Understanding and controlling the stability and reactivity of noble metal nanoparticles for CO oxidation

Ph. D. Thesis.Heterogeneous catalysts comprising of noble metal nanoparticles (such as Pt, Pd and Rh) supported on oxides, play a key role in a wide range of important chemical transformations including automotive exhaust control. However, due to the increasing demand for these catalysts and scarce resources of noble metals, there is a pressing need to reduce the consumption of noble metals in catalysts. Sintering of nanoparticles can cause catalyst deactivation, hence stabilizing nanoparticles can be a solution to reduce the loss of noble metals. Moreover, by improving the catalytic activity of noble metal nanoparticles, the amount of noble metals in catalysts can be reduced while still maintaining the required catalytic performance. Therefore, this thesis focuses on some novel nanostructures of noble metal catalysts that can potentially lead to enhanced stability and improved activity, in order to use noble metals more efficiently. CO oxidation was chosen as the model reaction in this study, because of its importance in automotive exhaust control. Interactions between metal nanoparticles and the support can have big influences on particle stability, as it was demonstrated in this thesis that weak particle-support interactions would lead to nanoparticle sintering under reaction conditions and hence destroy the dedicatedly designed nanostructures. In addition, the particle-support interactions may also bring some emergent functionalities that can affect the catalytic activity. Hence, different approaches were attempted to enhance the particle-support interactions, and their effects on the stability and activity of the catalysts were investigated. In the first approach, noble metal nanoparticles (Pd) were enclosed into porous organic cages (POCs, a class of hollow, cage-like macromolecules). The POCs were able to confine the nanoparticles, which resulted in a uniform particle size distribution. However, the limited accessibility of active sites in POCs and the thermal decomposition of the POC support (~300 °C) largely restricted the activity of the resulted catalysts hence their practical applications. The alternative approach was to partially embed (socket) noble metal nanoparticles in perovskite oxides via redox exsolution method. This has been previously demonstrated to produce highly stable transition metal nanoparticles. For the first time this thesis investigates the in situ formation of the socketed particles while at the same time providing valuable mechanistic insight for designing more efficient exsolved materials. Experiments have been conducted in situ in a latest generation environmental transmission electron microscope (ETEM) which allowed for the direct observation of the socket formation, metal particle nucleation and growth. The socket was found to form simultaneously with the particle growth due to the rise of perovskite lattice around particles. The particle growth seemed to be limited by the availability of exsolvable ions near the perovskite surface, which highlighted the importance to reduce the perovskite grain size when attempting to exsolve from dilute compositions. All the above mechanistic insight was employed to design materials that can exsolve from dilute substitution of noble metals thus potentially allowing for more efficient use of noble metals. Parameters such as substitution levels and reduction time and temperature were used to control exsolved particle characteristics and relate them to the catalytic activity. By comparing with the state-of-the-art Rh catalyst, the exsolved Rh catalyst with the same nominal metal loading exhibited similar activity despite that only a part of Rh in the bulk of perovskite exsolved on the surface. This indicates that the activity of exsolved catalysts can be enhanced probably due to the emergent functionalities that arise from the strained particles, which could potentially reduce the amount of noble metals in catalysts if the extent of exsolution can be increased. This thesis highlights the following design principles for noble metal catalysts. The stability of metal nanoparticles on the support must be high enough to maintain the designed nanostructures. That means that we need to have stronger particle-support interactions. Attempting to do this by full encapsulation was successful but compromised activity. Therefore, a partial embedding via the exsolution method results in a combined stabilizing effect and increased activity due to strain. Ultimately, this appeared to be the most promising method for designing efficient noble metal catalysts.CEA

Time Crystal in a Single-mode Nonlinear Cavity

Time crystal is a class of non-equilibrium phase with broken time-translational symmetry. Here we demonstrate the time crystal in a single-model nonlinear cavity. The time crystal originates from the self-oscillation induced by the linear gain and is stabilized by the nonlinear damping. We show this time crystal model exhibits four different characteristics: the emergence of classical limit cycle under the mean-field approximation, the dissipative gap closing in the thermodynamic limit, the quantum oscillation in the Husimi function, and the emergence of quantum limit cycle in the steady state. These properties provide a complete description of the time crystal and thus pave the way to investigate the time crystal in nonlinear systems

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

Preparation Method of Co 3

Co3O4 nanoparticles were fabricated by a novel, facile, and environment-friendly carbon-assisted method using degreasing cotton. Structural and morphological characterizations were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The component of the sample obtained at different temperatures was measured by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Nitrogen adsorption and desorption isotherms were utilized to reveal the specific surface areas. The formation mechanism of Co3O4 nanoparticles was also proposed, demonstrating that the additive degreasing cotton played an indispensable role in the process of synthesizing the sample. The resultant Co3O4 sample calcined at 600°C exhibited superior electrochemical performance with better specific capacitance and long-term cycling life, due to its high specific surface areas and pores structures. Additionally, it has been proved that this facile synthetic strategy can be extended to produce other metal oxide materials (e.g., Fe3O4). As a consequence, the carbon-assisted method using degreasing cotton accompanied a promising prospect for practical application

Trends and prospects of bimetallic exsolution

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

Emergence and future of exsolved materials

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

STW-MD: A Novel Spatio-Temporal Weighting and Multi-Step Decision Tree Method for Considering Spatial Heterogeneity in Brain Gene Expression Data

Motivation: Gene expression during brain development or abnormal development is a biological process that is highly dynamic in spatio and temporal. Due to the lack of comprehensive integration of spatial and temporal dimensions of brain gene expression data, previous studies have mainly focused on individual brain regions or a certain developmental stage. Our motivation is to address this gap by incorporating spatio-temporal information to gain a more complete understanding of the mechanisms underlying brain development or disorders associated with abnormal brain development, such as Alzheimer's disease (AD), and to identify potential determinants of response. Results: In this study, we propose a novel two-step framework based on spatial-temporal information weighting and multi-step decision trees. This framework can effectively exploit the spatial similarity and temporal dependence between different stages and different brain regions, and facilitate differential gene analysis in brain regions with high heterogeneity. We focus on two datasets: the AD dataset, which includes gene expression data from early, middle, and late stages, and the brain development dataset, spanning fetal development to adulthood. Our findings highlight the advantages of the proposed framework in discovering gene classes and elucidating their impact on brain development and AD progression across diverse brain regions and stages. These findings align with existing studies and provide insights into the processes of normal and abnormal brain development. Availability: The code of STW-MD is available at https://github.com/tsnm1/STW-MD.Comment: 11 pages, 6 figure

Chemically functionalised suspended-core fibre for ammonia gas detection

An optical fibre ammonia gas sensor utilising a functionalised four-leaf-clover-shaped suspended-core fibre (SCF) is demonstrated. The fibre is functionalised by depositing a thin layer of an ammonia sensitive dye (tetraphenylporphyrin tetrasulfonic acid hydrate) on the wall of the inner cavities of the SCF through capillary action. An in-line fibre sensing structure is designed for light transmission through the SCF and also allows gas exchange with the atmosphere through two porous polyethylene housings. The sensing structure exhibits a temperature-dependent transmission due to the thermal expansion of the housings. A ratiometric method is applied for signal processing which is demonstrated to reduce significantly the effect of temperature change in the tested range (20-30 C). The sensor is tested in ammonia concentration ranges from 0-10 ppm and demonstrates capability of detecting ammonia at ppb levels. The minimum tested concentration is 150 ppb in the experiment with a calculated limit of detection of 20 ppb. The sensor response time (T10-90%) is 160 s and it is demonstrated to be reusable after treatment with hydrogen chloride vapour. Numerical simulation of evanescent absorption features of such an SCF indicates that approximately 0.02% of the optical power exists in the air holes for the fundamental propagation mode. Distinctive absorption bands observed in the transmission spectrum of the fabricated sensor can also be observed in the simulated model after adding the TPPS absorption layer in the holes