Role of oxygenates and effect of operating conditions in the deactivation of a Ni supported catalyst during the steam reforming of bio-oil

Abstract

This work investigates the correlation of the reaction conditions and the reaction medium composition with the deactivation behavior of a Ni/La2O3-αAl2O3 catalyst used in the steam reforming of bio-oil.

Oxidative Steam Reforming of Raw Bio-Oil over Supported and Bulk Ni Catalysts for Hydrogen Production

“…The results in Section 2.3 prove that the total removal of coke is not enough for the recovery of the activity of the Ni catalysts used in the OSR of raw bio-oil, which should be attributed to the existence of other deactivation causes besides coke deposition, such as metal sintering or changes in the metal species. The sintering of Ni at 700 • C has been previously reported for Ni/LaAl catalyst used in the SR of bio-oil [15,61] for the commercial G90 catalysts used in the SR of biomass pyrolysis volatiles [73] and for spinel NiAl 2 O 4 catalysts prepared by different methods and used in the OSR of bio-oil [66]. Besides the changes in the metallic structure originated in the reaction step, the effect of the operating conditions in the regeneration step upon the physical-chemical properties of the catalyst (especially the metal properties) should be also considered, as they would be responsible for the activity of the regenerated catalyst.…”

“…On the one hand, the existence of an additional deactivation cause, besides coke deposition, such as Ni sintering due to the high temperature and water content in the reaction medium [61,66]; on the other hand, there could be metal loss due to the detachment during coke combustion of some Ni 0 particles located at the edges of the coke filaments. Consequently, the regeneration treatment should be able not only to remove coke but also to re-disperse the Ni particles, and moreover, the loss of Ni during the regeneration should be avoided.…”

“…For all the catalysts, two different combustion domains are observed: the main peak burns at low temperature, in the 300-450 • C range, and the position of its maximum differs depending on the nature of the support, in the order Ni/Ce < NiAl 2 O 4 < Ni/LaAl; the minority peak burns at high temperature, with a maximum near 600 • C. In the literature, the first peak is attributed to the combustion of encapsulating coke of amorphous nature and deposited on the metal sites (that catalyse the combustion reaction at a lower temperature), thus having a high impact on deactivation. The coke burning at high temperature is attributed to a more structured coke (with a high content of condensed polyaromatics) deposited on the support and, consequently, with a lower impact on deactivation [26,27,61]. The low content of coke deposited on the support for Ni/Ce catalyst could be explained by the redox properties of the CeO 2 support and its capacity for O 2 storage, which enhances the lattice oxygen exchange with O 2 in the gas phase and favours coke gasification during the reaction [62].…”

Temperature Programmed Oxidation Coupled with In Situ Techniques Reveal the Nature and Location of Coke Deposited on a Ni/La2O3‐αAl2O3 Catalyst in the Steam Reforming of Bio‐oil

“…Temperature programmed oxidation (TPO) using thermogravimetric analysis (TGA or TG) is by far the most common method used for the characterization of coke deposited on catalysts, gaining insights into the amount, nature or location of coke . The standard experimental procedure involves a linear heating program, and it allows a coke characterization based on the general criterion that fractions burning at lower temperatures have: (i) more hydrogenated nature, higher proportion of aliphatics with respect to aromatics; (ii) more oxygenated nature; (iii) higher accessibility within the catalyst porous structure; and/or (iv) higher proximity of coke to metallic sites, which catalyze coke combustion . TG‐TPO also allows the determination of coke combustion kinetics and quantification of the different coke fractions.…”

“…Additionally, other authors have observed two differentiated coke morphologies by SEM and TEM microscopy: (i) an amorphous or encapsulating coke, pointed as the responsible for a higher deactivation, since it blocks the metallic sites, and (ii) a structured or filamentous coke, whose contribution to deactivation is hardly noticeable, unless a massive filaments growth takes place and the entrance of reactants through the catalyst pores is hampered . In a previous work, Ochoa et al . studied the nature and location of the total coke deposited on a Ni/La 2 O 3 ‐αAl 2 O 3 catalyst in the steam reforming of bio‐oil at different operating conditions, using SEM and TEM microscopy, TPO, and XPS, Raman and FTIR spectroscopy.…”