S. Perathoner

Environmental catalysis: trends and outlook

Abstract Environmental catalysis has continuously grown in importance over the last 2 decades not only in terms of the worldwide catalyst market, but also as a driver of advances in the whole area of catalysis. The development of innovative “environmental” catalysts is also the crucial factor towards the objective of developing a new sustainable industrial chemistry. In the last decade, considerable expansion of the traditional area of environmental catalysis (mainly NOx removal from stationary and mobile sources, and VOC conversion) has also occurred. New areas include: (i) catalytic technologies for liquid or solid waste reduction or purification; (ii) use of catalysts in energy-efficient catalytic technologies and processes; (iii) reduction of the environmental impact in the use or disposal of catalysts; (iv) new eco-compatible refinery, chemical or non-chemical catalytic processes; (v) catalysis for greenhouse gas control; (vi) use of catalysts for user-friendly technologies and reduction of indoor pollution; (vii) catalytic processes for sustainable chemistry; (viii) reduction of the environmental impact of transport. Therefore, a significant change has occurred in the last decade in the areas of interest regarding environmental catalysts and in the modality of approaching the research. This review, based on but not limited to the workshop “Environmental Catalysis: A Step Forward” (Maiori, Italy, May 2001), introduces the proceedings of this workshop reported in this issue of Catalysis Today and has the objective of providing an overview to the topic and setting the basis for a step forward in environmental catalysis research.

Catalytic decomposition of N2O over noble and transition metal containing oxides and zeolites. Role of some variables on reactivity

Abstract The catalytic behavior in the decomposition of N2O of (i) Rh supported over A12O3, ZSM5, TiO2, ZrO2 and MgAl oxide, (ii) Ni- and Cu-containing mixed oxides obtained from hydrotalcite-type anionic clays, and (iii) Cu/zeolites (ZSM-5 and Beta) with different characteristics is reported and some of variables which influence the activity are discussed. In particular, the following aspects of the reactivity are shown: (i) the effect of prereduction of the samples, (ii) the reversible or irreversible deactivation by O2, NO, H2O and SO2, and (iii) the role of the ion-exchange method used to prepare the Cu/ZSM-5. The interesting catalytic behavior of Rh/TiO2 and the comparison of its characteristics with those of Rh/ZSM-5 are also shown.

Novel catalysts and catalytic technologies for N2O removal from industrial emissions containing O2, H2O and SO2

The catalytic behavior and some aspects of the development of novel catalysts for N2O decomposition (rhodium-on-zirconia based catalysts) or selective reduction with propane (Fe/ZSM-5 type catalysts) in the presence of the typical gas phase components of industrial emissions containing N2O in concentrations below 0.1% are discussed. In particular: (i) the possibility to improve the catalytic reactivity of rhodium-on-zirconia based catalysts by modification of the zirconia support with lanthanide ions or other dopants; (ii) the stability of the catalytic behavior of these samples in the presence or absence of SO2; and (iii) some characteristics and the stability behavior of Fe/ZSM-5 catalysts in the presence of SO2 for the reduction of N2O with propane/O2 are reported. The catalytic technologies of decomposition and selective reduction of N2O are compared in terms of economics of the processes and type of emissions for which they are preferably suited.

Role of the support and of adsorbed species on the behavior of Cu-based catalysts for No conversion

Abstract The catalytic behavior in NO conversion with NH3 in the presence or absence of O2 and in NH3 oxidation to N2 was studied on a series of copper samples on Al2O3, SiO2, ZrO2, TiO2 and ZSM5 supports. The results provide evidence for the role of the support in the change in catalytic behavior of supported copper species, as well as the different influence on the reactivity in NO conversion and on the side reaction of ammonia oxidation. The latter reaction is inhibited considerably by the chemisorption of NO. In addition, the presence or absence of O2 has considerable influence on the reactivity in NO conversion by ammonia and samples more active in the presence of O2 are less active in the absence of O2.

Benzene Selective Oxidation with N2O on Fe/MFI Catalysts: Role of Zeolite and Iron Sites on the Deactivation Mechanism

The catalytic performances of Fe-zeolites having MFI structures and in which the Fe introduced either by ion exchange or during the hydrothermal synthesis has undergone partial framework to extra-framework migration induced by controlled heat treatment are reported. In particular, the catalytic behavior as function of time-on-stream and the formation of carbonaceous species were studied. The results suggest that only a small fraction of the iron is active in the selective oxidation of benzene to phenol in the presence of N2O. It is suggested that the active fraction is formed by isolated iron ions in a pseudo-octahedral configuration with the sites positioned in hydroxyl nests (defects) of the zeolite and is selective in phenol formation as a result of in situ reduction during the catalytic tests. Two possible pathways of carbonaceous species were identified, the first through the intermediate further hydroxylation of phenol and the second through the coupling of phenol with benzene or another phenol molecule. This second pathway is the dominant mechanism of formation of carbonaceous species, although the relative rate of the two pathways depends on the zeolite characteristics and iron loading. It is also suggested that the second pathway depends on the strong chemisorption of phenol, probably on Lewis acid sites, which hinders the fast back-desorption of phenol out from the zeolite channels and thus favors the formation of carbonaceous species. Catalysts prepared by hydrothermal treatment show a lower rate of deactivation than those prepared by ion exchange, although the latter show a comparable productivity to phenol for amounts of iron in extra-framework positions around 20 to 30 times lower. The results also indicate that the presence of Al in the zeolite framework is beneficial for reducing the rate of deactivation as compared to that of Fe-silicalite samples.

On the Nature of Selective Palladium-Based Nanoparticles on Nitrogen-Doped Carbon Nanotubes for the Direct Synthesis of H2O2

Catalysts based on Pd and Pd–Au nanoparticles supported on N‐doped carbon nanotubes (N‐CNTs) are studied in the direct synthesis of H2O2. The initial selectivity in H2O2 formation is rather high (>95 %); however, there is a fast initial decrease during the first hour of time on stream. This was due to the initial presence of an organic capping agent (polyvinyl alcohol, which is used in the catalyst synthesis to obtain a high dispersion of metal particles). The removal of this capping agent during the reaction leads to a high mobility of metal nanoparticles. The high initial selectivity, when the capping agent is present, is due to small Pd terraces fully covered with chemisorbed O2 and limited H2 chemisorbed sites that consecutively hydrogenate the formed H2O2. The alloying of Pd with Au decreases the intrinsic reaction rate (per mg of Pd) and increases the selectivity in H2O2 formation, whereas Au alone is inactive. Au also has a minor effect on the consecutive conversion of H2O2 in both the decomposition and hydrogenolysis (in the presence of H2 only) reactions. These results suggest that Au does not block the unselective sites of H2O2 conversion but mainly creates isolated small terraces of Pd that can limit H2 chemisorption sites, which thus leads to higher selectivity to H2O2 under given reaction conditions.

In situ DRIFT study of the reactivity and reaction mechanism of catalysts based on iron–molybdenum oxides encapsulated in Boralite for the selective oxidation of alkylaromatics

Abstract An in situ DRIFT investigation of the behavior of iron–molybdenum oxides encapsulated in Boralite (FeMo/Bor) during the oxidation of toluene is reported. The study was carried out to obtain a better understanding of the differences between this catalytic material and (i) V–TiO2 based catalysts and (ii) bulk Fe2(MoO4)3. V–TiO2 based catalysts show a severe decrease in the selectivity to benzaldehyde with increasing conversion of toluene, in contrast FeMo/Bor samples. The effect was attributed to the presence of stronger Lewis acid sites in vanadium-based catalysts which, activating the carbon atom of the carbonyl groups, facilitate its nucleophilic attack to form benzoate species which further degrade to carbon oxides. FeMo/Bor shows higher selectivity at low conversion than bulk Fe2(MoO4)3, probably due to the presence of nanosized iron–molybdate particles inside the zeolite channels, and lower selectivity at high conversion. Due to back-diffusion limitations inside the zeolite pores, the aromatic ring of the alkylaromatic is oxidatively attacked to form maleic anhydride, precursor of the further oxidation to carbon oxides. In FeMo/Bor a different main pathway is responsible for the lowering of selectivity at high conversion with respect to V–TiO2 based catalysts.

Nanostructured electrocatalytic Pt-carbon materials for fuel cells and CO2 conversion

The recent growing possibilities for the preparation, in large quantities and at low cost, of a number of different types of nanostructured carbons (carbon nanotubes, nanofibers, nano-and meso-porous materials, nanocoils and nanohorns, etc.) have open new possibilities in a range of applications: H2 storage, electronic and field emission devices, advanced sensors, polymer reinforcement, and catalyst support. Nonetheless, most authors consider the use of these advanced nanostructured carbons with respect to carbon black only for the possibility of improving metal dispersion and/or utilization. However, these nanostructured carbons offer several additional aspects that make them highly interesting to develop advanced electrocatalysts.

Assessment of copper-vanadium oxide on mixed alumina-titania supports as sulphur dioxide sorbents and as catalysts for the selective catalytic reduction of NOx by ammonia

Abstract A series of materials featuring copper oxide and/or vanadium oxide supported on titanium oxide, aluminium oxide or a composite alumina-titania carrier were tested for suitability as sorbents/catalysts for the simultaneous removal of SO2 and NOx from flue gases. Sulphur dioxide sorption at 300°C was associated with the formation of copper sulphate and some aluminium sulphate, the latter via a bridging Cu-SO4-Al surface species. Only part of the sorption capacity could be regenerated by reduction in hydrogen at 450°C. Selective catalytic reduction of nitric oxide by ammonia over CuO/TiO2, CuO/Al2O3-TiO2 and CuO/Al2O3 was lessened following exposure of the catalysts to SO2 and this feature was assigned to competition for ammonia between nitric oxide and surface sulphate species, the latter involving formation of (NH4)2SO4. Vanadia supported on a composite support featuring 20 wt.-% alumina, balance titania, was as active as V/TiO2 for nitric oxide reduction by ammonia and the Al2O3 component rendered the catalyst resistant to SO2 poisoning because sulphate formed on vanadia could be transferred to Al2O3 where it did not interfere with the activity. Catalysts with copper and vanadium oxides supported on alumina-titania were not resistant to SO2 poisoning during selective catalytic reduction of nitric oxide because the copper component attracted too much SO2 onto the support, thus exceeding the protective capacity of the alumina component.

Disruptive catalysis by zeolites

The analysis of the new scenario for the industrial production of energy vectors and chemicals evidences the need to foster research in the field of catalysis by zeolites towards a novel, potentially disruptive, type of applications. To stimulate research in this direction, this perspective paper analyses a series of emerging concepts in catalysis by zeolites: i) the role of confinement, ii) the use of the Lewis acidity of zeolites, iii) the new possibilities to extend the concept of confined reactivity, iv) the role of defect sites, and iv) the organo-catalysis by guest species in zeolite cages. Then, two areas of novel possibilities for catalysis by zeolites are discussed more specifically: i) metallo-zeolites for methane conversion and ii) functionalized zeolites for reaction with CO2.