In this study, we present a combined spectroscopic and kinetic analysis of methane Dry Reforming (MDR) reaction on Rh by using a spatially-resolved operando-Raman annular reactor. MDR runs have been carried out in isothermal conditions (400-700°C) at different CH4 concentrations and CO2/CH4 ratios. When CO2 is supplied in excess with respect to the stoichiometry of the reaction (CO2/CH4 &gt; 1), no carbonaceous deposits are detected. Instead, when an equimolar feed (CO2/CH4 = 1) is supplied to the reactor, the typical G and D peaks of graphitic and disordered carbonaceous species can be identified in the Raman spectra. This is accompanied by a simultaneous decrease of the methane conversion. At fixed CO2/CH4 ratio, the increase of CH4 concentration leads to a progressive decrease of the MDR activity with the detection of coke deposition at the catalyst surface. These operando-Raman kinetic tests are then analysed using a detailed microkinetic model, which suggests a series of dehydrogenation steps of the hydrocarbon until the formation of C-species at the surface of the catalyst. This de-hydrogenation process is accompanied by a reduction pathway for CO2 which eventually forms OH species at the surface. Subsequently, OH oxidizes the carbonaceous species, leading to CO and H2. When the co-reactant is fed in excess with respect to CH4, the CO2 activation is quasi-equilibrated and the solely kinetically relevant step is the CH4 dehydrogenation. Thus, the overall reaction rate is not influenced by CO2 concentration. At CO2/CH4 = 1, the CO2-activation pathway is no longer equilibrated. This introduces a kinetic dependence on the CO2 concentration of the reaction rate and results in a reduction of the coverage of OH-species at the surface. Consequently, this reduced concentration of OH-species is not able to efficiently remove the C-species, which then accumulate, as revealed by the appearance of the G and D peaks in the Raman spectra. This mechanistic interpretation is further confirmed by adding a small amount (0.1%) of O2 to the feed stream for CO2/CH4 = 1. The co-feeding of O2 in the system is able to provide an additional route for the formation of OH and, as a result, C-species do not accumulate, and G and D peaks are not detected in the Raman spectra. As a whole, this study clearly shows the potential of the operando-Raman annular reactor in the detailed kinetic analysis of processes involving C-species.

Kinetic analysis of methane dry reforming on Rh in operando-Raman annular reactor

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.