Thermally Stable TiO₂‐ and SiO₂‐Shell‐Isolated Au Nanoparticles for In Situ Plasmon‐Enhanced Raman Spectroscopy of Hydrogenation Catalysts

Abstract

Raman spectroscopy is known as a powerful technique for solid catalyst characterization as it provides vibrational fingerprints of (metal) oxides, reactants, and products. It can even become a strong surface‐sensitive technique by implementing shell‐isolated surface‐enhanced Raman spectroscopy (SHINERS). Au@TiO2 and Au@SiO2 shell‐isolated nanoparticles (SHINs) of various sizes were therefore prepared for the purpose of studying heterogeneous catalysis and the effect of metal oxide coating. Both SiO2‐ and TiO2‐SHINs are effective SHINERS substrates and thermally stable up to 400 °C. Nano‐sized Ru and Rh hydrogenation catalysts were assembled over the SHINs by wet impregnation of aqueous RuCl3 and RhCl3. The substrates were implemented to study CO adsorption and hydrogenation under in situ conditions at various temperatures to illustrate the differences between catalysts and shell materials with SHINERS. This work demonstrates the potential of SHINS for in situ characterization studies in a wide range of catalytic reactions.

In SituShell‐Isolated Nanoparticle‐Enhanced Raman Spectroscopy of Nickel‐Catalyzed Hydrogenation Reactions

“…Figure b shows the before and after SHINER spectra for the Ni(Pr)/Au@SiO 2 catalyst/SHIN systems using Ni(NO 3 ) 2 (top), Ni(acac) 2 (middle) and NiCl 2 (bottom). The fresh catalysts showed the signals of the corresponding anions and ligands, namely NO 3 stretching at around 1000 cm −1 for Ni(NO 3 ) 2 /Au@SiO 2 and Ni−Cl stretching at approximately 360 cm −1 for NiCl 2 /Au@SiO 2 . For Ni(acac) 2 /Au@SiO 2 some Raman peaks can be observed in the 1250–1600 cm −1 region that may originate from the organic anion, but they do not match assignments previously made in literature.…”

“…The SHINERS‐enhancement and quality of these SHINs was tested using Rhodamine 6G and pyridine as probe molecule, as demonstrated in Figure S3. Rhodamine 6G is a dye molecule with a high Raman cross‐section often used to probe the SERS activity of plasmonic NPs and the subsequent loss in activity upon coating them with dielectric layers . Pyridine is used to detect pinholes present within the SiO 2 layer surrounding the Au NPs .…”

“…The biggest limitation for such Shell‐Isolated Nanoparticles (SHINs) to be generally applied in the field of catalysis is their moderate thermal stability (∼450 °C). Studies are thus so far limited to noble metal catalysts, which are easily reduced to their metallic, active form, without the need for high temperatures . Some recent examples from our own group involve the hydrogenation of CO into higher hydrocarbons, including alcohols, over SHINERS‐active Rh/Au@SiO 2 and RhFe/Au@SiO 2 catalysts, as well as the hydrogenation of phenylacetylene into styrene and ethylbenzene over SHINERS‐active Pt/Au@SiO 2 …”

Plexciton for surface enhanced Raman scattering and emission

“…Among these shells with good chemical stability, semiconductor shells show excellent potential for SERS studies and has been used for solar cells and photocatalysis . Semiconductor materials, such as TiO 2 and MnO 2 , could degrade organic molecules under the irradiation of light and thus ensuring the cyclic utilization of Raman substrate .…”

Extending Surface‐Enhanced Raman Spectroscopy to Liquids Using Shell‐Isolated Plasmonic Superstructures

“…For example, the group of Tian recently studied CO oxidation over Pt‐Fe and Pd catalysts deposited on core@shell Au@SiO 2 Shell‐Isolated Nanoparticles (SHINs) . Our own group has recently reported on support effects in CO adsorption and hydrogenation over Ru and Rh catalysts assembled on Au@SiO 2 and Au@TiO 2 SHINs . However, these studies were all limited to gas‐solid phase heterogeneous catalysis, with the application of SERS and SHINERS to liquid phase heterogeneous catalysis still lacking.…”