We had demonstrated that we can deposit overcoats using novel solution-phase synthetic techniques based on stoichiometric precursor injections, and use them to stabilize base metal hydrogenation catalysts during liquid-phase processing of biomass-derived molecules (Héroguel et al. App. Cat. B, 2017). We have now demonstrated that by carefully measuring ligand release, we can precisely inject the necessary precursor quantities to achieve exact monolayer by monolayer growth of multiple materials leading to coating qualities in solution that are indistinguishable from gas-phase ALD (Le Monnier et al. Advanced Materials, 2019). This work demonstrated that it was possible to grow atomically thin films onto high surface area dispersed materials. Various types of layers (oxide, phosphate, and sulfide) were deposited onto a wide range of substrates with varied shapes and surface properties. The ability to deposit such layers on dispersed materials is not only desirable in the field of heterogeneous catalysis but also in the fields of microelectronics, optics, sensing and energy conversion.
In doing this work, we clearly identified and were able to take advantage of the growth of clusters as opposed to films, which is a feature that is uniquely accessible in this liquid phase technique. This has opened up a new area of research where we build catalytic clusters atom-by-atom, which fits into our general goal of controlling catalytic active sites but does so in an unexpected way. We have notably used this technique to show that we can systematically explore catalytic promotional effects using different combinations of metal atoms (Le Monnier et al. ACS Sus. Chem. Eng. 2022). More recently, we have used this tehcnique to make CO2 hydrogenation catalysts with unprecedented activities both on a per metal basis and based on turnover frequencies that are normalized by active site.
In parallel, we have also developed slightly less precise, but extremely simple, coating methods that use non-hydrolytic sol-gel or chelation chemistry. This has resulted in controlled increases in activity, selectivity, and reduced deactivation for several reactions (Héroguel et al. J. Cat., 2018, Du et al. J. Mat. Chem. A, 2019 and Du et al. Small, 2018). We have used this method for controlling the particle size of nanoparticles and accessing particularly small Pd nanoparticles (Du et al. ACS Catalysis, 2020). This approach relied on the synthesis of an atomically dispersed material using coordinated Pd complexes. Unlike with nanoparticles, using atomically dispersed Pd before reductive treatment enabled us to limit the particle growth during thermal activation steps, yielding highly accessible, sinter-resistant Pd clusters less than 2 nm in diameter. Notably, engineering the Pd-ZrO2 interface of Pd/ZrO2 into inverted ZrO2-Pd interface using ZrO2 overcoat leads to an unprecedented 100% CO selectivity during CO2 hydrogenation, which is an important result in the context of CO2 utilization.
Because we were interested in building renewable processes notably from lignin, which is the largest natural source of renewable aromatic molecules, we have to control not only our catalytic material but also our substrate. The issue with lignin is that it rapidly condenses upon extraction, which makes it very difficult to actually upgrade it catalytically once isolated. Catalyst engineering can thus be completely ineffectual if it is employed on condensed/destroyed lignin. We recently reported a strategy that prevents condensation by functionalization with aldehydes during lignin extraction procedure (Shuai et al. Science 2016). However, we had not developed a method to successfully isolate this lignin to explore catalyst engineering independent of other biomass fractions. To address this, we recently developed a protocol, to protect lignin with aldehydes and isolate it in the first scalable procedure for producing 100 gram-scale quantities of bench-stable aldehyde-stabilized lignin, , which can be catalytically upgraded at near theoretical yields (Talebi et al. Nature Protocols, 2019). We have been successful in using the overcoating chemistry described above to stabilize these lignin conversion catalysts, which could help address a major challenge in biorefining (Talebkeikhah et al. Adv. Energy Materials, 2023).
This work has been disseminated in many of the most prestigious catalysis conference (Gordon conference on catalysis, North American Catalysis Meeting, Europacat, etc.) but also featured in mainstream venues like Swiss Public Radio programs or highlighted in start-up presentations. In particular, these technologies are now exploited (and often presented) by our EPFL spin-off Bloom Biorenewables that has been ranked in the top 10 of the best start-ups in Switzerland for two straight years.
