The deactivation of the Ni/La2O3–aAl2O3 catalyst in the SRE process<br>is a consequence of the coke formation rate and its nature,<br>which depend on the concentration of reaction medium components.<br>Moreover, coke evolution causes a change in Ni morphology,<br>which contributes to modifying catalyst activity. To understand the<br>deactivation, three stages have been identified, whose duration<br>depends on the operating conditions: (i) almost steady performance,<br>followed by (ii) a severe drop in ethanol conversion and<br>finally (iii) a steady and slow decrease in ethanol conversion until<br>total deactivation of the catalyst. The different deactivation rate at each step is explained by the formation of coke with different natures<br>(filamentous and nonfilamentous) and the different role of<br>these cokes in the blockage of Ni active sites.<br>The combination of diverse techniques for coke analysis has<br>allowed determining that progressive growth of filamentous and<br>nonfilamentous coke takes place in these stages. In the first reaction<br>stage the highest filamentous coke yield is reached. This coke<br>drags Ni particles during its growth, which are located at the end of<br>the fibers and separated from Ni crystallites, thus causing the<br>decrease in Ni particle size, but without blocking the sites, and<br>therefore without causing significant effect on deactivation. In<br>the second stage, both the dragging and the decrease in the size<br>of Ni particles continue. However, the change in the coke formation<br>mechanism, and therefore the change in its nature, plays an essential<br>role in the deactivation. In this second stage the yield of filamentous<br>coke decreases, whose structure becomes progressively<br>more condensed and graphitic, while the formation of nonfilamentous<br>coke becomes significant, which is the primary cause of deactivation<br>under these conditions by Ni sites blockage. In the third<br>stage, corresponding to a highly deactivated catalyst and a high<br>ethanol concentration in the medium, the formation of coke is very<br>slow and is mainly of a nonfilamentous nature, whose structure<br>evolves toward a high graphitization degree, while the progressive<br>blocking of Ni sites continues.<br>The coke formation rate and the nature of coke depend on the<br>concentration of reaction medium components. Thus, operating<br>conditions with low ethanol concentration (high conversion and<br>high concentration of CO and CH4) lead to a high formation rate<br>of coke with a filamentous nature. Operating with high ethanol<br>concentrations leads to a low coke formation rate, but the coke<br>formed under these conditions is nonfilamentous.
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