Realizing the high loading amount of active Cu on Al2O3 to boost its CO catalytic oxidation

Catalytic oxidation of carbon monoxide (CO) by Cu/Al2O3 has garnered increasing interest in recent years due to its promising application prospects. Numerous investigations conducted on the Cu/Al2O3 system, but its catalytic performance for CO oxidation is still not as promising as that of precious metal catalysts. Increasing the loading amount of the active Cu on Al2O3 surface is a feasible method for improving its activity. However, with the increase of Cu loading, the agglomeration and enlargement of Cu particles is inevitable, which reduces the active Cu amount. Therefore, the utilization rate of Cu atoms is not high and the catalytic performance often can not further rise. Enhancing active Cu loading amount as high as possible is a prerequisite to further enlarge the activity of Cu/Al2O3 catalyst. Herein, self-synthesized Al2O3 nanofibers (Al2O3-nf) with high specific surface area and abundant penta-coordinated aluminum (AlV) are used as the support to maximize the Cu loading amount by chemical vapor deposition (CVD). And commercially available α-Al2O3 is used for comparative experiment. The high specific surface area could make Cu high dispersion on Al2O3, even at 20 wt% Cu loads, which is beneficial to high concentration load of active Cu. The catalytic activity of Cu/Al2O3-nf-CVD gradually increases with the increase of Cu loading from 2 wt% to 20 wt%, exhibiting a clear linear correlation with the surface content of Cu0 on the catalyst. Meanwhile, this result confirms that Cu0 plays a crucial role in CO oxidation of Cu/Al2O3. However, commercial α-Al2O3 reaches its highest activity when the Cu load is 5%, and then its activity begins to decrease due to the agglomeration of particles. Moreover, Cu/Al2O3-nf-CVD also exhibits remarkable thermal stability for CO oxidation. This work highlights a new strategy to synthesis of high Cu loading amount, high activity and thermostable Cu/Al2O3 catalyst for low-temperature oxidation of CO.

Role of H2O Adsorption in CO Oxidation over Cerium-Oxide Cluster Anions (CeO2)nO- (n = 1-4)

Water (H2O) is ubiquitous in the environment and inevitably participates in many surface reactions, including CO oxidation. Acquiring a fundamental understanding of the roles of H2O molecules in CO oxidation poses a challenging but pivotal task in real-life catalysis. Herein, benefiting from state-of-the-art mass-spectrometric experiments and quantum chemical calculations, we identified that the dissociation of a H2O molecule on each of the cerium oxide cluster anions (CeO2)nO- (n = 1-4) at room temperature can create a new atomic oxygen radical (O•-) that then oxidizes a CO molecule. The size-dependent reactivity of H2O-mediated CO oxidation on (CeO2)nO- clusters was rationalized by the orbital compositions (O2p) and energies of the lowest unoccupied molecular orbitals of active O•- radicals modified by H2O dissociation. Our findings not only provide new insights into H2O-mediated CO oxidation but also demonstrate the importance of H2O in modulating the reactivity of the O•- radicals.

Unlocking the chemical environment of nitrogen in perovskite-type oxides

Nitrogen (N) doping of perovskite-type oxides is an effective method for enhancing their photocatalytic performance. Quantitative and qualitative analyses of the doped N species are essential for a deeper understanding of the catalytic activity enhancement mechanism. However, examining the N environment in perovskite-type oxides, particularly in the bulk, using conventional analytical techniques, such as X-ray photoelectron spectroscopy (XPS), is challenging. In this study, we propose a new analytical technique, advanced temperature-programmed desorption (TPD) up to 1600 °C, to complement the conventional methods. TPD can quantify all N species in bulk oxides. Moreover, it facilitates chemical speciation of N environments, such as substitutional and interstitial N species. This is verified by XPS, CHN elemental analysis, X-ray absorption spectroscopy, and in situ diffuse reflectance infrared Fourier-transform spectroscopy. This study demonstrates the feasibility of advanced TPD as a new analytical method that offers comprehensive information on the N species within N-doped oxide materials at the bulk level.

CO oxidation reactions on 3-d single metal atom catalysts/MgO(100)

The present work deals with a comprehensive computational theoretical study of the molecular CO and O2 adsorption on 3d single atoms (M/MgO(100)). The study is based on the chemical elements of the 3d row, as they represent an economic advantage compared with the so-called noble metals. The present study has been performed employing density functional theory calculations. Through the representation of the metastable states, we perform a synergetic analysis of the CO oxidation reaction to find trends that suggest the possible use of new candidates such as Ni/MgO(100) or Cu/MgO(100) single-atom catalysts, for this type of redox reaction. We found that Ni and Cu produce energetically viable CO to CO2 reactions. Ni and Cu atoms show the greatest diffusion barrier and are the best candidates due to their low sintering capability. The energetic and electronic properties of the single Cu and Ni atoms on MgO (100) give them the best characteristics to help in the CO oxidation process.

Improving the Catalytic Performance of BaMn0.7Cu0.3O3 Perovskite for CO Oxidation in Simulated Cars Exhaust Conditions by Partial Substitution of Ba

The sol-gel method, adapted to aqueous media, was used for the synthesis of BaMn0.7Cu0.3O3 (BMC) and Ba0.9A0.1Mn0.7Cu0.3O3 (BMC-A, A = Ce, La or Mg) perovskite-type mixed oxides. These samples were fully characterized by ICP-OES, XRD, XPS, H2-TPR, BET, and O2-TPD and, subsequently, they were evaluated as catalysts for CO oxidation under different conditions simulating that found in cars exhaust. The characterization results show that after the partial replacement of Ba by A metal in BMC perovskite: (i) a fraction of the polytype structure was converted to the hexagonal BaMnO3 perovskite structure, (ii) A metal used as dopant was incorporated into the lattice of the perovskite, (iii) oxygen vacancies existed on the surface of samples, and iv) Mn(IV) and Mn(III) coexisted on the surface and in the bulk, with Mn(IV) being the main oxidation state on the surface. In the three reactant atmospheres used, all samples catalysed the CO to CO2 oxidation reaction, showing better performances after the addition of A metal and for reactant mixtures with low CO/O2 ratios. BMC-Ce was the most active catalyst because it combined the highest reducibility and oxygen mobility, the presence of copper and of oxygen vacancies on the surface, the contribution of the Ce(IV)/Ce(III) redox pair, and a high proportion of surface and bulk Mn(IV). At 200 °C and in the 0.1% CO + 10% O2 reactant gas mixture, the CO conversion using BMC-Ce was very similar to the achieved with a 1% Pt/Al2O3 (Pt-Al) reference catalyst.

Hybrid Amyloid-Chitin Nanofibrils for Magnetic and Catalytic Aerogels

In the quest for a sustainable and circular economy, it is essential to explore environmentally friendly alternatives to traditional petroleum-based materials. A promising pathway toward this goal lies in the leveraging of biopolymers derived from food waste, such as proteins and polysaccharides, to develop advanced sustainable materials. Here, we design versatile hybrid materials by hybridizing amyloid nanofibrils derived by self-assembly of whey, a dairy byproduct, with chitin nanofibrils exfoliated from the two distinct allomorphs of α-chitin and β-chitin, extracted from seafood waste. Various hydrogels and aerogels were developed via the hybridization and reassembly of these biopolymeric nanobuilding blocks, and they were further magnetized upon biomineralization with iron nanoparticles. The pH-phase diagram highlights the significant role of electrostatic interactions in gel formation, between positively charged amyloid fibrils and negatively charged chitin nanofibrils. Hybrid magnetic aerogels exhibit a ferromagnetic response characterized by a low coercivity (<50 Oe) and a high specific magnetization (>40 emu/g) at all temperatures, making them particularly suitable for superparamagnetic applications. Additionally, these aerogels exhibit a distinct magnetic transition, featuring a higher blocking temperature (200 K) compared to previously reported similar nanoparticles (160 K), indicating enhanced magnetic stability at elevated temperatures. Finally, we demonstrate the practical application of these hybrid magnetic materials as catalysts for carbon monoxide oxidation, showcasing their potential in environmental pollution control and highlighting their versatility as catalyst supports.

CO2 and CO Capture on the ZnO Surface: A GCMC and Electronic Structure Study

The amount of polluting gases released into the atmosphere has grown drastically. Among them, it is possible to cite the release of CO2 and CO gases on a large scale as one of the products of the complete and incomplete combustion of petroleum-derived fuels. It is worth noting that the production of energy by burning fossil fuels supplies the energy demand but causes environmental damage, and several studies have addressed the reduction. One of them is using materials with the potential to capture these gases. The experimental and theoretical studies have significant contributions that promote advances in this area. Among the materials investigated, ZnO has emerged, demonstrating the considerable potential for capturing various gases, including CO2 and CO. This work used density functional theory (DFT) and Grand Canonical Monte Carlo Method (GCMC) to investigate the adsorption of CO2 and CO on the surface of Zinc oxide (ZnO) to obtain adsorption isotherms and interaction energy and the interaction nature. The results suggest that CO2 adsorption slightly changed the angle of the O-C-O to values less than 180°. For the CO, its carbon atom interacts simultaneously with Zn and O of the ZnO surface. However, CO interactions have an ionic character with a lower binding energy value than the CO2 interaction. The energies calculated using the PM6 and DFT methods generated results compatible with the experimental values. In applications involving a mixture of these two gases, the adsorption of CO2 should be favored, and there may be inhibition of the adsorption of CO for high CO2 concentrations.

Improvement of CO Oxidation and CH4 Combustion by Pd and Pt Partial Substitution on LaMn0.5Cu0.5O3 Perovskite

LaMn0.5Cu0.5O3 (LMC) as the parent perovskite and Pd- and Pt-doped LaMn0.5Cu0.5O3 catalysts (LMCPd and LMCPt) instead of Cu were synthesized in a new solid-state synthesis technique at a low temperature. Perovskite lattice formation of the LMC catalyst was successfully performed at 600 °C. All perovskites were investigated by X-ray diffraction, HRTEM, O2-TPD, H2-TPR, BET, and XPS analyses. The prepared perovskites were used as heterogeneous catalysts for CO oxidation and methane combustion reactions. The catalytic performance of the LMC catalyst was noticeably enhanced via Pd and Pt substitution instead of Cu. The enhancement in the mobility of lattice oxygen and specific surface area has triggered this catalytic performance improvement, which play an important role in CO oxidation and methane combustion. The Mn 2p and Mn 3s XPS spectra showed that by doping Pd and Pt in the LMC perovskite, Mn was affected in different states and the Mn 3s peaks were only observed in the LMCPt catalyst. XPS spectra of the LMCPd1 sample showed a high oxidation state of Pd3+ or Pd4+, from which it can be concluded that Pd was successfully incorporated into the LMC perovskite lattice. The H2-TPR profiles of the LMCPd and LMCPt perovskites revealed that the reduction peaks of Cu and Mn were shifted to lower temperatures by increasing Pd and Pt partial substitution due to the synergetic effect of the cation and the H2-spillover effect of palladium and platinum.

The Role of Grain Boundary Sites for the Oxidation of Copper Catalysts during the CO Oxidation Reaction

The oxidation of transition metal surfaces is a process that takes place readily at ambient conditions and that, depending on the specific catalytic reaction at hand, can either boost or hamper activity and selectivity. Cu catalysts are no exception in this respect since they exhibit different oxidation states for which contradicting activities have been reported, as, for example, in the catalytic oxidation of CO. Here, we investigate the impact of low-coordination sites on nanofabricated Cu nanoparticles with engineered grain boundaries on the oxidation of the Cu surface under CO oxidation reaction conditions. Combining multiplexed in situ single particle plasmonic nanoimaging, ex situ transmission electron microscopy imaging, and density functional theory calculations reveals a distinct dependence of particle oxidation rate on grain boundary density. Additionally, we found that the oxide predominantly nucleates at grain boundary-surface intersections, which leads to nonuniform oxide growth that suppresses Kirkendall-void formation. The oxide nucleation rate on Cu metal catalysts was revealed to be an interplay of surface coordination and CO oxidation behavior, with low coordination favoring Cu oxidation and high coordination favoring CO oxidation. These findings explain the observed single particle-specific onset of Cu oxidation as being the consequence of the individual particle grain structure and provide an explanation for widely distributed activity states of particles in catalyst bed ensembles.

Preparation of Ce-MnOx Composite Oxides via Coprecipitation and Their Catalytic Performance for CO Oxidation

Ce-MnOx composite oxide catalysts with different proportions were prepared using the coprecipitation method, and the CO-removal ability of the catalysts with the tested temperature range of 60-140 °C was investigated systematically. The effect of Ce and Mn ratios on the catalytic oxidation performance of CO was investigated using X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), H2 temperature programmed reduction (H2-TPR), CO-temperature programmed desorption (CO-TPD), and in situ infrared spectra. The experimental results reveal that under the same test conditions, the CO conversion rate of pure Mn3O4 reaches 95.4% at 170 °C. Additionally, at 140 °C, the Ce-MnOx series composite oxide catalyst converts CO at a rate of over 96%, outperforming single-phase Mn3O4 in terms of catalytic performance. With the decrement in Ce content, the performance of Ce-MnOx series composite oxide catalysts first increase and then decrease. The Ce MnOx catalyst behaves best when Ce:Mn = 1:1, with a CO conversion rate of 99.96% at 140 °C and 91.98% at 100 °C.

In situfabrication of Cu-Mn-O nanostructure catalysts on Ti mesh and their catalytic property optimization for low-temperature and stable CO oxidation

A series of interlaced 'tripe-shaped' nanoflake catalysts made of CuMn2O4werein situprepared on Ti mesh substrate through the associated methods of plasma electrolyte oxidation and hydrothermal technique. The surface morphology, elemental distribution and chemical state, phase composition and microstructure of CuMn2O4nanostructures prepared under different conditions were systemically investigated. To evaluate the catalytic activity, the CO oxidation as a probe reaction was used, and the results showed that 12h-Cu1Mn2-300 (hydrothermal reaction at 150 °C for 12 h, Cu/Mn = 1/2 in initial precursor, heat treatment temperature at 300 °C) exhibited the best CO oxidation capability withT100= 150 °C owe to the formation of uniform CuMn2O4nanosheet layersin situgrown on flexible Ti mesh and the synergistic effect of Cu and Mn species in spinel CuMn2O4, which makes it more active towards CO oxidation than pure copper/manganese oxides.

Insights into the Contribution of Oxidation-Reduction Pretreatment for Mn0.2Zr0.8O2-δ Catalyst of CO Oxidation Reaction

A Mn_0.2Zr_0.8O_2-δ mixed oxide catalyst was synthesized via the co-precipitation method and studied in a CO oxidation reaction after different redox pretreatments. The surface and structural properties of the catalyst were studied before and after the pretreatment using XRD, XANES, XPS, and TEM techniques. Operando XRD was used to monitor the changes in the crystal structure under pretreatment and reaction conditions. The catalytic properties were found to depend on the activation procedure: reducing the CO atmosphere at 400-600 °C and the reaction mixture (O₂ excess) or oxidative O₂ atmosphere at 250-400 °C. A maximum catalytic effect characterized by decreasing T₅₀ from 193 to 171 °C was observed after a reduction at 400 °C and further oxidation in the CO/O₂ reaction mixture was observed at 250 °C. Operando XRD showed a reversible reduction-oxidation of Mn cations in the volume of Mn_0.2Zr_0.8O_2-δ solid solution. XPS and TEM detected the segregation of manganese cations on the surface of the mixed oxide. TEM showed that Mn-rich regions have a structure of MnO₂. The pretreatment caused the partial decomposition of the Mn_0.2Zr_0.8O_2-δ solid solution and the formation of surface Mn-rich areas that are active in catalytic CO oxidation. In this work it was shown that the introduction of oxidation-reduction pretreatment cycles leads to an increase in catalytic activity due to changes in the origin of active states.

Operando Revealing the Crystal Phase Transformation and Electrocatalytic Activity Correlation of MnO2 toward Glycerol Electrooxidation

In this work, we report for the first time a comprehensive operando investigation of the intricate correlation between dynamic phase evolution and glycerol electrooxidation reaction (GEOR) performance across three primary MnO₂ crystallographic phases (α-, β-, and γ-MnO₂). The results showed that all three electrocatalysts exhibited comparable selectivity toward three-carbon products (∼90%), but γ-MnO₂ exhibited superior performance, with a low onset potential of ∼1.45 V_RHE, the highest current density of ∼1.9 mA cm^-2 at 1.85 V_RHE, and reasonable stability. Operando Raman spectroscopy revealed the potential-induced surface reconstruction of different MnO₂ structures from which a correlation among the applied potential, electrocatalytic activity, and product distribution was identified. The higher the applied potential, the greater conversion from the original structure to δ-MnO₂, resulting in lower C-C cleavage and higher 3C product selectivity. This study not only provides a systematic understanding of structure-controlled electrocatalytic activity for high selectivity toward 3C products of MnO₂ but also contributes to the development of a non-noble and environmentally friendly catalyst for valorizing glycerol.

Tailoring the Composition of BaxBO3 (B = Fe, Mn) Mixed Oxides as CO or Soot Oxidation Catalysts in Simulated GDI Engine Exhaust Conditions

Mixed oxides with perovskite-type structure (ABO₃) are promising catalysts for atmospheric pollution control due to their interesting and tunable physicochemical properties. In this work, two series of Ba_xMnO₃ and Ba_xFeO₃ (x = 1 and 0.7) catalysts were synthesized using the sol-gel method adapted to aqueous medium. The samples were characterized by μ-XRF, XRD, FT-IR, XPS, H₂-TPR, and O₂-TPD. The catalytic activity for CO and GDI soot oxidation was determined by temperature-programmed reaction experiments (CO-TPR and soot-TPR, respectively). The results reveal that a decrease in the Ba content improved the catalytic performance of both catalysts, as B0.7M-E is more active than BM-E for CO oxidation, and B0.7F-E presents higher activity than BF for soot conversion in simulated GDI engine exhaust conditions. Manganese-based perovskites (BM-E and B0.7M-E) achieve better catalytic performance than iron-based perovskite (BF) for CO oxidation reaction due to the higher generation of actives sites.

Role of La-based perovskite catalysts in environmental pollution remediation

Since the advent of the industrial revolution, there has been a constant need of efficient catalysts for abatement of industrial toxic pollutants. This phenomenon necessitated the development of eco-friendly, stable, and economically feasible catalytic materials like lanthanum-based perovskite-type oxides (PTOs) having well-defined crystal structure, excellent thermal, and structural stability, exceptional ionic conductivity, redox behavior, and high tunability. In this review, applicability of La-based PTOs in remediation of pollutants, including CO, NO x and VOCs was addressed. A framework for rationalizing reaction mechanism, substitution effect, preparation methods, support, and catalyst shape has been discussed. Furthermore, reactant conversion efficiencies of best PTOs have been compared with noble-metal catalysts for each application. The catalytic properties of the perovskites including electronic and structural properties have been extensively presented. We highlight that a robust understanding of electronic structure of PTOs will help develop perovskite catalysts for other environmental applications involving oxidation or redox reactions.

Rational design of trimetallic AgPt-Fe3O4 nanozyme for catalyst poisoning-mediated CO colorimetric detection

Carbon monoxide (CO) is not only a highly poisonous gas that brings great health risk, but also a significant signaling molecule in body. However, it is still challengeable for development of alternative colorimetric probes to traditional organic chromophores for simple, sensitive and convenient CO sensing. Here, for the first time, we rationally design a novel hydrophilic AgPt-Fe₃O₄ nanozyme with a unique heterodimeric nanostructure for colorimetric sensing of CO based on the excellent peroxidase-like catalytic activity as well as highly poisonous effect of CO on the nanozyme's catalytic activity. Both experimental evidence and theoretical calculations reveal the trimetallic AgPt-Fe₃O₄ nanozyme is susceptible to poisoning with the strongest affinity towards CO compared to individual Fe₃O₄ or Ag-Fe₃O₄, which is attributed to the adequate exposure of the active metallic sites and efficient interfacial synergy of unique heterodimeric nanostructure. Accordingly, a novel nanozyme-based colorimetric strategy is developed for CO detection with a low detection limit of 5.6 ppb in solution. Furthermore, the probe can be prepared as very convenient test strips and integrated with the portable smartphone platforms for detecting CO gas samples with a low detection limit of 8.9 ppm. Overall, our work proposes guidelines for the rational design of metallic heterogeneous nanostructure to expand the analytical application of nanozyme.

Multiple Interface Coupling in Ultrathin Mn-based Composites for Superior Catalytic Oxidation: Implications of Interface Coupling on Structural Defects

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. The modulation of the interfacial coupling effect of manganese oxides by chemical means is considered a critical and effective way to improve the catalytic performance. Herein, a novel one-step synthetic strategy of highly-efficient ultrathin manganese-based catalysts is proposed through optimal regulation of metal/manganese oxide multi-interfacial coupling. Carbon monoxide (CO) and propane (C₃H₈) oxidation are employed as probe reactions to investigate the structure-catalytic mechanism - catalytic performance relationship. The ultrathin manganese (Mn)-based catalyst exhibits superior low-temperature catalytic activity with a 90% conversion of CO/C₃H₈ realized at 106℃ and 350℃. Subsequently, the effect of "interfacial effect" on the intrinsic properties of manganese oxides is revealed. The ultrathin appearance of two-dimensional (2D) manganese dioxide (MnO₂) nanosheets changes the binding force in the vertical direction, thus resulting in an increase in the average manganese-oxygen (Mn-O) bond length and exposing more surface defects. Besides, the introduction of Copper (Cu) species into the catalyst further weakens the Mn-O bond and promotes the generation of oxygen vacancies, which subsequently enhances the oxygen migration rate. This study provides new insights into the optimal design of transition metal oxide interfacial assemblies for efficient catalytic reactions.

Boosting CO Catalytic Oxidation Performance via Highly Dispersed Copper Atomic Clusters: Regulated Electron Interaction and Reaction Pathways

Copper-loaded ceria (Cu/CeO₂) catalysts have become promising for the catalytic oxidation of industrial CO emissions. Since their superior redox property mainly arises from the synergistic effect between Cu and the CeO₂ support, the dispersion state of Cu species may dominate the catalytic performance of Cu/CeO₂ catalysts: the extremely high or low dispersity is disadvantageous for the catalytic performance. The nanoparticle catalysts usually present few contact sites, while the single-atom catalysts tend to be passivated due to their relatively single valence state. To achieve a suitable dispersion state, we synthesized a superior Cu/CeO₂ catalyst with Cu atomic clusters, realizing high atomic exposure and unit atomic activity simultaneously via favorable electron interaction and an anchoring effect. The catalyst reaches a 90% CO conversion at 130 °C, comparable to noble-metal catalysts. According to combined in situ spectroscopy and density functional theory calculations, the superior CO oxidation performance of the Cu atomic cluster catalyst results from the joint efforts of effective adsorption of CO at the electrophilic sites, the CO spillover phenomenon, and the efficient bicarbonate pathway triggered by hydroxyl. By providing a superior atomic cluster catalyst and uncovering the catalytic oxidation mechanism of Cu-Ce dual-active sites, our work may enlighten future research on industrial gaseous pollutant removal.

Highly Effective Pt-Co/ZSM-5 Catalysts with Low Pt Loading for Preferential CO Oxidation in H2-Rich Mixture

New Pt-Co catalysts of hydrogen purification from CO impurities for fuel cells were fabricated via the deposition of monodispersed 1.7 nm Pt nanoparticles using laser electrodispersion on Co-modified ZSM-5 prepared by the Co(CH3COO)2 impregnation. The structure of prepared Pt-Co zeolites was studied by low-temperature N2 sorption, TEM, EDX, and XPS methods. The comparative analysis of samples with different Pt (0.01–0.05 wt.%) and Co (2.5–4.5 wt.%) contents on zeolites with the ratio of Si/Al = 15, 28, and 40 was performed in the CO-PROX reaction in H2-rich mixture (1%CO + 1%O2 + 49%H2 + 49%He). The synergistic catalytic action of Pt and Co on zeolite surface makes it possible to completely remove CO from a mixture with hydrogen in a wide temperature range from 50 to 150 °C; the high efficiency of designed composites with low Pt loading is maintained for a long time. The enhancement of PROX performance originates from the formation of new active sites for the CO oxidation at the Pt-Co interfaces within zeolite channels and at the surface. In terms of their activity, stability, and selectivity, such composites are significantly superior to known supported Pt-Co catalysts.

C-O Bond Activation in Mononuclear Lanthanide Oxocarbonyl Complexes OLn(η2-CO) (Ln = La, Ce, Pr, and Nd)

Fundamental investigation of metal-CO interactions is of great importance for the development of high-performance catalysts to CO activation. Herein, a series of side-on bonded mononuclear lanthanide (Ln) oxocarbonyl complexes OLn(η²-CO) (Ln = La, Ce, Pr, and Nd) have been prepared and identified in solid argon matrices. The complexes exhibit uncommonly low C-O stretching bands near 1630 cm^-1, indicating remarkable C-O bond activation in these Ln analogues. The η²-CO ligand in OLn(η²-CO) can be claimed as an anion on the basis of the experimental observations and quantum chemistry investigations, although the CO anion is commonly considered to be unstable with electron auto-detachment. The CO activation in OLn(η²-CO) is attributed to the photoinduced intramolecular charge transfer from LnO to CO rather than the generally accepted metal → CO π back-donation, which conforms to the traditional Dewar-Chatt-Duncanson motif. Energy decomposition analysis combined with natural orbitals for chemical valence calculations demonstrates that the bonding between LnO and η²-CO arises from the combination of dominant ionic forces (>76%) and normal Lewis "acid-base" interactions. The fundamental findings provide guidelines for the catalyst design of CO activation.

Insight into the Effect of Oxygen Vacancy Prepared by Different Methods on CuO/Anatase Catalyst for CO Catalytic Oxidation

In this study, CuO loaded on anatase TiO2 catalysts (CuO/anatase) with oxygen vacancies was synthesized via reduction treatments by NaHB4 and H2 (CuO/anatase-B, CuO/anatase-H), respectively. The characterizations suggest that different reduction treatments bring different concentration of oxygen vacancies in the CuO/anatase catalysts, which finally affect the CO catalytic performance. The CuO/anatase-B and CuO/anatase-H exhibit CO conversion of 90% at 182 and 198 °C, respectively, which is lower than what occurred for CuO/anatase (300 °C). The XRD, Raman, and EPR results show that the amount of the oxygen vacancies of the CuO/anatase-H is the largest, indicating a stronger reduction effect of H2 than NaHB4 on the anatase surface. The in situ DRIFTS results exhibit that the Cu sites are the adsorption sites of CO, and the oxygen vacancies on the anatase can active the O2 molecules into reactive oxygen species. According to the in situ DRIFTS results, it can be concluded that in the CO oxidation reaction, only the CuO/anatase-H catalyst can be carried out by the Mvk mechanism, which greatly improves its catalytic efficiency. This study explained the reaction mechanisms of CO oxidation on various anatase surfaces, which offers detailed insights into how to prepare suitable catalysts for low-temperature oxidation reactions.

Highly Resistant LaCo1−xFexO3 Perovskites Used in Chlorobenzene Catalytic Combustion

The stability of LaCo1−xFexO3 perovskite structures (x = 0; 0.25; 0.5; 0.75; 1) was studied in the combustion of chlorobenzene. This family of catalysts was synthesized by the citrate method obtaining pure structures. The Fe doping in the original structure induces electronic environments capable of generating the Co2+/Co3+ redox couple. The characteristics observed in bulk are perfectly reflected on the surface, favoring a high resistance of the solids to chlorine poisoning. Superior stability under reaction conditions was observed in the phase with the lowest Fe content (x = 0.25), remaining stable at 100% combustion of chlorobenzene during 100 h, not observing intermediate reaction products. These results open up a new avenue for designing and fabricating high-performance catalysts in the environmental field

Facile Microwave Synthesis of Hierarchical Porous Copper Oxide and Its Catalytic Activity and Kinetics for Carbon Monoxide Oxidation

The synthesis of copper oxide (CuO)-based nanomaterials has received a tremendous deal of interest in recent years. Particularly, the design and development of novel CuO structures with improved physical and chemical properties have attracted immense attention, especially for catalysis applications. We report on a rational, rapid, and surfactant-free microwave synthesis (MWS) of hierarchical porous copper oxide (HP-CuO) with a three-dimensional (3D) sponge-like topology using an MWS reactor. The activity of the microwave (MW)-synthesized HP-CuO catalysts for carbon monoxide (CO) oxidation was studied and compared to CuO prepared by the conventional heating method (CHM). Results showed that HP-CuO catalysts prepared by MWS for 10 and 30 min surpassed the CuO catalyst prepared by CHM, exhibiting T 80 of 98 and 115 °C, respectively, as compared to 185 °C of CuO prepared by CHM (T80 is the temperature corresponding to 80% CO conversion). In addition, the MW-synthesized HP-CuO catalysts outperformed the CHM-synthesized CuO, achieving a 100% CO conversion at 150 °C compared to 240 °C in the case of CuO prepared by CHM. Interestingly, the HP-CuO catalyst expressed workable CO conversion kinetics with a reaction rate of c.a.35 μmol s-1 g-1 at 150 °C and apparent activation energy (E a) of 82 kJ mol-1. The HP-CuO catalyst showed excellent cycling and long-term stabilities for CO oxidation up to 4 cycles and 72 h on the stream, respectively. The enhanced catalytic activity and stability of the HP-CuO catalyst appear to result from the unique topological and structural features of HP-CuO, which were revealed by SEM, XRD, Raman, BET, TGA, XPS, and TPR techniques.

Porous unsupported CuO nanoplates for efficient photothermal CO oxidation

It is a significant issue for environmental protection and industrial production to eliminate CO, a gas harmful to life and some important reaction sites. Real environmental conditions require catalytic CO oxidation to occur at relatively low temperature. Nowadays, photothermal catalysis has been exploited as a new way to achieve CO elimination, different from thermal catalysis. CuO, as cheap and abundant substitute for precious metals, is considered to have potential in photothermal catalysis. Oxygen vacancies (O_V) and lattice oxygen (O_L) activity are considered extremely crucial for oxide catalysts in CO oxidation, according to Mars-van Krevelen mechanism. Herein, porous CuO nanoplates with adjustable O_Vand O_Lactivity were prepared by a facile method via controlling the morphology and phase composition of precursors. The light-off temperature (50% conversion) of the best sample obtained under the optimal conditions was ∼110 °C and an almost complete conversion was reached at ∼150 °C. It also achieved nearly 70% conversion under 6 standard Suns (6 kW cm^-2irradiation) and could work in infrared radiation (IR) regions, which could be attributed to the photo-induced thermal effect and activation effect. The simple synthesis and characterization provide a good example for the future photothermal catalysis.

Pt-Modified Nano-Sized Mn2O3 Oxide Prepared from the Mn3O4 Phase with Tetragonal Symmetry for CO Oxidation

One of the current problems in the environmental catalysis is the design of an effective and less costly catalytic system for the oxidation of CO. The nano-sized α-Mn2O3 oxide has been prepared and modified with 0.5 wt.% Pt. The catalysts have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), temperature-programmed reduction (TPR) and diffuse-reflectance infrared spectroscopy (DRIFTS). Finely divided PtO and Pt(OH)2 are being formed on the Mn2O3 surface as a result of the strong interaction between platinum and the nano-oxide. Based on DRIFTS investigations and the model calculations, a Langmuir–Hinshelwood type of mechanism is supposed for CO oxidation on Pt/Mn2O3. The CO and oxygen are adsorbed on different types of sites. The Mars–van Krevelen mechanism is the most probable one over pure Mn2O3, thus suggesting that CO2 is adsorbed on the oxidized sites. The CO adsorption in the mixture CO + N2 or in the presence of oxygen (CO + N2 + O2) leads to a partial reduction in the Pt+ surface species and the formation of linear Pt1+−CO and Pt0−CO carbonyls. Both of them take part in the CO oxidation reaction.

Study of the Cu(111) Surface by Scanning Tunneling Microscopy: The Morphology Evolution, Reconstructions, Superstructures and Line Defects

The Cu(111) surface is an important substrate for catalysis and the growth of 2D materials, but a comprehensive understanding of the preparation and formation of well-ordered and atomically clean Cu(111) surfaces is still lacking. In this work, the morphology and structure changes of the Cu(111) surface after treatment by ion bombardment and annealing with a temperature range of 300-720 °C are investigated systematically by using in situ low-temperature scanning tunneling microscopy. With the increase of annealing temperature, the surface morphology changes from corrugation to straight edge, the number of screw dislocations changes from none to numerous, and the surface atomic structure changes from disordered to ordered structures (with many reconstructions). In addition, the changing trend of step width and step height in different stages is different (first increased and then decreased). A perfect Cu(111) surface with a step height of one atom layer (0.21 nm) and a width of more than 150 nm was obtained. In addition, two interesting superstructures and a new surface phase with a large number of line defects were found. This work serves as a strong foundation for understanding the properties of Cu(111) surface, and it also provides important guidance for the effective pretreatment of Cu(111) substrates, which are widely used.

Modified BaMnO3-Based Catalysts for Gasoline Particle Filters (GPF): A Preliminary Study

Gasoline engines, mainly gasoline direct injection engines (GDI) require, in addition to three-way catalysts (TWC), a new catalytic system to remove the formed soot. Gasoline Particle Filters (GPF) are, among others, a possible solution. BaMnO3 and copper-doped BaMnO3 perovskites seem to be a feasible alternative to current catalysts for GPF. The physical and chemical properties of these two perovskites determining the catalytic performance have been modified using different synthesis routes: (i) sol-gel, (ii) modified sol-gel and iii) hydrothermal. The deep characterization allows concluding that: (i) all samples present a perovskite-like structure (hexagonal), except BMC3 which shows a polytype one (due to the distortion caused by copper insertion in the lattice), and ii) when a low calcination temperature is used during synthesis, the sintering effect decreases and the textural properties, the reducibility and the oxygen mobility are improved. The study of soot oxidation simulating the hardest GDI scenarios reveals that, as for diesel soot removal, the best catalytic performance involves the presence of oxygen vacancies to adsorb and activate oxygen and a labile Mn (IV)/Mn (III) redox pair to dissociate the adsorbed oxygen. The combination of both properties allows the transport of the dissociated oxygen towards the soot.

Biomimetic Tremelliform Ultrathin MnO2/CuO Nanosheets on Kaolinite Driving Superior Catalytic Oxidation: An Example of CO

Highly efficient three-dimensional (3D) kaolinite/MnO₂-CuO (KM@CuO-NO₃) catalysts were synthesized by a mild biomimetic strategy. Kaolinite flakes were uniformly wrapped by ultrathin tremelliform MnO₂ nanosheets with thicknesses of around 1.0-1.5 nm. Si-O and Al-O groups in kaolinite hosted MnO₂ nanosheets to generate a robust composite structure. The ultrathin MnO₂ lamellar structure exhibited excellent stability even after calcination above 350 °C. Kaolinite/MnO₂ exhibited abundant edges, sharp corners, and interconnected diffusion channels, which are superior to the common stacked structure. Open channels guaranteed fast transportation and migration of CO and O₂ during CO oxidation. The synthesized KM@CuO-NO₃ achieved a 90% CO conversion efficiency at a relatively low temperature (110 °C). Furthermore, the abundant oxygen vacancies on KM@CuO-NO₃ assisted the adsorption and activation of oxygen species and thus enhanced the oxygen mobility and reactivity in the catalytic process. The mechanism results suggest that CuO introduced to the catalyst not only acted as CO active sites but also weakened the Mn-O bond, subsequently improved the mobilities of the oxygen species, which was found to contribute to its high activity for CO oxidation. This study provides new conceptual insights into rationally regulating structural assembly between transition metal oxides and natural minerals for high-performance catalysis reactions.

High-Surface-Area Synthesis of Iron-Doped CaTiO3 at Low Temperatures: New Insights into Oxygen Activation, Iron States, and the Impact on Methane Oxidation

In the present work, a series of CaTi_1-xFe_xO_3-δ (0 < x < 0.5) materials are prepared using a modified Pechini method based on citric acid and a polyol as chelating agents. The synthesis conditions are optimized with respect to the specific surface area and phase purity by varying polyols (ethylene glycol, glycerol, and 1.6-hexanediol) and the ratio between citric acid, polyols, and cations. The impact of the polyols and the iron content (up to 40 mol % on the B site) is studied with respect to the oxygen exchange rate, reducibility using H₂-TPR, and catalytic performance for methane total oxidation. A correlation between the oxygen exchange rate studied using ¹⁸O exchange in powdered samples of CaTi_1-xFe_xO_3-δ (0 < x < 0.5) and ferric sites determined using Mössbauer spectroscopy and H₂-TPR is established. The oxygen activation and diffusion in CaTi_1-xFe_xO_3-δ (0 < x < 0.5) continuously increase in the studied range of Ti substitution. The methane oxidation performance does not increase above x = 0.3, showing that methane oxidation is not limited by surface oxygen activation and CH₄ is activated by specific iron sites in Fe-doped perovskites.

Inexpensive and easily replicable precipitation of CuO nanoparticles for low temperature carbon monoxide and toluene catalytic oxidation

Herein CuO nanoparticles (NPs) with nanostructures were prepared by precipitation method using hydrate copper sulfate (CuSO₄.5H₂O) and sodium hydroxide followed by heat treatment at 400 °C. The as-prepared CuO NPs with nanostructures were investigated using X-ray diffraction (XRD), Fourier Transformed Infra-red spectroscopy (FTIR), Raman spectroscopy, Scanning electron microscopy (SEM), X-ray photochemical spectroscopy (XPS), Energy dispersive spectroscopy (EDS), and Ultra-violet-visible (UV-vis) spectroscopy. In order to evaluate the reducibility, temperature programmed reduction (H₂-TPR) was applied. More importantly, CuO NPs was successfully tested as catalyst towards the total conversion of carbon monoxide (CO) and toluene (C₇H₈). Both XRD and Raman analysis as well as FTIR show that the sample exhibited a monoclinic spinel structure. SEM images indicate that CuO NPs are well-covered by grains size exhibiting homogeneous morphology composed of very fine interconnected particles with an apparent porosity. The sample was made up of Cu and O, according to the XPS and EDS measurements. The band gap energy obtained from optical property analysis is ∼2.65 eV. The catalytic performances of CuO NPs can be assigned to the combined effects of crystal structure, morphology, surface oxygen mobility, redox property and the higher specific surface area (∼87 m²/g). More precisely XPS and H₂-TPR data suggests that, the conversion of CO and C₇H₈ over CuO NPs follows a Mars-van Krevelen type mechanism. More importantly CuO NPs catalysts is reusable and exhibited good stability in the prolonged isothermal test. Thus, CuO NPs is confirmed as an efficient and inexpensive catalysts for CO and C₇H₈ conversion at low temperatures.

The noble metals M (M = Pd, Ag, Au) decorated CeO2catalysts derived from solution combustion method for efficient low-temperature CO catalytic oxidation: effects of different M loading on catalytic performances

The noble metal nanoparticles have attracted attention due to their excellent catalytic performance for CO oxidation at low temperatures. M-CeO₂(M = Pd, Ag, Au) catalysts with different atomic ratios of M/Ce were deposited via solution combustion method. Among them, 3 at% Pd-CeO₂, 5 at% Ag-CeO₂and 1 at% Au-CeO₂catalysts have better catalytic performances. Especially, 5 at% Ag-CeO₂catalyst shows better low-temperature CO oxidation performance. The catalytic activity for CO oxidation follows the follows the following sequence: 5 at% Ag-CeO₂(T₅₀ = 69 °C) > 3 at% Pd-CeO₂(T₅₀ = 99 °C) >1 at% Au-CeO₂(T₅₀ = 115 °C). Meanwhile, the catalysts are characterized by means of powder x-ray diffraction, scanning electron microscope, transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, Brunauer-Emmett-Teller and H₂-TPR. The characterization results show that the 5 at% Ag-CeO₂catalyst has excellent catalytic activity due to the good dispersion of Ag nanoparticles, the specific surface area of the material, and the reduction catalyst between different valence ions. Moreover, the surface of the catalyst enhances the mutual synergy, effectively promotes the generation of oxygen vacancies, and increases the active oxygen content of the catalyst surface. Finally, the catalytic mechanism of M-CeO₂catalysts is summarized.

Quality or Quantity? How Structural Parameters Affect Catalytic Activity of Iron Oxides for CO Oxidation

The replacement of noble metal catalysts by abundant iron as an active compound in CO oxidation is of ecologic and economic interest. However, improvement of their catalytic performance to the same level as state-of-the-art noble metal catalysts requires an in depth understanding of their working principle on an atomic level. As a contribution to this aim, a series of iron oxide catalysts with varying Fe loadings from 1 to 20 wt% immobilized on a γ-Al2O3 support is presented here, and a multidimensional structure–activity correlation is established. The CO oxidation activity is correlated to structural details obtained by various spectroscopic, diffraction, and microscopic methods, such as PXRD, PDF analysis, DRUVS, Mössbauer spectroscopy, STEM-EDX, and XAS. Low Fe loadings lead to less agglomerated but high percentual amounts of isolated, tetrahedrally coordinated iron oxide species, while the absolute amount of isolated species reaches its maximum at high Fe loadings. Consequently, the highest CO oxidation activity in terms of turnover frequencies can be correlated to small, finely dispersed iron oxide species with a large amount of tetrahedrally oxygen coordinated iron sites, while the overall amount of isolated iron oxide species correlates with a lower light-off temperature.

Experimental research on rapid removing characteristics of carbon monoxide generated during gas explosions

A large amount of gas, such as CO, accumulates in a coal mine after an explosion, leading to CO poisoning. In this study, a self-developed platform was used to eliminate CO from coal mines and determine the mass of the rapidly eliminated CO and its concentration in the eliminated gases. Equations were derived to calculate the amount of CO eliminated and the removing rate. The results showed that a rapid removing reagent in the form of nonprecious metal catalysts is useful for removing CO. Removing agents with larger masses facilitated the activation, irrespective of the CO concentration. For removing reagent amounts of 10, 15, 20, 25, and 30 g, the amount of CO eliminated, the removing rate, and the time required to complete catalytic oxidation increased sequentially. The CO removing process could be divided into three stages (I, II, and III) based on the variations in the CO, CO₂, and O₂ concentrations during CO removing. The removing reagent first chemically adsorbs CO and O₂, and then desorbs CO₂. The final CO concentration tends to 0, the O₂ concentration remains stable, and the CO₂ concentration decreases. This shows that the ablation agent has an impact on the changes in the CO and CO₂ concentrations.

Designing oxide chemiresistors for detecting volatile aromatic compounds: recent progresses and future perspectives

Oxide chemiresistors have mostly been used to detect reactive gases such as ethanol, acetone, formaldehyde, nitric dioxide, and carbon monoxide. However, the selective and sensitive detection of volatile aromatic compounds such as benzene, toluene, and xylene, which are extremely toxic and harmful, using oxide chemiresistors remains challenging because of the molecular stability of benzene rings containing chemicals. Moreover, the performance of the sensing materials is insufficient to detect trace concentration levels of volatile aromatic compounds, which lead to harmful effects on human beings. Here, the strategies for designing highly selective and sensitive volatile aromatic compound gas sensors using oxide chemiresistors were suggested and reviewed. Key approaches include the use of thermal activation, design of sensing materials with high catalytic activity, the utilization of catalytic microreactors and bilayer structures with catalytic overlayer, and the pretreatment of analyte gases or post analysis of sensing signals. In addition, future perspectives from the viewpoint of designing sensing materials and sensor structures for high-performance and robust volatile aromatic compounds gas sensors are provided. Finally, we discuss possible applications of the sensors and sensor arrays.