Atomic Insight into the Local Structure and Microenvironment of Isolated Co-Motifs in MFI Zeolite Frameworks for Propane Dehydrogenation.

Embedding metal species into zeolite frameworks can create framework-bond metal sites in a confined microenvironment. The metals sitting in the specific T sites of zeolites and their crystalline surroundings are both committed to the interaction with the reactant, participation in the activation, and transient state achievement during the whole catalytic process. Herein, we construct isolated Co-motifs into purely siliceous MFI zeolite frameworks (Co-MFI) and reveal the location and microenvironment of the isolated Co active center in the MFI zeolite framework particularly beneficial for propane dehydrogenation (PDH). The isolated Co-motif with the distorted tetrahedral structure ({(≡SiO)2Co(HO-Si≡)2}, two Co-O-Si bonds, and two pseudobridging hydroxyls (Co···OH-Si) is located at T1(7) and T3(9) sites of the MFI zeolite. DFT calculations and deuterium-labeling reactions verify that the isolated Co-motif together with the MFI microenvironment collectively promotes the PDH reaction by providing an exclusive microenvironment to preactivate C3H8, polarizing the oxygen in Co-O-Si bonds to accept H* ({(≡SiO)CoHδ- (Hδ+O-Si≡)3}), and a scaffold structure to stabilize the C3H7* intermediate. The Co-motif active center in Co-MFI goes through the dynamic evolutions and restoration in electronic states and coordination states in a continuous and repetitive way, which meets the requirements from the series of elementary steps in the PDH catalytic cycle and fulfills the successful catalysis like enzyme catalysis.

First-Principles Study of Electrocatalytically Reversible CO2 Capture on Graphene-like C3 N.

Developing advanced materials and new technologies for efficient CO2 capture and gas separation can enormously alleviate its impact on global climate change. In this study, we report a comprehensive density functional theory investigation of N2 , CH4 , H2 , and CO2 adsorption on a graphene-like C3 N monolayer. Our calculation results show that the four gas molecules are all physisorbed on the neutral C3 N monolayer. However, the interaction between CO2 and C3 N can be significantly boosted via the strategies of electrochemical methods such as introducing negative charge or applying external electric field to the system. While the adsorption of N2 , CH4 and H2 on C3 N monolayer is slightly influenced with the above strategies. Moreover, CO2 will release spontaneously from C3 N monolayer once the extra charge or electric field is removed from the system. These results demonstrate that the CO2 capture, regeneration and separation on C3 N monolayer can be controllable with the method of switching on/off the charge state or electric field during the adsorption. In addition, as a new synthesized 2D material (PNAS, 2016, 113, 7414-7419), C3 N possesses an extremely narrow band gap of 0.39 eV, which guarantees applying negative charge or electric field to it can be easily realized in experiment by electrochemical methods.

A theoretical insight into a feasible strategy for the fabrication of borophane.

Borophane, hydrogenated borophene, is an ideal material for nanoelectronic applications because of its high stability and its excellent mechanical and electronic properties. However, the fabrication of borophane has not been realized. Through a comprehensive density functional theoretical study, we propose a novel and feasible strategy for the fabrication of borophane, which is accomplished through an electrochemical method by modulating the charge that the borophene carries to activate hydrogen molecule decomposition on it. Our computational results show that by modulating the charge state of borophene, the energy barrier of H2 dissociation on it can be dramatically reduced to 0.27 eV, and the reaction is exothermic by 2.08 eV. This study demonstrates that the reaction of hydrogen decomposition on charged borophene to produce borophane is kinetically and thermodynamically feasible. In addition, the modulation of the charge state of borophene is feasible and less energy consuming due to its metallic character.

Methyl radical chemistry in non-oxidative methane activation over metal single sites.

Molybdenum supported on zeolites has been extensively studied as a catalyst for methane dehydroaromatization. Despite significant progress, the actual intermediates and particularly the first C-C bond formation have not yet been elucidated. Herein we report evolution of methyl radicals during non-oxidative methane activation over molybdenum single sites, which leads selectively to value-added chemicals. Operando X-ray absorption spectroscopy and online synchrotron vacuum ultraviolet photoionization mass spectroscopy in combination with electron microscopy and density functional theory calculations reveal the essential role of molybdenum single sites in the generation of methyl radicals and that the formation rate of methyl radicals is linearly correlated with the number of molybdenum single sites. Methyl radicals transform to ethane in the gas phase, which readily dehydrogenates to ethylene in the absence of zeolites. This is essentially similar to the reaction pathway over the previously reported SiO2 lattice-confined single site iron catalyst. However, the availability of a zeolite, either in a physical mixture or as a support, directs the subsequent reaction pathway towards aromatization within the zeolite confined pores, resulting in benzene as the dominant hydrocarbon product. The findings reveal that methyl radical chemistry could be a general feature for metal single site catalysis regardless of the support (either zeolites MCM-22 and ZSM-5 or SiO2) whereas the reaction over aggregated molybdenum carbide nanoparticles likely facilitates carbon deposition through surface C-C coupling. These findings allow furthering the fundamental insights into non-oxidative methane conversion to value-added chemicals.

Unveiling Potential Dependence in NO Electroreduction to Ammonia.

Recently, electrochemical NO reduction (eNORR) to ammonia has attracted enormous research interests due to the dual benefits in ammonia synthesis and denitrification fields. Herein, taking Ag as a model catalyst, we have developed a microkinetic model to rationalize the general selectivity trend of eNORR with varying potential, which has been observed widely in experiments, but not understood well. The model reproduces experiments well, quantitatively describing the selectivity turnover from N2O to NH3 and from NH3 to H2 with more negative potential. The first turnover of selectivity is due to the thermochemical coupling of two NO* limiting the N2O production. The second turnover is attributed to the larger transfer coefficient (ß) of HER than NH3 production. This work reveals how electrode potential regulate the selectivity of eNORR, which is also beneficial to understand the commonly increasing HER selectivity with the decrease of potential in some other electroreduction reactions such as CO2 reduction.

Mo-doped boron nitride monolayer as a promising single-atom electrocatalyst for CO2 conversion.

The design of new, efficient catalysts for the conversion of CO2 to useful fuels under mild conditions is urgent in order to reduce greenhouse gas emissions and alleviate the energy crisis. In this work, a series of transition metals (TMs), including Sc to Zn, Mo, Ru, Rh, Pd and Ag, supported on a boron nitride (BN) monolayer with boron vacancies, were investigated as electrocatalysts for the CO2 reduction reaction (CRR) using comprehensive density functional theory (DFT) calculations. The results demonstrate that a single-Mo-atom-doped boron nitride (Mo-doped BN) monolayer possesses excellent performance for converting CO2 to CH4 with a relatively low limiting potential of -0.45 V, which is lower than most catalysts for the selective production of CH4 as found in both theoretical and experimental studies. In addition, the formation of OCHO on the Mo-doped BN monolayer in the early hydrogenation steps is found to be spontaneous, which is distinct from the conventional catalysts. Mo, as a non-noble element, presents excellent catalytic performance with coordination to the BN monolayer, and is thus a promising transition metal for catalyzing CRR. This work not only provides insight into the mechanism of CRR on the single-atom catalyst (Mo-doped BN monolayer) at the atomic level, but also offers guidance in the search for appropriate earth-abundant TMs as electrochemical catalysts for the efficient conversion of CO2 to useful fuels under ambient conditions.

Electric field controlled CO2 capture and CO2/N2 separation on MoS2 monolayers.

Developing new materials and technologies for efficient CO2 capture, particularly for separation of CO2 post-combustion, will significantly reduce the CO2 concentration and its impacts on the environment. A challenge for CO2 capture is to obtain high performance adsorbents with both high selectivity and easy regeneration. Here, CO2 capture/regeneration on MoS2 monolayers controlled by turning on/off external electric fields is comprehensively investigated through a density functional theory calculation. The calculated results indicate that CO2 forms a weak interaction with MoS2 monolayers in the absence of an electric field, but strongly interacts with MoS2 monolayers when an electric field of 0.004 a.u. is applied. Moreover, the adsorbed CO2 can be released from the surface of MoS2 without any energy barrier once the electric field is turned off. Compared with the adsorption of CO2, the interactions between N2 and MoS2 are not affected significantly by the external electric fields, which indicates that MoS2 monolayers can be used as a robust absorbent for controllable capture of CO2 by applying an electric field, especially to separate CO2 from the post-combustion gas mixture where CO2 and N2 are the main components.