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Abstract: Unravelling kinetic oscillations, which arise spontaneously during catalysis, has been a challenge for decades but is important not only to understand these complex phenomena but also to achieve increased activity. Here we show, through temporally and spatially resolved operando analysis, that CO oxidation over Rh/Al2O3 involves a series of thermal levering events—CO oxidation, Boudouard reaction and carbon combustion—that drive oscillatory CO2 formation. This catalytic sequence relies on harnessing localized temperature episodes at the nanoparticle level as an efficient means to drive reactions in situations in which the macroscopic conditions are unfavourable for catalysis. This insight provides a new basis for coupling thermal events at the nanoscale for efficient harvesting of energy and enhanced catalyst technologies.

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Abstract: Co3O4 nanoparticles were supported on different TiO2 polymorphs, namely, rutile, anatase, and a 15[thin space (1/6-em)]:[thin space (1/6-em)]85 mixture of rutile and anatase (also known as P25), via incipient wetness impregnation. The Co3O4/TiO2 catalysts were evaluated in the preferential oxidation of CO (CO-PrOx) in a H2-rich gas environment and characterised in situ using PXRD and magnetometry. Our results show that supporting Co3O4 on P25 resulted in better catalytic performance, that is, a higher maximum CO conversion to CO2 of 72.7% at 200 °C was achieved than on rutile (60.7%) and anatase (51.5%). However, the degree of reduction (DoR) of Co3O4 to Co0 was highest on P25 (91.9% at 450 °C), with no CoTiO3 detected in the spent catalyst. The DoR of Co3O4 was lowest on anatase (76.4%), with the presence of TixOy-encapsulated CoOx nanoparticles and bulk CoTiO3 (13.8%) also confirmed in the spent catalyst. Relatively low amounts of CoTiO3 (8.9%) were detected in the spent rutile-supported catalyst, while a higher DoR (85.9%) was reached under reaction conditions. The Co0 nanoparticles formed on P25 and rutile existed in the fcc and hcp crystal phases, while only fcc Co0 was detected on anatase. Furthermore, undesired CH4 formation took place over the Co0 present in the P25- and rutile-supported catalysts, while CH4 was not formed over the Co0 on anatase possibly due to encapsulation by TixOy species. For the first time, this study revealed the influence of different TiO2 polymorphs (used as catalyst supports) on the chemical and crystal phase transformations of Co3O4, which in turn affect its activity and selectivity during CO-PrOx.

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Abstract: Sn-Beta has emerged as a state-of-the-art catalyst for a range of sustainable chemical transformations. Conventionally prepared by bottom-up hydrothermal synthesis methods, recent research has demonstrated the efficiency of several top-down methods of preparation. One attractive top-down approach is Solid-State Incorporation, where a dealuminated Beta zeolite is physically mixed with a solid Sn precursor, in particular Sn (II) acetate, prior to heat treatment at 550 °C. This procedure is fast and benign, and metal incorporation requires no solvents and hence produces no aqueous Sn-containing waste streams. Although the performances of these catalysts have been well explored in recent years, the mechanism of heteroatom incorporation remains unknown, and hence, opportunities to improve the synthetic procedure via a molecular approach remain. Herein, we utilise a range of in situ spectroscopic techniques, alongside kinetic and computational methods, to elucidate the mechanisms that occur during preparation of the catalyst, and then improve the efficacy of the synthetic protocol. Specifically, we find that successful incorporation of Sn into the lattice occurs in several distinct steps, including i) preliminary coordination of the metal ion to the vacant lattice sites of the zeolite during physical grinding; ii) partial incorporation of the metal ion into the zeolite framework upon selective decomposition of the acetate ligands, which occurs upon heating the physical mixture in an inert gas flow from room temperature to 550 °C; and iii) full isomorphous substitution of Sn into the lattice alongside its simultaneous oxidation to Lewis acidic Sn(IV), when the physically mixed material is exposed to air during a short (<1 h) isotherm period. Long isotherm steps are shown to be unnecessary, and fully oxidised Sn(IV) precursors are shown to be unsuitable for successful incorporation into the lattice. We also find that the formation of extra-framework Sn oxides is primarily dependent on the quantity of Sn present in the initial physical mixture. Based on these findings, we demonstrate a faster synthetic protocol for the preparation of Sn-Beta materials via Solid-State Incorporation, and benchmark their performance of the catalyst for the Meerwein-Ponndorf-Verley transfer hydrogenation reaction and for the isomerisation of glucose to fructose.

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Abstract: We have studied the effect of different supports (CeO2, ZrO2, SiC, SiO2 and Al2O3) on the catalytic performance and phase stability of Co3O4 nanoparticles during the preferential oxidation of CO (CO-PrOx) under different H2-rich gas environments and temperatures. Our results show that Co3O4/ZrO2 has superior CO oxidation activity, but transforms to Co0 and consequently forms CH4 at relatively low temperatures. The least reduced and least methanation active catalyst (Co3O4/Al2O3) also exhibits the lowest CO oxidation activity. Co-feeding H2O and CO2 suppresses CO oxidation over Co3O4/ZrO2 and Co3O4/SiC, but also suppresses Co0 and CH4 formation. In conclusion, weak nanoparticle-support interactions (as in Co3O4/ZrO2) favour high CO oxidation activity possibly via the Mars-van Krevelen mechanism. However, stronger interactions (as in Co3O4/Al2O3) help minimise Co0 and CH4 formation. Therefore, this work reveals the bi-functional role required of supports used in CO-PrOx, i.e., to enhance catalytic performance and improve the phase stability of Co3O4.

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Abstract: Hydrothermal degreening and ageing procedures were applied to a tri-metal (Pt-Pd-Rh) fully formulated lean NOX Trap catalyst to evaluate the effects of thermal stress on the performances and structural properties. X-ray absorption fine structure (XAFS) analysis revealed that the average size of the platinum particles was the same after degreening and ageing treatments. The formation of a new phase of alloyed Pt-Pd was observed to increase with the thermal load. The size of the ceria particles also increased after the ageing treatment. NOX storage capacity experiments revealed a substantial decrease of the concentration of active NOX storage sites which correlated with both ageing and degreening protocols. The performances of the treated catalyst were evaluated through spatially resolved (SpaciMS) lean-rich cycles. During the lean phase, the impact of the decrease in storage sites was significant on the aged sample, where an enlargement of the area required to achieve full storage was observed. On the other hand, the regeneration functionalities did not appear to be particularly affected by ageing. Rather, the aged sample showed a decrease of oxygen storage capacity (OSC), which promoted a lower reductant consumption and therefore a quicker and more efficient reduction process. On the other hand, the different distributions of adsorbed species by the end of the lean phase produced greater spread presence of NH3 and NOX slip along the channels of the aged sample during the reduction.