Methane Dry Reforming (MDR) represents an effective route for the valorization of bio-gas from biomass anerobic digestion to syngas. Metal-based catalysts are efficient for this reaction but are severely affected by carbon coking which eventually leads to catalyst deactivation. Hence, understanding the mechanism of carbon formation and its kinetic consequences in MDR is of primary importance to improve the catalytic materials and optimize the operating conditions.
In this study, we performed a combined spectroscopic-kinetic analysis by using an Operando Raman Annular reactor1. The experimental tests, carried out under nearly-isothermal conditions, were aimed at the analysis of the effect of dilution in MDR. We performed three different tests at constant space velocity, catalyst load (4% w/w of Rh on α-alumina) and CO2/CH4 ratio (2). The outlet gas composition was analyzed between 300 to 750°C, with steps of 50°C. The experimental results showed that higher reactant concentration negatively affects the activity of the system (Figure 1a).
Figure 1 – CH4 conversion during DR experiements with different dilution in the T range 300-750°C. a) Comparison between experimental data (dots) and microkinetic model predictions with fixed catalyst dispersion (solid lines). b) Comparison between experimental data (dots) and microkinetic model predictions with variable catalyst dispersion (solid lines).
The measured data were then compared with the predictions of a microkinetic model in order to relate the operando information to the mechanistic interpretation of the working catalyst2,3. In particular, the Reaction Path Analysis (RPA) showed that the mechanism consists of two different routes, one for the CH4 and the other for CO2. CH4 dehydrogenates till the formation of C ad-atoms at the catalyst surface. C atoms are then oxidized to CO by OH. CO2 decomposes to O, which is involved in the formation of OH, and CO. By performing a sensitivity analysis, the RDS of the whole process is identified as the dehydrogenation of CH3 to CH2.
The microkinetic model is able to predict with good agreement the experimental results for the test with a 4% of CH4 in the inlet stream but it only qualitatively reproduced the decrease in CH4 conversion due to the increase of reactants concentration. The analysis of the surface coverage predicted by the microkinetic model shows that an increase of reactant concentration results in a decrease of the free-active sites, thus affecting the rate of the RDS and the activity of the whole system. The mismatch between the model predictions and the experimental data appeared to be related to a phenomenon that involves the catalyst surface. To clarify this point, we performed long term MDR tests by keeping constant the T at 600°C and the CH4 inlet concentration at 8%. The CH4 conversion was monitored for more than 2 hours, showing a decreasing trend (Figure 2a). At the same time, we performed spatially resolved Raman analyses in order to monitor the catalyst surface (Figure 2b).
Figure 2 – a) CH4 conversion during DR run with CH4/CO2 = 8/16 performed at 600°C; b) Spatially Resolved Raman analysis performed during DR CH4/CO2=8/16: spectra recorded starting from the beginning (zone 1) to the end (zone 5) of the catalyst bed.
Two peaks, related to graphitic (G peak at 1580 cm-1) and disordered (D peak at 1350 cm-1) carbonaceous species, appeared in the Raman spectra. Thanks to the spatially resolved analysis, it was also evident that carbon is not uniformly present along the axis of the catalyst, but more deposits accumulated at the end of the 2 cm catalyst bed compared to the inlet, which maintained almost clean. This result suggests that formation of the stable carbonaceous species are stratified along the reactor length and they tend to accumulate towards the outlet of the reactor. CH4 is responsible for the formation of C intermediates while CO2 provides OH for converting it. However, CO2 is only weakly adsorbed at the surface and as CO2 decreases with the time on stream CH4 dehydrogenation prevails over the OH formation, which is not longer equilibrated. As a consequence, the C-species are not oxidized by OH and tend to organize at the surface. This leads to the formation and accumulation of stable C-deposits at the surface, covering the active sites and thus reducing the conversion of CH4. The nature of these stable C-deposits is confirmed by the fact that agreement between the model and experiment is recovered by imposing that the availability of catalytst free sites (Rh) decreases as a result of a coverage of the active sites by C-deposits (Figure 1b).
To further assess this mechanistic picture we considered an additional test by boosting the formation of OH species via the co-feed of a small amount of O2. In particular, the time evolution of the CH4 conversion at 600°C with CO2/CH4=1 showed a decreasing trend due to catalyst deactivation. The same test was repeated adding to the inlet stream 0.1% of O2. The decay of the conversion is substantially reduced. The model clarifies that this small amount of oxygen is converted to H2O at the very entrance of the reactor and the water produced provides an additional route for OH formation, which is independent from the concentration of CO2. Consequently, OH is not only related to the CO2 activation ot CO and O and thus it sustains the conversion of C to CO instead to C-deposits. This is further confirmed by Raman spectra, which provide no evidence of formation of stable C-deposits at the surface.
In conclusion, operando spatially-resolved Raman-spectroscopy was applied to the study of catalyst deactivation in MDR reaction. In particular, this methodology helps revealing that the deactivation of the catalyst is mainly due to the accumulation of C-deposits at the catalyst surface caused by the failure of CO2 activation route in MDR mechanism. By means of spatially-resolved Raman analyses, it was possible to observe that these C-deposits are not uniformly present along the axis of the catalyst bed, but their formation and accumulation are faveored towards the outlet. Moreover, these pieces of information has allowed us to extend the microkinetic model to the description of the relevant routes which lead to the formation of stable C-despositsat the catalyst surface.
References
1. Maghsoumi, A.; Ravanelli, A.; Consonni, F.; Nanni, F.; Lucotti, A.; Tommasini, M.; Donazzi, A.; Maestri, M. React. Chem. Eng. 2017, 2, 908-918.
2. Maestri, M.; Vlachos, D.G.; Beretta, A.; Groppi, G.; Tronconi, E. J. Catal. 2008, 259, 211-222.
3. Maestri, M.; Livio, D.; Beretta, A.; Groppi, G. Ind. Eng. Chem. Res. 2014, 53 (27), 10914–10928.