Converting Carbon Dioxide to Formic Acid by Tuning Quantum States in Metal Chalcogenide Clusters

Finding an efficient catalyst to convert carbon dioxide into valuable products is a challenging problem. Here, we have used DFT calculations to show how the attached ligands on the metal chalcogenide cluster can be utilized to design effective catalysts for carbon dioxide conversion to formic acid.
Published in Chemistry
Converting Carbon Dioxide to Formic Acid by Tuning Quantum States in Metal Chalcogenide Clusters

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The rapid growth of greenhouse gases in the atmosphere and its adverse effect on the environment is one of the significant challenges that is currently faced by humanity.1,2 Catalytic conversion of CO2 into chemicals, e.g., formic acid (HCOOH), serves as a cost-efficient alternative strategy for mitigating the adverse effect of CO2. Formic acid is a low-toxic liquid that can be easily transported and stored at room temperature. It can also act as a precursor of high-value-added chemicals, a hydrogen storage vector, and a possible future replacement for fossil fuels. As a result, the rational designing of CO2 conversion catalyst for the sustainable commercial synthesis of formic acid is a popular subject for investigation. In this work, using the first principle density functional methodology, we have explored the CO2 →HCOOH conversion using a metal chalcogenide cluster as a catalyst.

Among the various categories of nanoclusters, clusters composed of metal and chalcogenide atoms have attracted considerable attention in recent years for various reasons. Whereas most clusters are metastable, ligated metal chalcogenide clusters are highly stable against dissociation due to the stability gained from the strong covalent and dative bonding within the structure. As a result, it is possible to synthesize them in solvents or utilize them as building blocks to construct large superatomic assemblies and architecture.3–6 However, the unique feature of metal chalcogenide clusters is how the attached ligands influence their electronic properties. In cluster chemistry, ligands are prominently used as a deactivating agent. Coating a cluster with suitable ligands protects the sensitive core from active reagents and prevents the smaller cluster from coalescence. However, our group recently showed that ligands could serve a different purpose.7,8 For a ligated cluster, the resulting internal electric field originating from the metal and surrounding ligands can shift the electronic spectrum in either direction depending on the nature of the attached ligands. Thus, utilizing this ligands-induced shift, one can convert a cluster into an effective donor or acceptor depending on the number and type of the attached ligands without perturbing the valence electron count or the orbital occupancy. We have further shown that such alteration in the redox properties can have many potential applications in semiconductors, spintronics, or as p–n junction diodes.9,10

Figure 1
Figure 1. The CO2 →HCOOH conversion pathways on (a) Ti6Se8(PMe3)3 and (b) Ti6Se8(CO)3 clusters. (c) The relative trend of the barrier heights for the two hydrogenation steps (d) the adiabatic ionization energies (AIE) and the net hydrogen binding energies of intermediate 2 of the [Ti6Se8(PMe3)3-m(CO)m] (m=0-3) clusters.

In this work, we ventured in a different direction and investigated the influence of the ligands on the reactivity of the metal chalcogenide clusters. Although we have widely explored many aspects of metal chalcogenide clusters in the recent past, judging their potential in the field of catalysis remains untraversed. To evaluate their applicability as the model catalyst for CO2 →HCOOH conversion, we have chosen ligated/unligated Ti6Se8 cluster as the template system. Our computational investigation reveals some fascinating trends and features. First, it was observed that attaching strong σ-donor ligands (e.g., PMe3) with the cluster can reduce the barrier heights (Figure 1a) of both the hydrogenation steps of the reaction resulting in an efficient catalyst for formic acid synthesis. The effect of the π-acceptor ligand (e.g., CO) is just the opposite (Figure 1b). We have shown that with only three PMe3 ligands, it is possible to reduce the first barrier height to ~0.12 eV, significantly lower than the pristine cluster. The second interesting observation is that using different ratios of σ-donor (PMe3) and π-acceptor (CO) ligands, it is possible to alter the barrier heights (Figure 1c) and, therefore, the CO2 conversion rate in a predictable manner. Upon observing the electronic structure alteration due to the ligand attachment, we have noticed that the upward shifting of the electronic spectrum induced by the σ-donor ligand results in a facile charge transfer toward the reactant and also facilitates the release of H atoms from the cluster surface (Figure 1d), resulting in the observed lower barrier heights. As a result, the barrier heights show a predictable pattern depending on the ratio of the type of ligands attached to the cluster.

Both observations are crucial considering the design of a sustainable commercial catalyst for CO2 hydrogenation. Since many donor ligands are now commercially available or can be chemically synthesized, it is possible to precisely tailor a series of high-performance CO2 hydrogenation catalysts using the strategy described herewith. A larger metal chalcogenide cluster will offer more metal sites for ligand attachment, further expanding the possibility. Using the same technique, it may also be possible to convert a relatively inert metal chalcogenide cluster to an active catalyst or provide the ability to precisely control the rate of CO2 conversion via in situ ligand exchange or substitution. We hope the present study will motivate further investigation into the domain of rational design of hydrogenation catalysts.

For more details, please check our paper ( in Communications Chemistry.


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