Redox dynamics of Ni catalysts in CO2 reforming of methane

Abstract

The influence of redox dynamics of a Ni/MgAl oxide catalyst for dry reforming of methane (DRM) at high temperature was studied to correlate structural stability with catalytic activity and coking propensity.Structural aging of the catalyst was simulated by repeated temperature-programmed reduction / oxidation (TPR/TPO) cycles. Despite a very high Ni loading of 55.4 wt.-%, small Ni nanoparticles of 11 nm were obtained from a hydrotalcite-like precursor with a homogeneous distribution. Redox cycling gradually changed the interaction of the active Ni phase with the oxide support resulting in a crystalline Ni/MgAl 2 O 4 -type catalyst. After cycling the average particle size increased from 11 to 21 nm -while still a large fraction of small particles was present -bringing about a decrease in Ni surface area of 72%. Interestingly, the redox dynamics and its strong structural and chemical consequences were found to have only a moderate influence on the activity in DRM at 900 °C, but lead to a stable attenuation of carbon formation due to a lower fraction of graphitic carbon after DRM in a fixed-bed reactor. Supplementary DRM experiments in a thermobalance revealed that coke formation as a continuous process until a carbon limit is reached and confirmed a higher coking rate for the cycled catalyst.

A Short Review on the Catalytic Activity of Hydrotalcite-Derived Materials for Dry Reforming of Methane

“…These results are in good agreement with those presented by other authors [58,64,65] testing HT-derived materials with low nickel content. On the contrary, the materials with high content of Ni in brucite-like layers were reported to promote CH 4 decomposition [54,[66][67][68]. Therefore, the catalytic properties of hydrotalcite-derived materials (activity, stability, promotion of side reactions) are dependent on the materials composition i.e., nickel content, Ni/Mg and Mg/Al molar ratio.…”

“…The best catalytic performance was observed for the catalyst containing 19% Ni and calcined at 650 • C. The calcination temperature influenced the amount of carbon formed during reaction, i.e., it decreased with increasing calcination temperature. Mette et al [67,68] investigated performance of Ni/Mg/Al hydrotalcite-derived catalysts with high Ni loading (55 wt %) calcined and reduced in various conditions. Calcination at 800 and 1000 • C resulted in the formation of spinel phases, inactive in DRM.…”

“…Higher calcination temperature resulted in the increased stability [56] NiMgAl HT/CP 500, 600, 700, 800 • C for 6 h No significant effect of calcination temperature on performance in DRM was observed [76] NiMgAl HT/CP 350, 600, 800, 1000 • C Calcination at 800 and 1000 • C resulted in formation of spinel phase. The optimal calcination temperature was selected to be 600 • C [67,68] Thouahra et al [61] examined the influence of the calcination temperature on the performance of Ni/Al catalysts. They concluded that the use of calcination temperatures below 500 • C did not practically affect the catalytic activity.…”

Evaluation of nickel and copper catalysts in biogas reforming for hydrogen production in SOFC

“…According to several works [26,39,45,48], various processes may occur at this temperature range such as the reduction of small nickel oxide particles in strong interaction with the support, nickel oxide containing aluminum species in the lattice or nickel aluminate. It can be also noted that the high temperature peak was shifted to higher temperatures as the calcination temperature was increased, indicating an increase of interaction of nickel with the support, as found in a previous work [49]. The amount of hydrogen consumed during the TPR experiments are shown in Table 2, as well as the degree of reduction of the solid, calculated from this consumption.…”

Coprecipitation: An excellent tool for the synthesis of supported metal catalysts – From the understanding of the well known recipes to new materials

“…Due to this highly stable microstructure, whose origin is the precursor-induced embedment of the Ni particles in the ceramic matrix, the catalyst can be employed at a reaction temperature of 900 °C and it showed a remarkable stability over 100 hours [18]. It was shown recently that the catalyst loses its beneficial microstructure inherited from the LDH precursor upon repeated TPR-O cycles leading to an increase in Ni particle size to 21 nm after 21 cycles [38]. These experiments show the general limit of the precursor approaches.…”