Dr Kelly Kousi

Understanding and controlling the formation of nanoparticles at the surface of functional oxide supports is critical for tuning activity and stability for catalytic and energy conversion applications. Here, we use a latest generation environmental transmission electron microscope to follow the exsolution of individual nanoparticles at the surface of perovskite oxides, with ultrahigh spatial and temporal resolution. Qualitative and quantitative analysis of the data reveals the atomic scale processes that underpin the formation of the socketed, strain-inducing interface that confers exsolved particles their exceptional stability and reactivity. This insight also enabled us to discover that the shape of exsolved particles can be controlled by changing the atmosphere in which exsolution is carried out, and additionally, this could also produce intriguing heterostructures consisting of metal-metal oxide coupled nanoparticles. Our results not only provide insight into the in situ formation of nanoparticles but also demonstrate the tailoring of nanostructures and nanointerfaces.

The production of syngas (H-2 and CO)-a key building block for the manufacture of liquid energy carriers, ammonia and hydrogen-through the dry (CO2-) reforming of methane (DRM) continues to gain attention in heterogeneous catalysis, renewable energy technologies and sustainable economy. Here we report on the effects of the metal oxide support (gamma-Al2O3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) on the low-temperature (ca. 500-750 & DEG;C) DRM activity, selectivity, resistance against carbon deposition and iridium nanoparticles sintering under oxidative thermal aging. A variety of characterization techniques were implemented to provide insight into the factors that determine iridium intrinsic DRM kinetics and stability, including metal-support interactions and physicochemical properties of materials. All Ir/gamma-Al2O3, Ir/ACZ and Ir/CZ catalysts have stable DRM performance with time-on-stream, although supports with high oxygen storage capacity (ACZ and CZ) promoted CO2 conversion, yielding CO-enriched syngas. CZ-based supports endow Ir exceptional anti-sintering characteristics. The amount of carbon deposition was small in all catalysts, however decreasing as Ir/gamma-Al2O3 > Ir/ACZ > Ir/CZ. The experimental findings are consistent with a bifunctional reaction mechanism involving participation of oxygen vacancies on the support's surface in CO2 activation and carbon removal, and overall suggest that CZ-supported Ir nanoparticles are promising catalysts for low-temperature dry reforming of methane (LT-DRM).

The effects of the metal oxide support on the activity, selectivity, resistance to carbon deposition and high temperature oxidative aging on the Rh-catalyzed dry reforming of methane (DRM) were investigated. Three Rh catalysts supported on oxides characterized by very different oxygen storage capacities and labilities (gamma-Al2O3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) were studied in the temperature interval 400-750 degrees C under both integral and differential reaction conditions. ACZ and CZ promoted CO2 conversion, yielding CO enriched synthesis gas. Detailed characterization of these materials, including state of the art XPS measurements obtained via sample transfer between reaction cell and spectrometer chamber, provided clear insight into the factors that determine catalytic performance. The principal Rh species detected by post reaction XPS was Rh, its relative content decreasing in the order Rh/CZ(100%) > Rh/ACZ(72%) > Fth/gamma Al2O3(55%). The catalytic activity followed the same order, demonstrating unambiguously that Rh is indeed the key active site. Moreover, the presence of CZ in the support served to maintain Rh in the metallic state and minimize carbon deposition under reaction conditions. Carbon deposition, low in all cases, increased in the order Rh/CZ < Rh/ACZ < Rh/gamma-Al2O3 consistent with a bi-functional reaction mechanism whereby backspillover of labile lattice O2- contributes to carbon oxidation, stabilization of Rh and modification of its surface chemistry; the resulting O vacancies in the support providing centers for dissociative adsorption of CO2. The lower apparent activation energy observed with CZ-containing samples suggests that CZ is a promising support component for use in low temperature DRM.

Supported bimetallic nanoparticles used for various chemical transformations appear to be more appealing than their monometallic counterparts, because of their unique properties mainly originating from the synergistic effects between the two different metals. Exsolution, a relatively new preparation method for supported nanoparticles, has earned increasing attention for bimetallic systems in the past decade, not only due to the high stability of the resulting nanoparticles but also for the potential to control key particle properties (size, composition, structure, morphology, etc.). In this review, we summarize the trends and advances on exsolution of bimetallic systems and provide prospects for future studies in this field.

Glycerol, a by-product of biodiesel industry, could be utilized for the production of hydrogen via steam reforming. In the present work, three Ru-based catalysts, namely Ru/Al2O3, Ru/B2O3-Al2O3 and Ru/MgO-Al2O3, have been studied regarding their physicochemical and catalytic properties for the title reaction. N-2 adsorption desorption, powder XRD, CO Adsorption, NH3-TPD, CO2-TPD, TPR, TEM and SEM-EDS were employed to investigate the textural and structural characteristics of the catalysts. Quantitative assessment of the carbonaceous deposits, formed under reaction conditions, was performed using TPH and TPO methods The catalytic performance of the synthesized catalysts for the glycerol steam reforming reaction has been investigated in a fixed bed plug flow reactor using a feed of water/glycerol at a molar ratio of 20:1 at 400, 500 and 600 degrees C. Results show that at the low reaction temperature (400 degrees C), the support has a determining role on the catalytic performance, the selectivity to various oxygenates depending on the acid-base properties of the catalyst. At higher temperature (600 degrees C) metal activity dominates, H-2 and CO2 being the main reaction products.

•Catalytic systems for the reformation of glycerol and production of renewable hydrogen.•Modification of Ni catalysts to improve their activity at low temperatures and enhance their resistance to deactivation.•Type of products defined by the support at low temperatures.•Utilization and valorization of biodiesel by-product as hydrogen and chemicals source. Glycerol, a by-product of biodiesel industry, could be utilized for the production of synthesis gas or hydrogen via steam reforming. Ni/Al2O3 catalysts are efficient for this process but could be improved regarding their activity at low temperatures, selectivity toward hydrogen production and stability with time-on stream. In the present work, the effects of addition of Β2Ο3 and La2O3 on Ni/Al2O3, on the physicochemical characteristics and catalytic performance are investigated. N2 adsorption–desorption, XRD, UV–vis DRS, TPR, NH3-TPD and HR-TEM were employed for the evaluation of the textural and structural properties of the catalysts, while quantitative assessment of the carbonaceous deposits was performed using TPH-TPO. Catalytic behavior was investigated in the temperature range of 400–800°C. In the absence of metal, the support presents considerable activity for the dehydration reactions and influences selectivity to oxygenate products. The catalytic behavior of the bare carriers seems to be dictated by their surface acidity. The presence of Ni enhances significantly catalytic activity and promotes the production of gaseous products, mainly carbon oxides and hydrogen. Conversion to gas-phase products and hydrogen yield are enhanced by the addition of La2O3 to the support while the opposite is observed upon addition of B2O3. These differences are more pronounced at lower temperatures. Lower amount of graphitic carbon was formed on NiLaAl at all temperatures. However, this catalyst has not been proven more stable than the unmodified NiAl, mostly due to its poorer textural properties.

Supported nanoparticle systems have received increased attention over the last decades because of their potential for high activity levels when applied to chemical conversions, although, because of their nanoscale nature, they tend to exhibit problems with long-term durability. Over the last decade, the discovery of the so-called exsolution concept has addressed many of these challenges and opened many other opportunities to material design by providing a relatively simple, single-step, synthetic pathway to produce supported nanoparticles that combine high stability against agglomeration and poisoning with high activity across multiple areas of application. Here, the trends that define the development of the exsolution concept are reviewed in terms of design, functionality, tunability, and applicability. To support this, the number of studies dedicated to both fundamental and application-related studies, as well as the types of metallic nanoparticles and host or support lattices employed, are examined. Exciting future directions of research are also highlighted.

Exsolution of surface and bulk nanoparticles in perovskites has been recently employed in chemical looping methane partial oxidation because of the emergent materials' properties such as oxygen capacity, redox stability, durability, coke resistance and enhanced activity. Here we attempt to further lower the temperature of methane conversion by complementing exsolution with infiltration. We prepare an endo/exo-particle system using exsolution and infiltrate it with minimal amount of Rh (0.1 wt%) in order to functionalize the surface and induce low temperature activity. We achieve a temperature decrease by almost 220 degrees C and an increase of the activity up to 40%. We also show that the initial microstructure of the perovskite plays a key role in controlling nanoparticle anchorage and carbon deposition. Our results demonstrate that microstructure tuning and surface functionalization are important aspects to consider when designing materials for redox cycling applications.

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. Das kontrollierte Wachstum von metallischen Nanopartikeln an der Oberfläche und im Volumen von Perowskitoxiden induziert eine Belastungsspannung und fördert den Sauerstoffaustausch mit einem Methanstrom. Das Verfahren bietet einen einfachen Zugang zur Produktion von Synthesegas.

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.

Catalysts with active phase Ni, Co or Cu supported on γ-alumina were synthesized at constant loading (8wt.%) and tested for the glycerol steam reforming reaction (GSR). The synthesized samples, at their calcined and/or their reduced form, were characterized by BET, ICP, XRD, DRS, NH3-TPD, CO2-TPD, TPR and SEM. The carbon deposited on their surface under reaction conditions was characterized by TEM, TPO, TGA and Raman. Catalytic performance for the glycerol steam reforming reaction was studied in order to investigate the effects of reaction temperature on: (i) glycerol total conversion, (ii) glycerol conversion to gaseous products, (iii) hydrogen selectivity and yield, (iv) selectivity of carbonaceous gaseous products, (v) selectivity of liquid products and (vi) molar ratios of H2/CO and CO/CO2 in the gaseous products' mixture. The stability of all catalysts was also investigated through time on stream experiments. It was concluded that catalytic performance, including liquid products' distribution, depends on the acid-base properties of the materials. Specifically, a drastic drop in the activity of the Ni/Al catalyst was observed, while Co/Al and Cu/Al catalysts deactivate in a slower rate, confirming that coke deposition, associated with dehydration, cracking and polymerization reactions, takes place on the catalyst's surface strong acid sites. Reaction pathway for the glycerol steam reforming reaction. [Display omitted] •Catalysts Ni, Co, and Cu (8wt.%) on γ-alumina tested for the glycerol steam reforming reaction•The presence of aluminate structures in calcined samples were confirmed by XRD, DRS and TPR•Liquid products distribution defined by the catalyst's acid-base properties•Coke deposition that resulted in deactivation takes place on the surface strong acid sites.•Valorization of biodiesel by-product for the production of renewable hydrogen.

Lowering the temperature at which CH(4)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 CH(4)activation processes operate at temperatures between 600 and 900 degrees 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 degrees 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.

Carbon dioxide and steam solid oxide co-electrolysis is a key technology for exploiting renewable electricity to generate syngas feedstock for the Fischer-Tropsch synthesis. The integration of this process with methane partial oxidation in a single cell can eliminate or even reverse the electrical power demands of co-electrolysis, while simultaneously producing syngas at industrially attractive H-2/CO ratios. Nevertheless, this system is rather complex and requires catalytically active and coke tolerant electrodes. Here, we report on a low-substitution rhodium-titanate perovskite (La0.43Ca0.32Rh0.06Ti094O3) electrode for the process, capable of exsolving high Rh nanoparticle populations, and assembled in a symmetrical solid oxide cell configuration. By introducing dry methane to the anode compartment, the electricity demands are impressively decreased, even allowing syngas and electricity cogeneration. To provide further insight on the Rh nanoparticles role on methane-to-syngas conversion, we adjusted their size and population by altering the reduction temperature of the perovskite. Our results exemplify how the exsolution concept can be employed to efficiently exploit noble metals for activating low-reactivity greenhouse gases in challenging energy-related applications.

The growing demand for H2 and syngas requires the development of new, more efficient processes and materials for their production, especially from CH4 that is a widely available resource. One process that has recently received increased attention is chemical looping CH4 partial oxidation, which, however, poses stringent requirements on material design, including fast oxygen exchange and high storage capacity, high reactivity toward CH4 activation, and resistance to carbon deposition, often only met by composite materials. Here we design a catalytically active material for this process, on the basis of exsolution from a porous titanate. The exsolved Ni particles act as both oxygen storage centers and as active sites for CH4 conversion under redox conditions. We control the extent of exsolution, particle size, and population of Ni particles in order to tune the oxygen capacity, reactivity, and stability of the system and, at the same time, obtain insights into parameters affecting and controlling exsolution.

The hydrogenation of CO2 to methanol has been investigated over CuO/ZnO/Al2O3 (CZA) catalysts, where a part of the Al2O3 (0, 25, 50, 75, or 100%) was substituted by La2O3. Results of catalytic performance tests obtained at atmospheric pressure showed that the addition of La2O3 generally resulted in a decrease of CO2 conversion and in an increase of methanol selectivity. Optimal results were obtained for the CZA-La50 catalyst, which exhibited a 30% higher yield of methanol, compared to the un-promoted sample. This was attributed to the relatively high specific surface area and porosity of this material, the creation of basic sites of moderate strength, which enhance adsorption of CO2 and intermediates that favor hydrogenation steps, and the ability of the catalyst to maintain a large part of the copper in its metallic form under reaction conditions. The reaction mechanism was studied with the use of in situ infrared spectroscopy (DRIFTS). It was found that the reaction proceeded with the intermediate formation of surface formate and methoxy species and that both methanol and CO were mainly produced via a common formate intermediate species. The kinetic behavior of the best performing CZA-La50 catalyst was investigated in the temperature range 190-230 degrees C as a function of the partial pressures of H-2 (0.3-0.9 atm) and CO2 (0.05-0.20 atm), and a kinetic model was developed, which described the measured reaction rates satisfactorily.

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 approximate to 19-22 nm with very high number densities thus creating a highly active catalytic material for CO oxidation which rivals traditionally created exsolved samples.

Innovating technologies to efficiently reduce carbon dioxide (CO2) emission or covert it into useful products has never been more crucial in light of the urgent need to transition to a net-zero economy by 2050. The design of efficient catalysts that can make the above a viable solution is of essence. Many noble metal catalysts already display high activity, but are usually expensive. Thus alternative methods for their production are necessary to ensure more efficient use of noble metals. Exsolution has been shown to be an approach to produce strained nanoparticles, stable against agglomeration while displaying enhanced activity. Here we explore the effect of a low level of substitution of Ni into a Rh based A-site deficient titanate aiming to investigate the formation of more efficient, low loading noble metal catalysts. We show that this design principle not only fulfils a major research need in the conversion of CO2 but also provides a step-change advancement in the design and synthesis of tandem catalysts by the formation of distinct catalytically active sites.    

The aim of the work was to investigate the influence of support on the catalytic performance of Ni catalysts for the glycerol steam reforming reaction. Nickel catalysts (8 wt%) supported on Al2O3, ZrO2, SiO2 were prepared by the wet impregnation technique. The catalysts’ surface and bulk properties, at their calcined, reduced and used forms, were determined by ICP, BET, XRD, NH3-TPD, CO2-TPD, TPR, XPS, TEM, TPO, Raman, SEM techniques. The Ni/Si sample, even if it was less active for T