PdIn intermetallic nanoparticles for the Hydrogenation of CO2 to Methanol

Abstract

Direct hydrogenation of CO 2 to methanol could offer significant environmental benefits, if efficient catalysts can be developed. Here, bimetallic Pd-In nanoparticles show good performance as catalysts for this reaction. Unsupported nanoparticles are synthesised by the thermal decomposition of Pd(acetate) 2 and In(acetate) 3 precursors in a high boiling point solvent (squalane), followed by reduction using dilute H 2 gas (210 C). Adjusting the ratio of the two metallic precursors allow access to 5-10 nm nanoparticles with different phase compositions, including metallic Pd(0), In 2 O 3 and intermetallic PdIn. Liquid phase methanol synthesis experiments (50 bar, 210 C, H 2 :CO 2 = 3:1) identify the intermetallic PdIn nanoparticles as the most efficient. The catalysts exhibit around 70% higher methanol rates (normalised to the overall molar metal content) compared to the conventional heterogeneous Cu/ZnO/Al 2 O 3 catalyst (900 and 540 mol mmol PdInorCuZnAl -1 h -1 , respectively). In addition, the optimum Pd/In catalyst shows an improved methanol selectivity over the whole temperature range studied (190-270 C), reaching >80% selectivity at 270 C, compared to only 45% for the reference Cu/ZnO/Al 2 O 3 catalyst. Experiments showed an improvement in stability; the methanol production rate declined by 20% after 120 h run for the optimum PdIn-based compared with 30% for the Cu/ZnO/Al 2 O 3 catalyst (after 25 h). The optimum catalyst consists of 8 nm nanoparticles comprising a surface In-enriched PdIn intermetallic phase as characterised by XRD, HR-TEM, STEM-EDX and XPS. Post-catalysis analysis of the optimum catalyst shows that the same PdIn bimetallic phase is retained with only a slight increase in the nanoparticle size.