Influence of catalase encapsulation on Cobalt@Nanoporous carbon with multiwall shell for supercapacitor and polyurethane synthesis using carbon dioxide

Abstract

Enzyme encapsulated ZIFs exhibit significant host–guest synergism and protect the relative activity of the enzymes. Elucidation of metallic active sites including the nature of electronics and valance state has been forefront in energy conversion studies supported by high-resolution transmission electron microscopy and synchrotron X-ray absorption fine structure (XAFS) spectrogram. Here, we access metallic cobalt (Co) sites that were set by pyrolysis of catalase encapsulated ZIF-67 (cat@ZIF-67) at a high temperature (700 °C) to obtain cat@ZIF-67-NC. The local environment of Co-sites from X-ray photoelectron spectroscopic (XPS) and extended X-ray absorption fine structure (EXAFS) analysis suggests the presence of Co sites in a metallic valance state. HR-TEM and STEM analysis further revealed the Co particles encapsulated with multiwall shell morphology as support. An activity reversal is witnessed, where cat@ZIF-67-NC exhibited higher activity in CO2-cycloaddition-polymerization but was found weaker in capacitance than ZIF-67-NC for 3-electrode supercapacitor applications. The ZIF-67-NC has exhibited maximum Cs of 223 F/g at a scan rate of 5 mV/s, which retained about 87 F/g on increasing scan rate to 200 mV/s, in contrast to cat@ZIF-67-NC with a Cs value of 133.4 F/g at 5 mV/s with improved retention (58 % at 200 mV/s). The contrasting nature of cat-ZIF-67-NC and ZIF-67-NC in supercapacitor and CO2 cycloaddition-polymerization originates from encapsulated catalase in ZIF-67 based on XPS and EXAFS analysis. The electronic configuration and surface composition analysis using EXAFS and XPS techniques revealed the presence of metallic Co-sites responsible for supercapacitor and CO2-cycloaddition-polymerization. An influence of encapsulated catalase is evident in a decrease in the capacitance for cat@ZIF-67-NC due to metallic Co-sites. Co-sites in cat-ZIF-67-NC are more active in producing polyurethane by a non-isocyanate route via CO2-cycloaddition.