Towards efficient use of noble metals : via exsolution exemplified for CO oxidation

Many catalysts and in particular automotive exhaust catalysts usually consist of noble metal nanoparticles dispersed on metal oxide supports. While highly active, such catalysts are expensive and prone to deactivation by sintering and thus alternative methods for their production are being sought to ensure more efficient use of noble metals. Exsolution has been shown to be an approach to produce confined nanoparticles, which in turn are more stable against agglomeration, and, at the same time strained, displaying enhanced activity. While exsolution has been extensively investigated for relatively high metal loadings, it has yet to be explored for dilute loadings which is expected to be more challenging but more suitable for application of noble metals. Here we explore the substitution of Rh into an A-site deficient perovskite titante aiming to investigate the possibility of exsolving from dilute amounts of noble metal substituted perovskites. We show that this is possible and in spite of certain limitations, they still compete well against conventionally prepared samples with higher apparent surface loading when applied for CO oxidation

Ultrafast transport of laser-excited spin polarized carriers in Au/Fe/MgO(001)

A time domain approach to probe hot carrier-induced spin dynamics is demonstrated. The experiments are performed in epitaxial Au/Fe/MgO(001), where spin-polarized hot carriers are excited in the Fe layer by 35 fs laser pulses. They propagate to the Au surface where the transient spin polarization is detected by magneto-induced second harmonic generation. Different energies of majority and minority hot carriers excited in the exchange-split Fe band structure lead to their spindependent lifetimes in Au. Accordingly, two spin-polarized current contributions which propagate superdiffusively at different velocities result in a spin current pulse of about 100 fs duration.Comment: 5 pages, 4 figure

Endogenous nanoparticles strain perovskite host lattice providing oxygen capacity and driving oxygen exchange and CH4 conversion to syngas

Particles dispersed on the surface of oxide supports have enabled a wealth of applications in electrocatalysis, photocatalysis, and heterogeneous catalysis. Dispersing nanoparticles within the bulk of oxides is, however, synthetically much more challenging and therefore less explored, but could open new dimensions to control material properties analogous to substitutional doping of ions in crystal lattices. Here we demonstrate such a concept allowing extensive, controlled growth of metallic nanoparticles, at nanoscale proximity, within a perovskite oxide lattice as well as on its surface. By employing operando techniques, we show that in the emergent nanostructure, the endogenous nanoparticles and the perovskite lattice become reciprocally strained and seamlessly connected, enabling enhanced oxygen exchange. Additionally, even deeply embedded nanoparticles can reversibly exchange oxygen with a methane stream, driving its redox conversion to syngas with remarkable selectivity and long term cyclability while surface particles are present. These results not only exemplify the means to create extensive, self-strained nanoarchitectures with enhanced oxygen transport and storage capabilities, but also demonstrate that deeply submerged, redox-active nanoparticles could be entirely accessible to reaction environments, driving redox transformations and thus offering intriguing new alternatives to design materials underpinning several energy conversion technologies

Low temperature methane conversion with perovskite-supported exo/endo-particles

Lowering the temperature at which CH4 is converted to useful products has been long-sought in energy conversion applications. Selective conversion to syngas is additionally desirable. Generally, most of the current CH4 activation processes operate at temperatures between 600 and 900 °C when non-noble metal systems are used. These temperatures can be even higher for redox processes where a gas phase–solid reaction must occur. Here we employ the endogenous-exsolution concept to create a perovskite oxide with surface and embedded metal nanoparticles able to activate methane at temperatures as low as 450 °C in a cyclic redox process. We achieve this by using a non-noble, Co–Ni-based system with tailored nano- and micro-structure. The materials designed and prepared in this study demonstrate long-term stability and resistance to deactivation mechanisms while still being selective when applied for chemical looping partial oxidation of methane

Tracking the evolution of a single composite particle during redox cycling for application in H-2 production

Composite materials consisting of metal and metal oxide phases are being researched intensively for various energy conversion applications where they are often expected to operate under redox conditions at elevated temperature. Understanding of the dynamics of composite evolution during redox cycling is still very limited, yet critical to maximising performance and increasing durability. Here we track the microstructural evolution of a single composite particle over 200 redox cycles for hydrogen production by chemical looping, using multi-length scale X-ray computed tomography. We show that redox cycling triggers a centrifugal redispersion of the metal phase and a centripetal clustering of porosity, both seemingly driven by the asymmetric nature of oxygen exchange in composites. Initially, the particle develops a large amount of internal porosity which boosts activity, but on the long term this facilitates structural and compositional reorganisation and eventually degradation. These results provide valuable insight into redox-driven microstructural changes and also for the design of new composite materials with enhanced durability

Sulfur-tolerant, exsolved Fe–Ni alloy nanoparticles for CO oxidation

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007–2013)/ERC Grant Agreement Number 320725 and from the EPSRC via the research grants EP/P024807/1, EP/P009050/1, EP/P007767/1, EP/J016454/1 and EP/L017008/1. Data supporting this publication is openly available under an ‘Open Data Commons Open Database License’. Additional metadata are available at: https://doi.org/10.17634/161340-1.Metallic nanoparticles exsolved at the surface of perovskite oxides have been recently shown to unlock superior catalytic activity and durability towards various chemical reactions of practical importance. For example, for the CO oxidation reaction, exsolved Ni nanoparticles in oxidised form exhibit site activities approaching those of noble metals. This is of particular interest for the prospect of replacing noble metals with earth-abundant metal/metal oxide catalysts in the automotive exhaust control industry. Here we show that for the CO oxidation reaction, the functionality of exsolved Ni nanoparticles can be further improved when Fe is co-exsolved with Ni, as Fe–Ni alloy nanoparticles, eventually forming mixed oxide nanoparticles. As compared to the Ni nanoparticles, the alloy nanoparticles exhibit higher site activities, greatly improved durability over 170 h of continuous testing and increased tolerance towards sulphur-based atmospheres. Similarly to the single metal nanoparticles, the alloys demonstrate outstanding microstructural stability and high tolerance towards coking. These results open additional directions for tailoring the activity and durability of exsolved materials for the CO oxidation reaction and beyond.Publisher PDFPeer reviewe

Prediction of solar proton event fluence spectra from their peak flux spectra

Solar Proton Events (SPEs) are of great importance and significance for the study of Space Weather and Heliophysics. These populations of protons are accelerated at high energies ranging from a few MeVs to hundreds of MeVs and can pose a significant hazard both to equipment on board spacecrafts as well as astronauts as they are ionizing radiation. The ongoing study of SPEs can help to understand their characteristics, relative underlying physical mechanisms, and help in the design of forecasting and nowcasting systems which provide warnings and predictions. In this work, we present a study on the relationships between the Peak Flux and Fluence spectra of SPEs. This study builds upon existing work and provides further insights into the characteristics and the relationships of SPE Peak flux and Fluence spectra. Moreover it is shown how these relationships can be quantified in a sound manner and exploited in a simple methodology with which the Fluence spectrum of an SPE can be well predicted from its given Peak spectrum across two orders of magnitude of proton energies, from 5 MeV to 200 MeV. Finally it is discussed how the methodology in this work can be easily applied to forecasting and nowcasting systems

Rapid Plasma Exsolution from an A-site Deficient Perovskite Oxide at Room Temperature

The research was supported by EPSRC (Award Nos. EP/R023522/1, EP/R023603/1, EP/R023921/1, EP/R023638/1, EP/R008841/1, and EP/V055232/1) and financial support from the UK Catalysis Hub funded by EPSRC Grant reference EP/R027129/1. J.W. and S.C.P. gratefully acknowledge support from the EPSRC (EP/P007821/1) and also thank the U.K. ARCHER HPC facility and the THOMAS HPC (the UK Materials and Molecular Modelling Hub, partially funded by EPSRC EP/P020194) for providing computation resources, via the membership of the UK's HEC Materials Chemistry Consortium (funded by the EPSRC Grant Nos. EP/L000202, EP/709 P007821/1, EP/R029431, and EP/T022213).High‐performance nanoparticle platforms can drive catalysis progress to new horizons, delivering environmental and energy targets. Nanoparticle exsolution offers unprecedented opportunities that are limited by current demanding process conditions. Unraveling new exsolution pathways, particularly at low‐temperatures, represents an important milestone that will enable improved sustainable synthetic route, more control of catalysis microstructure as well as new application opportunities. Herein it is demonstrated that plasma direct exsolution at room temperature represents just such a step change in the synthesis. Moreover, the factors that most affect the exsolution process are identified. It is shown that the surface defects produced initiate exsolution under a brief ion bombardment of an argon low‐pressure and low‐temperature plasma. This results in controlled nanoparticles with diameters ≈19–22 nm with very high number densities thus creating a highly active catalytic material for CO oxidation which rivals traditionally created exsolved samples.Publisher PDFPeer reviewe