Engineering topics
Chang, Xin
Publications and source records attributed to Chang, Xin.
HJ-aggregates of donor–acceptor–donor oligomers and polymers
A vibronic exciton model is developed to account for the spectral signatures of HJ-aggregates of oligomers and polymers containing donor–acceptor–donor (DAD) repeat units. In (DAD) N π-stacks, J-aggregate-promoting intrachain interactions compete with H-aggregate promoting interchain interactions. The latter includes Coulombic coupling, which arises from “side-by-side” fragment transition dipole moments as well as intermolecular charge transfer (ICT), which is enhanced in geometries with substantial overlap between donors on one chain and acceptors on a neighboring chain. J-behavior is dominant in single (DAD)N chains with enhanced intrachain order as evidenced by an increased red-shift in the low-energy absorption band along with a heightened A 1 /A 2 peak ratio, where A 1 and A 2 are the oscillator strengths of the first two vibronic peaks in the progression sourced by the symmetric quinoidal–aromatic vibration. By contrast, the positive H-promoting interchain Coulomb interactions operative in aggregates cause the vibronic ratio to attenuate, similar to what has been established in H-aggregates of homopolymers such as P3HT. An attenuated A 1 /A 2 ratio can also be caused by H-promoting ICT which occurs when the electron and hole transfer integrals are out-of-phase. In this case, the A 1 peak is red-shifted, in contrast to conventional Kasha H-aggregates. With slight modifications, the ratio formula derived previously for P3HT aggregates is shown to apply to (DAD) N aggregates as well, allowing one to determine the effective free-exciton interchain coupling from the A 1 /A 2 ratio. Applications are made to polymers based on 2T-DPP-2T and 2T-BT-2T repeat units, where the importance of the admixture of the excited acceptor state in the lowest energy band is emphasized.
Vibronic exciton model for low bandgap donor–acceptor polymers
In this study, a vibronic exciton model is introduced to describe the excited state band structure and associated absorption spectra of low bandgap donor–acceptor conjugated polymers. The Hamiltonian is represented in a diabatic basis consisting of Frenkel-like donor and acceptor fragment excitations as well as charge-transfer (CT) excitations between neighboring fragments. States are coupled to each other through electron and hole transfer as well as Coulombically, through interacting fragment transition dipole moments. Local vibronic coupling involving the prominent aromatic-quinoidal vibrational mode, which is responsible for pronounced vibronic progressions in most conjugated oligomers and polymers, is also included. The DAD repeat unit is shown to behave like a J-aggregate trimer, driven by both the sizable in-phase electron and hole transfer integrals between donor and acceptor fragments as well as negative Coulomb coupling between donor and acceptor fragment excitations. The J-aggregate behavior is enhanced in the polymer limit through inter-repeat unit coupling, with the 0–0 vibronic peak significantly enhanced in the lowest-energy near-IR band. In addition, the radiative rate is enhanced by the number of coherently connected repeat units. The near-IR band is shown to possess roughly equal admixtures of CT and Frenkel-like excitations. Applications are made to the polymer PffBT4T-2DT, with the simulated absorption spectrum quantitatively capturing the salient features of the measured spectrum.
Propane Dehydrogenation on Single-Site [PtZn4] Intermetallic Catalysts
Propane dehydrogenation (PDH) is a commercial propylene production technology that has received much attention, but high reaction temperature results in decrease of propylene selectivity and catalyst stability. This paper describes a single-site [PtZn 4 ] catalyst by assembling atomically ordered intermetallic alloy (IMA) as a selective and ultrastable PDH catalyst. The catalyst enables more than 95% propylene selectivity from 520 to 620 oC. No obvious deactivation is observed within 160-hours test, superior to PtSn/Al 2 O 3 and state-of-the-art Pt-based catalysts. Additionally, based on in situ X-ray absorption fine-structure, X-ray photoelectron spectroscopy measurements and density functional theory calculations, we reveal that the surface [PtZn 4 ] ensembles in PtZn IMAs serve as the key active site structures, wherein the geometry-isolated and electron-rich Pt1 site in [PtZn 4 ] ensembles readily promotes the first and second C–H cleavage of propane, but inhibits further dehydrogenation of surface-bounded propylene. This significantly improves the selectivity and stability by prohibiting coke side reactions.