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Enabling Overall Water Splitting on Photocatalysts by CO Covered Noble Metal Co-catalysts Tobias F. Berto, Kai E Sanwald, J. Paige Byers, Nigel D. Browning, Oliver Yair Gutiérrez Tinoco, and Johannes A. Lercher J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02151 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 21, 2016

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The Journal of Physical Chemistry Letters

Enabling Overall Water Splitting on Photocatalysts by CO Covered Noble Metal Co-Catalysts Tobias F. Berto,† Kai E. Sanwald,† J. Paige Byers,§ Nigel D. Browning,# Oliver Y. Gutiérrez,*† and Johannes A. Lercher*†,‡ †

Department of Chemistry and Catalysis Research Center, TU München, Lichtenbergstrasse 4,

85747 Garching, Germany. §

Department of Chemical Engineering and Materials Science, University of California, Davis,

California 95616, United States. #

Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory,

P.O. Box 999, Richland, Washington 99352, United States. ‡

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999,

Richland, Washington 99352, United States.

ACS Paragon Plus Environment

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ABSTRACT

Photocatalytic overall water splitting requires co-catalysts, which efficiently promote the generation of H2, but do not catalyze its reverse oxidation. We demonstrate, that CO chemisorbed on metal co-catalysts (Rh, Pt, Pd) suppresses the back reaction, while maintaining the rate of H2 evolution. On Rh/GaN:ZnO, the highest H2 production rates were obtained with 4 - 40 mbar CO, the back reaction remaining suppressed below 7 mbar O2. The O2 and H2 evolution rates compete with CO oxidation and the back reaction. The rates of all reactions increased with increasing photon absorption. However, due to different dependencies on the rate of charge carrier generation, the selectivities to O2 and H2 formation increased in comparison to CO oxidation and the back reaction with increasing photon flux and/or quantum efficiency. Under optimum conditions, the impact of CO to prevent the back reaction is identical to that of a Cr2O3 layer covering the active metal particle.

TOC GRAPHICS


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Photocatalytic overall water splitting is an endergonic (∆G° = 237 kJ mol-1 = 1.23 eV/e), carbon-free route to H2, which does not require thermal energy input. The half reactions, i.e., H2 evolution reaction (HER) and O2 evolution reaction (OER), are driven by photogenerated charge carriers with defined potentials. Visible light absorbing semiconductors with efficient electron hole separation and appropriate co-catalysts for HER and OER are crucial or technical realization.1-3 Co-catalysts must enable efficient charge carrier transfer at the semiconductor/cocatalyst interface and optimum H binding energy.1,2,4 In addition, the co-catalyst must not catalyze the thermodynamically favored back reaction (BR) to water.5 Thus, it does not surprise that only few effective co-catalysts for overall water splitting are known. Transition elements, such as Pt, Rh, Cu, Ni or Pd, are efficient metallic or oxidic HER cocatalysts when coupled with oxidation of organic compounds (mostly used as “sacrificial agents”). However, these co-catalysts have low activity in the absence of sacrificial agents due to their relatively high BR activity.6-12 The most reported approach to prevent BR has been to combine the metal co-catalyst with oxides. It has been concluded that the oxide phases cover the metal as membranes, selectively allowing transport of protons and H2 to/from the metal core.13-15 Similar effects have been reported by covering the whole photocatalyst particle with oxyhydroxides of transition metals,16 or by covering, TiO2 photoanodes with hydroxides.17 The oxides might also perform as O2 evolution co-catalysts.18 As alternative to the solid-state approach, we hypothesized that selective metal poisoning could minimize BR rates. On testing this hypothesis, the results presented here demonstrate that a molecular layer of CO on the cocatalyst enhances H2 production during overall water splitting by hindering BR. Influence of CO on Overall Water Splitting over Rh/GaN:ZnO. Stable overall water splitting was observed over Rh/GaN:ZnO at 303 K under UV-Vis and visible light illumination


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(λ > 420 nm) in the presence of 40 mbar of CO as shown in Figure 1 and SI-Figure 1 (sacrificial agents were not used in this study). In contrast, the same photocatalyst yielded low H2 and O2 amounts under pure Ar (SI-Figure 2). The rate of H2 evolution was zero after 1 hour, because the co-catalyst efficiently catalyzed the back reaction.5 We propose that CO chemisorbs on the Rh particles as observed for CO on Rh-19,20 Pt-,20,21, or Pd-electrodes22 in aqueous phase within large potential ranges. CO occupies the sites for the more structure sensitive BR, suppressing it, whereas protons can still access metal sites for the structure insensitive HER. The H2 to O2 ratios were higher than 2 because CO was oxidized to CO2 (the amount of consumed CO equals the amount of CO2 detected). Thus, we introduce a corrected ratio (H2 to O2*-ratio) that accounts for the oxygen equivalents evolved as O2 and CO2. This ratio was close to 2 as expected for overall water splitting (Figure 1). It is worth noting that methanation of CO was not observed.

Figure 1. Overall water splitting over Rh/GaN:ZnO in the presence of 40 mbar CO. Dashed horizontal line represents the expected stoichiometric ratio of H2 to O2. Black lines serve to guide the eye. Reaction conditions: 75 mg photocatalyst, 100 mL H2O (pH = 4.5, H2SO4), 303 K, 40 mbar CO, 1 bar, 300 W Xe-lamp (CM1).


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Photocatalysts containing Rh/chromia and CO-Rh showed identical activities for overall water splitting (Table 1). HRTEM characterization showed Rh particles on the semiconductor covered by amorphous layers (SI-Figure 3), which we interpreted as Rh@Cr2O3 (core@shell) in agreement with Refs.13,15 Thus, the CO-layer is able to suppress the BR as efficiently as a Cr2O3 shell independent from the metal deposition method. Table 1: Comparison of evolved H2 after 5 h over CO-covered Rh/GaN:ZnO and Rh@Cr2O3/GaN:ZnO prepared via photodeposition or impregnation. Photocatalyst

Photodeposition

Impregnation

µmol H2 after 5h CO-Rh/GaN:ZnO

116

145

Rh@Cr2O3/GaN:ZnO

112

142

Reaction conditions: 75 mg photocatalyst, 100 mL H2O (pH = 4.5, H2SO4), 303 K, 40 mbar CO in case of CORh/GaN:ZnO, 1 bar, 300 W Xe-lamp (CM1).

Influence of CO Partial Pressure on Overall Water Splitting. The overall water splitting activity sharply increased with the addition of small amounts of CO (Figure 2). A plateau in activity was found between 8 to 40 mbar CO and declining rates were observed at higher partial pressures. Hence, whereas CO covers sites for BR without affecting the HER sites in the 8 40 mbar range, the decrease in activity above 40 mbar CO is attributed to hindered H+ reduction. In the presence of ~4 mbar of CO, the maximum H2 production rate was 28 µmol h-1 (similar to that observed in the 8 - 40 mbar CO range). After 2.5 h, the oxidative CO consumption decreased its partial pressure to 3.6 mbar and back reaction sites become available. Accordingly, the H2 evolution rate decreased to < 10 µmol h-1 (SI-Figure 4). However, at optimum conditions, (as in the experiment reported in Figure 1), the CO partial pressure decreased by ∼5% (from 40 mbar to


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38 mbar) after 5 h, which did not influence the HER as shown in Figure 2 while CO coverages on the metal barely change (see below). Quantification of the CO coverage on the co-catalyst. CO2 evolution rates follow a Langmuirtype rate equation as a function of dissolved CO (SI-Figure 5), regardless of the oxidation state of Rh under operating conditions (influenced by illumination as discussed in the SI). This Langmuir behavior contrasts the negative reaction orders in CO for thermal oxidation on metallic Rh at high surface coverages. We conclude that, in contrast to thermal CO oxidation, O2 and CO are activated on two different sites: CO on the metal reacts with oxidizing O-species generated at the semiconductor-metal interphase as shown by isotope experiments described below.

Figure 2. Amount of H2 evolved after 5 h and initial H2-evolution rate in function of CO partial pressure over Rh/GaN:ZnO. Black and grey lines serve to guide the eye. H2 to O2*-ratio is 2 in any experiment. Reaction conditions: 75 mg photocatalyst, 100 mL H2O (pH = 4.5, H2SO4), 303 K, 0 – 200 mbar CO, 1 bar, 300 W Xe-lamp (CM1). According to the Langmuir model, the coverage changes from 25 % to 60 % under CO partial pressures varying from 8 to 40 mbar on Rh2O3/GaN:ZnO. With ongoing in situ photoreduction it changes to 45-80 % in the same pressure range (SI-Figure 5 A, C, E). Stable overall water


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splitting rates were obtained with CO coverages of at least 45 % (8 mbar). Induction periods or small initial H2-evolution rates at low CO partial pressures (< 8 mbar, Figure 2) are attributed to low CO-coverages, which do not completely hinder the back reaction on the co-catalyst (SIFigure 5A). Reaction Mechanism. H2 and O2 evolution over CO-covered Rh/GaN:ZnO are an interplay of overall water splitting (coupled HER and OER), back reaction (BR), and CO oxidation (Scheme 1). The effects of the latter two, which lower the efficiency of the target reaction, were explored using (isotopelabeled) co-feeds ((18)O2, D2), and varying light intensities at 40 mbar CO. Scheme 1: Reactions occurring during overall water splitting over Rh/GaN:ZnO.

CO Oxidation. The O-species that oxidize CO can originate from water oxidation or from activated O2. By co-dosing

18

O2 prior to water splitting, C16O16O and C16O18O were detected

with 65 µmol and 1 µmol, respectively, after 5 hours. CO conversion was not observed in the dark (SI-Figure 6). Thus, CO is oxidized mainly by O-species generated through light-driven water oxidation. Photon flux, number of photogenerated charge carriers, and concentration of active O-species are proportionally correlated.1,23,24 Thus, overall water splitting was studied at varied photon flux at 365 nm to understand the influence of atomic O-species on CO oxidation rates. The H2 evolution rate was first order in photon flux (SI-Figure 7), which reflects the quasi-first-order kinetics of charge carrier recombination under the applied illumination conditions.25 The


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dependences of CO oxidation and O2 evolution on photon flux were 0.5 and 1.3, respectively (SI-Figure 8). Thus, with rising concentration of active O-species, the O2 evolution rate increase faster than the CO oxidation rate. In turn, the O2 to CO2-ratio must increase with increasing the rate of electron-hole generation by increasing the photon flux, the wavelength range of absorbed photons, or the quantum efficiency of the semiconductor. Those parameters were varied to verify this hypothesis. The O2 to CO2-ratio over Rh/GaN:ZnO increased from 1.9 to 5.6 by a four-times enhanced photon flux (SI-Figure 9), and from 0.3 to 1.4 by replacing visible-light with UV-Vis illumination (compare Figure 1, and SI-Figure 1). Rh/Al-SrTiO3,26 with higher apparent quantum efficiency than Rh/GaN:ZnO (10.9 % and 1.4 %, respectively, at 365 nm) yielded the O2 to CO2 ratio of 14.8 (1.9 over Rh/GaN:ZnO) using 365 nm UV-LEDs. Back Reaction. The stability of the CO-shell and the H2 production, were explored in dependence of D2 and O2 partial pressures. D2 was not consumed, when co-dosed prior to the experiments (SI-Figure 10A), while the expected H2 to O2*-stoichiometry was observed. The rate of HER was independent of the D2 partial pressure (SI-Figure 11) and HD was not formed (SIFigure 10B). Thus, dissociative adsorption of D2 over CO-covered Rh does not occur. This is in line with the suppressed H2 oxidation on CO poisoned Pt-catalysts in fuel cell applications.27 The amounts of evolved H2 were not influenced by low partial pressures of O2 dosed prior to irradiation (Figure 3), which points to the absence of BR at such conditions. In contrast, above 7 mbar of O2, the reaction order of hydrogen production in O2 was -1.3. The comparison between a water splitting experiment in the presence of 40 mbar CO and an experiment where

18

O2 (60

mbar) was co-dosed showed that in the presence of 18O2, 130 µmol less H2 was evolved, whereas the O2 evolution was almost invariant. The missing H2 matched almost stoichiometrically with the amount of consumed

18

O2 (55 µmol). The difference (10 µmol of

18

O2) is assigned to the


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reaction of H2 with 16O2 or 16O intermediates produced from water oxidation. Thus, back reaction occurs due to competitive adsorption of O2 and CO and is limited by increasing the CO to O2 ratio. Indeed, H2 evolution rate raised by a factor of four by increasing CO partial pressure from 40 to 120 mbar in the presence of 60 mbar O2 (SI-Figure 12).

Figure 3: Influence of O2 partial pressure on H2 evolution. The black dotted line serves to guide the eye. Reaction conditions: 75 mg photocatalyst, 100 mL H2O (pH = 4.5, H2SO4), 303 K, 40 mbar CO, 0 – 100 mbar O2, 1 bar, 300 W Xe-lamp (CM1). Thermal reaction of H2 and O2 is discarded due to hindered D2 activation at optimum CO coverages (SI-Figure 10). Thus, we define a light-driven BR, which may proceed via two pathways on the co-catalyst: (i) O2 is reduced via photoadsorption and captures protons forming water, or (ii) protons are reduced to H-atoms and react with dissociatively adsorbed O2 (SIScheme 1). In the latter case, water formation directly competes with HER (two hydrogen atoms to H2) and varying the concentration of H atoms in the presence of co-dosed O2 must impact HER and BR selectivities. Thus, overall water splitting was performed at varied photon flux in the presence of co-dosed O2. H2 production followed a second order dependence on photon flux (Figure 4 and SI-


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Figure 13). This result, which contrasts the first order dependence in the absence of co-dosed O2, points to the transfer of electrons to protons instead of to O2 as the main BR pathway (further explanation in SI-Figure 14). It also indicates that increasing photon flux decreases the share of BR to the overall reaction. Increasing the quantum efficiency has the same effect. In the presence of 30 mbar O2, the H2 evolution rates over Rh/GaN:ZnO (Q.E. 1.4 %) and Rh/Al-SrTiO3 (Q.E. 10.9 %) decreased by 85 % and 60 %, respectively, compared to the rates observed in the presence of less than 7 mbar O2.

Figure 4: Influence of photon flux on H2 evolution activity in the presence of 60 mbar O2. The black line serves to guide the eye. Reaction conditions: 75 mg photocatalyst, 100 mL H2O (pH = 4.5), 303 K, 40 mbar CO, 60 mbar O2, system pressure 1 bar, UV-LEDs (50 – 200 mA, 365 nm). Water splitting over Various Co-Catalysts. The generality of our approach was tested by modifying Al-SrTiO3 with Rh, Pt, or Pd. These metals are commonly used as co-catalysts for hydrogen evolution, and strongly adsorb CO (SI-Table 1). Considerable H2 amounts were produced in all cases only in the presence of CO, pointing to the suppression of BR (Table 2).


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Interestingly, an O2 to CO2 ratio of 13.5 on Rh/Al-SrTiO3 indicates that OER is favored over CO oxidation, whereas the opposite was observed over Pt/Al-SrTiO3 and Pd/Al-SrTiO3. Table 2: Evolved H2 and O2 to CO2-ratios after 5 h over different precious metal decorated AlSrTiO3. Photocatalyst

H2 after 5h

O2 to CO2-ratio after 5 h

[µmol] Rh/Al-SrTiO3

625

13.5

Pt/Al-SrTiO3

250

0.2

Pd/Al-SrTiO3

58

0.1

Reaction conditions: 125 mg photocatalyst, 100 mL H2O, 303 K, 40 mbar CO, 1 bar, 300 W Xe-lamp (CM1). In conclusion, a molecular layer of CO chemisorbed on precious metal co-catalysts is able to quantitatively suppress the back reaction of water splitting by poisoning the active sites for this reaction, while proton reduction and subsequent hydrogen evolution are unaffected. Over Rh/GaN:ZnO, CO coverages of 40 – 80 % lead to water splitting rates identical to those observed for core shell Rh@Cr2O3/GaN:ZnO. We demonstrate that a molecular protection layer is a powerful tool to enable overall water splitting by eliminating thermodynamic constraints and introducing kinetic control.

ASSOCIATED CONTENT Supporting

Information.

Experimental

section;

physicochemical

characterization

of

photocatalysts; supplementary kinetic plots for overall water splitting over Rh/GaN:ZnO and


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Rh/SrTiO3 with co-dosed CO; this material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank the Federal Ministry of Education and Research (BMBF) for financial support (project no. 01RC1106A), and Clariant for productive discussions within the framework of MuniCat and the iC4 PhotoCOO project. HRTEM-imaging was funded by the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL), and the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is a multiprogram national laboratory operated by Battelle for the DOE under Contract DE-AC05-76RL01830. K.E.S. gratefully acknowledges financial support by the Fond der Chemischen Industrie (FCI). The authors thank Kazuhiro Takanabe, Garry Haller, and Hany El-Sayed for fruitful discussions, as well as Xaver Hecht, Martin Neukamm, and Udishnu Sanyal for technical support, physicochemical characterization and TEM measurements.

REFERENCES


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