report
6.2.3 State of R&D

Catalytic traps

It is estimated that the world-market for oxide catalysts is over €3 billion (Morrison, 2006). Due to the wide-spread use of catalysts in the synthesis of materials (some 95% of materials will have undergone catalysed steps at some point during production) the economic impact will be even two orders higher (Morrison, 2006). The understanding and control of metal oxide catalytic activities is seen as critical to the development of sustainable chemistry (Morrison, 2006). Catalysts find application not only in remediation processes, but they can also contribute to a more efficient use of energy and raw materials.

Challenges in the synthesis of nanostructured catalysts are that many reactions employ mixed catalysts consisting of different oxides or (noble) metals, and that the function of active centres is not only determined by the constituent atoms but also by the surrounding crystal or surface structures; it is thus necessary to accurately control the synthesis of nanostructured catalysts (Morrison, 2006; Rickerby and Morrison, 2007). There is also a need to understand the chemical reactivity of the catalytic active centres and how this is affected by reactor conditions (Morrison, 2006). Different material mixes, particle sizes, and operating temperatures must be tested to ensure effective catalysis and the production of the expected less hazardous compounds (and not other pollutants) (Morrison, 2006).

A study by Ma et al. analysed the effects of particle size, catalyst loading and flow residence time on the reactivity of platinum nanoparticles in a methanol-air mixture. They found an enhanced reactivity with decreasing particles size of Platinum (Ma et al., 2008).

Noble-metal free wall-flow catalyitc traps have been tested for their efficiency in eliminating soot and NOx. It was found that catalyst crystals with a size similar to that of the particulate pollutant provide the highest number of contact points (Rickerby and Morrison, 2007). Such crystals (size range 20-50nm) can be prepared by solution combustion synthesis (Rickerby and Morrison, 2007). First applications in a pilot plant showed that the CoCr2O4 catalyst gave the best compromise between soot reduction and NOx abatement compared to MnCr2O, CoFe2O4 and currently used catalysts (Rickerby and Morrison, 2007). Also LiCrO2 catalysed traps showed an increased efficiency (Fino et al., 2007). Additionally, the regeneration of CoCr2O4 and LiCrO2 catalysts was found to be faster than the regeneration of non-catalytic ceramic filters. But in the case of CoCr2O, a significant quantity of nanoparticles was generated during regeneration (Rickerby and Morrison, 2007). Russo et al. evaluated the catalytic activity of a series of nano-structured soot combustion catalysts. Their performance was compared to the standard LaCoO3-performance. The La0.9Rb0.1CoO3 catalyst was found to allow the best compromise between satisfactory catalytic activity and stability (Russo et al., 2008). Different modified perovskite-type oxides were also synthesized and analysed by Seyfi et al. (Seyfi et al., 2009). The materials showed good structural and chemical stability up to 600°C and  high activity for the catalytic CO oxidation reaction. The composition La0.8Sr0.2Co0.8Cu0.2O3 achieved 100% CO oxidation at 355 ◦C.

Other studies investigated the effectivity of nano-gold catalysts on different metal oxides.

Qiao et al. analysed the oxidation of carbon monoxide by ferric hydroxide supported gold catalysts (Qiao et al., 2008) while Choudhary et al. (Choudhary et al., 2008) focused on the degradation of methane. Choudhary et al. found the Au/Fe2O3 catalyst to show the best performance compared to other catalysts such as Pd/Al2O3 and Pt/Al2O3. A study by Li et al. (Li et al., 2008) found nano-Au/Co3O4 to be a good catalytic material for the elimination of ethylene. They see potential applications in warehouse fruit storage to keep fruits fresh.

The oxidation of naphtalene by nano-crystalline cerium oxide catalyists was studied by Garcia et al. (Garcia et al., 2005). They found that ceria synthesised by precipitation with urea was the most active catalyst for oxidation of naphthalene to carbon dioxide. The urea precipitated CeO2 (CeO2 (U)) demonstrated over 90% naphthalene conversion to carbon dioxide at a temperature of 175 °C. Comparing the efficiency of CeO2 (U) with known high activity oxidation catalysts (Mn2O3 and 0.5% Pt/c-Al2O3) showed that CeO2 (U) was a more effective catalyst for naphthalene total oxidation. The high adsorption capacity of the material provides the advantage that it can be used as a combined catalyst and adsorbent to remove PAHs from waste streams (Garcia et al., 2005).

Photocatalysis

Photocatalytic degradation of pollutants such as NOx and VOCs is a very promising application. TiO2 has shown significant efficacy as oxidising agent and has already been incorporated in paints, surface coatings, glasses and cement for construction where it is expected to develop its self cleaning and de-polluting properties (Parkin and Palgrave, 2005; Rickerby and Morrison, 2007). The reaction efficiency of TiO2 can be increased by doping the semiconducter with noble metals. TiO2 can also be doped with B, N, C or S to harness visible light which is crucial for indoor application (Rickerby and Morrison, 2007; Zhang et al., 2009) (Dong et al., 2008). N-doped TiO2 can make use of artificial illumination by moving the light absorbing spectrum towards longer wavelengths (In et al., 2006; Orlov et al., 2006).

In the frame of the PICADA project (PICADA, 2005) several laboratory scale test, macro scale tests and tests on a pilot site were conducted. The laboratory tests showed an 80% reduction in levels of NOx (especially NO) near TiO2 containing paints and cements. In canyon street tests, NOx concentrations were reduced by 40-80% depending on the differences in emission sources, wind direction and orientation of the walls.

In further tests with NO and VOCs it was observed that the photocatalytic conversion of NO and toluene increased with decreasing relative humidity and toluene degradation was enhanced by the presence of NO  (Leva in (Rickerby and Morrison, 2007)). The degradation rate for NO was up to 95% and 32% for toluene (Leva in (Rickerby and Morrison, 2007)). Newer studies found that copper-cobalt oxides supported on nTiO2 enhanced the NO conversion to 98.9% at a temperature of 200°C in the presence of CO. The product was mainly N2O (at a temperature below 100°C). In the presence of CO2 and H2O, the catalyst deactivated within short time (Chen et al., 2009).

So far the photocatalytic efficiency (percentage of incident photons used) is only around 0.01% (Rickerby and Morrison, 2007). But according to Rickerby and Morrison (Rickerby and Morrison, 2007) 10% might be possible by enhancing the photocatalytic activity with noble metals (Pt, Au, Ag), SnO2/TiO2 composite materials or by doping with Fe(III). It is though not clear how stable these doped materials are.

Quiang and Meng (Qiang and Meng, 2009) developed a nTiO2-containing spray which can be applied on concrete pavement for the degradation of vehicle emissions. They report the efficiency to be 6-12%. It is unclear though which substance was targeted. The original article is in Chinese. A similar study probably with the same material investigated the decontamination rate of NOx by TiO2 photocatalyst immobilized on asphalt roads(Chen and Liu, 2010). They also found degradation efficiencies of around 6-12%.

Another study presented a new technique of indoor air cleaining by plasma discharge in the presence of nTiO2 as catalyst (Gao et al., 2008). According to the authors, VOCs could be decomposed with a reduced production of ozone as byproduct.

Kwon et al. (Kwon et al., 2008) summarize in their comprehensive review of photocatalytic nano-TiO2 different environmental applications and their efficiency. This review also includes information on the mechanism of degradation and the special properties of nano-TiO2 (compared to micro-TiO2).

Static filter: CNT

Vaseashta et al. (Vaseashta et al., 2007) tested CNT filled in a polymer composite matrix to create a static discharge to remove the PM from incoming air. According to Vaseashta et al. (Vaseashta et al., 2007) several products using static discharge are currently being commercialized.

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