Study on the roles of WO3 and MoO3 in the acid, redox, and re-oxidative features of CoOx adsorbents for Hg0 capture

In this article, serial MoO3- or WO3-modified CoOx adsorbents were synthesized using the sol-gel method. With the introduction of WO3, an enhanced Hg0 removal ability was achieved within the investigated temperature range; compared to the virgin CoOx, the theoretical Hg0 capture capacity of WO3-CoOx almost tripled, doubled, and even increased by sixteen times at 150, 200, and 250 °C, respectively. While adding MoO3 resulted in a shift of the adsorbent's operation temperature window towards higher temperatures. Doping WO3 or MoO3 introduced extra acid sites onto the CoOx surface, which facilitated the adsorption of Hg0. The existing certain amount of active Co3+ could still efficiently oxidize the adsorbed Hg0, thus enabling W-Co with an improved Hg0 capture capacity. By contrast, the relatively low oxidative ability of Co3+ could not effectively oxidize the adsorbed Hg0, thus interfering with the Hg0 removal process over Mo-Co at lower temperatures. As the Hg0 capture process continued, the consumption of Co3+ and acid sites caused a gradual increment in the outlet Hg0 concentration. The modification of MoO3 or WO3 accelerated the re-oxidation of Co3+, which alleviated the aforementioned negative effects, thus aiding in retaining a relatively stable Hg0 removal performance during the continual Hg0 capture process.