Catalyst nanostructures refer to catalysts that are designed and synthesized at the nanometer scale, typically between 1 and 100 nanometers. These nanostructures can include nanoparticles, nanoclusters, nanowires, and nanosheets, among others. The unique properties of materials at the nanoscale, such as high surface area to volume ratio, quantum effects, and tunable electronic properties, make them highly effective in catalysis.