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Global warming has been a serious concern worldwide. Greenhouse gases like CO2 majorly contribute to the rise in average global temperature. Attempts have been made to reduce the CO2 in the atmosphere. However, carbon capture alone was not found to be commercially viable. Various other green and cost-effective methods to convert CO2 into more useful products have been researched. CO2 methanation was found to be one such vital reaction of converting CO2 to useful fuel, like methane, using different catalysts. In this paper, this vital reaction has been reviewed comprehensively. Studies conducted with respect to materials that have proven catalysts such as Ni, synthesised by impregnation, to be the most suitable, where use of promotors (like Si and Ce) and supports (like zeolites, ceria and MOFs), high pressures, low humidity and optimum temperatures have led to increased selectivity, have been presented. The different reactors/contactors that have been used in this process at various scales, their design parameters, process conditions, performance, and limitations have been discussed which have shown the use of annular fixed bed reactors to be the most favourable. The different mechanisms proposed for the reaction, involving various intermediates, mainly carbenes and formates, and problems incurred in low temperature operation and finding of an appropriate support and promotor have been presented. Thermo-kinetic modelling studies on this reaction have also been presented and discussed. Every section has been summarized in the form of a table. The recent advances as well as future challenges and prospects of the above-said aspects of CO2 methanation have also been cited. Recent advances suggest methods of electrocatalytic reduction potentially employing Cu based compounds and perovskite oxides as catalysts, bio-electrocatalytic reduction using microbes as catalysts, and photocatalytic reduction using noble or critical metals including Ni and TiO2 as catalysts and graphene as a support, for the conversion of carbon dioxide to methane.

The influence of an anatase-TiO2 support on the diesel soot oxidation catalytic activity of manganese doped ceria is investigated. The soot conversion light-off temperature is significantly lowered with the application of anatase-TiO2 as the support in comparison to the unsupported and γ-Al2O3 supported CeO2–MnOx catalysts. Additionally, considerable enhancement in bulk and surface defects is observed for CeO2–MnOx/TiO2, which is attributed to the promotional role of the CeO2/TiO2 interface in the formation and stabilization of defect sites. The temperature programmed desorption of oxygen (O2-TPD) study of CeO2–MnOx/TiO2 indicates a sharp increase in oxygen desorption after a temperature of 500 K. In good correlation, the diesel soot conversion substantially increases after 550 K, but below this temperature the TiO2 supported catalyst exhibits comparable activity to that of the γ-Al2O3 supported catalyst. Increased oxygen mobility at elevated temperatures might play a key role in the performance of the TiO2 supported catalyst. Moreover, its structural stability and appreciable catalytic activity were retained even after high temperature annealing treatment.

We have described herein the synthesis and characterization of a uniquely designed mesoporous silica shell@Pd nanoparticle tethered amine functionalized silica core catalyst and its catalytic properties in the hydrogenation of levulinic acid, a key platform molecule in many biorefinery schemes, into γ-valerolactone, using formic acid as a sustainable H2 source. Monodispersed silica core particles (∼300 nm in diameter) were prepared and further functionalized by amine groups and then the in situ loading of Pd nanoparticles was carried out. Pd0-NPs are sandwiched between the nonporous silica core and the mesoporous silica shell, leading to the exceptional stability of the catalyst. The nanostructured material was thoroughly characterised by means of powder XRD patterns, N2 sorption, FE-SEM, UHR-TEM, TG-DTA, and XPS analysis. Our core–shell nanostructure catalyst encapsulated with Pd nanoparticles exhibited a significant increase in catalytic activity and excellent selectivity towards γ-valerolactone (99%) compared with control catalysts for levulinic acid hydrogenation, including Pd@C and Pd@SiO2 (without a mesoporous SiO2 shell). Our results suggest that the core–shell silica based nanocatalyst offers tremendous recyclability (up to the 10th catalytic run with consistent conversion and selectivity of γ-valerolactone), stability (no leaching of Pd and structure collapsing) and no sign of deactivation.

A series of MnOx/CeO2 (Mn/Ce), MnOx/ZrO2 (Mn/Zr), and MnOx/Ce0.75Zr0.25O2 (Mn/CZ) catalysts prepared by an impregnation method were tested for their ability to catalyse the oxidation of Hg0 at relatively low temperature (423 K). Various characterization techniques, namely, Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and H2-temperature programmed reduction (H2-TPR) were employed to understand the structural, surface, and redox properties of the prepared catalysts. Specific aspects of the catalysis of Hg0 oxidation that were investigated included the influence of MnOx loading (5, 15, and 25%) and the influence of HCl and O2. Among the catalysts tested, the 15Mn/CZ catalyst achieved the best Hg0 oxidation performance (~83% conversion of Hg0 to Hg2+) in the presence of HCl and O2. The higher activity of the 15Mn/CZ catalyst was most likely due to the presence of more oxygen vacancies, enhanced Mn4+/Mn4+ + Mn3+ + Mn2+ ratio and more surface adsorbed oxygen, which were proved by XRD, BET, Raman, and XPS. H2-TPR results also show that the strong interaction between the Ce0.75Zr0.25O2 support and MnOx improved the redox properties significantly as compared to pure CeO2 and ZrO2 supported MnOx catalysts.

Ceria (CeO2) is a well-known material for various industrial applications due to its unique redox properties. Such properties, dominated by structural defects that are primarily oxygen vacancies associated with the Ce3+/Ce4+ redox couple, can be easily modulated and optimized by different approaches. In this paper, nanosized Mn and Fe codoped CeO2 solid solutions, Ce0.7−xMn0.3FexO2−δ (x = 0.05–0.2), were prepared by a simple coprecipitation method and tested towards elemental mercury (Hg0) oxidation and adsorption. The obtained solid solutions were characterized in detail at the structural and electronic level by various techniques, namely, XRD, ICP-OES, BET surface area, TEM, Raman, H2-TPR, and XPS. The XRD results suggest that the Mn and/or Fe dopant cations are effectively incorporated into the CeO2 lattice. BET surface area results suggest that the addition of Mn and/or Fe dopants to CeO2 significantly reduces its crystallite size and thereby improves the surface area. Raman, H2-TPR, and XPS results reveal that the Mn and/or Fe dopant cations in the ceria lattice increased the concentration of structural oxygen vacancies and the reducibility of the redox pair Ce4+/Ce3+. The Hg0 oxidation and adsorption studies indicate that Ce0.7−xMn0.3FexO2−δ solid solutions exhibited the highest activity compared to pure CeO2. In particular, the Ce0.5Mn0.3Fe0.2O2−δ (CMF20) solid solution shows an Hg0 oxidation efficiency (Eoxi) of 86.5%. It was found that the doping of both Mn and Fe led to lattice distortion and restrained growth of CeO2, resulting in synergistic increase in oxygen vacancies and catalytic activity.

Co3O4 promoted CeO2 nanocubes have been found to exhibit outstanding catalytic activity for the oxidation of diesel soot at low temperatures (50% soot conversion = 606 K). This remarkable performance is attributed to the superior reducible nature of cerium oxide and the preferential exposure of CeO2 (100) and Co3O4 (110) facets. A probable mechanism based on the cooperative effect of cerium- and cobalt-oxides has been proposed, offering new possibilities for the design of promising materials for catalytic soot oxidation.

Doping ceria with heterocations is a commonly applied strategy to alter its structural and chemical properties including its key feature oxygen storage capacity (OSC). Although a few papers have been published on the structural properties and chemical reactivity of Zn-doped ceria, one lacks a comprehensive investigation on the effect of zinc incorporation in the ceria lattice on its structural properties, and how it could be correlated to the changes in its chemical reactivity including OSC and catalytic activities. Here, we have established an interesting correlation between the structural properties of dual-phase Zn-modified ceria nanocrystals, their OSCs, and their catalytic performance for the reverse water-gas shift (RWGS) reaction and soot oxidation. Upon incorporation of zinc in the ceria lattice, the degree of crystallinity is decreased according to XRD. Raman spectroscopy reveals a concomitant increase of oxygen vacancy concentrations within the Zn-modified ceria samples in comparison to pure ceria. The incorporation of zinc increases the reducibility of ceria according to H2-TPR and doubles the OSC as revealed by thermogravimetric studies. When the variations of the degree of crystallinity, the oxygen vacancy concentrations and the OSCs within the Zn-modified ceria samples are compared, an excellent correlation is established. Catalytic testing shows that Zn-modified ceria exhibits higher activities for the RWGS reaction, especially at the lower temperatures of 400 °C and 600 °C, while at 800 °C the catalyst deactivates rapidly. Such deactivation at high temperature can be totally eliminated by impregnating additional cobalt oxide on the ceria support. Some improvement in the soot oxidation activity is achieved with Zn-modified ceria, attributed to enhanced OSCs of the materials. Within the set of Zn-modified ceria samples, the activities are essentially identical, which is again correlated to their nearly identical OSCs, irrespective of the zinc concentration. Upon impregnation of additional cobalt oxide, the materials' activities for soot oxidation are markedly enhanced, as observed with the RWGS reaction. Pure ceria and Zn-modified ceria samples perform significantly better for soot oxidation after being leached with citric acid. Such enhancement is more pronounced with CZ10 (10 mol% zinc) in comparison to pure ceria. This observation suggests that the removal of non-incorporated ZnO increases the activity for soot oxidation.

The present work has been undertaken with an aim to synthesize valuable bio-additive fuels from glycerol acetalization using SnO2-based solid acids. Various promoters, namely SO42−, MoO3 and WO3 were incorporated to the SnO2 using a wet-impregnation method. An extensive physicochemical characterization has been achieved by means of XRD, BET surface area, BJH analysis, FT-IR, pyridine adsorbed FT-IR, NH3-TPD, ICP-OES and XPS techniques. The BET surface area of SnO2 is significantly improved from 11 to 32, 56 and 41 m2 g−1 after the addition of the WO3, MoO3, and SO42− promoters, respectively. The XPS studies revealed that Sn is present in the +4 oxidation state, whereas Mo, W and S are in the +6 oxidation state in the prepared samples. In addition, the SO42−/SnO2 sample contained super acidic sites, along with strong- and medium-acidic sites. The amount of acidic sites was found to be 46.47, 61.81, 81.45 and 186.98 μmol g−1 for the SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 samples, respectively. The pyridine adsorbed FT-IR studies revealed the existence of a superior quantity of Bronsted acidic sites than Lewis acidic sites in the synthesized catalysts. Promoted SnO2 catalysts exhibited a promising catalytic performance for glycerol acetalization with acetone and furfural, and the activity of the catalysts was found to increase in the following order: SnO2 < WO3/SnO2 < MoO3/SnO2 < SO42−/SnO2. The outstanding performance of the SO42−/SnO2 catalyst is mainly due to the existence of a large amount of acidic sites associated with the super acidic sites. The achieved optimum glycerol conversions with acetone and furfural were ~98 and 99% over the SO42−/SnO2 catalyst, respectively.

The intriguing research toward the exploitation of ceria-based materials for various applications has been growing significantly. In the present investigation, we describe the preparation, characterization and utilization of CeO2–La2O3 (CL) and CeO2–La2O3/SiO2 (CLS) solid solutions for the synthesis of two industrially useful chemicals namely 4-methylpent-1-ene and dehydroacetic acid. Coprecipitation and deposition coprecipitation from ultrahigh dilute solutions were used for the synthesis of CL and CLS catalysts, respectively. The physicochemical characterization has been achieved with the help of various techniques namely X-ray diffraction (XRD), BET surface area, transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Raman spectroscopy (UV-RS and Vis-RS), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) measurements. The structure–activity relationships helped to correlate different parameters that are necessary for obtaining desired products in good yields. The inclusion of silica support has an optimistic influence on the acid–base properties of the ceria–lanthana, in terms of both amount and strength of sites. The presence of silica not only manipulates the acid–base properties but also causes numerous benefits, for instance, it improves the dispersion, stabilizes the active component against sintering and enriches the oxygen vacancy concentration. The meticulous analysis of characterization and activity studies revealed the significant role of acid–base sites in directing the desired products. Interestingly, the CLS catalyst has shown better performance in the production of both 4-methylpent-1-ene and dehydroacetic acid compared to the unsupported CL sample due to well-balanced acid–base sites.

We report large-scale synthesis of high surface area nano-oxides of Ce0.8M0.2O2/Al2O3 (M = Tb and Hf) possessing cuboctahedral shape of 3–5 nm size, which exhibit excellent oxygen storage capacity (OSC) and superior CO oxidation activity in comparison to the most advanced ceria–zirconia/alumina catalyst supports employed in the existing three-way-catalytic converters (TWCs).

Titania supported ceria–lanthana solid solutions (CexLa1−xO2−δ/TiO2; CLT) have been synthesized by a facile and economical route. Existence of synergism between ceria–lanthana (CL) solid solutions and titania-anatase phase, which leads to decrease in the crystallite size, retarded titania phase transformation, and improved redox properties, has been thoroughly investigated by various techniques, namely, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV–vis DRS), Raman spectroscopy (UV–RS and Vis–RS), BET surface area analysis, and temperature programmed reduction (TPR). Two key observations made from the whole exercise were (i) mutual interaction of Ce and Ti ions could impose typical Ce–O–Ti modes at the interfacial region and (ii) the La3+ ion as a dopant provokes a large number of oxygen vacancies via a charge compensation mechanism. The promising role of these factors in the CO oxidation (one of the most formidable challenges) has been comprehensively described. The observed enhanced activity for the CLT sample is primarily attributed to an apparent specific orientation of the active component over the support, which is endorsed by the interfacial interaction. This specific mode could facilitate the CO adsorption with simultaneous bulk oxygen diffusion for more consumption and in turn better activity.

A novel copper (II) hydrotalcite catalyst has been investigated for its use in the cascade synthesis of β-hydroxy triazoles. It is found to be a dynamic catalyst for regioselective organic synthesis in water. The copper (II) hydrotalcite catalyst, in various Cu : Al molar ratios, was prepared by adopting a coprecipitation method and characterized using different techniques. The three component (epoxides, sodium azide and terminal alkynes) reactions under investigation were performed in water at ambient temperature without any additives. The formation of 2-azido alcohol, generated in situ, was observed to be the key step in the [3+2] cycloaddition of the click reaction. The optimized reaction procedures described herein are not only regioenriched and high yielding but also eco-friendly.

Alumina-supported nanosized ceria–lanthana solid solutions (CeO2−La2O3/Al2O3 (CLA) = 80 : 20 : 100 mol% based on oxides) were synthesized by a modified deposition coprecipitation method from ultra-high dilute aqueous solutions. The synthesized materials were subjected to various calcination temperatures from 773 to 1073 K to understand the surface structure and the thermal stability. Structural and redox properties were deeply investigated by different characterization techniques, namely, X-ray diffraction (XRD), Raman spectroscopy (RS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H2-TPR), and Brunauer–Emmett–Teller (BET) surface area. The catalytic efficiency was evaluated for CO oxidation at normal atmospheric pressure. BET surface area measurements revealed that synthesized samples exhibit reasonably high specific surface area. As revealed by XRD measurements, samples maintain structural integrity up to 1073 K without any disproportionation of phases. XPS results suggested that there is no significant change in the Ce3+ amount during thermal treatments due to the absence of undesirable cerium aluminate formation. A significant number of oxygen vacancies were confirmed from Raman and UV-vis DRS measurements. The CLA 773 sample exhibited superior CO oxidation activity. The better activity of the catalyst was proved to be due to a high dispersion in the form of nanosized ceria–lanthana solid solutions over the alumina support, facile reduction, and a high oxygen storage capacity.