Catalysts For Partial Oxidation Research Articles

A Thermodynamic Investigation of the Redox Properties of VPO Catalysts

The redox properties of a vanadium phosphorus oxide (VPO) catalyst with a V:P ratio of one were inves- tigated using Coulometric Titration at 873 K. Equilibrium between (VO)2P2O7 and VOPO4 exists at a P(O2 )o f 3 9 10 -4 atm, corresponding to DG of -60 kJ/mol O2. This value for VPO is significantly lower than that mea- sured with other vanadium-containing catalysts that have been studied. Furthermore, compared to other vanadium catalysts, V ?4 was stabilized against further reduction at lower P(O2). These redox thermodynamics may help to explain the unique catalytic properties of VPO catalysts for partial oxidation of butane to maleic anhydride.

Partial oxidation of methane into syngas (H2 + CO) over effective high-dispersed Ni/SiO2 catalysts synthesized by a sol–gel method

A Ni catalyst supported on mono dispersed silica spheres, Ni/SiO2-Sph (SG), has been successfully synthesized by a sol–gel method. By comparing it with other Ni catalysts (supported on commercial silica and silica spheres) prepared by an impregnation method, we find that the size of Ni particles and their dispersion are closely related to performances of the catalysts in partial oxidation of methane (POM) into synthesis gas (CO + H2). Several means such as H2-TPR, TEM, and XRD are employed to characterize these catalysts. Although the catalyst Ni/SiO2-Sph (SG) in specific surface area is not large, the Ni particles are the smallest in size (3–5 nm) among the three catalysts, and are uniformly distributed, high dispersed over the silica surfaces, being not much changed as Ni loading. It is notable that the smaller size of the NiO particles is corresponding to the stronger NiO–SiO2 interactions. The catalyst Ni/SiO2-Sph (SG) shows the best catalytic performances and the longest lifetime among the three catalysts at the POM conditions.

Influence of phosphate addition on activity of Ag and Cu catalysts for partial oxidation of alcohols

Phosphate-containing silica-supported silver and copper catalysts prepared by impregnation method were investigated using TEM, H2 TPR and CO TPR methods and tested in the reaction of selective ethylene glycol oxidation into glyoxal. It was shown that reversible oxidation–reduction of the active component took place in case of Ag- and Cu-containing catalysts. In case of Cu-containing systems formation of copper oxides on the catalyst surface led to the decrease in catalytic activity. Silver oxide-like species did not form in the presence of phosphate modifier on the surface of Ag-containing systems, and that led to the increase in glyoxal selectivity during ethylene glycol oxidation.

Recent Progress on Ni-Based Catalysts in Partial Oxidation of Methane to Syngas

The partial oxidation of methane (POM) to syngas has attracted the attentions of scientists in the recent 20–30 years due to the depletion of methane and its obvious advantages. Ni-based catalysts were most widely employed in the POM because of their comparable catalytic performance with noble metal-based catalysts as well as their cheap prices. However, Ni-based catalysts were suffered from carbon deposition and sintering which were the main reasons for catalyst deactivation. Therefore it’s necessary to improve the abilities of Ni-based catalysts to resist carbon deposition and sintering. In this review, types of Ni-based catalysts that could be used in the POM to syngas are reviewed and at last, the future of Ni-based catalysts in the POM is discussed.

Further performance enhancement of a DME-fueled solid oxide fuel cell by applying anode functional catalyst

In order to increase the coking resistance of SOFCs operating on DME fuel, a Pt/Al2O3–Ni/MgO mixture catalyst was investigated for internal partial oxidation of DME. Catalytic test demonstrated the mixture catalyst has higher activity for DME partial oxidation and lower CH4 selectivity than the individual Pt/Al2O3 and Ni/MgO catalysts. O2-TPO analysis demonstrated that the mixture catalyst also had much higher coke resistance than sintered Ni-YSZ anode, especially at high O2 to DME ratio. Raman spectroscopy of the carbon-deposited catalysts demonstrates that the graphitization degree of carbon is reduced with introducing O2 into DME, and the carbon deposited on the mixture catalyst is almost in amorphous structure. Two operation modes of the mixture catalyst for indirect internal partial oxidation of DME, i.e, directly depositing on the anode surface and locating in the anode chamber were tried. The performance of the cells operating on DME fuel through both operation modes was studied by I–V polarization test and EIS characterization. The cells delivered attractive peak power density of around 750 mW cm−2 by operating on DME-O2 mixture gas, modestly lower than 1012–1065 mW cm−2 operating on pure hydrogen fuel at 700 °C. The direct deposition of Pt/Al2O3–Ni/MgO onto anode surface to perform as a functional layer and a DME to O2 ratio of 2:1 in the mixture gas is preferred to minimize coke formation and maximize power output for the cell to operate on DME fuel.

A CFD Model for Methane Autothermal Reforming on Ru/γ-Al2O3 Catalyst

Hydrogen is the preferred fuel for fuel cells due to high reactivity for electrochemical reaction at anode. In the present study, a three dimensional CFD (Computational Fluid Dynamics) code was developed and validated to simulate the performance of a catalytic monolith fuel processor used for hydrogen generation. Methane autothermal reforming on 5% Ru/γ-Al2O3 catalyst was selected as the reaction mechanism. Ruthenium catalyst is a suitable catalyst for low temperature catalytic partial oxidation (LTCPO) process. This catalyst has good reforming activity and high hydrogen yield is obtained for ruthenium/γ-alumina. This catalyst also demonstrated to be stable within the investigation time. The computational domain of the simulations was selected to be the catalytic section of the reformer. The results provided an adequate match to the experimental data from literature with respect to the outlet and maximum reactor temperature and also distribution of the products. The reactor performance was thereafter studied by numerically revealing the effects of variations of O2/C and S/C feed molar ratios, and feed temperature on the profiles of temperature and species concentrations. Moreover, effects of using air instead of pure oxygen were also investigated. It was concluded that at higher O2/C and S/C feed molar ratios and also at higher feed gas temperature, more hydrogen will be achieved at the reactor outlet, which is very suitable for fuel cell applications.

Highly efficient base catalysis and sulfide oxidation reactions over new functionalized mesoporous polymers

Base catalyzed aldol condensation reactions and selective partial oxidation of sulfides are very important classes of reactions in synthetic organic chemistry. We have prepared a new alkali exchanged form of mesoporous polytriallylamine, OH-MPTA-1 and a Cr-grafted material Cr-MPTA-1 via the simple and facile in situ radical polymerization of a triallylamine template by organic–organic self-assembly of anionic surfactant systems. The materials have been characterized by elemental analysis, powder X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Powder XRD and TEM image analyses suggests the presence of mesophase and disordered wormhole-like mesopores in these samples. Our experimental results reveal that the alkali exchanged material OH-MPTA-1 acts as a very efficient base catalyst for the direct aldol reaction of different aromatic aldehydes with acetone. On the other hand, the Cr-grafted material Cr-MPTA-1, obtained through the reaction of potassium chromate with MPTA-1, acts as a very efficient catalyst for the liquid phase partial oxidation of sulfides to sulfoxides. Both OH-MPTA-1 and Cr-MPTA-1 can be recycled more than five times without an appreciable loss in activity for their respective catalytic reactions.

Oxygen adsorption and dissociation on yttria stabilized zirconia surfaces

We have modelled the nature of oxygen species on the surfaces of yttrium stabilized zirconia (YSZ) in relation to its application as a catalyst for partial oxidation of methane. Quantum mechanical DFT-GGA calculations have been carried out to characterize the interaction between oxygen molecules and the perfect ZrO2 and defective YSZ (111) planar and stepped surfaces. In general, the results suggest that the creation of oxygen vacancies by yttrium doping provides an active site for oxygen adsorption. Depending on the topography and defect dispersion, various oxygen intermediates have been identified and analyzed by their density of states. The low-coordinated Zr cations on the YSZ surfaces can attract strongly reduced oxygen species and the most stable adsorption state of oxygen achieves a higher bond saturation of the neighbouring Zr site. In addition, the probable reaction pathways have been predicted for oxygen dissociation on the plane and stepped (111) surfaces of YSZ.

A novel nanoengineered VOx catalyst supported on highly ordered TiO2 nanotube arrays for partial oxidation reactions

A new class of nanoengineered partial oxidation catalyst has been prepared through the use of chemical vapor deposition of a vanadium oxide precursor over a highly ordered TiO2 nanotube array. A battery of characterization techniques including X-ray diffraction (XRD) together with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman, and DRS-UV/vis indicate that the vanadia species are extremely well dispersed onto the surface of the nanotube array. Studies of the catalytic activity of this material were performed using the partial oxidation of ethanol as a probe reaction and compared with those obtained using typical P-25 TiO2 powdered support. Results showed that the nanoengineered approach to catalyst fabrication leads to a highly active catalytic material where most of the vanadium oxide phase is located on the surface and accessible for catalysis.

Effect of tellurium promoter on vanadium phosphate catalyst for partial oxidation of n-butane

Te-promoted (1%) vanadium phosphate catalyst (VPDTe) was prepared via VOPO 4·2H 2O by calcining its precursor VOHPO 4·0.5H 2O in a flow of n-butane/air. VPDTe catalyst has resulted a higher existence of V 5+ phase with V 5+/V 4+ ratio of 0.23. SEM micrographs show that Te addition altered the arrangement of the platelets from “rose-like” clusters to layer with irregular shape. Te addition has also markedly lowered the reduction activation energies of the vanadium phosphate catalyst as revealed by TPR profile. The amount of active oxygen species associated with V 4+ phase of the Te promoted catalyst was significantly higher than those of the unpromoted catalyst. These observations suggest that high mobility and availability of reactive oxygen species contributed to the enhancement of n-butane conversion up to 80% at 673 K, while only 47% over unpromoted catalyst (2400 h −1, 1.7% n-butane in air).

Nanostructured of Vanadium Phosphorus Oxide (Vpo) Catalyst Prepared by Sonochemical Treatment

A series of six nanostructures vanadium phosphorus oxide catalysts were synthesized through the starting material vanadium pentoxide which subjected to high intensity ultrasound irradiation with different duration of time. XRD patterns of all nanostructure vanadium phosphorus oxide catalyst were indexed to (VO)2P2O7 with one additional small peak which corresponds to the present of small amount of V5+ phase except for the catalyst that undergo 120 min of ultrasound irradiation time. The surface morphologies of the catalysts are uniform, well defined shapes and surface structures in nanoscale range but tend to agglomerate with each other to form larger particles. Furthermore, high amount of active oxygen species were released from V4+ phase (O--V4+ pair) for the nanostructures VPO catalyst which were synthesized with 30 min of ultrasound irradiation time and drastic increment in n-butane conversion (9 %) in catalytic evaluation test, suggested that these nanostructure VPO is an active catalyst for partial oxidation of n-butane towards malefic anhydride.

Visible-Light-Induced Partial Oxidation of Cyclohexane by Cr/Ti/Si Ternary Mixed Oxides with Molecular Oxygen

Cr/Ti/Si ternary mixed oxides (CrTiSi) were prepared by a hydrolysis of tetraethyl orthosilicate (TEOS), Cr(NO3)3, and titanium tetraisopropoxide (TTIP) followed by calcination. These oxides were used as catalysts for partial oxidation of cyclohexane with molecular oxygen under irradiation of visible light (λ > 400 nm). The CrTiSi catalysts produce cyclohexanol and cyclohexanone with high selectivity (>91%) and show much higher activity than the CrSi binary oxides. Among them, the catalyst with equimolar amounts of Cr and Ti shows the highest activity. Visible-light irradiation of the CrSi catalyst promotes a reduction of tetrahedrally coordinated Cr oxide species (Td6+) and produces the excited state (Td4+*), which behaves as an active site for oxidation. In contrast, the CrTiSi catalyst contains tetrahedrally coordinated Cr and Ti oxide species that are connected through the Cr–O–Ti bond. The photoformed Td4+* species is strongly stabilized due to delocalization of excited electrons on the Cr–O–Ti species. This suppresses rapid deactivation of Td4+* and results in enhanced photocatalytic activity.

Synthesis and applications of molybdenum (IV) oxide

Molybdenum dioxide (MoO2) is a transition metal oxide with unusual metal-like electrical conductivity and high catalytic activity toward reforming hydrocarbons. This review covers the synthesis techniques used to fabricate MoO2 in a variety of morphologies and particle sizes. Processing from molybdenum ore and reduction from MoO3 are also covered, with emphasis on reduction mechanisms and kinetic considerations. Discussions of various solution-based and gas phase synthesis techniques shed light on strategies to achieve various unique morphologies, which leads into a brief discussion of nanoscale MoO2. Nanoscale MoO2 is of interest for important technological applications including catalysts for partial oxidation of hydrocarbons, solid oxide fuel cell anodes, and high-capacity reversible lithium ion battery anodes.

The effect of the nature of the support on catalytic properties of ruthenium supported catalysts in partial oxidation of methane to syn-gas

The effect of the carrier on catalytic properties of ruthenium supported catalysts in partial oxidation of methane (POM) was investigated. A variety of supports differed in texture and reducibility (Al2O3, SiO2, TiO2, Cr2O3, CeO2 and Fe2O3) were used. The catalyst activity is governed by ruthenium phase formation (RuO2 → Ru0), and it depends on redox properties of the support as well as support-ruthenium phase interaction. The activity of Ru supported catalysts decreases in the order Al2O3 ≈ SiO2 > Cr2O3 > TiO2 > CeO2 > Fe2O3. No significant effects of the specific surface area and porosity of catalysts on the methane conversion and selectivity of CO formation were found. The selectivity of CO2 formation (total oxidation of CH4) under conditions of POM (a ratio of CH4/O2 = 2) is associated with the contribution of reducible support oxides into the catalytic performance.

Surface and structural features of Pt/CeO2-La2O3-Al2O3 catalysts for partial oxidation and steam reforming of methane

Mixed xCeO2-yLa2O3-Al2O3 oxides (where x+y=12wt%) were prepared by sol–gel method. It was shown that the mixed oxides are suitable carriers for supported Pt catalysts. The catalytic performances of the catalysts were evaluated in the reaction of stem methane reforming (SMR) and partial oxidation of methane (POM). The effect of the support kind on the surface and structural properties, as well as on the redox and catalytic behavior of the catalyst was studied. It was shown that the coexistence of the Pt0/Ptδ+ and Ce4+/Ce3+ redox couples leads to the increase of the CH4 conversion and carbon resistance over Pt catalysts. The CeO2-containing catalysts showed the highest TOFCH4 values in SMR in spite of the low metal dispersion that is related to the high capacity of the surface to carbon cleaning caused by the highest oxygen mobility at metal–support interface. The strong initial deactivation of alumina-supported Pt catalyst in POM was related to Pt sintering.

DFT study of the reactivity of methane and dioxygen with d 10-L 2M complexes

A density functional theory analysis of the reactions of methane and O 2 with d 10-L 2M complexes (M = Pd, Pt; L = N-heterocyclic carbene (NHC), PMe 3) is presented. Computations suggest that reaction of L 2M 0 with O 2/CH 4 to form cis- (L) 2M(OOH)(CH 3) is only slightly uphill (ΔG ∼ 10–11 kcal/mol). Based on calculated thermodynamics, reaction of (L) 2Pt 0 with CH 4 and O 2 is predicted to be more favorable by first addition of CH 4 and then reaction of O 2 with the resulting methyl–hydrido complex. However, oxidative addition for either the C–H bond of methane or of O 2 to d 10-L 2M complexes is kinetically prohibitive. If barriers to oxidative addition to d 10-L 2M systems could be reduced, conversion of L 2M(H)(CH 3) to L 2M(OOH)(CH 3) via hydrogen atom abstract/radical rebound is calculated to be thermodynamically and kinetically feasible, particularly for NHC and Pd. As (NHC) 2Pd also has a lower free energy to methane C–H oxidative addition than does (NHC) 2Pt, the former combination would appear to be a promising starting point in the search for catalysts for partial hydrocarbon oxidation.

Carbon formation in catalytic partial oxidation of methane on platinum: Model studies on a polycrystalline Pt foil

A polycrystalline Pt foil has been investigated as model catalyst in methane catalytic partial oxidation to synthesis gas. It is demonstrated that a substantial amount of carbonaceous deposits forms on the Pt foil upon reaction light-off blocking a large fraction of Pt surface atoms. By using in situ Raman spectroscopy and quantitative spectral analysis the evolution and spatial distribution of these carbonaceous compounds with reaction temperature and reaction time have been characterized. The chemical composition of the carbon material changes from highly reactive and strongly disordered directly after reaction light-off to highly ordered, oxidation and steam reforming resistant after several hours time on stream at 800 °C reaction temperature. Remarkably the carbon distribution at the Pt surface was found to be inhomogeneous and related to the nature of the microcrystals forming the polycrystalline foil in a yet unknown manner.

Porous nickel based catalysts for partial oxidation of methane to synthesis gas

The phase composition and texture of nickel catalysts supported on ribbon porous nickel with a magnesium oxide underlayer were investigated by X-ray diffraction, low-temperature nitrogen adsorption, and electron microscopy combined with energy dispersive X-ray microanalysis. The MgO underlayer was obtained by impregnation the porous nickel with a Mg(NO 3) 2 solution followed by calcination at 550 °C; nickel was supported additionally by impregnation with a Ni(NO 3) 2 solution followed by calcination at 450 °C. In the supported reduced nickel catalysts (800–900 °C, H 2) the phases of Ni and a solid solution of NiO in MgO were observed; a considerable part of nickel crystallites was epitaxially bound with MgO. The partial oxidation of methane to synthesis gas (800 °С, O 2/C = 0.65, GHSV = 127 L/g h) over porous nickel based catalysts was studied. The activity of initial nickel support decreased considerably during the partial oxidation of methane; the value of methane conversion decreased slightly after 100 h of testing on the supported nickel catalysts.

Effect of Calcination Atmospheres on the Performance of Ru/Al<SUB>2</SUB>O<SUB>3</SUB> Catalyst for Partial Oxidation of Methane to Syngas

The catalytic performance for partial oxidation of methane(POM) to syngas over the Ru/Al2O3 catalyst calcined in air(Ru/Al2O3-Air) and Ar(Ru/Al2O3-Ar) was studied.Steady-state reactivity was observed on the Ru/Al2O3-Ar sample,whereas oscillations in CH4,CO,H2,CO2,and H2O were observed on the Ru/Al2O3-Air sample.Comparative studies using CO and O2 chemisorption,X-ray diffraction,Raman,and temperature-programmed reduction(TPR) techniques were carried out in order to elucidate the property-structure relationship of the samples.The Ru dispersion on Ru/Al2O3-Air was only 1%,whereas that on Ru/Al2O3-Ar was about 9%.The significant difference in the Ru dispersion on the two samples is the major factor affecting the POM reaction.Two types of Ru species,RuO2 species weakly interacting with Al2O3 and the Ru-O-Al species strongly interacting with the support,were identified by H2-TPR experiments on the fresh catalyst.The former species could be easily reduced by H2(below 200 °C),while the latter can only be reduced by H2 at above 700 °C.During the POM reaction at above 600 °C,the RuO2 species were in the metal state,whereas the Ru-O-Al species could undergo cyclic redox transformation.Such transformation is responsible for the oscillation on Ru/Al2O3-Air sample.

Additive effects of alkaline-earth metals and nickel on the performance of Co/γ-Al 2O 3 in methane catalytic partial oxidation

Abstract Nano-sized γ-alumina (γ-Al 2O 3) was first prepared by a precipitation method. Then, active component of cobalt and a series of alkaline-earth metal promoters or nickel (Ni) with different contents were loaded on the γ-Al 2O 3 support. The catalysts were characterized by N 2 adsorption-desorption, X-ray diffraction (XRD) and thermogravimetry analysis (TGA). The activity and selectivity of the catalysts in catalytic partial oxidation (CPO) of methane have been compared with Co/γ-Al 2O 3, and it is found that the catalytic activity, selectivity, and stability are enhanced by the addition of alkaline-earth metals and nickel. The optimal loadings of strontium (Sr) and Ni were 6 and 4 wt%, respectively. This finding will be helpful in designing the trimetallic Co-Ni-Sr/γ-Al 2O 3 catalysts with high performance in CPO of methane.