On the Potential of Gallium- and Indium-Based Liquid Metal Membranes for Hydrogen Separation

The concept of liquid metal membranes for hydrogen separation, based on gallium or indium, was recently introduced as an alternative to conventional palladium-based membranes. The potential of this class of gas separation materials was mainly attributed to the promise of higher hydrogen diffusivity. The postulated improvements are only beneficial to the flux if diffusion through the membrane is the rate-determining step in the permeation sequence. Whilst this is a valid assumption for hydrogen transport through palladium-based membranes, the relatively low adsorption energy of hydrogen on both liquid metals suggests that other phenomena may be relevant. In the current study, a microkinetic modeling approach is used to enable simulations based on a five-step permeation mechanism. The calculation results show that for the liquid metal membranes, the flux is limited by the dissociative adsorption over a large temperature range, and that the membrane flux is expected to be orders of magnitude lower compared to the membrane flux through pure palladium membranes. Even when accounting for the lower cost of the liquid metals compared to palladium, the latter still outperforms both gallium and indium in all realistic scenarios, in part due to the practical difficulties associated with making liquid metal thin films.

Plasma-catalytic NOx production in a three-level coupled rotating electrodes air plasma combined with nano-sized TiO2

A novel three-level coupled rotating electrodes air plasma with nano-sized TiO2 photocatalysts is developed for plasma-catalytic NOx production. The effects of plasma catalysis on NOx production with different air flow rates, different N2 fractions and different humidity levels are evaluated. Final results show that the exceptionally synergistic effect between TiO2 and three-level coupled rotating electrodes air plasma significantly increases the NOx concentration by 68.32% (from 4952 to 8335 ppm) and reduces the energy cost by 40.55% (from 2.91 to 1.73 MJ mol-1) at an air flow rate of 12 l min-1 and relative humidity level of 12%, which beats the ideal thermodynamic energy limit ~2.5MJ/mol for the thermal gas-phase process. A possible mechanism for enhanced NOx production with TiO2 is discussed: Highly energetic electrons in plasma contribute to the formations of the electron–hole pairs and oxygen vacancy (Vo) on the TiO2 catalyst surface, it may facilitate the dissociative adsorption of O2 molecules to form superoxide radical groups (like O2.-), and H2O molecules to form surface hydroxyl groups (like OH.), and thus, improving energy efficiency.

Defect Formation, T-Atom Substitution and Adsorption of Guest Molecules in MSE-Type Zeolite Framework—DFT Modeling

We used computational modeling, based on Density Functional Theory, to help understand the preference for the formation of silanol nests and the substitution of Si by Ti or Al in different crystallographic positions of the MSE-type framework. All these processes were found to be energetically favorable by more than 100 kJ/mol. We suggested an approach for experimental identification of the T atom position in Ti-MCM-68 zeolite via simulation of infrared spectra of pyridine and acetonitrile adsorption at Ti. The modeling of adsorption of hydrogen peroxide at Ti center in the framework has shown that the molecular adsorption was preferred over the dissociative adsorption by 20 to 40 kJ/mol in the presence or absence of neighboring T-atom vacancy, respectively.

Water Splitting on Multifaceted SrTiO3 Nanocrystals: Computational Study

Recent experimental findings suggest that strontium titanate SrTiO3 (STO) photocatalytic activity for water splitting could be improved by creating multifaceted nanoparticles. To understand the underlying mechanisms and energetics, the model for faceted nanoparticles was created. The multifaceted nanoparticles’ surface is considered by us as a combination of flat and “stepped” facets. Ab initio calculations of the adsorption of water and oxygen evolution reaction (OER) intermediates were performed. Our findings suggest that the “slope” part of the step showed a natural similarity to the flat surface, whereas the “ridge” part exhibited significantly different adsorption configurations. On the “slope” region, both molecular and dissociative adsorption modes were possible, whereas on the “ridge”, only dissociative adsorption was observed. Water adsorption energies on the “ridge” ( −1.50 eV) were significantly higher than on the “slope” ( −0.76 eV molecular; −0.83 eV dissociative) or flat surface ( −0.79 eV molecular; −1.09 eV dissociative).

Tuning the selectivity of catalytic nitriles hydrogenation by structure regulation in atomically dispersed Pd catalysts

The product selectivity in catalytic hydrogenation of nitriles is strongly correlated with the structure of the catalyst. In this work, two types of atomically dispersed Pd species stabilized on the defect-rich nanodiamond-graphene (ND@G) hybrid support: single Pd atoms (Pd1/ND@G) and fully exposed Pd clusters with average three Pd atoms (Pdn/ND@G), were fabricated. The two catalysts show distinct difference in the catalytic transfer hydrogenation of nitriles. The Pd1/ND@G catalyst preferentially generates secondary amines (Turnover frequency (TOF@333 K 709 h−1, selectivity >98%), while the Pdn/ND@G catalyst exhibits high selectivity towards primary amines (TOF@313 K 543 h−1, selectivity >98%) under mild reaction conditions. Detailed characterizations and density functional theory (DFT) calculations show that the structure of atomically dispersed Pd catalysts governs the dissociative adsorption pattern of H2 and also the hydrogenation pathway of the benzylideneimine (BI) intermediate, resulting in different product selectivity over Pd1/ND@G and Pdn/ND@G, respectively. The structure-performance relationship established over atomically dispersed Pd catalysts provides valuable insights for designing catalysts with tunable selectivity.

2-step reaction kinetics for hydrogen absorption into bulk material via dissociative adsorption on the surface

We have demonstrated that the process of hydrogen absorption into a solid experimentally follows a Langmuir-type (hyperbolic) function instead of Sieverts law. This can be explained by independent two theories. One is the well-known solubility theory which is the basis of Sieverts law. It explains that the amount of hydrogen absorption can be expressed as a Langmuir-type (hyperbolic) function of the square root of the hydrogen pressure. We have succeeded in drawing the same conclusion from the other theory. It is a 2-step reaction kinetics (2sRK) model that expresses absorption into the bulk via adsorption on the surface. The 2sRK model has an advantage to the solubility theory: Since it can describe the dynamic process, it can be used to discuss both the amount of hydrogen absorption and the absorption rate. Some phenomena with absorption via adsorption can be understood in a unified manner by the 2sRK model.

Associative and Dissociative Adsorption of N2O Molecule Over C24 Fullerene Doped With V, Mn, Ru, Pd and Rh: A DFT Investigation

DFT calculations using the PBE-D3 level of theory were conducted on the C24 and MC23 (M = V, Mn, Ru, Pd, Rh and Au) clusters in order to determine their stability and electronic and magnetic properties. The interaction of the N2O molecule with the above clusters has also been examined in order to assess their adsorption and catalytic properties. The results show that the reactivity of the C24 fullerene was greatly improved after doping with a TM atom, and the metal atom which was replaced the carbon atom in the pure C24 fullerene is considered as the most favorable site to adsorb and dissociate the N2O molecule. The molecular adsorption of the N2O on the surface of the MC23 clusters from its oxygen atom was found to be less energetically favorable than by nitrogen atom, and the calculated adsorption energies (Eads) are in the range of - 6.5 to - 24.7 kcal mol-1. The N2O adsorption on the metal site of the clusters could also lead to the dissociation of the N2O into O* and N2 molecule with adsorption energies which vary from - 2.3 to - 86.8 kcal mol-1. The mechanism of the N2O dissociation on the clusters surface was studied, and the activation energies for each system were calculated in order to find the most catalytically active clusters in the N2O decomposition. The results indicate that the activation energies of the VC23 and MnC23 clusters are much lower than those obtained for the other clusters, reflecting high catalytic activity of the two clusters in the N2O decomposition reaction compared to the other studied clusters. The values of Eac of the reaction over the VC23 and MnC23 cluster are 9.3 and 7.9 kcal mol-1, respectively, indicating that both clusters could be employed as greatly active and efficient nanocatalysts for the N2O decomposition reaction.