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Effects of morphology and cesium promotion over silver nanoparticles catalysts in the styrene epoxidation

Chimentao, Ricardo José, Medina, Francesc, Sueiras, Jesús Eduardo, Fierro, José Luís García, Cesteros, Yolanda, Salagre, Pilar
Journal of materials science 2007 v.42 no.10 pp. 3307-3314
X-ray diffraction, X-ray photoelectron spectroscopy, aluminum oxide, catalysts, catalytic activity, cesium, epoxidation reactions, nanosilver, nanowires, oxidation, oxygen, phenylacetaldehyde, polyols, polyvinylpyrrolidone, scanning electron microscopy, silver, silver nitrate, styrene, temperature, transmission electron microscopy, ultraviolet-visible spectroscopy
Silver nanowires have been obtained by polyol reduction of silver nitrate in presence of polyvinyl-pyrrolidone (PVP). The as-synthesized silver nanowires were deposited on α- Al₂O₃. For comparison silver catalysts were also prepared by wetness impregnation obtaining irregularly shaped silver particles. Epoxidation of styrene to styrene oxide (SO) by molecular oxygen was studied using the silver catalysts. The main products were styrene oxide (SO) and phenylacetaldehyde (Phe). The promotion effect of the Cs on the silver nanowires catalysts was investigated. The Cs loading was in the range of 0–1 wt.% (refereed to silver). Furthermore, the effect of O₂:C₈H₈ molar ratio on the catalytic epoxidation was also investigated. Silver nanowires catalysts showed superior catalytic activity compared to those prepared by impregnation method. Besides, higher O₂:C₈H₈ ratios improved the selectivity to SO. The catalytic activity showed a maximum performance for silver nanowires promoted with 0.25 wt.% of Cs, achieving 94.6% of conversion and total selectivity to desired oxidation products (styrene oxide and phenylacetaldehyde). Moreover, the cesium promotion also contributed to the increase in the selectivity to styrene oxide. Temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) were employed to detect the presence of different species of oxygen in the catalysts indicating that subsurface oxygen was beneficial for the epoxidation. The samples were also structurally characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible absorption spectra and selected area electron diffraction pattern (SAED).