In-operando elucidation of bimetallic CoNi nanoparticles during high-temperature CH 4 /CO 2 reaction
- Citation data:
Applied Catalysis B: Environmental, ISSN: 0926-3373, Vol: 213, Page: 177-189
- Publication Year:
- Chemical Engineering; Environmental Science; Dry Reforming of Methane; Nickel; Cobalt; Bimetal; Carbon Deposition; Kinetics; Density Functional Theory; In-operando X-ray Absorption Spectroscopy
Dry reforming of methane (DRM) proceeds via CH 4 decomposition to leave surface carbon species, followed by their removal with CO 2 -derived species. Reactivity tuning for stoichiometric CH 4 /CO 2 reactants was attempted by alloying the non-noble metals Co and Ni, which have high affinity with CO 2 and high activity for CH 4 decomposition, respectively. This study was focused on providing evidence of the capturing surface coverage of the reactive intermediates and the associated structural changes of the metals during DRM at high temperature using in-operando X-ray absorption spectroscopy (XAS). On the Co catalysts, the first-order effects with respect to CH 4 pressure and negative-order effects with respect to CO 2 pressure on the DRM rate are consistent with the competitive adsorption of the surface oxygen species on the same sites as the CH 4 decomposition reaction. The Ni surface provides comparatively higher rates of CH 4 decomposition and the resultant DRM than the Co catalyst but leaves some deposited carbon on the catalyst surface. In contrast, the bimetallic CoNi catalyst exhibits reactivity towards the DRM but with kinetic orders resembling Co catalyst, producing negligible carbon deposition by balancing CH 4 and CO 2 activation. The in-operando X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements confirmed that the Co catalyst was progressively oxidized from the surface to the bulk with reaction time, whereas CoNi and Ni remained relatively reduced during DRM. Density functional theory (DFT) calculation considering the high reaction temperature for DRM confirmed the unselective site arrangement between Co and Ni atoms in both the surface and bulk of the alloy nanoparticle (NP). The calculated heat of oxygen chemisorption became more exothermic in the order of Ni, CoNi, Co, consistent with the catalytic behavior. The comprehensive experimental and theoretical evidence provided herein clearly suggests improvement to the catalyst design protocol by selecting the appropriate composition of Co-Ni alloy.