The effect mechanism of HCl on chromium removal by CaO sorbent

Abstract

A combination of experiments and density functional theory (DFT) calculations was conducted to explore the effect mechanism of HCl on Cr adsorption over CaO surface. The experimental results suggested that CaO showed good removal performance for Cr. The best Cr adsorption efficiency was 74.51% at 1000 °C in N2. HCl had an adverse effect on Cr removal due to the competition with Cr for active sites and the promotion effect on Cr volatilization by HCl. O2 can offset the inhibitory effect of HCl on Cr capture by CaO with the coexistence of HCl. The XPS results suggested that Cr capture by CaO contained adsorption and oxidation. The combined action of HCl and O2 can further promote the oxidation of chromium on CaO. DFT calculations demonstrated that HCl, CrCl3, and CrO2Cl2 adsorptions on CaO (001) surface were chemisorptions with the adsorption energies of −170.32, −288.25 and −125.56 kJ/mol, respectively. CrCl3 (Cr(III)) was formed on CaO surface via a five-step reaction pathway (Cr → Cr(ads) → CrCl(ads) → CrCl2(ads) → CrCl3(ads) → CrCl3). CrCl2 formation was the rate-determining step, and the energy barrier was 160.81 kJ/mol. CrO2Cl2 (Cr(VI)) preferred to form via chlorination-oxidation process with four steps (Cr → Cr(ads) → CrCl2(ads) → CrO2Cl2(ads) → CrO2Cl2). The rate-determining step of CrO2Cl2 formation on CaO surface in the pathway was CrCl2(ads) → CrO2Cl2(ads). The activation energy barrier of CrO2Cl2 formation on CaO surface was 238.60 kJ/mol. This study of reaction mechanism contributed to deeply understand the effect of HCl on the transformation of chromium species over CaO surface.