Influence of the support on the preferential oxidation of CO in hydrogen-rich steam reformates over the CuO–CeO2–ZrO2 system

Effects of Metal Promoters (M = Fe, Co, and Cu) in Pt/M x Zr y O z Catalysts and Influence of CO2 and H2O on the CO Oxidation Activity (PROX): Analysis of Surface Properties After Reaction

In the present paper, the effects of metal promoters (M = Fe, Co, and Cu) in Pt/M x Zr y O z catalysts and the influence of CO2 and H2O on the CO oxidation activity (PROX) were investigated. To do that, characterizations of catalyst structures and surfaces were performed and reported here. The catalyst Pt/Fe x Zr y O z (PFeZ) was the most active at low temperatures among the analyzed ones. The addition of platinum caused strong interaction with the mixed oxide, affecting the structure and the surface composition, blocking basic sites, and thus preventing catalyst deactivation. Particularly, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results evidenced the formation of carboxylate and carbonate species. Besides, the addition of CO2 and H2O in the gas feed stream affected the observed CO oxidation results, showing that CO2 competes with O2 on metallic sites. Moreover, DRIFTS and temperature-programmed desorption (TPD) analyses suggested the occurrence of OH- oxidation by CO, leading to the formation of highly reactive compounds that can be easily oxidized.

Bi-Doped and Bi Nanoparticles Loaded CeO2 Derived from Ce-MOF for Photocatalytic Degradation of Formaldehyde Gas and Tetracycline Hydrochloride

Bi/CeO2 (BC-x) photocatalysts are successfully prepared by solvothermal loading Bi nanoparticles and Bi-doped CeO2 derived by Ce-MOF (Ce-BTC). Formaldehyde gas (HCHO) and tetracycline hydrochloride (HTC) are used to evaluate the photocatalytic activity of the synthesized Bi/CeO2. For BC-1000 photocatalyst, the degradation of HTC by 420 nm < λ < 780 nm light reaches 91.89% for 90 min, and HCHO by 350 nm < λ < 780 nm light reaches 94.66% for 120 min. The photocatalytic cycle experiments prove that BC-1000 has good cyclic stability and repeatability. The results of photoluminescence spectra, fluorescence lifetime, photocurrent response, and electrochemical impedance spectroscopy showed that the SPR (Surface Plasmon Resonance) effect of Bi nanoparticles acted as a bridge and promoted electron transfer and enhanced the response-ability of Bi/CeO2 to visible light. Bi-doping produced more oxygen vacancies to provide adsorption sites for adsorbing oxygen and generated more ·O2 - thus promoting photocatalytic reactions. The mechanism of photocatalytic degradation is analyzed in detail utilizing active free radical capture experiments and electron paramagnetic resonance (EPR) characterization. The experimental results indicate that ·O2 - and h+ active free radicals significantly promote the degradation of pollutants.

Influence of Oxygen Vacancy-Induced Coordination Change on Pd/CeO2 for NO Reduction

The byproduct formation in environmental catalysis is strongly influenced by the chemical state and coordination of catalysts. Herein, two Pd/CeO2 catalysts (PdCe-350 and PdCe-800) with varying oxygen vacancies (Ov) and coordination numbers (CN) of Pd were prepared to investigate the mechanism of N2O and NH3 formation during NO reduction by CO. PdCe-350 exhibits a higher density of Ov and Pd sites with higher CN, leading to an enhanced metal-support interaction by electron transformation from the support to Pd. Consequently, PdCe-350 displayed increased levels of byproduct formation. In situ spectroscopies under dry and wet conditions revealed that at low temperatures, the N2O formation strongly correlated with the Ov density through the decomposition of chelating nitro species on PdCe-350. Conversely, at high temperatures, it was linked to the reactivity of Pd species, primarily facilitated by monodentate nitrates on PdCe-800. In terms of NH3 formation, its occurrence was closely associated with the activation of H2O and C3H6, since a water-gas shift or hydrocarbon reforming could provide hydrogen. Both bridging and monodentate nitrates showed activity in NH3 formation, while hyponitrites were identified as key intermediates for both catalysts. The insights provide a fundamental understanding of the intricate relationship among the local coordination of Pd, surface Ov, and byproduct distribution.

Restructuring Co-CoOx Interface with Titration Rate in Co/Nb-CeO2 Catalysts for Higher Water-Gas Shift Performance

H2 production via water-gas shift reaction (WGS) is an important process and applied widely. Cobalt-modified CeO2 are promising catalysts for WGS reaction. Herein, a series of Co/Nb-CeO2 catalysts were prepared by varying the rate of precipitant addition during the coprecipitation method and examined for hydrogen generation through WGS reaction. The rates of precipitant addition were 1, 5, 15, and 25 mL/min. We obtained ceria supported cobalt catalysts with different sizes and morphology such as 3, 8 nm nanoclusters, 30 nm cubic nanoparticles, and 50 nm hexagonal nanoparticles. The well dispersed small cobalt particles in Co/Nb-CeO2 that was prepared at 5 mL/min titration rate exhibit strong interaction between cobalt oxide and CeO2 that retards the reduction of CoOx producing Co-CoOx pairs. In contrast, 1-Co/Nb-CeO2 and 25-Co/Nb-CeO2 result in bigger and aggregated Co particles, resulting in fewer interfaces with CeO2. The Co0, Coδ+, Ce3+, and Ov species are responsible for improved reducibility in Co/Nb-CeO2 catalysts and were quantitively measured using XPS, XAS, and Raman spectroscopy. The Co-CoOx interface assists dissociation of the H2O molecule; CO oxidation requires low activation energy and realizes a high turnover frequency of 9.8 s-1. The 5-Co/Nb-CeO2 catalyst achieved thermodynamic equilibrium equivalent CO conversion with efficient H2 production during WGS reaction at a gas hourly space velocity of 315,282 h-1. Successively, the 5-Co/Nb-CeO2 catalyst exhibited stable performance for straight 168 h attributed to stable CO-Coδ+ intermediate formation, achieving efficient inhibition of typical CO chemistry over the Co metal, suitable for hydrogen generation from waste derived synthesis gas.

Magnesium as a Methanation Suppressor for Iron- and Cobalt-Based Oxide Catalysts during the Preferential Oxidation of Carbon Monoxide

The preferential oxidation of CO (CO-PrOx) to CO2 is an effective catalytic process for purifying the H2 utilized in proton-exchange membrane fuel cells for power generation. Our current work reports on the synthesis, characterization and CO-PrOx performance evaluation of unsubstituted and magnesium-substituted iron- and cobalt-based oxide catalysts (i.e., Fe3O4, Co3O4, MgFe2O4 and MgCo2O4). More specifically, the ability of Mg to stabilize the MgFe2O4 and MgCo2O4 structures, as well as suppress CH4 formation during CO-PrOx was of great importance in this study. The cobalt-based oxide catalysts achieved higher CO2 yields than the iron-based oxide catalysts below 225 °C. The highest CO2 yield (100%) was achieved over Co3O4 between 150 and 175 °C, however, undesired CH4 formation was only observed over this catalyst due to the formation of bulk fcc and hcp Co0 between 200 and 250 °C. The presence of Mg in MgCo2O4 suppressed CH4 formation, with the catalyst only reducing to a CoO-type phase (possibly containing Mg). The iron-based oxide catalysts did not undergo bulk reduction and did not produce CH4 under reaction conditions. In conclusion, our study has demonstrated the beneficial effect of Mg in stabilizing the active iron- and cobalt-based oxide structures, and in suppressing CH4 formation during CO-PrOx.

Combustion-Synthesized Porous CuO-CeO2-SiO2 Composites as Solid Catalysts for the Alkenylation of C(sp3)-H Bonds Adjacent to a Heteroatom via Cross-Dehydrogenative Coupling

A series of mixed oxides of CuO, CeO2, and SiO2 were prepared by gel combustion and employed for the first time as efficient solid catalysts in a solvent-less liquid-phase cross-dehydrogenative coupling. The facile one-pot catalyst synthesis resulted in highly porous materials presenting large specific surface areas and strong metal–support interactions. The interaction with highly dispersed CeO2 enhanced the redox properties of the CuO species. The CuO-CeO2-SiO2 composites exhibited excellent catalytic performance for the selective coupling between 1,1-diphenylethylene and tetrahydrofuran with a yield up to 85% of 2-(2,2-diphenylvinyl)-tetrahydrofuran in the presence of di-tert-butyl peroxide (DTPB) and KI. Albeit both CuO and CeO2 species are proved to be responsible for the catalytic conversion, a great synergistic improvement in the catalytic activity was obtained by extended contact between the oxide phases by high porosity in comparison with the reactions using individual Cu or Ce catalysts. The activity of the composite catalyst was shown to be highly stable after five successive reaction cycles. Furthermore, the study scope was extended to the synthesis of different derivatives via composite-catalyzed coupling of C(sp2)-H with C(sp3-H) adjacent to a heteroatom. The good yields recorded proved the general validity of this composite for the cross-dehydrogenative coupling reaction rarely performed on solid catalysts.

General and Efficient Synthesis of Two-Dimensional Monolayer Mesoporous Materials with Diverse Framework Compositions

Two-dimensional (2D) mesoporous materials have received substantial research interest due to their highly exposed active sites and unusual nanoconfinement effect. However, controllable and efficient synthesis of 2D mesoporous materials and investigation of their intrinsic properties have remained quite rare. Herein, a general and effective surface-limited cooperative assembly (SLCA) method enabled by leveling precursor solutions on KCl crystals via centrifugation is employed to conveniently synthesize two-dimensional (2D) monolayer mesoporous materials with different compositions. This novel strategy is performed in a manner similar to spin coating, not only enabling generation of ultrathin mesostructured composite film on KCl particles and recycling excessive precursor solution but also providing favorable solvent annealing environment for the film to form ordered mesostructures. Taking monolayer mesoporous Ce_0.8Zr_0.2O₂ solid solutions as a sample, they display ultrathin nanosheet morphology with a thickness of ∼20 nm, highly open porous structure, and easily accessible active sites of surface superoxide species. Upon decoration of 2D mesoporous Ce_0.8Zr_0.2O₂ nanosheets with Pt nanoparticles, the obtained catalyst exhibits superior catalytic activity and stability toward CO oxidation with a low onset temperature of 30 °C and a 100% conversion temperature of 95 °C, which are 35-70 °C lower than those for their counterpart materials, namely, three-dimensional (3D) mesoporous Pt/Ce_0.8Zr_0.2O₂. Moreover, their TOF_Pt value is ∼11.3 times higher than that of 3D mesoporous Pt/Ce_0.8Zr_0.2O₂. Characterizations based on various techniques indicate that such an outstanding catalytic performance is due to the ultrashort distance (20 nm) of mass diffusion, highly exposed active sites, rich surface-chemisorbed oxygen, and the synergistic effect between the Ce_0.8Zr_0.2O₂ matrix and Pt species.

Effects of precipitants on the catalytic performance of Cu/CeO2 catalysts for the water–gas shift reaction

The ratio of the precipitant (K2CO3 : KOH) was confirmed to affect the Cu dispersion and OSC of the Cu/CeO2 catalyst, and the Cu/CeO2 catalyst prepared with the K2CO3 : KOH ratio of 3 : 1 showed the highest activity.

Noble Metal Nanoparticles Incorporated Siliceous TUD-1 Mesoporous Nano-Catalyst for Low-Temperature Oxidation of Carbon Monoxide

This report, for the first time, demonstrated the low-temperature oxidation of carbon monoxide (CO) using nano-catalysts consisting of noble metal nanoparticles incorporated in TUD-1 mesoporous silica nano-structures synthesized via a one-pot surfactant-free sol–gel synthesis methodology. Herein, we investigated a nano-catalyst, represented as M-TUD-1 (M = Rh, Pd, Pt and Au), which was prepared using a constant Si/M ratio of 100. The outcome of the analytical studies confirmed the formation of a nano-catalyst ranging from 5 to 10 nm wherein noble metal nanoparticles were distributed uniformly onto the mesopores of TUD-1. The catalytic performance of M-TUD-1 catalysts was examined in the environmentally impacted CO oxidation reaction to CO₂. The catalytic performance of Au-TUD-1 benchmarked other M-TUD-1 catalysts and a total conversion of CO was obtained at 303 K. The activity of the other nano-catalysts was obtained as Pt-TUD-1 > Pd-TUD-1 > Rh-TUD-1, with a total CO conversion at temperatures of 308, 328 and 348 K, respectively. The Au-TUD-1 exhibited a high stability and reusability as indicated by the observed high activity after ten continuous runs without any treatment. The outcomes of this research suggested that M-TUD-1 are promising nano-catalysts for the removal of the toxic CO gas and can also potentially be useful to protect the environment where a long-life time, cost-effectiveness and industrial scaling-up are the key approaches.

Catalysts with Cerium in a Membrane Reactor for the Removal of Formaldehyde Pollutant from Water Effluents

We report the synthesis of cerium oxide, cobalt oxide, mixed cerium, and cobalt oxides and a Ce-Co/Al₂O₃ membrane, which are employed as catalysts for the catalytic wet oxidation (CWO) reaction process and the removal of formaldehyde from industrial effluents. Formaldehyde is present in numerous waste streams from the chemical industry in a concentration low enough to make its recovery not economically justified but high enough to create an environmental hazard. Common biological degradation methods do not work for formaldehyde, a highly toxic but refractory, low biodegradability substance. The CWO reaction is a recent, promising alternative that also permits much lower temperature and pressure conditions than other oxidation processes, resulting in economic benefits. The CWO reaction employing Ce- and Co-containing catalysts was carried out inside a slurry batch reactor and a membrane reactor. Experimental results are reported. Next, a mixed Ce-Co oxide film was supported on an γ-alumina membrane used in a catalytic membrane reactor to compare formaldehyde removal between both types of systems. Catalytic materials with cerium and with a relatively large amount of cerium favored the transformation of formaldehyde. Cerium was present as cerianite in the catalytic materials, as indicated by X-ray diffraction patterns.

Influences of doping and thermal stability on the catalytic performance of CuO/Ce20M1Ox (M = Zr, Cr, Mn, Fe, Co, Sn) catalysts for NO reduction by CO

The doping of variable valence metal cations into CeO₂ is beneficial for catalytic NO reduction by CO.

Hard-template synthesis of three-dimensional mesoporous Cu–Ce based catalysts with tunable architectures and their application in the CO catalytic oxidation

In this paper, 3D Cu–Ce-Ox catalysts with different pores were synthesized by controlling the calcination temperaturre. The CeCu20-600 showed the highest catalytic activity, where CO can be fully oxidized at 55 °C and expressed good stability.

The role of Cu species in electrospun CuO–CeO2 nanofibers for total benzene oxidation

Better reducibility of Cu ions in Cu_xCe_1−xO_2−δ mainly contributes to the enhanced low-temperature benzene oxidation over high surface area electrospun CuO–CeO₂.

Mesoporous NiCu–CeO2 oxide catalysts for high-temperature water–gas shift reaction

Mesoporous NiCu–CeO₂ oxide catalysts were synthesized by using the evaporation-induced self-assembly method applied to the high-temperature, water–gas shift reaction (HT-WGS) between 350 to 550 °C.

Structural features and catalytic performance in CO preferential oxidation of CuO–CeO2 supported on multi-walled carbon nanotubes

MWCNT supported CuO–CeO₂ catalysts show enhanced performance in CO-PROX due to unusual structure features induced by interactions between metal oxides and MWCNT.