Abstract
Porous silica xerogels were synthesized within 10 h by a two-step sol–gel process under atmospheric conditions. In the first step, tetraethylorthosiloxane (TEOS) was hydrolyzed with water using sulfuric acid as a catalyst. In the second step, water and ammonia were added to the prehydrolyzed silica sol prior to the drying of the samples at 150 °C. The influence of the ammonia concentration and the water content on the physicochemical properties and the water sorption behavior of silica xerogels produced in the aforementioned way was investigated. The resulting silica xerogels were characterized by helium pycnometry, scanning electron microscopy, fourier-transform infrared spectroscopy, nitrogen sorption, and water sorption. It was shown that a higher ammonia concentration leads to an increased pore size of the silica xerogel which starts to adsorb water at a higher relative pressure. With an increased water content during gelation, the silica xerogel has a higher specific surface area and a higher water sorption capacity. Furthermore, silica xerogels can be tailored by a controlled addition of ammonia and water to have a higher water sorption capacity than the reference silica gel over the whole range of relative pressure.
Highlights
Adsorption heat pumps (AHPs) have gained a lot of interest in the past couple of decades since they are considered an environmentally friendly and cost-effective alternative to compressor-based heat pumps
We report the synthesis of silica xerogels derived from tetraethylorthosiloxane (TEOS) within 10 h including hydrolysis, gelation, aging and drying
Chan et al [31] found that the pore volume is increasing when the gel is synthesized with a higher concentration of NaF which is in agreement with our observed data
Summary
Adsorption heat pumps (AHPs) have gained a lot of interest in the past couple of decades since they are considered an environmentally friendly and cost-effective alternative to compressor-based heat pumps. In order to maximize efficiency, the amount of cycled water within the operation window has to be maximized meaning that the sorbent has to be tailored for its application To address this issue, silica gels with tailored porosity and pore size have been proposed to enhance mass transport properties. Water vapor adsorption on silica gel has been investigated intensively owing to the fact that silica gel is used in industry as a drying agent, catalyst support, or filler [11, 24,25,26]. There exists only a few studies that investigate the influence of the pore size on the water sorption behavior through inhouse synthesis of silica gels [2, 27]. By a controlled addition of ammonia and water, silica xerogels can be tailored to have a higher water sorption capacity than the reference silica gel over the whole range of relative pressure
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