Hydroxyapatite-based biphasic catalysts with plasticity properties and its potential in carbon dioxide fixation

Abstract

• Polarized hydroxyapatite (HAp) fixates carbon into C1, C2 and C3 molecules. • Polarized brushite (Bru) is not catalytically active. • The synergy between Hap and Bru gives plasticity to Hap-Bru catalysts. • The HAp/Bru ratio in the catalyst regulates the C1/C3 ratio in the products. • The HAp-Bru system is a promising green alternative to metal-based electrocatalysts. The design of catalysts with controlled selectivity at will, also known as catalytic plasticity, is a very attractive approach for the recycling of carbon dioxide (CO 2 ). In this work, we study how catalytically active hydroxyapatite (HAp) and brushite (Bru) interact synergistically, allowing the production of formic acid or acetic acid depending on the HAp/Bru ratio in the catalyst. Raman, wide angle X-ray scattering, X-ray photoelectron spectroscopy, scanning electron microscopy and electrochemical impedance spectroscopy studies, combined with an exhaustive revision of the crystalline structure of the catalyst at the atomic level, allowed to discern how the Bru phase can be generated and stabilized at high temperatures. Results clearly indicate that the presence of OH – groups to maintain the crystalline structural integrity in conjunction with Ca 2+ ions less bonded to the lattice fixate carbon into C1, C2 and C3 molecules from CO 2 and allow the evolution from formic to acetic acid and acetone. In this way, the plasticity of the HAp-Bru system is demonstrated, representing a promising green alternative to the conventional metal-based electrocatalysts used for CO 2 fixation. Thus, the fact that no electric voltage is necessary for the CO 2 reduction has a very favorable impact in the final energetic net balance of the carbon fixation reaction.