Microgravity Effect on Ammonia Oxidation at Platinum Nanoparticles Modified Mesoporous Carbon Supports

Microgravity effect on the electrochemical oxidation of ammonia at platinum nanoparticles modified mesoporous carbon (MPC) substrates, with three different pore diameters, e.g. 64, 100, and 137 Å, have been studied in a parabolic flight between 24,000 and 32,000 ft. Microgravity effects on the chronoamperometric ammonia oxidation current density was a function of the mesoporous carbon support used. This support may be described as graphitic multilayer hollow globules, even though the carbon globules do not present a specific shape as seen in the HRTEM images.  Platinum nanoparticles were successfully chemically deposited, with a high dispersion, throughout the support’s topography.  An onboard accelerometer was the trigger in order to start each chronoamperomtric ammonia oxidation experiments when the microgravity condition of less than 0.02g (i.e. gravitational force) was achieved. Pt/MPC₆₄ sustained the current density between terrestrial and microgravity conditions by a slight margin over Pt/MPC₁₀₀. However, Pt/MPC₁₃₇ resulted with the smallest current density decrease under microgravity conditions versus ground based experiments.  Pt/MPC₁₃₇ has the largest pore diameter and shows a better capacity to sustain the oxidation current, which involves N₂ formation.  This effect can be ascribed to an easier diffusional process obtained by the larger pore diameter, which improves the access for the mobility of the ammonia molecules towards the electroactive sites and the corresponding detachment of the gaseous N₂molecules (bubbles) from the MPC cavities. Pt/MPC₁₃₇ catalyst resulted with the smallest current density decrease under microgravity conditions versus ground based experiments. This MPC support has the largest average pore diameter (137 Å) and shows a better capacity to sustain the oxidation current. This effect can be ascribed to an easier diffusional process facilitated by the larger pore diameter, which improves the access for the mobility of the ammonia molecules towards the electroactive sites and the corresponding detachment of the gaseous N₂ molecules (bubbles) away from them. In conclusion, it was demonstrated that under microgravity environment a porous infrastructure for a catalyst support has an impact on the mass transfer process of electroactive species, and a current density decreasing factor of ca. 50 - 67% must be taken into account. The lowest being the MCP average pore diameter 137 Å. In order to improve the current densities for ammonia oxidation, (100) faceted Pt nanoparticles need to be achieved.