High-temperature Reduction Treatment Research Articles

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.

Advanced Etching Techniques of LiNbO3 Nanodevices.

Single LiNbO3 (LNO) crystals are widely utilized in surface acoustic wave devices, optoelectronic devices, and novel ferroelectric memory devices due to their remarkable electro-optic and piezoelectric properties, and high saturation and remnant polarizations. However, challenges remain regarding their nanofabrication that hinder their applications. The prevailing etching techniques for LNO encompass dry etching, wet etching, and focused-ion-beam etching, each having distinct merits and demerits. Achieving higher etching rates and improved sidewall angles presents a challenge in LNO nanofabrication. Building upon the current etching researches, this study explores various etching methods using instruments capable of generating diverse plasma densities, such as dry etching in reactive ion etching (RIE) and inductively coupled plasma (ICP), proton exchange-enhanced etching, and wet chemical etching following high-temperature reduction treatment, as well as hybrid dry and wet etching. Ultimately, after employing RIE dry etching combined with wet etching, following a high-temperature reduction treatment, an etching rate of 10 nm/min and pretty 90° sidewall angles were achieved. Furthermore, high etching rates of 79 nm/min with steep sidewall angles of 83° were obtained using ICP dry etching. Additionally, using SiO2 masks, a high etching rate of 108 nm/min and an etching selectivity ratio of 0.86:1 were achieved. Distinct etching conditions yielded diverse yet exceptional results, providing multiple processing paths of etching for the versatile application of LNO.

Rudomain Iron Ore Treatment by High-Temperature Reduction

The purpose of this study was to conduct experiments comprising the high-temperature reduction treatment of commercially produced iron ore fines and lumps aimed at increasing the use value of the ore. The analysed ore was Ukrainian iron ore sold under the Rudomain commercial name, mined from a bed located in the southern part of the Saksagan region (Kryvyi Rih, Ukraine). The study describes in detail the basic physical, chemical, and physico-chemical properties of the analysed ore, and it comprised a thermodynamic analysis, which is typically used as the basis for defining reduction conditions. The Ukrainian ore—Rudomain—exhibited a lower total Fe content (58.20 wt%) and, by contrast, the highest SiO2 content (13.40 wt%), whereas SiO2 is present in this type of ore not only in form of silica (SiO2) but also in form of hydrated iron silicate (Fe3Si2O5(OH)4), i.e., the form of iron that is the most difficult to reduce. In the study, tests of thermal stability and thermal shock stability were carried out in various conditions, while the hardened pellets were thermally stable up to temperatures of 950 °C. The results of the performed experiments in high-temperature reduction of Rudomain iron ore were then compared with the results obtained with two other types of iron ores, in particular Krivbas and Carajas. Krivbas and Carajas ores show higher degrees of reduction and degrees of metallization than Rudomain ore. High-temperature experiments in thermal stability and carbothermic reduction have brought favourable information that is useful for the treatment of lower-grade ores with higher contents of SiO2, while Rudomain iron ore exhibited a rather good potential for effective metallisation.

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.

NiMo solid-solution alloy porous nanofiber as outstanding hydrogen evolution electrocatalyst

Developing a high-efficiency hydrogen evolution reaction (HER) electrocatalyst for the large-scale production of hydrogen is essential but challenging. In this study, we used NiMo solid-solution alloy porous nanofibers to develop a robust HER electrocatalyst through electrospinning, oxidization, and high-temperature reduction treatment. In 1 M KOH electrolyte, the fabricated NiMo solid-solution alloy porous nanofibers exhibited higher HER activity than Ni nanofibers, which required a low overpotential of 69, 208, and 300 mV at 100, 500, and 1000 mA cm−2, respectively, and had outstanding durability at 100 mA cm−2 over 60 h. We developed a promising candidate for a high-efficiency HER electrocatalyst, and our findings provided valuable information for fabricating highly robust alloy-based electrocatalysts.

Effect of calcination temperature on cation arrangement and oxygen storage/release capacity in cation-ordered Ce oxides

Cation-ordered κ-Ce2Zr2O8 exhibits a high oxygen storage/release capacity (OSC); however, the cation-ordered phase transforms into a cation-disordered phase during high-temperature calcination, thereby decreasing the OSC. Ti substitution in Ce0.5Zr0.4Ti0.1O2 facilitates the formation of a cation-ordered structure compared to the conventional κ-Ce2Zr2O8, which requires a high-temperature reduction treatment to obtain an ordered structure. In this study, the thermal stabilities of Ce0.5Zr0.4Ti0.1O2 and κ-Ce2Zr2O8 are compared. Powder X-ray diffraction reveals that the onset temperature for the cation disordering of Ce0.5Zr0.4Ti0.1O2 (700 ​°C) is lower than that of κ-Ce2Zr2O8 (1000 ​°C). Furthermore, scanning transmission electron microscopy suggests that the cation disordering of Ce0.5Zr0.4Ti0.1O2 is induced by the high mobility of Ti within the crystal lattice; thus, the OSC decreases and κ-Ce2Zr2O8 exhibits a higher thermal stability than Ce0.5Zr0.4Ti0.1O2. Hence, Ti substitution facilitates the formation of a cation-ordered structure; however, it stimulates cation disordering that lowers the OSC and reduces thermal stability.

Atomic-level understanding on the evolution behavior of subnanometric Pt and Sn species during high-temperature treatments for generation of dense PtSn clusters in zeolites

To achieve high-loading of stable subnanometric metal clusters on solid carriers is a challenge since those small metal clusters have strong tendency to sinter into larger nanoparticles. Development of facile synthesis methodologies to obtain subnanometric metal catalysts with high metal loading and high stability against sintering at high temperature (>500 °C) in reductive atmosphere (such as H2) is critical for the practical applications. In this work, we will present and discuss the generation of high-loading (~1.4 wt%) subnanometric Pt clusters confined in the sinusoidal channels of MFI zeolite, on the basis of the atomic-level understanding on the evolution of Pt and Sn species during high-temperature oxidation–reduction treatments. It will be shown that the structural evolution of Pt and Sn species is dependent on the post-synthesis treatments. The Pt particles on the external surface can disintegrate into subnanometric Pt species and get stabilized in the zeolite channels during high-temperature calcination in air while Sn species migrate from surface region to internal region during high-temperature reduction treatment at 650 °C. The resultant material containing bimetallic PtSn clusters confined in the 10MR sinusoidal channels of the purely siliceous MFI zeolite show excellent catalytic activity and stability, as demonstrated for dehydrogenation of light alkanes at high reaction temperature.

Correlation between surface chemistry and morphology of PtCu and Pt nanoparticles during oxidation-reduction cycle

Process conditions during catalytic reactions induce significant changes in surface chemistry and structure of bi- (mono) metallic nanoparticles leading to their deactivation, and this can ultimately affect the reactions long-term performance. Here PtCu and Pt model nanoparticles are prepared by microwave synthesis and characterized by X-ray diffraction (XRD). Surface chemical and morphological changes of the nanoparticles during high-temperature oxidation and reduction treatments cycle are correlated by near in situ X-ray photoelectron spectroscopy (XPS) and ex situ transmission electron microscopy (TEM) - energy-dispersive X-ray spectroscopy (EDS) studies. At 300 °C the surface atomic composition of the PtCu nanoparticles switches reversibly upon the cycle and at the same time their morphology and composition are maintained. At 400 °C, the surface atomic composition does not fully restore and, while the shape is maintained, the size and composition are not. This occurs by a mechanism of Cu leaching out from the nanoparticles. These data delineate potential operating conditions for stable PtCu nanocatalysts.

Thermostable Rh Metal Nanoparticles Formed on Al2O3 by High-Temperature H2 Reduction and Its Impact on Three-Way Catalysis

The influence of high-temperature H2 reduction treatment on Rh and Pd catalysts supported on Al2O3 was studied in relation to thermal aging in air. After air-aging at ≥900 °C, the Rh/Al2O3 catalyst was more strongly deactivated compared with the Pd/Al2O3 catalyst. As has been widely recognized, the solid-state reactions between Rh oxide and Al2O3 decreased the active surface area and stabilized inactive Rh3+ species. The activity was restored by the postreduction treatment with 20% H2/He at 200 °C, whereas a striking enhancement of activity was achieved by the reduction at 800–1100 °C, where substantial deactivation occurred for Pd/Al2O3. A mechanistic interpretation is proposed based on local structural characterization, which explains these contrasting thermal behaviors. The high-temperature reduction treatment produced active and thermostable Rh metal nanoparticles, which were highly dispersed on Al2O3. The observed dispersion (as high as ∼20% after reduction at 1000 °C) is among the highest for suppor...

Effect of iron precursor on the activity and stability of PtFe/C catalyst for oxygen reduction reaction

The stability of iron species in PtFe/C catalyst in acidic medium is a grand challenge to maintain a high activity and stability for oxygen reduction reaction (ORR) via the geometric and electronic effect. Herein, we report that the use of ammonium ferric citrate with a high thermal stability, instead of iron nitrate, greatly improves the stability of iron species in PtFe/C catalyst in acidic medium after high-temperature reduction treatment. The resulting PtFe/C catalyst with ammonium ferric citrate as iron precursor shows much improved activity and stability for ORR than the counterpart using iron nitrate due to enhanced proportion of PtFe alloy in the PtFe/C catalyst.

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.

Effect of Ti3+ Ions and Conduction Band Electrons on Photocatalytic and Photoelectrochemical Activity of Rutile Titania for Water Oxidation

Although TiO2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti3+ ions on its photocatalytic activity is not fully understood. In this study, we investigated the effects of high-temperature calcination and H2 reduction treatment on the water oxidation activity of rutile TiO2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO2 owing to strong oxidation, but its initial activity was restored by H2 treatment at above 500 °C. Electron spin resonance (ESR) spectra showed that the high-temperature calcination created O•– radicals (trapped hole in oxygen lattice site), while the H2 reduction treatment created Ti3+ ions (trapped electron in titanium lattice site) with oxygen vacancies. Diffuse reflectance ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy indicated an increase in the amount of electrons in shallow traps and the conduction band with H2 treatment temperature. Measurements of the sheet...

Stabilizing cobalt catalysts for aqueous-phase reactions by strong metal-support interaction

High-temperature calcination and reduction treatments of cobalt particles (17–20nm) supported on TiO2 create cobalt particles covered with a TiOy layer. The layer thickness ranges from 2.8 to 4.0nm. These phenomena, commonly called strong metal-support interaction (SMSI), can be used to improve the catalyst stability and change the catalyst selectivity. For example, non-overcoated cobalt catalysts leached during aqueous-phase hydrogenation (APH) of furfuryl alcohol, losing 44.6% of the cobalt after 35h time-on-stream. In contrast, TiOy-overcoated cobalt catalysts did not lose any measurable cobalt by leaching and the cobalt particle size remained constant after 105h time-on-stream. The 1,5-pentanediol selectivity from furfuryl alcohol hydrogenolysis increased with increasing TiOy layer thickness. The stabilized cobalt catalyst also had high yields for APH of xylose to xylitol (99%) and APH of furfural to furfuryl alcohol (95%). These results show that the SMSI effect produces a catalyst with a similar structure as catalysts prepared by atomic layer deposition, thereby opening up a cheaper and more industrially relevant method of stabilizing base-metal catalysts for aqueous-phase biomass conversion reactions. In addition, the SMSI effect can be used to tune catalyst selectivity, thus allowing the more precise atomic scale design of supported metal catalysts.

Semi-Metallic TiO2 Nanotubes: Growth, Properties and Applications

Self organized nanotubular structures of transition metal oxides grown on their metallic substrates, especially titanium, have attracted great scientific and technological interest due to the possibility to exploit their functional properties (such in photo-catalysis, electro-catalysis, solar energy conversion, DSSCs and as host for Li storage) in nanotubular morphology. In contrast to the conventional way to grow anodic TiO2 nanotubes, which is an optimized electrochemical treatment of Ti in fluoride containing electrolyte (for an overview see [1]) this work reports on the growth of nanotubes by so-called RBA (rapid breakdown anodization).[2] This nanotube growth-process is in comparison extremely fast (seconds instead of hours) and seems to be initiated by a pit. This leads to surface coatings consisting of dense packed bundles of nanotubes with very high aspect ratios (tube length up to 50 micrometer, tube diameter of ~40 nanometer). In addition, this method provides a new route for an insitu electrochemical doping during the growth of TiO2 nanotubular layers with various transition metals to adjust the electronic levels in the material which is important for charge transfer reactions through the semiconductor/electrolyte interface [3]. The second part of the presentation will demonstrate that alternatively the electronic and optical properties [4] of these TiO2 nanotubes can be significantly modified, showing semi-metallic behavior by a high temperature reduction treatment in acethylene or ammonia creating a suitable platform for highly conductive surface area electrodes which is important i.e. sensing, methanol oxidation or supercap applications [5]

Unusual Redox Behavior of Rh/AlPO4 and Its Impact on Three-Way Catalysis

The influence of the redox behavior of Rh/AlPO4 on automotive three-way catalysis (TWC) was studied to correlate catalytic activity with thermal stability and metal–support interactions. Compared with a reference Rh/Al2O3 catalyst, Rh/AlPO4 exhibited a much higher stability against thermal aging under an oxidizing atmosphere; further deactivation was induced by a high-temperature reduction treatment. In situ X-ray absorption fine structure experiments revealed a higher reducibility of Rh oxide (RhOx) to Rh, and the metal showed a higher tolerance to reoxidation when supported on AlPO4 compared with Al2O3. This unusual redox behavior is associated with an Rh–O–P interfacial linkage, which is preserved under oxidizing and reducing atmospheres. Another effect of the Rh–O–P interfacial linkage was observed for the metallic Rh with an electron-deficient character. This leads to the decreasing back-donation from Rh d-orbitals to the antibonding π* orbital of chemisorbed CO or NO, which is a possible reason for ...

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.

Ultrafast Anodic Growth of TiO2-Nanotubes with Semi-Metallic Properties

Self organized nanotubular structures of transition metal oxides grown on their metallic substrates, especially titanium [1], have attracted great scientific and technological interest due to the possibility to exploit their functional properties (such in photo-catalysis, electro-catalysis, solar energy conversion and as host for Li storage) in a nanotubular morphology. In contrast to the conventional way to grow anodic TiO2 nanotubes, which is an optimized electrochemical treatment of Ti in fluoride containing electrolyte (for an overview see [2]) this work reports on the growth of nanotubes by so-called RBA (rapid breakdown anodization).[3] This nanotube growth-process is in comparison extremely fast (seconds instead of hours) and leads to surface coatings consisting of dense packed bundles of nanotubes with very high aspect ratios (tube length up to 50 micrometer, tube diameter of ~40 nanometer). In addition, this method provides a new route for an insitu electrochemical doping during the growth of TiO2 nanotubular layers with various transition metals to adjust i.e. the photocatalytic activity [4]. The second part of the presentation will demonstrate that alternatively the electronic and optical properties [5] of these TiO2 nanotubes can be significantly modified, showing semi-metallic behavior by a high temperature reduction treatment in acethylene or ammonia creating a suitable platform for highly conductive surface area electrodes which is important i.e. sensing applications. [6] Literature: [1] V. Zwilling, M. Aucouturier, E. Darque-Ceretti, Electrochim. Acta, 45, 921 (1999).[2] S. Berger, R. Hahn, P. Roy, P. Schmuki Phys. Status Solidi B247, 10, 2424 (2010).[3] R. Hahn, J. M. Macak, P. Schmuki, Electrochem. Commun.,9, 947 (2007).[4] M. Stark, R. Hahn, M.S. Killian, P. Schmuki Catal. Sci. Technol., 3, 1765 (2013).[5] R. Hahn, F. Schmidt-Stein, J. Salonen, S. Thiemann, Y.Y. Song, J. Kunze,V.-P. Lehto, P Schmuki, Angewandte Chemie - International Edition 48 (39), 7236 (2009).[6] R. Hahn et al submitted

Photodeposition of Pt nanoparticles on TiO 2–carbon xerogel composites

Carbon xerogels synthesized from polycondensation of resorcinol with formaldehyde, having specific surface areas in the range 650 to 990 m 2 g −1 and variable degrees of surface oxidation, are used to prepare TiO 2–carbon xerogel composites by sol–gel methods. These composite materials are used to support Pt nanoparticles (5 wt.%) by the photodeposition technique. After a high temperature reduction treatment at 773 K, the obtained materials were characterized in order to assess the interactions between the phases Pt, TiO 2 and carbon xerogel. It is observed that the carbon xerogel acts as an adhesive agent of the TiO 2 and Pt particles, enhancing the interaction between the metal and the composite support.

Effect of preparation atmosphere of Pt–SnOx/C catalysts on the catalytic activity for H2/CO electro-oxidation

Pt–SnOx/C catalysts were prepared in a polyol process under Ar, sequential Ar and air, and air atmospheres in combination with a high temperature reduction treatment. The composition, structure, morphology and oxidation state of the prepared catalysts were characterized by Inductively Coupled Plasma-Atom Emission Spectroscopy, X-ray diffraction, scanning transmission electron microscopy and X-ray photoelectron spectroscopy. The electrochemical activities were evaluated by CO stripping voltammetry and single cell test in combination with in situ IR reflection absorption spectroscopy (IRRAS). The polyol-synthesized Pt–SnOx/C catalysts prepared under different atmospheres had a similar bulk composition, particle size and lattice parameter, however, the Pt–SnOx/C catalyst prepared under Ar atmosphere possessed a greater proportion of Sn(II) species than the other Pt–SnOx/C catalysts. Electrochemical and in situ IRRAS measurements indicated that the Pt–SnOx/C catalyst under Ar atmosphere had the greatest CO tolerance in proton electrolyte fuel cell among the Pt–SnOx/C catalysts.

Nanoscale compositional and structural evolution in ceria zirconia during cyclic redox treatments

We report changes in nanoscale composition and structure in individual ceria zirconia nanoparticles (nominal composition Ce0.7Zr0.3O2), before and after redox treatment, determined by energy dispersive X-ray (EDX) analysis using scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM) imaging. According to the chemical analysis of individual nanoparticles, the redox treatments promote the formation of more compositionally homogenous ceria zirconia solid solution nanoparticles, resulting in a uniform low-temperature reduction. Local pyrochlore-type cation ordering in individual nanoparticles was also observed after the high temperature reduction step. For nanoparticles with 70% ceria content, redox cycling treatment was critical to obtain material which exhibits a strong low temperature reducibility. The origin of significant X-ray diffraction line broadening and particle sintering mechanism in the solid solution nanopowders are discussed based on compositional and structural variation within and between individual nanoparticles and oxygen vacancies formed during high temperature reduction treatment.