Stable Heterometallic Cluster-Based Organic Framework Catalysts for Artificial Photosynthesis.

A series of stable heterometallic Fe2 M cluster-based MOFs (NNU-31-M, M=Co, Ni, Zn) photocatalysts are presented. They can achieve the overall conversion of CO2 and H2 O into HCOOH and O2 without the assistance of additional sacrificial agent and photosensitizer. The heterometallic cluster units and photosensitive ligands excited by visible light generate separated electrons and holes. Then, low-valent metal M accepts electrons to reduce CO2 , and high-valent Fe uses holes to oxidize H2 O. This is the first MOF photocatalyst system to finish artificial photosynthetic full reaction. It is noted that NNU-31-Zn exhibits the highest HCOOH yield of 26.3 µmol g-1 h-1 (selectivity of ca. 100 %). Furthermore, the DFT calculations based on crystal structures demonstrate the photocatalytic reaction mechanism. This work proposes a new strategy for how to design crystalline photocatalyst to realize artificial photosynthetic overall reaction.

Ionic bonding of lanthanides, as influenced by d- and f-atomic orbitals, by core-shells and by relativity.

Lanthanide trihalide molecules LnX3 (X = F, Cl, Br, I) were quantum chemically investigated, in particular detail for Ln = Lu (lutetium). We applied density functional theory (DFT) at the nonrelativistic and scalar and SO-coupled relativistic levels, and also the ab initio coupled cluster approach. The chemically active electron shells of the lanthanide atoms comprise the 5d and 6s (and 6p) valence atomic orbitals (AO) and also the filled inner 4f semivalence and outer 5p semicore shells. Four different frozen-core approximations for Lu were compared: the (1s(2) -4d(10) ) [Pd] medium core, the [Pd+5s(2) 5p(6) = Xe] and [Pd+4f(14) ] large cores, and the [Pd+4f(14) +5s(2) 5p(6) ] very large core. The errors of LuX bonding are more serious on freezing the 5p(6) shell than the 4f(14) shell, more serious upon core-freezing than on the effective-core-potential approximation. The LnX distances correlate linearly with the AO radii of the ionic outer shells, Ln(3+) -5p(6) and X(-) -np(6) , characteristic for dominantly ionic Ln(3+) -X(-) binding. The heavier halogen atoms also bind covalently with the Ln-5d shell. Scalar relativistic effects contract and destabilize the LuX bonds, spin orbit coupling hardly affects the geometries but the bond energies, owing to SO effects in the free atoms. The relativistic changes of bond energy BE, bond length Re , bond force k, and bond stretching frequency vs do not follow the simple rules of Badger and Gordy (Re ∼BE∼k∼vs ). The so-called degeneracy-driven covalence, meaning strong mixing of accidentally near-degenerate, nearly nonoverlapping AOs without BE contribution is critically discussed.

Does the 4f-shell contribute to bonding in tetravalent lanthanide halides?

Lanthanide tetrahalide molecules LnX4 (Ln = Ce, Pr, Tb; X = F, Cl, Br, I) have been investigated by density functional theory at the levels of the relativistic Zero Order Regular Approximation and the relativistic energy-consistent pseudopotentials, using frozen small- and medium-cores. The calculated bond lengths and vibrational frequencies are close to the experimental data. Our calculations indicate 4f shell contributions to bonding in LnX4, in particular for the early lanthanides, which show significant overlap between the Ln 4f-shell and the halogen np-shells. The 4f shells contribute to Ln-X bonding in LnX4 about one third more than in LnX3.

On structure and bonding of lanthanoid trifluorides LnF3 (Ln = La to Lu).

The trends in the series of lanthanoid (lanthanide) trifluoride molecules LnF3 (Ln = La to Lu) are governed by the valence-active Ln(4f,5d,5p,6s) shells. The series is investigated by quasi-relativistic density functional theory at both the scalar and spin-orbit-coupled levels. Integrating many of the previous experimental and theoretical deductions, we obtain the following comprehensive picture: (1) The comparatively small Ln-F bond length contraction of 14 pm from La to Lu is rather smooth but weakly modulated by spin-orbit coupling. (2) From La to Lu the floppy structure becomes more quasi-planar. (3) The heterolytic LnF bond energies (⅓LnF3→⅓Ln(3+) + F(-)) at the spin-orbit averaged level increase smoothly from 15.3 to 16.3 eV for La to Lu, only the 'divalent' lanthanoids Eu and Yb are outliers with 0.2 eV higher bond energies. (4) The homolytic LnF bond energies (⅓LnF3→⅓Ln + F) however show an overall W-shaped double-periodicity with maxima for LaF3, GdF3 and LuF3, decreasing from La to Eu and from Gd to Yb, the large individual variations being caused by different spin-orbit coupling and Coulomb interaction effects in Ln(0) and LnF3. (5) The Ln-F interaction is basically ionic (increasing with decreasing ionic radii) with some dative Ln(3+)← F(-) bonding. (6) The latter is of the Ln(5d)-F(2p) type with a rather constant bond order from La to Lu, with small Ln(5p) and very small Ln(4f) semi-core contributions decreasing from La to Lu. All these trends are rationalized.

Spiderweb-inspired all-weather CoS quantum dots confined in N-doped carbon for boosted sulfate radical evolution.

Inspired by the working principle of natural spiderweb and long-persistence phosphors, we have synthesized a spiderweb-like nanocomposite in which CoS quantum dots are confined in N-doped carbon frameworks/carbon nanotubes (CNTs). The intimate combination of three-dimensional conductive networks of CoS/CNTs with abundant active sites allows effective capture of sulfate radicals via both physical confinement and chemical bonding and accelerates the redox kinetics significantly. Furthermore, in virtue of the light storing and luminescence behaviors of long-persistence phosphors, the all-weather CoS/CNTs produced can realize an optimum degradation efficiency of 64% under dark conditions. Overall, this work reveals a significant step forward for building a desirable all-weather catalyst with abundant active sites for potential use in degradation under dark conditions.

Identification of the activity source of CO2 electroreduction by strategic catalytic site distribution in stable supramolecular structure system.

Identification of the real catalytic site in CO2 reduction reaction (CO2RR) is critical for the rational design of catalysts and the understanding of reactive mechanisms. In this study, the catalytic activity of pyridine-containing materials was for the first time structurally demonstrated in CO2RR by crystalline supramolecular coordination compounds model system. The system consists of three stable supramolecular coordination compounds (Ni-TPYP, Ni-TPYP-1 and Ni-TPP) with different numbers (4, 2 and 0) of active pyridine groups (i.e. uncoordinated pyridine nitrogen atoms). The electrocatalytic test results show that with the decrease of the number of active pyridine groups, the CO2RR performance is gradually reduced, mainly showing the reduction of highest FECO (99.8%, 83.7% and 25.6%, respectively). The crystallographic, experimental and theoretical evidences prove that the CO2RR activity is more likely derived from uncoordinated pyridine nitrogen than the electrocatalytic inert metal nickel in porphyrin center. This work serves as an important case study for the identification of electrocatalytic activity of pyridine-containing materials in CO2RR by simple supramolecular model system.

Effects of acid ionization on the formation mechanism of bimodal mesoporous Al-MCM-41s from coal gasification fine residue and evaluation of adsorption capabilities.

The development of synthetic methods to obtain high value-added mesoporous Al-MCM-41 from a low-cost silicon-aluminum source with low toxicity is an active research topic in solid waste resource utilization. In particular, the controlled synthesis of MCM-41 with a two-level pore distribution is a challenging task. In this work, the synthesis of unimodal and bimodal mesoporous Al-MCM-41s was achieved using acids with different degrees of ionization from coal gasification fine residue (CGFR) as bulk solid waste generated by the coal gasification process. We determined that the degree of acid ionization affected the self-assembly of inorganic/organic species as well as condensation processes, resulting in some changes of the hexagonal mesoscopic structure. The unimodal mesoporous Al-MCM-41 with acetic acid HAc and bimodal mesoporous Al-MCM-41s with an inorganic acid environment (HCl, HNO3 or H2SO4) could be effectively prepared in a controllable manner by the silicon and aluminum source obtained at alkali dissolution time 6 h and crystallization conditions at pH 10.5 and 383 K in 72 h. Moreover, the synthesis of Al-MCM-41-HAc with different SiO2/Al2O3 molar ratios (18-89) could also be realized by different alkali dissolution times. And alkali dissolution time (2-24 h) and the crystallization conditions (pH 4.5-11.5, temperatures 373-393 K, and time 48-96 h) also affected the formation of unimodal and bimodal mesoporous Al-MCM-41-HAc. In addition, the maximum adsorption amount onto bimodal mesoporous Al-MCM-41-H2SO4 (476.19 mg g-1 at 308 K) was larger than that onto unimodal mesoporous Al-MCM-41-HAc (243.90 mg g-1 at 303 K). The mesoporous Al-MCM-41s showed good stability.

Efficient Charge Migration in Chemically-Bonded Prussian Blue Analogue/CdS with Beaded Structure for Photocatalytic H2 Evolution.

The design of a powerful heterojunction structure and the study of the interfacial charge migration pathway at the atomic level are essential to mitigate the photocorrosion and recombination of electron-hole pairs of CdS in photocatalytic hydrogen evolution (PHE). A temperature-induced self-assembly strategy has been proposed for the syntheses of Prussian blue analogue (PBA)/CdS nanocomposites with beaded structure. The specially designed structure had evenly exposed CdS which can efficiently harvest visible light and inhibit photocorrosion; meanwhile, PBA with a large cavity provided channels for mass transfer and photocatalytic reaction centers. Remarkably, PB-Co/CdS-LT-3 exhibits a PHE rate of 57 228 µmol h-1 g-1, far exceeding that of CdS or PB-Co and comparable to those of most reported crystalline porous material-based photocatalysts. The high performances are associated with efficient charge migration from CdS to PB-Co through CN-Cd electron bridges, as revealed by the DFT calculations. This work sheds light on the exploration of heterostructure materials in efficient PHE.

Investigation on spin-flip reaction of Re + CH3CN by relativistic density functional theory.

To explore the integrated reaction mechanisms for Re atom with acetonitrile theoretically, density functional theory with zero-order regular approximation (ZORA) relativistic corrections has been employed at the BP86/TZ2P level. There have been three adiabatic potential energy surfaces in the study along sextet, quartet and doublet spin states. However, the detailed minimum energy reaction pathway altogether contains six stationary states () to (), five transition states (), and two intersystem crossings with spin inversion (marked by ⇒): (6)Re + CH3CN → η(1)-ReNCCH3 () → ⇒ η(2)-Re(NC)CH3 () → → η(3)-HRe(NCCH2) () → → CH3-ReNC () → → CH2[double bond, length as m-dash]Re(H)NC () ⇒ → CH[triple bond, length as m-dash]Re(H)2NC (). Thereinto, the lowest energy crossing points (LECP) have been determined by the DFT fractional-occupation-number (FON) approach. The first spin inversion has transferred the potential energy surfaces from high-spin sextet to the quartet intermediate () with the subsequent C-C bond breakage. The second one from the quartet to the low-spin doublet state accompanies the C-H activation, decreasing the transition barrier by 157 kJ mol(-1). The overall reaction could be exothermic by about 210 kJ mol(-1). Harmonic vibration frequencies and NBO, WBO analysis are also applied to verified the experimental observed information.