Accelerated growth of carbon nanofibers using physical mixtures and alloys of Pd and Co in an ethylene–hydrogen environment

The rate of catalytic carbon nanofiber formation from a mixture of ethylene and hydrogen at 550 °C was found to be dramatically faster over physical mixtures of palladium and cobalt micron scale particles than over either metal independently. The rate correlated with the metal fraction nearly identically for either Pd or Co rich mixtures. The highest rate increase over either pure metal was observed for a 1:1 mass ratio (∼150 times faster), although significant increases were found even at metal ratios of 11:1 (∼45 times faster). There was no direct evidence of extensive alloy formation from the mixed powders which suggests that a synergistic mechanism driven by proximity only may be responsible for the observed rate increases. It is thought a species (e.g. hydrogen atoms) formed at one metal (e.g. palladium) diffuses to the other where it accelerates carbon deposition by affecting the other catalyst material directly, or by generating favorable radical species. Kinetic synergism was also observed for Pd–Co alloys, although it was clearly less dramatic than that found for mixtures. Still, the fundamental similarity in behavior suggests that on the alloy surface two site types exist: one primarily Pd and one primarily Co.

Graphical abstractDramatic increases in solid carbon deposition rates using palladium and cobalt are achievable at 550 °C while flowing ethylene and hydrogen by simple mixing of the powders of the two otherwise inactive metals. The synergistic mechanism is attributed to a favorable change in gas chemistry coupled with close proximity.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Palladium and cobalt mixtures catalyze carbon deposition readily. ► When separate, neither metal is effective at carbon deposition. ► No extensive alloying was observed under normal reaction conditions. ► The synergism is attributed to favorable gaseous species being created. ► Close proximity is necessary for utilization of radicals by the other metal.