Redox properties and VOC oxidation activity of Cu catalysts supported on Ce₁-xSmxOδ mixed oxides

Copper-Based Silica Nanotubes as Novel Catalysts for the Total Oxidation of Toluene

Cu (10 wt%) materials on silica nanotubes were prepared via two different synthetic approaches, co-synthesis and wetness impregnation on preformed SiO2 nanotubes, both as dried or calcined materials, with Cu(NO3)2.5H2O as a material precursor. The obtained silica and the Cu samples, after calcination at 550 °C for 5 h, were characterized by several techniques, such as TEM, N2 physisorption, XRD, Raman, H2-TPR and XPS, and tested for toluene oxidation in the 20-450 °C temperature range. A reference sample, Cu(10 wt%) over commercial silica, was also prepared. The copper-based silica nanotubes exhibited the best performances with respect to toluene oxidation. The Cu-based catalyst using dried silica nanotubes has the lowest T50 (306 °C), the temperature required for 50% toluene conversion, compared with a T50 of 345 °C obtained for the reference catalyst. The excellent catalytic properties of this catalyst were ascribed to the presence of easy copper (II) species finely dispersed (crystallite size of 13 nm) on the surface of silica nanotubes. The present data underlined the impact of the synthetic method on the catalyst properties and oxidation activity.

Rational Design of Non-Precious Metal Oxide Catalysts by Means of Advanced Synthetic and Promotional Routes

Catalysis is an indispensable part of our society, involved in numerous energy and environmental applications, such as the production of value-added chemicals/fuels, hydrocarbons processing, fuel cells applications, abatement of hazardous pollutants, among others [...]

Gold and Ceria as Catalysts for VOC Abatement: A Review

Due to its excellent oxygen storage capacity, ceria is a well-known oxidation catalyst. However, its performance in the oxidation of volatile organic compounds can be improved by the introduction of gold. Depending on the type of VOC to be oxidized, the surface of gold nanoparticles and the gold/ceria interface may contribute to enhanced activity and/or selectivity. Choosing a proper preparation method is crucial to obtain optimal gold particle size. Deposition–precipitation was found to be more suitable than coprecipitation or impregnation. For industrial applications, monolithic catalysts are needed to minimize the pressure drop in the reactor and reduce mass and heat transfer limitations. In addition to the approach used with powder catalysts, the method employed to introduce gold in/on the washcoat has to be considered.

Facet-Dependent Reactivity of Ceria Nanoparticles Exemplified by CeO2-Based Transition Metal Catalysts: A Critical Review

The rational design and fabrication of highly-active and cost-efficient catalytic materials constitutes the main research pillar in catalysis field. In this context, the fine-tuning of size and shape at the nanometer scale can exert an intense impact not only on the inherent reactivity of catalyst’s counterparts but also on their interfacial interactions; it can also opening up new horizons for the development of highly active and robust materials. The present critical review, focusing mainly on our recent advances on the topic, aims to highlight the pivotal role of shape engineering in catalysis, exemplified by noble metal-free, CeO2-based transition metal catalysts (TMs/CeO2). The underlying mechanism of facet-dependent reactivity is initially discussed. The main implications of ceria nanoparticles’ shape engineering (rods, cubes, and polyhedra) in catalysis are next discussed, on the ground of some of the most pertinent heterogeneous reactions, such as CO2 hydrogenation, CO oxidation, and N2O decomposition. It is clearly revealed that shape functionalization can remarkably affect the intrinsic features and in turn the reactivity of ceria nanoparticles. More importantly, by combining ceria nanoparticles (CeO2 NPs) of specific architecture with various transition metals (e.g., Cu, Fe, Co, and Ni) remarkably active multifunctional composites can be obtained due mainly to the synergistic metalceria interactions. From the practical point of view, novel catalyst formulations with similar or even superior reactivity to that of noble metals can be obtained by co-adjusting the shape and composition of mixed oxides, such as Cu/ceria nanorods for CO oxidation and Ni/ceria nanorods for CO2 hydrogenation. The conclusions derived could provide the design principles of earth-abundant metal oxide catalysts for various real-life environmental and energy applications.

A review of confined-structure catalysts in the catalytic oxidation of VOCs: synthesis, characterization, and applications

This picture depicts the process of the catalytic oxidation of VOCs on confined-structure catalysts, which possess excellent activity and can effectively protect the active phase from aggregation and poisoning.

Preparation of MnO2 decorated Co3Fe1Ox powder/monolithic catalyst with improved catalytic activity for toluene oxidation

In this paper, KMnO₄ was used to pre-treat Co₃Fe-layered double hydroxides (LDH) precursor to prepare MnO₂ decorated Co₃Fe₁O_x catalyst. The toluene oxidation performance of the catalyst was investigated systematically. The optimized 0.1MnCF-LDO catalyst exhibited the best catalytic performance, and the temperatures of 50% and 90% toluene conversion (T₅₀ and T₉₀) were 218 and 243°C, respectively. The apparent activation energy (E_a) was 31.6 kJ/mol. The characterization results showed that the pre-redox reaction by KMnO₄ could increase the specific surface area, Co³⁺ species amount and oxygen defect concentration of the catalyst, which are the main reason of the improved toluene catalytic activity. Besides, this method was also applied to enhance toluene oxidation of iron mesh based monolithic catalyst. The 0.1MnCF-LDO/Iron mesh (IM) catalyst showed a 90% toluene conversion at around 316°C which was much lower than that of without MnO₂ addition (359°C). In addition, the water resistant of all the catalysts was studied as well, all the samples showed relatively good water resistance. The toluene conversion still remained to be over >80% even in the presence of 10 vol.% water vapor.

Recent Advances on the Rational Design of Non-Precious Metal Oxide Catalysts Exemplified by CuOx/CeO2 Binary System: Implications of Size, Shape and Electronic Effects on Intrinsic Reactivity and Metal-Support Interactions

Catalysis is an indispensable part of our society, massively involved in numerous energy and environmental applications. Although, noble metals (NMs)-based catalysts are routinely employed in catalysis, their limited resources and high cost hinder the widespread practical application. In this regard, the development of NMs-free metal oxides (MOs) with improved catalytic activity, selectivity and durability is currently one of the main research pillars in the area of heterogeneous catalysis. The present review, involving our recent efforts in the field, aims to provide the latest advances—mainly in the last 10 years—on the rational design of MOs, i.e., the general optimization framework followed to fine-tune non-precious metal oxide sites and their surrounding environment by means of appropriate synthetic and promotional/modification routes, exemplified by CuOx/CeO2 binary system. The fine-tuning of size, shape and electronic/chemical state (e.g., through advanced synthetic routes, special pretreatment protocols, alkali promotion, chemical/structural modification by reduced graphene oxide (rGO)) can exert a profound influence not only to the reactivity of metal sites in its own right, but also to metal-support interfacial activity, offering highly active and stable materials for real-life energy and environmental applications. The main implications of size-, shape- and electronic/chemical-adjustment on the catalytic performance of CuOx/CeO2 binary system during some of the most relevant applications in heterogeneous catalysis, such as CO oxidation, N2O decomposition, preferential oxidation of CO (CO-PROX), water gas shift reaction (WGSR), and CO2 hydrogenation to value-added products, are thoroughly discussed. It is clearly revealed that the rational design and tailoring of NMs-free metal oxides can lead to extremely active composites, with comparable or even superior reactivity than that of NMs-based catalysts. The obtained conclusions could provide rationales and design principles towards the development of cost-effective, highly active NMs-free MOs, paving also the way for the decrease of noble metals content in NMs-based catalysts.

Low-temperature catalytic oxidation of benzene over nanocrystalline Cu–Mn composite oxides by facile sol–gel synthesis

We prepare a series of nanocrystalline copper–manganese oxides by a facile citric acid sol–gel method, and test their activities for catalytic oxidation of benzene.

Catalytic combustion of VOC on sandwich-structured Pt@ZSM-5 nanosheets prepared by controllable intercalation

A sandwich-structured Pt@ZSM-5 nanosheets (Pt@PZN-2) was fabricated by controllable intercalating Pt nanoparticles between ZSM-5 single-layer sheets via electrostatic adsorption with ammonium ions in diquaternary ammonium surfactant, and the following calcinations. Pt clusters (ca. 4.3 nm) confined between nanosheet layers (thickness of ca. 2.9 nm) exhibits better dispersion and higher thermal stability even after high-temperature treatment, while the Pt clusters also served as pillars which well protected the inter-layers mesopores. Catalytic combustion of toluene (a model compound of large molecules VOCs) shows catalytic activity of Pt@PZN-2 is significant higher than that of Pt-loaded nanosheet ZSM-5 zeolite prepared by conventional impregnation (Pt/ZN-2). For example, the temperature at 98% toluene conversion of Pt@PZN-2 is only 176 °C which is 24 °C and 34 °C lower than that of Pt/ZN-2 and bulky ZSM-5 supported Pt by impregnation (Pt/CZ-500), respectively. Meanwhile, Pt@PZN-2 also gives 100% toluene conversion for more than 360 h at 230 °C with a higher metal stability. The good performance of Pt@PZN-2 was attributed to the high accessibility and enhanced diffusion of the bulky reactant to active site confined in the stable inter-layer mesopores resulting from the pillar of Pt nanoparticles.

Investigation of synergistic effects and high performance of La-Co composite oxides for toluene catalytic oxidation at low temperature

Cobalt oxides have been considered as a kind of highly efficient catalyst for the oxidation of volatile organic compounds (VOCs). In this work, lanthanum-cobalt composite oxides were prepared by using the co-precipitation method, and toluene was used as the model compound. Diversified techniques including XRD, SEM, Raman spectra, XPS, H₂-TPR, and N₂ adsorption-desorption were applied to investigate the physicochemical properties of as-prepared materials. The composite catalysts showed different morphology including larger specific surface area and higher pore volume which would accelerate the adsorption of toluene and improve the amount of active sites on surface. Moreover, the addition of lanthanum could enhance the low-temperature reducibility, and it could be also beneficial to expose more Co³⁺ and adsorbed oxygen species on the surface of catalysts which could accelerate the oxidation of toluene and lower onset oxidation temperature. 0.05La-Co (with a molar ratio of lanthanum against cobalt is 0.05) showed the best catalytic performance. The complete conversion of toluene was achieved at 225 °C under the condition of toluene concentration = 1000 ppm and SV = 20,000 ml·g^-1·h^-1. Stability test over 0.05La-Co was conducted at 225 °C and it could maintain the 100% conversion of toluene for 720 min, indicating the excellent stability of as-prepared catalysts. Undoubtedly, lanthanum-cobalt composite oxide is a kind of promising material for the catalytic oxidation of VOCs.

Promoting effect of Nb doping on catalytic performance for deep oxidation of 1, 2-dichloroethane over (Ce,Cr)xO2-Nb2O5 catalysts

A series of Nb-doped (Ce,Cr)_xO₂-Nb₂O₅ mixed oxides with varying (Ce,Cr)_xO_2/Nb₂O₅ mass ratio were prepared by a co-precipitation method and evaluated for the catalytic performance of eliminating 1,2-dichloroethane (DCE). The results indicate that there exists a strong synergistic effect between acid sites and redox species in (Ce,Cr)_xO₂-Nb₂O₅ improving the catalytic activity for DCE oxidation. Appropriate Nb doping could promote the high dispersion and the interaction of metal oxides in the (Ce,Cr)_xO₂-Nb₂O₅ catalysts, resulting in the formation of more Cr⁶⁺ species with strong oxidizing ability and excellent mobility of oxygen species from bulk to surface to create more active sites for DCE deep oxidation. The (Ce,Cr)_xO₂-Nb₂O₅ catalysts with (Ce,Cr)_xO₂/Nb₂O₅ ratios of 2/1~1/2 exhibit excellent catalytic activity and durability for DCE degradation in dry air as well as benzene or water vapor, and less chlorinated byproduct is produced during the degradation of DCE.

Effect of Y Modified Ceria Support in Mono and Bimetallic Pd–Au Catalysts for Complete Benzene Oxidation

Mono metallic and bimetallic Pd (1 wt. %)–Au (3 wt. %) catalysts were prepared using two ceria supports doped with 1 wt. % Y2O3. Yttrium was added by impregnation or co-precipitation. The catalyst synthesis was carried out by deposition–precipitation method, with sequential deposition–precipitation of palladium over previously loaded gold in the case of the bimetallic samples. The obtained materials, characterized by X-ray powder diffraction (XRD), High resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (TPR) techniques, were tested in the complete benzene oxidation (CBO). The results of the characterization analyses and the catalytic performance pointed to a close relationship between structural, redox, and catalytic properties of mono and bimetallic catalysts. Among the monometallic systems, Pd catalysts were more active as compared to the corresponding Au catalysts. The bimetallic systems exhibited the best combustion activity. In particular, over Pd–Au supported on Y-impregnated ceria, 100% of benzene conversion towards total oxidation at the temperature of 150 °C was obtained. Comparison of surface sensitive XPS results of fresh and spent catalysts ascertained the redox character of the reaction.

Removal of volatile odorous organic compounds over NiAl mixed oxides at low temperature

In this paper, a series of NiAl hydrotalcite-like compounds (HTLCs) were prepared by the urea-decomposition method. Removal of carbonyl sulfide (COS) and methyl mercaptan (CH₄S) over the hydrotalcite-derived oxides (HTO) at low temperature was studied. The Ni3Al-HTO exhibited higher catalytic activities than Ni3Al mixed oxides prepared by physical mixing method (Ni3Al-PM) or impregnation/calcination method (Ni3Al-IC). Based on the characterization, it was found that desulfurization activities are closely tied to the surface acid-base properties of catalysts. CO₂-TPD indicates that the basic characteristic of the Ni3Al-HTO is prominent. XPS results showed that there was a strong interaction between Ni and Al element on Ni3Al-HTO. The first principle calculation based on density function theory was applied with the aim to study the change of basic sites. The results showed that Ni3Al-HTO presents a half-metallic characteristic. Electron transfer from the Al and O atom to the Ni atom was observed, which is helpful for the transfer of electrons from the surface and improves the catalytic activity. Analysis of the DRIFT spectra suggests that sulfate species was formed by the action of surface basic sites, resulting in the formation of H₂O on the surface.

The synthetic evaluation of CuO-MnOx-modified pinecone biochar for simultaneous removal formaldehyde and elemental mercury from simulated flue gas

A series of low-cost Cu-Mn-mixed oxides supported on biochar (CuMn/HBC) synthesized by an impregnation method were applied to study the simultaneous removal of formaldehyde (HCHO) and elemental mercury (Hg⁰) at 100-300° C from simulated flue gas. The metal loading value, Cu/Mn molar ratio, flue gas components, reaction mechanism, and interrelationship between HCHO removal and Hg⁰ removal were also investigated. Results suggested that 12%CuMn/HBC showed the highest removal efficiency of HCHO and Hg⁰ at 175° C corresponding to 89%and 83%, respectively. The addition of NO and SO₂ exhibited inhibitive influence on HCHO removal. For the removal of Hg⁰, NO showed slightly positive influence and SO₂ had an inhibitive effect. Meanwhile, O₂ had positive impact on the removal of HCHO and Hg⁰. The samples were characterized by SEM, XRD, BET, XPS, ICP-AES, FTIR, and H₂-TPR. The sample characterization illustrated that CuMn/HBC possessed the high pore volume and specific surface area. The chemisorbed oxygen (O_β) and the lattice oxygen (O_α) which took part in the removal reaction largely existed in CuMn/HBC. What is more, MnO₂ and CuO (or Cu₂O) were highly dispersed on the CuMn/HBC surface. The strong synergistic effect between Cu-Mn mixed oxides was critical to the removal reaction of HCHO and Hg⁰ via the redox equilibrium of Mn⁴⁺ + Cu⁺ ↔ Mn³⁺ + Cu²⁺.

Optimization of N2O decomposition activity of CuO–CeO2 mixed oxides by means of synthesis procedure and alkali (Cs) promotion

The fine-tuning of CuO–CeO₂ mixed oxides by means of synthesis procedure (co-precipitation) and alkali promotion (1.0 at Cs per nm²) towards highly active deN₂O catalysts is demonstrated.

Rare-earth element doping-promoted toluene low-temperature combustion over mesostructured CuMCeOx (M = Y, Eu, Ho, and Sm) catalysts: the indispensable role of in situ generated oxygen vacancies

The synergetic effect between Ho and the CuCeO_x framework creates abundant active oxygen vacancies and significantly enhances the toluene destruction activity.

Effect of cobalt loading on the solid state properties and ethyl acetate oxidation performance of cobalt-cerium mixed oxides

Cobalt-cerium mixed oxides were prepared by the wet impregnation method and evaluated for volatile organic compounds (VOCs) abatement, using ethyl acetate (EtAc) as model molecule. The impact of Co content on the physicochemical characteristics of catalysts and EtAc conversion was investigated. The materials were characterized by various techniques, including N₂ adsorption at -196°C, scanning electron microscopy (SEM), X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) to reveal the structure-activity relationship. The obtained results showed the superiority of mixed oxides compared to bare CeO₂ and Co₃O₄, demonstrating a synergistic effect. The optimum oxidation performance was achieved with the sample containing 20wt.% Co (Co/Ce atomic ratio of ca. 0.75), in which complete conversion of EtAc was attained at 260°C. In contrast, temperatures above 300°C were required to achieve 100% conversion over the single oxides. Notably, a strong relationship between both the: (i) relative population, and (ii) facile reduction of lattice oxygen with the ethyl acetate oxidation activity was found, highlighting the key role of loosely bound oxygen species on VOCs oxidation. A synergistic Co-Ce interaction can be accounted for the enhanced reducibility of mixed oxides, linked with the increased mobility of lattice oxygen.

Nanostructured Samarium Doped Fluorapatites and Their Catalytic Activity towards Synthesis of 1,2,4-Triazoles

An investigation was conducted into the influence of the amino acids as organic modifiers in the facile synthesis of metal incorporated fluorapatites (FAp) and their properties. The nanostructured Sm doped fluorapatites (Sm-FAp) were prepared by a co-precipitation method using four different amino acids, namely glutamic acid, aspartic acid, glycine and histidine. The materials were characterized by various techniques including X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), high resolution transmission electron microscopy (HR-TEM), N₂-adsorption/desorption isotherm, temperature programmed desorption (TPD) and fluorescence spectrophotometry. Under similar conditions, Sm-FAp prepared using different amino acids exhibited distinctly different morphological structures, surface area and pore properties. Their activity as catalysts was assessed and Sm-FAp/Glycine displayed excellent efficiency in the synthesis of 1,2,4-triazole catalyzing the reaction between 2-nitrobenzaldehyde and thiosemicarbazide with exceptional selectivity and 98% yield in a short time interval (10 min). The study provides an insight into the role of organic modifiers as controllers of nucleation, growth and aggregation which significantly influence the nature and activity of the catalytic sites on Sm-FAp. Sm-FAp could also have potential as photoactive material.

Ethyl Acetate Abatement on Copper Catalysts Supported on Ceria Doped with Rare Earth Oxides

Different lanthanide (Ln)-doped cerium oxides (Ce_0.5Ln_0.5O_1.75, where Ln: Gd, La, Pr, Nd, Sm) were loaded with Cu (20 wt. %) and used as catalysts for the oxidation of ethyl acetate (EtOAc), a common volatile organic compound (VOC). For comparison, both Cu-free (Ce-Ln) and supported Cu (Cu/Ce-Ln) samples were characterized by N₂ adsorption at −196 °C, scanning/transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction in H₂. The following activity sequence, in terms of EtOAc conversion, was found for bare supports: CeO₂ ≈ Ce_0.5Pr_0.5O_1.75 > Ce_0.5Sm_0.5O_1.75 > Ce_0.5Gd_0.5O_1.75 > Ce_0.5Nd_0.5O_1.75 > Ce_0.5La_0.5O_1.75. Cu addition improved the catalytic performance, without affecting the activity order. The best catalytic performance was obtained for Cu/CeO₂ and Cu/Ce_0.5Pr_0.5O_1.75 samples, both achieving complete EtOAc conversion below ca. 290 °C. A strong correlation was revealed between the catalytic performance and the redox properties of the samples, in terms of reducibility and lattice oxygen availability. Νo particular correlation between the VOC oxidation performance and textural characteristics was found. The obtained results can be explained in terms of a Mars-van Krevelen type redox mechanism involving the participation of weakly bound (easily reduced) lattice oxygen and its consequent replenishment by gas phase oxygen.

Catalytic oxidation of ethyl acetate and toluene over Cu–Ce–Zr supported ZSM-5/TiO2 catalysts

Schematic of VOCs oxidation mechanism over the CCZ/Z (A) and CCZ/T (B) catalysts.

Effect of preparation method on the solid state properties and the deN2O performance of CuO–CeO2 oxides

The superiority of Cu–Ce mixed oxides prepared by precipitation (versus impregnation and exotemplating) was ascribed to their excellent redox properties, linked to the Ce⁴⁺/Ce³⁺ and Cu²⁺/Cu⁺ redox pairs.

The influence of manganese-cobalt oxide/graphene on reducing fire hazards of poly(butylene terephthalate)

By means of direct nucleation and growth on the surface of graphene and element doping of cobalt oxide (Co3O4) nano-particles, manganese-cobalt oxide/graphene hybrids (MnCo2O4-GNS) were synthesized to reduce fire hazards of poly(butylene terephthalate) (PBT). The structure, elemental composition and morphology of the obtained hybrids were surveyed by X-ray diffraction, X-ray photoelectron spectrometer and transmission electron microscopy, respectively. Thermogravimetric analysis was applied to simulate and study the influence of MnCo2O4-GNS hybrids on thermal degradation of PBT during combustion. The fire hazards of PBT and its composites were assessed by the cone calorimeter. The cone test results had showed that peak HRR and SPR values of MnCo2O4-GNS/PBT composites were lower than that of pure PBT and Co3O4-GNS/PBT composites. Furthermore, the incorporation of MnCo2O4-GNS hybrids gave rise to apparent decrease of pyrolysis products containing aromatic compounds, carbonyl compounds, carbon monoxide and carbon dioxide, attributed to combined impact of physical barrier for graphene and cat O4 for organic volatiles and carbon monoxide.

Dual function adsorbent-catalyst CuO-CeO2/NaX for temperature swing oxidation of benzene, toluene and xylene

A less common cyclic process of decontamination of benzene, toluene and o-xylene (BTX) using a dual function adsorbent-catalyst, referred to as the temperature swing oxidation, is introduced and discussed in this research. Preparation technique and characterization of the dual function adsorbent-catalyst CuO-CeO2/NaX are presented. The temperature swing oxidation of BTX consists of two stages: adsorption of the VOC from the stream saturating the adsorbent-catalyst at different levels and catalytic oxidation of concentrated VOC induced by raising bed temperature at different flow rates of regenerative air. The results indicate that at lower saturation levels and lower flow rates of regenerative air a complete oxidation performance is better. The highest obtained values of the overall conversion of toluene, o-xylene and benzene into CO2 and H2O were 99.3, 99.8 and 77.5%, respectively, proving that the temperature swing oxidation using a dual function adsorbent-catalyst is a promising VOC decontamination technique under properly selected operating conditions.