Heterogeneous catalysis - where the catalyst is in a different phase to the reactants - is a stalwart of the chemicals and energy industries. The development of active, selective, and energy-efficient heterogeneous catalysts is, therefore, a crucial pillar of our transition to more sustainable technologies. There has been a recent focus on single-atom heterogeneous catalysts (SAHCs), due to their maximum metal utilisation and unique reactivity. However, while carbon-nitrogen supports are widely used for SAHCs, most studies of their dynamics through a reaction have used oxide supports.
It is not yet clear whether single atoms or sub nanometre clusters that form under reaction conditions are active species. In work recently published in the Journal of Catalysis, a team of researchers used aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) to investigate the dynamics of palladium SAHCs on graphitic carbon nitride supports during two reactions - ethylene hydrogenation and H2-D2 exchange. Their results show that, at 100°C in a gas containing ethylene and hydrogen, the palladium single atoms form clusters, and suggest that these clusters are the active species. Their work offers new insights into the effect of gas atmosphere on speciation.
Prof Regina Palkovits, from RWTH Aachen University, says:
In heterogeneous catalysis, the current trend is towards single atoms, so that you have the ultimate utilisation of the metal. Tailoring the support can improve activity or selectivity, but analysing the dynamics, stability and real properties of these single atoms is really challenging.
The team compared the catalytic activity of two different samples - a palladium nanoparticle catalyst on exfoliated graphitic carbon nitride (ECN) and a palladium single-atom catalyst on ECN. They chose to study the catalysts during two reactions. Firstly ethylene hydrogenation, as that reaction covers the highly important area of C-C double bond hydrogenation in gas phase, and allows comparison with previous studies on the dynamics of single atoms on oxide supports. The second reaction studied was the much simpler H2-D2 exchange.
On Diamond's I20-EDE beamline, the team collected X-ray absorption spectra of catalysts after treatment in the reactions at different temperatures: 50°C, 100°C, 150°C and 200°C. As synchrotron-based XAS has a lower detection limit than laboratory-based XPS, and penetrates further into the sample, the spectra collected at Diamond offered more information about the electronic state of palladium and the chemical environment.
Prof Palkovits continues:
During our test reaction, ethylene hydrogenation, the results showed that the palladium nanoparticle catalyst was immediately active. For the SAHC, there was a temperature-dependent change. It wasn't active at low temperature, but it activates after some time at 100°C.
It's a highly dynamic system, but it seems that the most active species are not the single atoms we put in. Although the differences are tiny, and hard to see, the synchrotron data suggest that the single atoms are coming together to form small clusters that are smaller than nanoparticles. The second reaction, hydrogen-deuterium exchange, is much simpler. It tests for hydrogen activation, which is a prerequisite for any hydrogenation. And again, the results showed that the SAHCs were inactive.
The results from the H2-D2 exchange experiments demonstrate that the formation of the small clusters is a prerequisite for hydrogen activation and requires the presence of both ethylene and hydrogen.
The synchrotron experiments were conducted by Dr Maurice Vennewald, during his PhD at RWTH Aachen University.
Prof Palkovits concludes:
Maurice had extensive discussions with the beamline staff beforehand, and they provided support in optimum sample preparation and selection of the right references. Then of course they provided local support during the beamtime, and their assistance was also invaluable in evaluating the data collected. Diamond provides an invaluable resource for our catalysis research. We're currently analysing data collected at Diamond on a similar catalytic system using ruthenium in a formic acid decomposition reaction. Understanding the dynamics of supported metals under reaction conditions is of the utmost importance for understanding the system behaviour.
Vennewald M et al. Dynamics of palladium single-atoms on graphitic carbon nitride during ethylene hydrogenation. Journal of Catalysis 421 (2023): 134-144. DOI:10.1016/j.jcat.2023.03.011
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