Surface Carbon as a Reactive Intermediate in Dry Reforming of Methane to Syngas on a 5% Ni/MnO Catalyst

Abstract

A 5% Ni/MnO catalyst has been tested for the dry reforming of methane at different temperatures and reactant partial pressures. Changing the reactant ratio with time on stream results in a decrease in the deactivation rate of the catalyst. Graphitic carbon growth and metal particle sintering have been observed by applying in situ transmission XRD using synchrotron radiation under actual reaction conditions. Both methane and carbon monoxide separately result in graphitic surface carbon, which can then be oxidized by carbon dioxide. The morphology of the surface carbon has been analyzed by TEM, and the reactions of both methane and carbon monoxide result in the same graphitic multiwalled carbon nanotubes. The present combination of catalytic experiments and in situ techniques suggests that surface carbon acts as an intermediate in the formation of CO and that catalyst deactivation happens via metallic particle sintering. These results enable a more rational choice of reaction conditions to ensure high catalyst activity and long-term stability. Future catalyst advances must aim to prevent metal particle sintering.

Steering the Methane Dry Reforming Reactivity of Ni/La2O3 Catalysts by Controlled In Situ Decomposition of Doped La2NiO4 Precursor Structures

“…Surface carbon formation does not necessarily deactivate the catalyst and, for some systems, is a necessary intermediate of reaction. 27 , 63 Previously shown in situ XRD data (cf. Figures 6 , 7 , and S6–S8 ) and microscopy data ( Figures 3 – 5 and S1–S4 ) also support the claim of the absence of graphitic carbon formation under DRM conditions.…”

Carbon Nanotube Formation on Cr-Doped Ferrite Catalyst during Water Gas Shift Membrane Reaction: Mechanistic Implications and Extended Studies on Dry Gas Conversions

“…The ability of CO/CH 4 mixture to form CNTs with faster kinetic rates and higher conversions than the single gas feeds was previously observed in the literature and attributed to reactions (2) and (3) but at much higher reaction temperatures (>700 • C) [28]. The reaction Gibbs free energy (∆G r ) of the exothermic CO disproportionation increases and that of the CH 4 decomposition decreases with increasing temperature with ∆G r = − 28 kJ/mol for the former and ∆G r = + 12.4 kJ/mol for the latter at 500 • C [29]. Thus, lowering reaction temperature increases the equilibrium conversion of CO and decreases that of CH 4 , which may be the reason for the CO conversion being much higher than the CH 4 conversion in the reaction of CO/CH 4 mixture at low temperature (500 • C) and low feed flowrates.…”

“…Thus, lowering reaction temperature increases the equilibrium conversion of CO and decreases that of CH 4 , which may be the reason for the CO conversion being much higher than the CH 4 conversion in the reaction of CO/CH 4 mixture at low temperature (500 • C) and low feed flowrates. Carbon deposits from nonoxidative CH 4 dehydrogenation and/or CO disproportionation on oxide-supported metal catalysts can occur at relatively low temperatures to act as intermediates for both CO formation through carbon oxidation by CO 2 and re-hydrogenation of surface carbon in dry gas reforming of CO and CH 4 [19,20,29]. Because of the reactions' reversibility and common intermediates and products of these nonoxidative reactions, the reaction rates and carbon product morphological properties are expected to depend on mutual influences between CO and CH 4 .…”

Kinetic Assessment of the Dry Reforming of Methane over a Solid Solution Ni–La Oxide Catalyst

“…These mechanisms were based on both the data collected herein and on the extensive literature on this topic. 4,5,10,11,17,19,25,35,36,71,73,74,[76][77][78][79][80] In this section, the kinetics of the process is analyzed under the light of the model that best fitted the experimental data and fulfilled the thermodynamic consistency criteria, Table 2. The competing models are comprehensively explained in the Supplementary Information, section E, and summarized in Table S8.…”

“…The latter two reactions are the sources of the chemisorbed carbon adatoms required for the diffusion and growth of carbon nanotubes. 5,6 As shown in Figure 1, thermodynamically, dry reforming is promoted by a temperature increase and it prevails over its competing reactions at temperatures higher than 923K. However, the competing reactions are significant up to about 1173K, 7,8 with which the formation of water and carbon are virtually inevitable at the typical operational temperatures (873-1073K).…”