Activity For Partial Oxidation Research Articles

Constructing the Ni-O-Ce interface to enhance the activity and stability for partial oxidation of methane to syngas under high temperatures

Catalyst interface determines the activity and stability of partial oxidation of methane (POM) toward syngas under high temperatures crucially. Herein, Ni catalysts supported on CeO2 were prepared under different pretreatment atmospheres to construct an optimal and robust interface. Ni/CeO2 catalyst pretreated in H2 (Ni/CeO2-H2) exhibits the higher activity and the better stability than Ni/CeO2-Ar and Ni/CeO2-air catalysts. The Ni-O-Ce interfacial site in Ni/CeO2-H2 catalyst shows the lower reduction temperature, indicating the enhanced H-spillover effect and enhanced oxidation resistance of Ni under POM conditions. Moreover, the XPS and in situ Raman results show that Ni/CeO2-H2 contains more surface oxygen vacancies for adsorbing and activating oxygen, further contributing to the reaction activity. The in situ DRIFTS results indicate that the CH4 could react with the lattice oxygen to form formate and carbonate, and further decompose to CO and CO2. These findings deepen the fundamental understanding of Ni/CeO2 catalysts for POM reaction.

Breaking Continuously Packed Bimetallic Sites to Singly Dispersed on Nonmetallic Support for Efficient Hydrogen Production.

We have synthesized Pt1Zn3/ZnO, also termed 0.01 wt %Pt/ZnO-O2-H2, as a catalyst containing singly dispersed single-atom bimetallic sites, also called a catalyst of singly dispersed bimetallic sites or a catalyst of isolated single-atom bimetallic sites. Its catalytic activity in partial oxidation of methanol to hydrogen at 290 °C is found to be 2-3 orders of magnitude higher than that of Pt-Zn bimetallic nanoparticles supported on ZnO, 5.0 wt %Pt/ZnO-N2-H2. Selectivity for H2 on Pt1Zn3/ZnO reaches 96%-100% at 290-330 °C, arising from the uniform coordination environment of single-atom Pt1 in singly dispersed single-atom bimetallic sites, Pt1Zn3 on 0.01 wt %Pt/ZnO-O2-H2, which is sharply different from various coordination environments of Pt atoms in coexisting PtxZny (x ≥ 0, y ≥ 0) sites on Pt-Zn bimetallic nanoparticles. Computational simulations attribute the extraordinary catalytic performance of Pt1Zn3/ZnO to the stronger adsorption of methanol and the lower activation barriers in O-H dissociation of CH3OH, C-H dissociations of CH2O to CO, and coupling of intermediate CO with atomic oxygen to form CO2 on Pt1Zn3/ZnO as compared to those on Pt-Zn bimetallic nanoparticles. It demonstrates that anchoring uniform, isolated single-atom bimetallic sites, also called singly dispersed bimetallic sites on a nonmetallic support can create new catalysts for certain types of reactions with much higher activity and selectivity in contrast to bimetallic nanoparticle catalysts with coexisting, various metallic sites MxAy (x ≥ 0, y ≥ 0). As these single-atom bimetallic sites are cationic and anchored on a nonmetallic support, the catalyst of singly dispersed single-atom bimetallic sites is different from a single-atom alloy nanoparticle catalyst. The critical role of the 0.01 wt %Pt in the extraordinary catalytic performance calls on fundamental studies of the profound role of a trace amount of a metal in heterogeneous catalysis.

Variations of Cobalt Catalytic Activity and Selectivity in Ethylene Oxidation against Stepwise Oxidation of Cobalt Surface

Catalytic activity of Co foil in ethylene oxidation was studied against oxidation degree of Co surface at stepwise foil oxidation. Experiments were conducted at temperatures of 500–800°C by a pulse method using alternative pulses of 0.2% C2H4–0.25% O2–1% Ar–He testing mixture and 1% O2–1% Ar–He oxidative mixture. Oxidation degree of Co foil varied from a totally reduced surface to an oxidation depth about a hundred of cobalt oxide “monolayers”. Using XRD, SEM and EDS, it was shown that CoO phase formed during a first stage of the stepwise oxidation (from 0 to ~60 oxide “monolayers”) at the all tested temperatures and modifications of surface morphology could be observed. At this stage the samples had a relatively high activity in both partial and total oxidation of ethylene at 500–600°C. On the contrary, at 700–800°C total oxidation was practically absent and the rate of partial oxidation was much lower than that at 500–600°C. During a second stage of Co surface oxidation (from ~60 to ~120 oxide “monolayers”) at 500–600°C also Co3O4 phase was found as well as a gradual ordering of the oxide crystals. In that state, the samples demonstrated a stationary (at 500°C) or an extremal (at 600°C) activity in total oxidation of ethylene. On the contrary, a temperature increase up to 800°C led to a sharp decrease of catalytic activity of the Co foil in this interval of oxidation degree.

Highly dispersed Pt clusters within ZSM-5 stabilized by alkali metal ions and Al sites for partial methane oxidation

Zeolite is an ideal material to encapsulate nanoparticles and avoid metal aggregation, but it is challenging to achieve a high degree of metal dispersion inside zeolite. In this work, the Na+ reagent was introduced to synthesize Pt@S-1 and Pt@ZSM-5 to explore the effect of Al sites and alkali metal ions on dispersion of Pt nanoparticles. The properties of four catalysts were characterized by XRD, TEM, CO-TPR, TG and FT-IR, and the effects of alkali metal ions were analyzed combined with POM test results. The introduction of a certain amount of alkali metal ions in Pt@ZSM-5 catalyst consumed part of the hydroxyl group to form stable Pt-Ox(OH)yNa active species, which further improves the stability and dispersity of platinum nanoparticles and promotes the activity of partial oxidation of methane. This stabilization strategy can be extended to other porous materials encapsulating metal nanoparticles.

Continuous partial oxidation of methane to methanol over Cu-SSZ-39 catalysts

The direct conversion of methane to methanol using molecular oxygen as the oxidant is a grand challenge in the chemical sciences. Here we describe our efforts in using copper-exchanged SSZ-39 for this chemistry and show it is a promising material for this reaction. Testing in a continuous flow reactor at 225 °C, Cu-SSZ-39 showed superior activity for methane partial oxidation compared to Cu-SSZ-13 (site time yields of 26.9 ± 0.8 molCH3OH·molCu−1·h−1 *10−3 versus 13.2 molCH3OH·molCu−1·h−1 *10−3 respectively). A clear maximum in methanol productivity, both in terms of a mass catalyst basis and copper atom basis is observed for Cu-SSZ-39 at a Cu/Al ∼ 0.2. Running this reaction in the absence of oxygen over samples with a Cu/Al > 0.1 the reactivity is markedly lower than that in the presence of oxygen unlike what is observed in SSZ-13.

Research progress of vanadium pentoxide photocatalytic materials.

Photocatalytic reactions convert solar energy into chemical energy through a clean and green reaction process. Photocatalytic technology based on semiconductor materials provides us with a new idea in energy utilization and environmental governance. It was found that vanadium pentoxide (V2O5) has a narrow band gap, wide response range in the visible region, high oxygen density in the V2O5 lattice, high oxidation state of V5+, small energy requirement, and superior catalytic activity in partial oxidation. Therefore, the utilization rate of sunlight and photocatalytic oxidation can be greatly improved using V2O5 materials. However, the narrow band gap of V2O5 also makes it easier for the photogenerated electrons and holes to recombine in the excited state, and the stored energy is instantly consumed by carrier recombination. Therefore, how to promote the carrier separation of V2O5 and improve the photocatalytic efficiency are the key problems to be solved. Herein, several methods to improve the photocatalytic performance of V2O5 are reviewed, including metallic ion doping, non-metallic ion doping, semiconductor recombination, and noble metal deposition. Finally, it is suggested that future research directions should focus on a variety of modification methods simultaneously to promote photocatalytic efficiency and lower the cost, which will enable V2O5 to have a broad development prospect in the field of photocatalysis.

Possible Fine-Tuning of Methane Activation toward C2 Oxygenates by 3d-Transition Metal-Ions Doped Nano-Ceria-Zirconia.

In this work, we demonstrate a simple sol-gel technique to prepare metal-ion(s)-doped ceria-zirconia solid solution for efficient catalytic methane activation. The cation-depicting formula units are Ce0.80Zr0.20 (CZ), Ce0.79Zr0.20M0.01 (CZM), and Ce0.79Zr0.20M0.005M10.005 (CZMM1) (M and M1 = V, Mn, Fe, Co, and Cu), employed for undoped, mono-metal-ion-doped, and bi-metal-ion-doped solid solutions, respectively. Methane activation with Mn, Fe, Cu mono-metal-ion-doped CZ favors the C1 product, while CZCo assists C-C coupling with the formation of acetaldehyde. On the other hand, the Co- and Fe-doped bi-metal-ion combination catalyst (CZCoFe) shows significant ethanol but predominant formic acid formation. This is further promoted by the Co + V bi-metal-ion combination (CZCoV) catalyst, and it shows ethanol as the major product along with methyl hydrogen peroxide, methanol, and formic acid as minor products. An impressive ethanol yield of 93 μmol/g h with 76% selectivity obtained with the CZCoV catalyst is at par with that obtained with noble-metal-based catalysts under comparable reaction conditions. When Co and V content was increased two and four times from 0.005 to 0.01 and 0.02, ethanol yield increased at the expense of formic acid. The 213 μmol/g h ethanol yield (86% selectivity) observed with Ce0.76Zr0.20Co0.02V0.02 is probably the highest observed. The partial oxidation of CH4 in Co-based bi-metal combinations (Co + V or Co + Fe) suggests the synergistic effect of doped metal ions owing to the heterogeneous near-neighbor environment. The present results are attributed to the surface heterogeneity between the host and the dopants, which selectively promotes methane activation as well as C-C coupling. This indicates a large scope to tune the activity of partial oxidation of methane and product selectivity with different metal-ion(s) combinations.

Effect of Support and Pd Cluster Size on Catalytic Methane Partial Oxidation to Dimethyl Ether Using a NO/O2Shuttle

The partial oxidation of methane to dimethyl ether (DME) was studied on palladium supported on SiO2, SiO2-ZrO2, and ZrO2 at 225–375 °C and 0.1 MPa using either O2 or a mixture of NO and O2 as the oxidants. The NO + O2 mixture was more reactive than the O2 toward methane, probably due to the formation of active surface nitrate species. The formation of DME on the catalysts was influenced by the support, with the Pd/ZrO2 catalyst producing DME and the Pd/SiO2 catalyst producing only CO2, irrespective of the oxidant used. The Pd/SiZrO2 catalyst formed DME only when O2 alone was used as the oxidant. For the reaction with NO + O2, in situ Fourier transform infrared (FTIR) measurements showed that the partial oxidation activity on Pd/ZrO2 was related to its ability to form surface-bridged nitrate species, likely due to the presence of basic sites. However, an inhibitory effect of basicity on CH4 turnover frequency (TOF) was attributed to surface nitrate-induced structural modification deduced from X-ray absorption fine structure spectroscopy measurements. For the reaction with only O2, the formation of DME over Pd/ZrO2 and Pd/SiZrO2 was attributed to the reported ability of ZrO2 to retain methoxy intermediates. Furthermore, a weak structural effect on the TOF observed with O2 was attributed to competing effects of decreasing activity for C-H activation and increasing resistance to Pd oxidation with increasing particle size.

Singly dispersed Ir1Ti3 bimetallic site for partial oxidation of methane at high temperature

The interaction between metal and support is an essential topic in heterogeneous catalysis. The role of strong metal-support interaction (SMSI) between support and metal particles, oxide, or single atom site in catalysis has been investigated recently. In this work, we report a singly dispersed bimetallic site (SDBM), Ir1Ti3, formed on TiO2 support during partial oxidation of methane. The Ir1Ti3 site exhibits an unusual “switch on/off” catalytic activity for partial oxidation of methane to produce syngas, which is due to the strong interaction between TiO2 support and in-situ formed Ir1Ti3 SDBM site. The high activity and stability of the catalyst are also due to the unique structure according to the strong interaction effect. This work reveals the SMSI between oxide and SDBM and further impacts on catalysis.

NiO/CeO2-Sm2O3 nanocomposites for partial oxidation of methane: In-situ experiments by dispersive X-ray absorption spectroscopy

In this work, we analyze Sm2O3-doped CeO2 (SDC) nanopowders and NiO/SDC nanocomposites in terms of sample reducibility and catalytic activity for partial oxidation of methane. We assess the role of the average crystallite size and specific surface area in Ni and Ce reduction kinetics by in-situ X-ray absorption spectroscopy experiments in diluted H2 and CH4/O2 mixtures. Our results indicate that crystallite size and surface area play a key role in CH4 activation through modification of the sample redox behavior. The oxidation of the metallic phase is the main cause of sample deactivation. A clear relationship is established between the temperature of maximum Ni oxidation rate and grain size. An interplay between Ce atoms from the support and Ni from the active phase was observed during the experiments, evidencing a complex relationship between oxygen vacancy concentration and catalytic activity. A high Ce3+ content in catalyst support was detrimental to catalytic activity

Partial Oxidation of Isooctane over Ru‐Promoted Nickel–Molybdenum/Cerium–Zirconium Oxide Catalyst at an Intermediate Temperature for Internal Reforming Solid Oxide Fuel Cell Applications

In previous work, nickel–molybdenum (NiMo) nanoparticles supported on cerium–zirconium oxide (CZ) catalyst showed excellent catalytic activity and stability for liquid fuel reforming with an improved coking resistance compared with Ni/CZ. Lowering the temperature of solid oxide fuel cells (SOFCs) below 700 °C is vital, because they can reduce the material costs and thermal stresses of SOFCs. However, the performance of NiMo/CZ is insufficient when operated at temperatures below 750 °C. This work aims to improve the activity and coking resistance of NiMo/CZ at a temperature of 700 °C by adding a minimal amount (0.1 wt%) of Ru. The Ru‐promoted sample shows a high reforming activity for partial oxidation of isooctane at 700 °C. The X‐ray diffraction (XRD) and STEM indicate a smaller Ni crystallite size for NiMoRu/CZ catalyst, which would decrease its tendency to facilitate coke formation. The tubular SOFC with the NiMoRu/CZ as its internal reforming catalyst displays good power density output and performance stability at 200 mA cm−2.

Highly selective production of syngas (>99%) in the partial oxidation of methane at 480 °C over Pd/CeO2 catalyst promoted by HCl

The partial oxidation of methane (POM) is an attractive catalytic route for producing syngas (CO/H2), although there are problems to overcome such as high reaction temperature (>650 °C) and the deactivation of the catalyst due to the coke formation. We addressed the problem by including HCl in the POM over a Pd/CeO2 catalyst. Pd/CeO2 produces CO, CO2, and coke with low CO selectivity in the absence of HCl, while it converts methane to chlorinated products at below 450 °C in the presence of HCl. At above 480 °C, however, highly selective CO (>99%) is produced over Pd/CeO2 via the POM. Moreover, it is found that coke is not formed during the reaction, leading to the stable partial oxidation activity and selectivity up to 40 h. Both surface and bulk characterization results strongly propose that HCl plays an essential role in transforming Pd/CeO2 into (PdClx + Pd0)/CeOCl (0 < x ≤ 2), which provides the ideal environment for the catalytic route of the POM.

A heteromesocrystal photocatalyst consisting of SnO2(head)-TiO2(tail) nanorod hybrids

Radial heteromesocrystals consisting of anisotropic SnO2(head)-TiO2(tail) nanorods with heteroepitaxial junction were synthesized by a hydrothermal reaction using SnO2 nanocrystals as the seeds. The heteromesocrystals exhibit photocatalytic activity for aerobic partial oxidation of ethanol to acetaldehyde much higher than radial TiO2 homomesocrystals. The striking photocatalytic activity of the former can stem from efficient light harvesting by multiple light scattering, effective charge separation by the one-directional electron transport through the TiO2 NR followed by the subsequent smooth interfacial electron transfer to SnO2, and electrocatalytic activity of SnO2 for two-electron oxygen reduction reaction.

The facet effect of ceria nanoparticles on platinum dispersion and catalytic activity of methanol partial oxidation.

The effect of platinum-supported nano-shaped ceria catalysts on methanol partial oxidation and methyl formate product selectivity has been investigated. A Pt-supported CeO2 nanocube catalyst had a higher turnover frequency than nanosphere catalysts; however, nanosphere catalysts showed higher selectivity towards methyl formate. The observed ceria shape effect in catalysis was associated with the shape-dependent Pt dispersion and its oxidation states. Furthermore, in situ studies revealed that the reduced platinum and mono-dentate methoxy group were responsible for the higher turnover frequency.

Effect of the Oxidation Degree of a Nickel Foil Surface on Its Catalytic Activity in the Reaction of Ethylene Oxidation

The effect of the oxidation degree of a nickel foil surface on the rate of catalytic oxidation of ethylene was studied by a pulse method at 600 and 700°C. It was shown that a reduced metallic surface demonstrated a high activity in partial ethylene oxidation, whereas a partially oxidized surface with an oxidation degree of ~24 formal O2 monolayers, in the total oxidation of C2H4. A SEM investigation has revealed that the oxidized surface was partially coated with nickel oxide nanocrystals. A further increase in the surface oxidation degree led to a continuous coverage of the Ni surface with oxide crystals and a dramatic decrease of catalytic activity. In addition, a low maximum of total ethylene oxidation was observed at 700°C in the range of surface oxidation degree of 95–135 O2 monolayers.

Regeneration behavior of reforming catalysts based on perovskite oxides LaM0.95Rh0.05O3 (M: Cr, Co, Fe) by redox treatment

Perovskite-based catalysts with a self-regeneration property, named intelligent catalysts, were developed for CH4 partial oxidation, and the effect of redox treatment on regeneration behavior of the catalytic activity was investigated. Rh-substituted catalysts such as LaM0.95Rh0.05O3 (M: Cr, Co, Fe) were prepared by the Pechini method. Among the catalysts, LaCr0.95Rh0.05O3 exhibited the highest and the most stable catalytic activity for partial oxidation of methane before and after redox treatment. The precipitation and dissolution of Rh species were confirmed by X-ray photoelectron microscopy and transmission electron microscopy, indicating that the catalyst regenerated itself via the redox process.

Visible-light-driven selective oxidation of methane to methanol on amorphous FeOOH coupled m-WO3

Direct conversion of methane into value-added fuels or chemicals under ambient conditions remains a great challenge. Constructing solar-energy-driven catalytic systems is considered as a promising strategy, but the conversion efficiency and products selectivity are still low, especially for producing alcohol derivatives. Herein, to promote the photocatalytic activity of methane partial oxidation to methanol, a series of FeOOH/m-WO3 consisting of ordered mesoporous WO3 (m-WO3) and highly dispersed amorphous FeOOH were synthesized by using KIT-6 silica as hard template. The prepared FeOOH/m-WO3 catalysts exhibit dramatically improved visible-light catalytic activities toward selective oxidation methane into methanol in the presence of H2O2. A methane conversion rate of 238.6 μmol·g−1·h−1 is achieved over the optimal 1.98% FeOOH/m-WO3, which is 3 times higher than that of pristine m-WO3 (79.2 μmol·g−1·h−1). Moreover, a methanol production rate of 211.2 μmol·g−1·h−1 with a selectivity of 91.0% is obtained on the optimum catalyst under 4 h visible-light irradiation. Significantly, the greatly improved methane conversion and methanol production can be attributed to efficient electron migration from the conduction band of m-WO3 to highly dispersed FeOOH, evidenced by in-situ XPS analysis, transient photocurrent response and photoluminescence spectra. Furthermore, based on radicals trapping experiments and electron spin resonance (ESR) results, methane is primarily activated by photoholes accumulated on the valence band of m-WO3 to generate methyl radicals (·CH3) and the produced hydroxyl radicals (·OH) via decomposing H2O2 by photoelectrons on FeOOH surface are predominant oxidant for methanol generation. Desired methanol is selectively produced via radicals reaction between ·CH3 and ·OH.

Thermodynamically unconstrained forced concentration cycling of methane catalytic partial oxidation over CeO2 FeCralloy catalysts

Converting waste associated natural gas from oil fields is uneconomic with current gas-to-liquid technology. Micro Gas-to-Liquids technology (μGtL) combines process intensification and numbering up economics to reduce capital costs to convert flared and vented natural gas to value-added synthetic fuel: Milli-second contact times in the catalytic partial oxidation of methane (CPOX) integrated with a tandem Fischer-Tropsch (FT) step meets the economic constraints together with remote process control.FeCralloy knitted fibres with high thermal conductivity and low pressure drop, resist thermal and mechanical stresses in the high pressure CPOX step. The FeCralloy catalysts are free of pre-reduction treatments. We deposited Pt and/or CeO2 over the fibre surface via solution combustion synthesis. Methane conversion was higher at ambient pressure compared to 2 MPa while the Pt/CeO2 FeCralloy was relatively inert from 0.1 MPa to 2 MPa. However, both catalysts demonstrated high activity in quasi-chemical looping partial oxidation of methane: during the reduction step while feeding methane, an on-line mass spectrometer only detected H2 while in the oxidation step it detected predominantly CO. Kinetic modeling of the oxidation-reduction cycles suggests that the reaction follows a direct mechanism to produce CO and H2 rather than an indirect mechanism that first produces CO2 and H2O followed by reforming.

Methanol Partial Oxidation Over Shaped Silver Nanoparticles Derived from Cubic and Octahedral Ag2O Nanocrystals

Ag-catalyzed methanol partial oxidation is an important industrial catalytic reaction. Herein we report the morphology effect of Ag catalysts on the catalytic activity in methanol partial oxidation. Shaped Ag nanoparticles were prepared by reducing the cubic and octahedral Ag2O nanocrystals under methanol vapor atmosphere. Ag nanoparticles were observed to undergo serious restructuring during methanol partial oxidation, but Ag nanoparticles derived from octahedral Ag2O nanocrystals were found to be more intrinsically active than those derived from cubic Ag2O nanocrystals. Ex situ characterizations demonstrated the more facile formation of active oxygen species on Ag nanoparticles derived from octahedral Ag2O nanocrystals than on Ag nanoparticles derived from cubic Ag2O nanocrystals. Therefore, although undergoing serious restructuring under the harsh reaction conditions of methanol partial oxidation, the original morphology of Ag nanoparticles exerts strong influences on the structure and catalytic activity of restructured Ag nanoparticles. These results demonstrate that strong correlations between original surface structure and restructured surface structure of catalyst nanoparticles even under very harsh reaction conditions and add fundamental understandings of methanol partial oxidation over Ag catalysts.

Study on Nanofibrous Catalysts Prepared by Electrospinning for Methane Partial Oxidation

Electrospinning is a simple and efficient technique for fabricating fibrous catalysts. The effects of preparation parameters on catalyst performance were investigated on fibrous Ni/Al2O3 catalysts. The catalyst prepared with H2O/C2H5OH solvent showed higher catalytic activity than that with DMF/C2H5OH solvent because of the presence of NiO in the catalyst prepared with DMF/C2H5OH solvent. The metal ion content of the precursor also influences catalyst properties. In this work, the Ni/Al2O3 catalyst prepared with a solution containing the metal ion content of 30 wt % demonstrated the highest Ni dispersion and therefore the highest catalytic performance. Additionally, the Ni dispersion decreased as calcination temperature was enhanced from 700 to 900 °C due to the increased Ni particle sizes, which also caused a high reduction temperature and low catalytic activity in methane partial oxidation. Finally, the fibrous Ni/Al2O3 catalysts can achieve high syngas yields at high reaction temperatures and high gas flow rates.