“Single Pd Atom–In2O3 Catalyzes Production of CH3CH2OH from Atom-Economic C

■ INTRODUCTION

Both CH 4 and CO 2 are greenhouse gases and have been believed to contribute to climate change. 1,2 Reducing CO 2 emissions to eventually achieve carbon neutrality has been recognized as essential to reach the Paris Agreement of limiting global warming to 1.5 or 2 °C. 3 However, CH 4 presents a more severe greenhouse effect than CO 2 and is responsible for at least a quarter of global warming according to the recent literature 4,5 and United Nations reports (26th and 27th Conference of the Parties of United Nations Framework Convention on Climate Change). Furthermore, methane has also an adverse effect on tropospheric ozone and ecosystems. 6,7 Therefore, reducing methane emissions is an urgent challenge.

Converting methane to valuable chemicals other than direct combustion is a promising method for methane optimization and utilization. Since the 1980s, the oxidative coupling of methane catalyzed by metal oxides has been a research emphasis. 8,9 Valuable C 2+ chemicals can be obtained by coupling the CH 3 • radicals produced on the catalyst surface. However, unavoidable overoxidation makes the selectivity of oxidative coupling a longstanding challenge. CH 4 can be used in synthesizing high-energy density chemicals with maximum atom economy when the methyl species from CH 4 activation can be stabilized on the active sites of the catalysts. 10 For example, Pt(II) salts have been reported to catalyze the oxidative conversion of CH 4 to methanol at low temperatures from a Pt-methyl intermediate. 11 Periana et al. employed Pd(II) in the liquid phase to activate the C-H bond of CH 4 , generating a Pd-CH 3 species, which could be condensed with methanol to produce acetic acid. 10 In organometallics, the insertion of CO into the Ni-CH 3 σ bond of [NiCH 3 (L)]BPh 4 to form a C-C bond has been demonstrated. 12,13 In heterogenous catalysis, the conversion of CH 4 via C-C coupling provides one of the most attractive routes to synthesize long-carbon chain chemicals and fuels. In previous studies, we demonstrated that C-C coupling of the methyl species from CH 4 activation and CO 2 could be realized by following either the Eley-Rideal mechanism or Langmuir-Hinshelwood mechanism. 14,15 Bader charge analysis showed that CO 2 acted as an electrophile, attacking the carbon of the methyl species. If an electrophile stronger than CO 2 reacts with CH 4 , the activity of C-C coupling is expected to be higher. Since formaldehyde is a stronger electrophile than CO 2 and widely available, it can be used as a reagent to convert CH 4 to longer carbon chain products. Formaldehyde is also an indoor volatile organic compound and severely threatens human health. 16,17 To meet the limitation criteria of formaldehyde emission, both adsorption [18][19][20][21] and catalytic oxidation [22][23][24][25] of formaldehyde to CO 2 have been studied. Converting HCHO with CH 4 could help reduce HCHO emission while also alleviating the CO 2 emission.

Although the C-C coupling of CH 4 with formaldehyde is rarely reported, there have been studies reporting the formation of C-N and C-O bonds using the formaldehyde cation with ammonia and water. 26 Furthermore, formaldehyde serves as an active reactant in aldol condensation with carbonyl reactants to grow the carbon chain. 27,28 Notably, Liu and coworkers reported the collision of the formaldehyde cation with CH 4 and studied the effect of collision energy and vibrational excitation on the reaction. 29 They showed that the H elimination channel, which generates an ethanol-like structure through C-C bond formation, is energetically more favorable than the H abstraction channel, although the H elimination channel accounts for only 15% of the total conversion. Among the four covalent complexes formed through C-C or C-O coupling, the transition state of CH 3 CH 2 OH + formation has the lowest energy. Zhang et al. demonstrate that insertion of CO into the Ni-CH 3 bond followed by reduction produces acetaldehyde, which can be further hydrogenated to ethanol, 30 whereas half of the adsorbed CO species were believed to convert to formaldehyde on Ni (111). 31 These studies indicated the possibility of C-C coupling of formaldehyde with activated CH 4 under the heterogenous conditions. Recently, Chang et al. showed that ethanol production from formaldehyde and methane was possible on a dual-function catalyst including a single Fe atom and a frustrated Lewis Pair, with the CH 4 dissociation of an activation barrier of 1.15 eV being the rate-limiting step. 32 In recent years, single-atom catalysis has become an emerging area in catalysis research. 33,34 For example, the singly dispersed Pd atoms anchored on TiO 2 showed an apparent activation barrier of 28.9 kJ/mol for more than 10 Sonogashira C-C coupling reactions. 35 Herein, we designed a single Pd atom on the In 2 O 3 catalyst that enables C-C coupling of formaldehyde and methane by combining the outstanding activity of Pd for both C-H bond activation and C-C coupling reaction [35][36][37] and the activity of In 2 O 3 for concerted C-C coupling reactions. 15 In the present work, we used the density functional theory (DFT) calculation to study the selective activation of CH 4 on the single Pd site and the adsorption of formaldehyde on the In site. The detailed reaction mechanism for the C-C coupling reaction and the elementary steps leading to the production of ethanol, acetaldehyde, and ethylene were discussed. We also studied the C-C coupling reaction on other single metal atoms supported on the In 2 O 3 surface and found a balanced relationship between activation of the C-H bond in CH 4 , C-C coupling, and ethoxy hydrogenation steps. DFT calculations together with microkinetic analysis showed that ethanol was the dominant product on the Pd single atom-In 2 O 3 catalyst.

■ METHODS

All spin-polarized calculations were performed using the Vienna Ab initio Simulation Package (VASP). 38,39 The Perdew-Burke-Ernzerhof (PBE) functional was used to determine exchange and correlation energies. 40 The effective cores were described by the projector-augmented wave (PAW) potentials. 41,42 The wave functions of valence electrons were expanded using the plane wave basis set with a cutoff energy of 400 eV. For the In atom, the semicore 5s, 5p, and 4d 10 states were treated explicitly as valence states. For geometry optimization, either the conjugate gradient algorithm or the quasi-Newton scheme was adopted until the forces on the unconstrained atoms were less than 0.03 eV/Å and a (2 × 2 × 1) k-point grid was used to sample the Brillouin zone. Reaction pathway mapping implemented in Materials Studio in combination with the geometry optimization in the VASP code was employed to isolate the transition states. Since the adsorption of the molecular products in this work is nonactivated, no transition states were considered for their desorption.

The body-centered cubic bixbyite In 2 O 3 bulk structure with calculated lattice parameters a = b = c = 10.2302 Å was used to generate the active low-index surfaces. According to the study of Walsh and Catlow, (110) and (111) surfaces are the two most stable surfaces and the surface energy of the (110) surface (1.070 J/m 2 ) is slightly higher than that of the (111) surface (0.891 J/m 2 ). 43 By analyzing the atomic configuration, the (111) surface was found to consist of a stepped In-O chain involved in the (110) surface and, therefore, chose corresponds to an endothermic adsorption. All optimized structures, including transition states, were confirmed by normal-mode frequency analysis. To analyze the product selectivity on the single-Pd atom-In 2 O 3 (110) catalyst, we performed a microkinetic analysis with a mean-field approximation based on the DFT-calculated energetics and the harmonic vibrational frequencies. The details are provided in the Supporting Information. ■ RESULTS AND DISCUSSION CH 4 Dissociation on Pd/In 2 O 3 (110). The Pd active site has been shown to have an enhanced activity for the activation of the C-H bond in CH 4 . 36,37 As shown in Figure 1, the single Pd site on the In 2 O 3 (110) surface was used to activate the C-H bond of CH 4 . In our previous study, CH 4 dissociation on both the In-O and O-O pairs was examined, and the results show that the C-H bond activation on the In-O pair has an activation barrier of 1.57 eV and the activation barrier on the O-O pair, with the value of 1.86 eV, is even higher. 15 Therefore, we considered only the Pd-O pair as the active site for C-H bond activation in the present study. The structures of CH 4 adsorption as well as the transition and final states of C-H bond activation are shown in Figure 2. As shown in the IS of Figure 2, CH 4 adsorbs on the Pd/ In 2 O 3 (110) surface through a Pd-H bond with a distance of 1.87 Å, in contrast to the physisorption configuration on the CeO 2 and In 2 O 3 surfaces. 14,15 The interaction of Pd with H resulted in a partial activation of the connecting C-H bond, as shown by the elongated C-H distance of 1.15 Å, from 1.09 Å of the gaseous CH 4 molecule. At the same time, the Pd-In 4 and Pd-O 5 bonds were broken and the Pd-O 2 bond was shortened from 2.16 to 2.02 Å. The Pd-In 2 bond distance remained almost at the same value, 2.61 Å. The adsorption energy of CH 4 was calculated to be -0.43 eV.

In the TS shown in Figure 2, the C-H bond was further stretched from 1.15 to 1.39 Å. The departing H atom moved toward the surface O 5 site, resulting in a H-O distance of 1.40 Å. Meanwhile, the distance of the carbon atom of CH 4 to the Pd site was reduced to 2.17 Å from 2.35 Å in the initial state. The Pd-C-H-O four-centered interaction resulted in facile activation of the C-H bond, with an activation barrier of only 0.39 eV. These results are consistent with the previous study of CH 4 activation on Pd and PdO clusters with different O coverages. 36 Chin and coauthors suggested that the heterolytic C-H bond dissociation on the Pd-O pair proceeded through a four-centered (

resulting in a lower activation barrier than that on the O-O pair as the later involved formation of the radical-like CH 3 fragment prior to the formation of the C-O bond with the surface O atom. Following the C-H bond activation, the methyl species binds the Pd site with a Pd-C bond distance of 2.03 Å and the H atom occupies O 5 , forming a H-O bond at a distance of 0.98 Å, as shown in FS of Figure 2. The overall reaction energy of this step was -0.33 eV. We also considered CH 4 dissociation over the Pd-O 4 , O 5 -O 6 , In 4 -O 4 , and In 4 -O 5 pairs. The reaction energies were -0.34 eV, -0.31 eV, and 0.19 eV over the Pd-O 4 , O 5 -O 4 , and In 4 -O 5 pairs, respectively, referenced to the gaseous CH 4 molecule. Relaxation of the dissociated state over the In 4 -O 4 pair resulted in the migration of CH 3 * to vicinal Pd. Consequently, Pd-O remains the most active for CH 4 dissociation. We further tested the stability of the resulting CH 3 species on the Pd site by activating the C-H bond of the adsorbed CH 3 species with the assistance of the neighboring O 4 and O 2′ sites. Our results showed that this C-H bond activation step is endothermic by 0.62 and 0.41 eV on the O 4 and O 2′ sites, respectively, and therefore energetically unfavorable. Taking these results together, we conclude that the single Pd atom supported on the In 2 O 3 (110) surface promotes the first C-H bond activation of CH 4 while stabilizing the CH 3 species. HCHO Adsorption and C-C Coupling Reaction. Starting from the coadsorbed CH 3 and H species, we introduced the HCHO molecule onto the surface of the catalyst. The optimized structures of HCHO adsorption, transition, and final states of C-C coupling are depicted in Figure 3. To facilitate the C-C coupling reaction, we considered the In 4 site next to the adsorbed CH 3 * species as the adsorption site for HCHO. The adsorption energy of HCHO at this site is -0.29 eV. As shown in Figure 3a, the HCHO molecule binds to the In 4 site through an In-O bond of 2.49 Å. The C-O bond was slightly elongated to 1.23 Å from 1.21 Å of the gasphase molecule. To evaluate the impact of the activation sequence of CH 4 and HCHO on the coupling reaction, we considered the alternative pathway in which HCHO adsorbs on the Pd site first, followed by the dissociation of CH 4 on the In sites. The calculated adsorption energy of HCHO on the Pd site is -0.77 eV, comparable with the dissociative adsorption energy of CH 4 on the Pd site. However, the nearby In 4 site is endothermic by 0.19 eV and not active toward CH 4 dissociation. Additional microkinetic results shown in SI (Figure S4) indicate that the competitive adsorption of HCHO on the Pd sites will result in a decrease of the coverage of the intermediates from the C-C coupling steps and, thereby, the reaction. Nevertheless, the C-C coupling pathway remains operative, and the maximized reactivity can be achieved by dissociating CH 4 on the Pd site followed by adsorption of HCHO on the nearby In 4 site. We tested the reactivity of HCHO direct oxidation by the vicinal O site on the In 2 O 3 surface and found a reaction energy of 0.94 eV. These results indicate that In 2 O 3 cannot oxidize HCHO, opening a possibility of In 2 O 3 catalyzing the C-C coupling reaction. On Pd/In 2 O 3 (110), the coupling of HCHO* with CH 3 * follows a Langmuir-Hinshelwood mechanism. As an electrophile, the carbon center of HCHO* attacks the carbon center of the CH 3 * species to form a C-C bond. In the transition state (Figure 3b), the C-O distance of HCHO was further extended to 1.28 Å. Meanwhile, HCHO was distorted from the coplanar structure, with a ∠H-CH-O dihedral angle of 151.3°. Consequently, the C atom was displaced toward the CH 3 * species, resulting in an increased Pd-C distance of 2.14 Å. The C-C distance in the transition state is 2.19 Å. The activation barrier for coupling of CH 3 * with HCHO* is 0.56 eV. This value is comparable to the activation barrier of gaseous CO 2 insertion into the M-CH 3 * bond on the Zndoped CeO 2 surface 14 but much lower than the coupling activation barrier of adsorbed CO 2 * with M-CH 3 * on the (ZnO) 3 -supported In 2 O 3 surface. 15 The results indicate that HCHO is a more potent electrophile than CO 2 in the coupling reaction with the CH 3 * species. Moreover, this C-C coupling process has a lower activation barrier than that of the formation of the CH 3 CH 2 OH + complex from the collision of HCHO + with CH 4 (0.87 eV). 29 In the final state (Figure 3c), the newly formed C-C bond has a distance of 1.54 Å. The C-O distance increased to 1.39 Å from 1.28 Å at the transition state, indicating a conversion of the C-O double bond in the HCHO molecule to a C-O single bond. The formation of the C-C bond results in an adsorbed ethoxy species on the surface in a bidentate configuration, with O binding the surface In 4 atom and one H atom of the methyl group interacting with the Pd atom. The reaction energy of the C-C coupling step is -0.36 eV. Formation of Ethanol. The resulting ethoxy species can react with the H adatom from CH 4 dissociation, producing ethanol. The optimized structures along the pathway of ethanol formation are given in Figure 4. In the transition state for ethoxy hydrogenation, the H adatom was displaced toward the O atom of the ethoxy species, reaching a O-H distance of 1.17 Å, while the H-O 5 distance was elongated to 1.29 Å, as shown in Figure 4a. This step overcomes a small activation barrier of 0.11 eV and has an overall reaction energy of 0.06 eV. The resulting ethanol adsorbs on the surface through a Pd-H bond, as shown in Figure 4b. The desorption of ethanol is endothermic by 0.64 eV.

Based on the above elementary steps, we mapped out the overall potential energy profile from gaseous CH 4 and HCHO to ethanol in Figure 5. The potential energy profile in Figure 5 clearly shows that the formation of ethanol through C-C coupling of CH 4 with HCHO has an overall reaction energy of -0.71 eV and is thermodynamically favorable. We note that there has been a report of ethanol formation from CO insertion into the M-CH 3 * bond in organometallics, followed by hydrogenation, 30 and formation of ethanol from CH 4 and HCHO has 100% atomic efficiency. In fact, HCHO has been believed to be an important intermediate in a heterogenous CO hydrogenation reaction. 31 Direct coupling of the CH 3 * species with HCHO* through the surface-catalyzed C-C coupling to produce ethanol provides an alternative pathway for utilizing methane through an energy-and atom-efficient process.

Ethanol Formation on Other Single Metal Atoms on In 2 O 3 (110). So far, we have demonstrated that the activation of the first C-H bond in CH 4 , C-C coupling, and ethoxy hydrogenation are key steps in ethanol formation from CH 4 and HCHO. We extended the study to other single metal atoms supported on the In 2 O 3 (110) surface to examine their performance for ethanol synthesis and develop an understanding of the underlying physicochemical origin of the activity. These atoms include the neighboring elements in the periodic table, i.e., Ni, Pt, Ag, and Rh. In each case, we studied the elementary steps for ethanol production from CH 4 and HCHO and tabulated the results of these steps, including CH 4 dissociation, the C-C coupling reaction, and ethoxy hydrogenation in Table 1. The relaxed structures of these steps on other single metal atoms supported on In 2 O 3 (110) are provided in the Supporting Information. Analysis of density of states was used to gain some insights into the interaction between the reaction intermediates and the active ensembles, as shown in Figure 6.

As shown in Table 1, Pd/In 2 O 3 has the lowest activation barrier for the activation of the first C-H bond of CH 4 . In contrast, the reaction on Ag/In 2 O 3 is the most exothermic, attributable to a significant overlap between d-orbitals of Ag and s-and p-orbitals of C. The overlaps are at the lowest orbital energy, as shown in Figure 6a 6o. These results demonstrate that different singlemetal atom catalysts affect each elementary step differently, and an optimal ethanol production is a result of balancing all the catalytic steps. A correlation of the three steps on different single metal atoms on In 2 O 3 (110) plotted in Figure 7 exemplifies such balance.

The blue line in Figure 7 clearly shows that there is a positive correlation between the dissociation energy of CH 4 and the C-C coupling energy, although Ni deviates from the trend line. This is consistent with the fact that Ni is an excellent catalyst for CH 4 dry reforming due to its extraordinary activity for CH 4 dissociation. 44 The red line in Figure 7 shows that the reaction energy of the ethoxy hydrogenation step correlates linearly and negatively with the C-C coupling energy. The C-C coupling step is the most exothermic on Ag/In 2 O 3 , but the hydrogenation step is the most endothermic. On Pt/In 2 O 3 and Ni/In 2 O 3 , the hydrogenation step is favorable, but the CH 4 dissociation and C-C coupling steps are impeded. We note that the blue line crosses the red roughly at Pd, indicating that Pd offers a balanced Following the intramolecular H transfer step, cleaving the C-OH bond will produce ethylene. The departing OH may combine with the H adatom from CH 4 dissociation to form a H 2 O molecule. The activation barrier and reaction energy for the C-OH bond breaking step were calculated to be 0.81 and -0.32 eV, respectively. Following the breaking of the C-OH bond, the resulting ethylene species adsorbs on the Pd site in a π-mode with two equivalent Pd-C bonds at a distance of 2.12 Å. The adsorbed ethylene in this configuration is very stable, with the six atoms of ethylene being almost coplanar and a desorption energy of 1.2 eV. To complete the cycle, OH would combine with H to form H 2 O. This step is endothermic by 0.50 eV and has an activation barrier of 0.57 eV. The desorption energy of water is 0.51 eV. The overall potential energy profile for ethylene production from CH 4 and HCHO is shown in Figure 9.

As shown in Figure 9, the overall reaction from CH 4 and HCHO to ethylene and water with a reaction energy of -0.02 eV is only slightly exothermic. Therefore, the formation of ethylene is less favorable than that of ethanol production (-0.71 eV). Furthermore, the high activation barrier for the H transfer step and the high desorption energy of the resulting ethylene would further impede the reaction along this pathway.

Acetaldehyde Formation. In addition to the production of ethanol and ethylene, dehydrogenation of the ethoxy species produces acetaldehyde. The process starts from dehydrogenation at the methylene end of the adsorbed ethoxy species, as shown in Figure 10a,b. One H of the methylene group tilts toward the surface O 4 site in the transition state of dehydrogenation. The C-H bond extends to 1.27 Å, while the H-O distance becomes 1.39 Å. The breaking of the C-H bond leads to the formation of an acetaldehyde species, which adsorbs on the surface through one Pd-C bond and one Pd-H bond. The cleavage of the C-H bond shortens the C-C distance from 1.52 to1.48 Å, while the C-O bond is reduced from 1.35 to1.23 Å. The calculated dehydrogenation activation barrier and reaction energy are 0.66 and -0.93 eV, respectively. These results indicate that production of the acetaldehyde species from the ethoxy species is thermodynamically favorable. The desorption energy of the acetaldehyde species is 0.71 eV. To complete the cycle, the remaining two H atoms on the surface would desorb as a H 2 molecule. The optimized structures along the pathway for H 2 formation are depicted in Figure 10c-f. The overall potential energy profile for the conversion of CH 4 and HCHO to acetaldehyde and H 2 is given in Figure 11.

We note that although the formation of acetaldehyde from the ethoxy species is both kinetically and thermodynamically possible, the final H 2 formation and desorption steps may prevent the reaction from proceeding through this pathway. According to the potential energy profile in Figure 11, the overall reaction from CH 4 and HCHO to acetaldehyde and H 2 is endothermic with a small reaction energy of 0.28 eV. However, the formation of acetaldehyde is thermodynamically the least favorable among ethanol, ethylene, and acetaldehyde. We further studied the selectivity toward the three possible products on Pd/In 2 O 3 (110) based on the microkinetic analysis.

Microkinetic Analysis. We performed a microkinetic analysis based on the DFT-calculated energetics, including reaction energies, activation barriers, and harmonic vibrational frequencies. To determine the selectivity, we included all elementary steps leading to the formation of ethanol, ethylene, and acetaldehyde. The forward and reverse rate constants of each elementary step as well as the corresponding equilibrium constant were calculated based on the transition state theory. 12a, where A* denotes the adsorbed state of species A on the surface. By calculating the reaction rate of the three products at the steady state, we determined selectivity and have shown them in Figure 12b.

As shown in Figure 12a, the steady-state surface coverage of ethanol increases as the temperature was raised, reaching ∼0.8 at 500 K and is the dominant species below 550 K. This result is in consistency with the potential energy profiles in Figures 5, 9 550 K, CH 2 CH 2 * becomes the dominant species and has a coverage higher than that of CH 3 CH 2 OH*. However, the steady-state coverage of CH 3 CHO* remains extremely low up to 600 K. As we mentioned before, the three possible products share the common CH 3 CH 2 O* species. The combination of CH 3 CH 2 O* with surface H* leads to the formation of CH 3 CH 2 OH*. If intramolecular hydrogen transfer occurred, i.e., CH 3 CH 2 O* → CH 2 CH 2 OH*, followed by dropping the OH* group, CH 2 CH 2 * will be formed. However, the formation of CH 3 CHO* needs an empty active site, i.e., CH 3 CH 2 O* + * → CH 3 CHO* + H*. As shown by the temporal distribution of surface coverages at 600 K in Figure 12c, the surface is covered primarily by CH 3 CH 2 OH* at the initial stage of the reaction and the coverage of CH 3 CHO* rises to ∼0.16, whereas the coverage of CH 2 CH 2 * remains extremely low. However, as acetaldehyde is formed and H* accumulates on the surface. Desorption of H* as H 2 needs to overcome a barrier of 1.19 eV for H transfer and a reaction energy of 1.26 eV for H 2 formation. This highly endothermic step, together with the high desorption energy of 0.71 eV for acetaldehyde and 1.20 eV for ethylene, hindered the regeneration of active sites. As a result, the CH 3 CH 2 O* + * → CH 3 CHO* step leads to a decrease of the CH 3 CHO* coverage. At the same time, the remaining H* will likely combine with surface OH* to form H 2 O*, which in return facilitates the C-OH* cleavage of CH 2 CH 2 OH* to form ethylene, as shown in Figure 12c.

The selectivity toward each product is shown in Figure 12b. Clearly, ethanol is the main product, with a selectivity of >99.9%, and the combined selectivity for ethylene and acetaldehyde is under 0.1% in a temperature range of 400-600 K. The observed selectivity correlates with the surface coverages of CH 3 CH 2 OH*, CH 2 CH 2 *, and CH 3 CHO* and reflects the fact that the desorption energy of ethanol is 0.64 eV, lower than that of ethylene (1.20 eV) and acetaldehyde (0.71 eV). Moreover, along the ethylene and acetaldehyde formation pathways, additional steps are needed. Increasing the reaction temperature would enhance the selectivity toward both ethylene and acetaldehyde. At 600 K, a sharp increase in the selectivity of ethylene can be observed although the selectivity is still very low (<0.1%). The selectivity toward acetaldehyde also shows an increase but remains to be less than 0.1%. As such, ethanol is the dominant product of the singleatom Pd/In 2 O 3 (110) catalyst. More comprehensive analysis may change some details of the results but is not expected to affect the qualitative conclusion. The microkinetic analysis based on the DFT results confirmed an atom-economic ethanol formation with high selectivity on the single Pd atom supported-on-In 2 O 3 catalyst.