Oxide Catalysts Research Articles

Real-world aging and deactivation of catalytic diesel particulate filters in heavy-duty vehicles

Low-temperature soot oxidation in catalytic diesel particulate filters (DPF) is important for maintaining high efficiency of heavy-duty vehicles. This can be achieved by coating DPFs with an oxidation catalyst. In this work, catalytic DPFs have been collected from real-world operating heavy-duty vehicles for assessment of their catalytic activity and subsequent characterization. Testing of catalytic activity revealed the diminishing nitric oxide (NO) oxidation of the aged catalysts. The apparent reaction rates showed that the number of available catalytic sites decreased with mileage explaining the loss in activity. Characterization of the samples showed a decreasing surface area as well as an accumulation of metals and poisonous elements. An important finding from SEM-EDS analysis is the evident accumulation of phosphorus and sulfur in the washcoat in the absence of other ash-related elements, potentially explaining the decreased activity.

Mo–V–Al–K–O catalyst for low-temperature oxidative dehydrogenation of propane

Compared with propane dehydrogenation (PDH) technology, oxidative dehydrogenation (ODH) of propane is an effective way to break through the thermodynamic constraints, which has changed the endothermic reaction into exothermic. Meanwhile, coking in PDH was significantly reduced by the presence of an oxidant in ODH of propane. Therefore, ODH of propane has attracted wide interest and has been studied in recent years. However, the contradiction between the conversion of propane and the selectivity of propene was the intrinsic problem of ODH of propane that required further study. In this work, a novel aluminum/potassium-doped mixed metal oxide catalyst Mo1V0.5Al0.02K0.03Ox was synthesized using a simple co-impregnation method, whose selectivity for propene (83.2 %) and conversion of propane (12.7 %) at 300 °C were the best among all other alkali metal/alkaline-earth metal-doped catalysts. The further discussion illustrated that doping potassium and alumina could synergistically adjust the density of acid sites on the catalyst, establishing an equilibrium between high propane conversion and the ideal propene selectivity. After modification, the density of Brønsted acid decreased, leading to an enhancement in the selectivity of propene. Simultaneously, the addition of steam in the feed gas promotes propene to oxygenates and improves the conversion of propane under the condition of constant contact time.

Localized surface plasmon resonance effect-mediated in-situ photochemical–formation of H2O2 for high epoxidation performance over LaSrCoNiO6 nanoparticles

Selective aerobic epoxidation of allylic alcohols and olefins presents a promising solution to the modern chemical industry. However, the development of non-noble metal catalysts with superior catalytic performance for this reaction remains a significant challenge. This study introduces a plasmonic photothermal-catalytic system centered around nano LaSrCoNiO6 (LSCNi-N) catalyst, enabling the epoxidation of cinnamyl alcohol and styrene mediated by LSPR effect under visible light illumination (>420 nm). This catalyst exhibits superior epoxidation catalytic performance, with selectivities of up to 72.3 % in a 93.4 % conversion of cinnamyl alcohol and 91.8 % selectivity of styrene oxide at almost 100 % conversion of styrene. Mechanistic studies reveal that the high selectivity derives from the in-situ photochemical formation of H2O2 mediated by the localized surface plasmon resonance effect of LSCNi-N and hole scavenger effect of cinnamyl alcohol. These findings highlight the potential of designing plasmonic transition-metal oxidic catalysts to overcome challenges in selectively synthesizing fine chemicals through visible light catalysis.

Enhanced degradation of malachite green through heterogeneous processes using an iron oxide catalyst

Enhanced degradation of malachite green through heterogeneous processes using an iron oxide catalyst

The Efficient and Environmentally Friendly Chlorination of Arene, Alcohol, Halobenzene, and Peroxide Catalyzed by Fe–Ba Binary Oxides Using Hydrochloric Acid as Chlorine Source and Aqueous H2O2 as Oxidant

A series of Fe–Ba mixed oxides, including a pure Fe-containing sample as a reference, have been synthesized via a sol–gel process using Fe3+ or Fe2+ salts and BaSO4 as raw materials, with Pluronic P123 serving as a template. These oxides have been thoroughly characterized and subsequently utilized as catalysts for the chlorination of various organic molecules. Commercial hydrochloric acid, known for its relative safety, and environmentally friendly aqueous hydrogen peroxide were employed as the chlorine source and oxidant, respectively. The pure Fe-containing catalyst displays excellent thermal stability between 600 and 800 °C and exhibited moderate to high conversions in the chlorination of toluene, benzene, and tert-butyl hydroperoxide, with remarkable ortho-selectivity in chlorination of toluene. The combination of Fe3+ salt with BaSO4 in the sol–gel process results in a Fe–Ba mixed oxide catalyst composed of BaO2, BaFe4O7, and Fe2O3, significantly enhancing the chlorination activity compared to that displayed by the pure Fe catalyst. Notably, the chlorination of tert-butyl hydroperoxide (TBHP) does not require additional oxidants such as H2O2, and involves both electrophilic substitution and nucleophilic addition. Notably, the chlorination of bromobenzene yields chlorobenzene as the sole product, a transformation that has not been previously reported. Overall, this catalytic chlorination system holds promise for advancing the chlorination industry and enhancing pharmaceutical production.

Engineering a Cu-Pd Paddle-Wheel Metal-Organic Framework for Selective CO2 Electroreduction.

Optimizing the binding energy between the intermediate and the active site is a key factor for tuning catalytic product selectivity and activity in the electrochemical carbon dioxide reduction reaction. Copper active sites are known to reduce CO2 to hydrocarbons and oxygenates, but suffer from poor product selectivity due to the moderate binding energies of several of the reaction intermediates. Here, we report an ion exchange strategy to construct Cu-Pd paddle wheel dimers within Cu-based metal-organic frameworks (MOFs), [Cu3-xPdx(BTC)2] (BTC=benzentricarboxylate), without altering the overall MOF structural properties. Compared to the pristine Cu MOF ([Cu3(BTC)2], HKUST-1), the Cu-Pd MOF shifts CO2 electroreduction products from diverse chemical species to selective CO generation. In situ X-ray absorption fine structure analysis of the catalyst oxidation state and local geometry, combined with theoretical calculations, reveal that the incorporation of Pd within the Cu-Pd paddle wheel node structure of the MOF promotes adsorption of the key intermediate COOH* at the Cu site. This permits CO-selective catalytic mechanisms and thus advances our understanding of the interplay between structure and activity toward electrochemical CO2 reduction using molecular catalysts.

Highly Active and Stable Cu-Cd Bimetallic Oxides for Enhanced Electrochemical CO2 Reduction.

Electrochemical reduction of carbon dioxide (CO2) can produce value-added chemicals such as carbon monoxide (CO) and multicarbon(C2+). However, the complex reaction pathways of CO2electro-reduction reaction (CO2RR) greatly limit the product selectivity and conversion efficiency. Herein, the Cu-Cd bimetallic oxides catalyst was designed and applied for the CO2RR. The optimized 4.73%Cd-CuO exhibits remarkable electrocatalytic CO2RR activity for selective CO production in H-cell using 0.5 M 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6)/MeCN as electrolyte. The Faradaic efficiency of CO (FE(CO)) can be maintained above 90% over a wide potential range of -2.0 to -2.4 V vs. Ag/Ag+. Particularly, the catalyst achieves an impressive FE(CO) of 96.3% with a current density of 60.7 mA cm-2at -2.2 V vs. Ag/Ag+. Furthermore, scaling up the 4.73%Cd-CuO catalyst into a flow cell can reach 56.64% FE of C2+products (ethylene, ethanol and n-propanol) with a current density as high as 600 mA cm-2steadily.The excellent CO2RR performance of the as-synthesized 4.73%Cd-CuOcan be mainly attributed to the introduction of CdO to improve the ability of CuO to activate CO2, the electronic interactions between Cu and Cd can boostactivation and conversionkey intermediates of CO2RR and ensure the continuous stability of4.73%Cd-CuOin electrolysis process.

Modulating RuO2 Electrocatalysis via Introducing Lanthanides for Enhanced Acidic Oxygen Evolution

AbstractThe low activity of ruthenium dioxide (RuO2) and the rapid dissolution of the ruthenium (Ru) site during acid oxygen evolution reaction (OER) at high current density restricts its application in proton exchange membrane (PEM) electrolyzers. In this work, a series of rare‐earth (RE) elements doped RuO2 nanorods is developed as high‐performance OER electrocatalysts. X‐ray absorption spectroscopy suggests that RE doping regulates the local coordination environment around Ru sites, lowers the valance of Ru, and shortens the Ru─O─Ru(M) bond length, which enhances structural stability. Among the RE‐RuO2 samples, Sm‐RuO2 exhibits remarkable performance with a low overpotential of 283 mV to reach 100 mA cm−2 and maintained stability for over 140 h at 10 mA cm−2. Moreover, the PEM electrolyzer using Sm‐RuO2 as the anode can be stably operated at 500 mA cm−2 for 15 h. Electrochemical analysis, X‐ray photoelectron spectroscopy, and in situ Raman spectroscopy show that Sm doping lowers the d‐band center, reduces the adsorption energy of O intermediates to enhance the OER activity, and restrains excessive oxidation of Ru sites while stabilizing lattice oxygen during the OER process, thereby enhancing OER stability. This work offers valuable insights into improving the stability of metal oxide catalysts in acidic electrolytes.

Effect of Calcination Temperature in Large-Aperture Medium-Entropy Oxide (FeCoCuZnNa)O on CO2 Hydrogenation for Light Olefins

The catalytic hydrogenation of carbon dioxide is not only a way to mitigate the greenhouse effect but also provides high-value chemicals. In this work, a medium-entropy oxide catalyst (FeCoCuZnNa)O was prepared by the sol–gel method for highly active and selective hydrogenation of CO2 to value-added hydrocarbons. When reacted at 290 °C, 2.5 MPa, and 2500 mL·gcat−1·h−1, the CO2 conversion and selectivity of olefin were affected by the calcination temperature of the catalyst, and the best performances were 39% and 41.3%. The large pore size and oxygen vacancies (Ov) formed by (FeCoCuZnNa)O promote the activation of CO2 and promote the C-C coupling reaction of Fe5C2 in a hydrogenation reaction. The promoted C-C coupling reaction was related to the surface enrichment of iron species. The presence of Ov also inhibited the excessive hydrogenation reaction, further improving the selectivity of light olefins. In addition, (FeCoCuZnNa)O did not show significant deactivation within 75 h, indicating that the catalyst has strong industrial potential.

An Angstrom-Scale Protective Skin Grown In Situ on Perovskite Oxide to Enhance Stability in Water.

The utilization of perovskite oxide as a catalyst for aqueous reactions is promising but challenging in stability. Here, we propose an in situ growth strategy that constructs an ultrathin protective skin on the Sr0.9Fe0.81Ta0.09Ni0.1O3-δ perovskite surface and thus effectively solves the stability issue. Using a spherical aberration-corrected transmission electron microscope, we observe the coexistence of an angstrom-scale (~7 Å) Fe2O3 protective skin and FeNi alloy nanoparticles. A number of alloy nanoparticles grow along with the skin and uniformly take root on the skin surface. Such a hierarchical structure can reconstruct the surface electronic structure and suppress the ion leaching of perovskite oxide in water. Benefiting from this unique structure, the catalyst has experienced a substantial increase (800 h, more than three orders of magnitude) in its stable operation time in water (for example, in a hydrogen evolution reaction). These results provide valuable insight into solid-solid phase transitions and have substantial implications for using structural defects at surfaces to modulate mass transport and transformation kinetics. Our strategy is sufficiently simple and can be used to subtly manipulate the catalyst structures to improve the performance of perovskite-based catalysts and potentially other oxide catalysts for a wide range of reactions.

An Angstrom‐Scale Protective Skin Grown In Situ on Perovskite Oxide to Enhance Stability in Water

AbstractThe utilization of perovskite oxide as a catalyst for aqueous reactions is promising but challenging in stability. Here, we propose an in situ growth strategy that constructs an ultrathin protective skin on the Sr0.9Fe0.81Ta0.09Ni0.1O3‐δ perovskite surface and thus effectively solves the stability issue. Using a spherical aberration‐corrected transmission electron microscope, we observe the coexistence of an angstrom‐scale (~7 Å) Fe2O3 protective skin and FeNi alloy nanoparticles. A number of alloy nanoparticles grow along with the skin and uniformly take root on the skin surface. Such a hierarchical structure can reconstruct the surface electronic structure and suppress the ion leaching of perovskite oxide in water. Benefiting from this unique structure, the catalyst has experienced a substantial increase (800 h, more than three orders of magnitude) in its stable operation time in water (for example, in a hydrogen evolution reaction). These results provide valuable insight into solid‐solid phase transitions and have substantial implications for using structural defects at surfaces to modulate mass transport and transformation kinetics. Our strategy is sufficiently simple and can be used to subtly manipulate the catalyst structures to improve the performance of perovskite‐based catalysts and potentially other oxide catalysts for a wide range of reactions.

Influence of WO3 Content in Phosphated Tungsten-Zirconium Oxide Catalysts on the Catalytic Pathways of Glycerol Transformation.

The catalytic performance of phosphate-stabilized WOx-ZrO2 compositions in gas-phase glycerol dehydration has been investigated. Results show that varying WO3 concentrations direct the process towards either acrolein or allyl alcohol formation. Catalysts with low WOx content exhibit strong Lewis acid sites (Zr4+ and W6+), where these metal ions likely function as redox sites, facilitating glycerol hydrogenolysis to produce allyl alcohol. Higher WO3 concentrations (exceeding 20 wt.%) lead to the shielding of some W6+ and Zr4+ sites by polytungstate surface complexes, which are strong Brønsted acid sites. This alteration promotes glycerol dehydration through the removal of two water molecules, thereby shifting the selectivity towards acrolein formation.

CuS-NiTe2 embedded phosphorus-doped graphene oxide catalyst for evaluating hydrogen evolution reaction.

The hydrogen evolution reaction (HER), a crucial half-reaction in the water-splitting process, is hindered by slow kinetics, necessitating efficient electrocatalysts to lower overpotential and enhance energy conversion efficiency. Transition-metal electrode materials, renowned for their robustness and effectiveness, have risen to prominence as primary contenders in the field of energy conversion and storage research. In this investigation, we delve into the capabilities of transition metals when employed as catalysts for the HER. Furthermore, we turn our attention to carbon nanomaterials like graphene, which have exhibited tremendous potential as top-performing electrocatalysts. Nevertheless, advancements are indispensable to expand their utility and versatility. One such enhancement involves the integration of phosphorus-doped graphene. Our research focuses on the synthesis of CuS-NiTe2/PrGO, a nanocomposite with a crystalline structure, through a straightforward method. This nanocomposite exhibits enhanced catalytic activity for the HER, boasting a Tafel slope of 57 mV dec-1 in an acidic environment. Consequently, our findings present a straightforward and efficient approach to developing high-performance electrocatalysts for HER.

Plasma-Assisted Synthesis of Methanol Through Hydrogenation of Carbon Dioxide With Non-Noble Metal Mixed Oxide Catalysts.

The utilization of carbon dioxide through chemical conversion is a promising approach for the recycling of carbon resources. Despite well-developed industrial processes for CO2 hydrogenation to methanol, the effective use of CO2 as a feedstock remains challenging because of the costly requirements of high temperature and reaction pressure. In this paper, we report the methanol synthesis from CO2 and hydrogen using a dielectric barrier discharge (DBD) reactor under atmospheric pressure with a nickel-cerium-aluminum mixed oxide (Ni/Ce-Al MOx) catalyst. The combined use of plasma and the Ni/Ce-Al MOx catalyst was observed to yield 13.3±0.4 % of methanol, favorably compared to the 2.6±0.5 % yield of the case without catalyst. Microscopy images, selected area electron diffraction patterns, and energy-dispersive X-ray analysis confirmed the presence of fluorite-structured ceria, aluminium, nickel, and nickel oxide particles in the catalyst. The reaction mechanism for the plasma-assisted hydrogenation of CO2 was hypothesized to involve a carbide formation pathway due to the presence of carbide confirmed by X-ray photoelectron spectroscopic characterization.

Bipolar Membranes With Controlled, Microscale 3D Junctions Enhance the Rates of Water Dissociation and Formation

AbstractA soft lithographic method is developed for making bipolar membranes (BPMs) with catalytic junctions formed from arrays of vertically oriented microscale cylinders. The membranes are cast from reusable polydimethylsiloxane (PDMS) molds made from silicon masters, which are fabricated on 2″ to 4″ wafer scales by nanosphere lithography. High‐aspect‐ratio junctions are made on a length scale similar to the thickness of optimized catalyst layers for water dissociation, creating a platform for probing the dual effects of catalysis and local electric field at the microscale BPM junction. Optimized polymer materials and nanoscale metal oxide catalysts are used in this study. 3D BPMs are tested under reverse and forward bias conditions, exhibiting superior performance relative to their 2D counterparts. Under forward bias in H2‐O2 fuel cells, 3D BPMs achieve a current density of 1500 mA cm−2, ≈7 times higher than 2D membranes made from the same materials.

Microscopic Investigation of CO Oxidation Reaction by Copper–Manganese Oxide Catalysts

Microscopic Investigation of CO Oxidation Reaction by Copper–Manganese Oxide Catalysts

Quantification of electrochemically accessible iridium oxide surface area with mercury underpotential deposition.

Research drives development of sustainable electrocatalytic technologies, but efforts are hindered by inconsistent reporting of advances in catalytic performance. Iridium-based oxide catalysts are widely studied for electrocatalytic technologies, particularly for the oxygen evolution reaction (OER) for proton exchange membrane water electrolysis, but insufficient techniques for quantifying electrochemically accessible iridium active sites impede accurate assessment of intrinsic activity improvements. We develop mercury underpotential deposition and stripping as a reversible electrochemical adsorption process to robustly quantify iridium sites and consistently normalize OER performance of benchmark IrOx electrodes to a single intrinsic activity curve, where other commonly used normalization methods cannot. Through rigorous deconvolution of mercury redox and reproportionation reactions, we extract net monolayer deposition and stripping of mercury on iridium sites throughout testing using a rotating ring disk electrode. This technique is a transformative method to standardize OER performance across a wide range of iridium-based materials and quantify electrochemical iridium active sites.

Dynamic control and quantification of active sites on ceria for CO activation and hydrogenation

Ceria (CeO2) is a widely used oxide catalyst, yet the nature of its active sites remains elusive. This study combines model and powder catalyst studies to elucidate the structure-activity relationships in ceria-catalyzed CO activation and hydrogenation. Well-defined ceria clusters are synthesized on planar CeO2(111) and exhibit dynamic and tunable ranges of Ce coordination numbers, which enhance their interaction with CO. Reduced ceria clusters (e.g., Ce3O3) bind CO strongly and facilitate its dissociation, while near-stoichiometric clusters (e.g., Ce3O7) adsorb CO weakly and promote oxidation via carbonate formation. Unlike planar ceria surfaces, supported ceria clusters exhibit dynamic properties and enhanced catalytic activity, that mimic those of powder ceria catalysts. Insight from model studies provide a method to quantify active sites on powder ceria and guide further optimization of ceria catalysts for syngas conversion. This work marks a leap toward model-guided catalyst design and highlights the importance of site-specific catalysis.

Ternary ZnO/Co3O4/CuO heterostructure nanocomposites derived from trimetallic metal-organic frameworks for efficient electrocatalytic water oxidation

Enhancing the electrocatalytic performance of transition-metal oxide catalysts by optimizing the active center at the multi-component heterointerface is a daunting task. Herein, the three-component ZnO/Co3O4/CuO heterostructure composite catalyst was effectively produced through a room-temperature precipitation reaction, followed by calcination in air. The ZnO/Co3O4/CuO ternary heterostructure catalyst showed superior oxygen evolution reaction (OER) performance, characterizedby a low overpotential (312 mV at 10 mA cm−2) and Tafel slope (83.5 mV dec-1) compared to the ZnO, Co3O4, ZnO/CuO, ZnO/Co3O4, and Co3O4/CuO catalysts. The research shows that the property of electrocatalytic oxygen evolution is greatly enhanced due to the combined impact of each component.

Selective catalytic reduction of NO with NH3 over HZSM-5/CeO2 hybrid catalysts: Relationship between acid structure and reaction mechanism

CeO2 is active in the NH3 selective catalytic reduction of NO (NH3-SCR) reaction due to its excellent redox properties. However, the low surface acidity of CeO2 limits its NH3-SCR activity. In this study, highly active HZSM-5-modified HZSM-5/CeO2 (Z/Ce) mixed oxide catalysts were prepared by a simple physical milling method. The HZSM-5 modification stimulated many thermally stable Brønsted-acid structures and high oxygen mobility on the surface of the Z/Ce catalysts, promoting the acid cycling pathways driven by redox macrocycles, and accelerating NO removal. The NO conversion of 25Z/Ce at 200–400 °C is about 34.8 %-62.3 % higher than that of CeO2. A series of physicochemical and in situ interfacial reactions were analyzed to explore the medium- and high-temperature reaction mechanism on the CeO2 surface. The Eley-Rideal reaction mechanism on the Lewis-acid structure with NO(g) as the reactant is deduced. When HZSM-5 modified CeO2, more NO was activated to NO2(g) on the surface of the Z/Ce catalyst at medium and high temperatures (300 °C). This significantly enhanced the Eley-Rideal reaction mechanism with NO2(g) as the main reactant on the Brønsted-acid structure as well as the occurrence of the “Fast-SCR” reaction. This work provides a simple method to improve the performance of CeO2 in the NH3-SCR reaction and elucidates the reaction mechanism.