Research ArticleCHEMICAL PHYSICS

Operando detection of single nanoparticle activity dynamics inside a model pore catalyst material

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Science Advances  19 Jun 2020:
Vol. 6, no. 25, eaba7678
DOI: 10.1126/sciadv.aba7678
  • Fig. 1 Nanofluidic chip design, implementation, and gas flow characterization.

    (A) Schematic of the nanofluidic chip (blue) mounted in the sample holder containing gas and electrical connections. (B) Schematic of the nanofluidic chip and the microfluidic channels that connect the nanofluidic model pores to the inlet and outlet holes. (C) Top: Dark-field optical micrograph of a single model pore (channel 3 in the bottom) decorated with individual catalyst nanoparticles (red). Bottom: Nanofluidic system consisting of six parallel channels acting as the model pores. Each channel contains nanofabricated model catalyst particles in the 100-nm size range (details in fig. S3). The roman numbers highlight different regions relevant for the flow model calculations shown in (E). (D) Representative scanning electron microscopy (SEM) micrographs of a model pore decorated with a single Cu nanoparticle in its center region and an array of nanoparticles placed in its tapered parts connecting to the microfluidic system. (E) Simulated pressure profile of the model pores. The widened black line corresponds to the pressure difference between the six channels that stem from the fact that they have a different total length (more details in fig. S6). The roman numbers denote the different regions depicted in (C). Black arrows denote the gas flow direction in all panels.

  • Fig. 2 Plasmonic nanospectroscopy from multiple single nanoparticles inside a nanofluidic model pore.

    (A) Optical dark-field microscope image of 10 hybrid Au-Pd nanostructures placed inside a nanofluidic model pore (channel 3 in Fig. 1C), schematically depicted in (D). (B) Scattering spectra of the 10 nanoparticles in (A) after insertion of a dispersive grating between the microscope and the CCD camera. (C) Single particle dark-field scattering spectra of an Au-Pd hybrid nanostructure [red outline in (B), side view SEM image in inset] inside the nanofluidic model pore in pure Ar and 100 mbar H2. These structures are composed of an inert Au plasmonic nanoantenna (shaded yellow) that serves as a nanoprobe of the Pd particle (shaded blue) deposited on top of a 7-nm SiO2 support layer sandwiched between the two (43). a.u., arbitrary units. (D) Pressure profile along the model pore containing the nanoparticles, which in the experiment are simultaneously optically probed by plasmonic nanospectroscopy. The black arrow denotes the gas flow direction. (E) Change in scattering peak FWHM (ΔFWHM) of a selection of five nanoparticles along the model pore, induced by H2 pulses with decreasing concentration in Ar carrier gas. Changes in ΔFWHM are due to the formation of Pd hydride at a critical H2 partial pressure (see also fig. S7).

  • Fig. 3 CO oxidation over single Cu nanoparticles in a nanofluidic model pore.

    (A) Schematic of the center part of a nanofluidic model pore (channel 2 in Fig. 1C) containing four disk-shaped Cu nanoparticles (100-nm diameter and 40-nm thickness; green circles) in its narrowest region and high-density “patches” of identical particles in the tapered wider regions (black dots; cf. Fig. 1D). The nanoparticles are separated by 20 μm, and the drawing is not to scale. (B) Nominal O2 concentration in the system (black) together with the measured QMS response for CO, O2, and CO2. The CO signal is scaled by a factor of 1/8. (C) Experimental (solid lines) and FDTD simulated (dashed lines) single particle scattering spectra of a single Cu nanoparticle in its metallic (orange) and oxidized state (black). Insets show TEM micrographs of a representative Cu particle in the metallic (left) and oxidized state (right). Scale bars, 100 nm. (D) Optical response of four Cu nanoparticles in the model pore (green lines) upon increasing O2 concentration (black line) during reaction with CO. (E) Scaled zoom-in on the optical response from (D) revealing distinct time delays on the order of 5 min between the oxidation of the four nanoparticles. Shaded regions indicate changes in inlet O2 concentration.

  • Fig. 4 Cu nanoparticle back-reduction in CO flow through the model pore at 493 K.

    Normalized optical center response of the four single Cu nanoparticles after an experimental sequence identical to the one depicted in Fig. 3. At time, t = 0, indicated by the black dashed line, the O2 concentration in the feed is reduced to zero while keeping the CO flow constant at 6% in Ar carrier gas. This results in the simultaneous back-reduction of all four Cu particles to the metallic state after ca. 20 min, in stark contrast to the reverse oxidation process during the CO oxidation reaction, where delays of ca. 5 min between the phase transformation of the particles from the metallic to the oxidized state were observed due to local reactant conversion (cf. Fig. 3E).

  • Fig. 5 Simulated reactant concentration profile in a nanofluidic model pore during CO oxidation on a single Cu nanoparticle.

    (A) Schematic illustration of the simulation geometry with representative 2D reactant concentration profiles obtained at different positions along a nanofluidic model pore with the same dimensions as in the experiment, that is, width x = 400 nm, length y = 100 μm, height z = 100 nm. The color coding corresponds to the concentration profile presented in the color bar, and the black area corresponds to the particle. (B) Simulated tracer molecule concentration profile along the flow direction due to reactant conversion over four individual nanoparticles placed in sequence. The color code indicates the number of “active” particles (i.e., Cu particles in the metallic state) along the model pore. The inset illustrates the corresponding particle positions, and the arrow indicates the reactant flow direction. (C) 2D contour plots of calculated reactant tracer concentration across the nanofluidic model pore at different positions along it. The three panels are the same as the ones presented in (A) and correspond to the positions: (i) at the nanoparticle center, (ii) 50 nm after, and (iii) 200 nm after the center of the nanoparticle. The color code is the same as in (A). Extended data set in fig. S15.

Supplementary Materials

  • Supplementary Materials

    Operando detection of single nanoparticle activity dynamics inside a model pore catalyst material

    David Albinsson, Stephan Bartling, Sara Nilsson, Henrik Ström, Joachim Fritzsche, Christoph Langhammer

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    • Sections S1 to S6
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