CO2 Activation at Au(110)-Water Interfaces: An Ab Initio Molecular Dynamics Study
Authors: Qin Xueping , Vegge Tejs , Hansen Heine Anton 
Published: July 2022

The electrochemical carbon dioxide reduction reaction (CO2RR) is a promising technology to establish a sustainable carbon cycle by minimizing the carbon footprint and producing useful chemicals.1-2 Gold is a highly active and selective electrocatalyst towards CO2RR to produce CO, being as one critical syngas component. Previous theoretical simulations are mainly focused on the key reaction intermediates (*COOH and *CO) using the computational hydrogen electrode (CHE) model, where the CO2 activation is usually neglected which is not accessed in the employed vacuum condition. Constructing the Au-water interface with explicit water layers is beneficial to evaluate the CO2 activation, which might be the rate-determining step (RDS) during CO2RR on Au surfaces.3

In this work, the multiple explicit water layers are introduced to simulate the Au(110)-water interface, and the role of applied potential is considered on Au surfaces with different surface charge densities where the K cations are artificially included into water solvents. Combining the standard ab initio molecular dynamics (AIMD) and state-of-the-art constrained AIMD (cAIMD) simulations, the CO2 activation is studied from both kinetic and thermodynamic aspects. Using the slow-growth sampling approach with thermodynamic integrations, we found that CO2 adsorption could be activated at Au-water interface with 2K+, and the free energy barrier is estimated to be 0.61 eV (Figure 1a, top). Additionally, 0.81 electrons are transferred to *CO2 during adsorption based on Bader charge analysis (Figure 1a, bottom). The key intermediates during CO2RR are shown in Figure 1b (including initial state, transition state, and final state), and the reversed CO2 desorption shows very similar free energy diagram, which validates both our sampling approach and interfacial models.4

Besides the CO2 activation, the full reaction pathway including the subsequent proton transfers during CO2RR will be explored in the further study, and the complete free-energy landscape will be constructed using this Au-water interfacial model. Such an AIMD simulation study from the atomic level can largely contribute to the complete understandings of catalytic reaction pathways on Au surfaces and the interfacial behaviors at solid-liquid interfaces.

References

(1) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendorff, I., Chem. Rev. 2019, 119, 7610-7672.
(2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F., Science 2017, 355, eaad4998.
(3) Wuttig, A.; Yaguchi, M.; Motobayashi, K.; Osawa, M.; Surendranath, Y., Proc. Natl. Acad. Sci. 2016, 113, E4585.
(4) Qin, X.; Vegge, T.; Hansen, H. A., J. Chem. Phys. 2021, 155, 134703.

Figure 1. (a) Free energy profile (top) and Bader charge analysis (bottom) for CO2 adsorption along SG-AIMD in Au(110)-42H2O-2K+. The dashed red lines represent the key intermediates with the CV of 2.74 (transition state, TS), and 2.12 Å (final state), respectively. (b) The key structures during CO2 adsorption, including the initial state (Initial), TS, and final state (Final). Color code: Au, golden; K, purple; C, blue; O, red; H, white.

Figure 1