Hydroxyapatite-based catalysts for CO2 fixation with controlled selectivity towards C2 products. Phenomenal support or active catalyst?

Abstract

Permanently polarized hydroxyapatite (p-HAp) has been reported as a feasible green alternative to conventional catalysts for the selective conversion of CO₂ into highly valuable chemical products. However, structural control and enhanced electrical properties achieved on p-HAp clearly contrast with other reported catalytic systems, where hydroxyapatite mainly acts as a support receiving much less attention. In this work we take advantage of the knowledge obtained on p-HAp to develop an HAp-based catalytic system composed of TiO₂ nanoparticles deposited on p-HAp. It is important to stress that p-HAp is not only considered as a mechanical support but has been put in the spotlight for catalyst preparation and as an active catalytic part. Therefore, the use of p-HAp in this system has unveiled exceptional synergies with TiO₂ attributed to the enhanced electrical properties of p-HAp, capable of attracting the photo-electrons generated in TiO₂ nanoparticles avoiding electron–hole recombination. CO₂ fixation reactions carried out under mild conditions (120 °C, 6 bar and under UV exposure) result in complete selectivity control of the C2 products, shifting from ethanol (201 μmol g_catalyst⁻¹) for p-HAp alone to acetic acid (381 μmol g_catalyst⁻¹) when TiO₂ nanoparticles are loaded in the system. Considering the challenging CO₂ activation energy and the high control of the selectivity achieved, we do believe that this novel approach can be considered as a starting point to explore other systems and reactions where control of the crystal structure and the enhanced electrical properties of HAp can play a crucial role in the final products, reaction conditions, yields and selectivities.