Influence of morphology and composition of spherical layered double hydroxide particles and derived mixed oxides on photocatalytic CO2 reduction

Considering the flexibility in the synthesis that allows the formation of materials with more than two metals, the present study reports the preparation of trimetallic layered double hydroxides (LDHs) having Al as structural tri-positive cation, Ti as photocatalytically active d⁰ transition metal and either Ni or Co as dipositive cation. In addition, these LDHs were used as precursors of the corresponding trimetallic mixed oxides (MO). LDH and MO materials in combination with Ru(bpy)₃Cl₂ as photosensitizer and triethanolamine as sacrificial electron donor were used as catalysts for CO₂ reduction under solar light irradiation. A different product selectivity, either CH₄ for Ni-LDH or CO and H₂ for Co-MO, was observed with production rates for CH₄ or CO that are among the highest reported for these systems. The role of the inorganic materials in the photocatalytic process was supported by transient absorption spectroscopy that revealed the quenching of the Ru(bpy)₃Cl₂ triplet excited state by Ni-LDH or Co-MO. An important finding was that the trimetallic Co-Ti-Al oxide with cobaltite structure is able to perform CO₂ reduction in spite that the reduction potential of its conduction band is not sufficient to perform the process, evidence by photoluminescence revealing the existence of an upper electronic state responsible for the reduction. These results show the interest in screening multimetallic materials in photocatalysis due to their improved performance and diverse properties.