(319e) Ni-MgO Based NOVEL Catalysts Prepared By Combustion Synthesisfor CO2 and Methane Reforming

In this talk we present mechanistic aspects of the solution combustion synthesis (SCS) method to prepare of high surface area Ni-MgO solid solutions as active and stable catalysts for the dry reforming of methane (DRM) with CO_2, two major greenhouse gases.

In SCS, an aqueous solution containing a metal nitrate precursor (oxidant) is mixed with glycine (fuel) and dried to obtain a thick reactive gel, which is preheated until at a certain temperature the highly exothermic combustion reaction is initiated resulting in the formation of metal oxides. A modification of SCS was used by impregnating the reactive solutions into a cellulose paper, referred as paper assisted combustion synthesis (PACS).

During the synthesis, the combustion reaction exothermicity, temperature, and front propagation velocity were monitored by high-speed Infrared thermography and differential scanning calorimetry (DSC). The Ni-MgO formed solid solutions were characterized with textural (BET, SEM), structural (XRD, TPR, TEM), and surface analysis (XPS) techniques. Activity and stability of catalysts during the DRM were studied as a function of reaction temperature, time on stream (TOS), carbon deposition, and the reaction turnover frequency (TOF) was evaluated at differential methane and CO₂ conversions.

NiOMgO catalysts with 10, 20 and 30% Ni concentration were first prepared by SCS. which formed NiOMgO solid solutions with surface areas of up to 37 m²/g, which were inactive for DRM. Upon reduction at 600 ^oC, Ni atoms segregated to the surface of the solid solutions, forming crystallites containing the active sites. On 20%Ni-MgO reduced catalyst, methane conversion reached about 50% at 600 ^oC, however, there was also carbon accumulation of about 80 wt.% after about 2 hours of TOS.

Unlike to SCS, PACS synthesized Ni-MgO solid solutions exhibited much higher surface areas, partly because during combustion Mg²⁺ ions strongly catalyzed cellulose pyrolysis, creating a porous media, which acted as a template to the NiO-MgO solid solutions. The 10%NiOMgO-PACS catalyst exhibited the highest surface area of 153 m²/g compared to only 23 m²/g prepared by SCS method. At 600 ^oC, the 10%NiOMgO-PACS_R catalyst exhibited methane conversion close to equilibrium and significantly inhibited carbon deposition (~3 wt.% during 24 h TOS), thus increasing the catalyst’s stability. The higher activity and stability of the 10%Ni-MgO catalyst is attributed to the higher dispersion and smaller size of Ni crystallites formed during the PACS synthesis, and the formation of a higher amount of Ni³⁺ surface site defects (2).

1) V. Danghyan, A. Kumar, A. Mukasyan and E. E. Wolf¹, Appl. Cat. B 2020, 119-156.

2) V Danghyan, T. Orlova, S. Roslyakov, E. Wolf, A. Mukasyan. Combust. Flame 2020, 221, 462-475.