Beatriz de Rivas

Role of water and other H-rich additives in the catalytic combustion of 1,2-dichloroethane and trichloroethylene.

In several practical applications gas streams containing chlorinated volatile organic compounds with variable chemical nature (namely, 1,2-dichloroethane and trichloroethylene) and a significant moisture content (15000ppm) must be addressed. In this paper the control of such emissions by catalytic oxidation over Ce/Zr mixed oxides was analysed. Results in terms of activity and selectivity were compared with those obtained when other H-rich additives (1000ppm), such as hexane or toluene, were fed. High activity was found from mixed oxides featuring a suitable combination of a large population of acid sites, easily accessible oxygen species, and hydrophobic nature attributable to cerium content. The presence of additional H-rich compounds in the feed stream (water, toluene or hexane) tended to decrease the catalytic activity due to the blockage and/or competition for actives sites. However, the increased presence of hydrogen atoms in the stream notably promoted the selectivity to hydrogen chloride instead of molecular chlorine.

Oxidation of chlorinated alkanes over Co3O4/SBA-15 catalysts. Structural characterization and reaction mechanism

Cobalt oxide-based catalysts have been synthesised via a hard template of mesoporous silica SBA-15 in the form of massive Co3O4 nanoparticles, whose dimensions are controlled and limited by the support mesoporosity. Over this family of catalysts, both weak Bronsted and Lewis acidities have been detected, with the relative abundance a function of the cobalt content. The mesoporosity of the support leads to the growth of oxide nanoparticles, mainly in the small pores, thus improving their redox properties. Catalysts possessing cobalt oxide loadings higher than 30% present an adequate activity for the deep DCE oxidation toward carbon dioxide, hydrogen chloride and chlorine. It is believed that the reaction is markedly accelerated due to the simultaneous participation of the acid sites (where the chlorinated feed is efficiently adsorbed) and the redox sites (oxidation of the adsorbed feed with lattice oxygen anions). FTIR data on dichloroalkane oxidation evidence that lattice oxygen species are mainly involved in the Cl-VOC combustion, likely through a Mars–van Krevelen mechanism.

Inverse gas chromatography as a technique for the characterization of the performance of Mn/Zr mixed oxides as combustion catalysts

Adsorption of different volatile organic compounds (trichloroethylene, TCE; 1,2-dichloroethane, DCE; n-hexane) over different manganese-zirconia mixed oxides (Mn(x)Zr(1-x)O(2)) - widely used as combustion catalysts - was studied by inverse gas chromatography. Adsorption isotherms (calculated in the Henry region), adsorption enthalpies (DeltaH(ads)), and dispersive (gamma(S)(D)) and specific (I(sp)) components of the surface energy have been determined at infinite dilution for the investigated compounds. Both the adsorption enthalpy and the specificity of the interaction of TCE and DCE over Mn(x)Zr(1-x)O(2) catalysts depend strongly on manganese content. Thus, the adsorption strength of the reactants over the active sites is closely related with both the surface acidity and the accessibility of the lattice oxygen. A great influence of the specific interaction on the catalytic pattern has been also noticed. Since I(sp) depends on the redox properties, it has been proved that the specific interaction is determined by the presence of bulk Mn(3)O(4), which hinders the mobility of the oxygen lattice, and MnO(x), with the contrary effect. Finally, the selectivity to oxidation products has been correlated with both the enthalpy of adsorption and the specific interaction parameter, decreasing the selectivity to HCl with the increase of the enthalpy of adsorption.

Analysis of the Behaviour of Different Mixed Oxides in the Treatment of Cl-VOC Containing Gas Streams

The present work deals with the catalytic behaviour of a series of ceria and manganese-based catalysts for the deep single combustion chlorinated compounds in air. Our attention is focused on both activity and product selectivity of Ce/Zr and Mn/Zr in the oxidation of 1,2-dichloroethane (1,000 ppm) and trichloroethylene (1,000 ppm). This study also makes an attempt to establish a correlation between physico-chemical properties and catalytic performance.Both series of mixed oxides exhibited a noticeable activity in the combustion of both compounds, Ce0.5Zr0.85O2, Ce0.5Zr0.5O2 and Mn0.4Zr0.6O2 being the samples with the most promising catalysts. A remarkable acidity and readily accessible active oxygen species played a key role in determining the catalytic behaviour. As for the product selectivity, CO2, CO, HCl and Cl2 were the main products. The yield of CO2 and Cl2 was favoured as cerium or manganese content was increased. However, selectivity to chlorine was noticeably limited by the addition of stream in favour of HCl.

Surface Characterization of Mesoporous CoOx/SBA-15 Catalyst upon 1,2-Dichloropropane Oxidation

The active combustion catalyst that is based on 30 wt % cobalt oxide on mesoporous SBA-15 has been tested in 1,2-dichloropropane oxidation and is characterized by means of FT-IR (Fourier transform infrared spectroscopy) and ammonia-TPD (temperature-programmed desorption). In this work, we report the spectroscopic evidence for the role of surface acidity in chloroalkane conversion. Both Lewis acidity and weakly acidic silanol groups from SBA support are involved in the adsorption and initial conversion steps. Moreover, total oxidation reaction results in the formation of new Bronsted acidic sites, which are likely associated with the generation of HCl at high temperature and its adsorption at the catalyst surface. Highly dispersed Co oxide on the mesoporous support and Co-chloride or oxychloride particles, together with the presence of several families of acidic sites originated from the conditioning effect of reaction products may explain the good activity of this catalyst in the oxidation of Chlorinated Volatile Organic Compounds.