Porous nanomaterials find wide-ranging applications in modern medicine, optoelectronics, and catalysis, playing a key role in today's effort to build an electrified, sustainable future. Accurate in situ quantification of their structural and surface properties is required to model their performance and improve their design. In this article, we demonstrate how to assess the porosity, surface area and utilization of a model nanoporous soft-landed copper oxide catalyst layer/carbon interface, which is otherwise difficult to resolve using physisorption or capacitance-based methods. Our work employs electron tomography to characterize the three-dimensional structure of the catalyst layer and combines it with in situ soft X-ray absorption spectroscopy and lead underpotential deposition data to probe the stability and utilization of the catalyst layer under potential bias. The analysis proves that a significant share of the original surface area is exploited, and thus explains product distribution and crossover trends in the electrosynthesis of C