Speaker
Description
To intensify long-term renewable energy storage and conversion, within the framework of CatLab, we propose to develop novel thin film catalysts for hydrogenation and dehydrogenation processes. Catalytic reactions usually take place on catalyst surface and subsurface. Specific thin film design simplify structure to explore catalytic interface evolution and furthermore guide functional interface design. As a first group of catalysts in Catlab, thin Pd films with thickness from 1, 3, to 11 nm were prepared on top of a Si wafer with a buffer layer of SiO$_{2}$, ZnO or SiNx in between. The catalytic activity of these thin films was measured for C2H2 hydrogenation at both lab-scale reactor and operando X-ray photoelectron spectroscopy. CO adsorption was performed using microcalorimetry and PM-IRRAS to identify the amount of accessible Pd. In addition, morphology structure of the films was analyzed by scanning/transmission electron microscopy (S/TEM). While we observed a high activity of thin film Pd with 20 or 200 nm SiO$_{2}$ buffer (Si-SiO$_{2}$-Pd), thin film Pd without any buffer shows almost no activity (Si-Pd) because of the clear Pd diffusion (TEM) and palladium silicide formation at the interface (XPS) (T=150 °C, C$_{2}$H$_{2}$:H$_{2}$ = 1:30). For example, the catalytic conversion of Si-SiO220nm -Pd3nm could initially reach 98% followed by a slow deactivation to 81.6 % after running over 20 hours, and selectivity could stable at around 80%. The optimal thickness for Pd is about 3 nm and for SiO2 buffer 20 nm, with better activity than for 200 nm, while both buffers significantly prevent silicide formation. Results show only few differences between SiO$_{2}$ and SiNx, while ZnO slightly promotes activity. TEM and SEM observations show that the Pd deposition on Si substrates with an amorphous SiO2 buffer layer leads to a polycrystalline thin film composed of nonhomogeneous distributed nanoparticles. TEM studies of spent Si-SiO$_{2}$-Pd reveal that the thickness of the particle-like film increases suggesting a reconstruction of the isolated entities. As a comparison, Pd foil and powder catalysts were tested catalytically under the same conditions. When normalized by Pd amount, the thin film catalyst has a rather higher catalytic efficiency by a factor of 50, which suggests that a thin layer (3nm) of Pd is sufficient for catalytic conversion of C$_{2}$H$_{2}$. However, thin film Pd is unstable and deactivates during catalysis. Furthermore, CO adsorption shows that the Pd film is composed of energetically and structurally slightly different Pd sites. Having the number of accessible Pd, we calculated the turnover frequency of thin film Pd (0.241 s$^{-1}$. $^{2}$).
| Abstract Number (department-wise) | AC 4.1 |
|---|---|
| Department | AC (Schlögl) |
