Preferential Oxidation Research Articles

Recent Advances in Regulating Ceramic Monolithic Catalyst Structure for Preferential Oxidation of CO in H2.

Preferential oxidation of CO (CO-PROX) has tremendous significance in purifying hydrogen for fuel cells to avoid catalyst poisoning by CO molecules. Traditional powder catalysts face numerous challenges, including high pressure drop, aggregation tendency, hotspot formation, poor mass and heat transfer efficiency, and inadequate thermal stability. Accordingly, ceramic monolithic catalysts, known as their excellent thermal stability, high surface area, and superior mass and heat transfer characteristics, are gaining increasing research attention. This review examines recent studies on ceramic monolithic catalysts in CO-PROX, placing emphasis on the regulation of active sites (e.g., precious metals like Pt and Au, and non-precious metals like CuO and CeO2), monolith structures, and coating strategies. In addition, the structure-catalytic performance relationships, as well as the potential and limitations of different ceramic monolithic catalysts in practical application, are discussed. Finally, the challenges of monolithic catalysts and future research prospects in CO-PROX reactions are highlighted.

Sustainable sonoprocess for synthesizing γ-Ga2O3/In3Sn core–shell submicron particles via acoustic emulsification and oxidation of molten EGaInSn at room temperature

This study investigated the sustainable room-temperature synthesis of In3Sn/γ-Ga2O3 core–shell particles via an acoustic route using molten eutectic Ga–In–Sn alloy (EGaInSn). Sonication was used for the emulsification and oxidation steps. During the emulsification step, the sonication of molten EGaInSn in ethanol (EtOH) at 45 kHz facilitated the formation of the smallest EGaInSn particles (average diameter, Dav = 782 nm). In terms of EGaInSn particle size, 45 kHz sonication was suitable for emulsification of molten EGaInSn and ethanol system than 24 kHz sonication.During the oxidation step, the preferential oxidation of Ga in the EGaInSn particles occurred via sonication in a solution of EtOH and hydrazine monohydrate (N2H4·H2O). This selective oxidation of Ga on the surface of the EGaInSn particles resulted in the formation of In3Sn/γ-Ga2O3 core–shell particles via sonication at 45 kHz and room temperature.The entire process eliminated the need for dispersants and high-temperature treatments. Additionally, the process did not generate waste fluid containing counter anions, such as chloride anions. This sustainable sonochemical method offers a carbon–neutral approach for synthesizing functional nanocomposites with improved safety, simplicity, and energy efficiency.

Revolutionizing multifunctional electrolyte additive design and synthesis for high-voltage nickel-rich batteries in diverse climates

The high-voltage LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode presents a promising avenue for advancing the energy density of lithium-ion batteries. Nevertheless, current limitations primarily arise from electrolyte systems unable to meet stringent criteria, including stable performance under elevated temperatures and efficient discharge at low temperatures. In this study, we address these challenges by designing a molecular structure incorporating boron oxygen functional groups, phenyl-, and fluorine moieties to simultaneously enhance ion conductivity and thermal stability under extreme voltages. A multifunctional electrolyte additive, 3, 4, 5-trifluorophenylboronic anhydride (TFPBA), has been successfully developed via synthesis. TFPBA plays a pivotal role in promoting the formation of a hierarchical cathode electrolyte interphase (CEI) through its preferential decomposition and oxidation processes. This CEI features a mechanically stable inner layer enriched with fluorophenyl- groups, crucial for high-temperature cycling, alongside an outer layer concentrated with BxOyFz complexes that significantly improve Li+ conductivity, ensuring superior performance under low-temperature conditions and at elevated rates. Moreover, TFPBA acts as an HF scavenger within the electrolyte, mitigating corrosive effects on the cathode surface, thereby curtailing CEI reformation and the transition metal dissolution. The Li/NCM622 half cells with TFPBA exhibit impressive capacity retention rates of 92.58%, 80.09%, and 81.77% after extensive cycling at temperatures of −20 °C, 25 °C, and 55 °C, respectively. Moreover, NCM622/graphite full cells with TFPBA achieve a remarkable capacity retention of 97.97% after 150 cycles at room temperature. Our findings highlight the exceptional electrochemical performance of NCM622 cathodes at elevated voltages across a wide temperature range, providing valuable insights for the development of advanced electrolytes in high-demand applications.

Rational design of highly corrosion-resistant chromium hydroxide coupled amorphous-crystalline heterostructure to achieve efficient and stable seawater oxygen evolution

The pressing issues of global energy scarcity and water shortages can be substantially mitigated through the electrolysis of seawater. In this study, we synthesized a unique lamellar amorphous-crystalline heterostructure electrocatalyst by integrating the amorphous chromium hydroxide (Cr-OH) with the crystalline NiFe layered double hydroxides (NiFe LDH). The resulting Cr-OH/NiFeLDH@NF exhibited an exceptionally low oxygen evolution reaction (OER) overpotential and strong corrosion resistance. The development of the crystalline-amorphous heterointerface significantly influenced electronic coupling and enhanced electron transfer rates. Through the methanol response mechanism, it was demonstrated that the Cr-OH enhanced the catalyst’s corrosion resistance in chloride-rich environments and significantly extended its durability in seawater. Operative Raman spectroscopy confirmed that the high OER activity and corrosion resistance of Cr-OH/NiFeLDH@NF originated from the preferential oxidation of the outer Cr-OH layer, which promoted the electrochemical oxidation of Ni2+ to Ni4+, and the stable Cl− resistant environment formed by the in-situ generated and dissolved CrO42−.

Design of corrosion-resistant alloys for preventing oxidation-induced nanoscale Cr-depletion by inclusion engineering

Due to the preferential oxidation of Cr in various corrosion-resistant alloys, Cr-depletion extensively forms around the oxides and impairs passivation ability. This harmful effect has been widely recognized, but available solutions are limited. Here, to prevent such oxidation-induced Cr-depletion in corrosion-resistant alloys, we proposed a general strategy based on inclusion engineering using oxygen-preferred elements in alloy design. The inclusion-Cr depletion-passivation relationship was systematically studied in 3.5 wt% NaCl using commercially cast original, Si-modified, Al-modified, and Ti-modified CoCrFeMnNi alloys, in which the prominent inclusions were identified as MnCr2O4, MnO-SiO2, AlN, and TiN, respectively. Unlike the original alloy, all modified alloys prevented Cr-depletion with improved corrosion resistance; however, the Si-modified alloy exhibited the most remarkable improvement due to few nitride by-products. Thermodynamic analysis on the free energy changes of potential reactions revealed that all the additives were preferential to be oxidized over Cr, whereas Al and Ti nitridation reactions have the most negative free energies compared to all oxidation reactions, which validated experimental results. From experiments and thermodynamics, an effective method was proposed, and its application was discussed with three newly-designed alloys prepared using arc melting. We believe the proposed strategy can be widely used in designing alloys to prevent oxidation-induced Cr-depletion.

Characterization of microstructure and oxidation behavior of Al modified MoSi2 coating on Nb-Si based alloy

The aim of this work is to investigate the effects of Al on the microstructure formation and oxidation behavior of MoSi2 coating on Nb-Si based alloy by slurry sintering. The results indicate that the addition of a small amount of Al has little effect on the formation of MoSi2 coating. The phase constituent and structure of the inter-diffusion zone (IDZ) at the interface change significantly with the increase of Al content. The Al2O3 formed during the coating preparation process can hinder the diffusion of Si and Fe, thus suppressing formation of Mo5Si3 and Nb4Fe3Si5 layers in IDZ of the coatings with higher Al, but instead forming Fe-rich (Ti,Nb)5Si4 layer and (Nb,X)5Si3 layer. The higher amount of Al2O3 will increase the sintering resistance between the Mo and Si, resulting in a high porosity of the MoSi2 coating. By comparison, the 0.1Al coating exhibits the best oxidation resistance at 1250 °C. The preferential oxidation of Al promotes the formation of SiO2, which is conducive to the rapid filling of pores in the lower part of MoSi2 coating, forming a dense mixed layer of MoSi2+Mo5Si3+SiO2. However, adding a higher content of Al reduces the oxidation resistance of the coating.

Construction of Y(Al)SixOy protective layer on SiC ceramic substrate through thermal diffusion

As the promising candidates for hot section components used in gas turbines, SiCf/SiC composites suffer severe gas steam/melt corrosion, so their long-term stability depends on the surficial environmental barrier coatings. Here, we provide an alternative method to construct the protective structure by thermal diffusion. Firstly, Y-Al diffusants is diffused into the SiC substrate, forming a Y-Al diffusion layer. Then the diffusion layer can transform to a corrosion-resistant Y(Al)SixOy layer after oxidative treatment. The compositions and morphologies of the as-received diffusion layer and the Y(Al)SixOy layer were contrasted. Results show that a Y(Al)SixOy protective layer, which thickens with the extension of oxidative treatment time and the increase of Y/Al ratio, can be formed on the SiC surface. Concentration gradient caused by preferential oxidation of Y-Al prompts the Y-Al diffusing towards surface. Meanwhile, the oxidation of Y-Al makes the Y(Al)SixOy layer denser and thicker, and finally form the Y(Al)SixOy protective layer.

Ultra-pure hydrogen from chemical looping preferential oxidation of CO over a Cu–O–Ce based dual function material

Hydrogen from hydrocarbon reforming contains small amounts of CO, necessitating further purification for its use in fuel cell applications. In this work, a novel chemical looping preferential oxidation (CL-PROX) process is proposed for efficient hydrogen purification. Enabled by a Cu–O–Ce based dual function material (DFM), the process uniquely combines both catalytic CO preferential oxidation and in-situ immobilization. This results in near-complete CO conversion and < 20 ppm COx concentration with hydrogen recovery of 97 % in the purification step. Experimental characterizations indicate that CO is converted into formate and carbonate on the DFM, which can easily decompose to CO2 during the regeneration step. The interaction between copper and ceria species increased the reducibility of the DFM, as well as the CO activation and immobilization. Facile and reversible transformations between Cu2+/Ce4+ and Cu+/Ce3+ were observed throughout the reduction and re-oxidation stages. This work suggests an intensification strategy for selective hydrogen purification, and provides insights into the redox chemistry of Cu–O–Ce DFMs.

Flame ignition mechanism of magnesium alloys controlled by oxide films based on the oxidation behaviors of Al, Nd and Y

Oxide films hinder diffusion and resist external forces, which determines the flame ignition mechanism of magnesium alloys. The effects of the continuity, compactness and mechanical properties of oxide films on the ignition mechanism were analyzed, by investigating the flame ignition behaviors of AZ80 (ZM5), EZ30K (ZM6) and WE43 Mg alloys. The results show that the rupture of the oxide films caused by liquid gravity was the key to causing ignition. According to thermodynamic calculations, compared with Mg, Al cannot be preferentially oxidized; while Nd can be preferentially oxidized through significant enrichment, resulting in a discontinuous Nd2O3 inner layer in the ZM6 alloy; in contrast, Y has a strong preferential oxidation ability, which gives the WE43 alloy a continuous Y2O3 inner layer and self-healing ability. In addition, the oxide film of the ZM5 alloy is loose and has poor mechanical properties, so it cannot effectively hinder diffusion and resist liquid gravity. Differently, the oxide films of the ZM6 and WE43 alloys are dense and have better mechanical properties, leading to higher ignition temperatures and longer ignition times. In addition, a criterion was proposed to predict the ignition time based on the law of energy conservation, and it was simplified to predict the ignition temperature. The errors between the predicted and measured values are within 11%.

Oxidation behavior of harmonic structured high-entropy cantor alloy

Present study deals with the oxidation properties of the harmonic structured high-entropy Cantor (CoCrFeMnNi) alloy at 600 °C, 700 °C, and 800 °C in zero-grade air. The microstructure of the Cantor alloy is modified to a harmonic distribution with grain sizes at shell of ∼2 μm and core of ∼20 μm. The oxidation behavior of the harmonic one is compared with its non-harmonic counterpart and as-received 304L stainless steel. Both the high-entropy Cantor alloys show inferior oxidation resistance than the 304L stainless steel in this temperature range. Studies show that an Mn-rich oxide scale forms on the surface of the Cantor alloys, whereas a Cr-rich oxide scale forms on the 304L stainless steel surface. The high Mn content in the Cantor alloys deteriorates their oxidation resistance. However, the harmonic structure improves the oxidation resistance in the Cantor alloy due to the preferential oxidation of the shell region and the formation of a more compact oxide scale than its non-harmonic counterpart.

In situ and Operando Characterisation in the Preferential Oxidation of Carbon Monoxide over Base Metal Oxide Catalysts: A Review

AbstractPolymer electrolyte membrane fuel cells (PEMFCs) are the core technology of the steadily growing hydrogen (H2) economy as they can convert chemical energy, in the form of H2, to electrical energy. If the H2 is derived from (green) hydrocarbons, via steam reforming and the water‐gas‐shift reaction, then it would contain small amounts of carbon monoxide (CO, 0.5–2 %), among other gases. CO poisons the platinum‐based anode catalyst in the PEMFC, and the current recommendation is to decrease its concentration to below 0.01 % (or 100 ppm) in the H2‐rich PEMFC feed. The preferential oxidation of CO (CO‐PrOx) is a promising strategy for decreasing the CO concentration, and base metal oxide catalysts have shown great potential in this regard. However, such catalysts tend to undergo physicochemical changes that cause undesirable catalytic activity and selectivity changes. This review discusses the different base metal oxide catalysts that have been evaluated in CO‐PrOx, while paying special attention to the various in situ and operando techniques that have been used to monitor the physicochemical changes of base metal oxides during operation. We conclude the review by highlighting the recent and possible future attempts of circumventing the undesired physicochemical changes of base metal oxides during CO‐PrOx.

Differential Mechanisms of Microbial As(III) and Sb(III) Oxidation and Their Contribution to Tailings Reclamation.

Mine tailings are extremely oligotrophic environments frequently contaminated with elevated As and Sb, making As(III) and Sb(III) oxidation potentially important energy sources for the tailing microbiome. Although they have been proposed to share similar metabolic pathways, a systemic comparison of the As(III) and Sb(III) oxidation mechanisms and energy utilization efficiencies requires further elucidation. In this study, we employed a combination of physicochemical, molecular, and bioinformatic analyses to compare the kinetic and genetic mechanisms of As(III) and Sb(III) oxidation as well as their respective energy efficiencies for fueling the key nutrient acquisition metabolisms. Thiobacillus and Rhizobium spp. were identified as functional populations for both As(III) and Sb(III) oxidation in mine tailings by DNA-stable isotope probing. However, these microorganisms mediated As(III) and Sb(III) oxidation via different metabolic pathways, resulting in preferential oxidation of Sb(III) over As(III). Notably, both As(III) and Sb(III) oxidation can facilitate nitrogen fixation and phosphate solubilization in mine tailings, with Sb(III) oxidation being more efficient in powering these processes. Thus, this study provided novel insights into the microbial As(III) and Sb(III) oxidation mechanisms and their respective nutrient acquisition efficiencies, which may be critical for the reclamation of mine tailings.

MOF-derived high oxygen vacancies CuO/CeO2 catalysts for low-temperature CO preferential oxidation

The CO preferential oxidation reaction (CO-PROX) is an effective strategy to remove residual poisonous CO in proton exchange membrane fuel cells, in which oxygen vacancies play a critical role in CO adsorption and activation. Herein, a series of CuO/CeO2 catalysts derived from Ce-MOFs precursors were synthesized using different organic ligands via the hydrothermal method and the CO-PROX performance was investigated. The CuO/CeO2-135 catalyst derived from homophthalic tricarboxylic acid (1,3,5-H3BTC) exhibited superior catalytic performance with 100 % CO conversion at a relatively low temperature (T100% = 100 °C), with a wide reaction temperature range and excellent stability. The superior catalytic properties were attributed to the structural improvements provided by the 1,3,5-H3BTC precursors and the promotional effects of oxygen vacancies. Additionally, in-situ Raman spectroscopy was performed to verify the dynamic roles of oxygen vacancies for CO adsorption and activation, while in-situ DRIFTS analysis revealed key intermediates in the CO-PROX reaction, shedding light on the mechanistic aspects of the catalytic process. This work not only demonstrates insights into the effective CuO/CeO2 catalysts for CO preferential oxidation, but also provides a feasible way to synthesize MOF-derived catalysts.

Effect of long-term thermal aging on lead-bismuth eutectic corrosion behavior of 9Cr ferritic/martensitic steel

In lead-cooled fast reactor (LFR) systems, the liquid lead-bismuth eutectic (LBE) coolant provides a corrosive environment that damages the steel components during high-temperature operation. This study investigated the microstructural deterioration of 9Cr ferritic/martensitic (F/M) steel under thermal aging at 550 °C for 2,000, 10,000, or 20,000 h and its effect on oxidation corrosion in an LBE environment using multiscale characterization techniques. The results indicated that the thickness of the internal oxidation zone (IOZ) increased significantly with extended thermal aging, whereas that of the spinel layer remained relatively constant. The abundant subgrain boundaries that emerged during extensive thermal aging facilitated Fe diffusion, and the enlarged Cr-rich M23C6 carbides contributed to the formation of preferential oxidation regions, accelerating IOZ layer growth. The spinel layer formed from the IOZ was influenced by microstructural defects within the IOZ. A theoretical model describing the accelerated oxide layer growth due to thermal aging was developed. These findings support the advancement of LFR technology.

Corrosion of FeCrAl-1 Nb alloy in deaerated water at 290 ℃ and 7 MPa: The oxidation behavior of grain boundary

An investigation into the grain boundary corrosion behavior of a FeCrAl-1 Nb alloy at 290 ℃ and 7 MPa deaerated solution comprising lithium hydroxide and boric acid was conducted. After 745 hours of corrosion exposure, observations revealed the existence of voids along grain boundary, and preferential oxidation with the transformation from spinel oxide to amorphous oxide at the grain boundary was detected. 18O corrosion labeling results suggested that the corrosion of the alloy exhibited uniformity within a brief timeframe. Further experiments are necessary to investigate the potential impact of intergranular voids and oxidation on long-term corrosion.

In situ XRD and operando XRD-XANES study of the regeneration of LaCo0.8Cu0.2O3 perovskite for preferential oxidation of CO

Combinations of perovskite-type oxides with transition and precious metals exhibit remarkable regenerating properties that can be exploited for catalytic applications. The objective of the present work was to study the structural changes experienced by LaCo0.8Cu0.2O3 under reducing/oxidizing atmosphere (redox) and Preferential Oxidation of CO (PrOx, with high H2 concentration) conditions and their reversibility. LaCo0.8Cu0.2O3 was prepared by ultrasonic spray combustion and was characterized by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Structural changes were followed by operando XRD and XAS. Metallic Co and Cu were segregated under both sets of reducing conditions and re-dissolved into the perovskite upon oxidation at 500 °C. Simultaneously, the perovskite-type oxide disappeared under reducing conditions and formed again upon high-temperature oxidation. The effects of this reversible reduction/dissolution of B-site metals on catalyst structure and activity were studied concerning the catalytic process of PrOx. The active phases of cobalt and copper oxides suffer a reduction during the PrOx reaction due to the high H2 concentration; thus, the application of an intermediate oxidation treatment can regenerate the catalytic system and the perovskite can be used for several cycles of reaction and regeneration. In contrast, when this intermediate oxidation treatment is not applied, the catalytic performance decreases in successive activity cycles.

Unveiling Core-Shell Structure Formation in a Ni3Fe Nanoparticle with In Situ Multi-Bragg Coherent Diffraction Imaging.

Solid-state reactions play a key role in materials science. The evolution of the structure of a single 350 nm Ni3Fe nanoparticle, i.e., its morphology (facets) as well as its deformation field, has been followed by applying multireflection Bragg coherent diffraction imaging. Through this approach, we unveiled a demixing process that occurs at high temperatures (600 °C) under an Ar atmosphere. This process leads to the gradual emergence of a highly strained core-shell structure, distinguished by two distinct lattice parameters with a difference of 0.4%. Concurrently, this transformation causes the facets to vanish, ultimately yielding a rounded core-shell nanoparticle. This final structure comprises a Ni3Fe core surrounded by a 40 nm Ni-rich outer shell due to preferential iron oxidation. Providing in situ 3D imaging of the lattice parameters at the nanometer scale while varying the temperature, this study─with the support of atomistic simulations─not only showcases the power of in situ multireflection BCDI but also provides valuable insights into the mechanisms at work during a solid-state reaction characterized by a core-shell transition.

Wet oxidation of TiZrHfNbTaV high-entropy alloys: Role of grain-boundary and interdendritic diffusion

Refractory high-entropy alloys (RHEAs) are sensitive to thermal oxidation, significantly restricting their service performance and potential applications. However, the wet oxidation behavior of RHEAs has remained poorly understood despite the potential risk of catastrophic oxidation caused by water vapor. In this study, the oxidation mechanism of an equiatomic TiZrHfNbTaV high-entropy alloy (HEA) has been investigated in a wet atmosphere at elevated temperatures. Synchronous oxidation of all elements occurs at temperatures below 800 °C. At temperatures above 800 °C, preferential oxidation of Nb, Ti, and Zr, and internal oxidation of V occur on HEAs, which are thermodynamically less preferred as compared with oxidations of more reactive Hf and Ta. Experimental and theoretical analyses show that the oxide grain boundaries and substrate interdendritic regions of HEAs act as diffusion pathways for V and oxidizing species during preferential and internal oxidation processes. This work provides a fundamental understanding of the oxidation of multicomponent alloys in wet environments, which is helpful in the development of effective oxidation resistance strategies and innovative surface engineering techniques.

Overlooked interaction between redox-mediator and bisphenol-A in permanganate oxidation

Research efforts on permanganate (Mn(VII)) combined with redox-mediator (RM), have received increasing attention due to their significant performance for bisphenol-A (BPA) removal. However, the mechanisms underpinning BPA degradation remain underexplored. Here we show the overlooked interactions between RM and BPA during permanganate oxidation by introducing an RM—N-hydroxyphthalimide (NHPI). We discovered that the concurrent generation of MnO2 and phthalimide-N-oxyl (PINO) radical significantly enhances BPA oxidation within the pH range of 5.0–6.0. The detection of radical cross-coupling products between PINO radicals and BPA or its derivatives corroborates the pivotal role of radical cross-coupling in BPA oxidation. Intriguingly, we observed the formation of an NHPI-BPA complex, which undergoes preferential oxidation by Mn(VII), marked by the emergence of an electron-rich domain in NHPI. These findings unveil the underlying mechanisms in the Mn(VII)/RM system and bridge the knowledge gap concerning BPA transformation via complexation. This research paves the way for further exploration into optimizing complexation sites and RM dosage, significantly enhancing the system's efficiency in water treatment applications.

Sodium Difluoro(oxalato)borate Additive‐Induced Robust SEI and CEI Layers Enable Dendrite‐Free and Long‐Cycling Sodium‐Ion Batteries

AbstractSodium‐ion batteries (SIBs) are a promising candidate for large‐scale energy storage due to the low cost and abundant sodium resources. However, the formation of sodium dendrites on the surface of hard carbon (HC) anodes is the most intractable challenge for full cells during charging, leading to severe performance degradation and safety hazards. Here, a robust additive‐induced borate and fluoride‐rich interphase is constructed by introducing sodium difluoro(oxalato)borate (NaDFOB) as additive in the ether‐based electrolyte to relieve the performance deterioration for SIBs. NaDFOB can participate in the passivation process of electrolyte‐electrode interfaces through preferential oxidation and reduction of DFOB− to effectively restrain the growth of sodium dendrites. Moreover, the electrolyte decomposition and dissolution of transition metal ions are effectively inhibited. Benefiting from that, FeMn‐based Prussian blue (FeMnHCF) || HC full cell with a negative/positive capacity ratio (N/P ratio) of 1.09 displays a capacity retention of 82.1%, especially with a low N/P ratio of 0.96 the cell still demonstrates a stable Coulombic efficiency of over 99.9% after 500 cycles via using NaDFOB as additive. As a practical demonstration, the designed 18650 full cells display enhanced cycling stability with NaDFOB additive. The findings provide insights into the additive‐induced inorganic‐rich interfacial layers for dendrite‐free SIBs.