However, as conversion is increased by lowering the gas space velocity there is a sharp transition from an inhibiting to a beneficial role of CO relative to a CO-free feed. With increasing conversion more water is formed, and as water is a far stronger inhibitor to Cu-based catalysts than CO, the beneficial effect of CO arises from the removal of water through the water–gas shift reaction. At low conversion the methanol synthesis rate is thus highest for a CO-free feed that minimizes CO inhibition, whereas the rate at high conversion is optimal with a CO-rich syngas that minimizes water inhibition. The transition between these two types of behavior occurs around the conversion range leading to 1 mol% of methanol in the effluent gas. Hence, CO has a beneficial role at commercial high conversion conditions. The ZnO support exerts a strong, beneficial support effect at low conversion conditions, where the strong reductant CO has a purely negative effect. This could suggest that reduced Zn-sites (oxygen vacancies in ZnO or Cu-Zn surface alloy sites), whose concentration are expected to depend on the reductive potential of the atmosphere, are not critical to the support effect from ZnO. At both industrial conditions (523 K, 50 bar), mild conditions (448 K, atm. pressure) and in a nominally oxidizing gas (498 K, 20 bar with CO2 > H2) the addition of CO to the feed is detrimental to the activity of Cu/ZnO/Al2O3 at low conversion conditions. This supports that CO plays no beneficial role by facilitating ZnO-reduction and possibly that Zn alloyed into the Cu surface is unimportant for catalytic activity.
Niels D. Nielsen, Anker D.Jensen, Jakob M.Christensen
Journal of Catalysis, Volume 393, January 2021, Pages 324-334
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