Spinel Catalysts Research Articles

Modulating charge transfer and constructing synergistic bimetallic active sites for CO-SCR reactions in inverse spinel

The selective catalytic reduction of NO by CO (CO-SCR) are frequently catalyzed by spinel due to its exceptional stability and redox capabilities. In this research, a ternary metal-based inverse spinel catalyst Cu0.67Mn0.33Fe2O4 was synthesized utilizing the glycerol self-templating technique. Some Fe3+ (Fe3+Td) sites are transferred to vacant octahedral sites through the occupancy of tetrahedral sites by Mn species. Mn3+ (Mn3+Td) sites synergistically interact with Cu2+ and Fe3+ ions on the octahedral sites to promote charge transfer, producing numerous oxygen-defective and highly mobile oxygen species and constructing Cu+-Ov-Fe2+ active sites on the surface. Various characterizations demonstrated that Cu+-Ov-Fe2+ has greater catalytic activity, resistance to alkali metals, and water toxicity than the Cu2+-Ov-Fe2+ active site of CuFe2O4. In situ DRIFTS further revealed the reaction mechanism of the ternary metal-based inverse spinel catalyst, and this study provides novel perspectives on the development of Cu-based catalysts for CO-SCR.

AFe2O4 (A=Cu, Ni, Co, Mg, Ce, Mn) Catalysts for Hydrogen‐Rich Syngas Production from Corn Straw Pyrolysis‐Catalytic Steam Reforming

CuFe2O4, an iron‐based spinel catalyst, along with NiFe2O4, CoFe2O4, MgFe2O4, CeFe2O4, and MnFe2O4 are prepared using the sol–gel method. Different modified transition metals have been investigated to determine the influence on hydrogen production in a fixed‐bed reactor. The results indicated that all the prepared catalysts exhibit a spinel structure. At a reaction temperature of 700 °C, with a water–carbon molar ratio of S/C = 1.5 and a biomass‐to‐catalyst mass ratio of 1:1, the performance ranking of the AFe2O4 spinel catalysts is as follows: CeFe2O4 > CuFe2O4 > MnFe2O4 > NiFe2O4 > CoFe2O4 > MgFe2O4 > no catalyst. CeFe2O4 and CuFe2O4 catalysts demonstrate superior performance, with hydrogen volume fractions of 42.26% and 41.63% respectively. The AFe2O4 catalyst exhibits effective catalytic activity in the production of hydrogen from corn straw using water vapor, with the synergistic effect of A metal and Fe enhancing the catalytic activity of AFe2O4.

Facet Engineering of Cobalt Manganese Oxide for Highly Stable Acidic Oxygen Evolution Reaction

AbstractDeveloping cost‐effective, robust, and durable catalysts for water oxidation in acidic conditions remains a significant challenge for the practical application of proton exchange membrane electrolyzers. Cobalt‐based spinel catalysts, known for their effective oxygen evolution reaction (OER) activity in acidic environments, are promising alternatives to the costly iridium‐based catalysts. However, their application is often limited by poor stability under corrosive acidic conditions and oxidative potential. Here it is demonstrated that doping Co2MnO4 with Ni effectively regulates the structural and electronic properties of the catalysts, enabling stable operation for over 285 h at 100 mA cm−2 in 0.5 m H2SO4. This stability enhancement is primarily due to the increased exposure of the (400) facet, a result supported by theoretical studies. The proton exchange membrane water electrolysis (PEMWE) performance of Ni(5%)Co2MnO4 further demonstrates its potential, achieving 1 A cm−2 at a cell voltage of 2.3 V, with minimal degradation over 100 h at 500 mA cm−2. This study not only provides insights into the design of advanced OER catalysts through the doping of heteroatoms but also offers a pathway to enhance the sustainability and economic viability of acidic OER applications.

Cation rearrangement at tetrahedral sites in the Cu/ZnAl2O4 spinel enhancing CO2 hydrogenation to methanol

Cu/ZnAl2O4 spinel is a promising catalyst for CO2-to-methanol due to its dual-site H2 activation. However, controlling surface properties and metal-support interactions (MSI) through the coordination environment of spinel cations remains underexplored. Herein, the occupancy of Zn2+ and Al3+ cations in Cu/ZnAl2O4 spinel is fine-tuned by manipulating the calcination atmosphere. More disordered structures (AlO4 and ZnO6) observed on the CZA-Ar catalyst benefit the creation of oxygen vacancies and surface hydroxyl groups, which facilitate the formation of highly dispersed small-sized Cu species. This enhances MSI and hydrogen spillover effects, thereby boosting CO2 conversion and the space-time yield (STY) of methanol. High-pressure in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results reveal that the crucial formate intermediates form at tetrahedral sites in ZnAl2O4 spinel, with more Al-HCOO- species observed on the CZA-Ar catalyst due to the formation of AlO4. This work advances the understanding of spinel catalysts in CO2 hydrogenation to methanol.

A Critical Review of the Synthesis and Applications of Spinel-Derived Catalysts to Bio-Oil Upgrading.

The transformation of renewable bio-oil into platform chemicals and bio-oil through catalytic processes embodies an efficient approach within the realm of advancing sustainable energy. Spinel-based catalysts have garnered significant attention owing to their ability to precisely tune metals within the framework, facilitating adjustments to structural, physical, and electronic properties, coupled with their remarkable thermal stability. This review aims to provide a comprehensive overview of recent advancements in spinel-based catalysts for upgrading bio-oil. Its objective is to shed light on their potential to address the limitations of conventional catalysts. Initially, a comprehensive analysis is conducted on different metal oxide composites in terms of their similarity and dissimilarity on properties. Subsequently, the synthesis methodologies are scrutinised and potential avenues for their modification are explored. Following this, an in-depth discussion ensues regarding the spinels ascatalysts or catalyst precursors for catalytic cracking, ketonisation, catalytic hydrodeoxygenation, steam and aqueous-phase reforming, andelectrocatalytic upgrading of bio-oil. Finally, the challenges and potential prospectsare addressed, with a specific focus on the machine learning - based approaches to optimise the structure and activity of spinel catalysts. This review aims to provide specific directions for further exploration and maximisation of the spinel catalysts in the bio-oil upgrading field.

Structural optimization of the NiFe2O4 spinel catalyst aimed at efficient electrocatalytic C-O bond cleavage of lignin

The intricate structure of lignin poses significant challenges to its high-value utilization. Electrocatalytic conversion of lignin into valuable aromatic compounds using spinel catalysts under mild conditions is a promising approach. The electrochemical conversion of lignin and its model compounds was conducted using Ni-Fe spinel catalysts, achieving selective cleavage of the C-O bond in lignin model with 72 % phenol selectivity and 82 % conversion. Additionally, efficient electrochemical cleavage of the C-O bond was achieved using alkali lignin. Structural characterization and theoretical calculations revealed that the Ni-Fe spinel catalyst exhibited a uniform electron distribution, enhancing the adsorption of the C-O bond and lowering the activation free energy associated with C-O bond cleavage. Moreover, the heterogeneous interface of NiFe2O4 further accelerated the reaction kinetics. This study represents a significant breakthrough in active site identification and mechanistic analysis, facilitating the selective conversion of lignin and providing an efficient method for lignin depolymerization.

High purity H2 resource from methanol steam reforming at low-temperature by spinel CuGa2O4 catalyst for fuel cell

Methanol steam reforming (MSR) is an effective way to provide high purity hydrogen for PEMFCs, however the main challenge is the toxic effect of CO. In this study, CuGa2O4 spinel catalyst was applied first for MSR at low temperature. The properties of catalysts were characterized by various techniques, the MSR catalytic performance was also studied in deeply. The surface of the catalyst is composed of particle chains that are interconnected, forming high-volume pores that increase the specific surface area. The catalyst also has a rich porous structure, exposing more active sites. Furthermore, the presence of oxygen vacancies facilitates the adsorption of reactive oxygen species, reducing CO generation. A possible pathway for methanol dehydrogenation has been determined using DFT calculations. The relatively low overall energy barrier on the catalyst surface facilitates methanol activation. Among the steps, formaldehyde dehydrogenation is the rate-determining step in the methanol dehydrogenation process. The CuGa2O4 catalyst exhibits good gas selectivity, with a hydrogen selectivity of 98 % and CO selectivity of 0. and no deactivation occurred within 50 h, demonstrating outstanding durability. These features allow it to serve as an efficient catalyst for MSR online hydrogen production and direct supply to PEMFCs.

Surface- modified preparation of CuFe2O4/sodium alginate hydrogel catalysts for the long-lasting degradation of tetracycline

Copper ferrate spinel (CuFe2O4) catalysts are widely used in advanced oxidation processes for environmental wastewater treatment due to their advantages of low price, mild reaction conditions and ease of operation. Therefore, it is crucial to investigate the factors influence the surface properties of catalysts on their catalytic activity. In this research, the effects of process parameters on the surface properties of CuFe2O4 were investigated, and the relationship between the surface properties of CuFe2O4 and its catalytic activity was clarified. The optimised CuFe2O4 exhibited 92.4 % degradation of tetracycline (TC) and 55 % mineralisation of total organic carbon (TOC) within 30 min, the pseudo-first-order reaction rate was 0.0315 min−1, which was 2.2 times higher than that of the unoptimised CuFe2O4. The mechanism of action for enhancing catalyst performance and the degradation mechanism of tetracycline were clarified through a combination of characterization analysis. To further solve the problems of secondary pollution caused by ion leaching and the difficulty of practical application of powder catalysts, novel hydrogel microspheres composed of sodium alginate and CuFe2O4 were prepared by using the property of metal ions and sodium alginate to form a solid gel (SA/Ort-6). SA/Ort-6 was characterized by low ion leaching, excellent performance and excellent stability. The mass ratio of catalyst to treated water could be up to 1:22,000, and the mass ratio of catalyst to treated water can reach up to 1:22000, the removal rate of TOC was stable above 35 %. This study provides valuable insights into the relationship between surface properties and catalytic performance of CuFe2O4, offers a new approach to effectively solve leaching of metal ions, and opens up a new avenue for practical application of powder catalysts.

LaMn-doped cobalt spinel catalysts for enhanced oxygen evolution performance in acidic media

While cobalt-based oxides can function as OER catalysts under acidic conditions, challenges such as high overpotential and inadequate stability persist, demanding solutions to fulfill practical application demands. We utilized a rapid and efficient one-step molten salt method to synthesize LaMn-doped Co3O4 supported on carbon cloth, with the aim of improving its OER activity and stability in acidic conditions. Subsequent characterization revealed that doping increased oxygen vacancies and lattice distortion within the spinel structure, while also altering the morphology of the catalyst surface, rendering it denser. In addition, the doping optimizes the electronic structure around cobalt, so that the electrons around oxygen are biased towards cobalt, which enhances the electrocatalytic activity of Co3O4 and the stability of Co–O bonds. Ultimately, under 0.5 M H2SO4 conditions, LaMn–Co3O4/CC exhibited superior OER activity and stability, displaying a lower overpotential of 370 mV at a current density of 10 mA cm−2 and maintaining stability for over 24 h, a notable enhancement compared to pure Co3O4.

Detachment Suppression of NiCo Oxide Catalyst Layer for Oxygen Evolution Anode in Alkaline Medium Using Start and Stop Simulated Accelerated Degradation Test Protocol

INTRODUCTION While the alkaline water electrolysis (AWE) has the advantage of low cost and mass production, it has the issue of catalyst coated electrode degradation caused mainly by catalyst layer detachment due to reverse currents generated by the start and stop of variable renewable electricity. A previous study reported that heat treatment of a NiCo2O4 spinel catalyst-supported electrode (NiCo) on a Ni substrate suppress delamination of the catalyst layer with decrease in initial activity due to increase of electrode resistance. [1] In this study, in order to find optimize preparation for both initial activity and durability of the anode electrode catalyst, we compared and evaluated the degradation behavior of NiCo electrodes prepared various conditions. EXPERIMENTAL The accelerated durability test (ADT) was performed in a three-electrode cell with 7 M KOH aqueous solution as electrolyte at 25℃. RHE, Ni coil and 1 cm2 electrode of NiCo spinel catalyst supported on Ni mesh substrate prepared with 8 to 24 of layered thermal decomposition coating at various temperature are used as reference, counter and working electrodes, respectively. Here, the thickness of the coting was determined by Co loading measured with XRF. Constant current electrolysis at 1 A cm-2 for 2 hours was conducted as pretreatment at 80℃ before ADT. The ADT protocol consisted of three steps [2] (Fig.1): (1) constant current electrolysis at 600 mA cm-2 for 1 min, (2) potential scanning from open-circuit potential (OCP) to anodic potential E min (= 0.5 V vs. RHE) at sweep rate of -500 mV s-1, and (3) holding at constant potential of E min for 1 min. To evaluate the anode activity, CV (potential range: 0.5 to 2.0 V vs. RHE, sweep rate: 5 and 50 mV s-1, 3 cycles) and AC impedance measurements (bias potential: 1.6, 1.7 V vs. RHE, frequency range: 106-10-1 Hz, amplitude: 10 mV) were performed at every 200 ADT cycles in 2,000 cycles of ADT. RESULTS AND DISCUSSION As shown in Fig. 2 of the anode potential as a function of the ADT cycle, the potential of the 350°C prepared anode significantly increased around 800 cycles toward around 1.9 V vs. RHE, while the 450°C prepared anode showed slowly and constantly increase in potential.As shown in Fig. 3 of the redox peaks in cyclic voltammograms, the NiCo redox peaks around 1.2 V vs. RHE disappeared and the Ni redox peaks around 1.2 V vs. RHE appeared at the end of the ADT,. The NiCo redox peak of the 350°C prepared anode was significantly larger than the Ni redox peak in the initial period, whereas the Ni redox peaks became larger than the NiCo redox peak around 700 cycles. The charge of NiCo redox peak of the 450°C prepared anode gradually decreased and that of the Ni oxidation peak increased and they reversed for 8, 16, and 24 layered anodes at 1000, 1400, and 1600 cycles, respectively, but NiCo redox peak still remained. These results means that the consumption of the NiCo oxide electrocatalyst occurs during the ADT and the 450°C preparation suppress rapidly detachment of the 450°C prepared anode around 700 cycles.As shown in Fig. 4 of the polarization curves, the OER activity of the 8 layered at the 450°C prepared was almost same as that of the 350°C prepared anode at 200 cycles, and the 16 and 24 layered at 450°C prepared were higher activity than the 350°C prepared and the 8 layered at 450°C prepared. The Tafel slopes of the initial ADT (200 cycles) were almost the same for the 350°C and 450°C. The slope for the 450°C prepared anodes after 2000 cycles were almost same as the initial regardless of the number of applications, but the slope for the 350°C prepared increased. This result also suggests NiCo oxide of the 450°C prepared still remained at the end of the ADT and it dominates oxygen evolution reaction, while the 350°C prepared electrocatalyst completely consumed during the ADT. ACKNOWLEDGEMENTS This study was based on results obtained from the Development of Technologies for Realizing a Hydrogen Society (P20003) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). REFERENCES [1] N. Todoroki, K. Nagasawa, H. Enjoji, S. Mitsushima., ACS Appl. Mater. Interfaces, 15, 20, 24399 24407 (2023).[2] A. Abdel Haleem, K. Nagasawa, Y. Kuroda, Y. Nishiki, A. Zaenal, S. Mitsushima., Electrochem., 89, 2, 186 191 (2021). Figure 1

PGM-Free Oxygen Evolution Reaction Catalyst for PEM Water Electrolysis – a Mechanistic Study on Activity and Durability for Practical Applications

Low temperature proton exchange membrane water electrolyzer (PEMWE) represents a critical technology for green hydrogen production. It offers the advantages of significantly higher current density and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential. Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the platinum group metal (PGM) materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve add a significant cost barrier to PEM electrolyzer, which contributes to the overall expense of hydrogen production next only to the cost of electricity. Replacing Ir with earth-abundant transition metal oxides could help to reduce the electrolyzer system cost.For PEMWE application, a PGM-free anodic catalyst must be highly active to match the performance of Ir. Furthermore, it must be stable against oxidative acid corrosion. At Argonne National Laboratory, we recently developed a new PGM-free OER of a nanofibrous cobalt spinel catalyst co-doped with lanthanum and manganese (LMCF). [1] The catalyst was synthesized from the precursor of cobalt zeolitic methyl-imidazolate framework (Co-ZIF) doped with manganese and lanthanum. The Co-ZIF was mixed with a polymer solution and electrospun into nanofiber before being converted to fibrous cobalt spinel under low temperature oxidation to remove all the organics and carbon. The new PGM-free catalyst demonstrated a low overpotential of 353 mV (RHE) at 10 mA/cm2 and a low OER degradation over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at its anode at ~2.0 mg/cm2 demonstrated a current density of 2000 mA/cm2 at 2.47 volts (Nafion® 115 membrane) or 4000 mA/cm2 at 3.00 volt (Nafion® 212 membrane). The membrane electrode was also subjected to accelerated-stress-test (AST) in both voltage cycling and galvanostatic run at different current densities. Low degradation rates under both ASTs were observed.In addition to catalyst design and synthesis, we also conducted extensive structural characterizations, stability measurement, as well as computational modeling. Our in situ study found that the OER process involves significant lattice oxygen movement, leading to lowering instead of raising the cobalt oxidation state during the electrolysis. The measurement of the stability number, combined with computational Pourbaix diagram, elucidated the importance of the operating potential in reducing catalyst dissolution. These findings offer in-depth understanding and direction of future development for PGM-free OER catalyst in PEM water electrolyzer. Acknowledgement: This work is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewal Energy, Hydrogen and Fuel Cell Technologies Office, and by Laboratory Directed Research and Development funding of Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DEAC02-06CH11357. Work performed at the Center for Nanoscale Materials and Advanced Photon Source, both U.S. Department of Energy Office of Science User Facilities, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.[1] “La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis” Lina Chong, Guoping Gao, Jianguo Wen, Haixia Li, Haiping Xu, Zach Green, Joshua D. Sugar, A. Jeremy Kropf, Wenqian Xu, Xiao-Min Lin, Hui Xu, Lin-Wang Wang, Di-Jia Liu, Science 380, 609–616 (2023)

Facet-Dependent Evolution of Active Components on Spinel Co3O4 for Electrochemical Ammonia Synthesis.

Spinel cobalt oxides (Co3O4) have emerged as a promising class of catalysts for the electrochemical nitrate reduction reaction (eNO3RR) to ammonia, offering advantages such as low cost, high activity, and selectivity. However, the specific role of crystallographic facets in determining the catalysts' performance remains elusive, impeding the development of efficient catalysts. In this study, we have synthesized various Co3O4 nanostructures with exposed facets of {100}, {111}, {110}, and {112}, aiming to investigate the dependence of the eNO3RR activity on the crystallographic facets. Among the catalysts tested, Co3O4 {111} shows the best performance, achieving an ammonia Faradaic efficiency of 99.1 ± 1.8% with a yield rate of 35.2 ± 0.6 mg h-1 cm-2 at -0.6 V vs RHE. Experimental and theoretical results reveal a transformation process in which the active phases evolve from Co3O4 to Co3O4-x with oxygen vacancy (Ov), followed by a Co3O4-x-Ov/Co(OH)2 hybrid, and finally Co(OH)2. This process is observed for all facets, but the formation of Ov and Co(OH)2 is the most rapid on the (111) surface. The presence of Ov significantly reduces the free energy of the *NH2 intermediate formation from 1.81 to -0.53 eV, and plentiful active sites on the densely reconstructed Co(OH)2 make Co3O4 {111} an ideal catalyst for ammonia synthesis via eNO3RR. This work provides insights into the understanding of the realistic active components, offers a strategy for developing highly efficient Co-based spinel catalysts for ammonia synthesis through tuning the exposed facets, and helps further advance the design and optimization of catalysts in the field of eNO3RR.

Surface oxygen vacancy regulation and active metal doping for Cu-Al based spinel catalysts synthesis toward high-efficiency reverse water-gas shift reaction

The reverse water-gas shift (RWGS) reaction is of great significance to convert CO2 to valuable feedstocks. Reasonable design and preparation of catalysts is necessary to achieve high CO2 conversion and CO selectivity. In this study, a series of Cu-Al based spinel catalysts were synthesized by coprecipitation-calcination method or A-site doped with active metal (Co, Zn, Mg, and Fe) for the RWGS reaction. The results indicated that the surface oxygen vacancies and crystal structure of CuAl2O4 can be regulated by calcination temperature. Remarkably, the CuAl-800 catalyst exhibits the highest CO2 conversion rate (62.7 %) with 100 % CO selectivity at 500 °C. Excessive calcination temperatures (> 800 °C) resulted in a well-defined spinel structure, but decreased surface oxygen vacancies and CO2 conversion rate. At calcination temperatures < 800 °C, surface oxygen vacancies increased, but the formation of CuO impurities which irreversibly converted to Cu2O during reduction, decreased catalytic activity. Compared with Zn, Mg and Fe, Co doping can significantly improved the reactivity of CuAl2O4 catalyst in RWGS reaction at 500 °C, increasing the CO2 conversion rate by 8 % while maintaining 100 % CO selectivity. The main reason is that Co doping not only effectively improves the integrity and stability of spinel structure of CuAl2O4, but also enhances its ability to dissociate and activate H species through interfacial sites of CuO-OV-CuXCo1-XAl2O4, thus further enhancing its catalytic conversion ability of CO2 to CO. These results enrich the understanding of surface chemistry of CuAl2O4 spinel and lay an important theory foundation for design of spinel-based catalysts for RWGS reaction.

Facet-dependent electrocatalytic oxygen evolution of porous network structure Ni-Fe-O materials

Constructing unique crystal facets can increase active sites and enhance electronic structure for electrocatalysts towards the oxygen evolution reaction (OER). Here, we successfully achieved the transformation between the (311) and (222) facets of the NiFe2O4 catalysts by adjusting the ratio of Ni and Fe elements, and it was found that the activity in OER is enhanced with the exposure of (311) facet. The optimal catalyst shows a lower overpotential of 238 mV at 100 mA/cm2 current density and a small Tafel slope of 26.4 mV/dec. Furthermore, benefiting from the porous network structure, the unique super hydrophilic surface and the low-coordination Fe3+ atoms characteristic, the catalyst has an efficient mass transfer channel and fast charge transfer in the electrocatalytic OER process. More metal-O clusters were exposed on the (311) facet surface as the active site, which further improved the catalytic activity. This work presents a successful approach to creating highly active spinel catalysts through facet-engineering for electrocatalytic water splitting.

Spinel-structured catalyst formed by Fe, Cu, and Co composites for efficient toluene oxidation

Due to its distinctive structure and properties, MFe2O4 has been extensively investigated in catalytic oxidation. In this study, we synthesized a ternary metal composite Fe-based spinel catalyst using the sol-gel method for toluene oxidation. The catalytic performance assessments showed that the catalytic performance of Cu0.7Co0.3Fe2O4 (T90 = 262 °C, 100,000 mL·g−1·h−1) was superior to that of CuFe2O4 (T90 = 270 °C) and CoFe2O4 (T90 = 310 °C), compared to binary metal spinel catalysts. The characterization results indicate that the strong interactions among Cu, Co, and Fe in the ternary metal composite structure increased the content of oxygen vacancy at lower temperatures, thereby increasing the content of surface reactive oxygen species and improving catalytic performance. This study highlights the potential application of Fe-based spinel structures to develop more efficient spinel catalysts.

Unveiling the Ni2+ coordination sites effect of Ni-Al spinel catalysts for phenanthrene hydrogenation saturation

Nickel–aluminum (Ni–Al) spinel catalysts with distinct Ni/Al molar ratios were synthesized to construct Ni2+ coordination sites. Owing to the favorable distribution of Ni2+ coordination sites and the presence of suitable concentration of Ni2+ in octahedral sites, Ni/Al-0.725 catalyst achieved an 86.1 % average yield of perhydrophenanthrene, which was higher than that obtained with Ni/Al-0.25 (6.3 %). Characterization results indicated that Ni2+ of the Ni/Al-0.725 catalyst was present in octahedral sites and ∼ 40.4 % Ni2+ was reduced to Ni, whereas the active Ni metal of Ni/Al-0.25 catalyst mainly comprised Ni2+ occupying tetrahedral sites as precursors. Furthermore, the Ni/Al-0.725 catalyst exhibited the smallest Ni particle size (2.82 nm) and the largest Ni available surface (19.62 m2/gsample), which helped alleviate competitive adsorption among aromatic hydrocarbons. The Ni reduced by the spinel catalysts was in an electron-deficient state, which enhanced the adsorption of polycyclic aromatic hydrocarbons.

Size Effect of Cu Nanoparticles from Cu–Al Spinel Catalysts on Product Distribution via CO Hydrogenation

Size Effect of Cu Nanoparticles from Cu–Al Spinel Catalysts on Product Distribution via CO Hydrogenation

Effects of Zn loading on ZnxCo1−xCo2O4 spinel catalysts for low-temperature catalytic decomposition of N2O in the presence of inhibitors

The recent advancement of NH3-based fuel demands the development of catalysts that can decompose N2O at low temperatures and are stable against various inhibitors. Zn-doped cobalt spinel (ZnxCo1−xCo2O4) catalysts have demonstrated superior activity for N2O decomposition at low temperatures. However, their optimization and deactivation mechanisms with respect to Zn loading when inhibitors are present require further investigation. Herein, we reviewed the effects of Zn loading on catalysts at low temperatures without or with the presence of inhibitors (3 vol% O2, 6 vol% H2O, 200 ppm NO). The results showed that adding an appropriate amount of Zn into Co3O4 spinel catalysts improves their N2O decomposition activity. The Zn-induced enhancement of the catalysts’ redox properties, and reducibility was the underlying mechanism behind the activity promotion. The presence of structural defects created through the addition of Zn enhanced the catalyst’s resistance to O2. The deactivation by H2O and NO upon the addition of Zn could be correlated with the reduced availability of surface Co species and Co2+ as adsorption-active sites and the stronger bond of Co with –NO, respectively. This study underscores the importance of controlling the Zn loading as a bulk redox promoter in ZnxCo1−xCo2O4 spinel catalysts for practical applications.

A Low-Noble-Metal Ru@CoMn2O4 Spinel Catalyst for the Efficient Oxidation of Propane.

Noble metals have become a research hotspot for the oxidation of light alkanes due to their low ignition temperature and easy activation of C-H; however, sintering and a high price limit their industrial applications. The preparation of effective and low-noble-metal catalysts still presents profound challenges. Herein, we describe how a Ru@CoMn2O4 spinel catalyst was synthesized via Ru in situ doping to promote the activity of propane oxidation. Ru@CoMn2O4 exhibited much higher catalytic activity than CoMn2O4, achieving 90% propane conversion at 217 °C. H2-TPR, O2-TPD, and XPS were used to evaluate the catalyst adsorption/lattice oxygen activity and the adsorption and catalytic oxidation capacity of propane. It could be concluded that Ru promoted synergistic interactions between cobalt and manganese, leading to electron transfer from the highly electronegative Ru to Co2+ and Mn3+. Compared with CoMn2O4, 0.1% Ru@CoMn2O4, with a higher quantity of lattice oxygen and oxygen mobility, possessed a stronger capability of reducibility, which was the main reason for the significant increase in the activity of Ru@CoMn2O4. In addition, intermediates of the reaction between adsorbed propane and lattice oxygen on the catalyst were monitored by in situ DRIFTS. This work highlights a new strategy for the design of a low-noble-metal catalyst for the efficient oxidation of propane.

Enhancing sulfur resistance of oxides in carbon monoxide oxidation by a high‐entropy‐stabilized strategy

AbstractIndustrial chemical processes require sulfur‐resistant catalysts, which reduce catalyst replacement costs and simplify process operations. Herein, a high‐entropy‐stabilized strategy was put forward for sulfur‐resistant catalysis. A spinel high entropy (Zn0.2Mg0.2Cu0.2Mn0.2Co0.2Al2O4) was introduced by ball milling process with aluminum isopropoxide as the main precursor. Zn0.2Mg0.2Cu0.2Mn0.2Co0.2Al2O4 possessed a high surface area of 171.2 m2 g−1, higher than typical high‐entropy oxides (HEOs). The high‐entropy spinel catalyst exhibited better SO2‐resistance performance in the oxidation of carbon monoxide, better than the simple oxides. The SO2‐resistance of Zn0.2Mg0.2Cu0.2Mn0.2Co0.2Al2O4 was primarily improved by reinforcing the stability of the oxide using a high‐entropy structure to decrease the absorption of SO2 on its surface. Any adsorbed SO2 on the surface of the HEO was then selectively trapped by sacrificial metal ions with stronger electron‐withdrawing ability, protecting the active center (Cu2+, Co2+) from poisoning. This work reveals the significance of high‐entropy structures in sulfur resistance.