In 2022, global energy-related carbon dioxide (CO2) emissions reached a new high of more than 36.8 billion tonnes and remain on an unsustainable growth trajectory. Tackling this challenge requires a multifaceted approach: increasing energy efficiency, exploiting renewable energy sources and developing ways to capture, use and store CO2. One vibrant area of research explores transforming captured CO2 into fuels and other useful chemicals. The hydrogenation of CO2 to methanol offers one potential pathway to producing more sustainable plastics, fuels and chemicals. Pd/In2O3 (Palladium on Indium Oxide) catalysts have been well investigated on account of their high methanol selectivity and CO2 conversion. However, these platinum group metals are expensive, and the reason for their activity in this reaction is not fully understood. In work recently published in Angewandte Chemie, an international team of researchers demonstrated that a cheaper alternative - Pd/In2O3 species dispersed on Al2O3 - can match the performance of pure Pd/In2O3 catalysts. Using X-ray Absorption Spectroscopy (XAS) and infrared (IR) spectroscopy under operando reaction conditions, they were able to determine the nature of the active sites and their influence on the catalytic mechanism.
With atmospheric levels rising quickly and adversely affecting the climate, it's easy to think of CO2 simply as a waste product. Cleaning up that pollution by capturing CO2 and storing it deep underground is one possible solution. However, CO2 has many industrial uses, and treating the gas as a valuable resource offers us new tools to reach net zero targets.
Prof Andrew Beale of University College London says,
There has been a lot of focus on storing CO2. This research looks at making use of it - the Utilisation aspect of Carbon Capture Utilisation and Storage (CCUS) initiatives - and specifically the development of next-generation catalysts for converting CO2 to methanol.
The current commercial process for producing methanol uses a Cu/ZnO/Al2O3 catalyst. Pd/In2O3 is a promising replacement, maintaining high methanol selectivity and CO2 conversion. However, its active state is not well understood. The prevailing theory was that, under operando reaction conditions, palladium and indium form an alloy that is responsible for the high levels of methanol produced. Prof Beale continues;
When our collaborators at the Università di Bologna started working on a "thriftier" (more metal-efficient) catalyst, supporting Pd and In2O3 onto alumina, they thought that would effectively dilute the catalyst and reduce its activity. However, they didn't see a reduction in activity. If anything, the thrifty catalyst produced more methanol per unit of palladium. So we worked with Diamond's B18 beamline to develop a sample environment where we could investigate the catalyst under operando reaction conditions.
CO2 hydrogenation to methanol has to take place at high pressures, but most catalyst studies are done at atmospheric pressure for simplicity. Working with the beamline staff to develop a high-pressure sample environment allowed the research team to investigate the catalyst under operating conditions - 20 bar gas.
The team used Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on B18 to probe the palladium and indium environments in the catalyst during the reaction. By collecting EXAFS measurements for both metals, they could correlate changes and explore any bimetallic interactions.
Complementary high-pressure operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) experiments, carried out at the UK Catalysis Hub, helped them understand the catalytic mechanism and synergistic enhancement in the Pd/In2O3/Al2O3 system.
The results confirm that Pd/In2O3/Al2O3 can match the catalytic performance of pure Pd/In2O3 systems. The team also determined the nature of the active sites and their influence on the catalytic mechanism.
Prof Beale concludes;
We found that it's not easy to form the alloy in the thrifty catalyst, but the activity remains. So we disproved the hypothesis that extensive alloying is necessary for the good catalytic activity. There's something important about the palladium and indium oxide proximity, but they don't need to form an extensive alloy.
While In2O3 readily activates CO2, it struggles to split hydrogen. Introducing other metals - in this case, palladium - improves hydrogen splitting. The results of this work suggest that careful balancing of these separate activation processes, and the precise location and interactions of Pd and In2O3, are key to future catalyst optimisation. These insights are likely to extend to other bimetallic CO2 hydrogenation systems.
To find out more about the B18 beamline or discuss potential applications, please contact Principal Beamline Scientist Diego Gianolio: [email protected].
Potter, ME et al. A High Pressure Operando Spectroscopy Examination of Bimetal Interactions in 'Metal Efficient' Palladium/In2O3/Al2O3 Catalysts for CO2 hydrogenation." Angewandte Chemie International Edition (2023): e202312645. DOI:10.1002/anie.202312645.
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