High-temperature Hydrogen Reduction Research Articles

Effect of carbon content on the microstructure and mechanical properties of ultrafine WC-15Co cemented carbides using a Co-based solid solution binder phase

In this paper, a new method was proposed to improve the uniformity of the distribution of grain-growth inhibitors using a Co-based solid solution prepared with Co3O4, Cr2O3, and V2O5 as the raw materials through high-temperature hydrogen reduction. The effect of carbon content on the microstructure and mechanical properties of ultrafine WC-15Co cemented carbides using a Co-based solid solution binder phase were studied. The results demonstrated that the carbon content exerted a considerable influence on the composition of the binder phase, thereby affecting the morphology of the WC grains. With increasing the carbon content, Cr and V precipitated from the Co-based solid solution, adhering to the habit planes of WC or segregating at the WC-Co interface. This hindered the growth of WC grains, yielding an average particle size smaller than that of alloys containing 6.0wt.% carbon content. However, increased carbon content may enhance grain growth owing to the dissolution-precipitation process of WC grains during the liquid-phase sintering process. Thus, an alloy comprising 6.31wt.% carbon content and featuring trigonal prismatic-shaped WC grains with a mean particle size of 0.310 μm exhibited excellent comprehensive performance, characterized by a hardness, transverse rupture strength, and fracture toughness of 1579kg/mm2, 2581MPa, and 10.56MPa·m1/2, respectively.

Thermal stable Pt clusters anchored by K/TiO2–Al2O3 for efficient cycloalkane dehydrogenation

Catalytic dehydrogenation of cycloalkanes is considered a valuable endothermic process for alleviating the thermal barrier issue of hypersonic vehicles. However, conventional Pt-based catalysts often face the severe problem of metal sintering under high-temperature conditions. Herein, we develop an efficient K2CO3-modified Pt/TiO2–Al2O3 (K–Pt/TA) for cycloalkane dehydrogenation. The optimized K–Pt/TA showed a high specific activity above 27.9 mol·mol−1·s−1(H2/Pt), with toluene selectivity above 90.0% at 600 °C with a high weight hourly space velocity of 266.4 h−1. The introduction of alkali metal ions could generate titanate layers after high-temperature hydrogen reduction treatment, which promotes the generation of oxygen vacancy defects to anchored Pt clusters. In addition, the titanate layers could weaken the surface acidity of catalysts and inhibit side reactions, including pyrolysis, polymerization, and isomerization reactions. Thus, this work provides a modification method to develop efficient and stable dehydrogenation catalysts under high-temperature conditions.

3 nm-Wide WO3–x Nanorods with Abundant Oxygen Vacancies as Substrates for High-Sensitivity SERS Detection

Oxygen vacancy (Vo) engineering is a powerful tool to improve semiconductor metal oxide-based surface-enhanced Raman scattering (SERS) activity due to the enriched surface states of substrates. However, the current energy-consuming high-temperature hydrogen reduction methods of introducing Vo and the poor sensitivity impede practical applications of semiconductor SERS. In this work, a facile solvothermal method was developed to prepare ultrafine WO3–x nanorods (∼3 nm width) for high-performance SERS substrates. The lowest detection limit of the rhodamine 6G (R6G) molecule on the Vo substrate can be as low as 10–10 mol/L and exceeds that of unmodified WO3 by more than two orders of magnitude. Experimental and calculated results suggest that the Vo-induced localized surface plasmon resonance (LSPR), diverse vibronic coupling, and enhanced charge transfer synergistically account for this outstanding activity. These findings can provide insights into the rational design of oxygen vacancy-tailored semiconductor SERS substrates.

The Effect of Lithium Ion Leaching from Calcined Li-Al Hydrotalcite on the Rapid Removal of Ni2+/Cu2+ from Contaminated Aqueous Solutions.

A layered double hydroxide (LDH) calcined-framework adsorbent was investigated for the rapid removal of heavy metal cations from plating wastewater. Li-Al-CO3 LDH was synthesized on an aluminum lathe waste frame surface to prepare the sorbent. The calcination treatment modified the LDH surface properties, such as the hydrophilicity and the surface pH. The change in surface functional groups and the leaching of lithium ions affected the surface properties and the adsorption capacity of the heavy metal cations. A zeta potential analysis confirmed that the 400 °C calcination changed the LDH surface from positively charged (+10 mV) to negatively charged (-17 mV). This negatively charged surface contributed to the sorbent instantly bonding with heavy metal cations in large quantities, as occurs during contact with wastewater. The adsorption isotherms could be fitted using the Freundlich model. The pseudo-second-order model and the rate-controlled liquid-film diffusion model successfully simulated the adsorption kinetics, suggesting that the critical adsorption step was a heterogeneous surface reaction. This study also confirmed that the recovered nickel and/or copper species could be converted into supported metal nanoparticles with a high-temperature hydrogen reduction treatment, which could be reused as catalysts.

Synergetic effect of the interface electric field and the plasmon electromagnetic field in Au-Ag alloy mediated Z-type heterostructure for photocatalytic hydrogen production and CO2 reduction

The challenge of synergistically optimizing different mechanisms limits the further improvement of plasmon-mediated photocatalytic activities. In this work, the CdS/Au-Ag/B-TiO2 photocatalyst, combining a heterojunction structure with localized surface plasmon resonance (LSPR), is prepared by a high-temperature hydrogen reduction and hot solvent method. The novel synergistic interaction at the interface between the electric field and the plasmon electromagnetic field drives effective charge separation with improved photocatalytic efficiency. The Cd3Ti2 catalyst exhibits the highest photocatalytic activity, and the H2 generation efficiency under the full solar spectrum is 15.97 mmol⋅h−1⋅g−1 with good stability as high as 81.3% in 24 h cycles. The CH4 and CO precipitation performances are achieved at 14.2 μmol⋅h−1⋅g−1 and 113.9 μmol⋅h−1⋅g−1, respectively, in the CO2RR process. The design of the synergistic mechanism of the dual electric field offers an important development in the field of plasma-mediated photoredox catalysis.

Comparative study between supported bimetallic catalysts for nitrate remediation in water

Abstract As the population grows and the demand for water rises, the development of efficient and sustainable water purification techniques is becoming increasingly important to ensure access to clean and safe water in the future. The pollution of surface and groundwater by nitrate ( NO 3 − {\text{NO}}_{3}^{-} ) is a growing global concern due to the rise in nitrogen-rich waste released from agriculture and industry. The removal of nitrate ions from aqueous media using bimetallic catalysts loaded on several supports was studied. Multiwalled carbon nanotubes, activated carbon, titanium dioxide, titanium dioxide/multiwalled carbon nanotubes, and Santa Barbara Amorphous-15 were used as supports to synthesize these bimetallic catalysts. The effects of the support type, supported metal, and catalyst reduction method on the nitrate reduction activity in water were investigated. The catalysts were characterized by X-ray diffraction, fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller isotherm, inductively coupled plasma spectroscopy, and field emission gun scanning transmission electron microscope. In terms of nitrate conversion, high-temperature hydrogen reduction of the catalysts was a more effective method of catalyst preparation than NaBH4 reduction. Except for the carbon nanotube-TiO2 composite, pH fixation using CO2 flow improved the efficiency of supported catalysts. The catalysts 1Pd–1Cu/TiO2 and 1Pd–Cu/SBA-15 presented the highest catalytic activity, but the latter was the most selective to nitrogen.

Preparation and properties of Ni-coated WC powder and highly impact resistant and corrosion resistant WC-Ni cemented carbides

WC powders were uniformly coated by Ni nanoparticles through a combined chemical co-precipitation and subsequent high temperature hydrogen reduction strategy (abbreviated as CM-WCN), and then were consolidated by vacuum sintering at 1450 °C for 1 h to obtain WC-Ni cemented carbides. The microstructure and properties of the as-consolidated CM-WCN were investigated. The average grain size of WC in the consolidated CM-WCN was calculated to be in the range of 3.0–3.8 μm and only few pores were observed. A relative density of 99.6%, hardness of HRA 86.5 and bending strength of 1860 MPa were obtained for the CM-WCN-10wt.%Ni, and the highest impact toughness of 6.17 J/cm2 was obtained for the CM-WCN-12wt.%Ni, surpassing those of the hand mixed WC-Ni (HM-WCN) cemented carbides examined in this study and the other similar materials in the literature. CM-WCN cemented carbides possess excellent mechanical properties, due to their highly uniform structure and low porosity that could be ascribed to the intergranular-dominated fracture mode accompanied by a large number of plastic deformation tears of the bonding phase. In addition, the corrosion resistance of CM-WCN was superior to that of HM-WCN at the Ni content of 6-12 wt.%.

Ni-MoO2 nanoparticles heterojunction loaded on stereotaxically-constructed graphene for high-efficiency overall water splitting

The development of heterostructures is an effective way to improve the efficiency of water decomposition. More heterojunction with smaller size is an important strategy to improve the utilization efficiency of non-noble metal active substances and further improve catalytic activity. Herein, the stereotaxically-constructed graphene (SCG) was synthesized by cationic exchange resin and was used as the substrate to prepared Ni-MoO2@SCG nanoparticles heterojunction (Ni is surrounded by MoO2) for the first time by hydrothermal process and annealing method. In urea solution, porous SCG restricted the transition of nanoparticles to microspheres, inhibited the aggregation of active substances, and provided a large number of anchoring sites for nanoparticles. In the process of high-temperature hydrogen reduction, Ni atoms precipitate to form nano-particles and form unique heterojunction particles of about 20 nm with MoO2. The synergistic effect at the interface between Ni and MoO2 significantly enhances the catalytic activity of MoO2. The as-prepared Ni-MoO2@SCG sample exhibits overpotentials of 79.97 mV and 278 mV to reach a current density of 10 mA cm−2 for HER and OER in 1.0 M KOH, respectively. More importantly, it performs stable overall water splitting with a cell voltage of 1.548 V at a current density of 10 mA cm−2 as both cathode and anode. Anchoring nano heterojunction particles on the surface of porous Stereotaxically-constructed graphene provides a new direction for improving the catalytic performance of powder's electrocatalysis.

Inducing the Metal–Support Interaction and Enhancing the Ammonia Synthesis Activity of Ceria-Supported Ruthenium Catalyst via N2H4 Reduction

Metal–support interactions, which are related to electronic metal–support interaction (EMSI), particle morphology, interfacial perimeter, and strong metal–support interaction (SMSI), have a strong impact on the catalytic performance for ceria-supported metal catalysts. High-temperature hydrogen reduction can induce strong metal–support interaction in the Ru/CeO2 catalyst. However, this treatment under severe conditions would lead to the lowering of metallic Ru species because some Ru metal would be covered by Ce suboxides, which is detrimental to the activity of ammonia synthesis that required metallic Ru as active species. Herein, the Ru/CeO2 catalyst was obtained by N2H4 treatment under mild conditions to induce a metal–support interaction between Ru and ceria. The result shows that N2H4 reduction not only results in an enhancement of the electronic metal–support interaction but also causes an increase in the fraction of metallic Ru and the amount of the exposed Ru species. Therefore, a larger amount as well as a lower adsorption strength of H2 would adsorb on the Ru catalyst. As a result, the Ru/CeO2 catalyst with the N2H4 treatment exhibits a relatively higher ammonia synthesis activity. This discovery offers an effective way for the design of ceria-supported metal catalyst by tuning the interaction between metal and ceria.

Improving the ammonia synthesis activity of Ru/CeO2 through enhancement of the metal–support interaction

The metal–support interactions induced by high-temperature hydrogen reduction have a strong influence on the catalytic performance of ceria-supported Ru catalysts. However, the appearance of the strong metal–support interaction leads to covering of the Ru species by Ce suboxides, which is detrimental to the ammonia synthesis reaction that requires metallic species as active sites. In the present work, the interaction between Ru and ceria in the Ru/CeO2 catalyst was induced by NaBH4 treatment. NaBH4 treatment enhanced the fraction of metallic Ru, proportion of Ce3+, content of exposed Ru species, and amount of surface oxygen species. As a result, a larger amount of hydrogen species would desorb by the H2-formation pathway and the strength of hydrogen adsorption would be weaker, weakening the inhibition effect of the hydrogen species on ammonia synthesis. In addition, the strong electronic metal–support interaction aids in nitrogen dissociation. Consequently, Ru/CeO2 with NaBH4 treatment showed higher ammonia synthesis rates than that with only hydrogen reduction.

Preparation of a novel TiO2-graphene 3D framework material for efficient adsorption-photocatalytic removal of micro-organic contaminants from water

Micro-organic contaminants (MCs) have become potential source of aqueous environmental micro-pollution and TiO2 based 3D framework materials show great superiority for effectively overcoming the mass transfer resistance of MCs. Herein, a novel TiO2-graphene 3D framework material (3D TiO2/rGO) was prepared by rapid freezing-drying followed by high temperature hydrogen reduction, in which TiO2 nanowires (TiO2 NWs) and graphene oxide (GO) nanosheets were used as precursors and no organic components and templates were added. The as-prepared 3D TiO2/rGO exhibited significantly enhanced adsorption-photocatalytic activity for low concentration ethenzamide, one of the representative MCs in water. When the adsorbent dosage was 7.5 mg/150 mL, the removal rate of 500 ppb ethenzamide attained 98.5% in 60 min and the photocatalytic rate was 2.2 times of pristine TiO2 NWs and 17.4 times that of TiO2/GO under UV light irradiation, respectively and 99.8% ethenzamide was eliminated in 3 min and the photocatalytic rate was 1.89 times that of pristine TiO2 NWs and 2.83 times that of TiO2/GO under VUV light irradiation, respectively. The joint action of h+, •OH, •O2− and oxygen vacancies contributed to the photocatalytic removal of MCs and the as-prepared TiO2 based 3D framework material shows great potential for highly efficient removal of MCs from water.

Nanosheet-based Nb12O29 hierarchical microspheres for enhanced lithium storage.

Conductive Nb12O29 hierarchical microspheres with nanosheet shells were synthesized based on a hydrothermal process and a high-temperature hydrogen reduction treatment. The obtained materials demonstrated comprehensively good electrochemical properties, including a significant pseudocapacitive contribution, safe operating potential, high reversible capacity, superior initial coulombic efficiency, increased rate capability, and durable cycling stability.

Evaluation of a cleaning method for the tritium environment

Solvents can be used to clean process equipment and piping that will come in contact with tritium containing gas streams; but, the ways in which solvents interact with catalysts is also of interest. This method of cleaning does not account for impurities within the bulk of a material that could be released from catalyst or containment barriers during high temperature operation. One solvent of interest which can be used because of its ability to both degrease and be analyzed for carbon content is Vertrel™ MCA. Tests were performed to determine carbon, sulfur, and chlorine concentrations for two candidate materials which were subjected to Vertrel MCA cleaning. Comparisons with high temperature hydrogen reduction are provided. Solvent analysis, LECO carbon/low sulfur determination testing, and neutron activation testing were performed to provide comparisons. Results show potential risks of using solvent cleaning and analysis as the only method of determining impurity concentration of materials which will be exposed to high temperature testing.

Hydrogen reduction of molybdenum oxide at room temperature

The color changes in chemo- and photochromic MoO3 used in sensors and in organic photovoltaic (OPV) cells can be traced back to intercalated hydrogen atoms stemming either from gaseous hydrogen dissociated at catalytic surfaces or from photocatalytically split water. In applications, the reversibility of the process is of utmost importance, and deterioration of the layer functionality due to side reactions is a critical challenge. Using the membrane approach for high-pressure XPS, we are able to follow the hydrogen reduction of MoO3 thin films using atomic hydrogen in a water free environment. Hydrogen intercalates into MoO3 forming HxMoO3, which slowly decomposes into MoO2 +1/2 H2O as evidenced by the fast reduction of Mo6+ into Mo5+ states and slow but simultaneous formation of Mo4+ states. We measure the decrease in oxygen/metal ratio in the thin film explaining the limited reversibility of hydrogen sensors based on transition metal oxides. The results also enlighten the recent debate on the mechanism of the high temperature hydrogen reduction of bulk molybdenum oxide. The specific mechanism is a result of the balance between the reduction by hydrogen and water formation, desorption of water as well as nucleation and growth of new phases.

Plasma DBD activated ceria-zirconia-promoted Ni-catalysts for plasma catalytic CO2 hydrogenation at low temperature

The activity of ceria-zirconia supported Ni catalysts was measured in a hybrid cold dielectric barrier discharge (DBD) plasma-catalytic process for the hydrogenation of carbon dioxide into methane at low temperatures. Hydrogen DBD plasma treatment at room temperature was chosen as an alternative activation to conventional reduction performed at high temperature. The plasma-catalytic tests performed in the presence of the plasma-treated catalyst clearly show that the hydrogen plasma treatment can replace the high-temperature reduction in hydrogen.

High-temperature hydrogen-reduction process for the preparation of low-oxygen Mo powder from MoO3

In this study, a new high-temperature hydrogen-reduction (HTHR) process was applied for the preparation of a low-oxygen Mo powder whereby MoO3 was used as a raw material. The HTHR-prepared Mo powder showed a low oxygen concentration of 1580 ppm, which was obtained at a reduction temperature of 1500 °C. From the oxidation-resistance test of the low-oxygen Mo powder, the oxidation resistance of the 1500 °C-prepared low-oxygen Mo powder is outstanding. The improved oxidation resistance is derived from the reduced oxygen concentration that can be diffused to the surface where it forms an oxidation layer on the Mo powder. When compared with the conventional hydrogen-reduction process, the HTHR process provided an improved oxidation resistance as well as a low oxygen concentration.

New insight into the heteroatom-doped carbon as the electrode material for supercapacitors

Heteroatom-doped carbon with 4.06% of nitrogen and 10.42% of oxygen was synthesized by pyrolysis of ortho-aminophenol/formaldehyde resin. After high temperature hydrogen reduction, 2.22% of the surface oxygen was removed while the nitrogen content remained almost no change. The specific surface area enlarged from 90m2g−1 to 226m2g−1 and abundant micropores appeared. But we found that the specific capacitance of the reduced carbon as electrode for supercapacitors was decreased nearly 25 %. By measuring the conductivity, surface wettability, and electrochemical impedance, we proposed that (a) surface oxygen played an important role in the capacitive performance, especially the pseudocapacitance; (b) though the doped-oxygen reduced the electronic conductivity, on the other hand, enhanced the surface wettability and ion diffusion rate of the carbon electrodes.

Controlled surface properties of Au/ZSM5 catalysts and their effects in the selective oxidation of ethanol

The catalytic activity of gold supported on ZSM5 (Au/ZSM5) was investigated for the selective oxidation of ethanol in the presence of excess oxygen. Au/ZSM5 catalyst pretreated by nonthermal O2 plasma method showed the best oxidative activity compared to low-temperature calcination in air and high-temperature reduction in hydrogen atmosphere. Results from microscopy and X-ray diffraction characterizations proved that plasma pretreatment afforded a small Au particle size and a uniform dispersion of Au nanoparticles on ZSM5 surfaces. Characterization results further demonstrated that the residual ammonia adsorbed on ZSM5 surfaces during the precipitation can be oxidized to nitrate ions by nonthermal O2 plasma treatment, while it converted to NO+ by low-temperature oxygen calcination and was completely removed by high-temperature hydrogen reduction. Dissimilar surface/interface properties caused the tremendously different interaction between gold nanoparticles and zeolite support, and consequently the catalytic performances in ethanol oxidation. In particular, under the nonthermal O2 plasma pretreatment, the formed NO3− species lowered the acidity of ZSM5 surfaces as well as anchored the Au nanoparticles, resulting in nearly 100% selectivity toward selective oxidation instead of acid-catalyzed reactions even under high reaction temperature.

Interrelationships of defects, nitride modification and excess nitrogen in nitrided Fe-4.75 at.% Al alloy

Abstract In order to investigate the influence of crystal-lattice defects (dislocations) and of a relatively large Al content on the development of different modifications of aluminium nitride in ferrite and on the associated uptake of excess nitrogen, recrystallised and cold rolled specimens of Fe-4.75 at.% Al alloy were subjected to gaseous nitriding treatments at 550 °C employing a nitriding potential of 0.104 atm−1/2. In contrast with earlier results for an alloy of relatively low Al content, in both the nitrided cold rolled and the nitrided recrystallised specimens nanosized AlN precipitates developed, both of the metastable, cubic, NaCl type modification and of the stable, hexagonal, wurtzite type modification, which exhibit with the ferrite matrix a Baker–Nutting orientation relationship and a Pitsch–Schrader orientation relationship, respectively. A high-temperature hydrogen reduction treatment confirmed the stoichiometry of the precipitated nitrides as AlN. The fraction of cubic, NaCl type, AlN is significantly higher in the nitrided cold-rolled specimens because of its preferential development along dislocations. As compared to the nitrided recrystallised specimens, the higher dislocation density and the higher amount of cubic AlN in the nitrided cold-rolled specimens leads to a larger amount of excess nitrogen in the nitrided cold-rolled specimen.

High-temperature calcination and hydrogen reduction of rutile TiO2: A method to improve the photocatalytic activity for water oxidation

Rutile titania (TiO2) is an efficient photocatalyst for oxidizing water to O2. The photocatalytic activity of particulate rutile for water oxidation was significantly improved by H2 reduction at 700°C, after calcination at 1100°C. The improved activity was due to an increase in crystalline size during calcination, and an increase in conduction band electron concentration by the creation of oxygen vacancies. In contrast to the consideration that oxygen vacancy increases the recombination of electron and holes, the hydrogenated TiO2 exhibited high apparent quantum efficiency for O2 evolution, 41% under irradiation at 365nm. It was found that H2 treatment improved the photocatalytic activity per unit of surface area not only for O2 evolution but also for H2 evolution and acetic acid decomposition. The effect of H2 reduction treatment was obtained only if the rutile particle was previously calcined at temperatures higher than 1000°C. This suggests that space charge layer in large crystalline particles is involved in the activation mechanism of hydrogenated rutile TiO2 particles.