Diesel/biodiesel soot oxidation with ceo2 and ceo2-zro2-modified cordierites: a facile way of accounting for their catalytic ability in fuel combustion processes

Quim. Nova, Vol. 34, No. 5, 759-763, 2011 Rodrigo F. Silva, Edimar DeOliveira, Paulo C. de Sousa Filho, Cláudio R. Neri* e Osvaldo A. Serra Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. Bandeirantes, 3900, 14040-901 Ribeirão Preto – SP, Brasil Artigo DIESEL/BIODIESEL SOOT OXIDATION WITH CeO2 AND CeO2-ZrO2-MODIFIED CORDIERITES: A FACILE WAY OF ACCOUNTING FOR THEIR CATALYTIC ABILITY IN FUEL COMBUSTION PROCESSES Recebido em 23/4/10; aceito em 22/11/10; publicado na web em 9/2/11 CeO2 and mixed CeO2-ZrO2 nanopowders were synthesized and efficiently deposited onto cordierite substrates, with the evaluation of their morphologic and structural properties through XRD, SEM, and FTIR. The modified substrates were employed as outer heterogeneous catalysts for reducing the soot originated from the diesel and diesel/biodiesel blends incomplete combustion. Their activity was evaluated in a diesel stationary motor, and a comparative analysis of the soot emission was carried out through diffuse reflectance spectroscopy. The analyses have shown that the catalyst-impregnated cordierite samples are very efficient for soot oxidation, being capable of reducing the soot emission in more than 60%. Keywords: ceria-zirconia catalysts; diesel/biodiesel soot; automotive catalysis. INTRODUCTION Nowadays, a large number of urban centers suffer from the effects due to the air pollution, which results mainly from the fossil fuel burning, as in the case of mineral coal and petroleum derivatives (gasoline and diesel) combustion.1 These two feedstocks are responsible for generating the energy that moves the electrical, industrial and transport sectors, which comprise a large part of the world economy. In particular, the current widespread use of diesel can be explained by its energetic efficiency compared with other fuels, durability of engines, and its relatively low cost in some countries.2 Nevertheless, the well-known environmental problems associated to non-renewable fuels has stimulated scientists around the world to substitute them and to develop sustainable technological processes. In this context, bio-fuels appear as a viable alternative to the substances commonly employed in the industry and in the transport sector, notably to diesel.3,4 Thus, the partial or complete replacement of diesel by biodiesel means to deliver sustainability, once the biodiesel production is viable, its use is environmentally advantageous compared with diesel, it promotes the creation of new job vacancies, and leads to high energy yields.4 Even in the situations where biodiesel replaces large volumes of diesel in the fuel composition (percentages as high as 40% in some countries), some environmental effects still must be carefully evaluated. For example, it is well known that the particulate matter (PM) generated from diesel/biodiesel combustions is highly toxic and has been classified as probably carcinogenic to humans.5,6 The large degree of soot generation is due to the fact that the defining feature of the diesel engine is the use of compression ignition to burn the fuel. So, a high level of incomplete diesel/biodiesel combustion occurs, thus producing undesirable byproducts such as nitrogen oxides, carbon monoxide, aromatic polycyclic hydrocarbons, as well as large amounts of PM.7 Due to the strictness of the laws regarding the reduction of soot emission by vehicles, there is a tendency for the technological update *e-mail: of more efficient engines, improvement of the fuel characteristics, and treatments aiming at reducing the emissions of air pollutants.8,9 In this context, cerium compounds (mainly cerium(IV) oxide and its precursors) have gained importance since they participate in several catalytic reactions, especially those concerning the reduction of automobile exhaust emissions. The growing interest in cerium stems from its redox properties, its high oxygen mobility, and its ability to thermally stabilize the catalytic substrates,10-13 which is applied, for example, in the widely diffused three-way catalysts (TWC).14 The role of the catalytic converters is to eliminate the gases generated from the incomplete combustion of the fuel in explosion engines (alcohol or gasoline) or compression (diesel) by converting them into less aggressive substances.15 In compression engines with an oscillating air/fuel ratio, CeO2 favors the transformation of products arising from the incomplete fuel burning because it is capable of releasing and absorbing O2 during these oscillations.10,13,16,17 Thus, aiming at the development of anti-pollution processes, two cerium-based catalysts in the oxide form (CeO2 and CeO2-ZrO2) were synthesized and deposited onto ceramic substrates made of cordierite. These catalysts were then used to control the emission of PM formed during the burning of diesel and diesel/biodiesel blends in a stationary engine, and a subsequent comparative evaluation of the degree of soot emission through diffuse reflectance spectroscopy was performed. EXPERIMENTAL Catalyst preparation Firstly, 0.10 mol L-1 Ce(NO3)3 and ZrO(NO3)2 stock solutions were prepared by dissolving appropriate amounts of cerium(IV) oxide (99.995%, Aldrich) and zirconium(IV) oxychloride octahydrate (ZrOCl2.8H2O, >99.0% Riedel de Haën) in concentrated nitric acid under mild heating. In the case of the Ce(NO3)3 solution, hydrogen peroxide (30% v. solution) was eventually added in order to promote the Ce4+/Ce3+ reduction. The pH of the solutions was adjusted to ~3.5 through evaporation of the excess acid. 760 Silva et al. The mixed cerium(IV)-zirconium(IV) oxides were formed by adding appropriate volumes of the Ce(NO3)3 and ZrO(NO3)2 solutions (Ce3+/ZrO2+=1:1) to another solution containing ammonium hydroxide (25 wt.%), hydrogen peroxide (30 wt.%), and deionized water in a 4:1:4 (v/v) ratio. The mixture was kept under vigorous stirring at ambient conditions for 7 h. The resulting yellowish suspension was filtered and the solid was dried at 80 °C for 24 h. Finally, the obtained powder was post-annealed at 550 °C in air for 3 h to yield the final mixed catalyst.18 The cordierite ceramic substrates (5SiO2.2Al2O3.2MgO, kindly provided by Umicore®, Brazil) were modified through CeO2 and CeO2-ZrO2 deposition. For the catalytic tests, the substrates were cut so that cylindrical shapes measuring 3.5 cm in height and 2.5 cm in diameter were obtained. The deposition was performed by immersing the ceramic substrate into a 0.10 mol L-1 cerium(III) nitrate solution (or a mixture of cerium(III) and zirconyl nitrates, 0.10 mol L-1, Ce3+/ZrO2+=1:1) at 50 °C for 5 min. Next, the substrate was heated at 550 ºC for 15 min; this procedure was repeated five times. The mass of catalyst adhered to the monolith after the impregnation procedure was gravimetrically determined for each preparation; for this, previously dried cordierite samples (in vacuum in a desicator over silica) wer (...truncated)