Volumetric thermal capacity of solids is higher than that of gases by three orders of magnitude. We show that this leads to a dynamic response of a fixed bed reactor involving a solid phase reaction which is sluggish compared to that involving a gas phase reaction of the same kinetic and thermodynamic parameters. In the dynamic regime where the inlet gas temperature is lower than the initial bed temperature, fixed bed reactors exhibit the wrong way behavior. We show that in this regime, the transient maximum temperature rise and the steepness of the temperature front are affected by the phase in which reaction occurs, by the inlet gas temperature and by the volumetric heat transfer rate. The maximum temperature rise in the bed is lower for the solid phase reaction and decreases with a decrease in the volumetric heat transfer coefficient. These and other new findings are obtained by our newly developed robust, completely implicit spatially and temporally adaptive numerical algorithm which can capture steep moving fronts in fixed bed reactors.