Heterogeneous Droplet Catalyst for Selective Oxidation of Methane

Solid catalysts play vital roles as heterogeneous catalysts in numerous applications in the chemical, petrochemical, pharmaceutical, energy, food and automobile industries, facilitating large-scale production and facile product separation. However, conventional heterogeneous catalysts possess three major shortcomings that hinder their wide-spread application, including i) inferior catalytic activity and selectivity caused by a diversity of active sites, ii) tendency towards deactivation caused by agglomeration of catalyst particles, and iii) undesired carbon deposition in transformation of hydrocarbons. 1-4 In this research, we present a new kind of heterogeneous catalyst that consists of a liquid metal with dissolved catalytically active metal atoms, denoted as a heterogeneous droplet catalyst (HDC). Taking galinstan as the liquid metal with Cu as catalytically active metal solute (denoted as Cu-HDC) as an example, we demonstrate that the dissolved Cu atoms promote selective oxidation of methane into primary oxygenates at room temperature with a superb activity of 1940 mmol·gCu-1·h-1 and a selectivity of over 90%, outperforming state-of-the-art industrial catalysts. The liquid metal solvent ensures a high, entropically driven dispersion of the Cu atoms and prevents carbon deposition. The highly promising Cu-HDC catalyst shows a stable operation over 240 hours. By employing in-situ X-ray absorption spectroscopy (XAS), near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT), we show that the narrow d-orbital of the dissolved Cu atoms in HDC facilitates formation of adsorbed methyl radicals *CH3 and impedes their transformation into *CH2 (where the * notation is used to denote an adsorption site) 5,6. These features enhance the selectivity towards primary oxygenates and prevent over-oxidization and formation of C2+ products. As the phase of the HDC is different from those of gas, aqueous and organic solution phases its separation is as facile as that of solid catalysts. Unlike conventional supported catalysts that are designed with well-dened crystal structures, crystal facets and defects, HDCs are characterized by weak interactions between the as an example, we demonstrate that the Cu promote selective of into primary oxygenates at room temperature with a superb activity of 1940 mmol·g -1 and a selectivity of over 90%, outperforming state-of-the-art industrial catalysts. The liquid metal solvent ensures a high, entropically driven dispersion of the Cu atoms and prevents carbon deposition. The highly promising Cu-HDC catalyst shows a stable operation over 240 hours. By employing in-situ X-ray absorption spectroscopy (XAS), near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT), we show that the narrow d-orbital of the dissolved Cu atoms in HDC facilitates formation of adsorbed methyl radicals *CH 3 and impedes their transformation into *CH 2 (where the * notation is used to denote an adsorption site) 5,6 . These features enhance the selectivity towards primary oxygenates and prevent over-oxidization and formation of C 2+ products. As the phase of the HDC is different from those of gas, aqueous and organic solution phases its separation is as facile as that of solid catalysts. Unlike conventional supported catalysts that are designed with well-dened crystal structures, crystal facets and defects, HDCs are characterized by weak interactions between the liquid metal support and the active metal solute, by having mobile surfaces and by dynamic catalytically active centers (Figure 1A). These unprecedented features endow HDCs unique adsorption behavior and adaptive activation of molecules. For this reason, HDCs have the potential to become a major category of catalysts in the eld of heterogeneous catalysis.


Abstract
Solid catalysts play vital roles as heterogeneous catalysts in numerous applications in the chemical, petrochemical, pharmaceutical, energy, food and automobile industries, facilitating large-scale production and facile product separation. However, conventional heterogeneous catalysts possess three major shortcomings that hinder their wide-spread application, including i) inferior catalytic activity and selectivity caused by a diversity of active sites, ii) tendency towards deactivation caused by agglomeration of catalyst particles, and iii) undesired carbon deposition in transformation of hydrocarbons. [1][2][3][4] In this research, we present a new kind of heterogeneous catalyst that consists of a liquid metal with dissolved catalytically active metal atoms, denoted as a heterogeneous droplet catalyst (HDC). Taking galinstan as the liquid metal with Cu as catalytically active metal solute (denoted as Cu-HDC) as an example, we demonstrate that the dissolved Cu atoms promote selective oxidation of methane into primary oxygenates at room temperature with a superb activity of 1940 mmol·gCu-1·h-1 and a selectivity of over 90%, outperforming state-of-the-art industrial catalysts. The liquid metal solvent ensures a high, entropically driven dispersion of the Cu atoms and prevents carbon deposition. The highly promising Cu-HDC catalyst shows a stable operation over 240 hours. By employing in-situ X-ray absorption spectroscopy (XAS), near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT), we show that the narrow d-orbital of the dissolved Cu atoms in HDC facilitates formation of adsorbed methyl radicals *CH3 and impedes their transformation into *CH2 (where the * notation is used to denote an adsorption site) 5,6. These features enhance the selectivity towards primary oxygenates and prevent over-oxidization and formation of C2+ products. As the phase of the HDC is different from those of gas, aqueous and organic solution phases its separation is as facile as that of solid catalysts. Unlike conventional supported catalysts that are designed with well-de ned crystal structures, crystal facets and defects, HDCs are characterized by weak interactions between the liquid metal support and the active metal solute, by having mobile surfaces and by dynamic catalytically active centers. These unprecedented features endow HDCs unique adsorption behavior and adaptive activation of molecules. For this reason, HDCs have the potential to become a major category of catalysts in the eld of heterogeneous catalysis.

Main Text
Solid catalysts play vital roles as heterogeneous catalysts in numerous applications in the chemical, petrochemical, pharmaceutical, energy, food and automobile industries, facilitating large-scale production and facile product separation. However, conventional heterogeneous catalysts possess three major shortcomings that hinder their wide-spread application, including i) inferior catalytic activity and selectivity caused by a diversity of active sites, ii) tendency towards deactivation caused by agglomeration of catalyst particles, and iii) undesired carbon deposition in transformation of hydrocarbons. [1][2][3][4] In this research, we present a new kind of heterogeneous catalyst that consists of a liquid metal with dissolved catalytically active metal atoms, denoted as a heterogeneous droplet catalyst (HDC). Taking galinstan as the liquid metal with Cu as catalytically active metal solute (denoted as Cu-HDC) as an example, we demonstrate that the dissolved Cu atoms promote selective oxidation of methane into primary oxygenates at room temperature with a superb activity of 1940 mmol·g Cu -1 ·h -1 and a selectivity of over 90%, outperforming state-of-the-art industrial catalysts. The liquid metal solvent ensures a high, entropically driven dispersion of the Cu atoms and prevents carbon deposition. The highly promising Cu-HDC catalyst shows a stable operation over 240 hours. By employing in-situ X-ray absorption spectroscopy (XAS), near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT), we show that the narrow d-orbital of the dissolved Cu atoms in HDC facilitates formation of adsorbed methyl radicals *CH 3 and impedes their transformation into *CH 2 (where the * notation is used to denote an adsorption site) 5,6 . These features enhance the selectivity towards primary oxygenates and prevent over-oxidization and formation of C 2+ products. As the phase of the HDC is different from those of gas, aqueous and organic solution phases its separation is as facile as that of solid catalysts. Unlike conventional supported catalysts that are designed with well-de ned crystal structures, crystal facets and defects, HDCs are characterized by weak interactions between the liquid metal support and the active metal solute, by having mobile surfaces and by dynamic catalytically active centers ( Figure 1A). These unprecedented features endow HDCs unique adsorption behavior and adaptive activation of molecules. For this reason, HDCs have the potential to become a major category of catalysts in the eld of heterogeneous catalysis.
Galinstan, a gallium-based eutectic alloy is a liquid at room temperature [7][8][9][10] . Different metals can be dissolved into galinstan, e.g., Cu, Ni, Fe, Co, Zn, Pt, Pd, or Au. The major challenge in order to facilitate practical application of HDCs as catalysts is to prevent their aggregation into larger droplets. To reduce the size of the droplets and to increase their speci c surface area, we suspended the Cu-HDC droplets in polydimethylsiloxane (PDMS) and solidi ed the obtained suspension by adding a cross-linking agent and subjecting it to an appropriate thermal treatment ( Figure 1B). 11,12 The cured PDMS rubber with embedded micro-sized Cu-HDC droplets in it (denoted as Cu-HDC@PDMS) can be used (see Supplementary   Information: Preparation of M-HDC@PDMS).
The uniform distribution of Cu in the galinstan alloy was con rmed by energy dispersive X-ray spectroscopy (EDX). The obtained elemental mappings are shown in Figure 2A. The aberration-corrected scanning transmission electron microscope (STEM) images and associated 3D intensity pro le demonstrate a disordered arrangement of the Cu atoms ( Figure 2B, Figure S1 and Figure 2C). For a better observation, we extracted intensity values from the inverse fast Fourier transform (IFFT) image ( Figure  2D) in six different directions. Every pro le ( Figure 2E) is found to exhibit aperiodicity in both amplitude and frequency, con rming the disordered nature of the Cu atoms. For a veri cation of the liquid characteristics of Cu-HDC, we employed cryogenic transmission electron microscopy (cryo-TEM). The ring-like selected-area electron diffraction (SAED) pattern recorded at room temperature con rms its amorphous nature ( Figure 2F), while the SAED pattern recorded at -196 o C, i.e., below the melting point of galinstan, shows bright spots indicative of a polycrystalline structure. Using X-Ray diffraction (XRD) it was found that galinstan is able to dissolve about 2 wt% of Cu at room temperature ( Figure S2). Similar conclusions can be drawn by analyzing oscillations in extended X-ray absorption ne structure (EXAFS) spectra. Figure 2G compares EXAFS spectra of Cu foil, Cu-HDC with 1 wt% and that with 2 wt% Cu, while Figure 2H shows corresponding wavelet transforms for the k 3 -weighted EXAFS signals (corresponding tting results are shown in Figure S3 and Table S1, respectively). Fitting of the oscillations indicate that Ga atoms form the rst coordination shell of Cu atoms in Cu-HDC, rather than In or Sn (having a higher Rfactor). The absence of second-nearest neighbors peaks above 4 Å in the spectrum of Cu-HDC with 1 wt % Cu indicates that agglomeration and crystallization did not occur. In contrast, these peaks emerge in the EXAFS spectrum of Cu-HDC with 2.2 wt% of Cu .
Methane catalytic oxidation experiments using Cu-HDC@PDMS catalysts with different Cu contents were carried out in an autoclave with H 2 O 2 as the oxidant at temperatures in the range 25 -75 o C [13][14][15][16] . The liquid and gas products were analyzed by 1 H nuclear magnetic resonance (NMR) and gas chromatography (GC), respectively. Corresponding data are shown in Figure 3A, revealing that generation of primary oxygenates, the most promising oxidation products of methane 13,17 , is dominant with a selectivity over 93% under all tested conditions. Control catalytic experiments were carried out using galinstan@PDMS (without Cu), Cu-HDC (without PDMS), pure PDMS and pure galinstan as catalysts, but none of these showed an activity and selectivity comparable to Cu-HDC@PDMS. A linear relationship between production rate and Cu content is found as shown in Figure 3C, implying that Cu-based moieties serve as catalytically active sites. As alluded to above, once the Cu content exceeds about 2 wt%, a solid solution or alloy with crystalline CuGa 2 appears as was con rmed by XRD and EXAFS. The formed CuGa 2 species is less active in methane oxidation, which explains the departure from the linear relationship in Figure 3C at high Cu loadings. The production rates of undesired carbon dioxide and acetic acid are not signi cantly affected by the Cu content. The latter shows that the formation of these products is not associated with the presence of Cu. Besides dispersing the liquid metal droplets, the PDMS rubber modulates the wettability and accessibility of the catalyst. We experienced that the liquid metal is quite wettable to water and methanol with contact angles of 8.6 o and close to 0 o , respectively ( Figure S4). This unfavorable wettability to water and methanol hinders catalytic sites from contact with gaseous methane and desorption of generated methanol. The use of PDMS modi es the wettability of the catalyst surface to become more aerophilic, which promotes the adsorption of methane and the desorption of methanol, thus enhancing the catalytic activity. It was found that Cu-HDC@PDMS shows a durability of at least 240 hours without any decline in catalytic performance. This outstanding performance originates mainly from two aspects: i) the solution characteristics of Cu-HDC prevents aggregation of Cu atoms, and ii) its uidity avoids carbon deposition, which was con rmed by EDX analysis of used catalysts ( Figure S5 and Table S2). We further investigated the catalytic performance of Cu-HDC@PDMS at different temperatures and methane partial pressures. Corresponding results are shown in Figure 3D. As we identi ed Cu atoms as the catalytically active centers, the catalytic activity of Cu-HDC@PDMS was normalized to its Cu content. Even at room temperature and 1.5 MPa CH 4 , a catalytic performance as high as 1940 mmol·g Cu -1 ·h -1 (with a turnover frequency (TOF) of 124.1 mol·mol Cu -1 ·h -1 ) can be achieved. According to the best of our knowledge, this is the highest production rate of primary oxygenates from methane reported to date (cf . Table S3). Figure S6 shows that the reaction is rst order in methane. A value of 31.3 ±1.7kJ mol -1 was obtained for the apparent activation energy ( Figure S7). This value is lower than reported for other catalysts and underscores the superior catalytic activity exhibited by Cu-HDC@PDMS 13,18-20 . Figure S8 con rms a linear relationship between the amount of oxygenates formed and the time of reaction. Within the duration of the experiment the catalyst does not show any sign of deactivation. Analysis by 13 C and 1 H nuclear magnetic resonance (NMR) showed that products (CH 3 OOH and HOCH 2 OOH) are only observed when 13 CH 4 is used as reactant, con rming that methane is the only source of carbon in the products ( Figure S9) 17 .
The remarkable catalytic performance of Cu-HDC prompted us to investigate the underlying mechanism of the catalytic reaction. In-situ XAFS was used to study the local chemical environment of the Cu atoms during reaction. Measurements were performed using a home-made gas-tight reaction chamber equipped with a vitreous carbon window which allowed us to obtain the EXAFS signal ( Figure S10). Spectra were collected of Cu-HDC in pristine form, immersed in 30% hydrogen peroxide and under reaction conditions ( Figures 3F-3H 21 . To verify these ndings, we performed ambient-pressure X-ray photoelectron spectroscopy (APXPS) ( Figure 3I, Figure S11). The C 1s spectra recorded under 0.5 mbar CH 4 reveal the existence of CH x (285.6 eV) and molecular CH 4 (287.0 eV). New peaks appear when adding 0.5 mbar of O 2 due to the formation of OCH 3 (286.5 eV) and CO x (289.8 eV). The C 1s peak corresponding to OCH 3 is found to dominate in a mixed CH 4 /O 2 atmosphere, suggesting that OCH 3 is a stable intermediate in methane oxidation ( Figure 3J) 22,23 . The data further con rms the high selectivity of the Cu-HDC catalyst as the relative intensity of the CO x peak is found very small (10%).
First-principle DFT calculations were performed to investigate the high catalytic activity of Cu-HDC droplets towards oxygenates. As expected, the computations disclose a low energy barrier for migration of Cu atoms from the interior of the droplets to their surface, suggesting a high possibility for formation of Cu-centered active sites for the catalytic surface reaction ( Figure S12). These calculations were performed assuming the liquid metal to consist of pure gallium (rather than a mixture of gallium, indium and tin). We further calculated the reaction pathways for partial oxidation of CH 4 on the surfaces of Cu-HDC, pure gallium and on Cu(111) as shown in Figures S13, S14 and S15, respectively. In these gures, Roman numbering is used to designate sequential stages in the mechanism. The calculations reveal that surface Cu and two adjacent Ga atoms abstract one O atom from H 2 O 2 (II), forming a Cu-O-Ga 2 motif on the surface of Cu-HDC (III), acting as the site for CH 4 adsorption ( Figure S13). A bond distance of 1.88 Å is calculated for the Cu-O bond, which is consistent with the value determined by in-situ XAFS. Upon adsorption of CH 4 (III), one of its H atoms is abstracted by the *O atom, whereby the *CH 3 methyl radical is formed (IV). This is the decisive step in the overall reaction with an energy barrier (TS1) of 1.07 eV (by comparison with 1.38 eV ( Figure S11) and 1.56 eV ( Figure S12) calculated for similar reaction steps occurring on pure gallium and Cu(111), respectively). Two reaction steps may follow: (V a ) *CH 3 reacts with H 2 O 2 to form CH 3 OH, or (V b ) *CH 3 is further dehydrogenated to *CH 2 . The *CH 2 radical is the precursor intermediate of oxidation of carbon to higher oxidation states. The calculated energy barrier for reaction step (V b ) is 1.36 eV (TS2), which is high and implies that oxidation of CH 4 on Cu-HDC would not proceed any further. It may be noted that the energy barrier for this reaction occurring on Cu(111) is found to be only 1.03 eV ( Figure S12). The obtained results clearly demonstrate that reaction via reaction step (V a ) is the more favorable pathway to produce CH 3 OH (VI) on Cu-HDC. The formed CH 3 OH may be adsorbed on the Cu atom (VII). Subsequent H-abstraction (VIII) and reaction of the formed intermediate *OCH 3 with H 2 O 2 (IX) will generate CH 3 OOH (X).
To understand the superior activity and selectivity of Cu-HDC over Cu(111) in methane oxidation, a comparative computational analysis of the electronic structures of the TS1 transition states was carried out. In both cases, the *CH 3 radical resides on top of the Cu atoms, denoted as Cu-HDC_TS1 ( Figure S13) and Cu(111)_TS1 (Figure S15) , respectively. As shown in Figure 4, the orbital interaction between *CH 3 and Cu-HDC is substantially stronger than that between *CH 3 and Cu(111), which leads to a substantial lower energy barrier for CH 4 activation on Cu-HDC by comparison with that on Cu(111). The projected density of states (pDOS) of the Cu 3d orbitals in Cu-HDC ( Figure 4A) exhibits as a narrow peak, suggesting a very weak coupling between the Cu 3d orbitals and the surrounding Ga orbitals. In contrast, the 3d orbitals in Cu(111) are split and broadly distributed ( Figure 4B). The difference in the Cu 3d orbital distributions accounts for the different orbital interactions between Cu and *CH 3 in the respective TS1 transition states. The Cu 3d and *CH 3 non-bonding (nb) orbitals are strongly coupled in Cu-HDC_TS1, but hardly interact with each other in Cu(111)_TS1. As a result, a Cu-C bond is formed when *CH 3 is adsorbed onto CuGa 125 , while *CH 3 almost retains its radical character when adsorbed onto the Cu(111) surface.
These results are strengthened by the projected crystal orbital Hamilton population (pCOHP) calculations (Table S4), which reveal a Cu-C bond strength of +0.24 |e| for adsorption of *CH 3 onto Cu-HDC and +0.03 |e| in the case of adsorption onto Cu(111). Moreover, the differential charge density pro les indicate that there is an appreciably higher charge density between the Cu and C atoms in Cu-HDC_TS1 ( Figure 4C) than in Cu(111)_TS1 ( Figure 4D). The *CH 3 radical resides directly atop of the Cu atom on the Cu-HDC surface, with a binding energy of -1.21 eV ( Figure 4E), while it moves to a hollow site on the Cu(111) surface, with a much larger binding energy of -1.81 eV ( Figure 4F). Such a big difference in the binding strength can be explained in two alternative ways. One, *CH 3 forms three Cu-C bonds of length 2.23 Å when it adsorbs at the hollow site on Cu(111), while there is only one Cu-C bond of length 2.02 Å when it adsorbs atop a Cu atom on the Cu-HDC surface. Albeit the individual Cu-C bond is weaker in CH 3 -Cu(111) than in CH 3 -Cu-HDC, two more Cu-C bonds are formed in the former complex resulting in an overall stronger binding strength. Another explanation is that the d-π orbital hybridization between Cu and C atoms contributing to the Cu-C bonding is appreciably stabilized in the Cu(111)_*CH 3 complex, while it is barely changed in Cu-HDC_*CH 3 , as shown in Figure 4A and 4B, respectively. The much stronger binding of *CH 3 on the Cu(111) surface is further con rmed by the pCOHP analysis (Table S4), showing a larger d-p population (integral of -pCOHP) on the Cu-C bond (+0.73 |e| for Cu(111)_*CH 3 vs. +0.62 |e| for Cu-HDC_*CH 3 ). On the basis of above analysis it is concluded that, because of the narrow 3d orbital distribution, activation of CH 4 is more favorable on Cu-HDC, while the occurrence of over-oxidation and formation of C 2+ products is less likely than on the Cu(111) surface. Hence, Cu-HDC outperforms Cu(111) in terms of both activity and selectivity of methane oxidation.
In conclusion, in this research we have demonstrated that a liquid metal with a catalytically active metal solute can act as a highly e cient and durable heterogeneous catalyst. By employing a liquid metal with dissolved Cu, we achieved a state-of-the-art catalytic performance in the selective oxidation of methane, with an activity of 1940 mmol·g Cu -1 ·h -1 and a selectivity over 90% at room temperature. In-situ XAFS and NAP-XPS experiments and DFT calculations reveal that the active Cu atoms present at the surface of the liquid metal droplet facilitates adsorption of methane, generates methyl radicals and prevents over-oxidation and formation of C 2+ products. We believe that our ndings offer new opportunities and exciting challenges for discovery of novel heterogeneous catalysts.