Heterogeneous catalytic hydrogenation is an important industrial process in which hydrogenation is carried out with molecular hydrogen (H2) in the presence of a catalyst. Typical catalysts are based on noble metal nanoparticles (NPs) such as platinum (Pt), 1 palladium (Pd), 2 rhodium (Rh), 3 and ruthenium (Ru), 4 which are immobilized on a support with a large surface area. The supported NPs are active in a wide range of hydrogenation reactions under relatively mild conditions, but do not always ensure high selectivity for the desired product. This reduced selectivity is mainly due to the high reactivity of noble metal NPs. To tune the reactivity of active metal NPs, individual atoms dispersed in reducible oxides have been investigated and found to be selective hydrogenation catalysts in many important reactions. 5,6 Notably, it has been found that the active site of the hydrogenation reaction is not a pure metal, but an oxidized metal species. Somorjai and colleagues reported that oxygen strongly bound to the platinum surface affects the selectivity as well as the activity of a hydrogenation reaction. 7 Catalytic ally active sites may thus be neither metals nor metal oxides, but may be in between.
Metal oxides generally act as supports or promoters for the active catalyst. In fact, metal oxides used as sole catalysts are rare, especially in hydrogenation reactions. Surprisingly, cerium oxide (CeO2) turned out to be an efficient catalyst for gaseous hydrogenation from alkyne to olefin without the need for noble metals. 8 Spectroscopic studies on well-ordered CeO2 films in combination with density functional theory (DFT) calculations suggest that oxygen vacancies within CeO2 play a crucial role in the incorporation of hydrogen (H) into the bulk and promote hydrogenation reactions. 9 Numerous efforts have been made to investigate the interaction between hydrogen and metal oxides, [10][11][12] but little is known about the potential of hydrogen insertion to confer metal oxides with superior catalytic activity under ambient conditions. Thus, it is desirable to investigate H-bearing metal oxides as candidates for selective hydrogenation catalysts.
Here, we present the synthesis, characterization, and catalytic activity of a new H-bearing ruthenium dioxide denoted as HxRuO2. Ru-based catalysts have been continuously studied because of the lower cost of Ru compared to Pd and Pt as well as the superior catalytic activity for a wide range of reactions. Moreover, they have been regarded as one of the most active catalysts for several industrially important reactions. The inclusion of hydrogen in reducible ruthenium oxide is exceptional and unprecedented. Unlike rutile RuO2, HxRuO2 acts as a standalone catalyst and exhibits excellent activity in the hydrogenation of various arenes and nitroarenes. Moreover, the HxRuO2 catalyst is recyclable and maintains structural integrity in both alkaline and acidic media. This combination of superior activity and high stability can be beneficial for the use of HxRuO2 in a wide range of potential applications, especially in catalysis and energy-related fields.
Hydrogen insertion requires special reaction conditions because annealing metal oxides under a hydrogen atmosphere typically forms metals or oxygen-deficient oxides rather than H-inserted oxides. Several specialized methods are known for the synthesis of H-inserted oxides, including high pressure-high temperature techniques, 13 hydrogen spillover, 14 electrochemical hydrogenation, 15 and acid treatment. 16 However, these methods did not succeed in inserting hydrogen into ruthenium dioxide (RuO2). We have developed a soft chemical approach based on the water gas shift reaction, which produces gaseous hydrogen at the expense of carbon monoxide (CO + H2O ⇌ CO2 + H2). 17 The reaction was performed at temperatures below 200 o C to prevent the formation of Ru NPs and to gently insert hydrogen into the host oxide lattice. Fig. 1a shows powder X-ray diffraction (XRD) patterns of newly synthesized HxRuO2, rutile RuO2, and hydrous RuO2•xH2O. The XRD spectrum of HxRuO2 shows several distinct peaks that do not match those of known materials, indicating that HxRuO2 adopts a new phase. A structural change might occur upon the insertion of hydrogen into the rutile RuO2 matrix. The successful synthesis of HxRuO2 demonstrates the occurrence of hydrogen spillover on oxide surfaces, followed by chemical and structural transformations induced by coupled proton-electron transfer. 18 Although the formation mechanism is currently not well understood, the insertion reaction appears to be facilitated by oxygen vacancies in ruthenium dioxide. 19 The presence of hydrogen in HxRuO2 was confirmed by solid-state 1 H NMR spectroscopy, which is a powerful technique capable of determining hydrogen species and their chemical environment in the oxide framework. Fig. 1b shows the magic angle spinning (MAS) 1 H NMR spectrum for HxRuO2, illustrating two peaks at 1.3 and 14.7 ppm. For comparison, we measured the 1 H NMR spectrum for hydrous RuO2•xH2O (see inset). The single broad peak at 8.3 ppm is attributed to water, suggesting that the two peaks in HxRuO2 are not ascribed to protons in the structural water moiety. Based on the analysis of 1 H NMR involved in hydrogen bonding, 20 one peak at 14.7 ppm can be assigned to protons involved in the hydrogen bond, while the other signal at 1.3 ppm can be attributed to interstitial protons interacting weakly with neighboring oxygen and ruthenium atoms. The experimentally obtained chemical shifts are in good agreement with those computed via DFT for structure models with interstitial protons, as discussed in the Electronic Supplementary Information. Raman spectroscopy was used to obtain additional information about the crystal structure and chemical bonding of HxRuO2, whose Raman spectrum (Fig. 1c) displays a new set of sharp peaks below 800 cm -1 , but no signals in the range of 800 to 4,000 cm -1 . This indicates that neither the hydroxyl (O-H) group nor the water H-O-H moiety are present in HxRuO2. 21 Below 800 cm -1 the calculated spectrum of the most stable structure identified via DFT calculations, a P21/c symmetry RuO2H phase, was in good agreement with experiment (Fig. S1). However, at higher wavenumbers it possessed peaks associated with O-H wagging and stretching modes, not seen in experiment. As described below, first principles molecular dynamics (FPMD) calculations suggest that O-H
proton exchange ought to be facile in this structure at room
temperature. This can result in broadening of the O-H peaks, making them experimentally undetectable. The chemical environments of Ru and O were evaluated by X-ray photoelectron spectroscopy (XPS). Particularly, the oxidation state of Ru and the bonding nature of O were examined through XPS analysis. The XPS spectra of HxRuO2 are shown in Fig. S2 and were analyzed based on the published literature for common Ru materials. 22 The fitting of the O 1s peak clearly indicates the presence of hydrogen that interacts with oxygen. The broad 3d peak structure, which can be fitted with several symmetric curves, suggests the presence of multiple oxidation states including Ru 3+ and Ru 4+ ions. The Ru metal peak, which usually appears around 280.0 eV, was not detected.
To obtain quantitative information on the hydrogen stoichiometry in HxRuO2, thermogravimetric analysis (TGA) was conducted. Differential scanning calorimetry (DSC) and temperature programmed desorption-mass spectrometry (TPD-MS) measurements were also performed to evaluate the thermal stability of HxRuO2. Fig. 1d shows typical TGA, DSC, and TPD-MS curves for HxRuO2, collected in an inert gas atmosphere, as a function of temperature. The TGA curve shows a gradual weight loss up to about 230 o C and no further weight loss above 230 o C. The DSC curve exhibits a weak exothermic peak at 150 o C and a distinct endothermic peak related to water loss at 260 o C. The exothermic peak appears to be associated with the structural transition from monoclinic to orthorhombic or tetragonal due presumably to hydrogen loss. The endothermic DSC signal at 260 o C corresponds well with a significant weight loss observed in the TGA curve. Moreover, the temperature-dependent tendency of the TPD curve is very similar to those of the TGA and DSC curves.
As shown in the inset of Fig. 1d, H2O (m/z = 18) evolution was clearly observed in the TPD-MS curve, which follows weight changes on the TGA curve. In addition to H2O signals, H2 (m/z = 2) gas evolution was detected. The H2O mass spectrum shows a small peak at around 150 o C and a large peak starting at around 230 o C, which also agrees with the TGA and DSC curves.
To determine the exact content of hydrogen in HxRuO2, TGA measurements were performed in air using five different samples prepared under identical conditions (Fig. S3). The average weight loss calculated from the TGA data was 6.61%. The overall composition can be established as HxRuO2 (x=0.98), which corresponds to nearly one equivalent of hydrogen per formula unit.
The structure of HxRuO2 was determined from Rietveld refinement of high-resolution synchrotron XRD data using the computationally predicted monoclinic P21/c RuO2H structure described in the Electronic Supplementary Information as a starting model. First, we refined the positions of the Ru and O atoms and then two partially occupied H positions were included in the refined structure. A Fourier map constructed from the difference between the observed pattern and hydrogen-free refined structural model was employed to determine the positions of the hydrogen atoms. 23,24 Two nonequivalent positions, H1 and H2, were found and their parameters were refined until a satisfactory fit was obtained. The occupation factors of the H1 and H2 sites were set to 0.49 each, based on the TGA data. This P21/c model, including hydrogen positions, fit superbly to the synchrotron XRD data. Fig. 2a shows the Rietveld refinement of the synchrotron data to the model, which includes the observed, calculated, and difference diffraction patterns. The final refined structure is illustrated in Fig. 2b. As seen in Fig. 2c and2d, the locations of the hydrogen atoms are imaged as dotted red circles that have high residual electron densities. The final crystallographic data and structural parameters are shown in Table S1. Selected interatomic distances and angles are given in Table S2.
Unlike the rutile structure with four long (2.026 Å) and two short (1.869 Å) Ru-O bonds, 25 HxRuO2 has a distorted monoclinic structure with six different Ru-O bonds ranging from 1.993 to 2.148 Å. Two short and long Ru•••Ru distances, 2.519 and 3.149 Å, alternate along the c axis. A notable feature is the shortest Ru•••Ru separation (2.519 Å), which is comparable to the Ru-Ru bond length (2.650 Å) in hexagonal Ru metal, indicating the presence of metallic Ru-Ru bonding. The H1 proton interacts closely with two oxygen atoms at distances of 1.224 and 1.269 Å, while the H2 proton interacts weakly with three oxygen atoms at distances of 1.474 to 1.889 Å (Fig. 2b). These bonding properties are consistent with the aforementioned spectroscopic results.
To test the catalytic activity of HxRuO2, we began our investigation by evaluating the hydrogenation of toluene. Toluene was chosen as the model substrate because it is considered an efficient liquid organic hydrogen carrier (LOHC). 26 Catalytic hydrogenation was performed without
This journal is © The Royal Society of Chemistry 20xx solvent in an autoclave. For comparison, toluene hydrogenation activity was also tested using rutile RuO2 and commercial Ru catalyst. The commercial catalyst, 5 wt% of Ru loaded onto Al2O3 (Ru@Al2O3), is widely used as a catalyst for numerous hydrogenation reactions. The surface areas of rutile RuO2, HxRuO2, and Ru@Al2O3 determined from the Brunauer-Emmett-Teller (BET) analysis, are 4.0, 27.8 and 118.9 m 2 /g, respectively (Fig. S4). Typical SEM images of HxRuO2 (Fig. S5) indicate micron-sized powders, which are consistent with the BET data.
The reaction results are summarized in Table 1. When using the HxRuO2 as a catalyst, complete conversion of toluene to methyl cyclohexane was achieved under 2 MPa of H2 at 50 o C within 1 h (Table 1, entry 1). Methyl cyclohexane was observed as the sole product, and no partial hydrogenation products such as methyl cyclohexene and methyl cyclohexadiene were identified. Performing the same reaction in the presence of rutile RuO2, however, the yield of methyl cyclohexane decreased to 30% (Table 1, entry 2). The Ru@Al2O3 catalyst exhibits only 57% conversion of toluene (Table 1, entry 3). Fig. 3a displays the conversion of toluene over the three catalysts as a function of time. We note that rapid hydrogen uptake occurs at the beginning of the reaction with the HxRuO2 catalyst, as shown in Fig. 3a. The conversions using the RuO2 and Ru@Al2O3 catalysts increase with increasing reaction time, but full conversions were not reached even after 12 hours of reaction. This clearly suggests that HxRuO2 has a standalone catalytic function, especially for hydrogenation reactions.
The effect of H2 pressure on toluene hydrogenation was also examined. When 4 MPa of H2 was applied, toluene hydrogenation in the presence of HxRuO2 was completed even at room temperature within 2 h (Table 1, entry 4). In contrast, the RuO2 catalyst did not show any activity and the Ru@Al2O3 catalyst gave considerably lower conversion (Table 1, entries 5 and 6). This indicates that pressure plays a key role in promoting the activity of HxRuO2, while the pressure had little effect on the RuO2 and Ru@Al2O3 catalysts. The pressureenhanced activity of HxRuO2 might be ascribed to interstitial protons in the RuO2 lattice. The protic hydrogen seems to be highly mobile and strongly reactive under high H2 pressure. Chen et al. reported a novel state of hydrogen with extremely high mobility in hydrogenated Nb2O5 and WO3, which quickly reduces Cu 2+ ions into Cu 0 via proton transfer but has a very short lifetime. 27 On the other hand, protons in HxRuO2 are mobile and also exhibit a high stability, as evidenced by the intact XRD pattern of HxRuO2 after annealing at 150 o C at 2 MPa of H2. Next, we studied the stability of the HxRuO2 catalyst because recycling used catalysts is an essential part of the economic evaluation of arene hydrogenation. Therefore, catalyst reuse experiments were conducted to prove the recyclability of HxRuO2. The catalytic activity of the used catalyst is shown in Fig. 3b. Complete conversion of toluene was evident even after ten cycles and any noticeable deactivation of the catalyst was not detected. In addition, no structural change was observed, which was confirmed by powder XRD. The recycling results demonstrate that the HxRuO2 catalyst retains high activity against repeated toluene hydrogenations and can be used in industrially relevant continuous processes. In contrast, the catalytic activity of rutile RuO2 decreased with increasing cycles under the same conditions, mainly because RuO2 is easily decomposed into Ru. This indicates that the presence of hydrogen in HxRuO2 plays an important role in stabilizing the oxide framework.
The catalytic performance of Ru-based catalysts for converting toluene to methyl cyclohexane reported in the literature is summarized in Table S3 for comparison. The Ru catalysts supported on various materials exhibit superior activity in toluene hydrogenation, but temperatures higher than 100 °C are required for complete conversion (Table S3, 1-6). Very recently, low temperature catalytic activity in aromatic hydrogenation has been identified using alkalimodified Ru-Na catalysts, 28 which was performed at 60 °C and 2 MPa of H2 (Table S3, entry 7). As seen in entry 8, HxRuO2 showed similar hydrogenation activity at lower temperatures and pressures than the Ru-Na catalyst. By simply raising the H2 pressure to 4 MPa, moreover, the hydrogenation of toluene is completed within 2 h even at 25 °C. (Table S3, entry 9).
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 catalysts, the HxRuO2 catalyst shows better catalytic performance for toluene hydrogenation. It is worth mentioning that catalysts operating at low temperatures are important because hydrogenation is an exothermic reaction that can break C-C bonds.
To expand the scope of arene hydrogenation using the HxRuO2 catalyst, we investigated the reactivity of various arene substrates in the absence of solvent. Table 2 shows the best results optimized for complete conversion of arenes into corresponding fully hydrogenated products. Like toluene hydrogenation, the conversion of benzene to cyclohexane proceeded smoothly at room temperature (Table 2, entry 1). However, for xylene, a full conversion was achieved at 60 o C (Table 2, entry 3). Benzene, toluene, and xylene, which are referred to as BTX in the petrochemical industry, were readily converted to the corresponding saturated hydrocarbons under mild conditions. Naphthalene, which is the simplest polycyclic aromatic hydrocarbon and is a crystalline solid, was converted to the fully hydrogenated product, decalin. Remarkably, decalin is formed as the sole product through a solid-state reaction (Table 2, entry 4). N-ethylcarbazole (H0-NEC) and dibenzyltoluene (H0-DBT) are also regarded as promising future LOHC materials. 29,30 The HxRuO2 catalyst was investigated for the hydrogenation of the two LOHCs. N-ethyl-dodecahydro-carbazole (H12-NEC) along with partially hydrogenated products were obtained under 4 MPa of H2 at 120 o C. However, increasing the pressure of H2 to 5 MPa completely converts H0-NEC to H12-NEC within 3 h. Next, the hydrogenation of dibenzyltoluene (H0-DBT) was evaluated under similar reaction conditions. Complete conversion of H0-DBT to perhydro dibenzyltoluene (H18-DBT) was achieved at 120 o C and 5 MPa for 3 h. Performing the hydrogenation reactions of H0-NEC and H0-DBT in the presence of the Ru@Al2O3 catalyst, however, the yield of the desired products drastically decreased under identical conditions. Most of the hydrogenation reactions with Ru-based catalysts reported thus far have been investigated at temperatures higher than 150 °C to obtain higher yields ranging from 90 to 99%.
All these results show that HxRuO2 is catalytically more active, especially for hydrogenation reactions of aromatic compounds than rutile RuO2 and commercial Ru@Al2O3 catalysts. Moreover, the catalytic reactions proceeded smoothly in the absence of solvent and the HxRuO2 is quite stable even under harsh reducing conditions. The solventless hydrogenation of various arenes using the HxRuO2 catalyst might be attractive for industrial applications because neither solvent nor purification steps are required. In addition, the lack of additional steps reduces energy consumption and lowers
This journal is © The Royal Society of Chemistry 20xx Ru@Al2O3 b p-Nitrophenol 40 0.5 1.0 9.1 a Reaction condition: batch reactor, stirring bar, substrate (1 mmol), solvent=methanol/H2O (1:1) 2 mL, catalyst (10 mg). b 5 wt% ruthenium on alumina.
To determine whether HxRuO2 could act as a chemoselective catalyst, we tested various nitroarenes that contained at least nitro and phenyl groups. They are challenging substrates because they have reducible moieties in addition to the nitro group. The resulting aniline and its derivatives are industrially important chemicals that are widely used precursors for the manufacture of agrochemicals, and pharmaceuticals. 31,32 Noble metal catalysts are well known for the reduction of nitroarenes to anilines. Because of their high reactivity, they often lead to the formation of undesirable byproducts such as hydroxylamine. 33 The desired selectivity might be achieved by an oxide-based HxRuO2 catalyst.
Nitrobenzene was chosen as an initial model substrate to determine the optimal conditions for the hydrogenation reaction. The reaction was completed in a water-methanol solvent within 1 h at 60 o C under 0.5 MPa of H2. The HxRuO2 catalyst produces the desired aniline in excellent yield (>99%) with no by-products (Table 3, entry 1), clearly showing that HxRuO2 is an active catalyst for the chemoselective reduction of the nitro group. On the other hand, when RuO2 and Ru@Al2O3 were used as catalysts under the same conditions, the yields dropped to less than 30% (Table 3, entries 2 and 3). Based on the optimized reaction conditions, 4-nitrophenol, which possesses a hydroxyl group para to the nitro group on the phenyl ring, was tested. The use of HxRuO2 produced the desired 4-aminophenol in a yield greater than 99% (Table 3, entry 4), whereas much lower yields were obtained with RuO2 and Ru@Al2O3 (Table 3, entries 5 and 6).
Next, the general scope of our HxRuO2 catalyst was demonstrated in the reduction of functionalized nitroarenes. As shown in Table S4, they afforded the corresponding anilines in excellent yields over 99%. A notable finding is that substrates having slightly bulky electron-donating groups (Table S4, entries 3 and 4) as well as nitroarenes containing electron-withdrawing elements (Table S4, entries 5 and 6) were smoothly hydrogenated under mild conditions. All these results clearly show that HxRuO2 not only outperforms rutile RuO2 and commercial Ru@Al2O3 catalyst, but also acts as a catalyst for highly selective hydrogenations.
To determine whether the interstitial protons are key for the catalytic activity, we investigated the hydrogenation of benzene using deuterated DxRuO2. Catalytic hydrogenation of benzene was performed under the same conditions. Deuterated cyclohexane derivatives were identified from the reaction products by gas chromatography-mass spectrometer and 1 H NMR, clearly showing a direct transfer of deuterium from DxRuO2 to benzene. The deuterium vacancies in DxRuO2 are replenished by dissociated H species. This suggests that interstitial protons within HxRuO2 play a crucial role in accelerating hydrogenation reaction.
Werner and coworkers reported spectroscopic evidence for the coexistence of surface and bulk H species upon H2 dissociation over CeO2, suggesting that the Ce-H species are reactive enough to participate in the hydrogenation reaction. 34 Like the H2-CeO2 interaction, the dissociation of H2 occurs on HxRuO2, producing dissociated H species. The H species diffuse over oxide surfaces and can easily jump from one site to another, which appears to be assisted by the presence of mobile protons in bulk HxRuO2. This proton-hopping process might be related to the Grotthuss mechanism. 35 The vacant interstitial sites appear to serve as highly efficient conduits for rapid proton-hopping leading to the exceptional catalytic activity of HxRuO2.
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To better understand the barriers involved in the proton transfer DFT nudged elastic band (NEB) calculations 36 were carried out on the P21/c RuO2H phase (Fig. 4a). When all four of the hydrogen atoms within a single unit cell (4H) were transferred simultaneously a barrier of ~37 meV/atom was obtained, with the initial, midpoint and final structures illustrated in Fig. 4b Å. Some protons that were transferred near the beginning of the FPMD run remained so during the full duration of the simulation, while others quickly returned to their respective short O-H bond lengths. The protons spent about 2.50% of the simulation time bonded to an oxygen atom that differed from the one in the starting geometry. Inclusion of anharmonic and quantum nuclear effects are likely to further enhance the degree of proton transfer. These computational studies suggest that the facile proton dynamics may be related to the remarkable catalytic activity of HxRuO2.
In conclusion, we have developed a new ruthenium dioxide HxRuO2 that adopts monoclinic structure with interstitial protons. HxRuO2 acts as a standalone catalyst, exhibits highly selective hydrogenation under mild conditions, and outperforms rutile RuO2 and commercial Ru@Al2O3 catalysts. Furthermore, HxRuO2 is recyclable and stable in both acidic and alkaline media. The mobile proton embedded in the oxide lattice plays an important role in stabilizing the distorted structure and maintains the hydrogen flux without diffusion limitation, substantially enhancing the catalytic properties through coupled proton-electron transfer. The discovery of 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx
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This journal is © The Royal Society of Chemistry 20xx