Perala Venkataswamy

Catalytic oxidation and adsorption of elemental mercury over nanostructured CeO2–MnOx catalyst

A nanostructured CeO2–MnOx catalyst was synthesized by a coprecipitation method and subjected to different calcination temperatures at 773 and 1073 K to understand the surface structure and the thermal stability. The structural and redox properties were deeply investigated by various techniques, namely, X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-OES), Brunauer–Emmett–Teller (BET) surface area, transmission electron microscopy (TEM), Raman spectroscopy (RS), hydrogen-temperature programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The CeO2–MnOx catalyst calcined at 773 K was tested towards elemental mercury (Hg0) oxidation and the achieved results are compared with the pure CeO2 and MnOx. The XRD and TEM results confirmed the incorporation of Mn ions into the ceria lattice and the formation of a nanostructured solid solution, respectively. The RS and TPR results showed that the CeO2–MnOx catalyst exhibits more oxygen vacancies with superior redox ability over CeO2 and MnOx. XPS analysis indicates that Ce and Mn existed in multiple oxidation states. Compared to pure CeO2 and MnOx, the CeO2–MnOx catalyst exhibited greater Hg0 oxidation efficiency (Eoxi) of 11.7, 33.5, and 89.6% in the presence of HCl, O2, and HCl/O2-mix conditions, respectively. The results clearly indicated that the HCl/O2-mix had a promotional effect on the catalytic Hg0 oxidation. This was most likely due to the presence of surface oxygen species and oxygen vacancies being generated by a synergetic effect between CeO2 and MnOx. In the presence of HCl, the CeO2–MnOx catalyst exhibited good adsorption efficiency (Eads) of 92.4% over pure CeO2 (46.5%) and MnOx (80.6%). It was found that increasing the operating temperature from 423 to 573 K resulted in considerable increase of Eoxi and a decrease in the sorption of Hg0 on the catalyst.

Low-temperature CO oxidation over manganese, cobalt, and nickel doped CeO2 nanorods

Surface active sites such as oxygen vacancies, Ce3+ ions, and unsaturated coordinated sites on nano ceria (CeO2) are significant in catalytic oxidation reactions. The recent development in nanoengineered CeO2 made a pathway to extend its use in various catalytic applications. In this study, transition metals (Mn2+, Ni2+, and Co2+) doped CeO2 nanorods (NRs) were prepared by hydrothermal method and tested towards CO oxidation. Furthermore, the samples were characterized by various physicochemical techniques, namely, TEM and HR-TEM, SEM-EDX, XRD, ICP-OES, BET surface area, Raman spectroscopy, XPS, and H2-TPR. The results demonstrated that the incorporation of dopants greatly enhances the surface defective sites (Ce3+ ions and a high degree of surface roughness) and redox properties of CeO2 NRs and thereby improved catalytic activity. Especially, the Co–CeO2 NR catalyst exhibited better CO conversion (T50 ∼ 145 °C) when compared to pure CeO2 NR (T50 ∼ 312 °C).

Nanowire Morphology of Mono- and Bidoped α-MnO2 Catalysts for Remarkable Enhancement in Soot Oxidation

In the present work, nanowire morphologies of α-MnO2, cobalt monodoped α-MnO2, Cu and Co bidoped α-MnO2, and Ni and Co bidoped α-MnO2 samples were prepared by a facile hydrothermal synthesis. The structural, morphological, surface, and redox properties of all the as-prepared samples were investigated by various characterization techniques, namely, scanning electron microscopy (SEM), transmission and high resolution electron microscopy (TEM and HR-TEM), powder X-ray diffraction (XRD), N2 sorption surface area measurements, X-ray photoelectron spectroscopy (XPS), hydrogen-temperature-programmed reduction (H2-TPR), and oxygen-temperature-programmed desorption (O2-TPD). The soot oxidation performance was found to be significantly improved via metal mono- and bidoping. In particular, Cu and Co bidoped α-MnO2 nanowires showed a remarkable improvement in soot oxidation performance, with its T50 (50% soot conversion) values of 279 and 431 °C under tight and loose contact conditions, respectively. The soot combustion activation energy for the Cu and Co bidoped MnO2 nanowires is 121 kJ/mol. The increased oxygen vacancies, greater number of active sites, facile redox behavior, and strong synergistic interaction were the key factors for the excellent catalytic activity. The longevity of Cu and Co bidoped α-MnO2 nanowires was analyzed, and it was found that the Cu/Co bidoped α-MnO2 nanowires were highly stable after five successive cycles and showed an insignificant decrease in soot oxidation activity. Furthermore, the HR-TEM analysis of a spent catalyst after five cycles indicated that the (310) crystal plane of α-MnO2 interacts with the soot particles; therefore, we can assume that more-reactive exposed surfaces positively affect the reaction of soot oxidation. Thus, the Cu and Co bidoped α-MnO2 nanowires provide promise as a highly effective alternative to precious metal based automotive catalysts.

Crucial role of titanium dioxide support in soot oxidation catalysis of manganese doped ceria

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.

Investigation of physicochemical properties and catalytic activity of nanostructured Ce0.7M0.3O2−δ (M = Mn, Fe, Co) solid solutions for CO oxidation

AbstractIn 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. Graphical AbstractThe presence of structural oxygen vacancies, low temperature reducibility and synergetic interaction between Ce−O and Mn−O oxides were responsible for superior CO oxidation performance of Ce−Mn−O nano oxide compared to pure CeO2, Ce−Fe−O and Ce−Co−O samples.

Superior catalytic performance of a CoOx/Sn–CeO2 hybrid material for catalytic diesel soot oxidation

The present work reports the synthesis and characterization of a ceria-based hybrid nano-catalyst composed of a Snx+ dopant incorporated in the CeO2 crystal lattice and a finely dispersed CoOx phase on its surface. Characterization studies showed that the Ce, Sn, and Co cations were present in their multivalent oxidation states. The CoOx was confirmed to be Co3O4. A HRTEM image depicted the presence of a stepped catalyst surface, which has a special importance in enhancing the heterogeneous catalytic reaction rate carried out on the solid catalyst surface. The prepared materials were subjected to diesel soot oxidation catalysis. Model soot was combusted in the presence of air under both tight and loose contact conditions of the catalyst and soot. The hybrid catalyst exhibited improved performance compared to the Sn-doped nano-CeO2 and nano-CeO2 supported CoOx catalysts. The improved catalytic activity was attributed to the existence of synergism among the multivalent cations and the stepped surface of the hybrid catalyst, which act as the potential active sites for oxidation catalysis.

Nanostructured KTaTeO 6 and Ag-doped KTaTeO 6 Defect Pyrochlores: Promising Photocatalysts for Dye Degradation and Water Splitting

In this study, the nanostructured parent KTaTeO6 (KTTO) and Ag-doped KTaTeO6 (ATTO) catalysts with defect pyrochlore structure were prepared by solid-state and ion-exchange methods, respectively. The synthesized materials were characterized by various techniques to determine their chemical composition, morphology and microstructural features. The XRD studies show that both KTTO and ATTO have cubic structure (space group Fd3m) with high crystallinity. The doping of Ag altered the BET surface area of parent KTTO. The nano nature of the samples was studied by TEM images. A considerable red-shift in the absorption edge is observed for ATTO compared to KTTO. Incorporation of Ag+ in the KTTO lattice is clearly identified from EDX, elemental mapping and XPS results. Degradation of methyl violet and solar water splitting reactions were used to access the photocatalytic activity of KTTO and ATTO. The results obtained suggest that compared to KTTO, the ATTO showed higher photocatalytic activity in both cases. The favourable properties such as high surface area, more surface hydroxyl groups, stronger light absorption in visible region and narrower band gap energy were supposed to be the reasons for the high activity observed in ATTO.