Using Host-Guest Chemistry to Examine the Effects of Porosity and Catalyst-Support Interactions on CO2 Reduction

Nanoporous materials and carbon nanotubes are common supports for immobilizing molecular electrocatalysts, but the effects of these nano-supports on catalysis are not well understood. Thus, we developed bis-porphyrin nanocages (M2BiCage) for examining how porosity and interactions with carbon nanomaterials (C60 or C70) affect the CO2 reduction activity of metalloporphyrins. The porous structure of Zn2BiCage was characterized by SC-XRD, and the Fe and Co derivatives were found to adsorb on carbon black to provide inks with excellent accessibilities of the metal sites (~50 %) to H+ and e− even at high metal loadings (2500 nmol cm−2). A complex of C70 bound in (FeCl)2BiCage achieved good current densities for CO formation at low overpotentials (jCO > |5| mA cm-2, η = −330 mV; > |10| mA cm-2, η = −530 mV) with 95 % FE, and Co2BiCage achieved high TOF (~1300 h-1, η = −530 mV) with 90 % FE. Blocking the pore with C60 or C70 improved the activity of the M2BiCages (M = Fe) or had little effect (M = Co), indicating that good catalytic performance of the cages cannot be attributed to porosity. Neither enhanced electron transfer rates nor metal-fullerene interactions appear to underlie the ability of C60/C70 to improve the catalytic performance of (FeCl)2BiCage, in contrast to effects on electrocatalysis often proposed for carbon nano-supports.