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Electrocatalytic hydrogenation of phenol over Pt and Rh: unexpected temperature effects resolved Nirala Singh, Yang Song, Oliver Yair Gutiérrez Tinoco, Donald M. Camaioni, Charles T. Campbell, and Johannes A. Lercher ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02296 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016
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ACS Catalysis
Electrocatalytic hydrogenation of phenol over Pt and Rh: unexpected temperature effects resolved Nirala Singh,†,‡ Yang Song,§ Oliver Y. Gutiérrez,§ Donald M. Camaioni,† Charles T. Campbell,‡ Johannes A. Lercher*,†,§ †
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354, United States
‡
Department of Chemistry, University of Washington, Seattle, Washington, 98105-1700, United States
§
Department of Chemistry and Catalysis Research Institute, Technische Universtät München, Garching 85747, Germany
ABSTRACT: Both electrocatalytic hydrogenation (ECH) and thermal catalytic hydrogenation (TCH) of phenol by Pt and Rh show a roll-over in rate with increasing temperature without changing the principal reaction pathways. The negative effect of temperature for aqueous-phase phenol TCH and ECH on Pt and Rh is deduced to be from dehydrogenated phenol adsorbates, which block active sites. ECH and TCH rates increase similarly with increasing hydrogen chemical potential whether induced by applied potential or H2 pressure, both via increasing H coverage, and indirectly by removing site blockers, a strong effect at high temperature. This enables unprecedented phenol TCH rates at 60-100 °C.
KEYWORDS. hydrogenation, electrocatalysis, poisoning, phenol, platinum, rhodium Carbon-neutral, energy-dense liquid hydrocarbons are excellent transportation fuels, and can be renewably derived from biomass thermal catalytic hydrogenation with H2 (TCH) followed by oxygen removal.1,2 When H2 is unavailable, biomass can be electrocatalytically hydrogenated (ECH), effectively storing renewable electric energy in the chemical bonds of hydrocarbons. However, the processes to produce hydrocarbons require more than hydrogen addition3, which necessitates to explore this electrochemical hydrogenation under conditions, which are compatible for all elementary steps. We use here phenol as a model for hydrogen-rich intermediates from lignocellulose. Several studies have investigated TCH1,2 and ECH4–9 of phenol to cyclohexanone and cyclohexanol in water using Rh7–9 and Pt1,4–6,8,9 catalysts at close to ambient conditions. Increasing temperature generally increases both thermal and electrochemical rates, as reported for TCH and ECH of phenol from 5-60 °C.4,9 However, both its TCH and ECH rates decreased above 60 °C,9 which precludes use of higher temperatures required to achieve competitive rates and to allow linking the reductive processes to acid-base catalyzed reactions required to synthesize the hydrocarbons. Here, we show that for TCH this decrease is due to poisoning by dehydrogenated adsorbates, and can be avoided by increasing the H2 partial pressure (PH2), which removes poisons and increases TCH activity 130fold at 80 °C. The ECH rate is controlled by applied potential in a manner qualitatively consistent with the effect of PH2 on TCH rates, implying that both control the rate
via the surface coverage of H adatoms (formed from H+ and H2, respectively). Figure 1 shows the rates of TCH of phenol in water and aqueous acetate buffer solvent on Pt/C. From 22-60 °C the TCH rates and activation energy (33 kJ/mol, Figure 1 inset) were similar to previous reports.9 The TCH activation energy was similar to ECH on Pt/C (Rh/C TCH and ECH activation energies were also similar)9, confirming that the rate-determining step for both TCH and ECH is identical, i.e., the reaction of adsorbed H atoms (H*) with adsorbed phenol.1,8 Thus, the hydrogen source (H+/e– versus H2) does not affect the apparent activation energy. This is attributed to the fact that the step to make H* has a comparatively low barrier whether via H+ transfer in ECH (according to calculations10) or dissociative adsorption of H2 in TCH.11 We show below that the rate has a high order in PH2, which further suggests that the H* coverage is low and is determined by the steady-state balance between its adsorption rate to form H* and the desorption rate of H* to form H2. The observation that H* is removed mainly as H2 even in ECH is consistent with the low Faradaic efficiency reported.9 Figure 1 shows that the rate of phenol TCH decreased dramatically from 60 to 80 °C at 1 bar total pressure (equilibrium water vapor pressure with the balance H2), i.e., it is 8 times slower than the rate predicted from the temperature dependence measured at temperatures below 60°C. A similar effect was reported for gas-phase TCH of phenol on Pd at >150 °C,12,13 and gas-phase benzene TCH
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on Pt and Pd at >200 °C,14,15 where the rate decrease was concluded to be caused by the poisoning of surface sites by dehydrogenated adsorbates. With high temperatures and low PH2, a high fraction of the active sites is poisoned by dehydrogenated adsorbates. The reaction rate then exhibits a very high order in PH2 due to the decrease in the coverage by these poisons until PH2 increases enough to re-hydrogenate the carbonaceous deposits and desorb them.15 Exploring the catalytic activity of carbon-supported Pt and Rh for aqueous phenol TCH consisted of measurements in both a glass reactor (1 bar H2 bubbled through solution) and an autoclave (batch reactor pressurized with H2). Sufficiently high impeller rotation and low catalyst loading ensured that the conversion rates were not limited by external diffusion (see Supporting Information Table S1, Figure S4 and discussion). A porous carbon electrode on which the carbon-supported catalyst was deposited was used for ECH at different potentials. Voltages were converted to RHE from Ag/AgCl. Turnover frequencies (TOFs) were calculated based on number of Pt or Rh sites from H2 chemisorption.
Figure 1. Temperature dependence of TCH rate of 18 mM phenol in water (squares) and 50 mM aqueous acetate buffer (circles) using a 5 wt% Pt/C catalyst at 1 bar total pressure (H2 plus water vapor). Filled points are from this work, open 9 points are from Song et al. Solid black line is to guide the eye, and dotted line extrapolates the rate based on an activa-1 tion energy of 33 kJ mol . Inset: Arrhenius plot.
The low reaction rate for phenol TCH at 80 °C was observed with and without acetate buffer (Figure 1). Thus, the presence of acetate anions is not responsible for the decreased activity. Consequently, only results for TCH in water are reported in the following analysis. For the conditions in Figure 1 (1 bar total, 18 mM phenol and 22-80 °C) there was no dependence of the rate on phenol concentration; it remained constant to >95% conversion (18 mM to 2 mM phenol, see Figure S1-3). Because of the vapor pressure of water (0.45 bar at 80 °C), PH2 in the reactor drops to only 0.55 bar (from 1 bar at 20°C and 0.8 bar at 60 °C). The 8-fold difference between the rate at 80 °C
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predicted from the temperature dependence of the rates below 60°C and the measured rate at PH2 = 0.55 bar and 80 °C is in part due to this drop in PH2. Since the rate is first-order in H2 at 60 °C (see below), this drop from 0.8 to 0.55 bar H2 should decrease the rate only by 31%. The remaining decrease in rate (factor of 5.5) must be caused by another effect. We show below that this is due to poisoning by dehydrogenated adsorbates, which also explains the high order in H2 (3) at 80 °C. By increasing PH2 to 20 bar in Figure 2, the TCH rates increased, and the reaction rate decrease observed above 60 °C at 1 bar total pressure was not seen. The reaction rates below 60% conversion from 60 °C to 100 °C at 20 bar led to an apparent activation energy of 34 kJ mol-1 (Figure 2 inset), nearly identical to the activation energy below 60 °C at PH2 = 1 bar.9 The turnover number (TON) was much higher for the PH2 = 20 bar reactions (Figure 2) compared to PH2 ≤1 bar (Figure 1), and deactivation did not occur, showing that the high PH2 is very effective in preventing it. Figure 2 also shows that the rates decreased above 5060% conversion. Since the moles of H2 in the reactor at 20 bar was 10 times the amount required for complete conversion of phenol to cyclohexanol, the decrease in PH2 was small, and not nearly enough to account for the decreased rate (the rate is near zero-order in H2 above 6 bar, see below.) This differentiates the decreased rate above 5060% conversion from the decreased rate observed in Figure 1 at 80 °C and 0.55 bar H2, which is related to PH2.
Figure 2. Phenol (172 mM) TCH conversion versus time in water at PH2 = 20 bar in an autoclave at 60 °C, 80 °C and 100 °C on 5 wt% Pt/C. TOFs are calculated based on points connected by solid lines. Conversion during heat-up of the reactor (gray section from -20 to 0 minutes) causes the non-zero conversion at time zero. Inset: Arrhenius plot of initial TOFs for PH2 = 20 bar (solid black) and PH2 = 1 bar (blue open squares, from Figure 1).
Figure 3 compares phenol TCH rates at different PH2, showing the reaction rate as first-order in H2 from 0.8 bar to 10 bar at 60 °C (Figure 3a inset), but approximately third-order from 0.55 bar to 1.5 bar, followed by first-order in (average) H2 pressure from 1.5 to 6 bar at 80 °C (Figure 3b inset). (See Supporting Information Methods and Ta-
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ble S2). From the rate measurements in Figure 3 for PH2 = 10-30 bar at 60 °C (Figure 3a) and for 6-30 bar at 80 °C (Figure 3b), the rate was independent of PH2 at higher pressures. Figure 3c shows the TCH rates were zero-order for initial phenol concentrations from 86 to 690 mM at 80 °C and PH2 = 20 bar, behavior identical with TCH rates measured at 1 bar and 23-60 °C (zero-order from 2 to 18 mM phenol). For initial phenol concentrations of 86 to 690 mM phenol, the rates did not decrease until the conversion was above 50-60% (at 60-80 °C, PH2 = 20 bar), similar to behavior shown in Figure 2. The zero-order in phenol concentration observed over a wide range of measured conditions (2 to 18 mM at 1 bar from 23-60 °C and 43 to 690 mM at 20 bar PH2 and 80 °C) for less than 50% conversion is consistent with previous studies of TCH of aromatic molecules.15,16 Assuming a bimolecular Langmuir kinetics model, with the surface reaction of H* with phenol as the rate-determining step,1,8,9 the independence of rate on phenol concentration implies a high coverage of phenol at 2-18 mM at 60 °C, such that phenol coverage does not appear to be responsible for the decrease in TCH rate above 60 °C at 1 bar total pressure. In Figure 2, the rate is shown to decrease above 50-60% conversion even though the concentration of phenol (69 mM phenol at 60% conversion of 172 mM) was still large enough to give initial rates that are zero-order in phenol. At this ratio of H2 to phenol (>10) the equilibrium phenol conversion is in excess of 99%. Also, the decrease in activity was not related to the TON, i.e., the TCH rate at 690 mM (Figure 3c) did not decrease even at ~20 moles phenol gcat-1 (23% conversion), compared to the lowered rate at only ~7.5 mol gcat-1 (74% conversion) at 86 mM. Including 45 mM cyclohexanone with 86 mM phenol as the initial concentration caused the rate to decrease ~20% relative to 86 mM phenol without added cyclohexanone (see Supporting Information). The lack of a dependence on TON, coupled with the lower initial rate if cyclohexanone is included in the reactor implies that the decrease in activity is not related to poisoning, but a parallel reaction (e.g., reduction of cyclohexanone) that ensues as the competition for adsorption sites between the products and phenol increases with conversion. Thus, although the rate is independent of phenol concentration in the absence of products, the rate depends on the ratio of phenol to cyclohexanone when it nears ~1:1 (see Figure S5 for product concentrations). As noted above, H* is mainly removed by H2 desorption. Thus, the rate should be ½ order in PH2 for simple second-order Langmuir kinetics, when the surface reaction of H* with phenol is the rate-determining step (or possibly 1st order if the second H addition is rate determining as predicted theoretically1). The high reaction order in H2 (~3 dropping to 1 from 0.55 to 6 bar at 80 °C, Figure 3b) is hypothesized to be caused by a reaction pathway similar to that which occurs for TCH of gaseous benzene on Pd, where dehydrogenated benzene species
block active sites, and their removal via hydrogenation accounts for a very high H2 order (up to 4) at elevated temperatures.14,15 Thus,
Figure 3. Phenol TCH conversion in water at (a) PH2 = 10, 20 and 30 bar in an autoclave at 60 °C, (b) PH2 = 1.5, 6, 10, 20 and 30 bar at 80 °C with 172 mM phenol and (c) moles of phenol reacted per gram of catalyst for different initial concentrations of phenol in water at PH2 = 20 bar at 80 °C. Catalyst was 5 wt% Pt/C. Conversion during heat-up of the reactor (gray section from -20 to 0 minutes) causes the non-zero conversion at time zero. Insets: Log of TOF versus log of PH2 or log of phenol concentration for conversions below 50%.
the decreased phenol TCH activity seen at 80 °C in Figure 1 is hypothesized to be caused by the formation of phenolderived dehydrogenated surface species, which are too strongly bound to desorb, but can be rehydrogenated and removed by H2. The zero-order H2 dependence from PH2 = 10-30 bar at 60 °C (Figure 3a) and from 6-30 bar at 80 °C (Figure 3b) may be attributed to either surface saturation
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of hydrogen or competitive adsorption between hydrogen and phenol. If hydrogen and phenol do not compete for sites, an increase in PH2 should increase the coverage, and the rate of reaction, unless the surface is saturated in hydrogen. If phenol and hydrogen compete for sites, an increase in PH2 may increase the hydrogen coverage, while decreasing phenol coverage and the overall rate may remain relatively unchanged, providing both coverages are sufficiently high. More consistent with competitive adsorption, the presence of phenol has been shown to inhibit formation of adsorbed H atoms.6,9
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(see Figure S6). This is 8.5-fold higher than the maximum ECH rate in Figure 4 (0.12 s-1 at -0.35 V at 23 °C). It indicates the coverage of H* at -0.35 V in ECH is much lower than at PH2 = 6 bar in TCH, despite the higher (theoretical) effective hydrogen chemical potential (see Supporting Information).
Figure 5. Conversion vs. time for ECH of phenol (18 mM) on 5 wt% Rh/C in aqueous acetate buffer.
Figure 4. Electrochemical (ECH) TOFs for phenol hydrogenation on Pt/C and Rh/C in acetate buffer (18 mM phenol) at 23 17 °C as a function of potential (bottom X-axis) . For comparison, TCH TOFs for phenol on Pt/C in water at 60 and 80 °C are shown as a function of the H2 pressure on a log scale (top X-axis), so that both X-axes reflect the hydrogen chemical potential in the same way. Dotted lines are to guide the eye.
Figure 4 shows the ECH rates of phenol on Pt/C and Rh/C as a function of potential from recent work17. The trends for Rh/C and Pt/C were essentially the same. Also shown for comparison here are the TCH rates on Pt/C at 60 and 80 °C replotted from Figure 3 a,b as a function of log(PH2), since the equilibrium value of log(PH2) varies linearly with potential. The X-axis scaling and offset were chosen to maximize the similarity in dependences of the TCH and ECH rates on the effective chemical potential (coverage) of H*. These TCH rates were measured at a different phenol concentration (172 mM), but since the TCH rate is zero-order in phenol (see above), the comparison to the ECH rate is still valid. At low applied potentials (0.05 to -0.05 V for Rh/C), the applied potential had a large impact on the rate (100 mV increased the rate 10fold). As the applied potential was increased from -0.05 to -0.2 V the rate increased, but more slowly (300 mV to increase the rate 10-fold). Above -0.45 V vs. RHE the rate did not increase with applied potential. The TCH rates increased rapidly with PH2 at low pressures, then reached a maximum at 6-10 bar. By extrapolating the maximum rate for TCH (5.1 and 8.5 s-1 at >6 bar for TCH at 60 and 80 °C, respectively) to 23 °C, assuming an Arrhenius relation with Ea = 34 kJ mol-1, the TCH TOF is predicted to be 1.0 s-1
Figure 5 shows the rate of phenol ECH on Rh/C at 23 and 60 °C, at -0.15 and -0.45 V vs. RHE. Although at both 23 and 60 °C the reaction rate increased with applied potential, the initial rates at 60 °C were comparable to 23 °C, for the same applied potential. This independence of the initial ECH rate on temperature is coincidental as the rate is known to increase with temperature up to 40 °C and then to decrease above 40 °C,9 so that by 60 °C the rate has decreased to be similar to ambient conditions. The maximum conversion achieved at 60 °C was only 60% for -0.15 V vs. RHE, and 75% at -0.45 V vs. RHE, whereas nearly 100% conversion was achieved at 23 °C at both voltages, with the rate decreasing rather abruptly to zero after ~50 minutes of reaction. We attribute this to the buildup of surface species, which poison sites for ECH of phenol. Since this deactivation takes the same amount of time at both voltages, we attribute it to some thermal (not electrochemical) process, like dehydrogenation of phenol as also seen in TCH at 80 °C. It would happen at lower temperature in ECH than TCH due to the much lower coverage of H* (see above). We further proved this deposition of a poison at 60 °C by dropping the temperature to 25 °C after 120 minutes at 60 °C and -0.45 V vs. RHE, whence both the rate and maximum conversion markedly decreased compared to starting at 20 °C. Similarly, after 120 minutes at 60 °C and -0.15 V vs. RHE, the rate measured at 60 °C and -0.25 V vs. RHE had decreased. Our results indicate that the rates for TCH and ECH are controlled by PH2 or applied potential through (i) hydrogenation of dehydrogenated phenol species, which poison the active sites and (ii) an increase in H-coverage. However, ECH has a lower maximum achievable rate and faster deactivation by deposition of dehydrogenated phenol at elevated temperatures than TCH, consistent with lower H* coverage at the same or greater effective hydrogen
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chemical potential. The results imply that catalyst modifications which decrease stability of the dehydrogenated phenol poison relative to adsorbed phenol could accelerate its removal via rehydrogenation, and thus achieve even higher rates. The maximum TOF for TCH on Pt/C at 80 °C measured here (8.5 s-1) matches the rates of a proton exchange membrane H2 electrolyzer (see Supporting Information), showing it is feasible to combine electrolyzer/phenol hydrogenation in one system.
Research Foundation Innovation Fellowship. CTC acknowledges the Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division Grant No. DE-FG0296ER14630 for support of this work.
REFERENCES (1) (2)
ASSOCIATED CONTENT Supporting Information
(3) (4)
Experimental details, mass transport, rates at high conversions and literature comparison. This material is available free of charge via the Internet at http://pubs.acs.org.
(5) (6)
AUTHOR INFORMATION
(7) (8)
Corresponding Author (9)
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[email protected] (10)
Author Contributions All authors have given approval to the final version of the manuscript
(11) (12)
Notes The authors declare no competing financial interests.
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ACKNOWLEDGMENT
(14) (15) (16) (17)
Thermal catalysis described in this paper was conducted under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. N.S. acknowledges the Washington
Yoon, Y.; Rousseau, R.; Weber, R. S.; Mei, D.; Lercher, J. A. J. Am. Chem. Soc. 2014, 136, 10287. Zhao, C.; He, J.; Lemonidou, A. A.; Li, X.; Lercher, J. A. J. Catal. 2011, 280, 8. Furimsky, E. Appl. Catal. A Gen. 2000, 199, 147. Amouzegar, K.; Savadogo, O. Electrochim. Acta 1994, 39, 557. Zhao, B.; Guo, Q.; Fu, Y. Electrochemistry 2014, 82, 954. Sasaki, K.; Kunai, A.; Harada, J.; Nakabori, S. Electrochim. Acta 1983, 28, 671. Miller, L. L.; Christensen, L. J. Org. Chem. 1978, 43, 2059. Amouzegar, K.; Savadogo, O. Electrochim. Acta 1997, 43, 503. Song, Y.; Gutiérrez, O. Y.; Herranz, J.; Lercher, J. A. Appl. Catal. B Environ. 2016, 182, 236. Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. J. Phys. Chem. B 2004, 108, 17886. Christmann, K. Surf. Sci. Rep. 1988, 9, 1. Neri, G.; Visco, A. M.; Donato, A.; Milone, C.; Malentacchi, M.; Gubitosa, G. Appl. Catal. A, Gen. 1994, 110, 49. Díaz, E.; Mohedano, A. F.; Calvo, L.; Gilarranz, M. A.; Casas, J. A.; Rodríguez, J. J. Chem. Eng. J. 2007, 131, 65. Lin, S. D.; Vannice, M. A. J. Catal. 1993, 143, 539. Chou, P.; Vannice, M. A. J. Catal. 1987, 107, 140. Singh, U. K.; Vannice, M. A. AIChE J. 1999, 45, 1059. Song, Y.; Chia, S. H.; Gutiérrez, O. Y.; Lercher, J. A. J. Catal. DOI: 10.1016/j.jcat.2016.09.030
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