Effect of nickel phosphide phase on the photo-thermal catalytic hydrogenation of carbon dioxide

Abstract

The CO₂ hydrogenation (HYD) reaction was investigated over Ni phosphide (Ni₃P, Ni₁₂P₅, Ni₂P) catalysts to probe the effect of the Ni_xP_y phase on photo-thermal catalytic properties in comparison to Ni metal. The light absorption properties of 2.5 wt% Ni_xP_y/SiO₂ catalysts differ substantially, with the extent of light absorption decreasing as the P/Ni molar ratio of the Ni phosphide phase increases. This finding directly impacts the photo-thermal catalytic properties as the photo-enhancement (light activity / dark activity) correlates linearly with the extent of light absorption. For comparison purposes, 2.5 wt% Ni/SiO₂ catalysts were also investigated and showed high activity but suffered from low CO selectivity (57–68 %). A Ni₃P/SiO₂ catalyst was the most active of the Ni phosphides with high CO selectivity (>95 %), while Ni₁₂P₅/SiO₂ and Ni₂P/SiO₂ catalysts had lower CO₂ HYD activities but CO selectivities above 98 %. Upon light exposure, the Ni_xP_y/SiO₂ (and Ni/SiO₂) catalysts exhibited significant rises of temperature (∼200 K increase from room temperature), indicating the importance of photothermal heating in increasing the CO₂ HYD rate. The findings highlight how a non-metal element (i.e., P) plays a crucial role in tailoring the photo-thermal catalytic properties of earth abundant nickel metal.