Bolla Govinda Rao

Designing CuOx Nanoparticle-Decorated CeO2 Nanocubes for Catalytic Soot Oxidation: Role of the Nanointerface in the Catalytic Performance of Heterostructured Nanomaterials

This work investigates the structure-activity properties of CuOx-decorated CeO2 nanocubes with a meticulous scrutiny on the role of the CuOx/CeO2 nanointerface in the catalytic oxidation of diesel soot, a critical environmental problem all over the world. For this, a systematic characterization of the materials has been undertaken using transmission electron microscopy (TEM), transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDS), high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM), scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS), X-ray diffraction (XRD), Raman, N2 adsorption-desorption, and X-ray photoelectron spectroscopy (XPS) techniques. The TEM images show the formation of nanosized CeO2 cubes (∼25 nm) and CuOx nanoparticles (∼8.5 nm). The TEM-EDS elemental mapping images reveal the uniform decoration of CuOx nanoparticles on CeO2 nanocubes. The XPS and Raman studies show that the decoration of CuOx on CeO2 nanocubes leads to improved structural defects, such as higher concentrations of Ce(3+) ions and abundant oxygen vacancies. It was found that CuOx-decorated CeO2 nanocubes efficiently catalyze soot oxidation at a much lower temperature (T50 = 646 K, temperature at which 50% soot conversion is achieved) compared to that of pristine CeO2 nanocubes (T50 = 725 K) under tight contact conditions. Similarly, a huge 91 K difference in the T50 values of CuOx/CeO2 (T50 = 744 K) and pristine CeO2 (T50 = 835 K) was found in the loose-contact soot oxidation studies. The superior catalytic performance of CuOx-decorated CeO2 nanocubes is mainly attributed to the improved redox efficiency of CeO2 at the nanointerface sites of CuOx-CeO2, as evidenced by Ce M5,4 EELS analysis, supported by XRD, Raman, and XPS studies, a clear proof for the role of nanointerfaces in the performance of heterostructured nanocatalysts.

Integrated Experimental and Theoretical Study of Shape-Controlled Catalytic Oxidative Coupling of Aromatic Amines over CuO Nanostructures

We have synthesized CuO nanostructures with flake, dandelion-microsphere, and short-ribbon shapes using solution-phase methods and have evaluated their structure–performance relationship in the heterogeneous catalysis of liquid-phase oxidative coupling reactions. The formation of nanostructures and the morphological evolution were confirmed by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, elemental mapping analysis, and Fourier transform infrared spectroscopy. CuO nanostructures with different morphologies were tested for the catalytic oxidative coupling of aromatic amines to imines under solvent-free conditions. We found that the flake-shaped CuO nanostructures exhibited superior catalytic efficiency compared to that of the dandelion- and short-ribbon-shaped CuO nanostructures. We also performed extensive density functional theory (DFT) calculations to gain atomic-level insight into the intriguing reactivity trends observed for the different CuO nanostructures. Our DFT calculations provided for the first time a detailed and comprehensive view of the oxidative coupling reaction of benzylamine over CuO, which yields N-benzylidene-1-phenylmethanamine as the major product. CuO(111) is identified as the reactive surface; the specific arrangement of coordinatively unsaturated Cu and O sites on the most stable CuO(111) surface allows N–H and C–H bond-activation reactions to proceed with low-energy barriers. The high catalytic activity of the flake-shaped CuO nanostructure can be attributed to the greatest exposure of the active CuO(111) facets. Our finding sheds light on the prospective utility of inexpensive CuO nanostructured catalysts with different morphologies in performing solvent-free oxidative coupling of aromatic amines to obtain biologically and pharmaceutically important imine derivatives with high selectivity.

Highly efficient continuous-flow oxidative coupling of amines using promising nanoscale CeO2–M/SiO2 (M = MoO3 and WO3) solid acid catalysts

The development of promising solid acid catalysts alternative to hazardous liquid acids is essential towards a sustainable chemical industry. This work reports the synthesis of nanostructured CeO2–MoO3/SiO2 and CeO2–WO3/SiO2 solid acids, along with CeO2–MoO3, CeO2–WO3 and CeO2 for continuous-flow oxidative coupling of benzylamine using O2 as a green oxidant. A systematic physicochemical characterization has been undertaken using XRD, Raman, N2 adsorption–desorption, TEM, NH3-TPD, and XPS techniques. It was found that the dispersion of CeO2–MoO3 and CeO2–WO3 species on the SiO2 support leads to remarkable structural and acidic properties, due to the synergetic effect of the respective components. TEM analysis reveals the presence of highly dispersed WO3 (0.8–1.2 nm) and MoO3 (0.8–1 nm) nanoparticles in the synthesized catalysts. Among the various catalysts developed, the CeO2–MoO3/SiO2 sample exhibited higher BET surface area (248 m2 g−1), abundant oxygen vacancy defects, and large amounts of strong acidic sites. Owing to improved properties, the CeO2–MoO3/SiO2 solid-acid showed a superior catalytic performance in the continuous-flow oxidative coupling of benzylamine: the obtained benzylamine conversions for 1 h are ∼11.8, 55, 70, 76, and 96%, respectively, for CeO2, CeO2–WO3, CeO2–WO3/SiO2, CeO2–MoO3, and CeO2–MoO3/SiO2 catalysts. Importantly, the CeO2–MoO3/SiO2 solid acid exhibited a remarkable steady performance in terms of benzylamine conversion (∼88–96%) and selectivity of N-benzylbenzaldimine product (∼96–97.8%) up to 6 h. The outstanding catalytic performance of CeO2–MoO3/SiO2 solid acid coupled with the application of continuous-flow synthesis, economical benefits of the respective oxides, and eco-friendly oxidant is expected to bring new opportunities in the design of industrially-favourable chemical processes.

Characterization of Ceria-Based Nano-Oxide Catalysts by Raman Spectroscopy

Ceria-based materials have drawn intense research focus due to variety of catalytic and energy related applications. Creation of oxygen vacancy defects imparts oxygen storage and release property (OSC) in the ceria, which facilitates the redox catalytic activity. Introduction of aliovalent dopant ions into the ceria nanocrystals enhances the OSC by generation of extrinsic defects. The current study describes synthesis and characterization of rare earth doped ceria-based mixed oxides. Characterization of the samples was carried out using XRD, BET surface area measurement, TEM, HRTEM, etc. The bulk defect features of the samples were studied employing visible Raman spectroscopy. F2g peak of the Raman spectra evidenced red shift and peak broadening, which could be attributed to change in lattice parameter, oxygen vacancy defects, and smaller crystallite size of the doped nanocrystals. Additional peak (D1) appeared due to the creation of oxygen vacancy defects. The ratio of intensity of D1 peak to F2g peak gave the defect concentration of the doped samples. O 2p and Ce 4f direct band gap energies of the samples were also evaluated. Decrease of band gap energy of the doped samples provided evidence of defect concentration enhancement. A correlation was found among the defect concentration and OSC of the prepared materials. Finally, CO oxidation reaction was performed with the doped materials and the activity was found to be in accordance with the enhancement of defect concentration and OSC.

Novel approaches for preparation of nanoparticles

Abstract There is a detonation of interest and investment in the field of nanoscience and nanotechnology over the last few years. The nanoscience revolution is one of the biggest things to happen since the beginning of modern science, and it is nowadays at the center of future technological progress owing to the increasing ability to manipulate the matter on the nanometer scale. One of the important driving forces for the rapidly developing field of nanoparticle synthesis is the distinctly different physicochemical properties exhibited by the nanoparticles compared to their bulk counterparts. It may be due to the surface effect, small size effect, quantum size effect, and so on which open up new opportunities for the development of materials with unusual or tailored properties. Like nanomaterials, bulk materials also exhibit surface dependent properties but these are dominant in the case of nanoparticles only because they possess a vast surface area per unit volume and a high proportion of atoms at the surface and near surface layers rather than in the particle interior. Many properties of the nanoparticles are directly connected to their small size. The small size leads to many distinct properties, which influence the lattice symmetry and cell parameters. Properties of the materials are largely dependent on the size and morphology, due to this the control over the synthetic methodologies became an issue. This is because the growth of the materials in nanoscale is largely dependent on the thermodynamic and kinetic barriers in the reaction as defined by the reaction trajectory and influenced by vacancies, defects, and surface reconstructions. The simultaneous control of particle size and shape together with their uniformity is one of the key objectives in many synthetic procedures. In the present chapter, new developments in various preparation methodologies for the fabrication of nanoparticles are addressed.