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The present work extensively discusses the carbon pathways in the dry reforming of methane with carbon dioxide (DRM) at 750 °C towards syngas (CO/H2 ˜ 1) over 5 wt% Ni, 0.5 wt% Pt and their bimetallic alloy (5 wt% Ni-0.5 wt% Pt) supported on the reducible Ce0.8Pr0.2O2-δ carrier for the first time. Various transient and isotopic experiments (use of 13CO2 and 18O2) were designed and performed aiming at providing important information about the effect of insertion of Pt in the Ni/Ce0.8Pr0.2O2-δ solid (one pot wet impregnation) on the amounts (mg gcat−1) and transient rates (μmol gcat−1 s−1) of “carbon” formation via CH4/He (methane decomposition), CO/He (reverse Boudouard reaction) and the combination of the two (CH4/CO/He). Also, the effect of Pt on the amount and transient rate of “carbon” removal via the participation of support’s mobile active oxygen species during DRM was probed. Hydrogen reduction of the Ce0.8Pr0.2O2-δ support alone at 750 °C followed by transient isothermal CO2/He treatment (750 °C) probed the existence of an alternative path of CO2 activation in the presence of support’s oxygen vacant sites. The quantification of the origin of “carbon” in the DRM (CH4 vs CO2 activation route) was performed after using isotopically labelled 13CO2 in the feed gas stream. It was found that “carbon” deposition on supported Ni and Ni-Pt alloy was largely due to the CH4 activation route on the metal surface, whereas for the supported Pt, both activation routes applied. Temperature-programmed oxidation (TPO) was used to estimate the amount and reactivity of the “carbon” deposited as a function of time-on-stream in CH4/He, CO/He, CH4/CO/He and DRM reactions. It was found that even though supported bimetallic Ni-Pt provided a small drop in CH4-conversion compared to the supported monometallic Ni catalyst, a remarkable decrease in the rate of “carbon” accumulation was obtained for the former catalyst. The latter was ∼32 times lower after 12 h in DRM (20%CH4, CO2/CH4 = 1) at 750 °C. This is largely due to the reduced rate of “carbon” deposition (via CH4 decomposition) and the enhanced rate of its gasification by lattice oxygen. The supported bimetallic Ni-Pt catalyst presented overall an excellent performance and stability after 50 h in DRM at 750 °C with a low amount of accumulated “carbon” (0.38 wt%), which is considered, to our knowledge, as one of the lowest values reported so far for bimetallic Ni-based supported catalysts.

Two commercial Co-based catalysts supported on carbon-coated and uncoated γ-Al2O3 (during Co deposition) of different Co particle size (∼7 and 10 nm) and surface structure were investigated to address the effect of preparation route on important kinetic parameters of the methanation reaction (5 vol% CO, H2/CO = 2, P = 1.1 bar) by advanced transient isotopic and operando DRIFTS experiments. 13CO-SSITKA-Mass Spectrometry experiments performed at 230 °C and after 32 h on TOS, showed that the surface coverage (θCO) and mean residence time (τCO, s) of reversibly adsorbed CO-s are not influenced by the different preparation route, while θCHx, TOFCH4 (s−1) and the τCHx (s) related to the active carbonaceous species were slightly increased with increasing Co particle size (in the 7–10 nm range). On the other hand, the kinetic rates of CO conversion and CH4 production (μmol g−1 s−1) were found to increase with decreasing Co particle size. The TOFCH4,ITK (s−1) estimated on the basis of the measured active CHx-s and CO-s species was found very similar for the two Co particle sizes. The effective rate constant of hydrogenation of CHx-s (keff, s−1) after short time on stream (1 h) was found to increase with increasing Co particle size as the result of the different preparation route applied. Transient isothermal hydrogenation (TIH) experiments following the 13CO-SSITKA-MS gas switch measured the concentration of inactive CHx (Cβ) species formed after 32 h of reaction and which were found to be readily hydrogenated at 230 °C. Temperature-programmed hydrogenation (TPH) experiments estimated the concentration of other types of inactive carbonaceous species (Cγ), which were hydrogenated at elevated temperatures (250–600 °C). The amounts of Cβ and Cγ species were found to significantly increase with increasing Co particle size, and their kinetics of hydrogenation was dependent of Co particle size. These results provided evidence for the lower deactivation rate observed under industrial FTS conditions over the Co/γ-Al2O3 (carbon-coated) catalyst. Operando SSITKA-MS-DRIFTS studies followed by transient isothermal hydrogenation coupled with kinetic modeling highlighted the influence of Co particle size on the relative reactivity (keff) of two different types of adsorbed CO-s towards hydrogen. Catalyst characterization was performed by HAADF-STEM, H2-TPR and transient hydrogen chemisorption at 100 °C followed by TPD.

The carbon pathways in the dry reforming of methane with carbon dioxide (DRM) towards syngas at 750 °C have been investigated for the first time over 5 wt% Ni supported on Ti4+-doped ceria using various transient and isotopic experiments. The obtained results revealed important information about the effect of Ce1-xTixO2-δ support chemical composition (x = 0, 0.2 and 0.5) on the transient rates of “carbon” deposition via CH4/He (methane decomposition) and CO/He (Boudouard reaction), and on the rate of “carbon” removal via the participation of labile active oxygen of support under DRM reaction conditions. The alternative path of CO2 activation on the oxygen vacant sites of support was also probed by transient isothermal reduction (use of hydrogen) at 750 °C followed by transient CO2/He reaction. Quantification of the amount (mg C g−1cat) of inactive “carbon” accumulated as a function of time on stream in DRM and support chemical composition as well as of the origin of it (CH4 vs CO2 activation route), was made after conducting temperature-programmed oxidation (TPO) experiments coupled with the use of 13CO2. It was illustrated that 5 wt% Ni (dNi ∼ 20 nm) supported on Ce0.8Ti0.2O2-δ solid solution improves the H2-yield and H2/CO product gas ratio compared to 5 wt% Ni/CeO2 of similar Ni mean particle size but remarkably caused a drastic reduction in the rate of “carbon” deposition. After 12 h of DRM (20% CH4, CH4/CO2 = 1), Ni/Ce0.8Ti0.2O2-δ accumulates ∼ 270 times less “carbon” the origin of which was the CH4 activation and not the CO disproportionation route. The 5 wt% Ni/Ce0.8Ti0.2O2 catalyst developed presented high CH4 and CO2 conversions (75% and 82%), a H2-yiled of 64% and an H2/CO product gas ratio of 1.3 after 100 h of DRM at 750 °C (40%CH4/40%CO2/He GHSV = 30,000 h−1) with a relatively low amount (73 mg “C” gcat−1 or 7.3 wt%) of inactive “carbon”, which is considered to the best of our knowledge, as one of the lowest values reported in the literature for monometallic Ni supported catalysts under the examined DRM reaction conditions. 579 116 129