Stéphane Siffert

Gold catalysts in environmental remediation and water-gas shift technologies

This review presents studies and investigations of the use of gold catalysts in environmental applications, from the oxidation of CO and volatile organic compounds (VOCs) in environmental technologies to the production of hydrogen, a non-polluting energy source, in the water-gas shift (WGS) reaction. It summarizes a variety of results, from the discovery made by Haruta et al. about the catalytic behaviour of gold in CO oxidation, to the use of gold-based catalysts in environmental remediation, by the catalytic removal of different types of VOC using a wide variety of oxide supports and finally to the use of this highly active noble metal in WGS applications. It also discusses gold loading methods, comparing them in terms of simplicity, efficiency and the resultant particle size and dispersion of Au over various supports. The application of different types of supporting materials is also presented, with a critical discussion of the parameters affecting the choice and use of such materials, i.e. how the support interacts with gold particles and with pollutant molecules along with the advantages a support offers to VOC oxidation and WGS reactions. It ends by highlighting the potential of gold catalyts in the future.

Noble‐Metal‐Based Catalysts Supported on Zeolites and Macro‐Mesoporous Metal Oxide Supports for the Total Oxidation of Volatile Organic Compounds

The use of porous materials to eliminate volatile organic compounds (VOCs) has proven very effective towards achieving sustainability and environmental protection goals. The activity of zeolites and macro-mesoporous metal-oxide supports in the total oxidation of VOCs has been investigated, with and without noble-metal deposition, to develop highly active catalyst systems where the formation of by-products was minimal. The first catalysts employed were zeolites, which offered a good activity in the oxidation of VOCs, but were rapidly deactivated by coke deposition. The effects of the acido-basicity and ionic exchange of these zeolites showed that a higher basicity was related to exchanged ions with lower electronegativities, resulting in better catalytic performances in the elimination of VOCs. Following on from this work, noble metals were deposited onto macro-mesoporous metal-oxide supports to form mono and bimetallic catalysts. These were then tested in the oxidation of toluene to study their catalytic performance and their deactivation process. PdAu/TiO(2) and PdAu/TiO(2) -ZrO(2) 80/20 catalysts demonstrated the best activity and life span in the oxidation of toluene and propene and offered the lowest temperatures for a 50 % conversion of VOCs and the lowest coke content after catalytic testing. Different characterization techniques were employed to explain the changes occurring in catalyst structure during the oxidation of toluene and propene.

Role of molybdenum against ceria sulphur poisoning in the combustion of soot particles and the oxidation of propene

Abstract The influence of molybdenum on ceria activity, CO2 selectivity and resistance to sulphur poisoning was investigated. Different molybdenum-cerium catalysts, with various atomic ratios (Mo/Ce=0.001, 0.01, 0.1, 0.2 and 0.3), have been prepared and tested with respect to carbon black (CB) combustion and the oxidation of propene before and after treatment under SO2. Based on their catalytic behaviour and according to molybdenum loadings, the catalysts have been classified into two groups: (i) ceria and low molybdenum loading catalysts (1Mo1000Ce, 1Mo100Ce) which present a good activity, an excellent CO2 selectivity and a bad resistance to sulphur poisoning; (ii) catalysts containing higher molybdenum loadings (1Mo10Ce, 2Mo10Ce, 3Mo10Ce) show lower activity and CO2 selectivity but a better resistance to sulphur poisoning. The role of molybdenum against ceria sulphur poisoning has been evidenced.

Efficiency of natural clay as heterogeneous Fenton and photo-Fenton catalyst for phenol and tyrosol degradation

AbstractAdvanced oxidation processes (AOPs) are particularly useful for cleaning toxic or non-biodegradable materials in wastewater. Among these processes, the advantage of the Fenton processes is the potentially complete mineralization of organic substances. In this work, a naturally containing Fe-clay naturally red clay (NRC) was used as heterogeneous catalyst for the Fenton and photo-Fenton oxidation of two organic pollutants (phenol and tyrosol). NRC was characterized by several complementary methods including chemical analysis, X-ray diffraction, temperature programmed reduction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The influence of different parameters like calcination’s temperature and initial H2O2 concentration were examined considering both the pollutant conversion and the total organic carbon (TOC) abatement. This catalyst was very efficient for the mineralization of both compounds in aqueous medium under mild experimental conditions. Indeed, 100% pollutant con...

Titanium oxide nanotubes as supports of Au or Pd nano-sized catalysts for total oxidation of VOCs

Abstract Titanium oxide nanotubes (TNTs) have been synthesized via laboratory-made TiO2 A and commercial TiO2 P25. 1.5wt% gold or palladium were deposited on TNTA and TNTP25 supports and the catalytic activity was evaluated in propene, methyl ethyl ketone (MEK) and toluene total oxidation. The catalytic properties of supported TNT were correlated with the structural peculiarity and the nature of the TNT, but also with the nature of the noble metal and the kind of the VOC.

Preparation and characterization of Co−Fe−Cu mixed oxides via hydrotalcite-like precursors for toluene catalytic oxidation

Different Co 2−x Cu x Fe 1 hydrotalcites with x=0, 0.5, 1, 1.5 and 2 have been synthesized in order to be used as precursors of mixed oxides for total toluene oxidation. The formation of hydrotalcite phase has been evidenced for x 3 O 4 and/or CoFe 2 O 4 spinels. At 500°C, besides the Co−Fe spinels, CuO formation has been related for x≥1. The presence of CuO phase was not the determining parameter since a catalytic synergetic effect has been obtained for Co 1.5 Cu 0.5 Fe 1 sample.

Investigation of the elimination of VOC mixtures over a Pd-loaded V-doped TiO2 support

The removal of a butanone–toluene mixture over Pd/5%V-TiO2 and Pd/TiO2 catalysts has been investigated. The V-doped sample showed a higher performance in the oxidation of butanone and also a butanone–toluene mixture, which was attributed to competition between oxygen species and VOCs on vanadium adsorption sites. However in the presence of both VOCs, Pd/5%V-TiO2 showed a higher performance than in the elimination of butanone alone. An operando DRIFT experiment coupled with a mass spectrometer revealed that both VOCs compete in terms of adsorption on the surface of the catalyst. However, toluene adsorption is decreased by MEK, meaning that the latter oxidizes more easily into its by-products, whose combustion causes variations in the surface temperature of the catalyst, thus enhancing the oxidation of toluene molecules present in the gaseous stream.

Effects of the treatment and the mesoporosity of mesostructured TiO2 impregnated with noble metal for VOCs oxidation

Abstract Catalytic oxidation of volatile organic compounds (VOCs) was carried out over catalysts prepared from different nanostructured mesoporous TiO 2 presenting high surface areas and mesoporosity from 2.9 to 10.1 nm. These Pd impregnated solids were found to be powerful catalysts for total oxidation of toluene. However, the pore diameter of 2.9 nm, leading to palladium particles accessible and reducible in the mesopores should explain the interesting catalytic behavior of the Pd/mesoporous TiO 2 sample prepared by CTMABr surfactant and calcined at 400°C.

Cyclodextrin-intercalated layered double hydroxides for fragrance release

In order to attain the controlled release of fragrance, the intercalation of cyclodextrins (CDs) and fragrance in layered double hydroxides (LDHs) was examined. Carboxymethyl-β-CDs (CMCDs) of various degrees of substitution as well as Mg–Al support were synthesized. CMCDs were intercalated into LDH by the reconstruction method. Powder X-ray diffraction, thermal gravimetric analyses and Fourier transform infrared indicated a successful intercalation of CMCDs into the LDH gallery. The retention capacities of the hybrid materials were investigated in aqueous phase and in gaseous solution by static headspace gas chromatography and multiple headspace extraction. The functionalization of the LDH with CMCD allowed the encapsulation of various organic guests and could prolong the fragrance release time in comparison to that from LDH without CMCD, which can be attributed to the inclusion of the fragrance compound in the CMCD cavity.

Characterisation of new Pd / hierarchical macro-mesoporous ZrO2, TiO2 and ZrO2-TiO2 catalysts for toluene total oxidation

Amorphous (macro)-mesoporous ZrO 2 , TiO 2 and ZrO 2 -TiO 2 have been synthesised and characterised for volatile organic compounds (VOCs) catalytic oxidation. These solids used as catalytic supports present high surface areas. The stability of the porous structure after the calcination at 400°C is observed for ZrO 2 -TiO 2 and TiO 2 but partial breakdown of the structure occurred for ZrO 2 by the crystallisation to tetragonal phase. All these Pd impregnated solids are found to be powerful catalysts for total oxidation of toluene. Pd/TiO 2 presents the highest catalytic potential. The lowest toluene adsorption enthalpy, the low coke content observed after the catalytic test and the highest pore diameter leading to palladium particles more accessible and reducible should explain the interesting catalytic behaviour of Pd/TiO 2 .