Endothermic effects in porous silica doped with silver nanoparticles and the formation of by atmospheric oxygen via chemisorption

Porous silica dispersed with silver (Ag) nanoparticles (about 3 nm in diameter) within its pores has been prepared by a new method. The microstructures and the size distribution of particles within pores have been examined by transmission electron microscopy and the Brunauer - Emmett - Teller technique. The Ag nanoparticles are uniformly dispersed within the pores of silica; the particle size follows the log-normal distribution function. The thermal effect of this porous composite, exposed to ambient air for different times, has been investigated by differential scanning calorimetry and thermogravimetric analysis. An endothermic peak has been found for all the doped samples measured. At room temperature (298 K), a short exposure (2 d) to dry air leads to a wide endothermic peak at around 400 K; long exposure (1 month) results in a wider endothermic peak at around 450 K; when the sample was exposed to ambient air with relative humidity greater than 60%, a much higher endothermic peak exists at around 460 K. From the experimental results and discussions of adsorption and oxidation, the endothermic peaks for the samples exposed to dry air can be mainly attributed to the desorption of oxygen physisorbed and chemisorbed on the surface of the Ag particles within the pores, for lower- and higher-temperature peaks, respectively, and the corresponding desorption enthalpy values were estimated to be about 0.26 eV and 0.90 eV, respectively. For the sample exposed to humid air, the endothermic peak originates from the decomposition of silver oxide () formed on the surface layer of the Ag particles, and the bond energy of Ag - O in the film was estimated to be about 1.8 eV.