PREPARATION AND CHARACTERIZATION OF CuMgAl HYDROTALCITES USED IN SOOT COMBUSTION.

MERINO NORA A.; M. LUCIA RUIZ; SANDRA M. MOSCONI; NORA A. COMELLI; ILEANA DANIELA LICK; ENRIQUE RODRÍGUEZ CASTELLÓN; MARTA I. PONZI

Lyon

Simposio; The fourth International Symposium on "Calorimetry and thermal analysis in Catalysis"; 2012

In this work,  results of the  CuMgAl  hydrotalcites  (HT)  characterization  are shown.  The influence of the temperature in the preparation steps (precipitation and precipitate washed) of HT was particularly studied. In previous works of our group, a beneficial effect of the nitrate presence was observed  in the soot combustion reaction  due to its contribution  to the redox cycle. As a consequence, samples with different conditions of washed (eg. amount of solvent) were  prepared, in order to  avoid the complete elimination of the nitrates and,  as a consequence, improve the combustion reaction. The thermal transformations  that could occur  in  HT  were analyzed by means of thermogravimetric analysis (TGA-DTA).  The surface areas and pore volumes were determined from N2 adsorption  isotherms measured,  the crystalline structure of the catalysts was studied by X-ray diffraction, and the presence of anions in the catalysts was studied by FTIR with a Perkin-Elmer Spectrum RX1 instrument.  Samples were also studied  by XPS. After being calcined, the HT  were used in the Diesel soot combustion reaction.  Two equipments were used to carry out  these  catalytic experiments: a thermogravimetric reactor with an O2/He feed and a fixed bed reactor with NO/O2/He or O2/He feed. In the first case, the particulate matter combustion reaction was performed in a thermobalance (TG-60 Shimadzu) and 10 mg of solid were used. The conditions used were heating rate of 10 ºC min-1 with a 40 ml min-1 Air flow and 20 ml min-1 N2 flow. The final temperature was 800 ºC. The particulate matter (Printex-U, Degussa) and the catalyst, in a 1/10 ratio, were milled carefully in an agate mortar (thing contact) before being placed  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. In the second case, a quartz reactor (id = 0.8 cm) was used. The reaction mixture was obtained from three lines controlled individually: NO/He, O2/He and He for balance. The mixture composition was 8% vol of O2 and 1500 ppm of NO (total flow = 50 ml min-1). The mass of particulate matter and catalyst (1/10  mass ratio) loaded in the reactor was 33 mg. The particulate matter was mixed with the catalyst with spatula (loose contact). The temperature range studied  was  between 200-600 °C and the heating rate  was  1.5 ºC min-1. Reaction products were analyzed with a gas chromatograph Shimadzu model GC-8A provided with a TCD detector. The separation of products was carried out in a concentric column CTRI of Altech. This system allowed identifying and quantifying O2, CO2 and CO peaks. The amount of combusted soot was calculated from the chromatographic information of CO2 and CO. Based on the results obtained,  it could be concluded that a  high  reproducibility in  the synthesis of these catalysts was attained, the precipitation temperature had not a  remarkable effect and the washed of the final precipitate produced the elimination of the free nitrate ions, which are necessary for the studied reaction. The temperature of the maximum combustion rate without catalyst was  612 ºC, while with the  slightly washed and  intensely washed HT were 477 ºC and 527 °C, respectively.