Total Oxidation of Propane Using CeO2 and CuO-CeO2 Catalysts Prepared Using Templates of Different Nature

Nanoshaped Cerium Oxide with Nickel as a Non-Noble Metal Catalyst for CO2 Thermochemical Reactions

Four different nanoshapes of cerium dioxide have been prepared (polycrystals, rods, cubes, and octahedra) and have been decorated with different metals (Ru, Pd, Au, Pt, Cu, and Ni) by incipient wetness impregnation (IWI) and ball milling (BM) methods. After an initial analysis based on oxygen consumption from CO₂ pulse chemisorption, Ni-like metal, and two forms of CeO₂ cubes and rods were selected for further research. Catalysts were characterized using the Brunauer-Emmett-Teller formula (BET), X-ray spectroscopy (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV-visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H₂-TPR) and CO₂ pulse chemisorption, and used to reduce of CO₂ into CO (CO₂ splitting). Adding metals to cerium dioxide enhanced the ability of CeO₂ to release oxygen and concomitant reactivity toward the reduction of CO₂. The effect of the metal precursor and concentration were evaluated. The highest CO₂ splitting value was achieved for 2% Ni/CeO₂-rods prepared by ball milling using Ni nitrate (412 µmol/g_cat) and the H₂ consumption (453.2 µmol/g_cat) confirms the good redox ability of this catalyst.

Roles of Oxygen Species in Low-Temperature Catalytic o-Xylene Oxidation on MOF-Derived Bouquetlike CeO2

To realize efficient low-temperature catalytic o-xylene oxidation, MOF-derived CeO₂-X catalysts were prepared via the pyrolysis of MOF precursors with different ratios of cerium nitrate to trimesic acid. Among the synthesized catalysts, the bouquet like CeO₂-1 exhibited the best activity with T₅₀ and T₉₀ of 156 and 198 °C and the lowest activation energy of 60.67 kJ·mol^-1 (WHSV= 48 000 mL·g^-1·h^-1, o-xylene concentration = 500 ppm). o-Xylene was completely mineralized, and no change in conversion efficiency or CO₂ yield was found at 5 vol % H₂O for over 50 h. The rich active oxygen species (XPS: O_sur/O_latt = 0.69) and abundant oxygen vacancies (Raman: I_D/I_F2g = 0.036) of CeO₂-1 made crucial contribution to its superior catalytic activity. The O₂-TPD and H₂-TPR results confirmed that CeO₂-1 had more surface active oxygen and better mobility of bulk oxygen. Moreover, the reaction routes under different atmospheres were probed through in situ DRIFTS, in which oxygen vacancy played a key role in promoting the adsorption and activation of molecular oxygen and facilitating the migration of the bulk lattice oxygen.

Confinement of nano-gold in 3D hierarchically structured gadolinium-doped ceria mesocrystal: synergistic effect of chemical composition and structural hierarchy in CO and propane oxidation

Gadolinium-doped ceria hierarchical gold catalyst shows four-fold TOF increase compared to undoped non-hierarchical system, proving the synergistic effect of doping and structural hierarchy in propane oxidation.

Recent advances in non-noble metal-based oxide materials as heterogeneous catalysts for C–H activation

This perspective article summarizes the recent developments of non-noble metal-based oxides, as a new class of catalysts for C−H bond activation, focusing on their essential surface properties.

A highly active Ca/Cu/YCeO2-TiO2 catalyst for the transient reduction of NO with CO and naphthalene under oxidizing conditions

The transient combustion of biomass leads to the evolution of a variety of pollutants (NO, CO, organic compounds, and many others) that can react with each other on a suitable catalyst to generate compounds of lower toxicity. Here, the transient reduction of NO with CO and naphthalene in the presence of oxygen was studied on a Ca/Cu/YCeO2-TiO2 catalyst. Response surface methodology was used to identify the optimum amounts of calcium, copper, and cerium. The optimized Ca/Cu/YCeO2-TiO2 catalyst was then extensively studied and characterized. The coupling of yttrium-stabilized ceria with TiO2 provided an active support that effectively activated naphthalene. When calcium and copper were added to the support, the obtained Ca/Cu/YCeO2-TiO2 catalyst achieved the full conversion of CO and naphthalene and 72% conversion of NO. The Ca/Cu/YCeO2-TiO2 catalyst possessed labile oxygen species, which might be related to the high catalytic activity.

The fascinating world of mayenite (Ca12Al14O33) and its derivatives

Mayenite (12CaO·7Al2O3) is a mesoporous calcium aluminum oxide, with a characteristic crystalline structure. The framework of mayenite is composed of interconnected cages with a positive electric charge per unit cell that includes two molecules [Ca24Al28O64]4+, and the remaining two oxide ions O2−, often labelled “free oxygen”, are trapped in the cages defined by the framework. Starting from mayenite structure several derivatives have been prepared through advanced synthetic protocols by free oxygen substitution with various anions. Mayenite and its derivates have been intensively investigated in many applications which include catalysis (oxidation and reduction, ammonia synthesis, pinacol coupling), environmental sensors and CO2 sorbent materials. In this review, we summarize our recent results on the main applications of mayenite and its derivatives.

Promoting Effect of Palladium on ZnAl2O4-Supported Catalysts Based on Cobalt or Copper Oxide on the Activity for the Total Propene Oxidation

Palladium-modified Co-ZnAland Cu-ZnAl materials were used and found active for the catalytic oxidation of propene and propane. According to the results obtained by XRD, TPR and XPS, the zinc aluminate-supported phases are oxide phases, Co₃O₄, CuO and PdO_x for Co-ZnAl, Cu-ZnAl and Pd-ZnAl catalysts, respectively. These reducible oxide species present good catalytic activity for the oxidation reactions. The addition of palladium to Co-ZnAl or Cu-ZnAl samples promoted the reducibility of the system and, consequently, produced a synergic effect which enhanced the activity for the propene oxidation. The PdCo-ZnAl sample was the most active and exhibited highly dispersed PdO_x particles and surface structural defects. In addition, it exhibited good catalytic stability. The H₂ pre-treated PdCu-ZnAl, PdCo-ZnAl and Pd-ZnAl samples showed higher activity than the original oxide catalysts, evidencing the important role of the oxidation state of the species, mainly of the palladium species, on the catalytic activity for the propene combustion. The synergic effect between metal transition oxides and PdO_x could not be observed for the propane oxidation.

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.

Propane and Naphthalene Oxidation over Gold-Promoted Cobalt Catalysts Supported on Zirconia

Zirconia-supported gold-promoted cobalt catalysts were synthesized and tested for the complete oxidation of propane and naphthalene. The catalysts were characterized by BET surface area, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). In both propane and naphthalene combustion reactions, the results obtained indicate that catalysts formulated with Co3O4 are more active than those containing only Au. Catalysts prepared using the deposit/precipitation (DP) method have better activity than those in which the traditional technique is used. Gold addition using the DP methods generates a promoting effect on the activity of cobalt-containing catalysts. The AuDpCoZt catalyst was found to be the most active for both propane and naphthalene combustion. The catalytic behavior of this sample is associated with a synergic effect between gold, cobalt, and the support, which is also evidenced by an increase in the reducibility of this catalytic system. The effect of the presence of NO in the feed was also analyzed for propane combustion.

Synthesis of mesoporous ceria using metal- and halogen-free ordered mesoporous carbon as a hard template

Ordered mesoporous cerias were synthesised by employing metal- and halogen-free ordered mesoporous carbons (OMCs) as the hard templates in a 'nanocasting' procedure. TEM, small angle (SA) and wide angle (WA) XRD, and N₂ physisorption analyses were used to characterise the templates, intermediates and ceria products and electron tomography (STEM-HAADF) was used to explore the 3D morphology of the ceria nanostructures grown within the carbon templates. This allowed the relationships between the structures of the OMC templates and the products to be considered in detail as two parameters were varied. These were: the method of impregnation of the ceria precursor; and the temperature of calcination of the OMC template. Of the four impregnation methods tested, the solid-liquid method was found to be the most successful. This gave a high quality product with the highest yield of uniform mesopores, and crystalline nanorods of ceria arrayed in clear long-range order, as viewed by TEM and determined in SA and WAXRD. The specific surface area and pore volume exhibited by this sample were 111 m² g^-1 and 0.39 cm³ g^-1, respectively. 3D electron tomography reconstructions revealed the presence of a network of ordered, nanoscale, rod-like structures interlinked in a complex fashion. The effect of calcination temperature of the template on uptake of the ceria precursor during impregnation was studied by calcining OMCs at temperatures from 350 to 800 °C and using these as hard templates for the nanocasting of ceria. Of these, the carbon template calcined at 400 °C gave the highest quality product.

A Novel Approach to High-Performance Aliovalent-Substituted Catalysts-2D Bimetallic MOF-Derived CeCuOx Microsheets

Mixed transition metal oxides (MTMOs) have enormous potential applications in energy and environment. Their use as catalysts for the treatment of environmental pollution requires further enhancement in activity and stability. This work presents a new synthesis approach that is both convenient and effective in preparing binary metal oxide catalysts (CeCuO_x ) with excellent activity by achieving molecular-level mixing to promote aliovalent substitution. It also allows a single, pure MTMO to be prepared for enhanced stability under reaction by using a bimetallic metal-organic framework (MOF) as the catalyst precursor. This approach also enables the direct manipulation of the shape and form of the MTMO catalyst by controlling the crystallization and growth of the MOF precursor. A 2D CeCuO_x catalyst is investigated for the oxidation reactions of methanol, acetone, toluene, and o-xylene. The catalyst can catalyze the complete reactions of these molecules into CO₂ at temperatures below 200 °C, representing a significant improvement in performance. Furthermore, the catalyst can tolerate high moisture content without deactivation.

Zirconia-Supported Silver Nanoparticles for the Catalytic Combustion of Pollutants Originating from Mobile Sources

This work presents the physicochemical characterization and activity of zirconia-supported silver catalysts for the oxidation of pollutants present in diesel engine exhaust (propane, propene, naphthalene and soot). A series of silver-supported catalysts AgxZ (x = 1, 5 and 10 wt.%, Z = zirconia) were prepared, which were studied by various characterization techniques. The results show that silver is mainly found under the form of small metal nanoparticles (<10 nm) dispersed over the support. The metallic phase coexists with the AgOx oxidic phases. Silver is introduced onto the zirconia, generating Ag–ZrO2 catalysts with high activity for the oxidation of propene and naphthalene. These catalysts also show some activity for soot combustion. Silver species can contribute with zirconia in the catalytic redox cycle, through a synergistic effect, providing sites that facilitate the migration and availability of oxygen, which is favored by the presence of structural defects. This is a novel application of the AgOx–Ag/ZrO2 system in the combustion reaction of propene and naphthalene. The results are highly promising, given that the T50 values found for both model molecules are quite low.

Catalytic Behaviour of Flame-Made CuO-CeO2 Nanocatalysts in Efficient CO Oxidation

CuO-CeO2 nanocatalysts with varying CuO contents (1, 5, 9, 14 and 17 wt %) were prepared by one-step flame spray pyrolysis (FSP) and applied to CO oxidation. The influences of CuO content on the as-prepared catalysts were systematically characterized by X-ray diffraction (XRD), N2 adsorption-desorption at −196 °C, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and hydrogen-temperature programmed reduction (H2-TPR). A superior CO oxidation activity was observed for the 14 wt % CuO-CeO2 catalyst, with 90% CO conversion at 98 °C at space velocity (60,000 mL × g−1 × h−1), which was attributed to abundant surface defects (lattice distortion, Ce3+, and oxygen vacancies) and high reducibility supported by strong synergistic interaction. In addition, the catalyst also displayed excellent stability and resistance to water vapor. Significantly, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) showed that in the CO catalytic oxidation process, the strong synergistic interaction led readily to dehydroxylation and CO adsorption on Cu+ at low temperature. Furthermore, in the feed of water vapor, although there was an adverse effect on the access of CO adsorption, there was also a positive effect on the formation of fewer carbon intermediates. All these results showed the potential of highly active and water vapor-resistive CuO-CeO2 catalysts prepared by FSP.

Influence of the synthesis method on the catalytic activity of mayenite for the oxidation of gas-phase trichloroethylene

Catalytic oxidation of trichloroethylene (TCE) in heterogeneous phase (gas-solid) is an effective strategy for the conversion of this pollutant in less harmful compounds, namely CO₂, CO and HCl. In this work, we have studied the use of mayenite, a cost-effective material, as an active catalyst for the TCE conversion. In particular, we have assessed the influence of the mayenite synthesis method (hydrothermal, sol-gel and ceramic) on the reaction performance. The materials have been characterized by different techniques, such as XRD, N₂-sorption (BET), TPR, Raman spectroscopy, FESEM-EDX and TEM. The analysis of the light-off curves and product distribution, has shown that the use of the hydrothermal method for the mayenite synthesis results in the most active and selective catalyst. This has been related with a higher surface area and with a higher concentration of oxygen anions in the mayenite prepared by this method. It has been found that the presence of water in the stream do not influence the catalytic performance of the material. A mechanism for the reaction and for the partial deactivation of the catalyst has been proposed.

A Novel Synthetic Route to Prepare High Surface Area Mayenite Catalyst for TCE Oxidation

Mayenite (Ca12Al14O33) was synthesized by a novel route based on the use of polymethyl methacrylate (PMMA) as a soft templating agent. The material was tested for the total oxidation of trichloroethylene in the gas phase and the catalytic performance was analysed when using different initial amounts of PMMA in the catalyst synthesis. The results were compared with those obtained with a mayenite synthetized by a classical hydrothermal method. The highest activity in terms of TCE conversion was achieved in the presence of mayenite prepared using 10% w/w of PMMA; its activity was also higher than that of the hydrothermal mayenite. The surface area and the number of superoxide anions (O2−) seem to be the main properties determining the catalytic activity of the material.

The transient reduction of NO with CO and naphthalene in the presence of oxygen using a core–shell SmCeO2@TiO2-supported copper catalyst

This work studied the reaction of common pollutants on a catalytic surface under oxidizing conditions.

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.

Study of Ce-Cu mixed oxide catalysts by in situ electrical conductivity measurements

Three Ce-Cu mixed oxides, namely Ce_0.95Cu_0.05, Ce_0.6Cu_0.4 and Ce_0.15Cu_0.85, along with pure CeO₂ and CuO were characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure, and was followed with time during successive exposure to air, nitrogen and different gaseous mixtures containing propane as a VOC model molecule, under conditions close to those of their catalytic applications. CeO₂ and CuO appeared to be n-type and p-type semiconductors, respectively, while the semiconducting behavior of the Ce-Cu mixed oxides depended on the oxide composition. The semiconductive and redox properties of the samples were correlated with their catalytic behavior in CO oxidation, ethene total oxidation and soot combustion.