Zhigang Deng

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.

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.

Mechanistic Insight into Catalytic Oxidation of Ammonia on Clean, O‐ and OH‐Assisted Ir(1 1 1) Surfaces

Periodic DFT calculations have been performed to systematically investigate the catalytic mechanisms of ammonia (NH3) decomposition on clean, O‐ and OH‐assisted Ir(1 1 1) surfaces. The adsorption configurations, reaction energies and barriers, and elementary steps were elaborated. Our results show that the NHx (x=1–3) decomposition prefers to proceed by means of the O‐ and OH‐assisted reaction mechanisms, NH3+O→NH2+OH, NH2+OH→NH+H2O, and NH+OH→N+H2O, rather than the direct NHx decomposition of NH3→NH2→NH→N as a result of the high energy barriers involved. The promotion effect of the O‐ and OH‐oxidizing agents are then discussed using energy barrier analysis. The relationships between the selectivity toward the final product and coverage, O to N coverage, and reaction temperature are elucidated. Finally, we compare our results with analogous investigations of NH3 decomposition on Pt, Rh, and Ir surfaces.

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 Organic Dyes Incorporating Triphenylamine-Based Donors and Binary -Conjugated Bridges for Dye-Sensitized Solar Cells

The design of light-absorbent sensitizers with sustainable and environment-friendly material is one of the key issues for the future development of dye-sensitized solar cells (DSSCs). In this work, a series of organic sensitizers incorporating alkoxy-substituted triphenylamine (tpa) donors and binary π-conjugated bridges were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Molecular geometry, electronic structure, and optical absorption spectra are analyzed in the gas phase, chloroform, and dimethylformamide (DMF) solutions. Our results show that properly choosing the heteroaromatic atoms and/or adding one more alkoxy-substituted tpa group can finely adjust the molecular orbital energy. The solvent effect renders the HOMO-LUMO gaps of the tpa-based sensitizers decrease in the sequence of DMF solution < chloroform solution < gas phase. The absorption spectra are assigned to the ligand-to-ligand charge transfer (LLCT) characteristics via transitions mainly from tpa, 3,4-ethylenedioxythiophene (edot), and alkyl-substituted dithienosilole (dts) groups to edot, dts, and cyanoacrylic acid groups. The binary π-conjugated bridges play different roles in balancing the electron transfer and recombination for the different tpa-based sensitizers. The protonation/deprotonation effect has great effect on the HOMO-LUMO gaps and thus has great influence on the bands at the long wavelength region, but little influence on the bands at the short wavelength region.