The electrochemical reduction of nitrate anions in aqueous 0.1 M perchloric acid has been studied using Pt(S)-[n{1 1 0} x {1 1 1}] and Pt(S)-[n{1 1 0} x {1 0 0}] single crystal electrodes. It is demonstrated that the presence of Pt{1 1 0} adsorption sites is associated with a single, broad nitrate reduction peak centred at 0.18 V (RHE). Moreover, depending on the cooling environment used after flame-annealing (CO, H-2, Ar, air, nitrogen), the surface concentration of such sites varies which in turn regulates the nitrate reduction current density achievable for a given stepped Pt{h k l} electrode. The origin of this phenomenon is the propensity of the clean Pt{1 1 0} basal plane (and vicinal surfaces containing this plane) to reconstruct towards a stable (1 x 2) phase with strong CO chemisorption favouring formation of larger Pt{1 1 0}-(1 x 1) domains. In contrast, argon/air-cooling appears to promote the development of a largely (1 x 2) reconstructed surface which is much less active for nitrate reduction since the surface density of Pt{1 1 0}-(1 x 1) terrace sites is significantly diminished. Interestingly, hydrogen-cooling affords nitrate reduction activity intermediate between these two extremes. We suggest that under this particular preparation condition, a partially deconstructed (1 x 1) phase forms containing the "excess" 50% of surface atoms (originating from the (1 x 2) phase) sitting proud of the surface in the form of small (1 x 1) islands, together with residual (1 x 2) missing row regions. Hence, after hydrogen cooling, the nominal Pt{1 1 0} surface plane is speculated to exhibit a wider distribution of smaller terrace widths than found with CO cooling together with residual areas of (1 x 2). The weaker chemisorption of hydrogen apparently limits the size of the Pt{1 1 0}-(1 x 1) domains achievable and consequently, nitrate reduction activity is diminished. Based on these findings, it is proposed that nitrate reduction may be used as a quantitative electrochemical probe of Pt{1 1 0}-(1 x 1) sites at Pt nanoparticles in an analogous fashion to the method of ammonia electrooxidation presently used to quantify the surface abundance of Pt{1 0 0} sites. (C) 2019 Elsevier Inc. All rights reserved.