Shuxian Wei

Strategies to enhance CO2 capture and separation based on engineering absorbent materials

Uncontrolled massive CO2 emission into the atmosphere is becoming a huge threat to our global climate and environment. Carbon capture and storage (CCS), starting with the crucial step of CO2 capture and separation, provides a promising approach to alleviate this issue. The major challenge for CO2 capture and separation is exploring efficient adsorbent materials with high storage capacity and selectivity. This review firstly summarized the significant advancement in a variety of state-of-the-art adsorbent materials. Then, particular attention was focused on the practical strategies to enhance CO2 capture and separation based on current adsorbent materials by topological structure design, chemical doping, chemical functionalization, open metal sites, and electric fields. These strategies paved constructive ways for the design and synthesis of novel adsorbent materials. Finally, we gave a perspective view on future directions in this rapidly growing field.

Methanol Oxidation on Pt3Sn(111) for Direct Methanol Fuel Cells: Methanol Decomposition

PtSn alloy, which is a potential material for use in direct methanol fuel cells, can efficiently promote methanol oxidation and alleviate the CO poisoning problem. Herein, methanol decomposition on Pt3Sn(111) was systematically investigated using periodic density functional theory and microkinetic modeling. The geometries and energies of all of the involved species were analyzed, and the decomposition network was mapped out to elaborate the reaction mechanisms. Our results indicated that methanol and formaldehyde were weakly adsorbed, and the other derivatives (CHxOHy, x = 1-3, y = 0-1) were strongly adsorbed and preferred decomposition rather than desorption on Pt3Sn(111). The competitive methanol decomposition started with the initial O-H bond scission followed by successive C-H bond scissions, (i.e., CH3OH → CH3O → CH2O → CHO → CO). The Brønsted-Evans-Polanyi relations and energy barrier decomposition analyses identified the C-H and O-H bond scissions as being more competitive than the C-O bond scission. Microkinetic modeling confirmed that the vast majority of the intermediates and products from methanol decomposition would escape from the Pt3Sn(111) surface at a relatively low temperature, and the coverage of the CO residue decreased with an increase in the temperature and decrease in partial methanol pressure.

First-principles insight into the photoelectronic properties of Ge-based perovskites

The crystal configuration, electronic structure, charge-carrier transport, and optical properties of Ge-based MAGeX3 perovskites (MA = CH3NH3+; X = Cl−, Br−, and I−) and AGeI3 (A = Cs+, MA, FA (HC(NH2)2+), MO (CH3C(NH2)2+), and GA (C(NH2)3+)) were investigated using first-principles theory. The results showed that the increase in Ge–X bonds (from Cl− to I−) in MAGeX3 increased the volumes, weakened the covalent coupling of Ge–X, lowered the bandgaps, reduced the electron and hole effective masses, and red shifted the absorption spectra. Different A cations in the AGeI3 systems altered the package of perovskite crystals and thus significantly influenced the electronic and optical properties of those perovskites. Electronic property analyses revealed that the valence band maxima (VBM) of AGeI3 perovskites were mainly contributed by the I 5p and Ge 4s orbitals, whereas the conduction band minima (CBM) were dominated by Ge 4p orbitals. In AGeI3 perovskites, the bandgap increased and the absorption spectrum blue shifted in the sequence of Cs+ → MA → FA → MO → GA. Our results highlighted the effects of A and X on the photoelectronic properties of Ge-based perovskites.

CO tolerance of a Pt3Sn(111) catalyst in ethanol decomposition

CO tolerance is one of the crucial factors to protect catalysts against inactivation during ethanol decomposition processes. Herein, the intrinsic essence of CO tolerance and the effect of the alloying element Sn in Pt3Sn(111) are investigated by combining periodic density functional theory (DFT) and microkinetic modelling. It is found that the most competitive route to CO proceeds via CH3CH2OH → CH3CH2O → CH3CHO → CH3CO → CH2CO → CH2 + CO. Resulting from the decomposition of easily-oxidized CH3CO and hard-forming but readily-desorbed CH2CO, and the less competitive C–C bond scission occurring before Cα–H, Cβ–H, and O–H bond scission, the formation of CO is not facile on Pt3Sn(111). The alloying element Sn plays “bifunctional” and “ligand effect” roles to effectively strengthen O-end species adsorption, adjust the alloy electronic structures and weaken the Pt–CO bonds, thus facilitating CO elimination from Pt3Sn(111). Microkinetic modelling confirms the substantially high CO tolerance of Pt3Sn, and easy desorption of the most abundant species such as CH2CO and CH2 from the surface above room temperature. This theoretical work sheds new light on the CO tolerance of Pt3Sn(111) in ethanol decomposition and provides a fresh perspective on understanding the effect of alloying elements.

Effect of the functionalized π-bridge on porphyrin sensitizers for dye-sensitized solar cells: an in-depth analysis of electronic structure, spectrum, excitation, and intramolecular electron transfer

A series of porphyrin sensitizers for dye-sensitized solar cells (DSSCs) have been systematically investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT) in tetrahydrofuran (THF) solution. The effects of π-bridge length, heteroaromatic unit, longitudinal conjugation, and relative position of functionalized groups on the optical and electrical properties are elucidated by analyzing the geometry, electronic structure, electron excitation, spectrum, photo-induced intramolecular electron transfer (IET), and light-harvesting efficiency (LHE). Our results show that the increase in π-bridge length by the addition of phenyl groups distances the electron distribution of LUMO away from the anchoring group and sharply decreases the effective electron excitation at the long wavelength region. The introduction of heteroaromatics in the π-bridge, especially electron-deficient units, stabilizes LUMO levels and improves the light-harvesting capability and donor-to-acceptor IET characteristics significantly. The extension of longitudinal π-conjugation in the π-bridge broadens the B band and slightly strengthens and redshifts the Q band but results in undesired orbital overlap. Repositioning the phenyl/thiophene group away from carboxylic acid increases the energy gap but leads to more effective long-range IET processes with more electrons, longer distance, lower orbital overlap, and moderate transfer rate. Our results highlight the significant effect of the functionalized π-bridge on porphyrin sensitizers, and provide a new insight into the design and screening of sensitizers for DSSCs.

Decomposition mechanism of methylamine to hydrogen cyanide on Pt(111): selectivity of the C–H, N–H and C–N bond scissions

Periodic density functional theory (DFT) calculations were performed to systematically investigate the decomposition mechanism of methylamine (CH3NH2) to hydrogen cyanide (HCN) on Pt(111). The geometries and energies for all species involved are analyzed, and the decomposition network is mapped out to elaborate the reaction mechanism. Our results show that the CH3NH2, methanimine (CH2NH) and HCN prefer to desorb, while the other species prefer to decompose; the decomposition pathway prefers the successive N–H bond scissions followed by the C–H bond scissions, that is, CH3NH2 → CH3NH → CH3N → CH2N → HCN. The electronic structure and energy barrier analysis are used to identify the initial competitive scissions of C–H, N–H and C–N bonds. The interaction between fragments and surface in the TS plays a decisive role in controlling the energy barrier of initial CH3NH2 decomposition on Pt(111). Finally, the Bronsted–Evans–Polanyi (BEP) relation identifies that the C–H and N–H bond scissions stay competitive, but the C–N bond scission is not facile to occur.

Methanol oxidation on Ru(0001) for direct methanol fuel cells: analysis of the competitive reaction mechanism

The competitive oxidation reaction mechanism of methanol on the Ru(0001) surface has been investigated by periodic density functional theory (DFT). Stable adsorption configurations, elementary reaction energies and barriers, the potential energy surface (PES), and the electrochemical potential analysis were elucidated. The results showed that O–H bond activation was more competitive than C–H and C–O bond activation during the initial methanol oxidation. Competitive pathways occurred for CH3OH oxidation to CH2O via CH3OH → CH3O → CH2O versus CH3OH → CH2OH → CH2O, further to COOH via the CO pathway CH2O → CHO → CO → COOH versus the non-CO pathway CH2O → CH2OOH → CHOOH → COOH, and finally oxidation to CO2. Taking PES and the electrochemical potential analysis into account, CH3OH → CH2OH → CH2O → CH2OOH → CHOOH → COOH → CO2 appeared to be the preferred oxidation pathway. The OH group could inhibit CO formation by directly reacting with CH2O to yield CH2OOH but could not efficiently remove the CO that had already been produced by the reactions.

Initial reduction of CO2 on perfect and O-defective CeO2 (111) surfaces: towards CO or COOH?

First-principle calculations were performed to explore the initial reduction of CO2 on perfect and O-defective CeO2 (111) surfaces via direct dissociation and hydrogenation, to elucidate the product selectivity towards CO, COOH, or HCOO. The results showed that CO2 prefers a bent configuration with the C atom of CO2 occupying the oxygen vacancy site. Reductive hydrogenation CO2 + H → COOH* was more competitive than CO2 + H → HCOO* on both perfect and O-defective CeO2 (111) surfaces. Comparatively, CO2 hydrogenation towards COOH was slightly more favorable on the perfect surface, whereas reductive dissociation of CO2 was predominant on the O-defective CeO2 (111) surface. Electronic localization function, charge density difference, and density of states were utilized to analyze the effect of charge accumulation and redistribution on the adsorption and reductive dissociation of CO2 caused by the presence of O vacancies. The results of this study provided detailed insight into the initial reduction mechanisms of CO2 towards different products on perfect and O-defective CeO2 (111) surfaces.

Theoretical Insight into the Spectral Characteristics of Fe(II)-Based Complexes for Dye-Sensitized Solar Cells—Part I: Polypyridyl Ancillary Ligands

The design of light-absorbent dyes with cheaper, safer, and more sustainable materials is one of the key issues for the future development of dye-sensitized solar cells (DSSCs). We report herein a theoretical investigation on a series of polypyridyl Fe(II)-based complexes of FeL2(SCN)2, [FeL3]2

Mechanistic insight into the gas-phase reactions of methylamine with ground state Co+(3F) and Ni+(2D).

The gas-phase reaction mechanisms of methylamine (MA) with the ground-state Co(+)((3)F) and Ni(+)((2)D) are theoretically investigated using density functional theory at both the B3LYP/6-311++G(d,p) and B3LYP/6-311++G(3df,2p) levels. The reactions for hydride abstraction and dehydrogenation are analyzed in terms of the topology of potential energy surfaces (PESs). Co(+) and Ni(+) perform similar roles along the isomerization processes to the final products. Hydride abstraction takes place via the key species of metal cation-methyl-H intermediate, followed by a charge transfer process before the direct dissociation of CH(2)NH(2)(+)···MH (M = Co, Ni). The enthalpies of reaction, stability of metal cation-methyl-H species, and competition between different channels account for the sequence of the hydride abstraction products: CoH < NiH < CuH. The most competitive dehydrogenation route occurs through a stepwise reaction, consisting of initial C-H activation, amino-H shift, and direct dissociation of the precursor CH(2)NHM(+)···H(2). This theoretical work sheds new light on the experimental observations and provides fundamental understanding of the reaction mechanisms of amine prototype with late first-row transition metal cations.