Selective in-situ exsolution of catalytically active metals from doped perovskites during methane dry reforming
Florian Schrenk, Lorenz Lindenthal, Hedda Drexler, Raffael Rameshan, Christoph Rameshan
Institute of Materials Chemistry, Technische Universität Wien, Austria
Perovskite type oxides, doped with catalytically active metals, have shown great potential to catalyse MDR (Methane Dry Reforming) . These materials can undergo exsolution, the formation of metallic nanoparticles on the surface, of B-site elements if reducing conditions are applied . These exsolved nanoparticles are leading to an increase in catalytic activity. A Ni-doped perovskite (Nd0.6Ca0.4Fe0.97Ni0.03O3) and a Co-doped perovskite (Nd0.6Ca0.4Fe0.9Co0.1O3) were investigated in MDR conditions with a CH4 excess of 2:1 over CO2. To investigate the changes the catalyst undergoes during the reaction in situ NAP XPS (Near Ambient Pressure X-ray Photoelectron Spectroscopy) as well as in situ XRD (X-Ray Diffraction) experiments were performed. The XRD data showed the formation of a bcc phase at higher temperatures which cannot be assigned to Fe or the dopant (Ni/Co) due to the bad signal to noise ratio (Figure 1). The XPS data reveals that Ni is exsolving before Fe, as the Ni2p signal is shifting from the oxidized state into a metallic state between 600 °C and 650 °C and the Fe2p signal does not show a metallic component even at 700 °C, as seen in Figure 2. For the Co-doped catalyst, the formation of a metallic Co component was observed while no metallic Fe was detected.
Figure 1: in-situ XRD diffractograms of the Ni-doped catalyst with a CH4:CO2 ratio of 2:1 and temperature steps from 500 °C to 700 °C. The phases observed could be assigned to a perovskite phase (blue) as well as a brownmillerite phase with ordered oxygen vacancies (grey) during all temperatures. Forming at higher temperatures a CaCO3 (violet) and a bcc Phase, stemming ether from Ni or Fe (green), could be detected.
Figure 2: Comparison of the Fe2p and Ni2p transitions in the in-situ NAP XPS experiment of the Ni-doped catalyst. The sample was oxidized at 1 mbar O2 before the reaction, afterwards an atmosphere of 1 mbar CH4 and CO2 in a ratio of 2:1 was set and temperature steps between 500 °C and 700 °C were performed. A metallic component (blue) is forming between 600°C and 650 °C for Ni, whereas the Fe transition does not so such a component even at 700 °C.
 D. Pakhare and J. Spivey, "A review of dry (CO2) reforming of methane over noble metal catalysts," Chemical Society Reviews, vol. 43, no. 22, pp. 7813-7837, Nov 2014, doi: 10.1039/c3cs60395d.
 L. Lindenthal et al., "Modifying the Surface Structure of Perovskite-Based Catalysts by Nanoparticle Exsolution," Catalysts, vol. 10, no. 3, Mar 2020, Art no. 268, doi: 10.3390/catal10030268.