Group 13 metal doped Cu/ZnO catalysts from phase pure precursors via an isomorphous substitution route: mechanistic insights into promotional effects for syngas hydrogenation to methanol

Abstract

In the mixture of coprecipitated precursors of Cu/ZnO/Al₂O₃ catalysts, only the fraction of Al³⁺ ions incorporated into a single-phase zinc malachite actually contributes to the dilution of Cu²⁺ and is responsible for the promotional action. However, the preparation of promoter ion doped phase pure zinc malachite is difficult via coprecipitation routes and further, the effect of such promotion on the microstructure properties and activity is poorly understood. The present isomorphous substitution method delivers a controlled and efficient incorporation of group 13 metal ions, viz., Al³⁺, Ga³⁺ and In³⁺ into the pure phase malachite structure. The formation of M³⁺-doped pure phase precursors ensured maximum dilution of Cu²⁺ ions that lead to a compelling improvement in the microstructure properties of the calcined catalysts. As compared to the undoped catalyst, the Al³⁺ and Ga³⁺-doped catalysts exhibited an ∼8 and ∼5.4-fold increase in the methanol formation rate, respectively. The best performing Al³⁺-doped catalyst exhibited superior microstructure properties and high (855 mg g_cat⁻¹ h⁻¹) and stable STY_MeOH, which could be correlated with the promoter induced O vacancy sites. As a consequence of the promotion, the Cu–ZnO synergy was found to be improved resulting in the formation of a reduced ZnO_x overlayer as evidenced by XPS-AES and N₂O coupled H₂ TPD analyses. The Ga3d XPS and ²⁷Al MAS NMR results suggested that in the reduced catalysts, the major fractions of Al³⁺ and Ga³⁺ were found to be incorporated into the tetrahedral sites of the ZnO lattice and led to the contraction of unit cell volume eventually resulting in the modification of electronic properties of ZnO by inducing defects in its structure. Surprisingly, In³⁺ exhibited a detrimental effect on the catalytic performance as a result of its revoked dilution effect in the reduced catalyst owing to its reactive support–metal interaction resulting in the formation of a Cu–In bimetallic alloy.