This paper examines how façade morphology affects the potential of photocatalytic air purification in an urban context. Addressing pollutant accumulation in street-canyon conditions, the façade is treated as an active system that, when coated with TiO₂, can contribute to the degradation of NOₓ and VOC compounds. The study applies a parametric workflow in Rhino/Grasshopper and compares four façade variants of identical dimensions: a flat reference panel and three gyroid-based TPMS structures with different porosity levels. The evaluation is carried out using three geometric indicators: total exposed surface area (A), projected openness (O), and a combined index (IEPO). Results indicate that the medium-porosity gyroid achieves the most favorable balance between active surface and permeability, and the A–O–IEPO method is proposed as a fast, repeatable decision-support tool for early-stage design of photocatalytic façades.