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Abstract: Ceria (cerium oxide) based materials have been widely used for catalytic applications including; fluid catalytic cracking, water gas shift reactions, solid oxide fuel cells and automotive catalytic after-treatment. It has often been the material of choice for supporting precious metals in many of the applications previously mentioned, as it can provide a high surface area and has excellent redox properties. This enables ceria to absorb oxygen into its structure and release it again, to facilitate oxidation reactions. Cerium is also one of the most abundant rare earth metals, making it a relatively cost effective material to use in industrial applications. For these reasons, cerium oxide remains to be a material of high interest for the development of cost-effective industrial catalysts and is, therefore, the focal material for this work. Modern internal combustion engine technology is being directed towards lean burn operation, involving higher air-fuel ratios in order to improve thermal efficiency. In turn, the operating temperature of modern engines is being reduced, providing less of the heat energy required to activate the catalysts in the after-treatment system. Therefore, in order to mitigate the emission from efficient lean burn engines and comply with emission regulations, the requirement for low temperature automotive catalysts is apparent. The objective of this work is to develop effective ceria based materials for the application of modern automotive after-treatment catalysis. The first approach is the use of a novel method for the preparation of a mixed oxide catalysts, which show high activity for CO and HC oxidation at low temperature. The preparation of ceria/manganese mixed oxide catalysts using a novel synthesis method has been studied and the materials were characterised to gain insights of their structure and morphology. Temperature programmed reactions using a complex mixture of reactants, before and after hydrothermal aging, were carried out to investigate their application for automotive emission control. The results from these studies were compared with more conventional ceria based materials that are often used for automotive after-treatment. A second approach used in this work is a post synthesis technique for the surface modification of ceria catalysts using ion bombardment. This technique was carried out on a commercially sourced, model catalyst. A Pt-CeO2/ZrO2 was treated with nitrogen ion irradiation and the effects of the adjustable parameters of the ion beam were investigated by carrying out temperature programmed reactions. The samples were also extensively characterised using XAFS and XPS techniques to understand the effects of the ion beam treatment on the structure, morphology and Pt dispersion of the materials.

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Abstract: The recent report of P2–Na2/3Mg0.28Mn0.72O2 (P2-NMM) demonstrated the possibility of utilizing the oxygen redox couple in a layered oxide cathode without the need for alkali ions or vacancies in the transition metal layer. In this work, we report the synthesis of a new P2-type compound, Na2/3Mg1/4Mn7/12Co1/6O2 (P2-NMMC), which exhibits reversible specific capacities as high as 173 mAh g−1 and an improvement of the first cycle voltage hysteresis over P2-NMM. The material was characterised using a combination of ex-situ and operando techniques including X-ray diffraction (XRD), differential electrochemical mass spectrometry (DEMS) and X-ray spectroscopy (XAS) to identify potential sources for this improvement.

Open Access

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Abstract: Lithium-rich disordered rocksalts such as Li1.3Nb0.3Mn0.4O2 and Li2MnO2F are being investigated as high energy density cathodes for next generation Li-ion batteries. They can support the (de)intercalation of lithium ions over large compositional ranges while preserving the same overall structure. Here, we present a new Ni-rich oxyfluoride cathode, Li2NiO2F, with a disordered rocksalt structure. Li2NiO2F and can deliver a discharge capacity of 200 mAh g-1 at an average voltage of 3.2 V.

Open Access

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Abstract: The research for surface active sites with outstanding catalytic activity and selectivity is continuing apace. The rationally designed catalyst with optimised energy level and geometric configuration is key to achieving novel reaction pathways with superior performance. Compared to the conventional supported nanomaterials as catalysts, single atomic site catalysts (SASCs) not only inherit the excellent recyclability but are also featured with ultimate atom efficiency, high structural uniformity and tunable coordination environment. These great advantages of SASCs are due to their unique atomic dispersion nature that preserves features of both heterogeneous and homogeneous catalysts. Especially the homogeneity of SASC enables convincing identification and characterisation of real active sites, making it an ideal platform to establish the definitive structure-activity relationship and to validate reaction mechanisms. Therefore, rational designed SASC has become the most prominent material to fabricate desired active sites with outstanding catalytic activity and selectivity. In this thesis, chemical synthesis strategies and characterisation techniques for SASCs are carefully reviewed. The limitations and future perspectives from a subjective view of the current methodology are discussed in detail as well. As inspired by pioneers’ work, rational designed Ru and Cu SASCs are prepared to investigate their distinct relationships between catalyst structures and reaction behaviours during CO oxidation reaction. With the help of combined in situ characterisation techniques, the structural evolution of active sites for both Ru and Cu catalysts were carefully studied. In the first project, the ultimate rational design of Ru active centre is realised by building surface single-sites to mimic molecular Ru catalysts. Inspired by a homogeneous Ru(II) complex, an air-stable surface -[bipy-Ru(II)(CO)2Cl2] single-site is designed through precise engineering of geometric and electronic structures from -[bipy-Ru(III)Cl4]- site. Such Ru(II) single-site enable oxidation of CO while the Ru(III) site is completely inert, providing an excellent prototype of the synthetic strategy which is generally applicable to transition metals. The second project focuses on the electronic metal-support interactions (EMSI) which describe electron flow between metal sites and a metal oxide support. For CuO-CeO2 catalysts, the electron withdrawing effect on Cu species introduced by electrophilic Ce4+ is maximised for atomically dispersed Cu sites over CeO2 surface. Experiment evidence shows the energy levels of 3d orbitals of isolated Cu(I)/(II) sites are decreased by Ce4+ cations in the support framework. It is demonstrated by in situ study that a [Cu(I)O2]3- site on CeO2 could selectively adsorb molecular O2 and form a rarely reported electrophilic 2-O2 species, leading to ten times higher activity than CuO clusters in CO oxidation. The third project is derived from the previous study of CuO-CeO2 catalysts, in which an unbalanced electron transfer between Cu and Ce is observed for CuO clusters dominant samples. To explain the reaction pathway of CeO2 supported CuO clusters in CO oxidation, an electronic metal-support-carbon interaction (EMSCI) based on EMSI is proposed. In the CuO-CeO2 redox, an additional flow of electron from metallic Cu to surface carbon species is observed by combined in situ studies, providing a complete picture of the mass and electron flow in the catalytic redox cycles.

Open Access

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Abstract: Indium oxide (In2O3) is a promising catalyst for selective CH3OH synthesis from CO2 but displays insufficient activity at low reaction temperatures. By screening a range of promoters (Co, Ni, Cu, and Pd) in combination with In2O3 using flame spray pyrolysis (FSP) synthesis, Ni is identified as the most suitable first-row transition-metal promoter with similar performance as Pd–In2O3. NiO–In2O3 was optimized by varying the Ni/In ratio using FSP. The resulting catalysts including In2O3 and NiO end members have similar high specific surface areas and morphology. The main products of CO2 hydrogenation are CH3OH and CO with CH4 being only observed at high NiO loading (≥75 wt %). The highest CH3OH rate (∼0.25 gMeOH/(gcat h), 250 °C, and 30 bar) is obtained for a NiO loading of 6 wt %. Characterization of the as-prepared catalysts reveals a strong interaction between Ni cations and In2O3 at low NiO loading (≤6 wt %). H2-TPR points to a higher surface density of oxygen vacancy (Ov) due to Ni substitution. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance analysis of the used catalysts suggest that Ni cations can be reduced to Ni as single atoms and very small clusters during CO2 hydrogenation. Supportive density functional theory calculations indicate that Ni promotion of CH3OH synthesis from CO2 is mainly due to low-barrier H2 dissociation on the reduced Ni surface species, facilitating hydrogenation of adsorbed CO2 on Ov.