Search for reactive intermediates in catalytic oxidation with hydrogen peroxide over amorphous niobium(V) and tantalum(V) oxides

The reactive oxygen intermediates generated on surface of amorphous Nb 2 O 5 and Ta 2 O 5 upon interaction with aqueous H 2 O 2 were identified. The role of dehydroxylation of the surface, pH of H 2 O 2 solution, and the presence or absence of oxygen in gas phase were studied in depth by UV–Vis, FTIR, Raman, and EPR techniques. The study revealed high ability of the amorphous Nb 2 O 5 to form peroxo, superoxo, and radical hydroxyl species upon contact with hydrogen peroxide. This process depends on the level of surface hydroxylation which is lower for amorphous Ta 2 O 5 than amorphous Nb 2 O 5 . The relationship between the formation of superoxo and peroxo species and pH of the H 2 O 2 /H 2 O solution was proved. Superoxo species interact with the excess of H 2 O 2 towards hydroxyl radicals, which are the most active species in catalytic oxidation. The role of pH was evident in the oxidation of glycerol. NaOH and link of superoxo species to niobium enhance the reaction rate by increase of hydroxyl radicals number. The hydroxyl radicals are also the active species in the formation of hydroxylated cyclohexene and finally cyclohexenediol in cyclohexene oxidation

Zeolite MCM-22 modified with Au and Cu for catalytic total oxidation of methanol and carbon monoxide

The goal of this work was to use MCM-22 zeolites for preparation of monometallic (Cu or Au) and bimetallic (Cu and Au) catalysts for oxidation reactions. The focus was on precise determination of the nature of gold and copper species and their activity in the oxidation processes. For that purpose several characterization techniques were applied (XRD, N 2 adsorption/desorption, TEM, SEM, UV − vis, H 2 -TPR, 27 Al MAS NMR, FT-IR with the adsorption of pyridine, NO, and CO, ESR spectroscopy). They allowed us to de fi ne the following species formed on MCM-22 surface: metallic gold particles (XRD, UV − vis), isolated Cu 2+ with octahedral coordination (UV − vis, ESR), square planar Cu 2+ cations (ESR, IR), Cu + species (ESR+NO, FTIR+CO, and FTIR+NO), and oligonuclear clusters (UV − vis) as well as CuO-like species (H 2 -TPR). The presence of gold on the MCM-22 surface modi fi ed further by copper species caused the interaction between two modi fi ers leading to much easier reduction of CuO-like species and higher mobility of oxygen-promoting oxidative properties. The bimetallic catalyst was highly active in total oxidation of methanol and CO in the temperature range 523 − 623 K. Cu/Au-MCM-22 zeolite appeared useful for simultaneous removal of CO and methanol (by total oxidation) from gases emitted from automotive devices and during a variety of industrial process operations

Confined Photodynamics of an Organic Dye for Solar Cells Encapsulated in Titanium-Doped Mesoporous Molecular Materials

[EN] A triphenylamine dye (TPC1) encapsulated in titanium-doped mesoporous silica structures as alternative materials for dye-sensitized solar cells has been studied by means of stationary absorption and emission as well as ultrafast emission spectroscopy. For the samples prepared by a grafting method, a TPC1 complex with titanium atoms within the mesoporous silica in dichloromethane (DCM) solution is formed, having a red shift of the visible absorption band by about 1300 cm 1 with respect to that of the TPC1 in DCM (from 455 to 485 nm). For the complexes, multi- exponential emission quenching of the relaxed singlet excited state occurs with time constants from 300 fs to 30 ps and is assigned to the con¿ned electron injection process into the Ti O chromophore. The averaged electron injection rate from the higher energy levels gets smaller values for less energetic probing, from 2.7 1012 s 1 at 600 nm to 1.5 1012 s 1 at 700 nm. However, in the titanium-doped samples prepared by an impregnation method, we observed about 2 3 times slower injection. The di¿erence is explained by di¿erent coupling between TPC1 and titania domains. As a reference to the con¿ning e¿ect on the dynamics, we also studied the behavior of TPC1 when interacting with amorphous silica and purely siliceous MCM-41 material in the same solvent. In amorphous silica, an equilibrium between neutral and anion structures of TPC1 is found to be shifted toward the anion form. For the MCM-41 material, the presence of a new absorption band at around 690 nm is revealed, assigned to the spontaneously created and remarkable stable TPC1 radical cation. The lifetimes of the normal and anion forms in both materials were found to be similar to those in solution. The femtosecond relaxation dynamics in these materials is also similar to that in solution (dominated by the solvation), but additional emission quenching in the TPC1/MCM-41 sample is observed, probably due to intermolecular energy transfer. The rate of energy transfer was estimated to decrease gradually when increasing the observation wavelength, from 1.11 1012 s 1 at 500 nm to 0.13 1012 s 1 at 700 nm. We believe that our results interrogating ultrafast dynamics of an e¿cient dye interacting with titania within the mesoporous materials will help in a better understanding and improvement of dye-sensitized solar cells.The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n 235286 (NANOSOL). This work was also supported by the JCCM and MICINN through projects PCI08-0037-5868 and MAT2008-01609, respectively, and Consolider-Ingenio 2010 (CDS2009- 00050). We thank Prof. Licheng Sun and Dr. Xichuan Yang for giving us the TPC1 dye.Ziólek, M.; Martín, C.; Navarro Ruiz, MT.; García Gómez, H.; Douhal, A. (2011). Confined Photodynamics of an Organic Dye for Solar Cells Encapsulated in Titanium-Doped Mesoporous Molecular Materials. JOURNAL OF PHYSICAL CHEMISTRY C. 115(17):8858-8867. https://doi.org/10.1021/jp201627tS885888671151