Catalisadores a base de Níquel suportados em perovskitas do tipo CexAl2-xO3 para a reforma a seco do Metano

Abstract

Cerium aluminate is a perovskite of the type AmBnO3 which has little studied catalytic properties when synthesized by microwave assisted self-combustion. Therefore, the objective of this work was to produce cerium aluminate via microwave-assisted combustion in a single step using urea (250 % excess) as fuel and apply them as support for catalysts for the production of synthesis gas via methane dry reforming. The supports were named by CexAl2-xO3, where x is the mol content of cerium (x = 0,25; 0,50; 0,75 e 1); After synthesis, the supports were impregnated in water with 10 % nickel by weight and then calcined at 500 ° C for 3 h at 10 ° C.min-1 , being denominate by Ni/CexAl2-xO3 (catalysts). An α-Al2O3 was used as a reference (Ni/α-Al2O3). The x-ray diffraction (XRD), scanning electron microscopy (SEM-EDS and SEM-FEG), thermogravimetric analysis, density the helium gas by pycnometry, programmed temperature reduction, absorption spectroscopy in the region of ultraviolet and visible were the techniques used in the characterizations, beyond the catalytic test "in situ" accomplished in XPD line in National Synchrotron Light Laboratory analyzing stability and evolution of the crystal structure by XRD. The results show that there was aluminate formation in the synthesis with the beginning of the reflectivity increase in 380 nm; the presence of microstructural and macrostructural defects impacted the reflectivity variation in the UV-visible region and in the variation of density values (5.06 to 5.96 g.cm- ³); heterogeneous morphologies expressed by diversified surfaces, rugged reliefs, particulates tending to equiaxial symmetry and signs of circumferential perforations, in addition to prismatic crystals growing parallel to the surface. From methane dry reforming, the conversions of CO2 and CH4 were above 60%, even though there was nanotube coke formation, however, there was no deactivation of the catalysts after 20 h of reaction. All cerium-containing catalysts showed higher CH4 and CO2 conversion and better stability than the catalyst Ni/α-Al2O3. The catalysts with 0.50 and 0.75 mols of cerium presented higher CO2 selectivity and higher coke content. The 0.25 mol percent cerium catalyst presented the highest conversion percentages, better stability and lower coke formation. The Ni/Ce1Al1O3 catalyst presented better ratio of H2/Co (1.02). The catalytic test “in situ” shows stability of the crystal structure (CeAlO3) and the CeAl11O18 phase in one of the catalysts. These results are pertinent, since the work produced in one step perovskite catalytic supports (CexAl2- xO3) with stable structures that positively influenced the performance of nickel-based catalysts applied in the synthesis gas production, besides optimizing the manufacturing process of these catalysts by eliminating the calcining step of the support.