Effects of the structure of ceria on the activity of gold/ceria catalysts for the oxidation of carbon monoxide and benzene

Distinct morphology-dependent behaviors for Au/γ-Al2O3 catalysts: enhanced thermal stabilization in CO oxidation reaction

The durability of supported metal catalysts usually suffers from sintering, the metal nanoparticles aggregating into larger sizes and subsequent loss of reactive surface, resulting in catalysts deactivation when heated at elevated temperatures. Herein, we investigate the evolution of Au species on different morphologies of γ-Al₂O₃ and surprisingly found vastly different behavior for the dispersion of surface Au nanoparticles. A nanorod-shaped γ-Al₂O₃ is prepared by the hydrothermal method resulting in an extraordinary catalyst support that can stabilize Au nanoparticles at annealing temperatures up to 700 °C. In contrast, the Au-supported catalyst prepared using commercial γ-Al₂O₃ shows a greater degree of inactivation under the same conditions. Remarkably, the unique morphology of such nanorod-shaped γ-Al₂O₃ is beneficial in preventing Au nanoparticles from sintering. The γ-Al₂O₃ nanorods are more effective than the commercial γ-Al₂O₃ at anchoring the Au nanoparticles. The results of X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and H₂-TPR, reveal the interfacial interactions between Au nanoparticles and γ-Al₂O₃ nanorods, yielding a sinter-stability of the obtained Au/γ-Al₂O₃ nanorods catalyst. This synthetic strategy is simple and amenable to the large-scale manufacture of thermally stable γ-Al₂O₃ for industrial applications. Here, we investigate the morphology-dependent behavior of Au nanoparticles dispersed on different morphologies of γ-Al₂O₃. The result of X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and H₂-TPR, reveal the interfacial interactions between Au nanoparticles and gamma alumina nanorods. Au nanoparticles on γ-Al₂O₃ nanorods exhibit higher sinter-resistant performance than those on commercial γ-Al₂O₃.

Cerium-Copper Oxides Synthesized in a Multi-Inlet Vortex Reactor as Effective Nanocatalysts for CO and Ethene Oxidation Reactions

In this study, a set of CuCeOx catalysts was prepared via the coprecipitation method using a Multi-Inlet Vortex Reactor: the Cu wt.% content is 5, 10, 20, 30 and 60. Moreover, pure CeO2 and CuO were synthesized for comparison purposes. The physico-chemical properties of this set of samples were investigated by complementary techniques, e.g., XRD, N2 physisorption at −196 °C, Scanning Electron Microscopy, XPS, FT-IR, Raman spectroscopy and H2-TPR. Then, the CuCeOx catalysts were tested for the CO and ethene oxidation reactions. As a whole, all the prepared samples presented good catalytic performances towards the CO oxidation reaction (1000 ppm CO, 10 vol.% O2/N2): the most promising catalyst was the 20%CuCeOx (complete CO conversion at 125 °C), which exhibited a long-term thermal stability. Similarly, the oxidative activity of the catalysts were evaluated using a gaseous mixture containing 500 ppm C2H4, 10 vol.% O2/N2. Accordingly, for the ethene oxidation reaction, the 20%CuCeOx catalyst evidenced the best catalytic properties. The elevated catalytic activity towards CO and ethene oxidation was mainly ascribed to synergistic interactions between CeO2 and CuO phases, as well as to the high amount of surface-chemisorbed oxygen species and structural defects.

Hierarchical macroparticles of ceria with tube-like shape – synthesis and properties

The hierarchical organization of CeO2 nanoparticles into tube-like macroparticles has a great influence on the properties of the material.

Au/CeO2-ZnO/Al2O3 as Versatile Catalysts for Oxidation Reactions: Application in Gas/Liquid Environmental Processes

The present work showcases the versatility of nanogold systems supported on Zn-doped ceria when applied in two important environmental processes, the total CO oxidation, and the liquid phase oxidation of glucose to gluconic acid. In the CO oxidation the suitability of these materials is clearly demonstrated achieving full conversions even at sub-ambient conditions. Regarding the glucose oxidation our materials display high conversion values (always over 50%) and very importantly full or almost full selectivity toward gluconic acid-an added value platform chemical in the context of biomass upgrading routes. The key factors controlling the successful performance on both reactions are carefully discussed and compared to previous studies in literature. To our knowledge this is one of the very few works in catalysis by gold combining liquid and gas phase reactions and represents a step forward in the flexible behavior of nano gold catalysts.

Effect of Nickel Oxide Doping to Ceria-Supported Gold Catalyst for CO Oxidation and Water-Gas Shift Reactions

Ceria-supported gold catalyst has drew much research interest owing to its high reactivity on CO oxidation and water-gas shift (WGS) reactions. However, till now, there were relatively limited studies on the effect of secondary metal/metal oxide component into gold-ceria system to enhance its catalytic performance. In this work, we synthetized the ceria supported gold-nickel samples via a deposition-precipitation method with the base of NaHCO3 to adjust final pH value of 8~9. We found that the addition of nickel oxide drove off the gold species from the stock solution during synthesis, and thus resulted in a dramatical decrease on doped Au concentration. No crystallized phases of gold and nickel were observed on the surface of ceria nanorods in both X-ray diffraction (XRD) and transmission electron microscopy (TEM). The valence of nickel was maintained as Ni2+ for all the measured samples by X-ray photoelectron spectroscopy (XPS), while gold was oxidized with the increased nickel amount after analysis of X-ray absorption near edge spectroscopy (XANES). The corresponding catalytic tests showed that with the introduction of nickel oxide, the activity of gold-ceria catalyst was promoted for the WGS reaction, but inhibited for the CO oxidation reaction.

Gas Phase Hydrogenation of Furaldehydes via Coupling with Alcohol Dehydrogenation over Ceria Supported Au-Cu

We have investigated the synthesis and application of Au-Cu/CeO₂ (Cu: Au = 2) in the continuous gas phase (P = 1 atm; T = 498 K) coupled hydrogenation of 5-hydroxymethyl-2-furaldehyde (HMF) with 2-butanol dehydrogenation. STEM-EDX analysis revealed a close surface proximity of both metals in Au-Cu/CeO₂ post-TPR. XPS measurements suggest (support → metal) charge transfer to form Auδ- and strong metal-support interactions to generate Cu⁰ and Cu⁺. Au-Cu/CeO₂ promoted the sole formation of 2,5-dihydroxymethylfuran (DHMF) and 2-butanone in the HMF/2-butanol coupling with full hydrogen utilisation. Under the same reaction conditions, Au/CeO₂ was fully selective to DHMF in standard HMF hydrogenation (using an external hydrogen supply), but delivered a lower production rate and utilised less than 0.2% of the hydrogen supplied. Exclusive -C=O hydrogenation and -OH dehydrogenation is also demonstrated for the coupling of a series of m-substituted (-CH₃, -CH₂CH₃, -CH₂OH, -CF₃, -N(CH₃)₂, -H) furaldehydes with alcohol (1-propanol, 1-butanol, 2-propanol, 2-butanol, cyclohexanol) dehydrogenation over Au-Cu/CeO₂, consistent with a nucleophilic mechanism. In each case, we observed a greater hydrogenation rate and hydrogen utilisation efficiency with a 3⁻15 times lower E-factor in the coupling process relative to standard hydrogenation. Our results demonstrate the feasibility of using hydrogen generated in situ through alcohol dehydrogenation for the selective hydrogenation of m-furaldehydes with important industrial applications.

Single Silver Adatoms on Nanostructured Manganese Oxide Surfaces: Boosting Oxygen Activation for Benzene Abatement

The involvement of a great amount of active oxygen species is a crucial requirement for catalytic oxidation of benzene, because complete mineralization of one benzene molecule needs 15 oxygen atoms. Here, we disperse single silver adatoms on nanostructured hollandite manganese oxide (HMO) surfaces by using a thermal diffusion method. The single-atom silver catalyst (Ag₁/HMO) shows high catalytic activity in benzene oxidation, and 100% conversion is achieved at 220 °C at a high space velocity of 23 000 h^-1. The Mars-van Krevelen mechanism is valid in our case as the reaction orders for both benzene and O₂ approach one, according to reaction kinetics data. Data from H₂ temperature-programmed reduction and O core-level X-ray photoelectron spectra (XPS) reveal that Ag₁/HMO possesses a great amount of active surface lattice oxygen available for benzene oxidation. Valence-band XPS and density functional theoretical calculations demonstrate that the single Ag adatoms have the upshifted 4d orbitals, thus facilitating the activation of gaseous oxygen. Therefore, the excellent activation abilities of Ag₁/HMO toward both surface lattice oxygen and gaseous oxygen account for its high catalytic activity in benzene oxidation. This work may assist with the rational design of efficient metal-oxide catalysts for the abatement of volatile organic compounds such as benzene.

Oxygen vacancies dependent Au nanoparticle deposition and CO oxidation

Au deposition on CeO₂ support is oxygen vacancies dependent. Optimized V_Os result in small gold particle size and positively charged Au^δ+ to promote CO oxidation; excess V_Os lead to agglomerated Au NPs and the reduction of Au³⁺ reactive species, with catalysis deactivation.

The promoting influence of nickel species in the controllable synthesis and catalytic properties of nickel–ceria catalysts

The nickel species in the nickel–ceria samples were confirmed to have multi effects in the morphology and the structure formation.

Hierarchical polymorphic MnCO3 series induced by cobalt doping via a one-pot hydrothermal route for CO catalytic oxidation

A series of cobalt-doped MnCO₃ hierarchical microstructures with different morphologies were synthesized by tuning a single variable (the dopant content) via a one-step, mild solvothermal synthesis in a N,N-dimethylformamide (DMF) solution system.

Solvothermal synthesis of 1D nanostructured Mn2O3: effect of Ni2+ and Co2+ substitution on the catalytic activity of nanowires

In this paper, cation modified one dimensional Mn₂O₃ nanowires were synthesized via a solvothermal synthesis and calcination free from the template-assisted method.

Reactive removal of surface oxygen by H2, CO and CO/H2 on a Au/CeO2 catalyst and its relevance to the preferential CO oxidation (PROX) and reverse water gas shift (RWGS) reaction

CO and H₂ compete for the same active surface (lattice) oxygen species, but with a much lower reduction efficiency of H₂ compared to CO and CO–H₂ mixtures, during removal of active oxygen from a Au/CeO₂ catalyst by H₂, CO or CO/H₂ at temperatures between 30 and 300 °C.

The synthesis of CeO₂ nanospheres with different hollowness and size induced by copper doping

In this paper, copper-doped ceria oxides with different hollowness and size are fabricated by changing the Cu(2+) doping concentration in the mixed water-glycol system. Results show that the copper-doped CeO₂ oxides undergo a morphology transformation from the solid nanospheres to core-shell, then to hollow nanospheres with the increase of the Cu(2+) doping concentration. The corresponding size becomes smaller during this transfer process. The Cu(2+) doping induced acceleration in the nucleation and growth process is further investigated. The resultant Cu(2+)-doped CeO2 oxides exhibit enhanced CO conversion performance and better reduction behaviors.