The effect of reduction of pretreated NiO–ZnO catalysts on the water–gas shift reaction for hydrogen production as studied by in situ DRIFTS/MS

• NiO–ZnO catalyst was reduced using the low temperature with good catalytic activity.
• In situ DRIFTS/MS studied the catalysts of process and species of WGS reaction.
• H400 catalyst has the stronger linearly adsorbed CO within the whole WGS reaction.
• H220 catalyst showed the higher activity due to the more active sites and species.
• The adsorbed species reacted to the catalysts that mechanisms were proposed.

The water–gas shift (WGS) reaction on co-precipitated NiO–ZnO catalysts at different reduction temperatures has been studied by a temperature-programmed reaction using in situ diffuse reflectance infrared Fourier Transform Spectroscopy, coupled with mass spectroscopic (in situ DRIFTS/MS) techniques. The results reveal that a catalyst reduced at 493 K (labeled H220) showed higher activity than one reduced at 673 K (labeled H400) due to the ability of NiO on the H220 catalyst to promote CO conversion of the WGS reaction. In situ DRIFTS/MS studies show that there are three adsorbed species over the H220 catalyst at room temperature: adsorbed CO bands, molecularly adsorbed H2O and carboxyl species. Increasing the temperature to 423 K led to the emergence of CO2 and H2 and the disappearance of carboxyl species. However, the low catalytic activity of the H400 catalyst could be attributed to the conversion of the NiO sites to reduced Ni metal sites, which (i) adsorbed CO as the strong linearly bonded CO on the catalyst surface, slowing down the CO reaction, and (ii) showed a lower H2O uptake.