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

Abstract

The influence of A-and/or B-site doping of Ruddlesden−Popper perovskite materials on the crystal structure, stability, and dry reforming of methane (DRM) reactivity of specific A 2 BO 4 phases (A = La, Ba; B = Cu, Ni) has been evaluated by a combination of catalytic experiments, in situ X-ray diffraction, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and aberration-corrected electron microscopy. At room temperature, B-site doping of La 2 NiO 4 with Cu stabilizes the orthorhombic structure (Fmmm) of the perovskite, while A-site doping with Ba yields a tetragonal space group (I4/mmm). We observed the orthorhombic-to-tetragonal transformation above 170 °C for La 2 Ni 0.9 Cu 0.1 O 4 and La 2 Ni 0.8 Cu 0.2 O 4 , slightly higher than for undoped La 2 NiO 4 . Loss of oxygen in interstitial sites of the tetragonal structure causes further structure transformations for all samples before decomposition in the temperature range of 400 °C−600 °C. Controlled in situ decomposition of the parent or A/B-site doped perovskite structures in a DRM mixture (CH 4 :CO 2 = 1:1) in all cases yields an active phase consisting of exsolved nanocrystalline metallic Ni particles in contact with hexagonal La 2 O 3 and a mixture of (oxy)carbonate phases (hexagonal and monoclinic La 2 O 2 CO 3 , BaCO 3 ). Differences in the catalytic activity evolve because of (i) the in situ formation of Ni−Cu alloy phases (in a composition of >7:1 = Ni:Cu) for

How the in situ monitoring of bulk crystalline phases during catalyst activation results in a better understanding of heterogeneous catalysis

“…This phenomenon is particularly pronounced if the active phase is formed via a pathway of self-activation, e.g., as observed in the decomposition of intermetallic compounds 8 or the exsolution of metallic particles from perovskite materials. 9,10 In this Highlight article we provide an introduction to the widescale possibilities of monitoring the in situ development of crystalline (transient) species through pre-treatments and selfactivation to derive a better mechanistic understanding of heterogeneous catalysts and their constituting entities. To generalize the concept, we focus on a set of case studies spanning materials from oxide and oxy-carbides over complex oxides to intermetallic compounds in a number of important reactions, where the monitoring of crystalline species contributes to a thorough understanding.…”

“…To illustrate the importance of monitoring transient crystalline species during catalyst activation, we will focus on the behavior of the archetypical perovskite material LaNiO 3 in methane dry reforming. 9,10 Mostly due to their outstanding redox properties, perovskite materials with the general formula ABO 3 have evolved as a particularly promising catalyst class for methane dry reforming. 13 To form active phases, perovskites are used as precursor materials and are converted into active metal/oxide materials by dedicated activation treatments, either in a hydrogen atmosphere or directly in the CO 2 /CH 4 dry reforming mixture.…”

“…[18][19][20][21] Although stable intermetallic compounds are used directly as catalyst materials, they are increasingly used as catalyst precursor structures, where the active species is formed through an activation step. The concept itself is very much related to the perovskite activation 9,10 in a sense that the well-defined intermetallic compound starting structure paves the way to structurally steer the decomposition towards a metal-oxide composite with superior catalytic properties. 8 We have shown the capabilities of this approach for a number of intermetallic compounds in a variety of reactions, 22 but exemplify it for Cu 51 Zr 14 in the methanol steam reforming reaction.…”

“…To show the general validity of the concept, we group the example materials into two classes: ZrO 2 -based materials 5,8 and general metal-oxide systems. [9][10][11] The latter are studied to highlight the beneficial properties of a crystalline metaloxide phase boundary, either formed by in situ activation or by a co-precipitation approach. The respective samples are (i) a LaNiO 3 perovskite that is in situ decomposed into Ni/La 2 O 3 / La 2 O 2 CO 3 (ref.…”

“…LaNiO 3 is a highly active DRM pre-catalyst that can be transformed into a Ni/La 2 O 3 /La 2 O 2 CO 3 composite by the activation treatment. 9,10 Ex situ characterization of the spent catalyst state in fact reveals the final structure of LaNiO 3 after activation, but gives no answer on the individual steps of LaNiO 3 activation and eventual differences between hydrogen or DRM self-activation. Establishment of structure-activity correlations is therefore basically excluded.…”