Why are nanoparticles important in heterogeneous catalysis?

By Wilbert Vrijburg, 6 minutes to read
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How the VSParticle spark ablation technology can help with heterogeneous catalysis

Developing the mainstream heterogeneous catalysts of tomorrow requires a fundamental understanding of today’s state-of-the-art catalysts. Today’s catalysts are generally composed of metallic nanoparticles (e.g. Pt, Ru, Ni, Co) deposited on an oxide support (e.g. Al2O3, TiO2, SiO2). For any catalytic reaction, the metal nanoparticle’s size and shape can have a profound impact on the catalytic performance of the overall catalyst.1 Generally, smaller nanoparticles will have a higher fraction of surface metal atoms which generally leads to higher catalytic activity. However, below a certain threshold size, nanoparticles may no longer accommodate the most active sites for specific catalytic reactions.

To unravel the specific features of a metallic nanoparticle that direct its catalytic performance, extensive (in situ/operando) characterization methods are employed. These methods, which include modern electron microscopy (examples include: in situ HR-TEM, HAADF-STEM, STEM-EDX, STEM-EELS)2,3 and spectroscopy (examples: in situ FTIR, XAS, etc.) techniques,4 require versatile and reproducible synthesis procedures to obtain model nanoparticles for catalytic studies.

For such fundamental studies, conventional heterogeneous catalyst synthesis methods (e.g. impregnation, coprecipitation, etc.) are often not suitable to prepare model catalysts required for advanced microscopy or spectroscopy measurements. More advanced approaches such as colloidal nanoparticle synthesis routes have cumbersome and time-consuming synthesis procedures. Furthermore, stabilizing ligands necessary for colloidal nanoparticles can severely impact the behaviour and catalytic activity of metal nanoparticles and must therefore be removed.5 This is usually achieved via a thermal treatment at high temperatures (> 500 °C) which can cause the nanoparticles to sinter or damage the substrate on which the nanoparticles are deposited. Versatile methods to produce model unsupported and supported catalysts are therefore vital to improve our understanding of catalytic systems and develop future catalysts.

Experimental setup of the VSP-G Nanoparticle Generator

Figure 1: Experimental setup of the VSP-G1 Nanoparticle Generator equipped with the VSP-A1 Diffusion Accessory.

Advantages of VSParticle’s spark ablation nanoparticle synthesis method

VSParticle’s spark ablation technology offers a versatile method to rapidly produce nanoparticles for today’s catalysis researchers. The VSP-G1 Nanoparticle Generator (VSP-G1) is able to produce tunable metallic nanoparticles at the push of a button. These surfactant-free nanoparticles can be deposited directly onto a TEM grid or 2D substrate using the VSP-A1 Diffusion Accessory (VSP-A1, Figure 1). This approach was directly implemented to prepare unsupported Au nanoparticles (AuNPs) on a TEM grid (Figure 2). These highly crystalline AuNPs are free of stabilizing ligands/surfactants that can influence the nanoparticle shape, making these nanoparticles ideal for in situ TEM studies. In addition, by equipping the VSP-G1 with pre-alloyed electrodes, bimetallic/multi-metallic nanoparticles can be produced and studied to unravel their unique catalytic properties.

Image of unsupported Au nanoparticles deposited directly on a TEM grid

Figure 2: HAADF-STEM image of unsupported Au nanoparticles deposited directly on a TEM grid using the VSP-G1 equipped with VSP-A1 (courtesy of Prof. Günther Rupprechter).

Clean nanoparticles ready for advanced catalyst characterization

Recent advances in many spectroscopic techniques have ensured that today’s highest impact catalysis researchers have embraced in situ/operando spectroscopy in their works. For example, vibrational techniques such as Raman spectroscopy can provide insights into both the surface and gas-phase species that are participating in the catalytic reaction. In order to enhance the Raman signal and effectively study these reactions, Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) employs catalytically active nanoparticles (e.g. Ni, Pt, Rh) deposited on SiO2 encapsulated Au nanoparticles (Au@SiO2) for in situ studies. The Shell-Isolated Nanoparticles (SHINS) have a low thermal stability (ca. 450 °C), therefore SHINERS studies have typically been limited to studying precious metal catalysts (e.g. Rh, Pt, Pd) that can be reduced at low temperatures to form metallic nanoparticles on the Au@SiO2. However, industrially relevant metals such as Ni are inactive for hydrogenation reactions unless a high temperature reduction step (ca. 500 °C) precedes the catalytic reaction. Methods to produce SHINERS active catalysts with industrially relevant metals (e.g. Ni, Co, Cu, Fe) are therefore highly desired.

Figure 3: TEM images of (a) Au@SiO2, on which Ni nanoparticles were deposited via (b) Ni-precursor impregnation, (c) colloidal deposition, and (d) spark ablation (SEM). Those prepared through spark ablation were the only materials that were active for in situ SHINERS measurements (reproduced from 6).

Industrially relevant nickel catalysts made easy with VSParticle Spark Ablation Technology

A recent publication by Wondergem et al. demonstrated how clean Ni nanoparticles were deposited on Au@SiO2 SHINS via spark ablation. Compared with more conventional synthesis approaches such as impregnation and colloidal Ni nanoparticle deposition, spark ablation provided SHINERS-active catalysts.6 Importantly, these conventional wet synthesis methods required harsh thermal conditions to remove contaminants and activate the Ni-based catalyst. Ultimately such conditions destroyed the SHINS and were avoided by preparing Ni/Au@SiO2 via spark ablation. The spark ablation-prepared Ni/Au@SiO2 catalysts were employed for in situ Raman spectroscopy studies of industrially relevant hydrogenation reactions. The work by Wondergem et al. showcased how low transition metal catalysts can now by studied using SHINERS – a technique that was initially limited to precious metal-based catalysts. In Figure 4 below, a spectra of Ni catalysts prepared via spark ablation using the VSP-G1 and VSP-A1 demonstrates this result. The spectra was obtained under acetylene hydrogenation conditions and highlight (a) the presence/absence of adsorbed surface species after exposure to the reactant gases, (b) the formation of the gas-phase product during the in situ reaction. These results allowed the authors to (c) determine which molecular species were present on the Ni surface during the Ni-catalyzed hydrogenation of acetylene (reproduced from 6).

Spectra of supported Ni catalysts prepared via spark ablation

Figure 4: In situ SHINERS spectra of supported Ni catalysts prepared via spark ablation using the VSP-G1 and VSP-A1.