Research Article pubs.acs.org/acscatalysis
Electrooxidation of Alcohols with Electrode-Supported Transfer Hydrogenation Catalysts Megan Buonaiuto,† Antonio G. De Crisci,† Thomas F. Jaramillo,‡ and Robert M. Waymouth*,† †
Department of Chemistry and ‡Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: A titanium electrode, modified with Ru(OH)x/TiO2, was prepared and observed to mediate both chemical transfer hydrogenation and electrochemical alcohol oxidation. Electro-oxidation of 2-propanol (1.3 M) at room temperature and pH 7.2 exhibits an onset of electrocatalytic current at 1000 mV vs RHE for the twoelectron oxidation of 2-propanol to acetone. XPS characterization, cyclic voltammetry, and electrolysis experiments confirm that electrochemically active ruthenium species catalyzed the electro-oxidation of 2-propanol to acetone. These modified electrode surfaces maintain >60% original activity upon reuse, despite low loadings of ruthenium. The applied potentials are consistent with an electrocatalytic mechanism mediated by surface-immobilized Ru−oxo species (1380 mV vs RHE). These results indicate that heterogeneous transfer hydrogenation catalysts can function as alcohol electro-oxidation catalysts. KEYWORDS: electrocatalysis, transfer hydrogenation, alcohol oxidation, heterogeneous catalysis, ruthenium
■
INTRODUCTION Fuel cells are attractive alternatives to fossil fuel combustion due to their high theoretical efficiencies and the potential for utilization of a variety of fuel types, including those derived from renewable resources.1,2 Despite recent advances,3−7 many electrooxidation catalysts for alcohol fuel cells suffer from high overpotentials to achieve reasonable rates.2,4,8−10 There is an increasing need for electro-oxidation catalysts that can function under a wide range of operating conditions (pH, temperature, substrate, fuel phase), especially those that can mediate the energy-efficient oxidation of alternative fuels, such as biologically derived alcohols.1,2,5,11,12 Transfer hydrogenation (TH) catalysts mediate the reversible oxidation of alcohols utilizing ketones as the terminal oxidant.13 Typical transfer hydrogenation catalysts use alcohols as the hydrogen source, frequently 2-propanol, to reduce a variety of ketone substrates.13−17 As transfer hydrogenation reactions are not strongly exergonic, the reversible oxidation of the alcohols occurs close to or at the equilibrium redox potential. We have thus targeted chemical transfer hydrogenation catalysts as a starting point for the development of reversible electro-oxidation catalysts.5,18−20 Our initial studies focused on homogeneous Ru(II) catalysts;18,21 herein we describe investigations of the electrocatalytic behavior of heterogeneous transfer hydrogenation catalysts.22 We targeted functionalized electrodes derived from known heterogeneous transfer-hydrogenation catalysts. Mizuno et al.22 reported that ruthenium hydroxides supported on anatase titanium dioxide nanoparticles are active transfer hydrogenation © XXXX American Chemical Society
catalysts. At elevated temperatures, under inert conditions, a simple synthetic preparation of Ru(OH)x /anatase TiO 2 catalyzes hydrogen transfer between 2-propanol and ketone substrates (Scheme 1). We reasoned that titanium foils, Scheme 1. Transfer Hydrogenation of Acetophenone by 2Propanol
containing a native TiO2 layer, might function analogously to anatase particles22 as a support for the Ru hydroxides, enabling us to simultaneously test these modified electrode surfaces as both transfer hydrogenation catalysts and electrooxidation23−27 catalysts (Scheme 2). Herein, we report the generation of Ti electrodes modified with Ru(OH)x/TiO2 and their activity as transfer hydrogenation catalysts and as alcohol electrooxidation catalysts. Proton NMR, GC-FID product quantification, cyclic voltammetry, extended electrolysis, and XPS surface characterizations Received: August 18, 2015 Revised: October 29, 2015
7343
DOI: 10.1021/acscatal.5b01830 ACS Catal. 2015, 5, 7343−7349
Research Article
ACS Catalysis
To confirm the presence of ruthenium on the foil, X-ray photoelectron spectroscopy was performed on the Ru(OH)x/ TiO2/Ti electrodes. A representative survey scan and highresolution scans for the C 1s/Ru 3d region confirms the presence of surface ruthenium, titanium, oxygen, and elemental carbon contaminant with no other species detected (Figure 2a).
Scheme 2. Proposed Transfer Hydrogenation/Alcohol Electroxidation on Ru(OH)x/TiO2/Ti Electrodes
before and after electrolysis reveal that hydrous ruthenium species supported on oxidized Ttitanium electrodes mediate the two-electron electrocatalytic oxidation of 2-propanol to acetone.
■
RESULTS AND DISCUSSION Synthesis and Surface Characterization. Ru(OH)x/ anatase TiO2 nanoparticle TH catalysts were synthesized through sol−gel procedures as previously described.22,28 Ru(OH)x/TiO2/Ti electrodes were prepared by sol−gel deposition of aqueous ruthenium chloride onto a high-purity annealed titanium foil (99.999% trace metals basis). The cyclic voltammogram of the Ru(OH)x/TiO2/Ti electrode in 0.1 M phosphate buffer electrolyte shows a broad, quasi-reversible feature at E1/2 = 620 ± 50 mV vs RHE which is attributed to a RuIII/IV couple, consistent with previous hydrated ruthenium oxo species (Figure 1).23,29−31 The RuIII/IV
Figure 2. (a) Survey XPS scan of Ru(OH)x/TiO2/Ti electrode before (blue, lower trace) and after (red, upper trace) electrolysis of 2propanol. The electrode before electrolysis contains peaks for Ti, Ru, O, and C (elemental contaminant), while the electrode after electrolysis contains additional peaks for K and P from electrolyte contamination. (b) High-resolution XPS scan of the C 1s and Ru 3d region including individual fit components from CasaXPS. Two elemental carbon contamination regions have been used in this fit due to XPS spectra of blank washed Ti foil containing alcohol and ketone carbon contaminants. Figure 1. Cyclic voltammogram of a Ru(OH)x/TiO2/Ti electrode (and washed annealed titanium control electrode) in 0.1 M phosphate buffer at a scan rate of 10 mV/s. The broad feature with E1/2 = 620 ± 50 mV vs RHE is attributed to the surface-adsorbed RuIII/IV couple, not present in the titanium control scan. The black dashed line (- - -) denotes the capacitance plot used when integrating to estimate electrochemically active ruthenium on the electrode, the blue solid line () denotes the Ru(OH)x/TiO2/Ti electrode, and the green dotted line (···) denotes the Ti electrode control.
The high-resolution scan of the C 1s region illustrates the overlapping features from elemental carbon environments and ruthenium (Figure 2b). Due to differences in binding energy of 300 h−1; Figure S5 in the Supporting Information), as previously reported.22 Notably, the Ru(OH)x/TiO2/Ti foil demonstrated activity for transfer hydrogenation similar to that for the nanoparticle catalyst at comparable loadings (TOF of ca. 0.70 h−1), implicating an analogous chemical mechanism. The slightly reduced activity of the foil relative to that of the anatase particle catalyst could be the result of the nature of the underlying TiO2 support of the oxidized titanium foil. While the nature of the TiO2 support was reported to be important to catalyst activity (Ru(OH)x/TiO2(anatase) > Ru(OH)x/TiO2(rutile) > Ru(OH)x),22 our synthetic methods did not control for the crystallinity or phase of the TiO2 layer. On the basis of literature reports,38 the native titanium dioxide layer of the Ru(OH)x/TiO2/Ti electrode is expected to be
