The pathways of the hydrogenation by NiCu/ZnO from DFT molecular dynamics simulations
Graphical abstract
Introduction
Carbon dioxide is considered a greenhouse gas, which contributes to the problem of climate change and is responsible for gradually increasing global temperatures [1,2]. Despite the attempts to reduce large emissions by countries around the world, in 2019 the level of anthropogenic emission reached another record value – 415 ppm [3]. These emissions are the result of the lack of the technology for large scale utilization of . On the other hand, this situation is a driving force for research aimed at reducing emissions.
Among the possible strategies to solve the problem of related global warming, the most promising solution is the conversion to energy carriers, as it allows the continuous use of coal based energy by many branches of the industries that still depend on it [1,4]. Such conversion, however, is a significant challenge because is thermodynamically very stable molecule ( = −394 kJ/mol). The investigation of this conversion restrictions and understanding the reaction mechanisms may lead to implementation of improved catalytic systems in utilization [5].
The hydrogenation of to formic acid (FA) is one of the simplest hydrogenation possibilities, because it requires a transfer of only two hydrogen atoms. Additionally, due to a good oxidation kinetics, easy reversibility to carbon dioxide and high cell potentials of FA, it shows the potential of application in hydrogen storage processes, transport and distribution of hydrogen [6] or in the formic acid fuel cells (DFAFCs) [7,8]. The reduction to CO is another process that requires a transfer of two hydrogens to . CO is a fundamental compound used in chemistry, most importantly it is used as a feedstock for the Fischer-Tropsch process of fuels synthesis [4,9].
As far as the reaction mechanisms of those 2-electron reductions are concerned, many possibilities have been investigated. In one case, the requires activation prior to the hydrogenation, which can either lead to a facilitated transfer of a hydrogen to C or O atom, giving rise to the formate or the carboxyl intermediate, respectively [10,11]. Subsequent – second hydrogen transfer to the O atom of the formate or carboxyl will close the catalytic cycle yielding FA or CO and respectively. As an alternative for the latter, the C–O bond can dissociate directly, also yielding CO and catalyst bound oxygen. Thus, such a mechanism requires a direct contact of the reacting with the active site of the catalyst [12,13]. On the other hand, the hydrogenation to formate can be achieved also upon outer sphere attack of a hydride [14,15].
Typically, conversion can be carried out in both homogeneous and heterogeneous conditions [16,17]. Due to their stability, ease of separation and recovery, heterogeneous catalysts are considered a better solution [18], yet the homogeneous systems are currently significantly outperforming the heterogeneous ones [14].
Virtually all heterogeneous catalysts used either in hydrogenation or reduction of are based on metal ions or surfaces [[19], [20], [21], [22], [23]]. Significant efforts have been devoted to replace noble metal based catalysts with inexpensive and abundant catalysts, that will show conversion industrially viable. In this context, copper is a good alternative, and Cu-based catalysts have been often investigated in hydrogenation [[22], [23], [24], [25]] and reduction [12,26]. In-depth studies have demonstrated the ability of copper to hydrogenate the to products such as , or . In addition, Cu-based catalysts exhibit resistance to formation of carbon deposits [[27], [28], [29]]. This led to the implementation of a commercial catalyst – is used in the methanol synthesis from enriched syn-gas (CO, and ) [24,25] and in the low-temperature water-gas shift reaction [30].
Despite many studies on the catalysts containing ZnO supported Cu, the role of particular components of these systems in the reactivity of has not been undoubtedly clarified. It is suggested that the activity and selectivity of Cu/ZnO result from the strong synergy between Cu and ZnO at the interface, where ZnO – besides its usual role of a support in dispersing the Cu active phase – can be responsible for hydrogen binding, structural modifications or activation [25]. The other possibility lies in highly active ZnCu alloy formation by partial ZnO reduction or decoration of Cu with metallic Zn [31]. In this context, a theoretical work of the group of Marx [22,23] shed more light on the complexity of the hydrogenation reactions network. Their study aimed at the description of electronic and structural properties of the Cu/ZnO catalyst, which arise due to a synergistic combination of copper nanoclusters and zinc oxide support.
Further modification of the catalyst, however, is necessary in order to further improve its properties – especially with respect to the affinity towards , better control over selectivity of the process and prevention of its deactivation [21,[32], [33], [34], [35]]. In the present paper we tackle the modification of the Cu/ZnO system in form of a nanocluster supported on O-terminated ZnO (0001) surface. Bimetallic Cu–Ni nanocluster is expected to improve the interaction strength with respect to Cu based catalyst, upon the mutual interaction of these metals and their altered electronic structure.
Section snippets
The model of the catalyst
The model of the catalyst used in the study has been constructed similarly to the system used by the Marx group [22,23]. It consists of the ZnO surface and the Cu–Ni nanocluster. The surface is built as a supercell out of 443 unit cells of . The surface is periodic in a and b directions. The vacuum slab of approximately 18 Å thickness has been added above the surface in order to avoid the interaction between periodic images in c direction. The Periodic Boundary Conditions (PBC) have
Structural properties of
The investigated CuNi nanocluster is a very dynamic structure, what most likely is a result of the small size and relatively strong interactions with the supporting . During the MD simulations, the geometry of the nanocluster is changing, however copper tends to reside close to the surface. This observation is consistent with the one described by Austin et al. [21]. In addition there is a clear difference between the charge on each metal – Ni atoms tend to be reduced with
Conclusions
In summary, we presented a first principles study of conversion catalyzed by the system. Based on the obtained results, we can conclude that the conversion is determined by the competition between different hydrogenation and dissociation pathways. From the investigated systems, the dissociation leading to partial oxidation of the cluster and a carbonyl is characterized by the smallest activation energy, and being a thermoneutral process – it is the preferred pathway. This
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank prof. Evert Jan Meijer for fruitful discussions on the key aspects of constrained Molecular Dynamics simulations.
The simulations have been performed at Poznań Supercomputing and Networking Center, Academic Computer Centre Cyfronet in Kraków and Interdisciplinary Centre for Mathematical and Computational Modeling in Warsaw. This research was supported in part by PL-Grid Infrastructure.
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