Adsorbed organic modifiers can alter selectivity of metal catalysts by modifying reactant, intermediate, or product adsorption affinities and configurations. Herein, we show how alkylamine self-assembled monolayers with varying surface densities can be used to tune selectivity to desired hydrogenation products of nitrobenzene (NB) reduction on a Pt (111) catalyst. Nitrobenzene is a toxic environmental pollutant with deleterious health effects, and its selective conversion to valuable chemicals can both convert this pollutant and improve catalytic process efficiency. Density functional theory (DFT) calculations demonstrate that the selectivity of NB reduction to phenylhydroxylamine (PHA) is achieved by controlling the surface crowding, with specific sites exposed for the selective reduction of NB on the Pt (111) surface through the selection of alkylamine modifier surface density. Surface crowding forces NB and subsequent reaction intermediates to bind with their long axis vertical to the Pt (111) surface, increasing the selectivity to the desired product, PHA. This surface crowding serves both to enhance selectivity and provide insight into the reaction mechanism of NB reduction.