A preferential oxidation reactor is provided including a plurality of reactor sections. The reactor sections are individually optimized for operating at a preferred reaction temperature. In one embodiment, each reactor subsection includes a respective coolant flow for manipulating the operating temperature of the respective subsection. In another embodiment, a first section includes a lower temperature catalyst substrate, a second reactor section includes a higher temperature (i.e. normal) catalyst substrate and a third reactor section includes a lower temperature catalyst substrate. Yet another embodiment includes modifying the catalyst substrates of the respective subsections through the inclusion of promoters. Still another embodiment includes varying a density of the catalyst substrate across the reactor sections. Each of the embodiments enable quick light-off of the reactor, while limiting a reverse water-gas shift reaction.