Speaker
Description
The layered double hydroxides (LDH) represent a broad class of inorganic lamellar compounds with a high versatility in composition and large range of technological applications1. We have studied the Ni-Cu-Al LDH where the charge of the trivalent ions (Al$^{3+}$) is compensated by carbonate anions in the interlayer space$^{2}$. Within the coprecipitation method the effects of different synthesis parameters such as aging time, washing methods and temperature have been investigated. The formation of 2-dimentional stacked layers has been confirmed with TEM microscopy. Moreover, we have studied the activation procedures necessary for a catalytic application: a calcination procedure followed by a reduction step (in 5% H$_{2}$) to obtain metallic Cu/Ni nanoparticles finely and uniformly dispersed over alumina substrate. The particle size ranged from ca. 3 nm, 3.5 nm, to 10 nm as function of increasing reduction temperature from 300 °C, 450 °C to 900 °C, respectively. The STEM-EDX mapping results show a homogenous distribution of Ni and Cu nanosized particles. It has been found that Ni reduction temperature is lowered by increasing content of Cu, that could significantly promote Ni reducibility probably due to the hydrogen spillover effect. Moreover, a lower calcination temperature (e.g., 290 °C) also promotes the Ni reducibility. Furthermore, a deep XRD analysis illustrates that the observed lattice parameters match with the expected values obtained between Ni and Cu reference values (i.e., assuming Vegard type behavior). The interaction between Cu and Ni is also visible in the near edge X-ray absorption fine structure (NEXAFS) spectra, where significant changes are observed in Cu L-edge and smaller change in Ni L-edge. The NEXAFS results show that the Cu L-edge is affected, both white line position and line shape, by presence of nickel.
Activated samples with different metal content (Ni$_{0.7}$Al$_{0.3}$, Ni$_{0.56}$Cu$_{0.14}$Al$_{0.3}$, Ni$_{0.35}$Cu$_{o.35}$Al$_{0.3}$) were tested for the reverse water gas shift (rWGS) reaction. Our catalytic results show that the material rich in Ni-content possesses the lowest for CO yield, but good stability characterized by an overgrowth around the particles that prevents the catalyst to sinter during catalysis. Both samples with 4:1 and 1:1 metal ratio, Ni$_{0.56}$Cu$_{0.14}$Al$_{0.3}$ and Ni$_{0.35}$Cu$_{0.35}$Al$_{0.3}$, present a higher yield, but only the material with Ni/Cu ratio of 4 features also a remarkable stability. The STEM-EDX results corroborated that the low stability observed for Ni$_{0.35}$Cu$_{0.35}$Al$_{0.3}$ sample is due to a Cu segregation.
References
1. Bukhtiyarova, M. V. , J. Solid State Chem. 269, 494–506 (2019).
2. De Roy, Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. A. Mol. Cryst. Liq. Cryst. 311, 173–193 (1998).
| Abstract Number (department-wise) | AC 4.2 |
|---|---|
| Department | AC (Schlögl) |
