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

Bis-porphyrin nanocages (MBiCage, M = FeCl, Co, Zn) and their host-guest complexes with C and C were used to examine how molecular porosity and interactions with carbon nanomaterials affect the CO reduction activity of metalloporphyrin electrocatalysts. The cages were found to adsorb on carbon black to provide electrocatalytic inks with excellent accessibilities of the metal sites (≈50%) even at high metal loadings (2500 nmol cm), enabling good activity for reducing CO to CO. A complex of C bound inside (FeCl)BiCage achieves high current densities for CO formation at low overpotentials (|j| >7 mA cm, η = 320 mV; >13.5 mA cm, η = 520 mV) with ≥95% Faradaic efficiency (FE), and CoBiCage achieves high turnover frequencies (≈1300 h, η = 520 mV) with 90% FE. In general, blocking the pore with C or C improves the catalytic performance of (FeCl)BiCage and has only small effects on CoBiCage, indicating that the good catalytic properties of the cages cannot be attributed to their internal pores. Neither enhanced electron transfer rates nor metal-fullerene interactions appear to underlie the ability of C/C to improve the performance of (FeCl)BiCage, in contrast to effects often proposed for other carbon nanosupports.