CO oxidation over CuO/Ce1−xCuxO2−δ and Ce1−xCuxO2−δ catalysts: Synergetic effects and kinetic study

Tracking of Active Sites as Well as the Compositing Effect over a Cu/Ce-Based Catalyst with Superior Catalytic Activity

The replacement of a noble metal catalyst by base metals presents a great challenge for low-temperature CO and volatile organic compounds oxidation. Cu/Ce-based catalysts are expected to achieve this goal with excellent performance, among which the main active sites still need to be further explored. For this reason, CuCe catalysts were further compounded with typical elements (cobalt, Co) to study the main active sites and compositing effect by in-situ enhanced Raman and in-situ ultralow-temperature DRIFTS technologies. The main active site for both CuCe and CuCoCe catalysts was the same Cu-OV-Ce at the copper-cerium interface, named as asymmetric oxygen vacancy (ASOv). The dispersion of CuO and CeO2 species was promoted, and the formation energy of ASOv was decreased significantly from 1.502 to 0.854 eV after the addition of Co, which leads to an increase in the ASOv concentration. A small cobalt added can form more Co2+ species, improving the activity and stability. The activity of Cu1Co0.5Ce3 catalyst was significantly improved with 100% conversion of CO and toluene at 96 and 227 °C. Here, the ASOv was studied in relative quantification, showing consistency of catalytic activity and ASOv concentration. Meanwhile, the dynamic exchange of ASOv in the reactions was tracked, indicating that the redox equilibrium of ASOv can continuously produce new ASOV in Cu/Ce-based catalysts that cause long-term catalytic stability. In addition, it is almost difficult for CoCe and CoCu samples to form the ASOv, and the interaction between metals and metals was also weaker than that of CuCe and CuCoCe catalysts.

Investigating the differences of active oxygen species and carbonate species on the surface of Ce0.95M (M = Mn and Zr)0.05O2-δ catalysts prepared by the aerosol method during CO oxidation using operando TPR-DRIFTS-MS

Surface oxygen species and carbonate species play an important role in CO oxidation. However, their essential relationsh with CO oxidation activity remains unclear. In this paper, Ce0.95M (M = Mn and Zr)0.05O2-δ catalysts are selected as the research target and operando TPR-DRIFTS-MS is used to investigate the changes of oxygen species and carbonate species on the catalyst surface. The Ce0.95Mn0.05O2-δ catalyst has the best CO conversion (145 °C) and CO2 selectivity (99%). Operando DRIFTS-MS results show that MO plays a key role on the catalyst surface and can react with CO at low temperatures. Importantly, the high content of MO is conducive to the formation of monodentate carbonate (M-O-CO2) (M-O-CO2 decomposes at 50 °C). As the temperature increases, CeO and M-O-Ce also react with CO and produce M-Ov-Ce (oxygen vacancies). CO can combine with O2 adsorbed on the M-Ov-Ce (M2+-O22-) to form bidentate carbonate (M-O2-CO). The decomposition temperature of M-O2-CO is much higher than that of M-O-CO2, and its existence is the decisive step of CO oxidation. The above results provide a new way to regulate the surface oxygen species and carbonate species of Ce based catalysts in the later stages.

Synthesis of Inexpensive Ternary Metal Oxides by a Co-Precipitation Method for Catalytic Oxidation of Carbon Monoxide

By using a simple co-precipitation method, new Fe2 O3 -based nanocatalysts (samples) were synthesized. The samples were composites of two or three transition metal oxides, MOx (M=Fe, Mn, Co, Ni, and Cu). The average size of CuO crystallites in the composites composed of two oxide components (CuO-Fe2 O3 ) was about 14.3 nm, while in those composed of three (CuO-MnOx -Fe2 O3 ), the composite's phase compositions were almost in the amorphous form when annealing the sample at 300 °C. The latter sample had a specific surface area higher than that of the former, 207.9 and 142.1 g/m2 , respectively, explaining its higher catalytic CO oxidation. The CO conversion over the CuO-MnOx -Fe2 O3 -300 catalyst (1 g of catalyst, 2600 ppm of CO concentration in air, and 1.0 L/min of gas flow rate) begins at about 40 °C; the temperature for 50 % CO conversion (t50 ) is near 82 °C; and CO removal is almost complete at t99 ≈110 °C. The activity of the optimal sample was tested in different catalytic conditions, thereby observing a high durability of 99-100 % CO conversion at 130 °C. The obtained results were derived from XRD, FTIR, BET, SEM, elemental analysis and mapping, as well as catalytic experiments.

The research on CO oxidation over Ce-Mn oxides: The preparation method effects and oxidation mechanism

A series of CeO₂-MnO_x for highly efficient catalytical oxidation of carbon monoxide were prepared by citrate sol-gel (C), hydrothermal (H) and hydrothermal-citrate complexation (CH) methods. The outcome indicates that the catalyst generated using the CH technique (CH-1:8) demonstrated the greatest catalytic performance for CO oxidation with a T₅₀ of 98 °C, and also good stability in 1400 min. Compared to the catalysts prepared by C and H method, CH-1:8 has the highest specific surface of 156.1 m² g^-1, and the better reducibility of CH-1:8 was also observed in CO-TPR. It is also observed the high ratio of adsorbed oxygen/lattice oxygen (1.5) in the XPS result. Moreover, characterizations by the TOF-SIMS method indicated that obtained catalyst CH-Ce/Mn = 1:8 had stronger interactions between Ce and Mn oxides, and the redox cycle of Mn³⁺+Ce⁴⁺ ↔ Mn⁴⁺+Ce³⁺ was a key process for CO adsorption and oxidation process. According to in-situ FTIR, the possible reaction pathway for CO was deduced in three ways. CO directly oxidize with O₂ to CO₂, CO adsorbed on Mn⁴⁺ and Ce³⁺ reacts with O to form intermediates (COO^-) (T > 50 °C) and carbonates (T > 90 °C), which are further oxidized into CO₂.

Soot Combustion over Cu-Co Spinel Catalysts: The Intrinsic Effects of Precursors on Catalytic Activity

In this work, a series of CuCo₂O₄-x (x = N, A and C) catalysts were synthesized using different metal salt precursors by urea hydrothermal method for catalytic soot combustion. The effect of CuCo₂O₄-x catalysts on soot conversion and CO₂ selectivity in both loose and tight contact mode was investigated. The CuCo₂O₄-N catalyst exhibited outstanding catalytic activity with the characteristic temperatures (T₁₀, T₅₀ and T₉₀) of 451 °C, 520 °C and 558 °C, respectively, while the CO₂ selectivity reached 98.8% during the reaction. With the addition of NO, the soot combustion was further accelerated over all catalysts. Compared with the loose contact mode, the soot conversion was improved in the tight contact mode. The CuCo₂O₄-N catalysts showed better textural properties compared to the CuCo₂O₄-A and CuCo₂O₄-C, such as higher specific surface areas and pore volumes. The XRD results confirmed that the formation of a CuCo₂O₄ crystal phase in all catalysts. However, the CuO crystal phase only presented in CuCo₂O₄-N and CuCo₂O₄-A. The relative contents of Cu²⁺, Co³⁺ and O_ads on the surface of CuCo₂O₄-x (x = N, A and C) catalysts were analyzed by XPS. The CuCo₂O₄-N catalyst displayed the highest relative content of Cu²⁺, Co³⁺ and O_ads. The activity of catalytic soot combustion showed a good correlation with the order of the relative contents of Cu²⁺, Co³⁺ and O_ads. Additionally, the CuCo₂O₄-N catalyst exhibited lower reduction temperature compared to the CuCo₂O₄-A and CuCo₂O₄-C. The cycle tests clarified that the copper-cobalt spinel catalyst obtained good stability. In addition, based on the Mars-van Krevelen mechanism, the process of catalytic soot combustion was described combined with the electron transfer process and the role of oxygen species over CuCo₂O₄ spinel catalysts.

CO Oxidation over Alumina-Supported Copper Catalysts

CO oxidation, one of the most important chemical reactions, has been commonly studied in both academia and the industry. It is one good probe reaction in the fields of surface science and heterogeneous catalysis, by which we can gain a better understanding and knowledge of the reaction mechanism. Herein, we studied the oxidation state of the Cu species to seek insight into the role of the copper species in the reaction activity. The catalysts were characterized by XRD, N2 adsorption-desorption, X-ray absorption spectroscopy, and temperature-programmed reduction. The obtained results suggested that adding of Fe into the Cu/Al2O3 catalyst can greatly shift the light-off curve of the CO conversion to a much lower temperature, which means the activity was significantly improved by the Fe promoter. From the transient and temperature-programmed reduction experiments, we conclude that oxygen vacancy plays an important role in influencing CO oxidation activity. Adding Fe into the Cu/Al2O3 catalyst can remove part of the oxygen from the Cu species and form more oxygen vacancy. These oxygen vacancy sites are the main active sites for CO oxidation reaction and follow a Mars-van Krevelen-type reaction mechanism.

Fabrication of Stable Cu-Ce Catalyst with Active Interfacial Sites for NOx Elimination by Flame Spray Pyrolysis

The complete conversion of NOx to harmless N2 without N2O formation is crucial for the control of air pollution, especially at low temperatures. Cu-based catalysts are promising materials due to their low cost and high activity in NO dissociation, even comparable to noble metals; however, they suffer from low stability. Here, we established a Cu-Ce catalyst in one step with strong metal–support interaction by the flame spray pyrolysis (FSP) method. Almost 100% NO conversion was achieved at 100 °C, and they completely transferred into N2 at a low temperature (200 °C) for the FSP-CuCe catalyst, exhibiting excellent performance in NO reduction by CO reaction. Moreover, the catalytic performance can stay stable, while 23% NO conversion was lost in the same condition for the one made by the co-precipitation (CP) method. This can be attributed to the synergistic effect of abundant active interfacial sites and more flexible surface oxygen created during the FSP process. The flame technology developed here provides an efficient way to fabricate strong metal–support interactions, exhibiting notable potential in the design of stable Cu-based catalysts.

Copper Supported on Ceria Mesocellular Foam Silica as an Effective Catalyst for Reductive Condensation of Acetone to Methyl Isobutyl Ketone

Copper-containing materials based on Ce- and Ca-Nb-mesocellular foam (MCF) silica supports are prepared, characterized and applied as catalysts for gas-phase reductive condensation of acetone to produce methyl isobutyl ketone (MIBK). The properties of the materials, the interaction of metal species, and their role in the catalytic process are examined by nitrogen physisorption, XRD, XPS, CO₂ -TPD, H₂ -TPR, and chemisorption of NO and pyridine combined with FTIR spectroscopy. A synergistic interaction of Cu²⁺ , Cu⁰ , and CeO₂ species incorporated in the MCF support enable the Cu/Ce-MCF catalyst to yield 34 % of acetone conversion with over 90 % MIBK selectivity at 250 °C. Moreover, this high catalyst selectivity is maintained during operation for 24 h despite a decline in catalyst activity. The catalytic performance is superior to that of hydroxyapatite-supported Cu and similar previously reported Pd-containing catalysts.

Porous stainless-steel fibers supported CuCeFeOx/Zeolite catalysts for the enhanced CO oxidation: Experimental and kinetic studies

To develop novel catalysts of high-performance and cost-effectiveness, and to investigate the reaction kinetics of CO oxidation, ternary CuCeFeO_x catalysts supported on zeolite/PSF (porous stainless-steel fibers) were synthesized for the first time. Effects of different Ce/Fe ratios, loading amounts, calcination temperatures, and reaction kinetics were investigated. Remarkably improved catalytic performance was achieved in the PSF-supported catalysts compared to the granular ones, owing to the increased mass/heat transfer efficiency and the high dispersion of active metal oxide species anchored on the zeolite layer. The Cu₃Ce₁₂Fe₄-400 sample exhibited the best catalytic activity with a temperature difference in T₉₀ of almost 40 °C lower than the worst one. Characterization results from XRD, FTIR, TEM, XPS, H₂-TPR, etc. revealed that the promoted reducibility of metal oxides and formation of more oxygen vacancies significantly contributed to the enhanced catalytic activity. Furthermore, a generalized rate expression was derived from intrinsic and macro kinetic studies by assuming the conversion of CO to CO₂ as the rate-determining step, in which CO oxidation over the PSF-supported catalysts followed the pseudo-first-order kinetic established by the Langmuir-Hinshelwood type mechanism.

Dispersion of copper oxide species on nanostructured ceria

Copper oxides species deposited on ceria rods, particles, and cubes were examined for low-temperature oxidation of CO. It was found that the shape of ceria altered the dispersion and chemical state of copper species considerably. CuO_x monolayers and bilayers were formed on ceria rods and particles, while multilayers and faceted particles co-existed on ceria cubes. The formation of Cu⁺ species at the copper-ceria interface involved a significant charge transfer from copper oxides to the ceria surface via a strong electronic interaction, which was more pronounced on ceria rods. The concentrations of surface Cu⁺ and oxygen vacancies followed the order rods > particles > cubes, in line with their catalytic activity for CO oxidation at 343 K.

Elucidating the Nature of the Cu(I) Active Site in CuO/TiO2 for Excellent Low-Temperature CO Oxidation

Stabilized Cu⁺ species have been widely considered as catalytic active sites in composite copper catalysts for catalytic reactions with industrial importance. However, few examples comprehensively explicated the origin of stabilized Cu⁺ in a low-cost and widely investigated CuO/TiO₂ system. In this study, mass producible CuO/TiO₂ catalysts with interface-stabilized Cu⁺ were prepared, which showed excellent low-temperature CO oxidation activity. A thorough characterization and theoretical calculations proved that the strong charge-transfer effect and Ti-O-Cu hybridization in Ti-doped CuO(111) at the CuO/TiO₂ interface contributed to the formation and stabilization of Cu⁺ species. The CO molecule adsorbed on Cu⁺ and reacted directly with Ti doping-promoted active lattice oxygen via a Mars-van Krevelen mechanism, leading to the enhanced low-temperature activity.

Facile preparation of highly active and stable CuO–CeO2 catalysts for low-temperature CO oxidation via a direct solvothermal method

CuO–CeO₂ catalysts prepared by a direct solvothermal method exhibit high activity and stability for low-temperature CO oxidation.

Enhanced CH4 and CO Oxidation over Ce1- xFe xO2-δ Hybrid Catalysts by Tuning the Lattice Distortion and the State of Surface Iron Species

CeO₂-Fe₂O₃ mixed oxides are very attractive as catalysts for catalytic oxidation. Herein, we report the structural dependence of the Ce_{1- x}Fe _xO_2-δ catalysts for CH₄ combustion and CO oxidation via changing lattice distortion degrees, surface Fe₂O₃ states, and oxygen vacancy concentrations. The lattice distortion degree and oxygen vacancy concentration of Ce-Fe-O solid solution can be tuned by changing the contents of Fe and the precipitation temperatures in the preparation process. The precipitation at relatively high temperature (70 °C) promotes the lattice distortion, whereas a lower temperature (0 °C) helps the formation of surface oxygen vacancies. The in situ diffuse reflectance infrared/Raman experiments and the physicochemical characterization suggest that both the CO and CH₄ oxidations mainly follow a Mars-van Krevelen mechanism. Both the lattice distortion and the surface iron species play a crucial role in determining the catalytic activity by affecting the redox property of the catalysts. The surface iron species, combined with the oxygen vacancies, improve the catalytic performance by enhancing the adsorption capacity of reactants and reducibility of catalysts. The lattice distortion of CeO₂ contributes to the catalytic activity by tuning the oxygen mobility in the bulk, which promotes the re-oxidation rate of catalysts.

Experimental and computational studies on copper–cerium catalysts supported on nitrogen-doped porous carbon for preferential oxidation of CO

Geometric characteristics improve the synergy between Cu2⁺/Cu⁺ and Ce4⁺/Ce3⁺ couples.