Our projects cover a wide spectrum, from fundamentals to industrial applications, and always target the prestigious journals of the field (Scheme 1). For example, we work on overcoming the challenges associated with the atomically dispersed supported metal catalysts. In this regard, we coat them with ionic liquids (ILs) to investigate the structural factors controlling the performance of IL-coated supported metal catalysts and use special supports, such as graphene aerogel (GA) reduced at different conditions, zeolites, and metal organic frameworks to control the surface electronic structure. We collaborate with Dr. Simon Bare of SSRL, Prof. Bruce Gates of UC Davis, Prof. Viktorya Aviyente and Prof. Ramazan Yıldırım of Boğaziçi University, Prof. Uğur Ünal and Prof. Seda Keskin of Koç University for these research efforts. We also work on supported metal nanoparticles and investigate the relationships between the surface characteristics of supports and the active sites located at different positions on the supported metal nanoparticles. These efforts also cover a reaction crucial for hydrogen economy: ammonia decomposition. We use ammonia as a hydrogen storage molecule and work on developing cheap catalysts capable of efficiently decomposing it to produce hydrogen. Our research also includes industrial applications with special focus on converting gaseous hydrocarbons into liquid fuels. We have been collaborating with industry for more than a decade to direct these efforts. For instance, we worked on developing selective catalysts for the conversion light olefins into liquid fuels and green fuel additives. Our long-lasting collaboration with industry now continues with a project focusing on the conversion of CO2 into liquid fuels. These efforts not only shed light into major energy related problems, but also provide broad opportunities for the members of our group to prepare themselves for the industry. As an excellent proof of this, we had several graduates appointed as research engineers/scientists at the R&D center of our collaborators from the industry. Besides these research efforts on catalysis, we also collaborate with Prof. Seda Keskin’s Group at Koç University to diversify our research focus with materials for gas storage and separation applications. In this regard, we apply post-synthesis modifications on porous materials to boost their gas storage and separation performance. Following is a brief overview on some of the projects we have been working on:

Atomically dispersed supported metal catalysts
One of the emerging topics in the field of heterogenous catalysis is the atomically dispersed supported metal catalysts. In these novel catalysts, noble metals are present as site-isolated active species anchored onto supports, offering a maximum possible metal dispersion of 100%. As a result, they offer the best possible utilization of expensive transition metals with structures simple and uniform enough to allow decisive characterization for the investigation of structure-performance relationships. Even though there exists a tremendous interest on this topic, there is still very limited insights into overcoming the major challenges related with these novel catalysts: (i) tuning their catalytic performance, (ii) improving their stability, and (iii) enhancing their metal loadings. These challenges set the focus of our research on atomically dispersed supported metal catalysts:

Tuning the catalytic performance of atomically dispersed supported metals requires an ability to precisely control their electronic structures. In this regard, we work on i) modifying their ligation environment, including supports, ii) coating them with an IL layer, and iii) controllably adjusting their metal nuclearity, while still maintaining the atomic dispersion. For instance, we synthesized Ir(CO)2 complexes on various high-surface-area metal oxides with increasing electron-donor strength. Spectroscopy data indicated that electron density on iridium centers increases with support’s electron-donor strength. Subsequently, results of catalytic performance tests presented a strong correlation between the electron density of iridium sites and their selectivity towards partial hydrogenation of 1,3-butadiene (Chem. Sci. 2019, 10, 2623).

Another way of changing the electronic structure on an active metal center is to vary its nuclearity by forming metal clusters, which are small enough to offer 100% metal dispersion. For example, recently we demonstrated the transformation of atomically dispersed graphene aerogel-supported iridium complexes into Ir4 and Ir6 clusters as a response to changes in both temperature and reactant composition during ethylene hydrogenation. Comparison of the turnover frequencies indicated a more than two-order-of-magnitude boost in performance with increasing metal nuclearity (J. Catal. 2022, 413, 603).

These examples show that one can tune the catalytic performance of an atomically dispersed supported metal complex by either changing the support or by controlling the metal nuclearity. However, these approaches are limited. Because it might not be possible to maintain atomic dispersion on every support, or metal aggregation might go beyond small clusters and form large nanoparticles. In this regard, our group is among the pioneers of a novel approach, which offers a highly flexible route for controlling the electronic structure of active metal centers by coating them with a very thin layer of an IL (Scheme 2). These coating materials are salts, which are mostly liquid below 100 °C, and offer almost limitless number of structural possibilities. When they are in contact with active metal centers, they can either work as a macro-sized ligand to adjust metal’s electronic structure or as a selective layer to control effective concentrations of reactants/intermediates/products. For instance, in a groundbreaking study, we demonstrated that the electron density of atomically dispersed Ir(CO)2 complexes supported on -Al2O3 can be directly tuned by coating them with a thin layer of IL (ACS Catal. 2017, 7, 6969). Spectroscopy data showed that the IL donates electrons to the metal site and that the degree of electron donation increases with an increase in interionic interaction energy. Catalytic performance measurements complemented these results and demonstrated a strong correlation between the selectivity for butenes during 1,3-butadiene partial hydrogenation and the electron density on iridium. In a follow up study, we presented that the ligand effect of ionic liquid layer becomes more dominant as the electron-donor strength of the support becomes weaker. Hence, catalytic performance of atomically dispersed supported metal complexes can be tuned by the synergistic interplay between the IL layer and the support enveloping the active species, both of which act as macro-sized ligands. Next, we investigated the relative strengths of these ligand effects as a function of metal nuclearity. We prepared iridium complexes and clusters on three different supports with varying electron-donor characters and coated them with IL again with varying electron-donor characters. Our data demonstrated that ligand effect of ILs become more dominant when the metal nuclearity is high; whereas the ligand effect of support becomes more dominant as the metal nuclearity decreases. All these results demonstrate the broad opportunities for tuning catalytic performance by the selection of IL coating layer-support combination, and the importance of making this selection based on the metal nuclearity (J. Catal. 2020, 387, 186).