Ce Mixed Oxides Research Articles

Novel regenerated V- and Ce-mixed oxide modified catalysts for the NH3-SCR of NOx displaying a distinctive broad temperature window

The challenge for selective catalytic reduction (SCR) of NOx with NH3 is to improve de-NOx activity at low temperatures while simultaneously improving the tolerance of catalysts towards SO2 poisoning. Herein, V and Ce oxides were loaded on deactivated SCR catalysts subjected to acid pickling (CAT) via impregnation to recover de-NOx activity. The resulting V–Ce@CAT catalyst exhibited 85–95% NOx conversions over a wide temperature window of 200–450 °C while conversions of only 40–78% over the same temperature range were observed with CAT (spent catalyst treated with acid). Interestingly, in addition to the high tolerance towards SO2 displayed by V–Ce@CAT, the catalyst exhibited improved reactivity after the introduction of 200 ppm SO2, with the NOx conversion increasing from 80% to 87%. The results indicated that a kinetic equilibrium was reached between the formation and decomposition rates of Ce2(SO4)3 on V–Ce@CAT which prevented the accumulation of surface sulfate species, thereby protecting the main active species V from catalytic inactivation. Furthermore, in situ DRIFTS revealed that the NH3-SCR reaction pathway of V–Ce@CAT follows the L-H mechanism.

The abundant oxygen vacancies of the Co–Ce mixed oxides towards catalytic combustion toluene

A series of catalysts (Co3O4/Beta-60, CeO2/Beta-60, CoCeOx/Beta-60) were prepared by constant volume impregnation method and were applied to the catalytic oxidation toluene. The characterization results showed that some Co ions were embedded into the lattice of CeO2 to form CoCeO2-δ solid solution that promoted the formation of a large number of oxygen vacancies and generation of active oxygen species for the deep oxidation of toluene. The mobility of active oxygen species and reducibility for CoCeOx/Beta-60 were both improved. Therefore, CoCeOx/Beta-60 showed the superior catalytic performance, the conversion of toluene was 90% when the temperature was 307 °C. The combination between the zeolite carrier and the active components endowed the supported catalyst with excellent stability. Additionally, the in situ DRIFTS results displayed that the oxidation of toluene over CoCeOx/Beta-60 followed the MvK mechanism and L-H mechanism. Adsorbed toluene could be oxidized by not only active adsorbed oxygen on oxygen vacancies but also the lattice oxygen species. However, gaseous oxygen was paramount importance for the total oxidation of toluene for the reason that the gaseous oxygen could be activated through the oxygen vacancies generated to fill the active oxygen species after the depletion of active adsorbed oxygen and lattice oxygen. The study systematically reviewed the data for the catalytic combustion of toluene, aiming to provide new avenues for the preparation and application of catalysts with excellent catalytic activity.

Promotion effect of niobium on ceria catalyst for selective catalytic reduction of NO with NH3

The CeO2, Ce–Nb–Ox and Nb2O5 catalysts were synthesized by citric acid method and the promotion effect of Nb on ceria for selective catalytic reduction (SCR) of NO with NH3 was investigated. The catalytic activity measurements indicate that the mixed oxide Ce–Nb–Ox presents a higher SCR activity than the single oxide CeO2 or Nb2O5 catalyst. In addition, the Ce–Nb–Ox catalyst shows high resistance towards H2O and SO2 at 280 °C. The Raman, X-ray photoelectron spectra and temperature programmed reduction with H2 results indicate that the incorporation of Nb provides abundant oxygen vacancies for capturing more surface adsorbed oxygen, which provides a superior redox capability and accelerates the renewal of active sites. Furthermore, the Fourier transform infrared spectra and temperature programmed desorption of NH3 results suggest that niobium pentoxide shows high surface acidity, which is partly retained in the Ce–Nb–Ox catalyst possessing a high content of Lewis and Brønsted acid sites. Therefore, the incorporation of Nb improves both the redox and acidic capacities of Ce–Nb–Ox catalyst for the SCR reaction. Here, the redox behavior is primarily taken on Ce and the acidity is well improved by Nb, so the synergistic effect should exist between Ce and Nb. In terms of the reaction mechanism, in situ DRIFT experiments suggest that both NH3 on Lewis acid sites and NH4+ on Brønsted acid sites can react with NO species, and adsorbed NO and NO2 species can both be reduced by NH3. In the SCR process, O2 primarily acts as the accelerant to improve the redox and acid cycles and plays an important role. This work proves that the combination of redox and acidic properties of different constituents can be feasible for catalyst design to obtain a superior SCR performance.

NiLaM (M = Ce and/or Zr) mixed oxide catalysts for synthesis gas production by biogas reforming processes

NiLaM (M = Ce and/or Zr) catalysts were prepared by the polymerized complex method and evaluated in the dry, oxidative dry, and tri-reforming of a synthetic biogas (CH4/CO2 = 1.5) at 800 °C and atmospheric pressure. The catalysts that possessed the highest number of total basic sites and stronger Ni-support interaction showed better activity and coke resistance. A lower level of carbon deposits, with higher reactivity, was found for these catalysts in tri-reforming experiments. However, catalyst deactivation was observed for NiLaCe, attributed to nickel re-oxidation.

What Are the Best Active Sites for CO2 Methanation over Ni/CeO2?

We examined active sites for CO₂ methanation over Ni/CeO₂ catalysts prepared by a wet impregnation method. Four types of Ni/CeO₂ with Ni loadings of 1, 3, 5, and 10 wt % were used in this study, assuming that the Ni sites are well dispersed in the catalysts when changing the Ni loading. According to powder X-ray diffraction and scanning transmission electron microscopy, the low-loading catalysts (1 and 3 wt %) consist mainly of Ni–Ce mixed oxides. The results of temperature-programmed reduction by H₂ suggested that the Ni–Ce mixed oxides under reducing conditions contain oxygen vacancies (Ni–Vₒₓ–Ce). Note that the CO₂ conversion rate was proportional to the Ni loading, which probably means that Ni–Vₒₓ–Ce sites on the Ni–Ce mixed oxides are active in CO₂ conversion. In contrast, when the Ni loading was high (5 and 10 wt %), the catalysts possessed many metallic Ni nanoparticles supported on CeO₂ and Ni–Ce mixed oxides. Because the turnover frequencies of CO methanation for 5 and 10 wt % Ni/CeO₂ were identical, the presence of a metallic Ni surface could be essential for activation in CO methanation. We focused on the fact that the CO₂ conversion rate was not related to the number of oxygen vacancies on CeO₂ (Ce–Vₒₓ–Ce) but was related to the number of the Ni–Vₒₓ–Ce sites. Hence, the formation of Ni–Vₒₓ–Ce sites (CO production via the reverse water-gas shift reaction) and the exposure of metallic Ni sites (methanation of the thus-formed CO) are essential for CO₂ methanation. Although it has been known that oxygen vacant sites on Ni/CeO₂ catalysts are important for the catalytic activity, this study suggested anew that there are two types of the sites, Ce–Vₒₓ–Ce and Ni–Vₒₓ–Ce. Furthermore, it was clarified that the latter oxygen defect is important for CO₂ methanation.

Synergistic effect of Cu on a Ag-loaded CeO2 catalyst for soot oxidation with improved generation of active oxygen species and reducibility

In this work, the promoting effects of Cu on Ag-loaded CeO2 catalysts on soot oxidation were investigated through a series of Cu-incorporated catalysts (AgCu(x)Ce) with various amounts of Cu. Ag particles were highly dispersed over Cu(x)Ce mixed oxide, and AgCu(x)Ce catalysts presented improved activity in comparison to Ag/CeO2. Raman spectra and H2-TPR showed that the complex effects of the Ag-CeO2 and CuO-CeO2 interactions enhanced the ability to generate highly active superoxide (O2−) and greatly improved the reducibility of AgCu(x)Ce catalysts. The amount of Cu affected degrees of both the Ag-CeO2 and CuO-CeO2 interactions, and consequently, the ratio of active oxygen species (Oxn−) and the reducibility of the catalysts varied. AgCu(0.4)Ce with adequate surface oxygen vacancies promoted the generation of O2−, and the promoting effects enhanced the reducibility, resulting in greatly improved activity. It is suggested that introduction of Cu to Ag/CeO2 significantly improves the activity of the catalyst and that the optimized Cu content induces positive interrelations, whereas excessive Cu hinders the improvement in catalytic performance due to decreased synergistic effects between the Ag-CeO2 and CuO-CeO2 interactions.

Comparative study of La1–xCexMnO3+δ perovskites and Mn–Ce mixed oxides for NO catalytic oxidation

A series of La1–xCexMnO3+δ (x = 0, 0.05, 0.1, 0.2, and 0.3) perovskites and Mn–Ce mixed oxides were prepared. Their physico-chemical properties were systematically characterized and the NO oxidation activities of the catalysts were investigated. The La0.9Ce0.1MnO3+δ has the best activity among all of the catalysts, with a maximum NO conversion of 85% at 300 °C. The characterization results indicate that the doping of Ce improves the properties of the perovskites in terms of the specific surface area, the average valence state of Mn ions, the number of reactive oxygen species and the NOx desorption behaviors. The Mn–Ce mixed oxide calcined at 500 °C shows a similar NO oxidation activity with La0.9Ce0.1MnO3+δ. However, the activity of the mixed oxide obtained at 750 °C decreases a lot, which results from the loss of active sites and active oxygen species.

Turning the activity of Cr–Ce mixed oxide towards thermocatalytic NO oxidation and photocatalytic CO2 reduction via the formation of yolk shell structure hollow microspheres

Cr–Ce mixed oxides microspheres will have considerable potential in thermocatalytic NO oxidation and photocatalytic CO2 reduction reaction (CO2RR) if their low porosity and expensive synthetic expenditure can be overcome. To avoid these obstacles, a facile and economical strategy was adopted to synthesize double-shelled hollow Cr–Ce mixed oxide microspheres. The highly active multipurpose catalyst achieved an ultrahigh thermocatalytic NO oxidation efficiency at low temperatures, and provided high yields of CH4 and CH3OH during the photocatalytic CO2RR. For NO oxidation, the optimized catalyst provided over 40% NO removal at 150 °C, which was ascribed to the reduced aggregation of the active subunits and the increased active species availability. Further, the exposed surface could generate an increased content of Cr6+ and Oβ, which played the key role in NO oxidation. For CO2 photocatalytic reduction, the superior behavior originated from the reduced band gap and the effective separation of photoinduced charge carriers. This study opens up new possibilities of using Cr–Ce mixed oxides for application in catalysis.

Highly dispersed Fe–Ce mixed oxide catalysts confined in mesochannels toward low-temperature oxidation of formaldehyde

Highly dispersed Fe–Ce mixed oxides confined in mesochannels are prepared by two-step impregnation–calcination, achieving superior catalytic oxidation of HCHO (9.8 μg L−1) with 65 and 94.9% at 30 and 60 °C, respectively.

Removal of Elemental Mercury from Flue Gas Using Microwave/Ultrasound-Activated Ce–Fe Magnetic Porous Carbon Derived from Biomass Straw

Magnetic adsorbent shows good development prospects for separation of Hg0 from flue gas because it can be recycled. In this work, a novel magnetic biomass porous carbon adsorbent was developed by loading active ingredients (Ce and Fe mixed oxides) on renewable maize straw carbon with large specific surface area. Microwave activation and ultrasound treatment were applied to improve porous structure of maize straw carbon and distribution of active components. The influence of process parameters on Hg0 capture and the removal mechanism were also investigated. The results reveal that CeFe11%(3/5)/MSWU700 possesses the optimal Hg0 removing performance and adsorption capacity at 140 °C. The characterization results show that microwave activation can greatly increase the specific surface area of biomass carbon to form excellent porous structure, and ultrasound-assisted impregnation can facilitate the dispersion of active ingredients on the surface of adsorbent. The presence of Hg2+ on the surface of CeFe11%(3/5)...

Process dynamic investigations and emission analyses of biodiesel produced using Sr–Ce mixed metal oxide heterogeneous catalyst

The present study explores the feasibility of Sr–Ce based mixed metal oxides for its performance in transesterification reaction of waste cooking oil. The catalyst synthesis was carried out through gel combustion route and was characterized through several techniques including thermal analysis (TGA-DTA), X-ray diffraction (XRD), attenuated total reflectance based Fourier transform infrared spectroscopy (ATR-FTIR), high resolution scanning electron microscopy (HR-SEM) assisted with EDX, BET specific surface area and Hammett indicator basicity. The enhanced activity of the catalyst was investigated at pH 7.0 with Sr–Ce atomic ratio of 3:1 at 900 °C of calcination temperature. Influences of various process parameters on transesterification efficiency were carefully investigated. The experimental results demonstrated that maximum transesterification efficacy of 99.5% was achieved under optimized reaction conditions with catalyst dose of 2.0 wt %, oil-to-methanol ratio 1:14, reaction time 120 min, reaction temperature 65 °C and stirring speed of 700 rpm. For better interpretation of the process, the reaction rate was computed by employing pseudo-first and pseudo-second order kinetics model at varying reaction temperature (50 °C–75 °C). The transesterification data agreed well with pseudo-first order model with highest rate constant value of 2.5 × 10−3 min−1 was evaluated at 65 °C. Activation energy and frequency of the reaction was quantified from the Arrhenius expression as 17.04 kJ/mol and 9.92 min−1, respectively. Thermodynamic analysis of the reaction system suggests that the transesterification of the waste cooking oil followed endergonic reaction pathway. Synthesis of biodiesel was ascertained from the H1-NMR and FTIR analysis of the transesterified product, further, the physicochemical properties of the biodiesel were also compared with that of diesel fuel and the resultant values were found to be within ASTM limits. Reusability study was also conducted and it indicated that the catalyst can be easily regenerated and could be effectively reused up to four runs.

Influence of CO addition on the toluene total oxidation over Co based mixed oxide catalysts

Hydrotalcite like compounds containing Co, Al and Ce were synthesized by co-precipitation. The mixed oxides obtained after calcination were characterized by several techniques: XRD, BET, H2-TPR and XPS. Activities of mixed oxides were evaluated in toluene total oxidation in presence or in absence of carbon monoxide. The benzene, benzyl alcohol and benzaldehyde are principal by-products observed during toluene oxidation in presence of CoAl(Ce) mixed oxides. Moreover, presence of carbon monoxide improves toluene total oxidation over CoAlCe mixed oxides. Stability of the two best catalytic materials has been tested in the two conditions and show no deactivation.

Removal of Elemental Mercury from Coal Pyrolysis Gas Using Fe–Ce Oxides Supported on Lignite Semi-coke Modified by the Hydrothermal Impregnation Method

In this paper, Fe–Ce mixed oxides supported on lignite semi-coke were prepared by the hydrothermal impregnation method. The sorbents were employed to capture elemental mercury from coal pyrolysis gas. The influences of hydrothermal impregnation temperature, mole ratio of Fe/Ce, reaction temperature, and H2S concentration on Hg0 removal efficiency were investigated in a fixed-bed reactor. The physicochemical properties of the sorbent samples were characterized by X-ray diffraction, Brunauer–Emmett–Teller, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The results showed that the Fe/Ce-modified semi-coke sorbents, synthesized by hydrothermal impregnated at 200 °C, had larger specific surface areas and pore volumes than the original semi-coke. The sorbent with a Fe/Ce molar ratio of 0.4:0.2 exhibited the highest Hg0 removal efficiency of 83.5% at 150 °C. On the basis of the XPS characterization of the sorbent, a mechanism of Hg0 removal over the Fe0.4Ce0.2/SC200 sorbent was propose...

Adsorption and catalytic oxidation of elemental mercury over regenerable magnetic Fe[sbnd]Ce mixed oxides modified by non-thermal plasma treatment

This study proposes the novel application of non-thermal plasma treatment to improve the oxidation capacity of regenerable magnetic FeCe mixed oxides (FCs) for the efficient removal of elemental mercury (Hg0) from coal combustion flue gas. Sample characterization shows that the textural property, crystalline phases, and magnetic property of FCs undergo no obvious changes after plasma treatment. But greater Ce4+ concentration and richer lattice oxygen are generated on the treated FCs. The treated FCs exhibit far better Hg0 removal performance compared to raw FC. The effects of treatment time (0–20 min), reaction temperature (100–250 °C), and flue gas components (SO2, NO, O2, HCl and H2O) on Hg0 removal performance are also discussed. Both Hg0 adsorption capacity and adsorption rate evaluated at 150 °C for the treated FCs are extremely close to those obtained with a commercial activated carbon manufactured specifically for mercury removal from flue gas. Furthermore, the Hg0 removal mechanism is proposed for the treated FCs. The treated FCs include separate active sites for Hg0 adsorption and catalytic oxidation. Ce4+ species with greater oxidation state and lattice oxygen are largely consumed during the Hg0 removal process. However, these components are replenished by subsequent non-thermal plasma treatment. Finally, the spent FCs can be effectively recycled through magnetic separation, thermal desorption, and non-thermal plasma treatment.

Low-temperature SCR of NO with NH3 over biomass char supported highly dispersed Mn[sbnd]Ce mixed oxides

Series of catalysts with MnCe mixed oxides loaded onto biomass char (BC) modified by nitric acid were prepared via impregnation method. And these catalysts were used for the selective catalytic reduction (SCR) of NO with NH3. MnCe (7:3)/BC catalysts with loading 6% (mass ratio) MnCe oxides showed the best NO conversion ratio of 99.2% at 175 °C. The changes of the microstructure, phase composition, metal valence state and functional groups were investigated through SEM, BET, XRD, XPS and FT-IR. It showed that nitric acid modification could increase surface acidic functional groups of biomass char. And surface functional groups on BC could increase NH3 and NO adsorption capacity. In the denitration process, oxygen was transferred from CeO2 to Mn2O3, which could promote the cyclic catalytic reaction rate, then significantly enhanced the NO conversion in the MnCe/BC catalysts. Based on the experimental results and theoretical analysis, the synergetic mechanism model of surface functional groups on the surface of BC, Mn and Ce on the catalysts was set up.

Role of cerium in improving NO reduction with NH3 over Mn–Ce/ASC catalyst in low-temperature flue gas

Mn–Ce mixed oxide was loaded onto activated semi-coke (ASC) via impregnation method and the low temperature selective catalytic reduction (SCR) of NO with NH3 was investigated. The NO conversion and NO oxidation rates were both influenced by the ratios of Ce/(Mn+Ce), and Mn–Ce(0.3)/ASC with loading 5% (mass ratio) Mn–Ce oxides performed the highest catalytic conversion rate of 96.6% and NO oxidation rate of 18.4% at 250°C and GHSV=6000h−1. The changes of physical properties, microstructure, functional groups, crystal structure, surface atomic state, redox property and acid strength for the catalysts were characterized by BET, SEM, FT-IR, XRD, XPS, H2-TPR and NH3-TPD. The results showed that surface functional groups were increased after nitric acid treatment and abundant oxygen groups were observed on semi-coke surface, which were considered to be beneficial for the gas adsorption. Ce doping could raise the Mn4+/Mnx+ rate which played a vital role in catalytic reaction, enhancing the amount of weak and strong acid sites, increasing oxygen vacancies and accelerating the molecular oxygen turning into reactive oxygen species, which could enhance the NO oxidation activity thus contributed to the denitration efficiency.

Fe–Ce Mixed Oxides Supported on Carbon Nanotubes for Simultaneous Removal of NO and Hg0 in Flue Gas

Developing simultaneous removal technology of NOx and Hg0 in flue gas is highly desirable but remains challenging. Multiwalled carbon nanotube supported Fe–Ce mixed oxide nanoparticles (Fe(2)Ce(0.5)Ox/MWCNTs) were prepared using the ethanol impregnation method for DeNOx and Hg0 removal. The NO and Hg0 removal efficiencies over catalyst reached to 99.1% and 88.9% at 240 °C and a space velocity of 30 000 h–1. The Fe–Ce mixed oxides supported both inside and outside of MWCNTs in highly dispersed form with particle size of 3–5 nm. The Ce4+ existed on MWCNTs in two forms, some of which are the fluorite-like crystal CeO2; the other portion of Ce4+ entered the spinel structure of γ-Fe2O3 and led to the distortion of the γ-Fe2O3 lattice. Furthermore, it is evident that the chemisorbed oxygen content and oxidation activity of catalysts increased dramatically after Ce doping, which could be attributed to the excellent NH3-SCR and Hg0 removal performance.

Optimum Preferential Oxidation Performance of CeO2–CuOx–RGO Composites through Interfacial Regulation

Interfacial regulation offers a promising route to rationally and effectively design advanced materials for CO preferential oxidation. Herein, we initiated an interfacial regulation of CeO2-CuO x-RGO composites by adjusting the addition sequence of the components during the support formation. The presence of RGO along with the sequence tuning of the components is confirmed to survey the changes of the oxidation state of copper species, the content and distribution of the Cu+ site, and the synergistic interactions between Cu-Ce mixed oxides and reduced graphene oxide (RGO) over the catalysts. These catalysts were systematically characterized by inductively coupled plasma, X-ray diffraction, transmission electron microscopy/high-resolution transmission electron microscopy, hydrogen temperature-programmed reduction, X-ray photoelectron spectra, thermal gravimetric analysis, Raman spectra, and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements. The results show that RGO is favorable for the generation of Cu+ and the dispersion of copper-cerium species in the as-prepared catalysts. Furthermore, by multi-interfacial regulation of the CeO2-CuO x-RGO composites, the catalyst CeO2/CuO x-RGO exhibits a strikingly high catalytic oxidation activity at a low temperature coupled with a broader operation temperature window (i.e., CO conversion >99.0%, 140-220 °C) in the CO-selective oxidation reaction, which has been attributed to the high content of the active species Cu+ enriched on the surface, the highly dispersed copper oxide clusters subjected to a strong interaction with ceria, and the synergistic interactions between Cu-Ce mixed oxides and RGO.

Generalised two-dimensional correlation analysis of the Co, Ce, and Pd mixed oxide catalytic systems for methane combustion using in situ infrared spectroscopy

The process of methane combustion over the surface of a catalyst is still not fully understood. The identification of the reaction path and the intermediates created during catalysis is crucial for understanding the transformation of methane molecules. Two-dimensional (2D) correlation spectroscopy was engaged as a tool for the quantitative analysis of a series of temperature-dependent infrared spectra registered in situ during methane combustion. The prepared samples of catalysts were based on a Co, Pd and Ce mixed oxide adsorbed on an aluminium oxide layer deposited on kanthal steel. The registered spectra were transformed into 2D synchronous and asynchronous contour maps. The sequential order of spectral intensity changes was determined, and the resolution enhancement of overlapping IR bands by 2D correlation was demonstrated. The changes in the bands' intensity and information about band position can be correlated with a specific bond, and thus, the possible process intermediates can be identified. The 2DCoS analysis proved to be a powerful tool for band enhancement and revealed the changes occurring within the analysed catalyst systems as responses to increased temperature.

Preparation of highly dispersed Mn–Ce mixed metal oxides supported on the nitric vapour functionalised CNTs for low‐temperature NO reduction with NH 3

Highly dispersed Mn–Ce mixed metal oxide catalysts supported on the nitric vapour functionalised carbon nanotubes (CNTs) were synthesised by a polyol process for the low-temperature NO reduction with NH3. The obtained samples possessed high Mn4+/Mn and Oα/Oα + Oβ ratios and exhibited 90–100% NO conversion at 180–240°C.