Introduction to microkinetic modeling

This book started out as two separate documents. One was a set of exercises for the Advanced Thermodynamics and Catalysis course and the other was a method and theory section at that time envisioned for my PhD thesis. Only a very small part of the material in this book eventually made it into the thesis, as the whole would be much too elaborate

Effect of reaction atmosphere on catalytic CO oxidation over Cu-based bimetallic nanoclusters on a CeO2 support

Understanding the nature of active sites and the catalytic properties of oxide-supported bimetallic clusters under reaction conditions remains challenging. In this study, we combine first-principles calculations with genetic algorithm and grand canonical Monte Carlo methods to reveal the structures and compositions of CeO2-supported Cu-based bimetallic clusters in an oxygen-rich environment. Oxidized Cu4X4 (X = Pd, Pt, and Rh) bimetallic clusters on CeO2(111) are stable and exhibit different catalytic properties during CO oxidation compared with the pristine bimetallic clusters. Microkinetic simulations predict that CeO2(111)-supported Cu4Pd4O10, Cu4Pt4O11, and Cu4Rh4O14 clusters have much higher CO oxidation activity than the supported Cu4Pd4, Cu4Pt4, and Cu4Rh4 clusters; this is ascribed to the moderate CO adsorption strength and active oxygen on oxidized alloy clusters. A mechanistic study suggests that CO oxidation occurs via the O2 associative reaction mechanism on the Cu4Pd4O10 and Cu4Pt4O11 clusters, while it proceeds through the O2 dissociative reaction mechanism on the Cu4Rh4O14 cluster. Our calculations further predict that CO oxidation on the Cu4Rh4O14 cluster exhibits a low apparent activation energy, indicating that the oxidized cluster possesses excellent CO oxidation activity. This work demonstrates that the catalytic activity and reaction mechanism vary with the composition and oxidation state of the alloy nanocluster under the reaction conditions and emphasizes the influence of the reaction atmosphere on the reaction mechanisms and catalytic activity of oxide-supported alloy catalysts

A computational study of CO2 hydrogenation on single atoms of Pt, Pd, Ni and Rh on In2O3(111)

Metal promoted indium oxide (In2O3) catalysts are promising materials for CO2 hydrogenation to products such as methanol and carbon monoxide. The influence of the dispersion of the promoting metal on the methanol selectivity of In2O3 catalysts is a matter of debate, which centers around the role of atomically dispersed single metal atoms vs. metal clusters as catalysts for methanol formation. In this study, we used density functional theory calculations to compare the role of single atoms (SAs) of Ni, Pd, Pt and Rh placed on the In2O3(111) surface to study CO2 hydrogenation to CO and methanol. Direct and hydrogen-assisted CO2 dissociation pathways leading to CO as well as methanol formation via either formate or CO intermediates are explicitly considered. Microkinetic simulations show that all SA models mainly catalyze CO formation via a redox pathway involving oxygen vacancies where adsorbed CO2 dissociates followed by CO desorption and water formation. The higher barriers for hydrogenation of formate intermediates compared to the overall barrier for the rWGS reaction explain the negligible CH3OH selectivity.</p

Unraveling the Role of Metal-Support Interactions on the Structure Sensitivity of Fischer-Tropsch Synthesis

Structure sensitivity plays a pivotal role in heterogeneous catalysis and the Fischer-Tropsch reaction is one of the prime examples of such a structure-sensitive reaction. The activity and selectivity of this reaction depend on the size of the nanoparticle and this trend is observed for a whole range of support materials. To understand why metal-support interactions do not affect this trend, a ReaxFF force field is developed that effectively mimics the broad variety of support materials and captures the metal-support interaction strength into a single structural parameter. Particles of 1-9 nm embedded on support materials are sampled using simulated annealing molecular dynamics and the effect of the metal-support interaction on the active site distribution is studied. It is found that although the size-dependency profile of various active site topologies depends on the interaction strength of the nanoparticle with the support, step-edge sites with an FCC(110) motif remain insensitive to the type of support. Based on microkinetic simulations, it is established that these sites are predominantly responsible for the observed atom-based FTS activity rationalizing why Fischer-Tropsch synthesis is structure-sensitive but support-insensitive.</p

Atomistic insights into the degradation of halide perovskites: a reactive force field molecular dynamics study

Halide perovskites make efficient solar cells due to their exceptional optoelectronic properties, but suffer from several stability issues. The characterization of the degradation processes is challenging because of the limitations in the spatio-temporal resolution in experiments and the absence of efficient computational methods to study the reactive processes. Here, we present the first effort in developing reactive force fields for large scale molecular dynamics simulations of the phase instability and the defect-induced degradation reactions in inorganic CsPbI$_{3}$. We find that the phase transitions are driven by a combination of the anharmonicity of the perovskite lattice with the thermal entropy. At relatively low temperatures, the Cs cations tend to move away from the preferential positions with good contacts with the surrounding metal halide framework, potentially causing its conversion to a non-perovskite phase. Our simulations of defective structures reveal that, although both iodine vacancies and interstitials are very mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, initiating the decomposition reactions of perovskites to PbI$_{2}$. Our work puts ReaxFF forward as an effective computational framework to study reactive processes in halide perovskites.Comment: 11 pages, 6 figure

Charge transport modulation by a redox supramolecular spin-filtering chiral crystal

The chirality induced spin selectivity (CISS) effect is a fascinating phenomena correlating molecular structure with electron spin-polarisation in excited state measurements. Experimental procedures to quantify the spin-filtering magnitude relies generally on averaging data sets, especially those from magnetic field dependent conductive-AFM. We investigate the underlying observed disorder in the IV spectra and the origin of spikes superimposed. We demonstrate and explain that a dynamic, voltage sweep rate dependent, phenomena can give rise to complex IV curves for chiral crystals of coronene bisimide. The redox group, able to capture localized charge states, acts as an impurity state interfering with a continuum, giving rise to Fano resonances. We introduce a novel mechanism for the dynamic transport which might also provide insight into the role of spin-polarization. Crucially, interference between charge localisation and delocalisation during transport may be important properties into understanding the CISS phenomena