The Reactive Sites of Methane Activation: A Comparison of IrC₃⁺ with PtC₃⁺

Abstract

The activation reactions of methane mediated by metal carbide ions MC₃⁺ (M = Ir and Pt) were comparatively studied at room temperature using the techniques of mass spectrometry in conjunction with theoretical calculations. MC₃⁺ (M = Ir and Pt) ions reacted with CH₄ at room temperature forming MC₂H₂⁺/C₂H₂ and MC₄H₂⁺/H₂ as the major products for both systems. Besides that, PtC₃⁺ could abstract a hydrogen atom from CH₄ to generate PtC₃H⁺/CH₃, while IrC₃⁺ could not. Quantum chemical calculations showed that the MC₃⁺ (M = Ir and Pt) ions have a linear M-C-C-C structure. The first C–H activation took place on the Ir atom for IrC₃⁺. The terminal carbon atom was the reactive site for the first C–H bond activation of PtC₃⁺, which was beneficial to generate PtC₃H⁺/CH₃. The orbitals of the different metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. This study investigates the molecular-level mechanisms of the reactive sites of methane activation.

1. Introduction

Methane has attracted attention as the main component of natural gas and the conversion of methane into value-added chemicals is very important [1,2]. However, methane is extremely stable, with high C–H bond strengths (439 kJ/mol), negligible electron affinity and low polarizability [3]. At present, most of the catalytic conversion of methane needs to be carried out under high-temperature or high-pressure conditions [4]. Metal carbides are a kind of molecules with the potential to activate methane and have been studied by several groups [5]. Research on the mechanisms of the activation of methane by metal carbides is of great value and it may be helpful to find new catalysts of metal carbides [6,7]. At the same time, it is very difficult to study the activation mechanisms of methane. In recent years, people have found that the study of the gas-phase reaction of methane is an important means to study related reaction mechanisms [8,9].

Past studies have shown that gas-phase clusters are an ideal model for studying the reaction mechanisms in condensed-phase systems. The study of gas-phase clusters can reveal the specific reaction mechanisms, including the active sites, and provide references for condensed-phase catalytical processes [10]. At present, researchers are concerned about the reactions of methane with metal ions such as Os⁺ [11], Pt⁺ [12], Ta⁺ [13] and Rh(0) [14] and metal oxides such as MgO⁺ [15], PbO⁺ [16], V₄O₁₀⁺ [17] and Re₂O₇⁺ [18]. In addition to metal ions and their oxides, studies have shown that metal carbides can also activate methane, such as FeC₆⁻ [19], Mo₂C₂⁻ [20], Ta₂C₄⁻ [21], FeC₃⁻ [6], AuC⁺ [22], FeC₄⁺ [23] and MC⁺ [7]. These studies have explored some possible mechanisms for the activation of methane by metal carbide clusters. For example, the study of AuC⁺ reveals a special hydride-transfer mechanism (HT) [22], while the study of FeC₃^- shows that methane and atomic clusters generate CC-coupling reaction products at high temperatures and explained the possible mechanism of non-oxidized methane aromatization at the molecular level [6]. The study of FeC₄⁺ shows that the cluster can activate the C–H bonds of methane via the hydrogen-atom transfer (HAT) mechanism at ambient temperature; the study used the frontier orbital theory to explain the root cause of the HAT reaction [23]. Although there have been some studies on the mechanisms of metal carbides to activate methane in the past, there is still no clear investigation on the reactive sites of methane activation. Therefore, further research is needed to study the reactive sites for the activation of methane. Here, we reported the reactions of MC₃⁺ (M = Pt and Ir) with methane.

2. Experimental and Computational Methods

The experiments were performed using an ion trap mass spectrometer equipped with a laser vaporization–supersonic expansion ion source that was reported previously [24,25]. The MC₃⁺ (M = Pt and Ir) ions were generated by pulsed laser ablation of a rotating and translating metal/carbon (metal:carbon = 1:4) target. The nascent ablated plasma was entrained by a helium carrier gas with a backing pressure of about 0.5 MPa. The ions were mass-selected by a quadrupole and then were sent into a linear ion trap, where the ions were accumulated and cooled by helium gas. The MC₃⁺ ions reacted with CH₄, CD₄ and ¹³CH₄, introduced by a pulsed valve. After a 10 ms reaction, the trapped ions were ejected for mass detection.

Theoretical calculations were performed using the Gaussian 09 package [26]. All of the calculations were performed using the BMK functional with the def2-TZVP basis sets [27,28]. Vibrational frequency calculations were employed to identify the nature of reaction intermediates, transition states (TSs) and products. The Molclus program [29] was used to search for the possible stable structures of the MC₃⁺, MC₂H₂⁺, MC₄H₂⁺ and MC₃H⁺ (M = Pt and Ir). The low-lying stable isomers were then re-optimized at the BMK/def2TZVP level to confirm the relative energy sequence. Transition-state optimizations were performed with the synchronous transit-guided quasi-Newton (STQN) method and were verified through intrinsic reaction coordinate (IRC) calculations [30,31]. Vibrational frequency calculations were performed to identify the nature of reaction intermediates, transition states (TSs) and products.

3. Results and Discussion

The mass spectra for the reactions of mass-selected MC₃⁺ (M = Pt and Ir) ions with He (a1 and a2), CH₄ (b1 and b2), CD₄ (c1 and c2) and ¹³CH₄ (d1 and d2) in the ion trap at room temperature are shown in Figure 1. No product ion was observed in the mass spectra when using pure He as reactant gas, while two main peaks, at m/z = 219 and 243 for the Ir-system, and m/z = 222 and 246 for the Pt-system for the reactions with CH₄, which can be attributed to the product ions with chemical formulas MC₂H₂⁺ and MC₄H₂⁺ (M = ¹⁹⁶Pt and ¹⁹³Ir), were observed to be the major reaction products. A weak mass peak at m/z = 233 assigned to the ion with chemical formula PtC₃H⁺ for PtC₃⁺/CH₄ was also observed, while no IrC₃H⁺ was found. The mass spectra suggest that two main reaction channels for both systems and one hydrogen-atom abstraction channel for PtC₃⁺ were observed. The first channel is the formation of the MC₂H₂⁺ cation with the release of a neutral C₂H₂ (reaction 1). The second channel is the generation of the MC₄H₂⁺ ion with concomitant elimination of a dihydrogen molecule (reaction 2). The third channel is the generation of the PtC₃H⁺ ion with the release of CH₃ as shown in reaction 3.

MC₃⁺ + CH₄ → MC₂H₂⁺ + C₂H₂

(1)

MC₃⁺ + CH₄ → MC₄H₂⁺ + H₂

(2)

PtC₃⁺ + CH₄ → PtC₃H⁺ + CH₃

(3)

Impurity PtC₃•H₂O⁺ and IrCO⁺ ions were formed due to the small amount of contaminant in the chamber of the instrument. Isotopic-labeling experiments conducted using the CD₄ sample showed that there were peaks of IrC₂D₂⁺, IrC₄D₂⁺, PtC₂D₂⁺, PtC₄D₂⁺ and PtC₃D⁺, which demonstrates that the H atoms in the products were all from methane. Both the PtC₂H₂⁺ and Pt¹³CCH₂⁺ product ions were observed to have approximately the same intensity, when using ¹³CH₄, indicating that one or both carbon atoms of the eliminated C₂H₂ neutral molecule came from the PtC₃⁺ ion. The intensity of the peak of Ir¹³CCH₂⁺ is similar with that of IrC₂H₂⁺, which eliminates the background peak. The peaks of Ir¹³CC₃H₂⁺ and Pt¹³CC₃H₂⁺ were observed, which confirms the reaction channels.

In order to gain insight into the reaction mechanisms, the various possible structures of the products MC₃⁺ (M = Pt and Ir) were obtained by calculations at the BMK/def2-TZVP level and are shown in Figure S1. The most stable structures of MC₃⁺ (M = Pt and Ir) have a linear structure M-C-C-C, with the ground state ¹Σ⁺ (IrC₃⁺) and ²Σ⁻ (PtC₃⁺). For the products (Figure S2), the most stable structures of MC₂H₂⁺ and MC₄H₂⁺ (M = Pt and Ir) are metal cations with C₂H₂ and linear H-C-C-C-C-H, respectively. For MC₃H⁺ (M = Pt and Ir), the most stable structures of MC₃H⁺ (M = Pt and Ir) have a linear structure M-C-C-C-H, given in Figure S2.

In order to gain insight into the reaction mechanism, the potential energy profiles (PESs) were calculated. All of the three Reactions (1)–(3) leading to the stable structures of the products were exothermic. The pathways for the IrC₃⁺ + CH₄ reaction leading to the IrC₂H₂⁺/C₂H₂ and IrC₄H₂⁺/H₂ products are shown in Figure 2 and details are given in Figures S3–S4. An encounter complex (³I1), that is −1.51 eV lower in energy than the ground state reactants, is formed initially. The Ir atom serves as the active site of the reaction and intermediate ¹I2 is formed via the first C–H bond activation. Then, the CH₃ moiety is transferred to C₃ to form a C–C bond; meanwhile, one H atom is transferred to the Ir atom (¹I2 → ¹TS2 → ¹I3). Subsequently, two hydrogen atoms transfer from the metal Ir atom to the C atom to form two new C–H bonds (¹I3 → ¹TS3 → ¹I4 → ¹TS4 → ¹I5). The H atom of the CH₂ moiety is activated and transferred to the Ir metal (¹I5 → ¹TS5 → ¹I6); then, the H atom migrates from the Ir metal to the C atom which is not coordinated to other H atoms (¹I6 → ¹TS6 → ¹I7). After the CC bond is cleft, the intermediate ²I8 is formed, which involves two equivalent C₂H₂ moieties—either one can be liberated to form the final product P1 (³IrC₂H₂⁺/C₂H₂). For the pathway for the generation of IrC₄H₂⁺/H₂, there is the rearrangement steps from ¹I3 to form intermediate ¹I10 (¹I3 → ¹TS8 → ¹I9 → ¹TS9 → ¹I10). The final product ¹IrC₄H₂⁺ is generated with the liberation of H₂. Another possible pathway is given in Figure S4.

The pathways for the PtC₃⁺ + CH₄ reaction are shown in Figure 3. Details and other possible pathways are given in Figures S5–S7. The reaction pathway starts from the encounter complex ²I1, followed by the formation of a stable intermediate ²I2 via the approaching of CH₄ to the terminal carbon of PtC₃⁺ (²I1 → ²TS1 → ²I2), where the first C–H activation takes place. The CH₃ moiety of ²I2 is transferred to form ²I3, where CH₃ is loosely coordinated to PtC₃H⁺. The final product P1 (−0.46 eV) is generated with the liberation of CH₃. The energy of P1 is higher than the transition states from ²I2 to P2/P3, which can explain the weak peak of PtCH₃⁺, compared with PtC₄H₂⁺ and PtC₂H₂⁺. From ²I2, the H atom from the CH₃ moiety is activated and transferred to the metal center to form intermediate ²I4 (²I2→ ²TS3 → ²I4). The H atom subsequently migrates from the Pt metal to the terminal C atom to form intermediate ²I5 (²I4 → ²TS4 → ²I5). After rearrangement (²I5 → ²TS5 → ²I6), the H atom of the CH₂ moiety is activated and transferred to the Pt metal again (²I6 → ²TS6 → ²I7). Then, the H atom migrates from the Pt metal to C atom which is not coordinated to other H atoms (²I7 → ²TS7 → ²I8). After the C and Pt atoms are bonded, the stable intermediate ²I9 with a five-membered ring is generated. After the CC bond is cleft, the intermediate ²I10 is formed, which involves two equivalent C₂H₂ moieties—either one can be liberated to form the final product P2 (²PtC₂H₂⁺/C₂H₂). For another pathway to generate PtC₄H₂⁺/H₂, after the rearrangement from ²I5 to form ²I11, the H atom of the CH₂ moiety is activated and transferred to the Pt metal again (²I11 → ²TS11 → ²I12). The subsequent reaction pathway is the H–H bond formation; then, the final product PtC₄H₂⁺ is generated with the liberation of H₂.

For the reactions of the IrC₃⁺ and PtC₃⁺ with CH₄, we could find the hydrogen abstraction reaction products PtC₃H⁺ and no IrC₃H⁺ was observed. Based on our calculations, the reactive sites play a key role. There are two possible sites, the metal and terminal carbon atom (the other two carbon atoms are fully bonded). For IrC₃⁺/CH₄, the barrier to the first C–H activation on the terminal C atom (+0.04 eV; see Figure S4) is higher in energy than ¹TS1 (−0.68 eV). Based on the lack of IrC₃H⁺ observed, we confirm that the reactive site of the first C–H activation is the Ir atom. For PtC₃⁺/CH₄, the energy required for the first C–H bond activation on Pt and C atoms is +0.01 (²TSX3) and +0.07 eV (²TS1), given in Figure 3 and Figure S6, respectively. Though the Pt atom, as a reactive site, is favorable, the transition state of H atom transfer as the next step (+0.19 eV, ²TSX4; Figure S4) is higher in energy than ²TS1. Based on the observation of PtC₃H⁺ in the experiment, we confirm that the terminal carbon atom can be a reactive site for PtC₃⁺/CH₄.

As we all know, metals have a stronger adsorption capacity for methane than non-metals. The different reactive sites can be explained by the analysis of the orbital. In the PtC₃⁺ species, platinum uses two of the six valence orbitals (s and d) to form a σ- and π-bond with the adjacent carbon atom. This leaves seven electrons occupying the four remaining non-bonding orbitals on platinum, such that there are no empty orbitals on the metal. Therefore, it is hard for the C−H bond to donate electron density to the metal and the energy of the transition state ²TSX3 is a little higher than the ²TS1 in Figure 3. For the IrC₃⁺ species, Ir uses two of the six valence orbitals (s and d) to form a bond with the adjacent carbon atom, which leaves six electrons occupying the four remaining non-bonding orbitals on the Ir atom, such that there are enough empty orbitals on the metal, which is beneficial for the C–H bond activation of CH₄. This can explain why the barrier to the first C–H activation on the terminal C atom (+0.04 eV) is higher than that of ¹TS1 (−0.68 eV). Our work demonstrates that the orbitals of the metal influence the selection of the reactive sites for the activation of methane. The different reactive sites result in different reaction channels.

In conclusion, the activation of methane mediated by MC₃⁺ (M = Ir and Pt) were comparatively studied at room temperature by gas-phase experiments with theoretical calculations. Mass spectrometric studies on the reactions of the MC₃⁺ (M = Ir and Pt) ions with CH₄ show that two main reaction channels were observed. The first channel is the formation of the MC₂H₂⁺ cation with the release of neutral C₂H₂. The second channel is the generation of the MC₄H₂⁺ ion with concomitant elimination of a dihydrogen molecule. Besides that, PtC₃H⁺ could be found in the experiments, while IrC₃H⁺ could not. Quantum chemical calculations suggest that the MC₃⁺ (M = Ir and Pt) ions have a linear M-C-C-C⁺ structure. The Ir atom is the reactive site for the reaction of IrC₃⁺/CH₄. The generation of the PtC₃H⁺ and PESs can confirm that the terminal carbon atom is the reactive site for PtC₃⁺/CH₄. The orbitals of the metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. Our work is helpful for understanding reactive sites of methane activation.