The pathways of the CO2 hydrogenation by NiCu/ZnO from DFT molecular dynamics simulations

https://doi.org/10.1016/j.jmgm.2020.107677Get rights and content

Highlights

  • ZnO supported Ni–Cu bimetallic cluster exhibits activity in CO2 conversion.

  • The Molecular Dynamics simulations reveal a formate rotation as the limiting step.

  • The formation of the carboxyl intermediate leads to undesired pathway.

  • CO2 dissociation is the energetically preferred pathway, but requires further conversion.

Abstract

Metal nanoparticles supported on semiconductor surfaces have been proposed as promising nanocatalyst candidates of CO2 conversion to energy carrier molecules such as formic acid or carbon monoxide, which can be used as a feedstock for fuels synthesis. This study is focused on the bimetallic Cu/Ni nanoparticles supported on the ZnO. The respective reaction mechanisms have been studied by means of the Molecular Dynamics with the DFT methodology. The results suggest that on CuNi/ZnO CO2 hydrogenation to formate pathway is more favorable than carboxyl route. These pathways are competitive with the CO2 reduction to CO.

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 CO2 emissions by countries around the world, in 2019 the level of anthropogenic CO2 emission reached another record value – 415 ppm [3]. These emissions are the result of the lack of the technology for large scale utilization of CO2. On the other hand, this situation is a driving force for research aimed at reducing CO2 emissions.

Among the possible strategies to solve the problem of CO2 related global warming, the most promising solution is the CO2 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 CO2 is thermodynamically very stable molecule (ΔH = −394 kJ/mol). The investigation of this conversion restrictions and understanding the reaction mechanisms may lead to implementation of improved catalytic systems in CO2 utilization [5].

The hydrogenation of CO2 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 CO2. CO is a fundamental compound used in C1 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 CO2 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 H2O 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 CO2 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, CO2 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 CO2 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 CO2 conversion industrially viable. In this context, copper is a good alternative, and Cu-based catalysts have been often investigated in CO2 hydrogenation [[22], [23], [24], [25]] and reduction [12,26]. In-depth studies have demonstrated the ability of copper to hydrogenate the CO2 to products such as CH3OH, HCOOH or CO. In addition, Cu-based catalysts exhibit resistance to formation of carbon deposits [[27], [28], [29]]. This led to the implementation of a commercial catalyst – Cu/ZnO/Al2O3 is used in the methanol synthesis from CO2 enriched syn-gas (CO, CO2 and H2) [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 CO2 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 CO2 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 CO2 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 CO2, 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 Cu4Ni4 nanocluster supported on O-terminated ZnO (0001) surface. Bimetallic Cu–Ni nanocluster is expected to improve the CO2 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 4×4×3 unit cells of ZnO. The surface is periodic in a and b directions. The vacuum slab of approximately 18 Å thickness has been added above the ZnO surface in order to avoid the interaction between periodic images in c direction. The Periodic Boundary Conditions (PBC) have

Structural properties of Cu4Ni4/ZnO

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 ZnO. During the MD simulations, the geometry of the nanocluster is changing, however copper tends to reside close to the ZnO 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 CO2 conversion catalyzed by the CuNi/ZnO system. Based on the obtained results, we can conclude that the CO2 conversion is determined by the competition between different hydrogenation and dissociation pathways. From the investigated systems, the CO2 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.

References (45)

  • J. VandeVondele et al.

    Quickstep: fast and accurate density functional calculations using a mixed Gaussian and plane waves approach

    Comput. Phys. Commun.

    (2005)
  • F. Solymosi

    The bonding structure and reaction CO2 adsorbed on clean and promoted metal surfaces

    J. Mol. Catal.

    (1991)
  • R.S. Haszeldine

    Carbon capture and storage: how green can black be?

    Science

    (2009)
  • C. Hepburn et al.

    The technological and economic prospects for CO2 utilization and removal

    Nature

    (2019)
  • G. Laurenczy

    Hydrogen storage and delivery: the carbon dioxide – formic acid couple

    CHIMIA Int. J. Chem.

    (2011)
  • S. Enthaler et al.

    Carbon dioxide and formic acid—the couple for environmental-friendly hydrogen storage?

    Energy Environ. Sci.

    (2010)
  • S. Uhm et al.

    Understanding underlying processes in formic acid fuel cells

    Phys. Chem. Chem. Phys.

    (2009)
  • H.-L. Jiang et al.

    Liquid-phase chemical hydrogen storage: catalytic hydrogen generation under ambient conditions

    ChemSusChem

    (2010)
  • G. Melaet et al.

    Cobalt particle size effects in the fischer–tropsch synthesis and in the hydrogenation of CO2 studied with nanoparticle model catalysts on silica

    Top. Catal.

    (2013)
  • G. Peng et al.

    CO2 hydrogenation to formic acid on ni(111)

    J. Phys. Chem. C

    (2012)
  • B.M. Szyja et al.

    A DFT study of CO2hydrogenation on faujasite-supported ir4clusters: on the role of water for selectivity control

    ChemCatChem

    (2016)
  • Cited by (4)

    View full text