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Aug 6, 2018 - Institute for Catalysis, Hokkaido University, N21, W10, Kita-ku, Sapporo, Japan, 001-0021. ‡. Elementary Strategy Initiative for Catal...
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Remarkable Enhancement in Hydrogenation Ability by Phosphidation of Ruthenium: Specific Surface Structure Having Unique Ru Ensembles Shinya Furukawa, Yukihiko Matsunami, Ikutaro Hamada, Yasushi Hashimoto, Yasushi Sato, and Takayuki Komatsu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02582 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Catalysis

Remarkable Enhancement in Hydrogenation Ability by Phosphidation of Ruthenium: Specific Surface Structure Having Unique Ru Ensembles Shinya Furukawa*,†,‡, Yukihiko Matsunami||, Ikutaro Hamada¶,°,‡, Yasushi Hashimoto§, Yasushi Sato§, and Takayuki Komatsu*,|| †

Institute for Catalysis, Hokkaido University, N21, W10, Kita-ku, Sapporo, Japan, 001-0021 Elementary Strategy Initiative for Catalysis and Battery, Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto, Japan, 615-8510 || Department of Chemistry, Graduate School of Science and Enginnering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan, 185-8550. ¶ Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada, Suita, Osaka, Japan 565-0871 ° Global Research Center for Environment and Energy based on Nanomaterials Science, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Japan, 305-0044 § Central Technical Research Laboratory, JXTG Nippon Oil & Energy Co., 8 Chidoricho, Naka-ku, Yokohama, Japan, 231-0815

‡

ABSTRACT: A series of transition metals and metal phosphides supported on silica (M/SiO2, MxPy/SiO2: M = Pt, Pd, Ir, Rh, Ru, Ni, and Co) were prepared and tested as catalysts for hydrogenation of toluene as a hydrogen carrier. Catalytic activities were reduced greatly by phosphidation of noble metals such as Pt, Pd, Ir, and Rh. Catalytic activity of Ru, however, showed a remarkable increase in turnover frequency—26 times higher than that of bare Ru. Ru2P/SiO2 with a much lower cost exhibited a catalytic performance comparable with those of other platinum group metals. A combination of kinetic, infra-red, and computational studies revealed that (1) strong adsorption of toluene on Ru2P promoted hydrogenation and (2) a specific surface structure of Ru2P having large ensembles of electron-deficient Ru allowed the strong adsorption.

KEYWORDS: toluene hydrogenation, Ru2P, ensemble effect, surface structure, intermetallic compound Hydrogenation is a highly important chemical transformation in view of organic, pharmaceutical, and industrial chemistry.1 Platinum group metals (PGM) such as Pt, Pd, and Rh are known as excellent hydrogenation catalysts, which easily convert various unsaturated organic compounds. However, due to their rarity, uneven distribution, and accordingly extremely high costs, it is highly required to replace these noble metals with less noble alternatives.2 Ni as a less noble metal for hydrogenation has been applied to practical use for various reactions such as hydrogenation of unsaturated fatty acids,3 nitroarenes,4 and aldehydes.5 However, in comparison with PGMs, severe reaction conditions such as high temperature and hydrogen pressure are typically required to obtain sufficient reaction rates. Therefore, drastic improvement in catalytic performance of less noble metals is an important task for both chemical science and human society. Alloying of main active metal with the second metal

element is a common approach to modify the catalytic performance of the parent monometallic catalyst.6 For example, for oxygen reduction reaction over Pt-based bimetallic materials, great enhancements in catalytic activity have been achieved by alloying with specific second metals that optimize d-band structures.7 However, for hydrogenation of unsaturated molecules, alloy formation typically weakens the adsorption strength of reactant molecules hence the catalytic activity.8 Therefore, an innovative concept in catalyst design, which is different from conventional alloying, is necessary to develop highly efficient hydrogenation systems. Intermetallic compounds, which typically have specific crystal structures, ordered atomic arrangements, and unique electronic states, have been paid increasing attention as attractive inorganic materials for sophisticated catalyst design. 9 During our attempt to develop catalytic chemistry of hydrogenation, we discovered that a metal phosphide as an intermetallic compound in a broad sense exhibited a remarkably high hydrogenation activity with much lower cost than typical PGMs. Herein, we report a novel type of efficient hydrogenation catalyst and an innovative concept for ideal active sites for hydrogenation. As a target reaction, we here focused on hydrogenation of toluene to methyl cyclohexane. This reaction is an important process to storage gaseous hydrogen in toluene as a liquid hydrogen carrier and has already been applied in practical use. 10 Moreover, the catalyst should possess hydrogenation ability as strong as Pt to activate stable aromatic ring. In the present study, a series of transition metals (M = Pt, Pd, Ir, Rh, Ru, Ni, and Co) and corresponding metal phosphides (MxP y: Pt5P 2, Pd3P, Ir2P, Rh2P, Ru 2P, Ni2P, and Co 2P) were tested for this reaction. The catalysts were prepared by pore-filling impregnation method using SiO2 as a support and their crystallite phases were analyzed using X-ray 1

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diffraction (XRD) (Figure S1). For most catalysts, the desired metal and metal phosphide phases were formed with high phase purities. For Ru–P/SiO2 catalyst, however, the broad diffractions made the assignment difficult. Figures 1a and 1b show the transmission electron microscopy (TEM) image of Ru–P/SiO2 and the size distribution of nanoparticles, respectively.

Figure 2. Catalytic performances of various SiO2-supported transition metal and metal phosphide catalysts in toluene hydrogenation: (a) conversion of toluene, (b) change in TOF upon phosphidation, and (c) price of the main metal (2018) in the catalyst plotted against the TOF.

Figure 1. (a) TEM image and (b) size distribution of Ru–P/SiO2. (c) HR-TEM image of a single nanoparticle. (d) Crystal structures of Ru2P viewed along [ 1 5 3 ] direction (left: single unit cell, right: periodic structure). Dihedral angle between (21 1 ) and (112) planes is shown (74°).

Particle sizes ranged from 2 to 4 nm (volume weighted mean diameter, Dv = 3.8 nm), which are consistent with the broad diffraction observed in the XRD pattern. The particle size was alsmost similar to that of Ru/SiO2 (Dv = 3.0 nm, Figure S2). Figure 1c shows the high-resolution (HR)-TEM image of a single nanoparticle. Lattice fringes with spacing of 0.235 and 0.220 nm were clearly observed, which are consistent with the interplanar distances of Ru 2P(112) and 21 1  planes (0.235 and 0.221 nm, respectively).11 The observed dihedral angle (74°) also agree well with that derived from the crystal structure (74°, Figures 1c and d). These results indicate that the observed nanoparticle is of a single crystal of Ru2P viewed along with the [15 3] direction and that the phosphide phase was indeed formed on the SiO2 support. We then tested the catalytic performances of monometallic catalysts supported on SiO2 (M/SiO2, M = Pt, Pd, Rh, and Ni) in the hydrogenation of toluene (Figure 2a) at 473 K. PGMs such as Pt, Pd, Ir, and Rh showed moderate to high toluene conversions (Ctol), whereas Ru and Ni gave very low catalytic activities. No reaction occurred when Co was used. The negligible activities of Ni and Co may be due to the temperature of reduction pretreatment (200°C), which is insufficient to reduce these metals. Thus, PGMs except Ru showed prominently high hydrogenation abilities compared with other transition metals. For phosphide catalysts, a quite different trend was observed. Metal phosphides with PGMs other than Ru showed much lower Ctol than the corresponding pure metal catalysts. In contrast, for Ru catalysts, Ru 2P gave a great enhancement in Ctol (11 times higher). A similar trend

was also observed in the scale of turnover frequency (TOF), where 26 times increase in TOF was obtained for Ru2P catalyst (Figure S3). It is worth noting that Ru 2P showed Ctol and TOF comparable with those observed for other monometallic PGM catalysts. Figure 2b summarizes the change in TOF over phosphidation in a logarithm scale, highlighting the specific and drastic effect of phosphidation for Ru compared with those for other PGMs. Figure 2c shows the cost of raw materials contained in the catalysts, which are plotted against their catalytic activities. Of course, developing a catalyst plotted in the low right region is economically desired. In this context, Ru 2P can be an attractive low-cost alternative to Pt, Pd, or Rh-based hydrogenation catalysts due to the much low cost of Ru. We also investigated the effect of catalyst support on the performance. Survey of various supports (SiO2, Al2O3, TiO2, CeO2, MgO, and active carbon) for Ru 2P revealed that SiO2 gave the highest Ctol and 100% selectivity to methylcyclohexane (S MCH) (Table S1). A small amount of benzene (~1%) was formed when TiO2, Al2O3, CeO2, and MgO were used, suggesting that acid-base property of the support may trigger undesired demethylation. Long-term stability of Ru 2P/SiO2 catalyst in toluene hydrogenation was also tested. Although Ctol decreased gradually during the first 24 h, almost no change in Ctol was observed after 24 h up to 60 h (Figure S4). S MCH kept 100% even after 60 h. Thermogravimetryic analysis for the spent Ru 2P/SiO2 showed a weight loss at 240°C with an exothermic peak probably due to combustion of carbonaceous deposit (Figure S5). Thus, the decline in the catalytic activity in the long run can be attributed to poisoning of the active sites by carbonaceous deposit. Next, we performed a kinetic study and several characterizations for Ru/SiO2 and Ru 2P/SiO2 catalysts to understand the role of phosphidation on the drastic improvement in catalytic performance. Firstly, we focused on a hydrogen isotope effect on the reaction rate in toluene 2

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ACS Catalysis hydrogenation. Interestingly, the use of D2 instead of H2 increased the reaction rate for Ru (rH/rD = 0.75) and Ru2P (rH/rD = 0.85). Such an inverse isotope effect can be attributed to a higher surface concentration of D atoms than that of H atoms and the rate-determining step being hydrogen-attack to adsorbed toluene. This is because surface D species is thermodynamically more stable relying on its lower zero-point energy. 12 Based on the result of isotope experiment, we consider that the hydrogenation step was accelerated by phosphidation of Ru. Then, the adsorption/desorption behavior of toluene was studied using an infra-red temperatureprogramed desorption (IR-TPD) technique. Figure 3a shows the change in IR spectra of toluene adsorbed on Ru2P/SiO2 during a TPD procedure.

Peaks assigned to stretching vibration of CO linearly adsorbed on Ru atoms13 was observed at 2017 and 2052 cm−1 for Ru and Ru 2P, respectively. The blue-shift in the peak position indicates that Ru atoms of Ru 2P had partially positive charges. The stronger adsorption of toluene on Ru2P than Ru can be attributed to the electro-deficient Ru, which should promote adsorption of the electron-rich aromatic rings. However, a similar blue-shift was also observed for Rh catalysts (Figure S7). As reported previously, phosphide formation generally makes the transition metal electron-deficient due to electron transfer from metal to phosphorus atoms.14 Therefore, the decrease in the electron-density of the active transition metal cannot be a sole reason to enhance toluene adsorption. According to the experimental results, we assumed that the surface structure of the metal phosphides critically determines the strength of toluene adsorption. In general, the formation of metal phosphide greatly dilutes the large metal-metal ensembles,15 which is necessary for stable π-adsorption of aromatic rings. Therefore, the formation of phosphide phases seems to be intrinsically unfavorable for toluene adsorption. To investigate the surface structure of Ru 2P, we estimated the surface energies of various planes of Ru 2P using van der Waals inclusive density functional theory (DFT) calculations.16 The calculations revealed that (112) and (210) planes, which also exhibited intense XRDs, were the most stable surfaces (Table S2). The equilibrium crystal shape of Ru 2P was determined by Wulff construction 17 based on the calculated surface energies, representing the dominance of these two planes at the surface (Figure 4a).

Figure 3. (a) Change in FT-IR spectra of toluene adsorbed on Ru2P/SiO2 during heating from 40°C to 150°C under vacuum. TPD profiles of toluene desorption for (b) Ru- and (c) Pd-based catalysts (d) FT-IR spectra of CO adsorbed on Ru/SiO2 and Ru2P/SiO2 with low CO coverages (< 0.3). The scale bar indicates absorbance.

Peaks assigned to C–H stretching vibrations of toluene gradually decreased upon heating under vacuum, reflecting thermal desorption of toluene. For Ru/SiO2 and Ru 2P/SiO2, the normalized intensities of the peaks at 3030 cm−1 (C−H stretching region) were plotted against temperature (Figure 3b), corresponding to the TPD profiles of toluene. These profiles revealed that toluene adsorbed on Ru2P/SiO2 desorbed at a higher temperature than that on Ru/SiO2, suggesting stronger adsorption on Ru2P. Conversely, the corresponding TPD profiles for Pd/SiO2 and Pd3P/SiO2 gave an opposite trend, where monometallic Pd/SiO2 showed higher desorption temperature than the phosphide catalyst (Figure 3c). No obvious difference was observed between the TPD profiles of Rh/SiO2 and Rh 2P/SiO2 (Figure S6). These trends agree with the changes in TOF by phosphidation as mentioned in Figure 2b, suggesting that the adsorption strength of toluene is a key factor to determine the hydrogenation activity. Based on these results, it can be said that the stronger adsorption of toluene on Ru 2P than on Ru accelerated hydrogen-attack to the adsorbed toluene, which is the rate-determining step. We also focused on the change in the electronic state of the metal upon phosphidation. Figure 3d shows FT-IR spectra of CO adsorbed on Ru/SiO2 and Ru2P/SiO2 at low coverages.

Figure 4. (a) Equilibrium crystal shape of Ru2P determined by Wulff construction using surface energies calculated by DFT. (b) Atomic arrangement of Ru2P(112) surface displaying relatively large Ru– Ru ensembles.

The (112) plane of Ru 2P displayed a unique surface structure, where Ru–Ru ensembles comprising 6 Ru atoms were created at the surface. This large ensemble is likely to allow strong πadsorption of toluene, which needs 3 or 4 atom ensembles, and surface hydrogen atoms adjacent to the adsorbed toluene (Figure 4b, bottom). We also calculated the adsorption energies of toluene (E ads) on Ru(0001) and Ru 2P(112) planes having various adsorption geometries. For Ru(0001) plane, E ads ranged from 2.10 to 2.35 eV (Figure S8a). The most stable configuration was the tri-σ model with the center of aromatic ring located above an hcp hollow site and with its methyl group pointing toward another hollow site. This configuration agrees with that reported in an experimental and theoretical study of benzene adsorption on Ru(0001).18 For Ru2P(112) plane, the stable adsorption geometry of toluene was di-σ-di-π configurations on 4-Ru-atom ensembles (Figure S8b), where 3

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substantially higher E ads (2.71~2.77 eV) were obtained. Thus, the DFT calculations demonstrate that Ru 2P provided a unique surface that captured toluene more strongly than pure Ru surface. However, strong adsorption of toluene might inhibit hydrogen adsorption, thereby limit the hydrogenation activity. Therefore, we also considered toluene adsorption at high coverages. Additional adsorption of one toluene molecule on another Ru-ensemble site showed a larger E ad (2.52 eV, Figure S9a), highlighting that toluene adsorption on the Ru-ensemble site is strong even at the middle coverage. Then, adsorption of two toluene molecules with one on the Ru-ensemble and the second covering the adjusent Ru sites (designated in Figure 4b as H adsorption sites) and a P atom was considered (Figure S9b). Geometry optimization of this initial structure did not converged and the second toluene molecules moved away from the surface. This indicates that toluene adsorption on the H adsorption site is kinetically unfavored. In a similar fashion, geometry optimizations at higher coverage (three and four toluene molecules on the slab, Figures S9c and d, respectiely) did not converged. Therefore, we concluded that full coverage of Ru2P(112) with toluene is not likely to occur. We also confirmed that dissociative adsorption of H2 at the H adsorption site is exothermic (Ead = 0.69 eV, Figure S10) even when toluene is co-adsorbed on the Ru-ensemble site. Thus, the DFT calculations support that H adsorption is compatible with the strong adsorption of toluene. This is probably due to the specific and approipriate size of Ru-ensembles and P atoms that block additional toluene adsorption. Based on the results of experimental and theoretical approaches, we concluded that the combination of electronic and geometric effects by phosphidation allowed a great enhancement in toluene adsorption. In other words, Ru2P provided appropriate ensembles of electron-deficient Ru at the surface, which promoted toluene adsorption. It should be noted that the specific promotion effect of Ru 2P relies strongly on the specific bulk and surface structures displaying large Ru ensembles. We also emphasize that the specific crystal and surface structures of Ru 2P allowed “electronic modification by phosphidation” and “retaining the large Ru ensemble” compatible. Typically, alloy formation not only modifies the electronic structure (ligand effect) but also dilutes active metal-metal ensembles necessary for hydrogenation (ensemble effect). Therefore, catalyst design based on alloying has long been accepted as ineffective for improving catalytic activity in hydrogenation. In this context, our discovery provides not only a highly efficient catalytic system, but also an innovative concept of catalyst design for hydrogenation. Besides, we emphasize that the novel chemistry of metal phosphide discovered in the present study differs completely from that reported for other systems, such as hydrodesulfurization and hydrodenitrogenation. In these systems, phosphide phases promoted C−S(N) bond scission but not aromatic hydrogenation.19 Another difference is that, for hydrodesulfurization, the enhancement in catalytic activity by phosphidation is observed not only for a specific element but also for a wide variety of transition metals.19 It is also known that transition metal phophides (M xP y: M = Mo, W, Fe, Ni, Co, and Cu) show great catalytic performances in electrochemical hydrogen evolution reaction (HER).20 In these systems, strong hydrogen adsorption is weakened due to an ensemble effect

derived from P incorporation, which enhances their HER activity.21 Besides, a charge transfer from P to M provides both proton-acceptor (P δ−) and hydride-acceptor (M δ+) centers that work to promote the HER.21 Thus, for hydrogen production, both the ensemble and ligand effects provide positive contribution. However, for hydrogenation, the ensemble effect negatively contributes to the catalysis as mentioned above. Finally, we discuss the reason why the strong adsorption of toluene enhanced the catalytic activity. A possible explanation may come from the viewpoint of molecular structure. Our calculations reveal that the strongly adsorbed toluene on Ru 2P(112) surface was highly distorted and had an sp 3-like conformation compared with that on Ru(0001) surface (Figure 5): angles between aromatic C−H bonds and the phenyl ring for Ru2P were much larger than those for Ru(0001).

Figure 5. Side views of toluene adsorbed on (a) Ru(0001) (atpo00°) and (b) Ru2P(112) (conformation 3) surfaces. Dotted line indicates the plane consisted of the aromatic ring.

Given that hydrogenation of aromatic compounds generally requires structural changes from sp 2 to sp 3 conformations, the structural similarity in the initial and final states as observed for Ru 2P(112) may provide a kinetically favorable pathway. A recent DFT study reported that the energy barrier for hydrogenation of benzene over Ru(0001) was evidently reduced when the adsorption strength of benzene was increased,22 which supports our scenario. In summary, we developed a highly efficient catalytic system for toluene hydrogenation using Ru 2P/SiO2. Phosphidation of Ru greatly enhanced the catalytic activity (more than 20 times higher) up to the levels of Pt, Pd, and Rh. Ru 2P/SiO2 can be a lower-cost alternative for PGM hydrogenation catalysts. The specific enhancement in catalytic performance by phosphidation of Ru relied on the unique electronic state and surface structure of Ru2P, where appropriate ensembles of electron-deficient Ru were arranged. Emphasis should be placed on the compatibility between the electronic modification and retaining metal−metal ensembles, which is hardly achieved using conventional bimetallic materials. The insight obtained in the present study provides a novel concept of catalyst design for developing more efficient hydrogenation catalysts.

ASSOCIATED CONTENT Supporting Information Experimental and computational details, XRD patterns, activity tests, FT-IR spectra, and DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org. 4

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ACS Catalysis

AUTHOR INFORMATION Corresponding Author [email protected], [email protected]

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI (grant numbers of 16H04565 and 17H04965), MEXT Program for Development of Environmental Technology using Nanotechnology, and the Cooperative Research Program of Institute for Catalysis, Hokkaido University (grant number of 17B1012). TEM observeation was aided by Analysis Center of Tokyo Institute of Technology. The DFT calculations were performed using the Numerical Materials Simulator at the National Institute for Materials Science.

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