The development of gas separation processes dealing with very low concentration ranges is a rapidly growing domain with key applications such as trace detection, air purification from harmful pollutants, etc. Yet, the design of efficient technologies in this field is hampered by the lack of robust strategies to predict the gas selectivity of optimal adsorbents from simple pure gas adsorption data. Here, the selectivity predicted using different methods, namely Henry's method and the ideal adsorbed solution theory (IAST), are compared with the true selectivity obtained using breakthrough experiments. As a case study, these methods are discussed when applied to Xe/Kr separation in two different process conditions using different adsorbents (an active carbon and two silver-doped adsorbents). Typical data show that Henry's method, in which selectivity is assumed to correspond to the ratio of Henry's constant measured for each gas of the mixture, should be considered with caution as it is very sensitive to the pressure range considered but also the number of points used for affinity assessment. IAST is found to be more accurate provided its applicability to predict gas coadsorption from pure gas adsorption data is first established. However, even when applicable, the case of very low concentrations remains a problem as it leads to very large uncertainties in the selectivity predicted using IAST. We discuss how typical errors in assessing the selectivities using the different methods lead to nonoptimal adsorbent choices for a given separation process. Finally we demonstrate that Agloaded zeolite shows xenon capacities and Xe/Kr selectivities that surpass all other materials.

Silver metal nanoparticles are among the most widely studied nanoparticles. They are widely used heterogeneous catalysts used for many purposes such as antisepsis, hydrogenation, and carboxylation but also for the trapping of xenon in nuclear test and detection facilities. The catalytic activity and adsorption capacity of silver nanoparticles, which depend on their size distribution and dispersion on the support, generally decrease with time because of agglomeration of the metal into larger particles. In this study, we quantified the sintering process of silver nanoparticles supported in Zeolite Socony Mobil 5 (ZSM-5) zeolite. It was found that 85% of the sintering process of the silver nanoparticles was driven by Ostwald ripening. We found that silver nanoparticles are trapped in porous cavities that are meso- or macroporous defects in the zeolite. Although this phenomenon limits the amount of silver that diffuses to the zeolite external surface, it does not prevent the formation of large particles by atom migration. The presence of chloride reactants facilitates the sintering phenomenon by lowering the energy barrier. This finding provides a rational basis for the design of silver-containing zeolite-based heterogeneous catalysts.

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.

The adsorption mechanism of xenon on three noble metal clusters (M = Ag, Au, and Cu) has been investigated in the framework of density functional theory (DFT) within generalized gradient approximation (GGA-PBE). The ab initio calculations were performed with the quantum molecular dynamics (QMD) package ABINIT using the projector augmented (PAW) formalism. The spin–orbit coupling (SOC) and dispersion effects (Van der Waals DFT-D3) have been taken into account. According to these calculations, the M–Xe bonds are partly covalent and electrostatic and their contribution depends on the cluster size and nature. This study underlines the importance of using the SOC and the Van der Waals (VdW) effects. Based on these results, copper nanoparticles have the highest affinity for interaction with xenon compared with silver and gold.