Dimensionally and compositionally tuned bimetallic nanoparticles are synthesized in an atmospheric pressure microplasma reactor and found to catalyze carbon nanotube (CNT) growth at lower temperatures and higher growth rates than monometallic nanoparticles. Kinetic studies show that the activation energy for CNT growth is lowered substantially with nanoparticle alloys, demonstrating that intrinsic catalytic activity is intimately related to chemical composition.