Husn-Ubayda Islam

Understanding the oxidation behavior of Pt nanoparticles (NPs) is crucial for developing durable and efficient catalysts. In this study, we investigate the oxidation of a realistic Pt NP, retrieved from scanning transmission electron microscopy (STEM) images. We use a multistep approach combining ReaxFF and MACE-MP-0 forcefields with Density Functional Theory (DFT) calculations. Our Monte Carlo simulations reveal high oxidation of the nanoparticle, with oxygen penetrating deep into the core. We explore the plausibility of these configurations by carrying out XRD, TEM and EXAFS measurements on samples of various average particle sizes. Progressing in our workflow, we find that 100 ns of thermostated dynamics at 350 K using the ReaxFF forcefield leads to the formation of detached Pt$_6$O$_8$ species. To explore the validity of this small platinum-oxide cluster, we first optimize the geometries using the recent MACE-MP-0 forcefield resulting in structures without the species. We then compare both forcefields to DFT calculations showing closer agreement for MACE-MP-0 compared to ReaxFF. Finally, we discuss the electronic structure of our oxidized nanoparticles spanning a whole range of oxygen coverages, finding substantial changes in the Pt-5$d$ and O-2$p$ projected density of states of the platinum structure as the coverage increases. Our findings emphasize the importance of accurately describing the potential energy surface and explicitly modeling oxygen coverage to predict catalytically relevant properties at high potentials. This study aims to provide a foundation for understanding the complex interplay between nanoparticle structure, oxidation state, and catalytic performance, aiming to guide the rational design of advanced catalytic materials.

To improve the understanding of catalysts, and ultimately the ability to design better materials, it is crucial to study them during their catalytic active states. Using in situ or operando conditions allows insights into structure–property relationships, which might not be observable by ex situ characterization. Spatially resolved X-ray fluorescence, X-ray diffraction and X-ray absorption near-edge spectroscopy are powerful tools to determine structural and electronic properties, and the spatial resolutions now achievable at hard X-ray nanoprobe beamlines make them an ideal complement to high-resolution transmission electron microscopy studies in a multi-length-scale analysis approach. The development of a system to enable the use of a commercially available gas-cell chip assembly within an X-ray nanoprobe beamline is reported here. The novel in situ capability is demonstrated by an investigation of the redox behaviour of supported Pt nanoparticles on ceria under typical lean and rich diesel-exhaust conditions; however, the system has broader application to a wide range of solid–gas reactions. In addition the setup allows complimentary in situ transmission electron microscopy and X-ray nanoprobe studies under identical conditions, with the major advantage compared with other systems that the exact same cell can be used and easily transferred between instruments. This offers the exciting possibility of studying the same particles under identical conditions (gas flow, pressure, temperature) using multiple techniques.

Small pore zeolites have shown great potential in a number of catalytic reactions. While Mo-containing medium pore zeolites have been widely studied for methane dehydroaromatisation (MDA), the use of small pore supports has drawn limited attention due to the fast deactivation of the catalyst. This work investigates the structure of the small pore Mo/H-SSZ-13 during catalyst preparation and reaction by operando X-ray absorption spectroscopy (XAS), in situ synchrotron powder diffraction (SPD), and electron microscopy; then, the results are compared with the medium pore Mo/H-ZSM-5. While SPD suggests that during catalyst preparation, part of the MoO_x anchors inside the pores, Mo dispersion and subsequent ion exchange was less effective in the small pore catalyst, resulting in the formation of mesopores and Al₂(M_OO₄)₃ particles. Unlike Mo/H-ZSM-5, part of the Mo species in Mo/H-SSZ-13 undergoes full reduction to Mo⁰ during MDA, whereas characterisation of the spent catalyst indicates that differences also exist in the nature of the formed carbon deposits. Hence, the different Mo speciation and the low performance on small pore zeolites can be attributed to mesopores formation during calcination and the ineffective ion exchange into well dispersed Mo-oxo sites. The results open the scope for the optimisation of synthetic routes to explore the potential of small pore topologies.