Effect of Chlorine-Containing VOCs on Silver Migration and Sintering in ZSM-5 Used in a TSA Process

Abstract

Silver nanoparticles are currently one of the most studied nanostructured nanomaterials. Because nanoparticle size and dispersion act together in determining a material’s physical and chemical properties, there is a continuous quest to develop size-controlled synthesis methods. Nonetheless, the instability of the nanometer-sized particles, which is caused by their tendency to aggregate irreversibly into larger particles, remains a recurrent problem. The use of confining scaffolds, such as the regular system of cages in a crystalline zeolite-type material, is often reported in the literature as an efficient solution to overcome particle migration at the surface. Silver nanoparticles encapsulated in ZSM-5 (Ag@ZSM-5) represent a new generation of adsorbent for Xe enrichment from the atmosphere that is currently being developed at the pilot scale in a Temperature Swing Adsorption (TSA) process. In this study, we have found that the presence of Cl-containing compounds in the air (VOCs) leads to a poisoning of the active silver phase by the formation of silver chloride. By a careful study of process parameters, we have found that most of the chlorine can be removed by heat treatment above 573 K so that the adsorption properties of silver are regenerated. That said, when applying 573 K temperature regeneration at the pilot scale, we observe a very minor but observable decay of xenon adsorption capacity that continues cycle after cycle. The mechanism of capacity decay is discussed in terms of (i) the residual presence of Cl at the surface of silver nanoparticles, (ii) the aggregation of silver nanoparticles into larger particles (sintering mechanism), and (iii) the acceleration of silver particle migration to the surface and sintering.