Ni–Nb-Based Mixed Oxides Precursors for the Dry Reforming of Methane

Abstract

Perovskite-related mixed-oxides based on La Ni Nb and La Sr Ni Nb were synthesized by the auto combustion method to use as precursors materials for the catalytic reforming of methane at 700 8C, atmospheric pressure, CH 4 :CO 2 = 1:1. LaNiO 3 and LaNbO 4 were used as reference. XRD analysis show that the synthesis method produce a new series of precursor family formed by a mixture of oxides where Ni crystallized as part of a perovskite and Ruddlesden-Popper structure while Nb formed lanthanum orthoniobate LaNbO 4 , a scheelite-type structure alternating with oxide layers, with phase distribution depending on niobium content. For Nb (x B 0.3) Ni crystallizes as LaNiO 3 perovskite-type oxide while for Nb (x C 0.7) it forms mainly the orthoniobate phase LaNbO 4 a scheelite-type structure. At higher calcined temperatures (*1100°C) La 2 Ni 0.8 Nb 0.2 O 4 was formed with a Ruddlesden-Popper structure consisting of three perovskite type layers along the c-axis alternating with a layer of the rock salt type phase. TEM analysis showed the presence of cubic particles with sizes varying between 5 and 60 nm depending on the extent of substitution of Ni by Nb. Reduction of the perovskite-related precursor oxides produced a series of Ni 0 /La 2 O 3 -NbOx oxides with high metallic dispersion which favors the activity and stability of the catalysts. Introduction of doping quantities of Sr into LaNi 0.8 Nb 0.2 O 3±k structure produced a mixture of oxides with Sr dissolved in the lanthanum orthoniobate LaNbO 4 scheelite-type structure due to the similarity of ionic radii of La and Sr. Under the reaction conditions conversions near the thermodynamic equilibrium were attained which remains for long periods of time assessing the stability of the synthesized catalysts.

In Situ-Determined Catalytically Active State of LaNiO3 in Methane Dry Reforming

“…In addition, perovskite-related structures, such as La 2 NiO 4 , can exhibit a high catalytic activity in DRM as well. 18,26 Sometimes referred to as a spinel phase, in fact, it represents a type of layered perovskite (a so called "Ruddlesden−Popper phase"), where the perovskite-type intergrows with the rock salt type and forms the observed crystal structure.…”

“…However, this only explains the high catalytic activity, but not the observed long-time coking stability. To understand the self-regenerating process of the entire catalyst entity, the mechanism of the supporting material La 2 O 3 upon reaction with CO 2 to La 2 O 2 CO 3 is crucial (eq )­ The in situ formation of La 2 O 2 CO 3 is the apparent key step in the self-regenerating process because the reaction of deposited carbon (via eq ) with La 2 O 2 CO 3 on the metallic Ni particles to form La 2 O 3 and CO supposedly takes place at the phase boundary (eq )­ In addition, perovskite-related structures, such as La 2 NiO 4 , can exhibit a high catalytic activity in DRM as well. , Sometimes referred to as a spinel phase, in fact, it represents a type of layered perovskite (a so called “Ruddlesden–Popper phase”), where the perovskite-type intergrows with the rock salt type and forms the observed crystal structure.…”

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

“…Hence, four doped La 2 NiO 4 samples (apart from pure La 2 NiO 4 ) were analyzed: La 1.8 Ba 0.2 NiO 4 , La 2 Ni 0.9 Cu 0.1 O 4 , La 2 Ni 0.8 Cu 0.2 O 4 , and La 1.8 Ba 0.2 Ni 0.9 Cu 0.1 O 4 . We opted to study the corresponding Ruddlesden–Popper phases in favor of LaNiO 3 because the latter, especially in doped samples, is structurally unstable toward La 2 NiO 4 upon synthesis and subsequent annealing. ,, To directly establish reactivity–structure relationships, we carried out the decomposition of the respective perovskites in the DRM reaction mixture and also analyzed the corresponding structural changes by synchrotron-based in situ X-ray diffraction. Besides the determination of structural changes of the catalysts, the in situ synchrotron XRD experiments allow us also to follow the chemical expansion , of each catalyst, being crucial for its respective catalytic behavior.…”

Elucidating the role of earth alkaline doping in perovskite-based methane dry reforming catalysts

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In recent years, several material classes have been screened for the development of promising catalysts and a better understanding of the reaction mechanism on an atomic level. [20][21][22][23][24][25] Materials include intermetallic compounds, oxides, or metal-oxide systems. Mechanistic-wise, there is consensus that a bifunctional operating mechanism for both methane and carbon dioxide activation usually prevails.…”