The significant increase in atmospheric CO₂ concentration, primarily owing to fossil fuel combustion, is a pressing environmental challenge. Catalytic hydrogenation of CO₂ into methanol is a promising route because methanol serves as a clean liquid fuel, an energy carrier, and a key chemical intermediate. However, the CO₂ molecule is highly stable and inert, making its conversion challenging. Furthermore, the reaction competes with the reverse water-gas shift (RWGS) reaction, which produces carbon monoxide (CO) as an undesired byproduct.
Pt/In₂O₃ is known as an effective catalyst for this reaction, but it has a major weakness: at the high temperatures required for catalyst preparation and reaction, both the Pt nanoparticles and the In₂O₃ support suffer from severe sintering, leading to particle growth and deactivation. Conventional post-impregnation methods are prone to Pt agglomeration, which decreases catalytic activity and promotes the unwanted RWGS reaction.
To address this, researchers at Huaqiao University, the National University of Singapore, and collaborating institutions developed an in situ growth and embedding strategy. They used MIL-68(In) nanorods as a precursor, growing Pt nanoparticles on the metal-organic framework (MOF) before calcination. During calcination, the MOF decomposed to form In₂O₃, embedding the Pt nanoparticles within the evolving oxide matrix.
The resulting Pt/In₂O₃ catalyst features ultrasmall In₂O₃ crystallites (11.8 nm) and a high surface area (48.3 m²·g⁻¹). Spectroscopic analyses revealed strong Pt-O-In electronic interactions, which generate a high density of surface oxygen vacancies (41%) while minimizing the number of exposed metallic Pt sites.
Under industrially relevant conditions (3 MPa, 300 °C, 12,000 mL·gcat⁻¹·h⁻¹), the optimized catalyst achieved a CO₂ conversion of 10.6% and a methanol production rate of 0.22 gMeOH·gcat⁻¹·h⁻¹. The apparent activation energy required for methanol formation was reduced to 62.2 kJ·mol⁻¹, significantly lower than that of pure In₂O₃ (122.8 kJ·mol⁻¹). The catalyst also exhibited exceptional stability over 180 hours, outperforming previously reported Pt/In₂O₃ catalysts.
The authors note that this MOF-mediated confinement strategy provides a versatile route for designing durable, high-performance metal/oxide catalysts, offering a promising pathway for the sustainable conversion of CO₂ into valuable chemicals.
