CeO2 Catalysts Research Articles

Steam reforming of ethanol to hydrogen formation: Kinetic modeling and experimental investigations

AbstractTo derive the kinetic model for steam reforming of ethanol, the experiment tests were accomplished at vast working conditions (i.e., temperature: 300–900°C, pressure: 1–11 bar, and H2O/C2H5OH ratio (S/E) of 3–21) on the Ni‐Pt/CeO2 catalyst in a fixed bed reactor. The physicochemical properties of the prepared catalyst were investigated by x‐ray fluorescence, thermal gravimetric analysis x‐ray diffractometer, and N2 adsorption/desorption Brunauer‐Emmett‐Teller (BET) tests. The kinetic model was developed based on the Langmuir–Freundlich approach, and the proposed mechanism included four reversible reactions on two types of the active site. The values of estimated activation energy for four reactions were in the range of reported values by various workers. The kinetic model error to anticipate the responses was 11.96% and a well forecasted H2 yield compared to other responses. The complete ethanol conversion reached above 700°C at 1 bar pressure irrespective of the S/E value. The variation of CO yield in terms of temperature was very low up to 700°C. An augmentation in S/E, enhanced ethanol conversion and H2 yield, while these trends were reversed for CO yield and CH4 selectivity.

Enhanced low-temperature catalytic performance for toluene combustion of CeO2-supported Pt-Ir alloy catalysts

Herein, bimetallic Pt-Ir alloy nanoparticles (NPs) were successfully fabricated by ethylene glycol reduction of H2PtCl6·6H2O and IrCl3 through a microwave-assisted method. The Pt-Ir alloy NPs were deposited on CeO2 catalysts for toluene combustion. The Pt2.5Ir/CeO2 sample exhibited better catalytic activity at low reaction temperature, compared with monometallic catalysts. The excellent low-temperature catalytic activity was resulted from the interaction between Pt and Ir, richer adsorbed oxygen species and low-temperature reducibility. Furthermore, the in-situ DRIFTS was used to investigate the possible reaction pathway of Pt2.5Ir/CeO2. It showed that the broken of benzene ring on Pt2.5Ir/CeO2 can be accomplished at lower temperature, resulting in enhancing the deep oxidation of toluene.

Catalytic upgrading of the polymeric constituents in Covid-19 masks

A substantial volume of various types of Covid-19 masks has been disposed of since the start of the global pandemic. These facemasks are made of non-degradable polymeric materials. As such, they currently signify a major source of microplastic pollution in the environment. Through incineration or pyrolysis, thermal processing has emerged as a mainstream approach in the waste management of the ever-increasing loads of generated facemasks. Via a combined experimental-theoretical framework, we report herein salient features that govern thermal decomposition of the distinct plastic-based components in 3 M N95 (with a respirator) and surgical facemasks. Protective three-four layers in the considered masks are composed of polypropylene (PP) that degrades in a one-step across the temperature window 330 – 480 °C. Char residues from the decomposition of ear straps (consisting of polyester) attain 24% and 15% fractions of the initial mass in the case of surgical and N95 masks, respectively. Thermo kinetic parameters are derived for the different components of the facemask by using the Coats-Redfern approach from the thermogravimetric analysis data. Here we also report the potentiality of producing value-added products from the face mask using the GCMS by virtue of the catalytic oxidation of the material expending the Niobium doped CeO2 catalyst under controlled conditions. Constructed mechanisms through density functional theory (DFT) computations illustrated the nature of chemical reactions that mark the two-stage decomposition curve of polyurethane (the material used in the nose area in N95 masks). These chemical events characterize rupture of C-C(O) bonds, sequential departure of CO2/C2H4 molecules, and fission of the C-C linkages. Outcomes from this investigation provide important information (i.e., thermal stability regions of the deployed polymers and potential emission profiles) needed in the urgent pursuit to safely and economically recycle polymeric constituents in Covid-19 masks, and potentially other types of medical wastes.

The influence of doping amount on the catalytic oxidation of formaldehyde by Mn-CeO2 mixed oxide catalyst: A combination of DFT and microkinetic study

Formaldehyde (HCHO) is a major environmental pollutant. The Mn-doped CeO2 catalyst has good catalytic performance for the oxidation of HCHO. The catalytic activity can be effectively tuned by changing the amount of metal doping. In this paper, density functional theory combined with micro-kinetic analysis are employed to provide a molecular level understanding to such effects. The CeO2(111) surface with different Mn doping content was used to study the oxidation mechanism of HCHO. Highly dispersed Mn doped ceria was dominant at low content of Mn. While with the increase of Mn doping, Mn begins to accumulate on the CeO2(111) surface. It is not conducive to the breaking of C-H bonds, the generation of oxygen vacancies and the adsorption of active oxygen species. Therefore, the low-content Mn-doped CeO2 catalyst has higher catalytic oxidation activity of HCHO. The present contribution is useful for further optimization of Mn-CeO2 catalysts towards HCHO oxidation.

Synthesis of diethyl carbonate from CO2 and orthoester promoted by a CeO2 catalyst and ethanol

The combination of a CeO2 catalyst and ethanol effectively promotes the direct synthesis of diethyl carbonate (DEC) from CO2 and an orthoester. The reaction temperature, initial CO2 pressure, and the ethanol/orthoester volume ratio were found to significantly influence DEC formation and were systematically studied to determine the optimum conditions. DEC amount of 5.2 mmol was obtained at 160 °C, 5 MPa of CO2, and a reaction time of 20 h, providing high productivity (1.21 mmolDEC mmolcatalyst−1 h−1) using a CeO2 catalyst. The recovered CeO2 catalyst was recyclable at least four times without any significant loss of activity when both triethyl orthoacetate and triethyl orthovalerate were used. A series of orthoesters bearing various alkyl substituents, aromatic substituents and halide moieties were evaluated for their activities with and without ethanol. In the presence of ethanol, orthoesters bearing longer alkyl substituents led to higher DEC yields, and orthoesters bearing aromatic and halide moieties negatively impacted DEC formation. In the absence of ethanol, triethyl orthoacetate provided the highest DEC yield; in all cases, the use of ethanol resulted in a greater DEC formation than when the reaction was performed without ethanol.

The Influence of Precursor on the Preparation of CeO2 Catalysts for the Total Oxidation of the Volatile Organic Compound Propane

CeO2 catalysts were prepared by a precipitation method using either (NH4)2Ce(NO3)6 or Ce(NO3)3, as CeIV or CeIII precursors respectively. The influence of the different precursors on catalytic activity was evaluated for the total oxidation of propane with water present in the feed. The catalyst prepared using the CeIV precursor was more active for propane total oxidation. The choice of precursor influenced catalyst properties such as surface area, reducibility, morphology, and active oxygen species. The predominant factor associated with the catalytic activity was related to the formation of either CeO2.nH2O or Ce2(OH)2(CO3)2.H2O precipitate species, formed prior to calcination. The formation of CeO2.nH2O resulted in enhanced surface area which was an important factor for controlling catalyst activity.

Catalytic Treatment of Organic Solvent Vapor in a Baking Oven

AbstractA catalytic treatment for organic solvent vapor using the heat of a baking oven was investigated. A Pt/Al2O3 catalyst showed high activity for toluene. A Co3O4‐CeO2 catalyst preferentially oxidized ethyl acetate and butyl acetate, but by‐products were generated. These organic solvents produce high emissions from painting and printing ink in Japan. However, treating all three solvents simultaneously at low temperatures is difficult. In the present study, when a Pt/Co3O4‐CeO2 catalyst was employed at oven temperatures of 230–250 °C, a gas containing toluene, ethyl acetate, and butyl acetate was able to be effectively treated with no carbon deposition. The application of the Pt/Co3O4‐CeO2 catalyst enabled treatment in the oven using the oven's own heat.

Deep Understanding into the Effect of Fe on CO2 Methanation: A Support-Dependent Phenomenon

An in-depth understanding of the influence mechanism of the nonprecious metal Fe promoter on CO2 methanation is of great significance to the optimal design of high-efficiency CO2 methanation catalysts. In this research, CeO2 and Al2O3-supported Ni-based catalysts were prepared and evaluated for the CO2 methanation reaction. Interestingly, it was found that the addition of Fe into the CeO2-supported Ni catalyst lowered the CO2 methanation performance, while it greatly enhanced the performance of the Al2O3-supported Ni catalyst. A variety of factors over Fe-modified catalysts were explored, in which surface basicity along with oxygen vacancies could contribute to the adjustment of the CO2 methanation performance.

Acid treatment of ZrO2-supported CeO2 catalysts for NH3-SCR of NO: Influence on surface acidity and reaction mechanism

BackgroundDue to excellent oxygen storage capacity and redox ability, Ce-based catalysts are usually considered as one of the promising candidates of traditional V-based catalysts for NH3-SCR of NO. However, surface acidity of Ce-based catalysts is often insufficient which restrains their SCR performance to some extent. MethodsHerein, ZrO2-supported CeO2 catalysts were prepared by precipitation and wet impregnation methods. Aiming to promote catalyst surface acidity, a novel method was proposed to pre-treat ZrO2 support with various acids (HCl, HNO3, H3PO4 and H2SO4) via impregnation process. and the effects of acid treatment on SCR performance of CeO2/ZrO2 catalysts had been investigated. Significant findingsIt was worthy to note that H2SO4-treated CeO2/ZrO2 catalyst exhibited a superior NO removal efficiency which was higher than 94% in a wide temperature range of 248–500 °C. NH3-TPD results revealed that the surface acidities acidity of CeO2/ZrO2 catalysts had been obviously enhanced after acid treatment with HNO3, H3PO4 and H2SO4, except for HCl. According to in-situ DRIFRS results, one could infer that SCR reactions occurred over CeO2/ZrO2 catalyst without acid treatment might proceed only in E-R mechanism. But for acid-treated CeO2/ZrO2 catalysts, both E-R and L-H mechanisms were might be followed in SCR reaction process.

Preparation of M/Ce1–xTixO2 (M=Pt, Rh, Ru) from sol-gel method and their catalytic oxidation activity for diesel soot

A series of Ce1–xTixO2 mixed oxide catalysts were synthesized by sol-gel method and then loading of noble metal (M = Pt, Rh, Ru) was used for soot oxidation. Ti-doped Ce1–xTixO2 catalysts (x is the molar ratio of Ti/(Ti + Ce) and ranges from 0.1 to 0.5) exhibit much better oxidation performance than CeO2 catalyst, and the Ce0.9Ti0.1O2 catalyst calcined at 500 °C has the best catalysis activity. Each noble metal (1 wt%) was supported on Ce0.9Ti0.1O2 (M/C9T1) and the properties of the catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, Brunauer–Emmett–Teller (BET) method, and H2-temperature programmed reduction (H2-TPR) results. Results show that the introduction of Ti into CeO2 forming Ti-O-Ce structure enhances the catalytic activity and increases the number of oxygen vacancies at the catalyst surface. The noble metal is highly dispersed over Ce0.9Ti0.1O2, and M/C9T1 catalysts present enhanced activity in comparison to Ce0.9Ti0.1O2. It is found that noble metals can greatly increase the activity of the catalyst and the corresponding oxidation rate of soot can enhance the electron transfer capacity and oxygen adsorption capacity of the catalyst. A small amount of Ti doping in CeO2 can significantly improve the activity of the catalyst, while a large amount of Ti reduces the performance of the catalyst because a large amount of Ti is enriched on the surface of the catalyst, which hinders the contact and reaction between the catalyst and the soot.

Determining the role of oxygen vacancies in palmitone selectivity and coke formation over acid metal oxide catalysts for the ketonization of methyl palmitate

In the present study, TiO2, CeO2, MnO2, and ZrO2 catalysts were used to investigate the catalytic performance in methyl palmitate ketonization. The reaction was accelerated by weak Lewis acid, while the oxygen vacancies promoted the palmitone selectivity. The use of various characterization techniques revealed that oxygen vacancies played an important role for trapping the hydrogen atom and inhibited its spillover. This caused the suppression of the Lewis acid transformation to new Bronsted acid. Thus, the cracking of palmitone over the new Bronsted acid was diminished, leading to the decrement of selectivity to undesired products. However, when the hydrogen atom was trapped in the oxygen vacancies, the coke deposition was dominant. This phenomenon arose because hydrogen could not suppress the deep dehydrogenation, resulting in the transformation of aliphatic coke to aromatic coke. The role of oxygen vacancies was determined, and it had the positive effect on the palmitone selectivity, whereas it raised the coke formation.

Enhanced Electrocatalytic Oxidation of Formate via Introducing Surface Reactive Oxygen Species to a CeO2 Substrate.

Direct formate fuel cells (DFFCs) as promising energy technologies have been applied for portable and wearable devices. However, for the formate oxidation reaction (FOR), the deficiency of catalysts has prevented DFFCs from practical applications. Herein, we prepared a Pd-loaded CeO2 catalyst through a simple steam treatment at 400 °C to enhance the catalytic FOR performance. In comparison with the counterpart of Pd/CeO2 without stream treatment, the as-prepared Pd/CeO2-ST catalyst has a lower onset potential of 381 mV and a lower peak potential of 0.64 V with a higher peak current of 10.62 mA cm-2. The experimental results show that the enhanced FOR properties of Pd/CeO2-ST are ascribed to the introduction of surface reactive oxygen species to the CeO2 substrate, which substantially promotes the desorption of adsorbed hydrogen (H*) intermediates. Density functional theory (DFT) calculations reveal that on the surface of CeO2, the abundant oxygen vacancies boost the OH* adsorption ability and accelerate the kinetics of the potential-limiting step. This work not only proposes a new strategy for enhancing the activity of FOR catalysts but also highlights the understanding of the FOR mechanism in alkaline media for DFFC applications.

Selective vapour-phase dehydrocyclization of biomass-derived 1,4-butanediol to γ-butyrolactone over Cu/ZnAl2O4-CeO2 catalyst

Production of γ-butyrolactone (GBL) from vapour-phase dehydrocyclization of biomass derived 1,4-butanediol is considered as more sustainable and significantly commercial industrial process. In this regards, Cu-based catalyst was designed with ZnAl2O4 spinel support and CeO2 as a promoter to obtain Cu/ZnAl2O4-CeO2 catalyst with different Cu:Ce mass ratio. The catalysts were characterized by XRD, H2-TPR, FT-IR, CO2-TPD, FE-SEM and BET analysis and then tested for the production of γ-butyrolactone (GBL) by dehydrocyclization of 1,4-butanediol (BDO). Further, various reaction parameters were optimized and their effects on the catalytic activity were studied to obtain maximum yield towards desired product at ambient reaction conditions. The results revealed that metallic Cu species and CeO2 as a promoter were highly dispersed on ZnAl2O4 support with appreciable surface area. Additionally, CeO2 as a promoter prevented agglomeration, increased the basic character of catalyst to selectively obtain GBL as a major product. 10 wt% Cu/ZnAl2O4-10 wt% CeO2 catalyst exhibited excellent yield (94%) towards GBL product with tetrahydrofuran (THF) as the only by-product and demonstrated consistent activity and selectivity during 26 h time on stream. Further, the optimized catalyst displayed appreciable recyclability performance up to seven recycles without much loss in its catalytic performance making the catalyst feasible at industrial scale.

Organic compound modification of CeO2 and 2-cyanopyridine hybrid catalyst in carbonate synthesis from CO2 and alcohols

The effect of surface modification of CeO2 and 2-cyanopyridine hybrid catalyst with organic compounds was investigated by using dialkyl carbonate synthesis from CO2 and alcohols. Phenol is the useful modifier for CeO2 and 2-cyanopyridine hybrid catalyst without drastic decrease of the activity. Based on the characterizations of phenol-modified CeO2 and 2-cyanopyridine hybrid catalyst such as TG-DTA, GC, FTIR and UV–vis, phenol and 2-cyanopyridine were adsorbed on CeO2 at the same time and the adsorption ratio to the surface Ce on CeO2 is estimated to be 0.11 and 0.12, respectively (2-cyanopyridine/Ce on CeO2 surface = 0.12, phenol/Ce on CeO2 surface = 0.11). The reactivity of alcohols was changed by the addition of phenol due to the steric hindrance of phenol adsorbed on CeO2. The effect of the steric hindrance was large at the C2 and C3 positions of alcohols, which corresponds to the C2 and C3 positions of phenol. The reactivity was drastically decreased in the cases of C2 position-branched alcohols compared with the case without phenol addition, and the yield ratio (The yield without phenol/the yield with phenol) increased: 61 in the case of 2-ethyl-1-butanol, 9.9 in the case of 2-methyl-1-butanol, and 3.7 in the case of 1-butanol. From the DFT calculations, phenol is adsorbed nearby 2-cyanopyridine adspecies by the π-π interaction between the aromatic rings of these chemicals to decrease the reaction space over CeO2, resulting in the reactivity decrease of the steric alcohols, particularly the alcohols with substituents at the C2 position. Therefore, phenol can restrict the reaction field of CeO2 and 2-cyanopyridine hybrid catalyst by the steric hindrance, enabling the control of the reactivity of alcohols in the synthesis of dialkyl carbonates from CO2 and alcohols.

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.

Facile reflux preparation of defective mesoporous ceria nanorod with superior catalytic activity for direct carbon dioxide conversion into dimethyl carbonate

Nanostructured ceria has drawn a great interest in sustainable catalysis research due to its unique oxygen release/storage capability. However, the conventional hydrothermal method for ceria preparation requires harsh conditions, which restrict its applications. Here we report a facile reflux process that is able to synthesize ceria nanorods in a mild environment. Particularly, it is found that the rapid reflux synthesis enables the formation of defective mesoporous structure in ceria catalysts, which enriches the surface with abundant trivalent Ce ions and oxygen vacancy sites. These features significantly improve the catalytic activity of ceria nanorod in synthesizing dimethyl carbonate (DMC) from methanol carbonation. Our result also reveals that the shorter reflux period for catalyst preparation can lead to a higher DMC yield due to the larger amounts of mesoporous defects and the higher concentrations of oxygen vacancies. Moreover, the reflux-synthesized ceria nanorods exhibit over 3 times higher DMC yield than hydrothermal counterpart, which can be attributed to their superior CO2 adsorption and activation capabilities as evidenced by combined surface characterizations. This study presents a practical strategy to synthesize CeO2 catalysts with exceptional catalytic activity by creating a defective mesoporous surface structure in a highly-efficient manner.

Front Cover: Effective Heterogeneous MoOx‐Modified CeO2 Catalyst for Michael Addition of Dimethyl Malonate to 2‐Cyclohexen‐1‐one (ChemCatChem 19/2021)

The Front Cover shows molybdenum oxide-modified CeO2 (MoOx-CeO2) catalyst particles dispersed in the solvent and Michael reaction over the catalysts. In their Communication, M. Tamura et al. developed the effective and reusable heterogeneous catalysts of MoOx-CeO2 for Michael addition of dimethyl malonate to 2-cyclohexen-1-one, providing higher activity than only CeO2 and a typical strong base of DBU. Catalyst characterization and experimental data showed that the isolated Mo6+ species was formed on the CeO2 surface and the interface of the isolated MoOx and CeO2 was the active site of MoOx-CeO2 catalyst. More information can be found in the Communication by M. Tamura et al.

CeO2-Catalyzed Synthesis of 2-Imidazolidinone from Ethylenediamine Carbamate.

CeO2 acted as an effective and reusable heterogeneous catalyst for the direct synthesis of 2-imidazolidinone from ethylenediamine carbamate (EDA-CA) without further addition of CO2 in the reaction system. 2-Propanol was the best solvent among the solvents tested from the viewpoint of selectivity to 2-imidazolidinone, and the use of an adequate amount of 2-propanol provided high conversion and selectivity for the reaction. This positive effect of 2-propanol on the catalytic reaction can be explained by the solubility of EDA-CA in 2-propanol under the reaction conditions and no formation of solvent-derived byproducts. This catalytic system using the combination of the CeO2 catalyst and the 2-propanol solvent provided 2-imidazolidinone in up to 83% yield on the EDA-CA basis at 413 K under Ar. The reaction conducted under Ar showed a higher reaction rate than that with pressured CO2, which clearly demonstrated the advantage of the catalytic system operated at low CO2 pressure or even without CO2.

NOx-assisted soot oxidation based on Ag/MnOx-CeO2 mixed oxides

In previous studies, Ag/CeO2 catalyst showed the high activity on NOx-free soot oxidation. For additional improvement of oxidation activity, Mn was introduced in Ag/CeO2 and NOx-assisted soot oxidation was performed. NOx-assisted soot oxidation activities showed the large difference between the catalysts. Especially, Ag/2MnOx-1CeO2 catalysts showed remarkably improved T20 at 349 °C compared with T20 of Ag/CeO2 at 464 °C (each T50 is 391 °C and 400 °C, and each Tmax is 514 °C and 509 °C). NO oxidation activity was increased as the mole ratio of Mn increased, while amount of NO adsorption was increased in Ag/MnOx-CeO2 mixed oxide compared to other catalysts. Especially, Ag/2MnOx-1CeO2 catalysts showed NO oxidation activity at low-temperature and NO desorption at appropriate temperature. As a result, it was confirmed that not only the NO oxidation activity but also the NOx adsorption characteristics are the most important factors in NOx-assisted soot oxidation. Accordingly, Ag/2MnOx-1CeO2 catalysts were proposed as optimal catalysts.

Investigating the efficacy of CeO2 multi-layered triangular nanosheets for augmenting cathodic hydrogen peroxide production in microbial fuel cell

The current investigation deals with the synthesis of multi-layered triangular nanosheets of CeO2 with a quasi-planar geometry and its application as a cathode catalyst for accelerated cathodic reduction of O2 to H2O2 in a dual chamber microbial fuel cell (MFC) treating simulated domestic wastewater. The unique structure of CeO2 facilitated local storage of oxygen at nanoscale level and high oxygen reduction activity as evident from the cyclic voltammogram, which enabled a higher rate of cathodic H2O2 electrosynthesis. The MFC containing CeO2 catalyst (MFC-CAT) exhibited the highest H2O2 production of 221.4 ± 18.94 mg L−1 in 24 h; while the MFC with bare carbon electrode (MFC-C) exhibited 133.4 ± 11.16 mg L−1 under similar operating conditions. The highest specific H2O2 yield achieved with an influent COD of 1000 mg L−1 in the anodic chamber and an external resistance of 10 Ω was 13.16 ± 2.22 mg L−1 h−1 at the end of 4 h for MFC-CAT. The power density of 179.23 mW m−2 was observed for MFC-CAT in the same operational stage. In comparison, MFC-C exhibited a maximum specific H2O2 yield of 7.44 ± 0.89 mg L−1 h−1 for a retention period of 8 h and a power density of 96.1 mW m−2. Results of the present investigation revealed that synthesized ceria as cathode catalyst can facilitate a higher yield of H2O2 in the catholyte, while simultaneously enhancing the electrical performance compared to bare carbon catalyst.