Paolo Ciambelli

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.

Supercritical antisolvent precipitation of nanoparticles of a zinc oxide precursor

Supercritical antisolvent (SAS) precipitation has been applied to the production of zinc acetate with the aim of evaluating the applicability of this new process to the production of controlled size nanoparticles of catalyst precursors. SAS process is based on the large volumetric expansion of the liquid solvent induced by the fast diffusion of the antisolvent inside the liquid phase. The main process parameter that controlled zinc acetate particle size and particle size distribution (PSD) was the concentration of the liquid solution. Zinc acetate nanoparticles with sizes down to 30 nm and with a mean particle size of 50 nm have been produced. The nanoparticles showed different porosities depending again on the concentration of the liquid solution. BET surface areas up to about 175 m2/g were measured using N2 adsorption.

Photocatalytic Degradation of Organic Dyes under Visible Light onN-Doped Photocatalysts

This study was focused on the application of white and blue light emitting diodes (LEDs) as sources for the photocatalytic degradation of organic dyes in liquid phase with visible light. The photocatalytic activity of N-doped titanium dioxide, synthesized by direct hydrolysis of titanium tetraisopropoxide with ammonia, was evaluated by means of a batch photoreactor. The bandgap energy of titanium dioxide was moved in the visible range from 3.3 eV to 2.5 eV. The visible light responsive photocatalysts showed remarkably effective activity in decolorization process and in the removal of total organic carbon. Methylene blue was also used as a model dye to study the influence of several parameters such as catalyst weight and initial concentration. The effect of dye on the photocatalytic performance was verified with methyl orange (MO). The results demonstrated that the right selection of operating conditions allows to effectively degrade different dyes with the N-doped TiO2 photocatalysts irradiated with visible light emitted by LEDs.

Catalytic oxidation of soot from diesel exhaust gases: 1. Screening of metal oxide catalysts by TG-DTG-DTA analysis

Abstract Thermal analysis was employed as a screening technique to evaluate the activity of catalysts for the oxidation of soot produced during combustion. Oxidation took place over metal-oxide-based catalysts at temperatures much lower than in the absence of catalyst. Best results were displayed by a Cu/V/K-supported catalyst which is very active at about 330 °C. In the absence of oxygen, 16% of soot was oxidised by the oxygen of the catalyst, suggesting that a redox mechanism is involved.

Catalytic combustion of carbon particulate

Abstract The catalytic combustion of carbon particulate was investigated with the aim of developing a catalyst for the abatement of soot in diesel exhausts. In the presence of a Cu/V/K based catalyst a strong increase of the rate of carbon combustion and a marked decrease of the apparent activation energy were observed. The carbon reactivity showed a linear dependence upon the initial ratio of the mass of catalyst to the mass of carbon and a square root dependence upon the oxygen partial pressure. However, catalytic combustion of diesel soot and partially graphitized carbon black confirmed previous results obtained for other carbon materials. In particular, kinetic data follow a model proposed for the catalytic combustion of amorphous carbon black [1]. An extension of this model, incorporating the influence of carbon-catalyst contact on the catalyst performances, has been presented.

Catalytic oxidation of an amorphous carbon black

Abstract The oxidation of an amorphous carbon black in the presence of a potassium-copper-vanadium catalyst supported on α-alumina has been studied. Burnoff temperatures in the presence of catalyst were lowered by about 300 K with respect to those for uncatalyzed combustion. Cycles of temperature-programmed reduction and oxidation, along with TG-MS analyses demonstrated that a redox mechanism is at the basis of the catalyst strong activity. A differential flow reactor was employed to perform kinetic tests. The progress of the catalytic combustion process was followed measuring the concentrations of carbon oxides in the product stream at the reactor outlet with on-line NDIR analyzers. The dependence of the reaction rate on the relevant variables was investigated. In particular, the apparent activation energy for the catalytic oxidation was found to be less than half that of the corresponding uncatalyzed process, while the carbon reactivity showed a linear dependence upon the amount of catalyst and a square root dependence upon the oxygen partial pressure. The results allowed the formulation of a mechanism for the catalytic oxidation of carbon black and suggest that the limiting step of the overall process is that of catalyst reduction. On the basis of such findings a kinetic equation for catalytic carbon black oxidation is proposed.

Multiwalled carbon nanotube films as small-sized temperature sensors

We present the fabrication of thick and dense carbon nanotube networks in the form of freestanding films (CNTFs) and the study of their electric resistance as a function of the temperature, from 4 to 420 K. A nonmetallic behavior with a monotonic R(T) and a temperature coefficient of resistance around −7×10−4 K−1 is generally observed. A behavioral accordance of the CNTF conductance with the temperature measured by a solid-state thermistor (ZnNO, Si, or Pt) is demonstrated, suggesting the possibility of using CNTFs as temperature small-sized (freely scalable) sensors, besides being confirmed by a wide range of sensitivity, fast response, and good stability and durability. Concerning electric behavior, we also underline that a transition from nonmetal to metal slightly below 273 K has been rarely observed. A model involving regions of highly anisotropic metallic conduction separated by tunneling barrier regions can explain the nonmetallic to metallic crossover based on the competing mechanisms of the metal...

Oxidative dehydrogenation of propane over vanadium and niobium oxides supported catalysts

Publisher Summary This chapter discusses the activity and selectivity of catalysts based on niobium and vanadium oxides supported on high surface area anatase TiO2 in ethane oxidative dehydrogenation (ODH). Specifically, the influence of the cooperation of vanadium and niobium oxides supported phases as components, inducing redox and acid properties, respectively, together with the effect of the preparation conditions on the catalytic performances have been studied. The vanadia–itania catalysts are very active, but with low selectivity, because of their high reducibility. Catalytic performances of VOx/TiO2 systems in ethane ODH are improved by the addition of niobium. When TiO2 is coimpregnated by vanadium and niobium oxides, the presence of niobium enhances the selectivity to ethylene at low vanadium content, whereas it slightly depresses the activity without enhancing the selectivity at high vanadium content. This should be because of the effect of niobium on vanadium reducibility, especially affected at low vanadium content. By changing the order of addition of vanadia and niobia to the support, catalysts with slightly different redox and acid properties are obtained. At low vanadium loading, supporting the two oxides at the same time results in the best catalytic performances, while at high loading a two steps impregnation gives the best results.

Nitric oxide decomposition over Cu-exchanged ZSM-5 with high Si/Al ratio

Abstract The catalytic properties of Cu-exchanged H-ZSM-5 ( Si Al = ca. 80 ) in NO decomposition were investigated. It was found that, both in the presence and in the absence of oxygen in the feed, the rate of NO decomposition increased with the copper content up to about 640% Cu over-exchange, while the turnover frequency reached a maximum value at about 500% over-exchange. The reaction order of NO decomposition in the absence of O2 was 1.6 in the range of NO concentration from 1000 to 5000 ppm. Addition of 1 % O2 in the feed resulted in the decrease of NO conversion to N2 depending on the contact time. In all the range of Cu% exchange the catalysts were active for the catalytic oxidation of NO to NO2, reaching the equilibrium conversion both in the absence (above 350°C) and in the presence (above 300°C) of O2 in the feed.

Catalytic alkylation of phenol with methanol: factors influencing activities and selectivities: II. Effect of intracrystalline diffusion and shape selectivity on H-ZSM5 zeolite

Abstract Six samples of H-ZSM5 zeolites, differing in the size of crystallites and acidity, have been tested as catalysts in the alkylation of phenol with methanol. We observed that the size of crystallites strongly influences activity, even for very small crystallites (220–280A), by the intervention of intracrystalline diffusion. Kinetic runs have been performed on the most active catalyst. A moderate shape selectivity has been observed only in the formation of p-methylanisole and xylenols. Catalyst deactivation occurs through two mechanisms which are independent of the contact time, the first evolving exponentially and the second linearly. The interpretation of this behaviour and of related consequences on the reaction is discussed in the paper together with the reaction path model, the mechanism and the kinetics.