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.
Highlights
Among the noble gases, xenon is of the utmost economic importance, with numerous applications such as energy-efficient lightning [1], medicine [2], and chemical analysis [3]
When used in SPALAX process conditions, after permeation membranes where the Xe concentration is approximately 5 ppm, the adsorption capacities of the Ag@ZSM-5 zeolite tend to decrease after several Temperature Swing Adsorption (TSA) cycles
Starting from cycle 6, the flow rate and the duration were progressively increased to reach a contact time of 19 h and a flow rate of 160 mL/min, and quickly the xenon adsorption capacities started to decrease. This experiment highlights the impact of the identified chlorinated compounds on the Ag@ZSM-5 zeolite. It seems that there is a mixture of two contributions: the reaction of gas with solid to produce an inactive phase (AgCl) and the sintering of silver nanoparticles promoted by the chlorinated compounds
Summary
Xenon is of the utmost economic importance, with numerous applications such as energy-efficient lightning [1], medicine [2], and chemical analysis [3]. The development of alternative enrichment and separation processes that are more cost-efficient should facilitate the use of xenon by decreasing its price and allowing the expansion of its market. The enrichment and separation of Xe and Kr is relevant for radioactivity release monitoring. In this context, the CEA in France has developed the SPALAX process [6,7,11] (Système de Prélèvement Automatique en Ligne avec l’Analyse du Xénon), which concentrates and purifies xenon from air samples prior to analytical measurement. A Xe/Kr/N2 separation is carried out in columns filled with active carbons in a Temperature Swing Adsorption (TSA) process, resulting in a xenon-enriched fraction
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