The effect of sodium species on methanol synthesis and water–gas shift Cu/ZnO catalysts: utilising high purity zincian georgeite

Abstract

The effect of sodium species on the physical and catalytic properties of Cu/ZnO catalysts derived from zincian georgeite has been investigated. Catalysts prepared with <100 ppm to 2.1 wt% Na⁺, using a supercritical CO₂ antisolvent technique, were characterised and tested for the low temperature water–gas shift reaction and also CO₂ hydrogenation to methanol. It was found that zincian georgeite catalyst precursor stability was dependent on the Na⁺ concentration, with the 2.1 wt% Na⁺-containing sample uncontrollably ageing to malachite and sodium zinc carbonate. Samples with lower Na⁺ contents (<100–2500 ppm) remained as the amorphous zincian georgeite phase, which on calcination and reduction resulted in similar CuO/Cu particle sizes and Cu surface areas. The aged 2.1 wt% Na⁺ containing sample, after calcination and reduction, was found to comprise of larger CuO crystallites and a lower Cu surface area. However, calcination of the high Na⁺ sample immediately after precipitation (before ageing) resulted in a comparable CuO/Cu particle size to the lower (<100–2500 ppm) Na⁺ containing samples, but with a lower Cu surface area, which indicates that Na⁺ species block Cu sites. Activity of the catalysts for the water–gas shift reaction and methanol yields in the methanol synthesis reaction correlated with Na⁺ content, suggesting that Na⁺ directly poisons the catalyst. In situ XRD analysis showed that the ZnO crystallite size and consequently Cu crystallite size increased dramatically in the presence of water in a syn-gas reaction mixture, showing that stabilisation of nanocrystalline ZnO is required. Sodium species have a moderate effect on ZnO and Cu crystallite growth rate, with lower Na⁺ content resulting in slightly reduced rates of growth under reaction conditions.