The unique physical properties of carbon nanotubes (CNTs) have attracted a great deal of interest for potential applications. Despite the availability of a wide range of synthesis techniques, the structure of CNTs has not been controlled sufficiently during growth. In chemical vapor deposition, transition metals are used to catalyze nucleation and growth of CNTs. However, the role of the catalyst in CNT growth remains unknown. Here, we present a systematic investigation of the catalytic properties of Fe and Ni nanoparticles towards CNT growth. In addition to pure monometallic catalysts, our unique synthetic approach allows the preparation of bimetallic nanoparticles which exhibit lower activation energies and growth temperatures. Nanometer-sized metal nanoparticles are produced in a continuous-flow, gas-phase process based on an atmospheric-pressure microplasma. Microplasmas are miniaturized versions of direct-current glow discharges formed between a hollow cathode with a microcavity (d~180 microns) and a counter electrode. Vapor-phase precursors such as nickelocene or ferrocene are introduced into the microplasma and decomposed non-thermally by electron impact to nucleate particles homogenously in the gas phase. Particle growth is limited to the small reactor volume (less than 1 nL) created by the microplasma geometry. As a result, ultrasmall nanoparticles (1-3 nm mean diameter) with narrow size distributions and tunable chemical composition are synthesized. To grow CNTs, a mixture of acetylene (C₂H₂) and hydrogen (H₂) gases are added to the particle flow and heated in a tube furnace at fixed temperatures between 300 and 650^oC. As-grown nanoparticles and nanotubes are continuously monitored in real time using a differential mobility analyzer. Kinetic studies have been performed to determine activation energies of 119 and 73 kJ/mol for monometallic Fe and Ni nanoparticles, respectively. In comparison, NixFe1-x bimetallic nanoparticles are observed to have dramatically different activation energies as low as 28 kJ/mol. Additionally, the nanoparticle alloys are found to catalyze CNT growth at 300^oC, significantly lower than temperatures normally required for monometallic catalysts. These results suggest that the bimetallic catalyst particles are characterized by synergistic effects during the nucleation and growth of CNTs. A possible mechanism for CNT growth based on the diffusion of carbon on the catalyst particles will be presented. Microcharacterization of the catalyst particles and CNTs by high-resolution TEM, X-ray diffraction, and micro Raman spectroscopy will also be discussed.