# Lattice Engineering in Hydroxyapatite Enables Direct Photocatalytic Synthesis of C4 Products from CO2

## TL;DR

This paper presents a new method using engineered hydroxyapatite to convert CO2 into C4 products efficiently under solar light.

## Contribution

The study introduces vacancy-engineered hydroxyapatite with thermal polarization for direct CO2-to-C3+ conversion.

## Key findings

- A 15% selectivity toward C3–C4 products was achieved under solar light and mild conditions.
- Crystal lattice vacancies were identified as key to CO2 bond cleavage and product formation.
- DFT and experimental methods confirmed the role of band gap trap states in photocatalytic activity.

## Abstract

Amid the burst of
carbon dioxide (CO2) capture and conversion
technologies, research prioritizing industrially feasible catalysts
is vital to minimize climate change effects. In the present work,
permanently polarized hydroxyapatite-based biphasic systems have been
strategically designed through vacancy engineering and a thermally
stimulated polarization (TSP) process, achieving a 15% selectivity
toward C3–C4 products through a single-step
CO2 continuous-flow reaction (CO2-to-C3+) under solar light irradiation and mild reaction conditions. To
elucidate the underlying catalytic mechanism, extensive experimental
characterization has been performed in combination with theoretical
density functional theory (DFT) calculations. More specifically, Raman
spectroscopy, X-ray diffraction, and high-resolution transmission
electron microscopy have been used for structural characterization,
and electrochemical impedance spectroscopy studies have been performed
to determine charge conduction customization. On the other hand, DFT
calculations have been employed to determine the photocatalytic contribution
by determining the density of states and band diagrams. The results
have been further supported by UV–vis experimental measurements,
facilitating the elucidation of the mechanisms behind the photoexcited
electrons through band gap trap state generation. Finally, additional
adsorption energy studies, combined with Bader charge analysis and
Nudge elastic band calculations, have allowed the identification of
the binding sites responsible for C3+ molecule growth as
far as the CO2 dissociation pathway and respective energy
barrier. These results highlight the role of the crystal lattice vacancies
in the CO2 bond-cleavage process and represent a huge step
toward the design of efficient and scalable catalysts for CO2-to-C3+ production.