Nanotubos de titanatos modificados com metais: desempenho catalítico na reação de redução catalítica seletiva de NOx por CO

Abstract

Titanate nanotubes were modified with metals (Cr, Co, Ni, Al and Ag) through ion exchange and wet impregnation (Pt). The solids were evaluated by X-Ray Diffraction (XRD), Raman Spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy coupled to Energy Dispersive Spectroscopy (SEM-EDS), Transmission Electron Microscopy (HRTEM), Electron Paramagnetic Resonance (EPR), X-ray Photoelectron Spectroscopy (XPS) and N2 adsorption-desorption isotherms. The evaluation of catalysts in the selective catalytic reduction of NOx in the presence of CO was carried out in a temperature range from 50 to 550 ºC, using a mixture of gases containing 500 ppm of NOx and 1000 ppm of CO. The Na2Ti3O7 phase, whose structure is monoclinic, was obtained in all solids, regardless on the insertion of metals in the interlamellar regions or deposition on the surface. The solids containing Pt and Ag, however showed the corresponding oxide phases, according to the results of XRD, Raman spectroscopy and FTIR. SEM and TEM techniques showed that the solids have particles with nanorod morphology after impregnation by Pt and Ni, while the titanate nanotubes were entangled after the insertion of Cr, Co Ag, and Al. The insertion of Al, Ni and Co in the interlamellar region of the nanotubes led to an increase in textural properties as a result of the increase in the distance between the walls of the nanotubes. In contrast, Pt, Cr and Ag had reduced textural properties, due to the formation of the nanoparticles and deposition of extra-lattice species on the surface of the solids. The catalytic tests in the CO-SCR reaction showed that all catalyst presented a conversion of less than 20% at temperatures below 100 ºC, while the solids containing Pt and Ni reached 92% conversion at 200 and 300 ºC, respectively. The sample containing Co presented conversion around 55% at 350 ºC while the other solids containing Cr, Ag and Al maintained conversion below 20%, regardless of temperature. The high conversion of the Pt-containing solid (NTiPt) at 200 ºC was related to the presence of extra-lattice platinum species, for example PtOx, PtOxCly or Pt(OH)xCly, which contributed to the increase in the acidity of the solid.