In situ dynamic tracking of heterogeneous nanocatalytic processes by shell-isolated nanoparticle-enhanced Raman spectroscopy

Surface molecular information acquired in situ from a catalytic process can greatly promote the rational design of highly efficient catalysts by revealing structure-activity relationships and reaction mechanisms. Raman spectroscopy can provide this rich structural information, but normal Raman is not sensitive enough to detect trace active species adsorbed on the surface of catalysts. Here we develop a general method for in situ monitoring of heterogeneous catalytic processes through shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) satellite nanocomposites (Au-core silica-shell nanocatalyst-satellite structures), which are stable and have extremely high surface Raman sensitivity. By combining operando SHINERS with density functional theory calculations, we identify the working mechanisms for CO oxidation over PtFe and Pd nanocatalysts, which are typical low- and high-temperature catalysts, respectively. Active species, such as surface oxides, superoxide/peroxide species and Pd–C/Pt–C bonds are directly observed during the reactions. We demonstrate that in situ SHINERS can provide a deep understanding of the fundamental concepts of catalysis.

CeO 2 . Cerium (III) nitrate, oleic acid, tert-butylamine, and toluene were added to a autoclave. The autoclave was sealed and heated to 180 o C, and kept at this temperature for 24 h. After cooling to room temperature, the mixtures were centrifuged to remove the solid impurities. CeO 2 nanoparticles were then obtained by precipitation of the supernatant solution with excess ethanol. The obtained CeO 2 nanoparticles were then washed with ethanol for several times, and dispersed in toluene or hexane. Space is segmented into box-shaped cells, with electric fields located on the edges and magnetic fields located on the faces so that the electric and magnetic fields can be obtained at different positions in time. This orientation of the fields is known as a Yee cell. A standard Cartesian Yee cell used in 3D-FDTD calculations is often set as a cubic voxel, and the 3D space lattice is comprised of a multiplicity of such Yee cells.
In order to obtain an accurate field distribution for a 3D object, the Yee cell size (Δs×Δs×Δs, where Δs is the side-length of each cell), which is the most important constraint in any 3D-FDTD simulation, must be much less than the excitation wavelength (λ).
Δs ≤ λ/12 (1) In this work, the simulations were conducted using the commercially available Lumerical Solutions software (version 7.5). Two close-packed SHINERS-satellite nanocomposites with 120 nm Au cores, 2 nm silica shells and 2 nm Pt nanocatalysts were modelled. In order to ensure calculation convergence and accuracy, the simulation time and Yee cell size were set at 1000 fs and 0.5 nm respectively.
DFT calculation method. Spin-unpolarized calculations were carried out at the level of RPBE 9 using the Vienna ab initio simulation package (VASP 5.3.5) 10,11 . It has been well documented that the commonly used density functionals, such as LDA, PBE, PW91 and HSE, overestimated the adsorption energies of CO on metal surface [12][13][14] .
RPBE functional was specially designed to remedy the notorious problem. Gajdos et al. 13   The minimum energy reaction pathways were calculated using the nudged elastic band method. The final transition state structures were refined using a quasi-Newton algorithm until the Hellman-Feynman forces on each ion were lower than 0.03 eV/Å.
The adsorption energies (ΔE ads ) were calculated using equation 2, in which E ad/sub , E ad , and E sub were the total energies of the optimized adsorbate/substrate system, the adsorbate in the gas phase, and the clean substrate respectively.     Fig. 13a). This indicates that CO desorbs from the catalyst surface and CO coverage decreases as temperature increases. If the feed is changed to pure O 2 after initial adsorption of CO, Raman bands for oxygen species appear ( Supplementary Fig. 13b). This result means that oxygen easily adsorbs on the catalyst surface if pure O 2 is present in the feed. With prolonged reaction times and increased temperatures, the intensities of the oxygen species peaks increase while the intensity of the Pt-C peak decreases ( Supplementary Fig. 13b). In-situ

SHINERS-satellite spectra of CO oxidation on Pt nanocatalysts at 30 o C show only
the Pt-C stretching band (Fig. 3c blue curve and Supplementary Fig. 13c