Yan-Ping Ma

Partial Oxidation of Propylene Catalyzed by VO3 Clusters: A Density Functional Theory Study

Density functional theory (DFT) calculations are carried out to investigate partial oxidation of propylene over neutral VO 3 clusters. C=C bond cleavage products CH 3CHO + VO 2CH 2 and HCHO + VO 2CHCH 3 can be formed overall barrierlessly from the reaction of propylene with VO 3 at room temperature. Formation of hydrogen transfer products H 2O + VO 2C 3H 4, CH 2=CHCHO + VO 2H 2, CH 3CH 2CHO + VO 2, and (CH 3) 2CO + VO 2 is subject to tiny (0.01 eV) or small (0.06 eV, 0.19 eV) overall free energy barriers, although their formation is thermodynamically more favorable than the formation of C=C bond cleavage products. These DFT results are in agreement with recent experimental observations. VO 3 regeneration processes at room temperature are also investigated through reaction of O 2 with the CC bond cleavage products VO 2CH 2 and VO 2CHCH 3. The following barrierless reaction channels are identified: VO 2CH 2 + O 2 --> VO 3 + CH 2O; VO 2CH 2 + O 2 --> VO 3C + H 2O, VO 3C + O 2 --> VO 3 + CO 2; VO 2CHCH 3 + O 2 --> VO 3 + CH 3CHO; and VO 2CHCH 3 + O 2 --> VO 3C + CH 3OH, VO 3C + O 2 --> VO 3 + CO 2. The kinetically most favorable reaction products are CH 3CHO, H 2O, and CO 2 in the gas phase model catalytic cycles. The results parallel similar behavior in the selective oxidation of propylene over condensed phase V 2O 5/SiO 2 catalysts.

Acidification and assembly of porphyrin at an interface: counterion matching, selectivity, and supramolecular chirality.

The interfacial diprotonation and assemblies of a free-base achiral porphyrin, 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-21H,23H-porphine, on various acidic subphases were investigated. It has been shown that the compound could be diprotonated in situ on an acidic subphase and can form assemblies. The interfacially organized supramolecular assemblies were transferred onto a solid substrate, and the assemblies showed supramolecular chirality. Interestingly, the supramolecular chirality of the assemblies of the diprotonated species showed a counterion-dependent behavior. For the assemblies fabricated from the aqueous HCl subphases, a strong Cotton effect (CE) could be observed, although the porphyrin itself is achiral. When an aqueous HBr solution was used as the subphase, the assemblies showed a weak CE, whereas no CE could be detected for the assemblies formulated from the HNO3 or HI subphase. Interestingly, when a mixture of HBr and NaCl, or HNO3 and NaCl, was employed as the subphase, the formed assemblies displayed chiral features similar to those fabricated on the HCl subphase, suggesting that the Cl(-) could be preferentially visualized in terms of supramolecular chirality, although the system itself is composed of achiral species. On the basis of the experimental facts and a theoretical calculation, an explanation with regard to the different sizes of the counterions and the distinct binding affinities of the counteranions to the diprotonated porphyrin species has been proposed. Our findings provide new insights into the assembly of the diprotonated porphyrins as well as the interfacially occurring symmetry breaking.

Density functional study on cage and noncage (Fe2O3)n clusters

Both cage and noncage structures of (Fe(2)O(3))(n) (n = 2-6 and 10) clusters are studied using density functional theory. All the cage structures are stable without imaginary vibrational frequency but the global minima are the noncage clusters for most cases. Our results show that oxidation of Fe(4)O(n) (n<6) clusters by O(2) at room temperature is exothermic, while oxidation of n > or = 6 clusters is endothermic. This is in qualitative agreement with an experimental observation that only Fe(4)O(n) (+) (n > or = 6) clusters are produced in a laser vaporization source under saturated O(2) growth conditions. Since (Fe(2)O(3))(n) clusters have high stability and different structural and bonding properties from those of the bulk Fe(2)O(3), they may serve as good models for predicting or interpreting novel properties of Fe(2)O(3) nanomaterials.

Acetylene Cyclotrimerization Catalyzed by TiO2 and VO2 in the Gas Phase : A DFT Study

Density functional theory (DFT) calculations have been used to investigate acetylene cyclotrimerization catalyzed by titanium and vanadium dioxides. The calculated results illustrate that the overall process is highly favorable at room temperature from both thermodynamic and kinetic points of view. The mechanism of C2H2 cyclotrimerization over MO2 (M = Ti, V) can be understood as four steps: (1) a four-membered ring (-O-M-C=C-) formation that coordinates and activates the first C2H2 molecule; (2) the second C2H2 insertion into the M-C bond to form a six-membered ring (-O-M-C=C-C=C-); (3) the third C2H2 insertion into the M-C bond to form an eight-membered ring (-O-M-C=C-C=C-C=C-); and (4) contraction of the eight-membered ring and benzene formation and desorption. All of the reaction steps are overall barrierless with respect to the separated reactants (MO2C2xH2x + C2H2, x = 0, 1, 2). This theoretical study predicts that the M=O double bond in MO2 is very catalytic toward the C2H2 cyclotrimerization. The metal center in this study can be considered always in the same +4 oxidation state (Ti4+ and V4+). In contrast, two-electron cycling of the metal center is present in the documented mechanism for the C2H2 cyclotrimerization. The C2H2 cyclotrimerization over the Ti atom and TiO molecule is also studied, and the documented mechanism applies in this case. The new mechanism is suggested to apply to reactions using titanium and vanadium oxides as catalysts.

Classification of VxOyq Clusters by Δ = 2y+q−5x

Vanadium oxide clusters VxOyq (x ≤ 8, q = 0, ±1) are classified according to the oxidation index (Δ=2y+q−5x) of each cluster. Density functional calculations indicate that clusters with the same oxidation index tend to have similar bonding characters, electronic structures, and reactivities. This general rule leads to the findings of new possible ground state structures for V2O6 and V3O6+ clusters. This successful application of the classification method on vanadium oxide clusters proves that this method is very effective in studying the bonding properties of early transition metal oxide clusters.

A Theoretical Study on the Mechanism of C2H4 Oxidation over a Neutral V3O8 Cluster

Density functional theory (DFT) calculations are used to investigate the reaction mechanism of V(3)O(8)+C(2)H(4). The reaction of V(3)O(8) with C(2)H(4) produces V(3)O(7)CH(2)+HCHO or V(3)O(7)+CH(2)OCH(2) overall barrierlessly at room temperature, whereas formation of hydrogen-transfer products V(3)O(7)+CH(3)CHO is subject to a tiny overall free energy barrier (0.03 eV), although the formation of the last-named pair of products is thermodynamically more favorable than that of the first two. These DFT results are in agreement with recent experimental observations. The (O(b))(2)V(O(t)O(t))(.) (b=bridging, t=terminal) moiety containing the oxygen radical in V(3)O(8) is the active site in the reaction with C(2)H(4). Similarities and differences between the reactivities of (O(b))(2)V(O(t)O(t))(.) in V(3)O(8) and the small VO(3) cluster [(O(t))(2)VO(t) (.)] are discussed. Moreover, the effect of the support on the reactivity of the (O(b))(2)V(O(t)O(t))(.) active site is evaluated by investigating the reactivity of the cluster VX(2)O(8), which is obtained by replacing the V atoms in the (O(b))(3)VO(t) support moieties of V(3)O(8) with X atoms (X=P, As, Sb, Nb, Ta, Si, and Ti). Support X atoms with different electronegativities influence the oxidative reactivity of the (O(b))(2)V(O(t)O(t))(.) active site through changing the net charge of the active site. These theoretical predictions of the mechanism of V(3)O(8)+C(2)H(4) and the effect of the support on the active site may be helpful for understanding the reactivity and selectivity of reactive O(.) species over condensed-phase catalysts.

Interfacial Molecular Assemblies of Metalloporphyrins with Two Trans or One Axial Ligands

Molecular assemblies of metalloporphyrins trans-dichloro(5,10,15,20-tetra-p-tolylporphyrinato)tin(IV) (SnCl2TPPMe) and trans-dihydroxo(5,10,15,20-tetra-p-tolylporphyrinato)tin(IV) (Sn(OH)2TPPMe), which have two trans axial ligands, as well 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (FeClTPPOMe) and 5,10,15,20-tetraphenyl-21H,23H-porphine manganese(III) chloride (MnClTPP), which have one axial ligand, are interfacially organized by Langmuir and Langmuir-Blodgett (LB) techniques. SnCl2TPPMe and Sn(OH)2TPPMe form nanofibrous structures which can display distinct supramolecular chirality, although the molecular units themselves are achiral, while FeClTPPOMe and MnClTPP form irregular nanoparticles that display negligible supramolecular chirality. An interpretation in terms of the effects of the axial ligands is proposed for this interesting phenomenon. Moreover, compared with assemblies of the diprotonated free-base porphyrins, which are fabricated by interfacial (air/2.4 M HCl) organization of free-base porphyrin, those of SnCl2TPPMe and Sn(OH)2TPPMe display higher stability in terms of supramolecular chirality. These results indicate that the assembly properties of metalloporphyrins can essentially be affected by the axial ligands that are attached to their chromophores, and that stable chiral porphyrin supramolecular associations can be easily produced by using achiral metalloporphyrins bearing two trans axial ligands.

Nearly monodispersed perylene nanotablets: Easy fabrication and unique optical properties

Here we describe a simple physical vapor deposition (PVD) method to prepare large quantity of highly monodispersed perylene nanotablets with a deposition-temperature controlled approach. The nanotablets we get are 1 um octagon with 100–200 nm thickness. Confocal microscopic images of the nanotablets show novel yellow emission which is quite different from perylene monomers when excited with a UV light of a mercury lamp. The morphology of the unique perylene nanotablets is significantly affected by the temperature of the deposition substrates. This approach is simple and template free and the as prepared nanotablets possess smooth surface which is important for the potential application in fabricating nanostructure based organic semiconductor nanoelectronic devices.