Steel mill offgas separation and purification

A method comprising:

• contacting an offgas stream comprising H2, H2O, CO, CO2, and at least one impurity comprising COS with a pretreating adsorbent in a pretreating adsorbent bed to catalyze a reaction of H2O and COS to form H2S and CO2 in the offgas stream and remove H2S from the offgas stream to produce a treated gas stream;
• and
• separating the treated gas stream to produce a carbon dioxide-enriched stream and a carbon dioxide-depleted stream;
• wherein the pretreating adsorbent comprises particles with a combination of an H2S-adsorbent and at least one metal oxide.