This analysis relates catalyst material composition and bimodal pore size characteristics in a direct, quantitative way to the reactivities for simultaneous NO reduction, Hg-0 oxidation, and SO3 production along utility selective catalytic reduction (SCR) reactors. SCR monoliths sustain two chemically distinct regions. In the inlet region, strong NH3 adsorption minimizes the coverage of chlorinated and sulfated surface sites, so NO reduction inhibits Hg-0 and SO2 oxidation. Once the NH3 has been consumed, however, the chlorinated surface coverage surges by orders of magnitude, and the Hg-0 oxidation rate rapidly increases, even while the HCl concentration in the gas phase remains uniform. Ammonia inhibition also eliminates the benefit of the rapid film mass transfer at the SCR inlet from promoting Hg-0 oxidation. In many cases, the Hg-0 oxidation rate becomes limited by film transport soon after the Hg-0 begins to oxidize, so that none of the catalyst internal surface area is utilized. Shifting the pore size distribution toward macropores in a final catalyst stage appears to be an effective means for directly enhancing Hg-0 oxidation. The predictions were validated with pilot-scale data to demonstrate the crucial impact of NH3 inhibition on SCR performance and with full-scale data for catalysts from a single vendor to show quantitative consistency across broad ranges of coal Cl content, gas hourly space velocity (GHSV), NH3/NO ratio, and catalyst specifications.