Zirconia-supported silver nanoparticles for the catalytic elimination of contaminants.

M. MONTAÑA; MARIA SILVIA LEGUIZAMON APARICIO; MARCO A. OCSACHOQUE; MONICA L. CASELLA; I. DANIELA LICK

Santa Fe

Conferencia; VI San Luis School and Conference on Surfaces, Interfaces and Catalysis; 2018

UNL-CONICET

This work presents the physicochemical characterization and the activity of zirconia-supported silver catalysts for the catalytic total oxidation at low temperatures of pollutants present in diesel engine exhaust (naphthalene and soot) which are released by the atmosphere generating serious environmental and human health problem. This process can be carried out in the catalytic filters located in the exhaust pipe of automobiles. A prominent characteristic of this catalyst is the ability of metal silver to activate de oxygen though to dissociative adsorption, a capacity that can be promoted synergistically with a mobility of oxygen provided by the support [1]. In this context, the zirconia with its capacity to generate oxygen vacancies is presented as an adequate support [2]. There are studies that indicate that some hydrocarbons can be adsorbed on silver metal [3]. A series of silver-supported catalysts AgxZ (x=1, 5 and 10wt.%, Z=zirconia) were prepared by impregnating the support with aqueous solution of silver nitrate in an ammoniacal medium. The support, zirconium oxide hydrate ZrO2.nH2O, was prepared in the laboratory by hydrolysis of zirconyl oxychloride, ZrOCl2.6H2O in an ammonium hydroxide medium. The catalysts were obtained by calcination of the precursors at 600°C for 2h. The catalysts were studied by various physical chemistry characterization techniques as textural and surface studies (BET), X-ray diffraction (XRD), morphological by microscopy SEM-EDS and TEM, programmed temperature reduction (TPR) and UV-vis diffuse reflectance spectroscopy. The results by BET show that the adsorption isotherm is of IV type and presents a hysteresis loop associated with filling process of the mesopores. The increase in pore size of the Ag5Z catalyst compared to the other catalysts is evident. The zirconia support exhibits a high specific surface value, 340m2/g. By increasing the metallic content, the surface area gets smaller (Ag5Z: Sg=24m2/g). The XRD analysis shows that all silver catalysts have a predominantly tetragonal metastable crystal structure of zirconia. The appearance of this phase indicates an interaction between zirconia and silver, which is generated in the process of oxolation-crystallization of the support. On the other hand, there are no line diffraction indicating the presence of crystal structures of Ag2O, nor of metallic silver, Ag°, if these species exist they are present with a very low crystal size and could not be observed by XRD equipment.The metallic catalyst presents small scattered particles over the support that can be visualized by the mapping with the SEM EDS analysis and shows a high concentration of silver. The semiquantitative results according to EDS show that the surface is enriched in silver for the catalysts Ag5Z and Ag10Z. The Ag1Z catalyst did not allow its quantification for its low concentration. The TPR analysis is seen that the silver is in great proportion in a reduced state (peak to low temperature 90°C). The UV-vis spectra is observed an energy absorption band located between 330 nm and 390 nm, where particles of metallic silver whose size is small (3-5 nm) are absorbed. The results of TEM for the Ag5Z catalyst indicate that the metal nanoparticles are dispersed on the surface of support and their size ranges from 2 to 10 nm, (dp=5.3nm).The catalytic performance for the oxidation of naphthalene and soot show that the catalysts are active for these reactions. In naphthalene this presents a T50 (temperature at which 50% conversion of the hydrocarbon to CO2 is reached) at 430°C and in soot at 650°C. In the case of naphthalene, the most active catalyst is Ag5Z and for the combustion reaction of soot, the most active catalyst is Ag10Z. Silver is introduced into the zirconia, generating Ag-ZrO2 catalysts with high activity for the oxidation of naphthalene and diesel soot. Silver can contribute with zirconia in the catalytic redox cycle, through a synergistic effect, providing sites that facilitate the dissociative adsorption of molecular O2 generating oxygen active species.