The critical role in surface reactions and heterogeneous catalysis of metal atoms with low coordination numbers, such as found at atomic steps and surface defects, is firmly established1,2. But despite the growing availability of tools that enable detailed in situ characterization3, so far it has not been possible to document this role directly. Surface properties can be mapped with high spatial resolution, and catalytic conversion can be tracked with a clear chemical signature; however, the combination of the two, which would enable high-spatial-resolution detection of reactions on catalytic surfaces, has rarely been achieved. Single-molecule fluorescence spectroscopy has been used to image and characterize single turnover sites at catalytic surfaces4,5, but is restricted to reactions that generate highly fluorescing product molecules. Herein the chemical conversion of N-heterocyclic carbene molecules attached to catalytic particles is mapped using synchrotron-radiation-based infrared nanospectroscopy6,7 with a spatial resolution of 25 nanometres, which enabled particle regions that differ in reactivity to be distinguished. These observations demonstrate that, compared to the flat regions on top of the particles, the peripheries of the particles—which contain metal atoms with low coordination numbers—are more active in catalysing oxidation and reduction of chemically active groups in surface-anchored N-heterocyclic carbene molecules.
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F.D.T. thanks the Director, Office of Science, Office of Basic Energy Sciences and the Division of Chemical Sciences, Geosciences, and Biosciences of the US Department of Energy at LBNL (DE-AC02-05CH11231) for partial support of this work. We thank the M. Raschke group at the University of Colorado for collaborating on the development of the SINS endstation. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract number DE-AC02-05CH11231. W.J.W. thanks the NSF for a predoctoral fellowship (DGE 1106400), and the Arnold Group (UCB) for use of their infrared spectrometer.
The authors declare no competing financial interests.
Nature thanks C. Campbell, G. Rothenberg, F. Tao, B. Weckhuysen and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, b, Preparation procedure of OH-functionalized 1 (a) and NO2-functionalized (b) NHCs that were anchored to the surface of Pt and Au particles, respectively.
a, b, Pt 4f XPS data before (a) and after (b) the adsorption of NHCs on Pt particles. c, d, C 1s XPS data following exposure of the NHC-coated Pt particles to oxidizing (O2, 70 °C, 10 h; c) and reducing (H2, 70°C, 10 h; d) conditions. XPS data (black curves) and the calculated Gaussians that construct the measured XPS signal (coloured curves) are shown. CPS, counts per second; B.E., binding energy.
ATR-IR measurements were conducted after the following treatments: blue, exposure of the sample to mild oxidizing conditions (1 atm O2, 40 °C, 10 h); red, exposure of the sample to harsher oxidizing conditions (1 atm O2, 70 °C, 10 h); black, exposure of the sample to reducing conditions (1 atm H2, 70 °C, 10 h).
a, b, Low-magnification (a) and high-magnification (b) HR-SEM images were taken following deposition of 20-nm Au film on Si(110) surface and surface annealing (under flow of N2) to 1,073 K for 2 h.
N 1s XPS data of NO2-functionalized NHCs that were attached to the surface of Au particles. XPS measurements were conducted before (red spectrum) and after the exposure of the sample to reducing environment, using NaBH3CN (green spectrum) or NaBD3CN (blue spectrum) as a reducing agent. The black dots represent the measured XPS data points; the coloured curves are the averaged XPS signals. The XPS signals that were measured after exposure of the sample to reducing agents (blue and green spectra) have been multiplied by three.
NHCs were attached to the surface of Si-supported Au particles. The sample was exposed to reducing conditions with a deuterated reagent. The dip in the scattering signal around 1,250 cm−1 is induced by a high Si signal from the reference spectra.
Infrared point measurements were conducted on the bare Si surface (green spectrum) and on the edge and centre of a single Pt particle (red and black spectra, respectively). High noise levels between 1,900 cm−1 and 2,600 cm−1 (indicated by the vertical dotted lines) are induced by the presence of a diamond window that separates the ultrahigh-vacuum part of the light source from the rough vacuum area and that highly absorbs the infrared light in this range.
The corresponding biased spectra are shown in Fig. 3. a–d, AFM topography scans and infrared nanospectroscopy line scans were conducted following exposure of the sample to various oxidizing (a, b) and reducing (c, d) conditions. The path of the infrared line scans along the surface is marked by red arrows in the AFM images (left). The colour and size of the different pentagons represent the chemical properties and surface densities of NHCs, as detected by the infrared line scans (blue, OH functional group; purple, acid functional group). The relatively narrow O–H peaks at high wavenumbers are correlated to a deteriorated signal-to-noise ratio in this region. Because only the most dominant part of the O–H peak is highlighted in this colour scheme, the peak width of high-wavenumber (>2,700 cm−1) vibrations is about 50 wavenumbers.
The corresponding biased spectra are shown in Fig. 4. Infrared nanospectroscopy line scans were performed across the centre and edge of NHC-coated Pt particles (marked by red and green arrows, respectively, in the AFM images). a, c, Infrared nanospectroscopy measurements were conducted after the samples were exposed to mild oxidizing (a) and reducing (c) conditions. The coloured pentagons (blue, OH functional group; purple, acid functional group) represent the molecules that reside on the centre and edge of the particle as detected by infrared line-scan measurements. The relatively narrow O–H peak of the carboxylic acid was correlated to a deteriorated signal-to-noise ratio at high wavenumbers. Because only the most dominant part of the O–H peak is highlighted in this colour scheme, the peak width is about 250 wavenumbers. b, d, Schematics of Pt particles with NHCs (coloured pentagons) following their exposure to oxidizing (b) and reducing (d) conditions.
a, AFM topography image of NHC-coated Pt particles that were deposited on Si surface. The white line indicates the path of the line scan along the sample. b. Cross-sectional analysis of the height of one Pt particle. The length of the profile of the particle is about 20 nm (indicated by the distance between the vertical dash-dotted lines; the positions of which are indicated by blue crosses in a). The error in the AFM height measurements is about 2%, as estimated by Si surface roughness measurements.
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Wu, CY., Wolf, W., Levartovsky, Y. et al. High-spatial-resolution mapping of catalytic reactions on single particles. Nature 541, 511–515 (2017). https://doi.org/10.1038/nature20795
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