Why do copper-based catalysts need to be deeply reduced?

The main chemical components of copper-based catalysts are CuO-ZnO-Al2O3; these are inactive before reduction. They only become active after reduction, during which CuO in the catalyst components is reduced to Cu+ or metallic copper, and then fuses together with ZnO present in the components. Therefore, copper-based catalysts must be reduced before being put into use. After production, the methanol catalyst exists in the form of CuO in its oxidized state. Since the properties of this oxidized state are relatively stable, it is suitable for storage and transportation. However, CuO does not serve as a catalyst in methanol reactions; therefore, the catalyst must be reduced to its active Cu–CuO form in order to facilitate catalytic reactions. This process is known as catalyst reduction.

Is Cu the main component playing a key role in the active ingredient? Are they still Cu+ ions? Deep restoration is aimed at restoring the activity of the difficult-to-reduce active centers within the catalyst particles. Due to the effect of diffusion, the reduction process usually occurs first on the surface of the catalyst particles; as the reduction progresses, it gradually moves toward deeper reduction (which requires an increasing temperature). The overall activity of the catalyst is related to many factors; for example, pressure drop control during the filling of each tube in a tubular reactor is crucial. The catalyst should be filled evenly, and care must be taken to prevent catalyst bridging. (1) The key to the reduction process is controlling the rate of the reduction reaction, which is primarily influenced by hydrogen concentration and temperature. Therefore, the hydrogen concentration must be strictly controlled within the allowable range, and a steady temperature increase as well as uniform water output are required. (2) During the reduction process, the principle of \"increasing hydrogen pressure without raising temperature, and increasing temperature without increasing hydrogen pressure\" should be followed.

It is the monovalent copper ions that are active; therefore, the oxidized form of copper must be reduced before use.