Deactivation of Ni/γ-Al2O3 Catalysts in CO Methanation: Effect of Zr, Mg, Ba and Ca Oxide Promoters

Abstract

reserves [1,2]. The SNG production process involves two steps: gasification of the carbon-containing feedstock and subsequent catalytic methanation of the resulting synthesis gas. Methanation of synthesis gas involves the two following reactions [3][4][5]:These reactions are thermodynamically favored at low temperature and high pressure [6]. It is therefore that the methanation process is designed to effectively remove the heat of reaction and thus maximize the methane yield [1,7]. There are currently two main methanation concepts: single fluidized bed reactors and series of adiabatic fixed bed reactors with intercooling [1]. The latter, also known as high temperature methanation, has already found commercial application [8,9].Alumina-supported nickel catalysts are usually employed in this application due to their high activity, selectivity to methane and relatively low price [7]. However, their stability is threatened by the severe conditions of the high temperature methanation process [10,11]. In this process, the catalyst at the inlet of the reactor is exposed to low temperatures and high CO partial pressures, fact that favors the formation of polymeric carbon and sintering via nickel carbonyl formation [4,[12][13][14]. The exothermic reactions cause a significant temperature rise. As a result, the major part of the catalyst in these reactors is exposed to high temperatures and high steam partial pressures fact that promotes sintering of the nickel nanoparticles and the support [11,15]. In order to limit the temperature rise and thus, catalyst deactivation, a high gas recycle is used in these reactors. Catalysts with improved stability at low andAbstract Catalyst deactivation is one of the major concerns in the production of substitute natural gas (SNG) via CO methanation. Catalysts in this application need to be active at low temperatures, resistant to polymeric carbon formation and stable at high temperatures and steam partial pressures. In the present work, a series of aluminasupported nickel catalysts promoted with Zr, Mg, Ba or Ca oxides were investigated. The catalysts were tested under low temperature CO methanation conditions in order to evaluate their resistance to carbon formation. The catalysts were also exposed to accelerated ageing conditions at high temperatures in order to study their thermal stability. The aged catalysts lost most of their activity mainly due to sintering of the support and the nickel crystallites. Apparently, none of these promoters had a satisfactory effect on the thermal resistance of the catalyst. Nevertheless, it was found that the presence of Zr can reduce the rate of polymeric carbon formation.

Mechanistic and multiscale aspects of thermo-catalytic CO2conversion to C1products

“…222 However, rapid coke deposition on the catalyst surfaces during the reaction limits the practical use of Ni-based catalysts. 217,223 To cope with such challenges, noble metals like Rh Ru, Pt, Ir, and Pd were also investigated, because of their higher coking resistance. [224][225][226] Good catalyst performances and resistance to deactivation by coking were showed by supported bimetallic catalysts such as Ni-Rh/LaAlO 3 , 227 Ni-Cu/MgO, 228 and Ni-Mn/Al 2 O 3 .…”

“…A linear correlation was observed between the amount of carbon formed and the degree of deactivation of the catalyst. 216 Barrientos et al 217 investigated various Ni/Al 2 O 3 catalysts promoted with MgO, CaO, BaO, and ZrO 2 under low-temperature conditions (300 °C), and they showed that the presence of Zn lowers the formation of polymeric carbon. Galhardo et al , 92 with in situ spectroscopic analyses, observed that the accumulation of carbon species on the surface of a Ni/SiO 2 catalyst at high temperatures leads to a Ni 3 C-like phase, which changes the process selectivity towards the formation of CO. After carbon depletion from the surface of the Ni particles by oxidation, the catalyst regains its high selectivity to CH 4 production.…”

“… 66,223 It is an inevitable process that can lead to a temporary or permanent catalyst deactivation via pore blockage, 320,321 metal particle encapsulation, 322 or breakage of the catalyst pellets. 217 In the processes reviewed in this work, carbon is primarily produced by CO disproportionation (2CO → C* + CO 2 ) and CH 4 dehydrogenation (CH 4 → C* + 2H 2 ). The latter reaction is favored at high temperature, while CO disproportionation is exothermic and thus thermodynamically favored at a temperature lower than 500 °C.…”

Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses

“…Dopants are typically metal oxides such as Zr, Ce, La, V and Mg. The addition of zirconia to catalyst materials reduces the carbon formation and the activity deactivation due to sintering [570]. Ceria remarkably prevents the formation of coke deposits and improves the catalytic activity and stability [571].…”

Catalytic CO Methanation over Mesoporous ZSM5 with Different Metal Promoters

“…They found that the dissociation of CO2 led to minor formation of RhOx is the reason for the enhanced activity in Rh/Al2O3 catalyst. But, it is noteworthy that the existing metal promoters still suffered from deactivation because of low surface area of support material and low dispersion of loaded metal, which can be avoidable by choosing a suitable supported metal material [20].…”