Combustion of soot particulate over Cu-Mg-Al oxides catalysts derived from hydrotalcite.

NORA A. MERINO; M. LUCÍA RUIZ; SANDRA M. MOSCONI; NORA A. COMELLI; E. RODRÍGUEZ-CASTELLÓN; A. JIMÉNEZ-LÓPEZ; MARTA I. PONZI

Munich

Congreso; 15th International Congress on Catalysis 2012; 2012

Hydrotalcites are classified as anionic clays with a general formula [M(II)1-xM(III)x(OH)2] (Am-)x/m×nH2O, where M(II) is a divalent cation, M(III) a trivalent cation, A an anion with charge m- and x the M(III)/(M(III)+M(II)) molar ratio. Synthesis of these materials containing transition metal ions, especially copper, in the sheets is of particular interest because of their selective oxidation properties. The Mg/Al with molar ratio 3:1 and Cu1.5Mg1.5/Al hydrotalcites were prepared by the coprecipitation method; simultaneously adding a mixed salt solution and mixed basic solution dropwise into distilled water under vigorous mechanical stirring, whilst maintaining a constant pH and temperature. The mixed salt solutions consisted of metal nitrates of Mg, Al and Cu. The slurry was aged under vigorous stirring at 70 °C for 30 minutes. The precipitate was washed several times until pH 7 was reached and then dried at 110°C for 12 h. The hydrotalcite-like compounds were obtained by calcining the hydrotalcites at 600°C for 4 h and at 900 ºC for 4 h. The solids were divided into two groups, the first one, without Cu, was labeled as HTMgAl and the second one, with copper, as HTCuMgAl. Hydrotalcites or oxides formed by calcination (hydrotalcite-like compounds) display interesting properties with multiple applications. They can be used as catalysts or catalyst supports due to their large surface areas, basic character, high metallic dispersion and high thermal stability. When these mixtures of oxides contain transition metals, they also display redox properties. X-ray diffraction was studying using Cu K radiation ( = 1.5378 Å, 40 kV, 30 mA). The surface areas and pore volumes were determined from N2 adsorption isotherms measured at -196 ºC. XPS spectra were collected with non-monochromatic Mg Kα radiation (300 W, 15 kV, 1253.6 eV). The PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. The presence of anions in the catalysts was studied by FTIR with a Perkin-Elmer Spectrum RX1 instrument. The surface areas of all catalysts range from 20 to 270 m2g-1. They decreased with the introduction of Cu and the increase of the calcination temperature, due to the growth of crystallite size. In contrast, the pore size distribution remains similar before and after calcination and between the two series of the catalysts. In agreement with these results, XRD analysis shows the presence of the oxides (MgO periclase and CuO tenorite) at 600ºC and the addition of the spinel phase (CuAl2O4) at 900ºC, which also explains the decrease of the specific surface areas. XPS analysis indicates the presence of Cu, Mg and Al ions at the surface. FTIR data gives evidence of the existence of free nitrate ions in all of the solids with copper. Hydrotalcites have been applied in the catalytic abatement of NOx and SOx and in the simultaneous removal of NOx and soot. The combustion of soot needs high reaction temperatures where the storage of NOx is poor. We report here on the catalytic performance of a series of MgAl and CuMgAl hydrotalcites with varying calcinations temperature for the combustion of particulate matter. This research is focused on reducing the temperature for the maximum combustion rate by adding a transitional metal to hydrotalcite-like compound. The particulate matter combustion was performed in a thermobalance with a heating rate of 10 ºC/min with a 40 cm3min-1 air flow and 20 cm3min-1 N2 flow. The particulate matter (Printex-U, Degussa) and the catalyst, in a 1/10 ratio, were milled carefully in an agate mortar (tight contact) before being introduced into the reactor. Weight loss and temperature were recorded as a function of time. The derivative curve (DTGA) was obtained from the weight loss information as a function of time and from this curve the temperature where the combustion rate is maximum (Tmax) was obtained. The observed activities follow the order HTCuMgAl600>HTCuMgAl900>HTMgAl600>HTMgAl900. HTCuMgAl600 catalyst exhibits a Tmax of 445 ºC, at least 100ºC less than the catalysts without Cu. The observed difference in the catalytic performance is related to the redox properties of the catalysts and the nitrate ions presence. Catalysts containing copper have the capacity to transform NO to NO2 and to oxidize CO to CO2.The active species might come from CuO, which act for redox-type mechanism for soot oxidation in the NOx-soot reaction.