DFT-Based Exploration of Transition Metal Phosphides for CO Reduction Reaction

1. Introduction

Following the Industrial Revolution, the first signs of an increase in carbon dioxide (CO₂) levels were discovered. This rise in CO₂ levels led to a significant deterioration of the climate, which was mostly caused by the overuse of fossil fuels. As a result, the transformation of carbon dioxide into a green fuel by electrochemical conversion is now a major topic of discussion among scientists and has garnered a great deal of interest [1,2,3,4,5]. Carbon dioxide reduction reaction (CO₂RR) is an efficient technique for the carbon-free synthesis of alcohol and hydrocarbon fuels. This method reduces CO₂ emissions and fosters the sustainable production of renewable fuels efficiently. CO₂RR may be conducted by four unique approaches: biochemical, photochemical, electrochemical, and thermochemical. Nevertheless, due to its configurable selectivity and reliable catalytic efficiency, electrochemical CO₂ reduction ranks among the most effective methods, since this process is exclusively reliant on electrode performance [6,7,8,9,10,11,12,13,14].

Historically, electrochemical CO₂RR have used specific metal-based catalysts, with copper being one of the most studied examples. Although copper is an exceptional catalyst for converting CO₂ into multiple products, it suffers from poor product selectivity. In a recent study, copper was shown to catalyze more than ten products at potentials exceeding -1.0 V, signifying its lack of selectivity and the challenge of product separation [15]. Similarly, another investigation identified copper as an electrode material characterized by both inadequate selectivity and a high overpotential requirement for CO₂RR [16]. Ultimately, improving activity and selectivity and the tailored design of catalysts are fundamental concerns of the present day. Consequently, researchers are focused on engineering new surfaces and more reliable reaction pathways to develop superior catalysts.

During CO₂RR, carbon monoxide and formic acid are the principal products that are observed during the initial phase of reduction, resulting from the first electron-proton transfer. However, further transition of electron-proton can lead to the formation of hydrocarbons and oxygenates, but this progression often results in a decline of selectivity and proficiency of CO₂RR [17,18,19,20,21,22,23,24,25,26,27]. To overcome this challenge, an improved strategy involves utilizing the carbon monoxide (first intermediate of CO₂RR) for direct electrocatalytic CO reduction (CORR). For instance, MoS₂ with the doping of transition metal atoms has been investigated for CORR, and chromium-doped MoS₂ having sulphur vacancies was reported as a promising surface to reduce the CO at -0.33V [28]. Similarly, metal-nitride phosphorene (MN₃@P) as a single-atom catalyst (SAC) has also been explored for CO reduction [29].

In this manuscript, we have done a comprehensive examination of TMPs as a catalytic surface for electrocatalytic reduction of CO under room conditions. The structures of TMPs under the current investigation are taken in the rocksalt crystalline orientation in (100) facets of 1:1 ratio of metal and phosphorus atoms. The theoretical framework based on density functional theory (DFT) is utilized here to assess their catalytic activity. Moreover, these surfaces can show excellent catalytic activities due to their adjustable structures, surprising conductivity and stability under the electrochemical environment [30,31]. It has already been reported that a decrease in thermal dissolution and improvement in stability of transition metal-based catalysts could be achieved through the introduction of phosphorous atoms [32]. Owing to their superior stability, effective activity, and economic pricing, these materials provide an appealing option for consideration as catalysts [33]. That is the main motive behind the selection of these surfaces in our analysis, and herein their catalytic activity is indicated by free energy diagrams based on their Gibbs free energy values.

2. Computational Research Details

In our present investigation, the Vienna ab initio Simulation Package (VASP) software has been used to examine TMPs as catalytic surfaces for CO reduction under ambient circumstances [34]. The electronic structural analysis was conducted using density functional theory (DFT), with the Revised Perdew-Burke-Ernzerhof (RPBE) as the exchange-correlation functional [35]. The computational precision and capabilities of RPBE for the development of certain properties, such as surface analysis and catalytic activity, which are crucial to the present inquiry, prompted its selection. Furthermore, the concordance between experimental findings and computational analyses conducted using RPBE was a primary reason for its selection in our current work [13,36,37,38]. For the optimization, a cut-off energy of 400 eV and a 4×4×1 Monkhorst-Pack K-point grid were used [39]. Secondly, we modified the essential conditions to achieve a relaxed structure with different adsorbates till their atomic forces went below 0.03 eV/Å. TMPs-based electrodes for catalyst design have 40 atoms and five layers, with a 1:1 ratio between the atoms of metal and phosphide, meaning that there are 20 atoms of metal and 20 phosphide atoms. During the investigation, the lower dual layers were restricted, but the remaining three layers were permitted to fully engage with reactants and participate entirely in chemical reactions. To illustrate this, we constructed Figure 1, whereby the borders of the TMPs in the x and y axes are treated as periodic, but a vacuum of 20 Å is implemented along the z-axis to mitigate the self-interaction of adjacent unit cells.

The reaction pathways identified during the CO reduction into distinctive renewable products on TMPs were investigated using the thermochemical model (TCM), and the effect of applied potential was analyzed using the computational hydrogen electrode (CHE) [40,41,42,43,44,45,46,47]. To compute the adsorption energy (ΔE_ads), we used the following equation [48]:

ΔE _ads = E _{Total system} – E _{System without adsorbate} – E _{adsorbed species}

(1)

To calculate the Gibbs free energy [49], we can use Eq. 2

ΔG = ΔE_DFT + ΔE_ZPE - T_ΔS + ΔG_pH + ΔG_U

(2)

Here

ΔE_DFT = Energy difference between two intermediate states,

ΔE_ZPE = Zero-point energy correction,

TΔS = Change in entropy,

Notably, ΔG_pH indicates the pH shift; the entire value of this component is 2.303 k_bTpH. K_b indicates the Boltzmann constant, T is temperature, and pH represents electrolytic pH; this is independent to overpotential; hence pH is 0 here. That is, ΔG_pH = zero. From Eq. 2, we have

ΔG = ΔE_DFT + ΔE_ZPE - T_ΔS + (0) + ΔG_U

ΔG = ΔE_DFT + ΔE_ZPE - T_ΔS + ΔG_U

(3)

ΔG_U = -neU, wherein n represents the number of electrons, e signifies the charge of an electron, and U symbolizes the applied potential [50,51,52]. In the absence of an applied potential, this term would equal 0, and Eq. 3 might be represented as such.

ΔG = ΔE_DFT + ΔE_ZPE - T_ΔS

(4)

To get the value of the onset potential (OP) that occurs between the two steps with the maximum energy, the following equation [51] is utilized:

OP = -ΔG_max /e

(5)

Where ΔG_max reflects the greatest variation in free energy seen between two steps that are next to each other in the reaction rout.

3. Results and Discussion

3.1. Surface Analysis

The surface analysis was the next phase in the forward examination, which came after we had obtained the optimal relaxed structures, which was the first step in our screening process. During this process, we looked at the slab's capacity to absorb a variety of different species. For example, the electrocatalytic reduction of carbon monoxide is the primary emphasis of our study. As a result, we studied the potential TMPs for carbon monoxide adsorption. Because of this, in addition to verifying the CO adsorption, we also examined the proton coverage at the various sites. The free energies of different adsorbed adsorbates have been summarized in Figure 2.

Figure 2 illustrates that the adsorption of CO is exergonic for HfP, TaP, TiP, VP, and ZrP; conversely, CrP and NbP exhibit unfavorable adsorption characteristics owing to the high energy demand for this process. Conversely, we examined the potential for water poisoning at each site by adsorbing water molecules to assess surface reactivity for this particular adsorption, then comparing the binding energies with CO adsorption. Figure 2 clearly demonstrates that water adsorption is consistently more endergonic than CO adsorption across all surfaces. Consequently, CO molecules are expected to occupy the catalyst surface prior to water, hence reducing the likelihood of water-induced surface poisoning. This indicates that all catalysts remain readily available and effective for CORR. In conclusion, only five TMPs (Hf, Ta, Ti, V, and Zr) exhibit high CO adsorption; thus, we will focus only on these surfaces for further investigation, since they are suitable for CORR with no risk of HER due to endergonic adsorption of proton on the surface when compared with CO adsorption.

3.2. Catalytic Activity Toward CO Reduction

CO reduction and conversion could lead to the production of methane, methanol and formaldehyde. These syntheses were observed after eight, six, and four electron proton transfers during electrocatalytic CORR. Additionally, we noticed that these formations were helped by different reaction routes, so we looked at the TMPs as catalyst surfaces and examined each route for the specific item. However, after thorough analysis, we only included the most promising free energy landscapes.

Our analysis demonstrated that VP has the highest activity among the several TMPs, since it generates formaldehyde at 0.69 eV, as seen in Figure 3. Additionally, the production of both methane and methanol was detected at the adjustable energy of 0.69 eV. Secondly, PDS during this reaction pathway was observed after the second protonation from CO*+PH* to CO*+PH*+MH*.

HfP was also identified as a suitable substrate for the electrocatalytic synthesis of both methane and methanol. As seen in Figure 4, the whole reaction pathway progresses seamlessly, exhibiting the minimal energy requirenment of around 0.82 eV for CH₃OH and CH₄. This low energy clearly identifies HfP's catalytic effectiveness in facilitating the multi-electron CO reduction reaction. PDS arises at the second proton-electron transfer, especially during the transition from CO*+MH* to CO*+MH*+MH*

The surfaces of TaP, TiP and ZrP exhibit good catalytic activity for CO reduction, as shown in Figure 5 (a-c). This activity enables the generation of environmentally friendly products such as CH₂O, CH₃OH, and CH₄ at energy of 0.90 eV. During the fourth protonation, notably the transition from CHO* to CHOH*, we were able to see that TaP has the PDS (Figure 5(a)). However, PDS for TiP, on the other hand, was seen early after the second protonation step (CO*+MH* to CO*+MH*+PH*), exhibiting a distinct reaction route while exhibiting equivalent thermodynamic efficiency (Figure 5(b)). In the meantime, ZrP (Figure 5(c)) is also capable of reducing CO into distinctive products at a slightly greater energy of 1.04 eV. Furthermore, in a manner comparable to that of TiP, ZrP demonstrates the PDS after the second protonation during the formation of CO*+ MH*+PH* from CO*+MH*.

3.3. Onset Potentials

To provide a concise review of our findings, we have included a comparative summary of the onset potential for CO reduction in Figure 6. This visual description offers a clear image of catalytic performance across all five TMPs, even though free energy layouts have previously been described.

4. Conclusions

This work used DFT to examine several surfaces composed of transition metals, such as TMPs, for the reduction of CO by electrocatalysis. We concentrated on the rock salt structures of TMPs in the (100) orientation. During the surface investigation, we noted that no surface was in the danger zone of water piousness, since each slab exhibited high CO adsorption compared to water molecules. Secondly, HER was not a concern for any surface, owing to the advantageous adsorption of CO on each TMPs. VP is designated the most active surface for CO reduction at a minimum potential of -0.69 V. VP facilitates the conversion of methane, methanol, and formaldehyde at this adjustable potential. This work presents distinct chemical pathways and a method to mitigate carbon footprints.