Adsorption and Catalytic Decomposition Mechanism of C₆F₁₂O on Cu Surfaces: A Density Functional Theory Study

Abstract

C₆F₁₂O has been recognized as an environmentally friendly substitute applied in the fire protection, insulation equipment, and refrigeration industry. The stability and catalytic decomposition characteristics of C₆F₁₂O in the presence of metals are crucial for evaluating the applicability of such alternatives across different scenarios and recycling treatment. In this study, the adsorption and decomposition mechanisms of C₆F₁₂O on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces have been investigated based on the density functional theory (DFT). The adsorption structures and energies of C₆F₁₂O and its key dissociation products are investigated to obtain the most stable adsorption configurations. Additionally, the projected density of states (PDOS) and electron density difference calculations are performed to explore the electronic properties of the adsorption systems. Four major dissociation reactions involving the C-C bond breakage of C₆F₁₂O that occurred on Cu surfaces are examined individually, with comparisons made to the corresponding homolytic reactions of free C₆F₁₂O. The results indicate that Cu surfaces exhibit a promising catalytic effect for C₆F₁₂O decomposition, which depends on both the kind of surfaces and the reaction pathway. Furthermore, most decomposition pathways of C₆F₁₂O on Cu surfaces are exothermic and C₆F₁₂O tends to decompose into C₅F₉O and CF₃ under the Cu catalytic effect.

1. Introduction

Fluorine-containing substances such as hydrofluorocarbons (HFCs) and sulfur hexafluoride (SF₆) with their excellent thermodynamic performance, radical capture, and insulating ability are widely applied in fire protection [1,2], the refrigeration industry [3], and gas-insulated equipment [4]. However, HFCs and SF₆ are recognized as strong greenhouse gases with a high global warming potential (GWP) and long atmospheric lifetime (ALT) [5,6,7,8]. Heptafluoropropane (also called HFC 227ea) is one of the most widely used fire suppressants with the GWP up to 3350 and the ALT 38.9 years, and, for the GWP of the insulating gas, the SF₆ is 23,500 and ALT is 3200 years. These greenhouse gases are scheduled to phase out according to the Kyoto Protocol (1997) [9] and the Kigali Amendment to the Montreal Protocol (2016) [10]. Hence, seeking environmentally friendly substitutes is of great importance [8,11]. In recent years, C₆F₁₂O (perfluro-2-methyl-3-pentanone, also called FK-5112, trade name Novec 1230 or Novec 649) has drawn much attention due to its environmentally friendly performance, and low GWP (approximately 1) and ALT (2 weeks). And its approximately nontoxicity, noncombustility, and outstanding thermodynamic and insulation performance has made C₆F₁₂O a promising substitute for HFCs and SF₆ in these fields [1,12,13,14,15].

In practical application, the stability of such a working fluid during storage and transport is of great importance for system operation. However, unwanted incidents such as discharge or being overheated may induce decomposition, which may lead to function failure and both environment and toxicity issues as reported in previous studies [12,16,17]. Moreover, the decomposition of C₆F₁₂O can also occur through photolysis, hydration, and hydrolysis [18,19]. The decomposition pathways of C₆F₁₂O have been investigate both through experiments and theoretical ways. Zhang et al. [20] studied the decomposition products of C₆F₁₂O and N₂ mixed gas under AC voltage by gas chromatography–mass spectrometry; the breakage of the C-C bond as the initial decomposition pathway was obtained through density functional theory (DFT). This dissociation mechanism of C₆F₁₂O was also found in overheated scenes by experiment and DFT analysis [17]. Besides, our previous work [16,21] examined the initial decomposition pathways that, mainly, through the break of C-C bonds by ReaxFF molecular dynamic simulations, further determined the dissociation mechanism of C₆F₁₂O.

Although some studies have explored the stability of the fluid itself and its decomposition mechanism, C₆F₁₂O inevitably interacts with container materials during storage and operation. Given that certain metal components act as effective adsorbents and catalysts, the aforementioned decomposition process may be accelerated. Compatibility thus emerges as a critical issue, as it impacts not only material corrosion but also working fluid stability. In terms of adsorption, Li et al. [22] studied the interaction between C₆F₁₂O and metals (Cu, Al, and Ag) through both experiment and DFT calculation, which found that the interaction between C₆F₁₂O and Cu, and Al introduces fluorine to the metal surface and generates some byproducts, and the interaction mechanism between C₆F₁₂O and Cu, and Al is chemical adsorption. Similarly, the interaction between C₅F₁₂O and Al (1 1 1) and Ag (1 1 1) was also studied by the DFT method and the interaction between Al (1 1 1) is chemical adsorption, which is stronger than physical interaction as C₅F₁₂O interacts with Ag (1 0 0) [23]. Zhang et al. [24] studied the decomposition of C₅F₉O and conducted the DFT calculations to investigate the compatibility of the decomposition products with Cu and Al. Furthermore, regarding the catalytic effects on the decomposition of the metals in contact with the working fluid, Cui et al. [25] analyzed the adsorption and decomposition of SF₆ on α-Al₂O₃ (0 0 0 1) through a DFT study and found that α-Al₂O₃ has certain catalytic properties. Huo et al. [26] investigated the catalytic mechanisms of Cu (1 1 1), Cu (1 1 0), and Cu (1 0 0) surfaces on HFO-1336mzz (Z) decomposition through DFT calculations, finding that Cu could obviously promote the dissociation of HFO-1336mzz (Z) and the Cu (1 0 0) surface has the highest catalytic activity while the Cu (1 1 1) surface has the lowest. Zhang et al. [27]. investigated the compatibility between the C₆F₁₂O-air gas mixture and copper and aluminum through experiments, and the results showed that C₆F₁₂O-air is incompatible with heated copper, resulting in the decomposition reactions, and the metal acts as a catalyst that could decrease the energy barrier of C₆F₁₂O. In addition to stability concerns, environmental considerations highlight that the catalytic decomposition of C₆F₁₂O after long-term use—including its recovery and treatment—will also emerge as a critical research topic in the future. Since previous studies have shown that metals may exert a catalytic effect on the decomposition of fluorine-containing substances, it is imperative that we explore the catalytic decomposition mechanism of C₆F₁₂O on metal surfaces.

In this work, considering that copper is a widely used material in application scenarios of C₆F₁₂O, the catalytic effect of Cu on the decomposition of different substances has been extensively investigated via DFT calculations [28,29,30,31,32], which shows this method is a powerful tool for illustrating the mechanism of surface reactions at the molecular level. The DFT method has been applied to explore the adsorption and catalytic decomposition mechanism of C₆F₁₂O on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces. Four initial dissociation reactions involving C-C bond breakage are considered to investigate the catalytic decomposition mechanism of C₆F₁₂O first, and then the adsorption configurations of C₆F₁₂O, the relative dissociation products, and co-absorbed species on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces are determined. Finally, four decomposition pathways of C₆F₁₂O on these surfaces are calculated compared to the pathways of free C₆F₁₂O to illuminate the catalytic decomposition mechanism of C₆F₁₂O on the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces.

2. Results

2.1. Adsorption

2.1.1. Adsorption of C₆F₁₂O and Products on Cu (1 0 0) Surface

Three typical adsorption sites (top, bridge, and hole) on the Cu (1 0 0) surface are considered for C₆F₁₂O and its decomposition products. The most stable adsorption states and the corresponding adsorption energies, configurations, and key bond length for each species absorbed on the Cu (1 0 0) surface are listed in Table 1. The most stable adsorption structures of C₆F₁₂O and the relevant products on the Cu (1 0 0) surface are shown in Figure 1.

Table 1. Adsorption site, configuration, bond length, and energy for C₆F₁₂O and its decomposition species on Cu (1 0 0).

Specie	Adsorption Site	Configuration	Bond Length/Å	Eads/eV
C6F12O	Top	O-bound	2.005 (O-Cu)	1.02
C5F9O	Top	C-bound, O-bound	2.040 (C-Cu), 2.102 (O-Cu)	2.07
C5F9O-(2)	Top	C-bound, O-bound	2.093 (C-Cu), 2.132 (O-Cu)	2.55
C4F7O	Hole	C=O-bound	2.030 (C-Cu1), 2.022 (C-Cu2), 2.131 (O-Cu3), 2.111 (O-Cu4)	2.62
C3F5O	Hole	C=O-bound	2.020 (C-Cu1), 2.026 (C-Cu2), 2.123 (O-Cu3), 2.109 (O-Cu4)	2.52
C3F7	Top	C-bound	2.051 (C-Cu)	2.38
C2F5	Top	C-bound	1.996 (C-Cu)	2.23
CF3	Top	C-bound	1.977 (C-Cu)	3.19

Cu1, Cu2, Cu3, and Cu4 represent the different Cu atoms of the Cu (1 0 0) surface.

Figure 1. The most stable adsorption structure of C₆F₁₂O and the relevant products on Cu (1 0 0) surface. Color code: carbon—gray, oxygen—red, fluorine—blue, Cu—orange.

As for obtaining the most stable adsorption structure of C₆F₁₂O, several initial geometric structures of C₆F₁₂O are built where the molecule with different orientations is placed on different sites of the surface, considering the particular functional groups in the molecule [22,23,24]. After geometric optimization calculations of the three different adsorption sites (top, bridge, and hole) on the Cu (1 0 0) surface, the most stable adsorption structure is the O atom adsorbed on the top site of the Cu (1 0 0) surface, as shown in the Figure 1a, which is consistent with the previous studies [22,23] and the electrostatic potential and dipole moment analysis of C₆F₁₂O. The O atom bonding with the Cu atom with the bond length of 2.005 Å and the O atom has a little movement from the top site after interaction. The adsorption energy reaches 1.02 eV.

The initial geometric structures of the dissociation species of C₆F₁₂O through the breakage of the C-C bond are established by the C atom with an unpaired electron paralleled to the different adsorption sites of the metal surface. There are two isomers of C₅F₉O: one is (CF₃)₂CF₂COCF₂ (C₅F₉O), the other is CF₃CF₂COCF₂CF₃ (C₅F₉O-(2)). For C₅F₉O, the most stable adsorption structure on the Cu (1 0 0) surface is shown in Figure 1b. C₅F₉O adsorbed on the Cu (1 0 0) surface through C-Cu and O-Cu has a relatively lower absorption energy, 2.07 eV, and the bond length of C-Cu is 2.040 Å and O-Cu 2.102 Å. Meanwhile, its isomer C₅F₉O-(2) adsorbs on the top site of the Cu (1 0 0) surface through C-Cu and O-Cu with an adsorption energy of 2.55 eV. The bond length of C-Cu is 2.093 Å and that of O-Cu is 2.132 Å, as shown in Figure 1c.

C₄F₇O prefers to absorb on the bridge site of the Cu (1 0 0) surface through the C=O bond, by which C interacts with two Cu atoms forming a C-Cu bond and O with another two Cu atoms forming an O-Cu bond, as shown in Figure 1d. The length of the C-Cu bonds is 2.030 and 2.022 Å, respectively, and that of the O-Cu bonds 2.131 and 2.111 Å, respectively. The adsorption energy of the most stable structure is 2.62 eV. The most stable adsorption structure of C₃F₅O on the Cu (1 0 0) surface is the C=O adsorbed on the bridge site (shown in Figure 1e), similar to C₄F₇O. The adsorption energy is 2.52 eV. The length of the C-Cu bonds is 2.020 and 2.026 Å, respectively, and the length of the O-Cu bonds is 2.123 and 2.109 Å, respectively.

Three initial adsorption structures have been considered for C₃F₇, C₂F₅, and CF₃, and the most stable adsorption structures are the species residing on the top site of the Cu (1 0 0) surface forming C-Cu bonds, as shown in Figure 1f–h. The length of the C-Cu bond of C₃F₇, C₂F₅, and CF₃ is 2.051 Å, 1.996 Å, and 1.977 Å, respectively. The adsorption energy of CF₃ is 3.19 eV, which is the largest among these three species, and the adsorption energy of C₃F₇ and C₂F₅ is 2.38 and 2.23 eV, respectively. Though CF₃ could adsorb on the bridge sites forming two C-Cu bonds, the adsorption energy is lower, which is not as stable as the top site adsorption.

2.1.2. Adsorption of C₆F₁₂O and Products on Cu (1 1 0) Surface

Four typical adsorption sites (top, hole, short bridge, and long bridge) on the Cu (1 1 0) surface are considered for C₆F₁₂O and the decomposition products. The most stable adsorption states and the corresponding adsorption energies, configurations, and key bond length for each species absorbed on the Cu (1 1 0) surface are listed in Table 2. The most stable adsorption structures of C₆F₁₂O and the relevant products on the Cu (1 1 0) surface are shown in Figure 2.

Table 2. Adsorption site, configuration, bond length, and energy for C₆F₁₂O and its decomposition species on Cu (1 1 0).

Specie	Adsorption Site	Configuration	Bond Length/Å	Eads/eV
C6F12O	Top	O-bound	1.967 (O-Cu)	1.21
C5F9O	Top	C-bound, O-bound	2.012 (C-Cu), 2.072 (O-Cu)	2.26
C5F9O-(2)	Top	C-bound, O-bound	2.068 (C-Cu), 2.073 (O-Cu)	2.76
C4F7O	Long Bridge	C=O-bound	1.919 (C-Cu), 2.060 (O-Cu)	2.81
C3F5O	Long Bridge	C=O-bound	1.925 (C-Cu), 2.067 (O-Cu)	2.67
C3F7	Top	C-bound	2.032 (C-Cu)	2.59
C2F5	Top	C-bound	1.979 (C-Cu)	2.35
CF3	Top	C-bound	1.967 (C-Cu)	3.32

Figure 2. The most stable adsorption structure of C₆F₁₂O and the relevant products on Cu (1 1 0) surface.

For the adsorption of C₆F₁₂O on the Cu (1 1 0) surface, the most stable adsorption configuration is the one where C₆F₁₂O adsorbs on the top site, with the length of the O-Cu bond being 1.967 Å. The calculated adsorption energy of C₆F₁₂O is 1.21 eV, as shown in Figure 2a. The adsorption of C₅F₉O and C₅F₉O-(2) on the Cu (1 1 0) surface is similar to the Cu (1 0 0) surface, which C₅F₉O prefers to adsorb on the top site through C-Cu and O-Cu bonds (Figure 2b,c). For C₅F₉O, the adsorption energy on the top site is 2.26 eV and the length of the C-Cu and O-Cu bonds is 2.012 and 2.072 Å, respectively, while the adsorption energy for C₅F₉O-(2) is 2.76 eV and the length of the C-Cu and O-Cu bonds is 2.068 and 2.073 Å, respectively. Both C₄F₇O and C₃F₅O tend to absorb on the long bridge site of the Cu (1 1 0) surface through C=O bonds (Figure 2d,e). The adsorption energy of C₄F₇O on the long bridge site of the Cu (1 1 0) surface is 2.81 eV with the length of the C-Cu and O-Cu bonds being 1.919 and 2.060 Å, respectively. And the adsorption energy of C₃F₅O on the Cu (1 1 0) surface is 2.67 eV with the length of the C-Cu and O-Cu bonds being 1.925 and 2.067 Å, respectively. For C₃F₇, C₂F₅, and CF₃, after geometric optimization, the most stable adsorption configurations are those where these species adsorb on the top site of Cu (1 1 0), forming C-Cu bonds, as shown in Figure 2f–h. The length of the C-Cu bonds is 2.032, 1.979, and 1.967 Å, respectively, and the adsorption energy of the most stable adsorption structures is 2.59, 2.35, and 3.32 eV.

2.1.3. Adsorption of C₆F₁₂O and Products on Cu (1 1 1) Surface

Four typical adsorption sites (top, bridge, hcp, and fcc) on the Cu (1 1 1) surface are considered for C₆F₁₂O and its decomposition products. The most stable adsorption states and the corresponding adsorption energies, configurations, and key bond length for each species absorbed on the Cu (1 1 1) surface are listed in Table 3. The most stable adsorption structures of C₆F₁₂O and the relevant products on the Cu (1 1 1) surface are shown in Figure 3.

Table 3. Adsorption site, configuration, bond length, and energy for C₆F₁₂O and its decomposition species on Cu (1 1 1).

Specie	Adsorption Site	Configuration	Bond Length/Å	Eads/eV
C6F12O	Top	O-bound	2.090 (O-Cu)	0.93
C5F9O	Top	C-bound, O-bound	2.045 (C-Cu), 2.136 (O-Cu)	1.77
C5F9O-(2)	Top	C-bound, O-bound	2.090 (C-Cu), 2.129 (O-Cu)	2.23
C4F7O	Bridge	C=O-bound	1.919 (C-Cu), 2.191 (O-Cu)	2.35
C3F5O	Bridge	C=O-bound	1.925 (C-Cu), 2.181 (O-Cu)	2.21
C3F7	Top	C-bound	2.080 (C-Cu)	2.15
C2F5	Top	C-bound	2.013 (C-Cu)	2.06
CF3	Top	C-bound	1.993 (C-Cu)	3.06

Figure 3. The most stable adsorption structure of C₆F₁₂O and the relevant products on Cu (1 1 1) surface.

For the Cu (1 1 1) surface, the most stable adsorption configuration of C₆F₁₂O is that the molecule binds to the top site of the surface forming an O-Cu bond with a length of 2.090 Å as shown in Figure 3a. The calculated adsorption energy is 0.93 eV. The adsorption configuration of C₅F₉O and C₅F₉O-(2) on the Cu (1 1 1) surface is similar to that on the Cu (1 0 0) and Cu (1 1 0) surface. The species reside on the top site of the surface, forming C-Cu and O-Cu bonds (Figure 3b,c). For C₅F₉O, the adsorption energy is 1.77 eV with the length of C-Cu and O-Cu being 2.045 and 2.136 Å, respectively, while the adsorption energy for C₅F₉O-(2) is 2.23 eV and the length of the C-Cu and O-Cu is 2.090 and 2.129 Å, respectively. C₄F₇O and C₃F₅O would absorb on the bridge site of the Cu (1 1 1) surface (Figure 3d,e). The adsorption energy of C₄F₇O on the bridge site of the Cu (1 1 1) surface is 2.35 eV with the length of the C-Cu and O-Cu bonds being 1.919 and 2.191 Å, respectively. Meanwhile, the adsorption energy of C₃F₅O on the Cu (1 1 1) surface is 2.21 eV with the length of the C-Cu and O-Cu bonds being 1.925 and 2.181 Å, respectively. C₃F₇, C₂F₅, and CF₃ tend to reside on the top site of the Cu (1 1 1) surface, forming C-Cu bonds after geometry optimization, as shown in Figure 3f–h. The length of the C-Cu bond of each species is 2.080, 2.013, and 1.993 Å respectively, and the adsorption energy of the most stable adsorption configurations is 2.15, 2.06, and 3.06 eV.

2.2. Co-Adsorption

As the dissociation reactions of C₆F₁₂O occurs on the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces, the co-adsorption configurations and adsorption energies of decomposition products including C₅F₉O + CF₃, C₅F₉O-(2) + CF₃, C₄F₇O + C₂F₅, and C₃F₅O + C₃F₇ have been studied to obtain the final states in the dissociation reactions of C₆F₁₂O on the metal surfaces.

The co-adsorption energy ($E_{co-ads}$) for the dissociation species adsorbed on Cu surfaces could be defined as follows:

$$E_{co-ads}=E_a+E_b+E_{slab}-E_{(a+b+slab)}$$

where $E_a$, $E_b$, $E_{slab}$, and $E_{(a+b+slab)}$ represent the total energy of the radical A and B, the Cu slab, and co-adsorption structure ($a+b+slab$).

Considering the initial co-adsorption structures in the geometry optimization calculations, the corresponding dissociation species are placed adjacently and at the most stable adsorption site on the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces [26,33]. For example, for the co-adsorption of C₅F₉O + CF₃ on the Cu (1 0 0) surface, the initial structure is that C₅F₉O is placed on the top site and CF₃ on the adjacent top site according to the calculation results before.

The optimized co-adsorption structures after calculations are shown in Figure 4, and the most stable co-adsorption sites, the corresponding co-adsorption energies, and the sum of the separated adsorption energies, E_sum, are listed in Table 4. After geometry optimizations of each configuration, the co-adsorption structures mainly maintain their initial states, except for some tiny movement around the adsorption sites due to the repulsive interaction between the related species.

Figure 4. Optimized co-adsorption on the Cu surfaces: (a) Cu (1 0 0), (b) Cu (1 1 0), and (c) Cu (1 1 1).

Table 4. Adsorption site and energy for C₆F₁₂O and its decomposition species on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1).

Surface	Species	Adsorption Site	Eco-ads/eV	Esum/eV
Cu (1 0 0)	C5F9O + CF3	Top + Top	5.17	5.26
	C5F9O-(2) + CF3	Top, Bridge + Top	5.65	5.74
	C4F7O + C2F5	Hole + Top	4.84	4.85
	C3F5O + C3F7	Hole + Top	4.84	4.90
Cu (1 1 0)	C5F9O + CF3	Top + Top	5.47	5.58
	C5F9O-(2) + CF3	Top + Top	5.96	6.08
	C4F7O + C2F5	Long Bridge + Top	5.14	5.16
	C3F5O + C3F7	Long Bridge + Top	5.25	5.26
Cu (1 1 1)	C5F9O + CF3	Top + Top	4.81	4.83
	C5F9O-(2) + CF3	Top + Top	4.89	5.29
	C4F7O + C2F5	Bridge + Top	4.38	4.41
	C3F5O + C3F7	Bridge + Top	4.39	4.36

From Table 4, it can be found that most of the co-adsorption energies are a little less than the sum of the corresponding separated adsorption energies, which may indicate the presence of interaction energies between the co-adsorbed species as well as tiny differences in the interaction strength of these species with the Cu surfaces.

2.3. Electronic Properties of C₆F₁₂O on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) Surfaces

To understand the interaction between C₆F₁₂O and different copper surfaces, the PDOS on the atomic orbitals of the O atom and the nearest Cu atom and the electron density difference for the most stable adsorption structures are calculated as shown in Figure 5.

Figure 5. PDOS and charge density difference isosurfaces of the most stable adsorption structures of C₆F₁₂O (the blue and yellow regions represent positive and negative values) in 0.03 isovalue: (a) Cu (1 0 0), (b) Cu (1 1 0), and (c) Cu (1 1 1).

It can be found that the PDOS of C₆F₁₂O on the different Cu surfaces is similar. From −10.6 to −8.2 eV and −0.5 to 1 eV, the density of states (DOS) is mainly contributed by the 2p orbital of the O atom. From −4.5 to −0.7 eV, the DOS is primarily contributed by the Cu 3d orbital. A large overlap between the O 2p orbital and the Cu 3d orbital is observed in the energy range of −4.5 to −0.7 eV and −0.5 to 1 eV, which indicates that the formation of the adsorption bonds is mainly due to the mixing between the O 2p orbital and Cu 3d orbital. The overlap in the energy range of −10.6 to −8.2 eV is attributed to the hybridization among the O 2s, O 2p, Cu 4s, and Cu 3d orbitals. Moreover, some slight peaks are observed nearby −14, −16, and −17 eV, which comes from the mixing between O 2s and O 2p.

The electron density difference of the most stable adsorption structure is also analyzed, as shown in Figure 5. The blue and yellow represent the electron accumulation and depletion region, respectively. It can be found that some electrons mainly transfer from the Cu atom to the O atom and the electron density in the vicinity of the O atom increases after the adsorption, which means that Cu surfaces acts as the electron donor and the O atom acts as the electron acceptor and may bind with the Cu atom.

According to the analysis of the electronic properties through the PDOS and electron density difference, the interaction between the O atoms in the C₆F₁₂O and Cu surfaces is relatively strong, which could be regarded as chemisorption.

2.4. Decomposition of C₆F₁₂O on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) Surfaces

In this section, the dissociation reactions including the breakage of C-C bonds at different positions of C₆F₁₂O on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces are fully investigated, respectively. The decomposition pathways of C₆F₁₂O are listed as follows:

C₆F₁₂O → C₅F₉O + CF₃

(1)

C₆F₁₂O → C₅F₉O-(2) + CF₃

(2)

C₆F₁₂O → C₄F₇O + C₂F₅

(3)

C₆F₁₂O → C₃F₅O + C₃F₇

(4)

The most stable adsorption configurations of C₆F₁₂O on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces are chosen as the initial reactants, and the products are the most stable co-adsorption structures of the corresponding dissociation species. The calculated energy barrier (E_b) and the reaction energy ($∆\mathrm{U}$) of each dissociation reaction pathway are listed in Table 5. The related transition states (TS) of each reaction are shown in Figure 6, Figure 7 and Figure 8.

Table 5. Energy of C₆F₁₂O decomposition reactions pathways on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces.

Surface	Reactions	TS	Eb/eV	$∆\mathbf{U}$/eV
Cu (1 0 0)	Pathway 1	TS 1-1	2.25	0.03
	Pathway 2	TS 1-2	2.49	−0.42
	Pathway 3	TS 1-3	3.56	−0.50
	Pathway 4	TS 1-4	3.70	−0.59
Cu (1 1 0)	Pathway 1	TS 2-1	2.77	−0.12
	Pathway 2	TS 2-2	2.57	−0.57
	Pathway 3	TS 2-3	3.48	−0.71
	Pathway 4	TS 2-4	3.36	−0.84
Cu (1 1 1)	Pathway 1	TS 3-1	2.75	0.32
	Pathway 2	TS 3-2	2.92	0.27
	Pathway 3	TS 3-3	3.59	−0.15
	Pathway 4	TS 3-4	3.50	−0.23
Free C6F12O	Pathway 1			4.20
	Pathway 2			4.22
	Pathway 3			3.28
	Pathway 4			3.22

Figure 6. The transition states of C₆F₁₂O dissociation on the Cu (1 0 0) surface.

Figure 7. The transition states of C₆F₁₂O dissociation on the Cu (1 1 0) surface.

Figure 8. The transition states of C₆F₁₂O dissociation on the Cu (1 1 1) surface.

For the dissociation of C₆F₁₂O on the Cu (1 0 0) surface, the most stable adsorption configuration where C₆F₁₂O adsorbs on the top site is chosen as the initial reactant, then the reactant would decompose into small species through the breakage of C-C bonds as shown in Figure 6. In pathway 1, C₆F₁₂O dissociates into the C₅F₉O and CF₃ via TS 1-1. The angle of the C-O-Cu bond decreases from 173.1° to 112.3° in TS 1-1, which means the C-O bond is closer to the Cu (1 0 0) surface. The length of the O-Cu bond is about 2.196 Å compared to the 2.005 Å in the initial state. After TS 1-1, C₅F₉O adsorbs on the top site of the surface, forming O-Cu and C-Cu bonds. The energy barrier and reaction energy of pathway 1 on the Cu (1 0 0) surface is 2.25 and 0.03 eV, respectively. Pathway 2 is where C₆F₁₂O decomposes into C₅F₉O-(2) and CF₃ through the breakage of the C-C bond at the other end of C₆F₁₂O. In TS 1-2, the initial O-Cu bond has been broken and C₅F₉O-(2) absorbs on the Cu (1 0 0) surface forming the O-Cu bond. After TS 1-2, C₅F₉O-(2) moves to the bridge site and CF₃ moves to the top site. The energy barrier of the reaction is 2.49 eV, which is a little higher than pathway 1, and pathway 2 on the Cu (1 0 0) surface is exothermic, the reaction energy of which is −0.42 eV. In pathway 3, C₆F₁₂O dissociates into C₄F₇O and C₂F₅ on the Cu (1 0 0) surface through TS 1-3 where the distance between the dissociated C atoms increases from 1.531 to 2.747 Å. The bond length of O-Cu changes from 2.005 to 2.039 Å. Passing through TS 1-3, the dissociated C₄F₇O would move to the hole site and C₂F₅ to the top site. The energy barrier of pathway 3 on the Cu (1 0 0) surface is 3.56 eV, and the reaction energy is −0.50 eV, which is also exothermic. For pathway 4, it starts from the most stable adsorption structure of C₆F₁₂O occupying the top site via the O atom on the Cu (1 0 0) surface, then the breakage of the C-C bond leads to the co-adsorption of C₃F₅O and C₃F₇ via TS 1-4. The O-Cu bond is stretched to 2.141 Å and the angle of C-O-Cu decreases from 173.1° to 132.2°. After TS 1-4, C₃F₇ adsorbs on the adjacent top sites forming the C-Cu bond and C₃F₅O adsorbs on the hole sites with the C-O almost parallel to the surface. The energy barrier of pathway 4 on the Cu (1 0 0) surface is 3.70 eV and the reaction energy is −0.59 eV, which indicates this pathway is an exothermic reaction.

For the dissociation of C₆F₁₂O on the Cu (1 1 0) surface, the four pathways mentioned before are fully investigated as shown in Figure 7. The most stable adsorption structure of C₆F₁₂O on the Cu (1 1 0) surface is selected as the initial reactant of each pathway. In pathway 1, the cleavage of the C-C bond in C₆F₁₂O generates C₅F₉O and CF₃ species through TS 2-1, where the distance of the dissociated C atoms increases from 1.564 to 2.903 Å. The angle of C-O-Cu changes to 127.3° from the initial state, 140.2° which indicates that C₅F₉O is much closer to the surface in TS 2-1. Then, C₅F₉O adsorbs on the top site of the Cu (1 1 0) surface, forming C-Cu and O-Cu bonds, and CF₃ resides on the top site. The energy barrier and the reaction energy of pathway 1 are 2.77 eV and −0.12 eV, respectively. In pathway 2, C₆F₁₂O decomposes into C₅F₉O-(2) and CF₃ via TS 2-2. C₅F₉O-(2) moves to the bridge site of the Cu (1 1 0) surface through O-Cu bonds in TS 2-2, and then it adsorbs on the top site of the surface similar to CF₃. Pathway 2 has an energy barrier of 2.57 eV and a reaction energy of −0.57 eV. As C₆F₁₂O dissociates to C₄F₇O and C₂F₅ (pathway 3) via TS 2-3 on the Cu (1 1 0) surface, the angle of C-O-Cu decreases from 140.2° to 124.8° in TS 2-3 with the length of the O-Cu bond rising to 2.126 Å. The distance between the dissociated C atoms is 3.127 Å compared with 1.518 Å at the beginning. Passing through TS 2-3, C₄F₇O adsorbs on the long bridge site and C₂F₅ adsorbs on the top site. Pathway 3 needs to conquer the energy barrier of 3.48 eV with an exothermic value of −0.71 eV. In pathway 4, the breakage of the C-C bond forming C₃F₅O and C₃F₇ needs to overcome the energy barrier of 3.36 eV via TS 2-4. The distance of breakage of C atoms increases to 3.369 Å in TS 2-4. Similar to the products of pathway 3, C₃F₅O finally binds at the long bridge site and C₃F₇ binds at the top site, and the reaction energy of pathway 4 is −0.84 eV.

For the dissociation of C₆F₁₂O on the Cu (1 1 1) surface (Figure 8), the most stable structure that C₆F₁₂O adsorbs on the top site of the surface is chosen as the initial reactant for each pathway. The breakage of the C-C bond forms the C₅F₉O and CF₃ species (pathway 1) via TS 1. The distance between the dissociated C atoms rises to 3.238 Å in TS 3-1 while the C-C bond length in C₆F₁₂O is 1.561 Å. In TS 3-1, the decomposition fragments are not bonding with the Cu atoms in the surface, and the length of the Cu-Cu bond below the O atom increases from 2.572 to 2.596 Å. After TS 3-1, C₅F₉O and CF₃ move to the top sites forming O-Cu and C-Cu bonds. The energy barrier of the pathway 1 is 2.75 eV and the reaction energy is 0.32 eV, meaning this reaction is endothermic. As for pathway 2, the length of the O-Cu bond increases from 2.090 to 2.211 Å and the distance between dissociated C atoms increases from 1.574 to 2.936 Å in TS 3-2. Passing through TS 3-2, CF₃ and C₅F₉O adsorb on the top sites of the surface. The energy barrier of this reaction is 2.92 eV and the reaction energy is 0.27 eV, which is also an endothermic reaction. In pathway 3, C₆F₁₂O dissociates into C₄F₇O and C₂F₅ on the Cu (1 1 1) surface, which need to conquer the energy barrier of 3.59 eV with an exothermic value of −0.15 eV. The bond length of O-Cu is 2.163 Å in TS 3-3 while it is 2.090 Å in the initial state, and the distance of separated C atoms increases to 2.749 Å. After TS 3-3, C₂F₅ moves to the top site, and C₄F₇O resides on the bridge site of the surface with the length of C-Cu being 1.929 Å and O-Cu 2.148 Å. For pathway 4, C₆F₁₂O decomposes into C₃F₅O and C₃F₇ on the Cu (1 1 1) surface via TS 3-4. In TS 3-4, the species are not bonded with Cu atoms on the surface. The distance between the O-Cu bond increases from 2.090 to 2.252 Å, and the distance between the dissociated C atoms rises to 2.594 Å. C₃F₇ and C₃F₅O would finally reside on the top site and bridge site of the surface after TS 3-4. The energy barrier and the reaction energy of pathway 4 on the Cu (111) surface are 3.50 and −0.23 eV, respectively.

Compared to the dissociation energies of the free C₆F₁₂O molecule in Table 5, the energy barriers of dissociation reactions (pathway 1 and 2) on the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces are much lower, which indicates the catalytic activity of Cu surfaces in the decomposition of C₆F₁₂O. The decomposition pathways of C₆F₁₂O on Cu surfaces are mainly exothermic except for the dissociation reactions of pathway 1 and 2 on the Cu (1 1 1) surface. Moreover, different from free C₆F₁₂O decomposing, the energy required for pathway 1 and 2 is lower than pathway 3 and 4 on the Cu surfaces, indicating that C₆F₁₂O tends to dissociate through pathway 1 and 2, where the Cu (1 0 0) surface exhibits a better catalytic effect.

3. Methods

A generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange correlation function [34] is employed in all DFT calculations [35,36]. A double numerical plus polarization (DNP) basis set and density functional semicore pseudopotential (DSPP) [37] are utilized, where DSPP accounts for the relativistic effects of transition atoms. Van der Waals interactions are described via the Grimme method [38,39]. The Brillouin zone is sampled by a 4 × 4 × 1 Monkhorst–Pack mesh of the k-point [40] with a Methfessel–Paxton smearing of 0.003 Ha. The convergence criteria for energy, maximum force, and maximum displacement are 2 × 10⁻⁵ Ha, 4 × 10⁻³ Ha/Å, and 5 × 10⁻³ Ha, respectively, with a self-consistent field (SCF) convergence accuracy of 1 × 10⁻⁶ Ha.

The Cu bulk structure is optimized first and the Cu lattice parameter is 3.61 Å, which is correlated with the experiment result of 3.62 Å [41], showing the reliability of the computational method. Considering the balance between the accuracy and computational cost, the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces are modeled with a (5 × 5) supercell with a four-layer plate (100 Cu atoms in a cell) compared with the previous studies [22,23]. The vacuum layer of 20 Å is added between the plates to avoid the periodic interactions. Moreover, the atoms of the bottom two layers are fixed while the upper two layers are relaxed. There are three adsorption sites of the Cu (1 0 0) surface, the top, bridge, and hole; four adsorption sites of the Cu (1 1 0) surface, the top, long bridge, short bridge, and hole; and four adsorption sites of the Cu (1 1 1) surface, the top, bridge, hcp, and fcc, as shown in Figure 9. The adsorption energy ($E_{ads}$) for the substances adsorbed on the Cu surfaces are calculated as follows:

$$E_{ads}=E_{slab}+E_{substance}-E_{substance-slab}$$

where $E_{slab}$ and $E_{substance}$ represent the total energy of the Cu slab and adsorbate, and $E_{substance-slab}$ represents the total energy of substances adsorbed on the slab.

Figure 9. The absorption sites of Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surface: from top views.

The charge density difference in the adsorption structures of C₆F₁₂O is calculated as the following equation:

$$∆\mathsf{\rho }={∆\mathsf{\rho }}_{substance-slab}-{∆\rho }_{slab}-{∆\rho }_{substance}$$

where ${∆\mathsf{\rho }}_{slab}$ and ${∆\rho }_{substance}$ represent the total density of the Cu slab and C₆F₁₂O, and ${∆\mathsf{\rho }}_{substance-slab}$ represents the total density of the C₆F₁₂O molecule adsorbed on the slab.

The projected density of states (PDOS) is also explored to further analyze the electronic properties and interaction mechanism between the C₆F₁₂O and Cu surfaces.

The transition states (TSs) are searched by the complete LST/QST method for the initial decomposition pathways [29,30,33,42,43]. For the decomposition reactions occurring on the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces, the reaction energy ($∆\mathrm{U}$) and activation energy ($∆E_b$) are defined as follows:

$$∆\mathrm{U}=E_{product}-E_{reactant}$$

$$∆E_b=E_{TS}-E_{reactant}$$

where $E_{product}$, $E_{reactant}$, and $E_{TS}$ are the total energies of the product (co-adsorbed state), reactant (C₆F₁₂O adsorbed state), and transition state, respectively.

To evaluate the catalytic effect of the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces on C₆F₁₂O decomposition, the energy of the initial decomposition pathways ($∆{E\prime }_b$) of C₆F₁₂O is calculated at the same calculation level for comparison, which is defined as follows:

$$∆{E^{\prime }}_b(∆\mathrm{U})={E\prime }_{product}-{E^{\prime }}_{reactant}$$

where ${E\prime }_{reactant}$ and ${E\prime }_{product}$ are the total energies of C₆F₁₂O and its decomposition products.

4. Conclusions

In this work, the adsorption and dissociation of C₆F₁₂O on the Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces are systematically investigated through DFT calculations. The main conclusions are listed as follows:

The most stable adsorption structures for C₆F₁₂O, C₅F₉O, C₅F₉O-(2), C₄F₇O, C₃F₅O, C₃F₇, C₂F₅, and CF₃ on Cu surfaces are determined: most of these species, except for C₄F₇O and C₃F₅O, prefer to adsorb on the top site of the surfaces. On the Cu (1 0 0) surface, C₄F₇O and C₃F₅O prefer to adsorb on the hole site while these species tend to reside on the long bridge and bridge site of the Cu (1 1 0) and Cu (1 1 1) surfaces, respectively. The adsorption energies of each species on the Cu surfaces are obtained and the adsorption energy of the dissociation species is much higher than C₆F₁₂O, especially for CF₃, indicating a relatively strong interaction with Cu surfaces.

The interaction between the C₆F₁₂O and Cu surfaces is relatively strong, which belongs to chemical adsorption according to the PDOS and charge density difference analysis. Moreover, the metal surfaces act as the electron donor and C₆F₁₂O would gain electrons through the electronic analysis.

The reaction barriers of some C-C bond decomposition pathways (pathway 1 and pathway 2) via thermodynamically stable intermediates on Cu (1 0 0), Cu (1 1 0), and Cu (1 1 1) surfaces are lower than the dissociation energies of C₆F₁₂O alone, which indicates the promising catalytic effect of Cu on the decomposition of C₆F₁₂O.

The decomposition pathways of C₆F₁₂O on Cu surfaces are mainly exothermic except for the dissociation reactions of pathway 1 and 2 on the Cu (1 1 1) surface. C₆F₁₂O tends to dissociate through pathway 1 and 2 on Cu surfaces, which is different from free C₆F₁₂O decomposing. The Cu (1 0 0) surface has a better catalytic effect on pathway 1 and 2.

The results also provide a molecular-level analysis of the storage compatibility and stability of C₆F₁₂O, as well as the potential post-treatment pathway via catalytic pyrolysis after long-term service in practical applications.