 catalyst, the formation of spinel Cu1.5Mn1.5O4 phase prevented good dispersion of the manganese-oxide phases and weakened the reducibility and oxygen mobility. In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) revealed that the intermediates accumulated continuously were difficult to be oxidized rapidly without the replenishment gaseous oxygen. Moreover, the complete transformation of the intermediate benzoate may be the key rate-controlling step in the reaction. The Facile method of preparing mixed-metal solid solution from LDH precursor system could be widely developed for the design of transition metals in catalytic oxidation of toluene.

The Effect of Metal Ratio and Precipitation Agent on Highly Active Iron-Manganese Mixed Metal Oxide Catalysts for Propane Total Oxidation

Iron-manganese mixed metal oxide catalysts with a range of Fe:Mn ratios were synthesised by co-precipitation using sodium carbonate and evaluated for total propane oxidation. The Fe0.50Mn0.50Ox catalyst was the most active, and this was due to increased surface area along with the formation of a Mn2O3 phase that was not present in the other catalysts. The effect of the precipitating agent was evaluated with the Fe0.50Mn0.50Ox catalyst, investigating preparation using (NH4)2CO3, K2CO3, NH4OH, KOH, and NaOH. In almost all cases, the activity of propane oxidation was increased compared to the Na2CO3-prepared catalyst, with the hydroxide-precipitated catalysts generally being more active than the carbonates. The NH4OH catalyst was the best performing and this was thought to be due to the formation of a highly active mixed defect spinel structure. Results demonstrate that highly active mixed metal oxide total oxidation catalysts can be prepared using abundant elements, and the choice of precipitating agent is important to maximise the activity.

Enhancement of ethylene production by alkali metal doping of MoVSb mixed oxide catalyst for ethane oxidative dehydrogenation

Hydrothermally prepared Mo-V-Sb mixed oxides doped with alkali metal cations (Li+, Na+, K+, Rb+, or Cs+) show a significantly improved catalytic behavior in the oxidative dehydrogenation of ethane (ODH) compared to conventional alkali metal-free MoVSbO catalysts. Initial selectivity to ethylene above 95 % was held up to 20 % of ethane conversion for all alkali metal promoted MoVSbO catalysts, as well as lower ethylene overoxidation at higher ethane conversion, with differences depending on the alkali metal employed. Thus, from Li to K, the enhanced catalytic performance follows a bottom-up trend as increasing the size of the cation. An intermediate behavior is observed though for the catalyst doped with Cs cations, with the highest atomic radius. The K-doped MoVSbO catalyst is the least prone to both ethane total oxidation and ethylene overoxidation, which enables up to 90 % selectivity to ethylene at ethane conversion higher than 65 %.

Low-temperature oxidative dehydrogenation of propane over NiV mixed oxides derived from LDH precursors

A series of NiV mixed metal oxide (MMO) catalysts were derived from the NiV layered double hydroxides (LDHs) synthesized by the constant pH coprecipitation method. The synthesis parameters, (i) Ni/V molar ratio and (ii) calcination temperature ( Tp), were controlled. The MMO catalysts were characterized by physicochemical characterization techniques and then tested for the oxidative dehydrogenation of propane (ODHP). The results showed that the calcination temperature affected the crystalline phase formation and grain size of NiO, as well as the activated temperature of propane, and the addition of V greatly regulated the activity and quantity of the surface oxygen species and improved the propylene selectivity. 4NiV-500 catalyst (Ni/V = 4, Tp = 500 °C) exhibited the best catalytic performance at low temperature (250 °C) with an initial propylene selectivity of about 70%. Therefore, it is an effective method to obtain NiV MMO catalysts with excellent low-temperature activity for ODHP, using LDHs as the precursors while controlling the appropriate amount of V and calcination temperature simultaneously.

Low-temperature catalytic combustion of trichloroethylene over MnO -CeO2 mixed oxide catalysts

The cerium-based catalysts were investigated for the catalytic combustion of trichloroethylene (TCE) and exhibit a surprising catalytic activity. MnOx was doped into CeO2 by a citric acid (CA) sol-gel method, and the effect of Mn content on the physicochemical properties and catalytic activities of MnOx-CeO2 mixed oxides was investigated systemically. The introduction of MnOx into CeO2 can evidently improve the catalytic activity and reaction stability of the CeO2 catalyst, and the catalytic activity can be maintained for more than 100 h at 250 °C under air atmosphere. Moreover, a possible reaction mechanism and a catalytic cycle for the catalytic combustion of TCE over the MnOx-CeO2 mixed oxide catalysts are proposed.

Lanthanum–bismuth mixed oxide catalyst with improved activity for carbonyl sulfide hydrolysis

The presence of carbonyl sulfide (COS) in industrial tail gas poses considerable risks to human health and ecosystems. Catalytic hydrolysis is efficient in COS removal, yet developing high-performance catalysts at low temperatures remains challenging. In this work, we designed novel nanorod-like lanthanum-bismuth (La-Bi) mixed oxides for the catalytic hydrolysis of COS. The COS hydrolysis activity of the Bi-doped catalyst was significantly improved as compared to the bare La oxide. The catalysts with various La/Bi ratios were thoroughly analyzed by microscopic and spectroscopic techniques to elucidate their structural and chemical properties. The results indicated that the Bi doping into the La oxide enhanced the basicity and electron transfer of the catalyst, leading to improved hydrolysis activity. A reaction pathway for the COS catalytic hydrolysis on La-Bi mixed oxide was proposed, in which COS adsorption was significantly strengthened due to enhanced basicity, and thereby the formation of intermediates such as HCO3− and HSCO2- was facilitated. In addition, the La-Bi mixed oxide catalyst demonstrated excellent sulfur tolerance and stability in long-term testing using simulated industrial gas.

A Review of the Catalysts Used in Propane Ammoxidation Reaction to Produce Acrylonitrile

Abstract: Background: Various catalysts were studied to synthesize acrylonitrile via propane ammoxidation. The catalysts used for this process can be categorized as antimony-based catalysts, molybdenum-based catalysts, zeolites, nitride material catalysts, and other catalysts. The catalyst which was proved to be an efficient catalyst among all other catalysts is the mixed metal oxide catalyst MoVNbTe due to its high selectivity and yield.

Synthesis and Characterization of Physically Mixed V2O5.CaO as Bifunctional Catalyst for Methyl Ester Production from Waste Cooking Oil

Synthesis of the solid bifunctional vanadium-calcium mixed oxides catalyst was accomplished by application of a simple physical mixing approach. In this work, we compared the catalytic activity of CaO and 2%V2O5.CaO catalyst. Various characterization methods, such as X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), BET surface area, and temperature-programmed desorption (TPD) of CO2 and NH3, were involved in studying the newly synthesized catalysts. The presence of CaO, CaCO3, and Ca(OH)2 compounds in the synthesized catalyst were detected by XRD and FTIR analysis. The existence of 2% V2O5 on the CaO catalyst surface was demonstrated by XRF analysis. From TPD-NH3, TPD-CO2, and BET surface area analysis, it was known that the 2% V2O5-CaO catalyst had a higher total number of acid-base sites and surface area than the CaO catalyst. In the fatty acid methyl esters (FAME) production from waste cooking oil (WCO) with higher free fatty acid (FFA), CaO could only catalyze the transesterification reaction. In contrast, 2%V2O5-CaO could successfully catalyze both the esterification of FFA and the transesterification of triglyceride (TG) simultaneously in a one-step reaction process. Thus, these results prove that 2%V2O5.CaO can act as a bifunctional catalyst in the production of biodiesel from WCO. Moreover, the synthesized 2%V2O5.CaO catalyst could achieve a maximum FAME yield of 51.30% under mild reaction conditions, including a 20:1 methanol to oil molar ratio, 60 °C reaction temperature, 1 wt% of catalyst loading, and 3 hours of reaction time.

Ni-Mg/Al Mixed Oxides Prepared from Layered Double Hydroxides as Catalysts for the Conversion of Furfural to Tetrahydrofurfuryl Alcohol

Ni-Mg/Al mixed oxide catalysts (Ni2Al, Ni2Mg1Al, and Ni1Mg1Al) obtained from layered double hydroxides (LDHs) were tested on the one-pot production of tetrahydrofurfuryl alcohol (TFA) from furfural (FF). Upon calcination at 400 °C and reduction at 500 °C, the LDHs gave catalysts containing small nickel crystallites (<4 nm) dispersed on mixtures of metal oxides and spinel structures. Complete conversion of FF (>99.5%) was achieved on all the catalysts after 4 h at 190 °C and 5.0 MPa of H2 using 5 wt.% FF in ethanol and a furfural-to-catalyst mass ratio of 7.44 g/g. TFA evolved from the sequential hydrogenation of FF to furfuryl alcohol (FA) to TFA. Competing reaction routes involved decarbonylation of FF to furan (FUR) followed by hydrogenation to tetrahydrofuran (THF) or hydrogenolysis to n-butane (BU) and the hydrogenation of the carbonyl group in FF to form 2-methyl furan (mFUR) and its hydrogenation to 2-methyltetrahydrofuran (mTHF). A third competing route consisted of the nucleophilic addition of FF with ethanol and with FA to form acetals (such as 2-(diethoxymethyl)furan, FDA), which were later converted to difurfuryl ether (DFE) and tetrahydrofurfuryl ethyl ether (TFEE) as final products. Hydrogen pressure favored the production of TFA and diminished the formation of acetals, while temperature reduced the capacity of the catalyst to hydrogenate the furan ring, thus reducing TFA and increasing FA and FUR. An 80% yield to TFA was achieved with the Ni2Mg1Al catalysts after 6 h at 190 °C and 50 bar H2, but a variety of coproducts were present at low concentration. Testing of the catalysts in gas-phase hydrogenation conditions at atmospheric pressure revealed a poorer performance, with FA as the main product.

Oxygen vacancy regulation strategy in V-Nb mixed oxides catalyst for enhanced aerobic oxidative desulfurization performance

Bimetallic oxide is a potential catalyst for oxidative desulfurization of fuel. Thus, an appropriate method is needed to improve its catalytic performance. Manufacturing defect is an effective means. In this contribution, an oxygen vacancies (OVs) regulation strategy for enhancing the catalytic activity of bimetallic oxide is proposed. Density functional theory (DFT) calculations show that the crystal phase has a huge influence on the generation energy of oxygen vacancies, so a series of V-Nb mixed oxide with different crystal phases are synthesized. Detailed characterizations show that the as-prepared tetragonal V-Nb mixed oxide (T-VNbOx) has lower OVs formation energy and larger OVs concentration (compared to orthorhombic V-Nb mixed oxides, O-VNbOx). Owing to the activation of OVs, the catalytic activity of T-VNbOx was significantly enhanced to form ultra-deep oxidative desulfurization. In addition, T-VNbOx can be cycled eight times without significantly degrading the desulfurization performance.

Unveiling the Role of In Situ Sulfidation and H2O Excess on H2S Decomposition to Carbon-Free H2 over Cobalt/Ceria Catalysts

The emerging energy and environmental concerns nowadays are highlighting the need to turn to clean fuels, such as hydrogen. In this regard, hydrogen sulfide (H2S), an abundant chemical compound found in several natural sources and industrial streams, can be considered a potential carbon-free H2 source through its decomposition. In the present work, the H2S decomposition performance of Co3O4/CeO2 mixed oxide catalysts toward hydrogen production is investigated under excess H2O conditions (1 v/v% H2S, 90 v/v% H2O, Ar as diluent), simulating the concentrated H2S-H2O inflow by the Black Sea deep waters. The effect of key operational parameters such as feed composition, temperature (550–850 °C), and cobalt loading (0–100 wt.%) on the catalytic performance of Co3O4/CeO2 catalysts was systematically explored. In order to gain insight into potential structure-performance relationships, various characterization studies involving BET, XRD, SEM/EDX, and sulfur elemental analysis were performed over the fresh and spent samples. The experimental results showed that the 30 wt.% Co/CeO2 catalyst demonstrated the optimum catalytic performance over the entire temperature range with a H2 production rate of ca. 2.1 μmol H2∙g−1·s−1 at 850 °C and a stable behavior after 10 h on stream, ascribed mainly to the in-situ formation of highly active and stable cobalt sulfided phases.

Oxygen Vacancy Promoted O2 Activation over Mesoporous Ni-Co Mixed Oxides for Aromatic Hydrocarbon Oxidation.

Whether the oxygen vacancies of heterogeneous catalysts improve their catalytic activity or not has recently been the topic of intense debate in the oxidation of hydrocarbons. We designed an effective strategy to construct mesoporous Ni-Co mixed oxides via a ligand-assisted self-assembly approach. The surface oxygen vacancy concentrations of the mesoporous Ni-Co mixed oxide catalysts were regulated by changing the doping amount of Ni or the reduction method, and the relationship between oxygen vacancies and catalytic activity was studied. Controlled experiments and DFT calculations revealed that oxygen molecules were more favorably adsorbed and activated on oxygen vacancies to form active oxygen species. Increasing the oxygen vacancy concentration within a certain range can effectively enrich the active oxygen species, therefore improving the oxidation rate of ethylbenzene. The optimized mCo3O4-0.1NiO catalyst exhibited a remarkable catalytic activity for the solvent-free oxidation of ethylbenzene to acetophenone, typically including 68.0% conversion and 95.4% selectivity (20 mg mCo3O4-0.1NiO, 10 mL ethylbenzene, and 0.6 MPa O2).

CO2 hydrogenation to methanol over Zr- and Ce-doped indium oxide

In recent decades, climate change has become a major issue that needs to be addressed. Many efforts have been made on the reduction of CO2 emissions and its conversion in energy carriers and high value-added products such as methane, methanol, dimethyl-ether, and hydrocarbons. The present study focuses on the development of catalysts for hydrogenating CO2 to methanol, which is a useful chemical and an alternative liquid fuel. According to the literature, In2O3-based catalysts are particularly selective in the hydrogenation of CO2 to methanol, reducing the production of CO even at high space velocities compared to the more common ternary catalysts such as Cu/ZnO/Al2O3 or Cu/ZnO/ZrO2. Therefore, the effects of CeO2 and ZrO2 on In2O3-based catalysts were investigated in the present study. The InxCe100−x and the InxZr100−x mixed oxides catalysts were synthesized via gel-oxalate coprecipitation by varying the atomic ratios between the elements. Subsequently, they were analysed with several characterisation techniques to rationalise the catalytic performances that were obtained by testing the samples in a fixed bed reactor under different reaction conditions. The addition of different amounts Ce or Zr modified the structure and morphology of the samples and promoted the adsorption of CO2 from 1.8 mmolCO2⋅gcat−1 up to 10.6 mmolCO2⋅gcat−1. ZrO2 stabilises the structure and the results suggests that the greater specific activity (168 mgCH3OH⋅gIn2O3−1⋅h−1 at 300 °C and 2.5 MPa of In40Zr60) could be ascribed to the electronic promotion of Zr. On the contrary, the addition of CeO2 did not reveal a beneficial effect on the activity. Concerning the stability, In2O3-ZrO2 binary oxides seemed to be affected mainly by sintering; whereas In2O3-CeO2 were affected by at least three deactivating phenomena: sintering, reduction of In2O3 to metallic indium and coking. Consequently, the deactivation rate of these binary oxides increased from 1.04 ⋅ 10−2 h−1 of the In100 to 4.13 ⋅ 10−2 h−1 of the In40Ce60.

Evaluation of Co-Fe, Cu-Fe, Ni-Al and Ni-Fe mixed oxides in catalytic combustion of methane: Comparison study and investigating the effect of preparation method

In this investigation, co-precipitation (Cp) and sol gel (Sg) technique were employed to fabricate CoFe2O4, CuFe2O4, NiAl2O4 and NiFe2O4 mixed oxide catalysts. The catalytic activity of these catalysts in methane combustion was studied at temperature range of 300 to 550 °C and gas hourly space velocity of 14,000 h − 1. Different analyses including the X-Ray Diffraction (XRD), N2 adsorption-desorption (BET), Energy Dispersive X-Ray Analysis (EDX), Scanning Electron Microscopy (SEM) and H2-TPR analysis were used in order to investigate the structural properties of the catalysts. The small crystal size improves the surface area and catalyst active site, thus increasing the rate of reaction and conversion factor. The results revealed that the method of preparation had an important role in controlling the structural features of the catalysts, resulting in an important difference in catalytic presentation. The reactor tests required CO2 selectivity of 100% for all synthesized catalysts, and the best catalytic activity (97% methane conversion) and structural properties (surface area of 41.81 m2g − 1 and crystal size of 13.20 nm) was demonstrated by CoFe2O4 (co-precipitation) sample. It was also revealed that catalysts synthesized using sol gel method had agglomeration impact on the surface of the catalyst, leading to an increase in crystalline size and a decrease in catalytic activity.

CO2 hydrogenation to methanol over hydrothermally synthesized Inx-Zry catalysts

In2O3-based catalysts are considered to be promising catalysts for CO2 hydrogenation to methanol, while ZrO2 exhibits a significant improvement of the catalytic activity of In2O3 catalysts. In this work, Inx-Zry mixed oxides catalysts were prepared by a hydrothermal method and tested for methanol synthesis from CO2 hydrogenation at 260–320 °C and 2–5 Mpa. The results showed that all Inx-Zry mixed oxides catalysts show superior catalytic performances than pure In2O3 due to the contribution of oxygen vacancies and the activation of CO2 by zirconia. Further investigation showed that the catalytic performance of Inx-Zry mixed oxide with the coexistence of In-VO-In and In-VO-Zr sites is better than that of In4Zr6Ox-CP solid solution which only possesses single In-VO-Zr sites, due to the higher oxygen vacancy content and stronger electronic interactions between In and Zr species.

CoCuAlOx mixed oxides for the CO2 purification from oxycombustion process exhaust feed

To produce CO2 rich effluents, oxyfuel combustion is a promising technology. Nevertheless, storage as well as valorization steps require the removal CO and NOx produced during combustion process to reach a CO2 as purest as possible. In view of development of transition metal-based catalyst for this application, CoCuAlOx mixed oxide derived from hydrotalcite like preparation way were used as catalyst for NO-CO abatement in a gas stream similar to those obtained in oxyfuel combustion. Materials characterization was explored by several techniques (XRD, Surface area analysis, TPR, HAADF-STEM coupled with EDX, in-situ XPS) that show good catalyst homogeneity and copper surface species modification. The best catalytic performances for the NO-CO abatement are obtained with the ratio Co/Cu= 2 mixed oxide catalyst. The impact of water was also investigated and following trends were obtained: (i) a shift towards CO oxidation higher temperature; (ii) a decrease of NO reduction yield due to copper surface species modification.

Pyrolysis and characterization of poplar mixed nanometal oxide catalysts

Poplar is an important wood bioresource and is one of the most dominant plantations in the world with high content of cellulose (52.99%). However, the pyrolytic products produced from poplar pyrolysis are not well studied and applied. In this study, pyrolysis of poplar mixed with different types and the ratio of nanometal oxide catalyst (NiO: 0.50 g, NiO–Ag: 0.25 + 0.25 g, Ag: 0.50 g, Fe2O3: 0.50 g, and Fe2O3–Ag: 0.25 + 0.25 g) were performed to identify the catalyst showing desirable performance in poplar pyrolysis. The thermogravimetry (TG) analysis revealed that adding Fe2O3 in pyrolysis has enhanced the decomposition of poplar where most of the weight loss (71.61 wt%) was in the temperature range of 25–780 °C. The results of the gas chromatography/mass spectrometry (GC/MS) analysis showed that the primary components of the produced bio-oil are acetaldehyde, methyl glyoxal, furfural, d-allose, and acetic acid, which can be used as raw materials in pharmaceuticals, industrial processes, and agricultural endeavours.

A novel MnTi mixed oxide catalyst engineered by metal–organic framework precursor for enhancing NOx removal performance

In this research, MnTi metal–organic frameworks (MnTi‐MOFs) were successfully prepared, and derived catalysts were innovatively applied to the removal of NOx by selective catalytic reduction with NH3 (NH3‐SCR). The test results show that the one‐step controllable synthesis of MnTi‐MOFs material is achieved through adjusting the Mn/Ti molar ratios and the proportion of metal to organic ligands, in which the ratio of organic ligands is vitally significant for the synthesis of MnTi‐MOFs (MnTi‐MOFs can only be synthesized with the ligand to metal molar ratio in the range of 3–4). Owing to the special heterogeneity of MOFs, the catalyst sample derived from the MnTi‐MOFs precursor prepared with suitable raw material ratio exhibits the best SCR performance. Series of characterization analyses show that the catalysts derived from MOFs precursors with ordered lattice structure exhibit an appropriate surface chemical adsorption oxygen, an increased Mn4+/Mnn+ content, and enhanced acidity of the surface of the sample. This research affords an innovative method to the controllable preparation of dual metal MOFs and expands the development of catalysts in the field of low‐temperature de‐NOx.

Transition metal-containing MgFe ex-LDH mixed oxides, effective catalysts in the hydrodeoxygenation of benzyl alcohol

Several M-MgFe-LDH precursors were prepared via co-precipitation and then calcined at 500 °C to yield the corresponding M-MgFeO mixed oxide catalysts, which were tested in the hydrodeoxygenation (HDO) of benzyl alcohol. The prepared LDH contained Mg and Fe in a mol ratio of 3:1 and 10 at% M with respect to cations, where M = Ni, Co, Ag and Cu. The Cu-MgFeO mixed oxide gave the best results, therefore we proceeded with the preparation of a Cu(x)-MgFeO series, where x = 2.5–20 at% Cu. The prepared solids were characterized by powder X-ray diffraction, nitrogen adsorption-desorption, temperature-programmed reduction with H2, temperature-programmed desorption of CO2 and energy dispersive X-ray spectroscopy. We studied the influence of the nature of the transition metal M on the activity and selectivity of the catalysts and, for the Cu-containing samples, the influence of Cu loading, reaction time and reaction temperature on their performance in the HDO reaction of benzyl alcohol. With 93.8% conversion and 93.8% selectivity towards toluene at 230 °C, 3 h reaction time and under a hydrogen pressure of 5 atm, the Cu(10)-MgFeO catalyst proved to have the best performance in the studied reaction. Therefore, its stability and reusability were checked. We also proposed a sequence of reactions for the entire HDO process, based on a reverse Mars-van Krevelen mechanism driven by oxygen vacancies formed on the catalyst surface.