Unravelling CO Activation on Flat and Stepped Co Surfaces: A Molecular Orbital Analysis

Structure sensitivity in heterogeneous catalysis dictates the overall activity and selectivity of a catalyst whose origins lie in the atomic configurations of the active sites. We explored the influence of the active site geometry on the dissociation activity of CO by investigating the electronic structure of CO adsorbed on 12 different Co sites and correlating its electronic structure features to the corresponding C-O dissociation barrier. By including the electronic structure analyses of CO adsorbed on step-edge sites, we expand upon the current models that primarily pertain to flat sites. The most important descriptors for activation of the C-O bond are the decrease in electron density in CO’s 1π orbital , the occupation of 2π anti-bonding orbitals and the redistribution of electrons in the 3σ orbital. The enhanced weakening of the C-O bond that occurs when CO adsorbs on sites with a step-edge motif as compared to flat sites is caused by a distancing of the 1π orbital with respect to Co. This distancing reduces the electron-electron repulsion with the Co d-band. These results deepen our understanding of the electronic phenomena that enable the breaking of a molecular bond on a metal surface.</p

Computational study of CO2 methanation on Ru/CeO2 model surfaces:On the impact of Ru doping in CeO2

The Sabatier reaction (CO 2 + H 2 → CH 4 + H 2O) can contribute to renewable energy storage by converting green H 2 with waste CO 2 into CH 4. Highly dispersed Ru on CeO 2 represents an active catalyst for the CO 2 methanation. Here, we investigated the support effect by considering a single atom of Ru and a small Ru cluster on CeO 2 (Ru 6/CeO 2). The influence of doping CeO 2 with Ru was investigated as well (Ru 6/RuCe x-1O 2x-1). Density functional theory was used to compute the reaction energy diagrams. A single Ru atom on CeO 2 can only break one of the C-O bonds in adsorbed CO 2, making it only active in the reverse water-gas shift reaction. In contrast, Ru 6 clusters on stoichiometric and Ru-doped CeO 2 are active methanation catalysts. CO is the main reaction intermediate formed via a COOH surface intermediate. Compared to an extended Ru(11-21) surface containing step-edge sites where direct C-O bond dissociation is facile, C-O dissociation proceeds via H-assisted pathways (CO → HCO → CH) on Ru 6/CeO 2 and Ru 6/RuCe x-1O 2x-1. A higher CO 2 methanation rate is predicted for Ru 6/RuCe x-1O 2x-1. Electronic structure analysis clarifies that the lower activation energy for HCO dissociation on Ru 6/RuCe x-1O 2x-1 is caused by stronger electron-electron repulsion due to its closer proximity to Ru. Strong H 2 adsorption on small Ru clusters explains the higher CO 2 methanation activity of Ru clusters on CeO 2 compared to a Ru step-edge surface, representative of Ru nanoparticles, where the H coverage is low due to stronger competition with adsorbed CO.</p

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

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

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

Ni-In Synergy in CO2Hydrogenation to Methanol

Indium oxide (In2O3) is a promising catalyst for selective CH3OH synthesis from CO2but displays insufficient activity at low reaction temperatures. By screening a range of promoters (Co, Ni, Cu, and Pd) in combination with In2O3using flame spray pyrolysis (FSP) synthesis, Ni is identified as the most suitable first-row transition-metal promoter with similar performance as Pd-In2O3. NiO-In2O3was optimized by varying the Ni/In ratio using FSP. The resulting catalysts including In2O3and NiO end members have similar high specific surface areas and morphology. The main products of CO2hydrogenation are CH3OH and CO with CH4being only observed at high NiO loading (≥75 wt %). The highest CH3OH rate (∼0.25 gMeOH/(gcath), 250 °C, and 30 bar) is obtained for a NiO loading of 6 wt %. Characterization of the as-prepared catalysts reveals a strong interaction between Ni cations and In2O3at low NiO loading (≤6 wt %). H2-TPR points to a higher surface density of oxygen vacancy (Ov) due to Ni substitution. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance analysis of the used catalysts suggest that Ni cations can be reduced to Ni as single atoms and very small clusters during CO2hydrogenation. Supportive density functional theory calculations indicate that Ni promotion of CH3OH synthesis from CO2is mainly due to low-barrier H2dissociation on the reduced Ni surface species, facilitating hydrogenation of adsorbed CO2on Ov © 2021 The Authors. Published by American Chemical Societ

Structure Sensitivity of CO2Conversion over Nickel Metal Nanoparticles Explained by Micro-Kinetics Simulations

Nickel metal nanoparticles are intensively researched for the catalytic conversion of carbon dioxide. They are commercially explored in the so-called power-to-methane application in which renewably resourced H2 reacts with CO2 to produce CH4, which is better known as the Sabatier reaction. Previous work has shown that this reaction is structure-sensitive. For instance, Ni/SiO2 catalysts reveal a maximum performance when nickel metal nanoparticles of ∼2-3 nm are used. Particularly important to a better understanding of the structure sensitivity of the Sabatier reaction over nickel-based catalysts is to understand all relevant elementary reaction steps over various nickel metal facets because this will tell as to which type of nickel facets and which elementary reaction steps are crucial for designing an efficient nickel-based methanation catalyst. In this work, we have determined by density functional theory (DFT) calculations and micro-kinetics modeling (MKM) simulations that the two terrace facets Ni(111) and Ni(100) and the stepped facet Ni(211) barely show any activity in CO2 methanation. The stepped facet Ni(110) turned out to be the most effective in CO2 methanation. Herein, it was found that the dominant kinetic route corresponds to a combination of the carbide and formate reaction pathways. It was found that the dissociation of H2CO∗ toward CH2∗ and O∗ is the most critical elementary reaction step on this Ni(110) facet. The calculated activity of a range of Wulff-constructed nickel metal nanoparticles, accounting for varying ratios of the different facets and undercoordinated atoms exposed, reveals the same trend of activity-versus-nanoparticle size, as was observed in previous experimental work from our research group, thereby providing an explanation for the structure-sensitive nature of the Sabatier reaction