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Two Au/TiO2 samples with different gold loadings (0.7 and 4.0 wt.% Au) were prepared by deposition-precipitation with urea and calcined at 673 K. TEM revealed gold particles of 3.2 and 3.9 nm for the 0.7 and 4.0 wt.% samples, respectively. The samples were subjected to different red-ox treatments and then the state of gold was determined by the FTIR spectra of CO adsorbed at low temperature. Several kinds of gold carbonyl species were detected during the experiments: (i) Au0–CO at around 2107 cm−1; (ii) Au+–CO at ca. 2175 cm−1; (iii) Auδ+–CO in the region of 2140–2137 cm−1 and (iv) Auδ′+–CO (δ′ > δ) at around 2155 cm−1. The 4.0 wt.% sample contained mainly metallic gold after evacuation at 673 K. Subsequent interaction with oxygen at 373 K leads to oxidation of a fraction of the surface metallic gold sites to Auδ+ sites. These sites were considered as cations located on the surface of the metal particles with a partially positive charge δ+ (0 < δ < 1) because of electron transfer from the gold bulk. Evacuation at 673 K leads to back reduction of the Auδ+ sites to metallic gold. The oxidation of gold particles was more efficient when performed with a NO + O2 mixture. It resulted in creation of Auδ′+ sites with a higher positive charge than that of the Auδ+ sites. In this case the oxidation involved a higher number of Au0 sites. A similar treatment of the 0.7 wt.% Au sample, however, resulted in formation of “isolated” Au+ species. The results indicate that small metal particles are more easily oxidized by a NO + O2 mixture. A model of the formation of the different sites, explaining well the experimental results, is proposed. [ABSTRACT FROM AUTHOR]

Wastewater originating in the chemical process industries contain high concentrations of organics that must be treated before discharge. Phenol is one of the most important of these pollutants because it istoxic, even at low concentrations. Recently, hydrogen peroxide treatment has emerged as a viable alternative oxidation technique for treatment. It does not form any harmful or intrusive by-products and it is an ecologically clean and non-toxic chemical. However, the methods for treatment of wastewater with hydrogen peroxide are based mostly on homogeneous catalysis by metal ions inducing additional pollution. From this, it was anticipated that the use of heterogeneous catalysiswould be a good alternative, thus opening up a new field of investigation. Previous studies, based on screening tests, evidenced the catalytic activity of an MFI zeolite Fe-ZSM-5 for phenol elimination by H2O2. The oxidation tests are carried out in a batch mode, using a stirred glass reactor. The influence of pH, temperature and H2O2 stoichiometric ratio on the phenol oxidation rate with Fe-ZSM-5 is studied. The temperature is below 100 deg.C in order to avoid pressurization of the treatment reactor. Using a H2O2 stoichiometric ratio equal to 1.5 avoids accumulation of any quinone-like by-products. Under these conditions, the catalytic system H2O2 /Fe-ZSM-5 allows a total elimination of phenol and a significant TOC removal (50%)without leaching-off more than 1 ppm of Fe(II) ions. At the end of the reaction period, only carboxylic acids (maleic, fumaric, oxalic, acetic) accumulate. These latter should not be considered as a drawback of this process. The chemical oxidation process would be used only as a detoxification step and such compounds could be treated with a biological process, in a second step. [ABSTRACT FROM AUTHOR]