In oxidation reactions catalysed by supported metal nanoparticles with oxygen as the terminal oxidant, the rate of the oxygen reduction can be a limiting factor. This is exemplified by the oxidative dehydrogenation of alcohols, an important class of reactions with modern commercial applications1,2,3. Supported gold nanoparticles are highly active for the dehydrogenation of the alcohol to an aldehyde4 but are less effective for oxygen reduction5,6. By contrast, supported palladium nanoparticles offer high efficacy for oxygen reduction5,6. This imbalance can be overcome by alloying gold with palladium, which gives enhanced activity to both reactions7,8,9; however, the electrochemical potential of the alloy is a compromise between that of the two metals, meaning that although the oxygen reduction can be improved in the alloy, the dehydrogenation activity is often limited. Here we show that by separating the gold and palladium components in bimetallic carbon-supported catalysts, we can almost double the reaction rate compared with that achieved with the corresponding alloy catalyst. We demonstrate this using physical mixtures of carbon-supported monometallic gold and palladium catalysts and a bimetallic catalyst comprising separated gold and palladium regions. Furthermore, we demonstrate electrochemically that this enhancement is attributable to the coupling of separate redox processes occurring at isolated gold and palladium sites. The discovery of this catalytic effect—a cooperative redox enhancement—offers an approach to the design of multicomponent heterogeneous catalysts.
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We thank L. Kang and R. Wang from University College London and Cardiff University for access and assistance with the electron microscopy; and the Diamond Light Source for access to beamline E01 (proposal number EM18909). C.J.K. acknowledges funding from the National Science Foundation Major Research Instrumentation programme (GR# MRI/DMR-1040229). S.M.A. thanks the Saudi Arabian government for his PhD scholarship. X.H. and Q.H. thank Cardiff University School of Chemistry for financial support. Q.H. also acknowledges the support by National Research Foundation (NRF) Singapore, under its NRF Fellowship (NRF-NRFF11-2019-0002). K.W. and L.Z. thank the Chinese Scholarship Council (CSC) for financial support. XPS data collection was performed at the EPSRC National Facility for XPS (‘HarwellXPS’), operated by Cardiff University and UCL, under contract number PR16195. We thank Cardiff University and the Max Planck Centre for Fundamental Heterogeneous Catalysis (FUNCAT) for financial support.
The authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 Diagrammatic representation of the sol-immobilisation method used for catalyst preparation.
a, Monometallic Au/C and Pd/C. b, Au=Pd/C. c, Au@Pd/C catalysts. d, alloyed Au–Pd/C. e, Schematic representation of our reactor set-up for the thermocatalytic experiments.
Extended Data Fig. 2 Time-on-line data of aqueous HMF oxidation over series of Au/Pd catalysts and their TOF.
a, Au/C. b, Pd/C. c, Au–Pd/C alloy. d, Physical mixture of Au/C + Pd/C. e, Au@Pd/C. f, Au/C followed by the addition of Pd/C after 30 min. g, Au/C followed by the addition of C after 30 min. Reaction conditions: HMF (0.1 M); NaHCO3 (0.4 M); H2O (16 ml); Au/C: 72.1 mg; Pd/C: 71 mg; Au–Pd/C alloy: 143.1 mg; Au@Pd/C: 143.1 mg; C: 71 mg; 80 °C; pO2 = 3 bar. Key: FDCA yield (■), FFCA yield (♦), HMFCA yield (▲), HMF conversion (●), mass balance (*). Associated error bars correspond to mean ± s.d. (n = 5). h, The influence on ODH activity when various quantities of Pd/C (▲) and C (●) are added to Au/C (72.1 mg); ODH activity exhibited by various quantities of Pd/C, in the absence of Au/C, is also displayed (◂). i, The influence of oxygen pressure (0.6–3.0 bar) on ODH activity over a physical mixture of Au/C + Pd/C (▲) and Au/C (♦) is displayed. The reaction conditions used for h and i are given in Methods. j, Summary of each catalyst in terms of HMF conversion, initial rate and TOF at a 5-min reaction time. The total active sites available in each catalyst was estimated using the Mackay model, based on the presented particle size distributions25. Further information relating to how the TOFs were calculated, can be found in Methods.
Extended Data Fig. 3 Catalytic performance over Au/C, Pd/C, Au–Pd/C alloy, Au/C + Pd/C physical mixture and Au@Pd/C catalysts in series of alcohol oxidation reactions.
a, Glycerol oxidation. b, Ethanol oxidation. c, 5-Formyl-2-furancarboxylic acid (FFCA) oxidation. d, 5-Hydroxymethylfuroicacid (HMFCA) oxidation. Reaction conditions were all listed in Methods. e, Conversion values for the HMF oxidation reaction after 5 and 15 min time-on-line for the various catalysts studied in this work. Reaction conditions: HMF (0.1 M); NaHCO3 (0.4 M); H2O (16 ml); Au/(C/TiO2/BN): 72.1 mg, Pd/(C/TiO2/BN): 71 mg, Au@Pd/C, Au–Pd/(C/TiO2/BN) and Au/(C/TiO2/BN) + Pd/(C/TiO2/BN): 143.1 mg; 80 °C; pO2 = 3 bar; reaction time: 5 and 15 min. * presents the test on bare supports in HMF oxidation, HMF (0.1 M); NaHCO3 (0.4 M); H2O (16 ml); C/TiO2/BN: 60 mg; 80 °C; pO2 = 3 bar; reaction time: 30 min.
Extended Data Fig. 4 Electron microscopy analysis of Au/C + Pd/C (physical mixture) catalyst after one cycle of use in the oxidation of HMF.
a, b, Representative complementary BF- and HAADF-STEM micrographs showing metal nanoparticle size and spatial distribution. c, Atomic resolution HAADF-STEM micrograph of a C grain supporting Au particles. d, A C grain supporting Pd particles confirming that the Au and Pd remain separated under our reaction conditions. e, f, Representative XEDS spectra of individual Au particles and Pd particles in the catalyst, respectively. No evidence of Au or Pd migration or intermixing after the catalytic reaction was observed.
a, b, Au 4f and c, d, Pd 3d /Au 4d regions for Au/C and Pd/C monometallic catalysts before and after a typical HMF oxidation reaction as a physical mixture. Among which, a, fresh Au/C; b, used Au/C; c, fresh Pd/C; and d, used Pd/C. TPR data for the physically mixed Au/C + Pd/C catalyst and the Au@Pd/C catalyst e, before and f, after HMF oxidation.
Extended Data Fig. 6 Electrochemical and thermal catalytic oxidation of aqueous HMF over Au/Pd catalysts.
a, Correlation between the thermo- and electro-catalytic HMF oxidation over the series of catalysts. For thermocatalytic experiments, the initial rates were from a 5-min reaction. The current densities were from the maxima observed in the corresponding CV experiments (Fig. 3a). Associated error bars correspond to mean ± s.d. (n = 3). b, Aqueous HMF oxidation over the mono- and bi-metallic Au–Pd catalysts. Reaction conditions: HMF (0.1 M); NaOH (0.4 M); H2O (16 ml); 25 °C; pO2 = 3 bar; 30 min; catalyst amounts for Au@Pd/C and Au–Pd/C:143.1 mg, Au/C: 72.1 mg, Pd/C: 71 mg, carbon balance: ca 92%. c, Catalytic performance in short circuit with current density (normalized by an electrode surface area of 0.07 cm2) generated as a function of time in the single cell. Reaction conditions: 0.1 M NaOH and 0.02 M HMF in 50 ml H2O; Au (working electrode) and Pd or C (counter electrode); 25 °C; O2 flow: 50 ml min−1. d, H-type dual cell consists of Au as the anode in an N2 flow, Pd as cathode in an O2 flow. The two cells connect via an anion exchange membrane. Reaction conditions: each cell contains 0.1 M NaOH and 0.02 M HMF in 35 ml H2O; 25 °C; gas flow O2/N2: 50 ml min-1. e, Reaction conditions: i: 0.1 M NaOH and 0.02 M HMF in 50 ml H2O, 25 °C, N2 flow: 50 ml min−1; ii: same as i, except for the O2 flow: 50 ml min−1; iii: each cell contains 0.1 M NaOH and 0.02 M HMF in 35 ml H2O, 25 °C, O2/N2 flow: 50 ml min−1; iv: same as iii, except for the disconnection of Au and Pd electrodes; v- same as iii, except the mass of Pd/C is doubled.
Extended Data Fig. 7 Representative STEM-HAADF images and X-ED spectra of nanoparticles in the Au=Pd/C catalysts and its corresponding activities.
a, Lower magnification STEM-HAADF image of the Au = Pd/C catalyst. b, c, X-ED spectra obtained from individual nanoparticles, showing a Au-only and a Pd-only nanoparticle. d, STEM-HAADF image and the corresponding X-ED spectrum (inlet) of a Janus-like particle occasionally found in this Au=Pd/C catalyst. e, Activity comparison to the physical mixture f. Reaction conditions: HMF (0.1 M); NaHCO3 (0.4 M); H2O (16 ml); Au/C: 72.1 mg; Pd/C: 71 mg; Au=Pd/C: 143.1 mg; 80 °C; pO2 = 3 bar; reaction time: 30 min. Associated error bars correspond to mean ± s.d. (n = 3).
Extended Data Fig. 8 Electron microscopy analysis of Au@Pd/C catalyst after one use in the oxidation of HMF.
a, b, Representative complementary pair of BF- and HAADF-STEM micrographs showing metal nanoparticle size and spatial distribution. c–e, Atomic resolution HAADF-STEM micrographs of particles. The yellow arrows in e highlight certain atomic columns that appear lower in contrast, indicating some alloying of Pd with the Au matrix. f, A representative XEDS spectrum obtained from a typical nanoparticle, showing the presence of both Au and Pd.
a, c, The Au@Pd/C catalyst. b, d, the physical mixture Au/C + Pd/C catalyst. Reaction conditions: HMF (0.1 M); NaHCO3 (0.4 M); H2O (16 ml); Au/C: 72.1 mg; Pd/C: 71 mg; Au@Pd/C: 143.1 mg; 80 °C; pO2 = 3 bar; reaction time: 60 min. Key: FDCA yield (■), FFCA yield (♦), HMFCA yield (▲), HMF conversion (●), mass balance (*).
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Huang, X., Akdim, O., Douthwaite, M. et al. Au–Pd separation enhances bimetallic catalysis of alcohol oxidation. Nature 603, 271–275 (2022). https://doi.org/10.1038/s41586-022-04397-7
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