Terahertz chemical microscopy (TCM) was applied to visualize the distribution of the work function shift of catalytic metals under hydrogen gas. TCM measures the chemical potential on the surface of a SiO2/Si/sapphire sensing plate without any contact with the plate. By controlling the bias voltage between an electrode on the SiO2 surface and the Si layer, the relationship between the voltage and the THz amplitude from the sensing plate can be obtained. As a demonstration, two types of structures were fabricated on the sensing plate, and the work function shifts due to catalytic reactions were visualized. (C) 2012 Optical Society of America