Photo-thermo-catalytic or PTC purification of process gasses (i.e., air, flue gases, and others) from NOx is presented in this study. A discussion of temperature’s role in photocatalytic NOx removal and the progress of photo-thermo-catalytic reactors for the NOx removal process are presented. Lab- and pilot-scale reactors are described. The impact of temperature on the photocatalytic conversion of hydrocarbons is analyzed with regard to its relation to the photocatalytic selective reduction of NOx (photo-SCR). Another important issue is light transfer in pilot-scale reactors due to the sensitivity of light sources to temperature. Examples of light transfer solutions in photo-thermo-catalytic reactors are presented. Finally, the further development of photo-thermo-catalytic reactors is discussed, including pressurized systems.