Generation of supported silver nanoparticles by reduction of silver-exchanged LTA zeolite

MAXIMILIANO R. GONZÁLEZ; ELENA I. BASALDELLA; GLADYS E. MACHADO; ANDREA M. PEREYRA

Santa Fe

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

Supported  nanoparticles  remain  a  subject  of  intense  studies  because  of  their  countless  applications  in  many technological  areas  such  as  catalysis,  sensor  design,  electronics, medicine,  etc.  Recent  developments  are  based  on  the control of properties in the nanometer range in order to obtain suitable materials for particular applications. In case of Ag nano species, the specific properties arise from variations in the chemical states and positions of silver entities dispersed in the matrix. Different meso- or microporous materials are being used as support in order to precipitate silver nanoparticles on their external and internal surfaces, such as mesoporous silica MCM-41, SBA-15, faujasites and NaA. Besides, several advantages  are  offered  by  using microporous materials  as  supports  because  they  allow  the  synthesis  of  particles with narrow size distributions and homogeneously dispersed in their internal surfaces. In this work, silver nanoparticles dispersed in a solid matrix consisting of micrometer-sized crystals of zeolite NaA were obtained. The  reduction method  for getting the metallic nanoparticles, as well as the presence of  silver  in the exchange sites  of  the  zeolite,  were  varied  in  order  to  establish  their  influence  on  the  generation  of  supported,  individual,  not segregated silver nanoparticles. Two procedures for obtaining silver nanoparticles were applied: (1) thermal treatment  in O2  atmosphere  and  (2)  chemical  reduction  using  sodium  citrate.  Transmission  electron  microscopy  (TEM)  and  UV-vis spectroscopy  (UV-vis)  analyses  confirmed  the  presence  of  rounded  crystalline  silver  nanoparticles,  homogeneously dispersed on the outer  surface of the zeolite crystals. Silver nanoparticles coexist with non-crystalline  clusters and  silver cations hosted inside the porous structure. The existence of Ag0 and Ag2O nanoparticles was confirmed by X-ray diffraction (XRD). The chemical treatment mostly  led to the  formation of Ag2O nanoparticles. According to the reduction treatment, the  size  of  synthesized  nanoparticles  can  oscillate  from  2  up  to  34  nm,  exceeding  the  pore  diameter  of  the three-dimensional matrix  network.  This  fact  suggests  that  the  Ag  ordering  starts  inside  the matrix  cavities with  cluster formation and subsequent occurrence of cluster linking and growth on the external zeolite surface.