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WOx-promoted TiO2–ZrO2 solid acid catalyst was prepared and applied in the catalytic acetalization and ketalization of glycerol with carbonyl compounds to produce biofuel additives. The presence of WOx promoter and TiO2 remarkably improved the catalytic activity of ZrO2. Approximately, 100% glycerol conversion was evidenced with non-bulky aliphatic aldehydes and ketones like, propanol and cyclohexanone. The physical characterization of WOx-promoted TiO2–ZrO2, revealed a higher formation of tetragonal crystalline phase of ZrO2, over monoclinic. The total surface acidity and the ratio of Bronsted to Lewis acidic site concentrations were determined by NH3-TPD and pyridine-chemisorbed FTIR spectroscopy, respectively. A considerably higher concentration of Lewis acidic sites, ~ 213.29 μmol/gm, was evidenced on the WOx-promoted TiO2–ZrO2 catalyst surface. Catalytic activity study revealed a direct correlation between the surface Lewis acidic site concentration and the activity of catalyst. This significant observation indicated the key role of Lewis acidic sites in this catalytic process. The WOx-promoted TiO2–ZrO2 catalyst was also considerably stable and showed good performance in the acetalization/ketalization of glycerol with other substituted carbonyl compounds. The WOx-promoted TiO2–ZrO2 solid acid catalyst exhibits superior catalytic performance for acetalization and ketalization of glycerol with carbonyl compounds to produce biofuel additives.

The fossil fuel run engines emit hazardous pollutants like CO, unburnt hydrocarbons, NOx, etc., due to partial burn of the fuel. These pollutants are converted to nontoxic elements like CO2, H2O and N2 by the catalytic convertor before releasing in the atmosphere. To carry out the redox conversion of the pollutants in the fluctuating O2 atmosphere of the real engine exhaust, CeO2-ZrO2 is used as an oxygen storage/release component of the catalytic convertor. High oxygen storage capacity (OSC) of the catalytic material usually ensures high conversion. The rare earth metal oxide CeO2 exhibits OSC property by virtue of its reversible redox capability. The redox conversion is carried out by the participation of the lattice oxygen and adsorbed active oxygen species present on the CeO2 surface. During the redox process, oxygen vacancies are generated on the catalyst surface, which consequently acts as the active centre for the next cycle of the catalytic reaction. Thus, the ease of oxygen vacancy formation along with other factors like the preparation/post-preparation treatment of the catalysts, presence of dopants, thermal treatment/ageing conditions, etc., strongly influences the OSC of CeO2. In the present article, these factors have been discussed in brief. Hopefully, this compendium will be helpful to gain a preliminary idea on this topic, especially for the student community.

In this study nanostructured MoO3 and WO3 promoted SnO2 solid acid catalysts were explored for the production of glycerol carbonate via carbonylation of bioglycerol with urea. The investigated reference SnO2 and promoted catalysts were synthesized by fusion and wet-impregnation methods, respectively. The physicochemical properties of the prepared catalysts were thoroughly analyzed by XRD, Raman, BET surface area, TEM, FTIR, pyridine adsorbed FTIR, NH3-TPD, and XPS techniques. It was found from the characterization studies that integration of SnO2 with MoO3 and WO3 promoters leads to remarkable structural, textural, and acidic properties. Especially, a high quantity of acidic sites were observed over the MoO3/SnO2 catalyst (~ 81.45 μmol g−1) followed by WO3/SnO2 (61.81 μmol g−1) and pure SnO2 (46.47 μmol g−1), which played a key role in the carbonylation of bioglycerol with urea. The BET specific surface area and oxygen vacancies of SnO2 were significantly enhanced after the addition of MoO3 and WO3 promoters. TEM images revealed the formation of nanosized particles with a diameter of around 5–25 nm for the synthesized catalysts. The MoO3/SnO2 catalyst exhibited a high conversion and selectivity towards glycerol carbonate in comparison to other catalysts. The observed better performance is attributed to unique properties of MoO3/SnO2 catalyst including smaller crystallite size, high specific surface area, abundant oxygen vacancies, and more number of acidic sites. This catalyst also exhibited remarkable stability with no significant loss of activity in the recycling experiments. Nanostructured MoO3/SnO2 solid acid catalyst exhibited an excellent catalytic activity and a high selectivity to glycerol carbonate in the carbonylation of bioglycerol with urea under solvent-free and mild conditions.

Shape-controlled metal nanomaterials are considered as a unique class of catalysts because of their synergistic size- and shape-dependent catalytic properties. This work reports the synthesis of a novel size- and shape-controlled catalyst, consisting of highly dispersed MnOx nanoparticles (average particle size of 4.5 nm) on shape-controlled SiO2 nanospheres (250–300 nm) for selective cyclohexene oxidation using air as the oxidant under solvent- and base-free conditions. The MnOx/SiO2 catalyst exhibited an excellent cyclohexene conversion (~ 92%) with a high selectivity (~ 96%) to the allylic products (2-cyclohexene-ol and 2-cyclohexene-one) under mild conditions, outperforming various SiO2 supported CoOx, FeOx, and CuOx catalysts. The better performance of shape-controlled MnOx/SiO2 nanocatalyst is due to high redox nature of Mn, uniform dispersion of smaller sized MnOx nanoparticles, and synergetic interaction between MnOx and SiO2 spheres, as evidenced by XPS and TEM studies. Further, the MnOx/SiO2 catalyst could be reused at least 5 times for selective cyclohexene oxidation with a negligible loss in its catalytic performance, indicating the excellent stability of shape-controlled metal nanocatalysts in organic synthesis under economically viable and mild conditions. Shape-controlled MnOx/SiO2 nanocatalyst shows an excellent catalytic activity and a high selectivity to allylic products in the oxidation of cyclohexene under mild conditions

Development of non-noble metal catalysts for oxidation of CO is an important subject for reducing the automotive emissions. Recently, shape-controlled synthesis of CeO2 has increasingly attracted the attention of researchers due to its size- and morphology-dependent unique properties. Following this line of thinking, herein, we successfully report the synthesis of Cr-doped CeO2 (Ce1−xCrxO2−δ; X = 0.05, 0.1, and 0.15) nanorods with various Cr contents by a facile hydrothermal method. Structural, surface, optical, and redox properties of the Cr-doped CeO2 nanorods were investigated by various techniques, namely, ICP-OES, TEM-HRTEM, FE-SEM/EDX/EDS, XRD, BET, Raman, UV–vis DRS, PL, XPS, H2-TPR, and O2-TPD. The catalytic performance was evaluated for CO oxidation. For comparison, the efficiency of Cr2O3 was also studied for CO oxidation under identical conditions. As revealed by various characterization results, the chromium ions were doped into the ceria lattice (formation of Ce–O–Cr solid solution), which enhanced the intrinsic properties such as oxygen vacancy concentration and surface area. It was found that the Cr-doped CeO2 nanorods show superior CO oxidation activity than the pristine counterparts (CeO2 nanorods and Cr2O3). The highest CO oxidation efficiency was achieved with the light-off temperature of T50 = 261 °C, when the Cr doping amount was 10% (Ce0.9Cr0.1O2−δ). A high specific surface area, more number of surface oxygen vacancies, a high concentration of Ce3+, and enhanced oxygen reducibility of Ce0.9Cr0.1O2−δ nanorods were found to be responsible for its superior catalytic performance. Further, the Ce0.9Cr0.1O2−δ nanorods exhibited a steady CO conversion over a period of 55 h investigated. The obtained results are expected to have a significant impact on the use of non-noble metal based Cr-doped CeO2 nanorods in environmental applications. The Cr-doped CeO2 nanorods with Ce0.9Cr0.1O2−δ composition showed enhanced CO oxidation performance at a lower temperature (~ 261 °C) than that of pristine CeO2 nanorods (338 °C) and Cr2O3 (361 °C) catalyst. This behaviour is a result of enhancement of oxygen vacancies, surface Ce3+ species, low-temperature reducibility, and high surface area.

Development of non-noble metal catalysts with improved structural, surface, and redox properties for catalytic soot oxidation has gained an enormous interest. Among practical alternatives, ceria-supported transition metal oxides have been proved to be the satisfactory catalysts for soot oxidation due to their outstanding redox properties and oxygen transfer capability promoted by the strong interaction between metal oxide and ceria interfaces. Following the above considerations, in the present work, CuO/Zn-CeO2 nanocomposite was prepared by a wet impregnation method and investigated for catalytic soot oxidation. To probe the significance of nanocomposites, pure counterparts, namely, CuO/CeO2, CuO/ZnO, and Zn-CeO2, were also synthesized. Various characterization techniques, namely, TEM-HRTEM, N2O chemisorption, XRD, ICP-OES, BET, Raman, XPS, and H2-TPR, were employed to investigate the structural, surface, and redox properties. Pure CuO/ZnO and CuO/CeO2 catalysts showed a soot oxidation activity with a T50 of 613 and 526 °C, respectively, under the practical conditions of NO concentration of 500 ppm and 20% O2. Interestingly, the CuO/Zn-CeO2 nanocomposite exhibited a remarkable higher soot oxidation activity with a T50 of 460 °C. The enhancement in the soot oxidation activity has been attributed to a strong interaction between the highly dispersed CuO and Zn-CeO2 support, which resulted in the desired textural properties and abundant surface defects (Ce3+ species as well as oxygen vacancies). In addition, the long-term stability test verifies an excellent reusability of the CuO/Zn-CeO2 nanocomposite towards soot oxidation without appreciable loss in the activity. The present study demonstrates the significance of ceria-based nanocomposite catalysts for environmental pollutant abatement.

The present work describes the role of oxide supports such as alumina (Al2O3), silica (SiO2), and titania (TiO2) on the structural and catalytic properties of europium ion-doped cerium oxide. In particular, the main motive of this study was to further improve the catalytic efficiency of Ce0.8Eu0.2O2-δ solid solutions for oxidation reactions. Diesel soot oxidation and oxidative coupling of benzylamine were examined as model reactions to assess the catalytic properties of the prepared materials. These were prepared by deposition coprecipitation method and characterized by various techniques namely, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and BET/BJH surface area and pore-size distribution methods. The characterization studies reveal that Ce0.8Eu0.2O2-δ is well-dispersed without the formation of CeO2 or Eu2O3 crystalline compounds over the surface of various supports. The TEM studies confirm the nano-crystalline nature of the Ce0.8Eu0.2O2-δ and Raman studies indicate the formation of oxygen vacancies which are significant for oxidation reactions. As expected, the supported Ce0.8Eu0.2O2-δ mixed oxides exhibited better catalytic activity than pure ceria and unsupported oxides towards the soot and benzylamine oxidation reactions. Supported Ce0.8Eu0.2O2-δ(CE) catalysts are exhibiting better performance than unsupported CE and pure ceria for soot and benzylamine oxidation owing to metal oxide-support and metal oxide-metal oxide interactions.

A series of Ce1 − x–ZrxO2 − δ (x = 0.2, 0.5, and 0.8) mixed oxides were prepared by coprecipitation method and explored for selective oxidation of vanillyl alcohol employing O2 and acetonitrile as the oxidant and solvent, respectively under base-free conditions. To ascertain the key factors responsible for vanillyl alcohol oxidation, the physicochemical properties of the synthesized catalysts were investigated by various characterization techniques namely, XRD, BET surface area, Raman, XPS, and H2-TPR. It was observed from this exercise that the catalytic activity dependents on the Ce:Zr mole ratio, which is related to the degree of reducibility of the catalyst. Interestingly, the catalytic activity is enhanced with the increase of Ce content in the Ce–Zr mixed oxide. Among the investigated catalysts, the Ce0.8Zr0.2O2 combination exhibited a high catalytic activity with ~ 98% conversion and ~ 99% selectivity to vanillin. Smaller crystallite size, large BET surface area, more number of oxygen vacancies, improved redox properties, and strong synergetic interaction are found to be the key factors to promote the oxidation ability of Ce0.8Zr0.2O2 catalysts towards vanillyl alcohol oxidation. Further, the influence of reaction parameters such as time, solvent, temperature, and oxygen pressure were also studied to optimize the catalytic process for vanillyl alcohol oxidation. As revealed by these studies, the high activity of Ce0.8Zr0.2O2 catalyst could be retained up to five cycles without appreciable loss in the activity and selectivity. Nanosized Ce0.8Zr0.2O2 catalyst exhibited an excellent catalytic activity and superior selectivity to vanillin in the liquid phase oxidation of vanillyl alcohol under ecofriendly conditions

Selective allylic oxidation of cyclohexene was investigated over nanostructured CeO2/SiO2 and CeO2–Sm2O3/SiO2 catalysts synthesized by a feasible deposition precipitation method. The CeO2–Sm2O3/SiO2 catalyst showed excellent catalytic efficiency with ~89 % cyclohexene conversion and ~90 % selectivity for allylic products (i.e., 2-cyclohexen-1-ol and 2-cyclohexene-1-one), while only ~50 and ~35 % cyclohexene conversion was observed, respectively, over CeO2/SiO2 and CeO2 catalysts. Systematic characterization of the designed catalysts was undertaken to correlate their catalytic activity with the physicochemical properties using X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) surface area measurements, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and NH3-temperature programmed desorption (TPD) techniques. The results revealed that doping of Sm3+ into the ceria lattice and simultaneous dispersion of resultant Ce–Sm mixed oxides on the silica surface led to improved structural, acidic, and catalytic properties. The better catalytic efficiency of CeO2–Sm2O3/SiO2 was due to high specific surface area, more structural defects, and high concentration of strong acid sites, stimulated by synergistic interaction between various oxides in the catalyst. The cyclohexene conversion and selectivity for allylic products depended on the reaction temperature, nature of solvent, molar ratio of cyclohexene to oxidant, and reaction time. Possible reaction pathways are proposed for selective allylic oxidation of cyclohexene towards 2-cyclohexen-1-ol and 2-cyclohexene-1-one products. SiO2-supported CeO2–Sm2O3 nanocatalyst exhibited outstanding catalytic performance with superior selectivity for allylic products in liquid-phase selective oxidation of cyclohexene under mild reaction conditions.

A series of Cr-TUD-1 catalysts with various loadings of chromium (3, 5, 7, and 9 wt%) were prepared by microwave irradiation and explored for oxidative dehydrogenation of propane to propylene utilizing CO2 as soft oxidant. The microwave irradiation reduced the synthesis time and the resulting Cr-TUD-1 catalysts exhibited a high specific surface area of more than 600 m2 g−1. The synthesized catalysts were characterized by various techniques including XRD, XPS, TEM, UV–vis DRS, BET surface area, and pore size distribution to understand the physicochemical properties, and to correlate with the catalytic activity. Among various compositions, the 7% Cr-TUD-1 catalyst exhibited a high propane conversion (~ 45%) with better propylene product selectivity (~ 75%). The Cr-TUD-1 catalyst was also found to be quite stable up to 8 h of time-on-stream investigated. As revealed by the characterization techniques, the inter-convertible Cr6+ to Cr3+/2+ species are very crucial for the observed better catalytic activity of these materials. The TUD-1 enables to encapsulate the chromium nanoparticles in the porous silica structure with high dispersion which help in maintaining the better catalytic performance.

In this work, various transition and rare earth metal ions (M3+; M = Mn3+, Fe3+, La3+, and Pr3+) doped CeO2 solid solutions were prepared by a simple sol–gel method for the degradation of an organic dye, Rhodamine B (RhB) as a model pollutant. The as-prepared samples were thoroughly studied by various characterization techniques to understand the surface and optical properties. The XRD results suggested formation of solid solutions, and TEM studies confirmed the nanosized nature of the ceria particles. The Raman and XPS results revealed that the doping of metal ions enhanced the formation of Ce3+ ions associated with the oxygen vacancies. The doping of M3+ ions in the CeO2 lattice strongly influenced the band gap tuning of undoped CeO2 (3.01 eV) from the UV to Visible region (i.e., 2.45–2.90 eV). Photoluminescence studies suggested that doping of M3+ cations suppresses the recombination rate of photogenerated electron–hole pairs. The photocatalytic activity results indicated that the doped CeO2 samples exhibit substantially enhanced photocatalytic performance for the degradation of RhB compared to undoped CeO2. The better catalytic activity of doped CeO2 samples could be attributed to the presence of defects (Ce3+ ions and oxygen vacancies), which play a prominent role as trapping centres for excited electrons and inhibit the recombination process. The scavengers tests confirmed that the generation of highly reactive hydroxyl (·OH) and super oxide (·O2−) radicals are actively involved in the photodegradation process. This work rendered a new concept for rational design and development of doped CeO2-based materials as better photocatalysts.

In this work, a novel nanostructured TiO2 supported CeO2–Sm2O3 (CS/T) catalyst was prepared by deposition coprecipitation method and then evaluated for CO oxidation. The role of TiO2 support in the CS/T catalyst was studied by comparing its physicochemical and catalytic properties with the unsupported CeO2–Sm2O3 (CS) and CeO2. Under identical conditions, the CS/T catalyst showed better activity with lower light-off temperature (T50 = ~547 K) compared to the CS and CeO2. Characterization results revealed the constructive structural modifications in the CS/T catalyst owing to the strong synergistic effect between the CeO2–Sm2O3 and TiO2 support, which significantly influenced the catalytic performance for CO oxidation. The highly dispersed Ce–O–Sm solid solution over the TiO2 support leads to large specific surface area, more oxygen vacancies and active oxygen species, which are responsible for better activity of CS/T catalyst. Further, the CS/T catalyst exhibits high reusability (up to three runs) and good long-term stability (48 h) without appreciable loss of catalytic activity.

In this investigation, Mn-doped CeO2(CM) catalysts were prepared by co-precipitation (CP), sol-gel (SG), and hydrothermal (HT) methods and characterized by XRD, ICP-OES, N2 adsorption–desorption, SEM, TEM-HREM, Raman, XPS, FT-IR, XPS and H2-TPR techniques, and evaluated for CO oxidation activity. The CM catalyst obtained by HT method showed superior CO oxidation performance at much lower temperature (T50 = ~355 K). The better performance of CM-HT catalyst is ascribed to its larger surface area, higher concentration of surface adsorbed oxygen, enhanced reducibility, and more oxygen vacancy sites. Further, it was found that the CM-HT catalyst is fully recyclable for over four runs while maintaining its high activity, which suggests that it is a better catalyst for CO oxidation at lower temperatures.

In the present study nanosized Sm3+, Eu3+, and Gd3+ doped ceria (Ce0.8M0.2O2−δ, M = Sm, Eu, and Gd) catalytic systems were synthesized by coprecipitation method. The prepared samples were characterized by X-ray powder diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, transmission electron microscopy (TEM), Raman spectroscopy, UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS), and temperature programmed reduction (TPR) and finally evaluated for CO oxidation studies. The XRD results suggested that lattice expansion occurred with the introduction of trivalent dopants into the ceria lattice and confirms the formation of solid solutions. Raman spectra corroborated the existence of oxygen defects. TEM results indicate the presence of nanocrystalline nature of the prepared catalysts. TPR analysis revealed that trivalent dopants decreased the surface reduction temperature of the pure ceria by minimum 141 and maximum 249 K. From the activity studies, it was noticed that for pure ceria the observed T50 is 613 K, at this temperature doped ceria exhibited minimum 75 % CO conversion. The above characterization studies demonstrate that trivalent dopants successfully tuned the structural and catalytic properties of pure ceria. Among the examined samples, Ce0.8Sm0.2O2−δ material improved the CO oxidation activity much better than other samples.

Herein we report the synthesis and photocatalytic evaluation of heterostructure $$\hbox {WO}_{{3}}/\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ (WMCN) and $$\hbox {CeO}_{{2}}/\hbox {g-C}_{{3}}\hbox {N}_{4}$$ (CMCN) materials for RhB degradation and photoelectrochemical studies. These materials were synthesized by varying the dosages of $$\hbox {WO}_{{3}}$$ and $$\hbox {CeO}_{2 }$$ on $$\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ individually and were characterized with state-of-the-art techniques like XRD, BET surface area, FT-IR, UV–Vis DRS, TGA, SEM, TEM and XPS. A collection of combined structural and morphological studies manifested the formation of bare $$\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ , $$\hbox {WO}_{{3}}$$ , $$\hbox {CeO}_{{2}}$$ , $$\hbox {WO}_{{3}}/\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ and $$\hbox {CeO}_{{2}}/\hbox {g-C}_{{3}}\hbox {N}_{4 }$$ materials. From the degradation results, we found that the material with 10 wt% $$\hbox {WO}_{{3}}$$ and 15 wt% $$\hbox {CeO}_{{2}}$$ content on $$\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ showed the highest visible light activity. The first order rate constant for the photodegradation performance of WMCN10 and CMCN15 is found to be 5.5 and 2.5 times, respectively, greater than that of $$\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ . Photoelectrochemical studies were also carried out on the above materials. Interestingly, the photocurrent density of WMCN10 photoanode achieved $$1.45\,\hbox {mA cm}^{-2}$$ at 1.23 V (vs.) RHE and this is much larger than all the prepared materials. This enhanced photoactivity of WMCN10 is mainly due to the cooperative synergy of $$\hbox {WO}_{{3}}$$ with $$\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ , which enhanced the visible light absorption and suppresses the electron–hole recombination. Synopsis The WMCN materials exhibited superior photocatalytic performance than CMCN and bare $$\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ materials due to the existence of synergism between $$\hbox {WO}_{{3}}$$ and $$\hbox {g-C}_{{3}}\hbox {N}_{{4}}$$ .

This work demonstrates that MoOx/CeO2–ZrO2 solid acid acts as a promising catalyst in the solvent-free oxidative-coupling of benzylamine into dibenzylimine with O2 as the oxidant. The benzylamine conversions were found to be ~27, 51, 74, and 99 % for CeO2, CeO2–ZrO2, MoOx/CeO2, and MoOx/CeO2–ZrO2 catalysts, respectively. The MoOx/CeO2–ZrO2 catalyst also efficiently converts various substituted- and secondary-benzylamines under solvent-free conditions with O2 resulting in moderate to excellent yields. Remarkably, a high selectivity of ~99.6–99.9 % towards the dibenzylimine product was found for all reaction conditions. The outstanding performance of MoOx/CeO2–ZrO2 catalyst was attributed to high specific surface area, improved structural defects, and superior acid properties. The MoOx/CeO2–ZrO2 catalyst show an outstanding performance in the oxidation of benzylamine and its derivatives, which is attributed to abundant acidic sites, large number of oxygen vacancies, and superior BET surface area.

In this work, nanosized Ce0.7M0.3O2−δ (M = Mn, Fe, Co) solid solutions were prepared by a facile coprecipitation method and evaluated for CO oxidation. The physicochemical properties of the synthesized samples were investigated by various characterization techniques, namely, XRD, ICP-OES, BET surface area, SEM-EDX, TEM and HRTEM, Raman, XPS, and H2-TPR. XRD studies confirmed the formation of nanocrystalline single phase Ce0.7M0.3O2−δ solid solutions. ICP-OES analysis confirmed actual amount of metal loadings in the respective catalysts. The BET surface area of Ce0.7M0.3O2−δ samples significantly enhanced after the incorporation of dopants. TEM studies confirmed nanosized nature of the samples and the average particle sizes of Ce0.7M0.3O2−δ were found to be in the range of ∼8–16 nm. Raman studies indicated that the incorporation of dopant ions into the CeO2 lattice promote the formation of more oxygen vacancies. The existence of oxygen vacancies and different oxidation states (Ce3+/Ce4+ and Mn2+/Mn3+, Fe2+/ Fe3+, and Co2+/Co3+) in the doped CeO2 samples were further confirmed from XPS investigation. TPR measurements revealed an enhanced reducibility of ceria after the incorporation of dopants. The catalytic activity results indicated that the doped CeO2 samples show excellent CO oxidation activity and the order of activity was found to be Ce0.7Mn0.3O2−δ > Ce0.7Fe0.3O2−δ > Ce0.7Co0.3O2−δ > CeO2. The superior CO oxidation performance of CeO2-MnOx has been attributed to a unique Ce-Mn synergistic interaction, which facilitates materials with promoted redox properties and improved oxidation activity.

This work attempts to gain information about the role of trivalent and tetravalent dopants on the structural and catalytic properties of ceria (CeO2). In this study, we have prepared Zr4+, La3+, and Eu3+ doped ceria (CZ, CL, and CE) by coprecipitation method and calcined at 773 K. The physicochemical characterization was achieved by using various techniques, namely, X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR) spectroscopy. The catalytic efficiency for soot oxidation was evaluated by thermogravimetric (TG) method and compared with undoped CeO2. Doped CeO2 catalysts decreased the soot oxidation temperature by more than 158 K compared to pure ceria. This is ascribed to mutual interaction and synergistic effect between the dopant species and the ceria. Among the synthesized nanocatalysts, the CE sample exhibited better performance. The observed better activity of CE was attributed to the presence of more number of oxygen vacancies, a high specific surface area, and easy reducibility as confirmed from Raman, BET surface area, and TPR measurements, respectively.

The development of noble-metal-free catalysts is currently a topic of paramount research investigation in the chemical industry. In this work, a promising CeO2–MoO3/SiO2 catalyst was synthesized for the oxidation of a wide variety of benzylamines with O2 as the oxidant under solvent-free conditions. For comparison, the efficiency of CeO2 and CeO2–MoO3 catalysts were also studied for the oxidation of benzylamine under identical conditions. The physicochemical properties of the catalysts were thoroughly analysed using X-ray diffraction, Raman spectroscopy, Barrett–Joyner–Halenda pore size distribution, Brunauer–Emmet–Teller (BET) surface area, Fourier-transform infrared spectroscopy, ammonia-temperature programmed desorption (NH3-TPD), and X-ray photoelectron spectroscopy techniques. Characterization studies revealed that the CeO2–MoO3/SiO2 catalyst shows favourable structural, textural and acidic properties, which are due to strong interaction between the Ce–Mo mixed oxide and the SiO2. In particular, the CeO2–MoO3/SiO2 sample exhibits smaller crystallite size and highest BET surface area compared with the CeO2–MoO3 and bare CeO2. Raman analysis revealed that the CeO2–MoO3/SiO2 sample exhibits greater number of oxygen vacancy defects. The NH3-TPD studies indicated that the CeO2–MoO3/SiO2 sample has large amount of acidic sites. Owing to the beneficial properties, the CeO2–MoO3/SiO2 sample exhibited an outstanding catalytic performance for the aerobic oxidation of various benzylamines. The achieved benzylamine conversions are ~16, 65 and 98 %, respectively, for CeO2, CeO2–MoO3 and CeO2–MoO3/SiO2 samples. Despite the nature of the catalyst used and the reaction conditions employed, almost ~100 % selectivity for the dibenzylimine product was found in the present study. Remarkably, the CeO2–MoO3/SiO2 catalyst can be repeatedly used up to 5 cycles without any considerable variation in the benzylamine conversion and imine product selectivity. The CeO2–MoO3/SiO2 catalyst showed an outstanding efficiency in the oxidation of benzylamine, which is attributed to abundant acidic sites, large number of oxygen vacancies, and superior BET surface area.

Y-doped ceria–zirconia (Ce0.8Zr0.12Y0.08O2−δ, CZY) and ceria–lanthana (Ce0.8La0.12Y0.08O2−δ, CLY) ternary oxide solid solutions were synthesized by a facile coprecipitation method. Structural, textural, redox, and morphological properties of the synthesized samples were investigated by means of X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP–OES), Raman spectroscopy (RS), UV–visible diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by hydrogen (H2-TPR), high resolution transmission electron microscopy (HRTEM), and Brunauer–Emmett–Teller surface area (BET SA) techniques. The formation of ternary oxide solid solutions was confirmed from XRD, RS, and UV–vis DRS results. ICP–OES analysis confirmed the elemental composition in the ternary oxide solid solutions. HRTEM images revealed irregular morphology of the samples. RS, UV–vis DRS, and XPS results indicated enhanced oxygen vacancies in the Y doped samples. H2-TPR profiles confirmed a facile reduction of CZY and CLY samples at lower temperatures. BET analysis revealed an enhanced surface area for CZY and CLY samples than the respective CZ and CL undoped mixed oxides. All these factors contributed to a better CO and soot oxidation performance of CZY and CLY samples. Particularly, the CLY sample exhibited highest catalytic activity among the various samples investigated.

In this study, nanosized Ce0.9Eu0.1O2−δ (CE 91) and Ce0.8Eu0.2O2−δ (CE 82) solid solutions were prepared by a modified co-precipitation method, followed by calcination at 773 and 1,073 K. Structural properties of the prepared materials were thoroughly investigated by different characterization techniques, namely, X-ray powder diffraction (XRD), Brunauer–Emmett–Teller surface area, high resolution transmission electron microscopy, Raman spectroscopy, UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). The catalytic activity for CO oxidation was assessed and compared with the well-established CeO2–ZrO2 (CZ) catalyst. As confirmed by powder XRD, the synthesized materials maintain the F-type crystalline structure characteristic of ceria (CeO2). The interaction of CeO2 with Eu3+ dopant promoted the formation of oxygen vacancies as revealed by Raman results. UV–Vis DRS and XPS data suggest that Ce exists in both 3+ and 4+ oxidation states and Eu in the 3+ state. Results of catalytic CO oxidation indicate that Eu doped ceria exhibits a better performance than Zr doped ceria irrespective of the calcination temperature. This may be due to the increased number of oxygen defects and the easily reducible nature of the material, which results from the substitution of Ce4+ species with Eu3+ ions.

Hydrogenolysis of glycerol to 1,2-propanediol is an alternative route for efficient utilization of biomass-derived glycerol to value-added chemicals. In this study, catalytic hydrogenolysis of glycerol was systematically investigated over various Ru/CeO2 catalysts. CeO2-support was prepared by different methods and Ru deposition by wet impregnation method. Synthesized catalysts were characterized by various techniques, namely, XRD, XPS, TPR, TPD, and other. Hydrothermally synthesized CeO2-supported Ru catalyst showed relatively more number of acid sites with mild acid strength and exhibited highest conversion and product selectivity. Effect of various parameters including Ru loading, reaction temperature, and hydrogen pressure was evaluated and addressed.

Ceria is an important rare earth metal oxide for numerous catalytic applications. In the present work, a combined study of structural characterization and catalytic activity of nanocrystalline Ce–Gd–O mixed oxides was thoroughly investigated towards CO oxidation. The Ce–Gd mixed oxides with different Gd-doping amounts (10 and 20 mol% Gd with respect to Ce) were prepared by means of an economical and simple coprecipitation method with aqueous NH3 solution as the precipitant. The resulting samples were treated at different calcination temperatures to evaluate their structural homogeneity and thermal stability. An extensive physicochemical characterization was done by means of XRD, Raman, TEM, BET analysis, H2-TPR, XPS, and UV–Vis DRS techniques. XRD studies revealed the formation of nanocrystalline single phase Ce–Gd solid solutions. Raman studies further disclosed the formation of Ce–Gd solid solutions associated with deformed F2g band and additional bands pertaining to oxygen vacancy defects. The Gd-doping remarkably reduced the CeO2 crystallite size, which is in nanoscale range as evidenced by TEM images. The BET surface area and oxygen vacancy defects of CeO2 were significantly enhanced after Gd3+ incorporation. The ability of CeO2 to store and release of oxygen (oxygen storage capacity, OSC) is markedly improved 3 and 4 times for Ce0.9Gd0.1O2−δ and Ce0.8Gd0.2O2−δ samples, respectively. Among the investigated catalysts, the Ce0.8Gd0.2O2−δ sample calcined at 773 K showed better catalytic activity due to smaller crystallite size, higher BET surface area, enhanced reducible nature, and superior OSC. It is found that the catalytic performance of the Ce–Gd sample strongly depends on the Gd-loading and calcination temperature.

CeO2–Gd2O3 oxides were synthesized by a modified coprecipitation method and subjected to thermal treatments at different temperatures to understand their thermal behaviour. The obtained samples were characterized by XRD, BET, TEM, Raman and TPR techniques. Catalytic efficiencies for oxygen storage/release capacity (OSC) and soot oxidation were evaluated by a thermogravimetric (TG) method. XRD and Raman results indicated the formation of Ce0.8Gd0.2O2−δ (CG) solid solutions at lower calcination temperatures, and TEM studies confirmed nanosized nature of the particles. Raman studies further confirmed the presence of oxygen vacancies and lattice defects in the CG sample. TPR measurements indicated a facile reduction of ceria after Gd3+ addition. Activity studies revealed that incorporation of Gd3+ into the ceria matrix favoured the creation of more structural defects, which accelerates the oxidation rate of soot compared to pure ceria.

The relative effects of Zr4+ and Pr3+/4+ dopants on the structure, redox properties, and catalytic performance of nanosized ceria was studied. The investigated ceria–zirconia and ceria–praseodymia (CP) solid solutions were prepared by a modified coprecipitation method, characterized by a variety of techniques, and evaluated for soot oxidation. The characterization results indicate that CP has more surface and bulk oxygen vacancies, redox sites, and lattice oxygen mobility, and better thermal stability. Besides having low specific surface area, CP is more active in soot oxidation. This better activity has been attributed to the presence of more surface and bulk oxygen vacancies, which promote the adsorption of gas-phase oxygen and the formation and mobility of large numbers of active oxygen species.

A series of TiO2–ZrO2 supported V2O5 catalysts with vanadia loadings ranging from 4 to 12 wt% were synthesized by a wet impregnation technique and subjected to various thermal treatments at temperatures ranging from 773 to 1,073 K to understand the dispersion and thermal stability of the catalysts. The prepared catalysts were characterized by X-ray powder diffraction (XRD), BET surface area, oxygen uptake, and X-ray photoelectron spectroscopy (XPS) techniques. XRD results of 773 K calcined samples conferred an amorphous nature of the mixed oxide support and a highly dispersed form of vanadium oxide. Oxygen uptake measurements supported the formation of a monolayer of vanadium oxide over the thermally stable TiO2–ZrO2 support. The O 1s, Ti 2p, Zr 3d, and V 2p core level photoelectron peaks of TiO2–ZrO2 and V2O5/TiO2–ZrO2 catalysts are sensitive to the calcination temperature. No significant changes in the oxidation states of Ti4+ and Zr4+ were noted with increasing thermal treatments. Vanadium oxide stabilized as V4+ at lower temperatures, and the presence of V5+ is observed at 1,073 K. The synthesized catalysts were evaluated for selective oxidation of o-xylene under normal atmospheric pressure in the temperature range of 600–708 K. The TiO2–ZrO2 support exhibits very less conversion of o-xylene, while 12 wt% V2O5 loaded sample exhibited a good conversion and a high product selectivity towards the desired product, phthalic anhydride.

Nanosized CexM1−xO2−δ (M = Zr, Hf, Tb and Pr) solid solutions were prepared by a modified coprecipitation method and thermally treated at different temperatures from 773 to 1073 K in order to ascertain the thermal behavior. The structural and textural properties of the synthesized samples were investigated by means of X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), BET surface area, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (RS) techniques. The catalytic efficiency has been performed towards oxygen storage/release capacity (OSC) and CO oxidation activity. The characterization results indicated that the obtained solid solutions exhibit defective cubic fluorite structure. The solid solutions of ceria–hafnia, ceria–terbia and ceria–praseodymium exhibited good thermal stability up to 1073 K. A new Ce0.6Zr0.4O2 phase along with Ce0.75Zr0.25O2 was observed in the case of ceria–zirconia solid solution due to more Zr4+ incorporation in the ceria lattice at higher calcination temperatures. The reducibility of ceria has been increased upon doping with Zr4+, Hf4+, Tb3+/4+ and Pr3+/4+ cations. This enhancement is more in case of Hf4+ doped ceria. Among various solid solutions investigated, the ceria–hafnia combination exhibited better OSC and CO oxidation activity. The high efficiency of Ce–Hf solid solution was correlated with its superior bulk oxygen mobility and other physicochemical characteristics.

We have been exploring the utilization of a simple and fast microwave-induced solution combustion synthesis technique for the preparation of various ceria-based mixed oxides for different catalytic applications. In our comprehensive investigation, CeO2–SiO2 (MWCS), CeO2–TiO2 (MWCT), CeO2–ZrO2 (MWCZ) and CeO2–Al2O3 (MWCA) mixed oxides were synthesized by solution combustion synthesis method using microwave dielectric heating and employed for CO and soot oxidation applications. The intricate relationship between ceria and other supporting oxides has been explored with the help of various analytical techniques namely, X-ray diffraction (XRD), temperature programmed reduction/oxidation (TPR/TPO), temperature programmed desorption (TPD) of ammonia and CO2, Raman spectroscopy (RS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), BET surface area and thermogravimetry analysis (TGA) methods. XRD results revealed amorphous nature of the material in case of ceria-silica mixed oxide and formation of a specific cubic fluorite type Ce0.5Zr0.5O2 solid solution in the case of ceria-zirconia mixed oxide. Ceria-titania and ceria-alumina mixed oxides exhibited diffraction lines only due to crystalline ceria. Zirconia-based mixed oxide exhibited a lower reduction temperature and better redox properties compared to other samples. TPD of ammonia and CO2 results revealed superior acid–base properties for MWCS mixed oxide. TGA measurements indicated a complete combustion in all preparations. RS results suggested defective structure of mixed oxides resulting in the formation of oxygen vacancies. XPS results revealed that ceria-zirconia mixed oxide contained more Ce3+ compared to other oxides. Among all the mixed oxides, the MWCZ sample exhibited a higher oxygen storage capacity, and better CO and soot oxidation activities. All these interesting findings have been elaborated in this publication.

Titania–silica, titania–alumina, and titania–zirconia mixed oxides (1:1 molar ratio) were prepared by a microwave-induced solution combustion synthesis technique. The prepared materials were characterized by thermogravimetry/differential thermal analysis, X-ray diffraction (XRD), Raman spectroscopy, BET surface area, X-ray photoelectron spectroscopy (XPS), ultraviolet–visible diffuse reflectance spectroscopic (UV–Vis DRS), and Fourier transform infrared (FTIR) techniques to assess their physicochemical properties. Their photocatalytic activity for the degradation of phenol in aqueous solution under sunlight was studied. XRD and Raman studies revealed the presence of titania in the form of anatase phase in all the mixed oxides synthesized. The XRD studies further suggested that titania–zirconia contains an additional (Ti,Zr)O2 phase. UV–Vis DRS results reveal that all samples exhibit absorption maxima near visible region. FTIR results revealed the presence of Ti–O–Si linkages in the titania–silica sample, which are responsible for its higher activity in the photocatalytic degradation of phenol under sunlight.

Various CeO2–MxOy (MxOy = SiO2, TiO2, ZrO2, and Al2O3) mixed oxides were prepared by microwave induced solution combustion method and analyzed by different complimentary techniques, namely, X-ray diffraction (XRD), Raman spectroscopic (RS), UV–Vis diffuse reflectance spectroscopy (UV-DRS), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG-DTA), and BET surface area. XRD analyses revealed that CeO2–SiO2 and CeO2–TiO2 mixed oxides are in slightly amorphous form and exhibit only broad diffraction lines due to cubic fluorite structure of ceria. XRD lines due to the formation of cubic Ce0.5Zr0.5O2 were observed in the case of CeO2–ZrO2 sample. RS results suggested defective structure of the mixed oxides resulting in the formation of oxygen vacancies. The UV-DRS measurements provided valid information about Ce4+ ← O2− and Ce3+ ← O2− charge transfer transitions. XPS studies revealed the presence of cerium in both Ce3+ and Ce4+ oxidation states. The ceria–zirconia combination exhibited better oxygen storage capacity (OSC) and CO oxidation activity when compared to other samples. The significance of present synthesis method lays mostly on its simplicity, flexibility, and the easy control of different experimental factors.

Vapour phase selective hydrogenation of acetophenone has been performed over a series of Pt/CeO2–MOx (MOx = SiO2, Al2O3, TiO2, and ZrO2) catalysts. The controlled hydrogenation was carried out in the 453–533 K temperature range at normal atmospheric pressure. The ceria-based mixed oxides were prepared through a co-precipitation or deposition-precipitation route. Platinum was deposited by a wet impregnation method. The obtained catalysts were calcined at 773 K and characterized by means of X-ray diffraction, Raman spectroscopy, BET surface area, temperature programmed reduction, temperature programmed desorption, thermogravimetry, and scanning electron microscopy. XRD analyses suggest that CeO2–SiO2 and CeO2–Al2O3 primarily consist of CeO2 nanoparticles dispersed over the amorphous silica or alumina surface. In the case of CeO2–TiO2, presence of segregated nanocrystalline CeO2 and TiO2-anatase phase were noted. Formation of cubic Ce0.75Zr0.25O2 solid solution was observed in the case of CeO2–ZrO2. No peaks pertaining to platinum could be detected from XRD profiles. Formation of zirconia rich tetragonal phase (Ce0.4Zr0.6O2) was observed in the case of Pt/CeO2–ZrO2 sample. Raman measurements revealed the fluorite structure of ceria and presence of oxygen vacancies in all samples. TPR results suggest that the presence of Pt facilitates the reduction of ceria. The catalytic performance of Pt-based catalysts was found to depend strongly on the nature of the support oxide employed. Among various catalysts investigated, the Pt/CeO2–SiO2 catalyst exhibited better product yields.

Nanosized Ce0.5Zr0.5O2 solid solution has been synthesized by microwave-induced solution combustion method (MWCZ) and compared with that of a Ce x Zr1−x O2 solid solution with the same composition but prepared by the conventional coprecipitation method (CPCZ) and calcined at 773 K. X-ray diffraction and cell parameter studies revealed the incorporation of more zirconia and formation of more defect sites in the ceria lattice of the material prepared by microwave method. Raman spectroscopic measurements suggested the presence of oxygen vacancies, lattice defects and displacement of oxygen ions from their ideal lattice positions. X-ray photo electron spectroscopic studies indicated a high reducibility and surface enrichment of Ce3+ ions in the MWCZ sample. Better oxygen storage capacity and CO oxidation activity was observed for MWCZ in comparison to that of CPCZ sample. The significance of the microwave method lies mainly in its simplicity, flexibility, and easy control of different factors that determine the activity of the mixed oxide.

The aza-Michael addition reactions of amines with α,β-unsaturated carbonyl compounds were efficiently carried out at room temperature under solvent-free condition employing sulfated zirconia as a reusable heterogeneous catalyst. The desired products were formed in short reaction times and in high yields. The bulk and surface properties of the synthesized catalyst was examined by X-ray powder diffraction, BET surface area, temperature programmed desorption of ammonia, scanning electron microscopy and thermogravimetric techniques. Characterization results reveal the super acidic nature of the catalyst.

We have been exploring the utilization of supported ceria and ceria–zirconia nano-oxides for different catalytic applications. In this comprehensive investigation, a series of Ce x Zr1−x O2/Al2O3, Ce x Zr1−x O2/SiO2 and Ce x Zr1−x O2/TiO2 composite oxide catalysts were synthesized and subjected to thermal treatments from 773 to 1073 K to examine the influence of support on thermal stability, textural properties and catalytic activity of the ceria–zirconia solid solutions. The physicochemical characterization studies were performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HREM), thermogravimetry and BET surface area methods. To evaluate the catalytic properties, oxygen storage/release capacity (OSC) and CO oxidation activity measurements were carried out. The XRD analyses revealed the formation of Ce0.75Zr0.25O2, Ce0.6Zr0.4O2, Ce0.16Zr0.84O2 and Ce0.5Zr0.5O2 phases depending on the nature of support and calcination temperature employed. Raman spectroscopy measurements in corroboration with XRD results suggested enrichment of zirconium in the Ce x Zr1−x O2 solid solutions with increasing calcination temperature thereby resulting in the formation of oxygen vacancies, lattice defects and oxygen ion displacement from the ideal cubic lattice positions. The HREM results indicated a well-dispersed cubic Ce x Zr1−x O2 phase of the size around 5 nm over all supports at 773 K and there was no appreciable increase in the size after treatment at 1073 K. The XPS studies revealed the presence of cerium in both Ce4+ and Ce3+ oxidation states in different proportions depending on the nature of support and the treatment temperature applied. All characterization techniques indicated absence of pure ZrO2 and crystalline inactive phases between Ce–Al, Ce–Si and Ce–Ti oxides. Among the three supports employed, silica was found to stabilize more effectively the nanosized Ce x Zr1−x O2 oxides by retarding the sintering phenomenon during high temperature treatments, followed by alumina and titania. Interestingly, the alumina supported samples exhibited highest OSC and CO oxidation activity followed by titania and silica. Details of these findings are consolidated in this review.

Uniform nanosized, mesoporous and high specific surface area ceria–zirconia (1:1 mole ratio) solid solutions were synthesized employing a poly-block copolymer surfactant combined with microwave or thermal treatment. For comparison purpose an identical composition Ce x Zr1−x O2 mixed oxide was also prepared by a conventional coprecipitation method. The surface and bulk structure of the synthesized samples were investigated using X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET surface area and BJH pore size distribution (PSD) methods. The catalytic activity was evaluated for CO oxidation at normal atmospheric pressure. The Ce x Zr1−x O2 solid solutions obtained through surfactant use exhibited high specific surface area and mesoporosity. The XRD measurements revealed the presence of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 phases in the case of conventional coprecipitation and surfactant controlled synthesized samples, respectively. The Raman measurements revealed existence of more oxygen defects in the surfactant-controlled and microwave treated sample. The SEM images suggested that all samples consist of typical spherical agglomerates with almost uniform size within the nanometer size range. The TEM measurements revealed nanosized crystallites in a narrow range between 4 and 8 nm, and densely packed in the case of conventional precipitation and microwave treated samples. Interestingly, the Ce x Zr1−x O2 solid solution obtained by surfactant-controlled method and treated with microwave radiation exhibited better CO oxidation activity than other samples. Enhanced activity of surfactant-controlled and microwave treated sample is correlated with its unique physicochemical characteristics.

This study was undertaken to know the influence of chloride ions in TiO2–ZrO2 mixed oxide and to explore chloride containing Fe2O3/TiO2–ZrO2 catalyst for oxidative dehydrogenation of ethylbenzene to styrene utilizing carbon dioxide as a soft oxidant. The TiO2–ZrO2 mixed oxide was synthesized by a co-precipitation method. Over the calcined support, a nominal 15 wt.% Fe2O3 was deposited by a wet impregnation method. The prepared catalysts were characterized using X-ray diffraction, temperature programmed desorption of NH3 and CO2, scanning electron microscopy and BET surface area methods. The catalytic activity measurements were made in a fixed-bed microreactor under normal atmospheric pressure. Characterization studies reveal that chloride ions exhibit a strong influence on the physicochemical and catalytic properties of the titania–zirconia composite oxides. The impregnated iron oxide was found to be in a highly dispersed form over the TiO2–ZrO2 support and influenced greatly on its acid–base properties. The chloride containing Fe2O3/TiO2–ZrO2 catalyst exhibited better activity and selectivity. The order of activity on various samples was found to be Fe2O3/TiO2–ZrO2 (chloride) > TiO2–ZrO2 (chloride) > TiO2–ZrO2.

The catalytic oxidation of soot particulates has been investigated over CeO2, CeO2–ZrO2 and CeO2–HfO2 nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under ‘tight contact’ conditions. The XRD results revealed formation of cubic CeO2, Ce0.75Zr0.25O2 and Ce0.8Hf0.2O2 phases in case of CeO2, CeO2–ZrO2 and CeO2–HfO2 samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce3+←O2− and Ce4+←O2− respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr4+ and Hf4+ into the CeO2 lattice. The CeO2–HfO2 combination catalyst exhibited better activity than the CeO2–ZrO2. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.

We have been exploring various new catalyst systems for the utilization of carbon dioxide as a soft oxidant in the catalytic dehydrogenation of ethylbenzene (EB) to styrene. The utilization of CO2 as a soft oxidant for the commercially important catalytic dehydrogenation of EB to styrene has received enormous attention recently due to its several attractive features. This review summarizes the results of our most recent findings on zirconia-based composite oxide catalyst systems exploited for this reaction. Under this systematic and comprehensive investigation various zirconia-based composite oxide catalysts namely, TiO2-ZrO2, MnO2-ZrO2, CeO2-ZrO2, K2O/TiO2-ZrO2, B2O3/TiO2-ZrO2 and CeO2-ZrO2/SBA-15 have been synthesized, characterized by various techniques and evaluated for the title reaction. Most of these composite oxide catalysts were found to exhibit very interesting physicochemical characteristics and exceptionally better catalytic properties for this reaction. As revealed by characterization results, a large number of acid–base sites with moderate strength are essential for a high conversion and product selectivity of this reaction with CO2 as the soft oxidant.

Nanocrystalline monophasic Ce0.5Zr0.5O2 solid solution (1:1 molar ratio) has been synthesized by microwave-induced combustion method in a modified domestic microwave oven (2.45 GHz, 700 W) in approximately 40 min from cerium nitrate and zirconium nitrate precursors using urea as ignition fuel. For the purpose of better comparison, a CexZr1 − xO2 solid solution (1:1 molar ratio) was also synthesized by a conventional co-precipitation method from nitrate precursors and subjected to different calcination temperatures. The synthesized powders of both methods were characterized by means of X-ray powder diffraction, thermogravimetry/differential thermal analysis, scanning electron microscopy, and BET surface area techniques. Oxygen storage capacity (OSC) measurements were performed to understand the usefulness of these materials for various applications. The characterization results reveal that the sample obtained by microwave-induced combustion-synthesis route exhibits homogeneous monophasic Ce0.5Zr0.5O2 solid solution whereas the co-precipitated sample displays compositional heterogeneity. The OSC measurements reveal that the materials synthesized by both methods exhibit comparable oxygen vacancy content (δ).

The influence of SiO2, TiO2, and ZrO2 on the structural and redox properties of CeO2 were systematically investigated by various techniques namely, X-ray diffraction (XRD), Raman spectroscopy (RS), UV–Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HREM), BET surface area, and thermogravimetry methods. The effect of supporting oxides on the crystal modification of ceria was also mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahigh dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO2–SiO2 sample primarily consists of nanocrystalline CeO2 on the amorphous SiO2 surface. Both crystalline CeO2 and TiO2-anatase phases were noted in the case of CeO2–TiO2 sample. Formation of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 (at 1073 K) were observed in the case of CeO2–ZrO2 sample. The cell ‘a’ parameter estimations revealed an expansion of the ceria lattice in the case of CeO2–TiO2, while a contraction is noted in the case of CeO2–ZrO2. The DRS studies suggest that the supporting oxides significantly influence the band gap energy of CeO2. Raman measurements disclose the presence of oxygen vacancies, lattice defects, and displacement of oxide ions from their normal lattice positions in the case of CeO2–TiO2 and CeO2–ZrO2 samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation states, Si(IV), Ti(IV), and Zr(IV) at the surface of the materials. Cerium is present in both Ce4+ and Ce3+ oxidation states. The HREM results reveal well-dispersed CeO2 nanocrystals over the amorphous SiO2 matrix in the case of CeO2–SiO2, isolated CeO2 and TiO2 (A) nanocrystals and some overlapping regions in the case of CeO2–TiO2, and nanosized CeO2 and Ce–Zr oxides in the case of CeO2–ZrO2 sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO2 is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of mixed oxides is more than that of pure CeO2 and the CeO2–ZrO2 exhibits highest OSC.

A nanocrystalline titania–silica (1:1 molar ratio) binary oxide material was synthesized by microwave-induced combustion process in a modified domestic microwave oven (operated at 2.45 GHz frequency and 700 W power) in approximately 60 min from in situ synthesized titanyl nitrate and siliconyl nitrate using urea as fuel. For the sake of comparison, two different types of TiO2–SiO2 powders were also synthesized by the sol-gel and the co-precipitation methods. All the synthesized powders were characterized with the help of thermogravimetriy/differential thermal analysis, x-ray diffraction, transmission electron microscopy (TEM), and Brunauer–Emmett–Teller surface area measurements and the results compared. The as-synthesized TiO2–SiO2 powder obtained by the combustion process showed an average crystallite size of 10 nm and the specific surface area of 115 m2g-1. Among the three differently synthesized TiO2-SiO2 powders, only the microwave-induced combustion synthesis yielded crystalline material. TEM in particular confirmed the presence of nano-sized particles in the microwave-induced combustion-synthesized powder. Among the three analogies, microwave synthesis was found to be superior in terms of ease of processing leading to time and power savings.

The vapor-phase catalytic O-alkylation of α- and β-naphthols with methanol has been investigated over lanthanum, cerium, samarium, and antimony phosphate catalysts promoted with cesium hydroxide. Among various catalysts investigated, the Cs-SmPO4 combination catalyst provided better catalytic activity and selectivity. The CsOH promoter suppressed formation of C-alkylated side products and enhanced the O-alkylation selectivity. Characterization of these catalysts by X-ray diffraction and FT-infrared techniques revealed that the cesium promoter has no influence on the bulk structure of the catalysts. Temperature programmed desorption of anhydrous ammonia over promoted and unpromoted catalysts revealed that the cesium promoter decreases total surface acidity of the catalysts.

The vapour phase selective oxidation of 4-methylanisole to anisaldehyde was investigated over different V2O5 /MgO-Al2O3 catalysts at 673 K and normal atmospheric pressure. Among various catalysts investigated the 16 wt% V2O5 /MgO-Al2O3 catalyst provided good conversion and product selectivity. The MgO-Al2O3 mixed oxide was obtained by a co-precipitation method and V2O5 was impregnated from ammonium metavanadate. The MgO-Al2O3 support and various V2O5 /MgO-Al2O3 catalysts were characterized by means of X-ray diffraction, FT-infrared, electron spin resonance, scanning electron microscopy, ammonia and carbon dioxide chemisorption methods. The characterization results suggest that vanadia does not form layer structures on the support surface, instead interacts very strongly with the support, in particular with MgO, and forms amorphous compounds. The NH3 and CO2 uptake results provide an interesting information on the acid-base characteristics of these catalysts and correlate with their catalytic properties.

A series of V2O5/MgO catalysts containing 1 to 20 wt % V2O5 were prepared and characterized by means of X-ray diffraction, FT-infrared, electron spin resonance, scanning electron microscopy, NH3 and CO2 uptake measurements. Activity and selectivity of the catalysts were evaluated for selective oxidation of 4-methylanisole to p-anisaldehyde. Characterization results suggest that the vanadia when impregnated on magnesia interacts strongly with the support and results in the formation of a magnesium orthovanadate compound instead of two-dimensional over layers on the support surface. The V−Mg−O compound formed exhibits good conversion and product selectivity for the title reaction. Attempts are made to correlate the acid-base characteristics of the catalysts, obtained from NH3 and CO2 uptakes, with their catalytic properties.

The vapour phase synthesis of isobutyraldehyde from methanol and ethanol in one step was investigated over titania-silica, titania-alumina, titania-zirconia, titania-silica-zirconia, and magnesia supported vanadium oxide catalysts at 623 K and under normal atmospheric pressure. Among various catalysts the titania-silica binary oxide supported vanadia provided higher yields than the other single or mixed oxide supported catalysts. The high conversion and product selectivity of V2O5/TiO2-SiO2 catalyst (20 wt% V2O5) was related to the better dispersion of vanadium oxide over titania-silica mixed oxide support in addition to other acid-base and redox characteristics. A reaction path for the formation of isobutyraldehyde from methanol and ethanol mixtures over these catalysts was described.

Titania-alumina binary oxide supported vanadia catalysts calcined at different temperatures were studied by X-ray diffraction, infrared, oxygen chemisorption and surface area measurements to establish the effects of vanadia loading and thermal treatments on the structure of the dispersed vanadium oxide species. The results suggest that vanadia when calcined at 773 K is in highly dispersed state on the support surface. This dispersion is mostly retained upto a calcination temperature of 873 K. However, thermal treatments above 873 K transform vanadia and titania into crystalline phases and then TiO2 anatase into rutile.

The oxidative methylation of α-, β- and γ-picolines with methane to the corresponding vinyl- and ethylpyridines was carried out at 750°C and under normal atmospheric pressure, over sodium, cesium or sodium-cesium promoted magnesia catalysts. Among the three picolines, the γ-picoline was the most reactive, followed by α- and β-picolines. The (5 mol% Na-5 mol% Cs)/MgO catalyst provided higher yields than the individual promoters, at the same total alkali content of 10 mol%.