The Effect of SrCrO4 Impurity Phase in (La​0.75​Sr0.25​)​0.95​Cr​X​Fe​1-X​O​3-Δ (LSCrF) Nano-Ceramic Powder Series As Catalyst for the Propane Dehydrogenation
Authors: Kucuk Kamil 
Published: February 2020

Lanthanum strontium chromite based materials are promising candidate for use as electrode components in Intermediate Temperature Proton Conductive Fuel Cell (IT-PCFC)[1]. In this work, (La0.75Sr0.25)0.95CrxFe1-xO3-δ (LSCrF) with x=0.3, 0.4, 0.5, 0.6 and 0.7 perovskite nano-ceramic powder series considered as anode material for IT-PCFCs[2] were focused on due to its structural, physical & chemical and also electrochemical properties corresponding with Cr amount, such as catalytic activities under the operation conditions. The nano-sized LSCrF perovskite powders were systematically synthesized by using sol-gel method[3], to figure out whether the increment of Cr amount have any positive or negative effect in terms of catalytic performance, impurity, particle size and surface area. In this regard, all powders were annealed at the different temperatures changing from 500°C to 700°C in order to determine the minimum formation temperature of the powders having different amounts of Cr and Fe in order to enhance smaller particle size with higher surface area.
LSCrF nano-powders were structurally characterized by means of X-ray Powder Diffraction (XRD) with Reitveld refinement by using GSAS software[3]. Cr rich samples shows more SrCrO4 impurity phase and bigger crystallite size than Fe-rich samples which leads to poor catalytic performance in the propane dehydrogenation process. Ex-situ X-ray Absorption Spectroscopy study was also performed for Fe (7112eV) and Cr (5989eV) K-edge in the fluorescence mode by using a 4-elements Vortex detector. When we looked at the XANES region of both Fe and Cr K edge XAS data to understand the electronic structure of Fe and Cr in the LSCrF powders, Cr K edge XAS data shows a sharp pre-edge peak resulting from SrCrO4 impurity phase having tetrahedral geometry of Cr+6 with 4-fold Oxygen atoms, whereas Fe K-edge XAS data does not show any pre-edge feature due to the octahedral geometry of Fe+3 in LSCrF perovskite structure, as we expect. the C3H8 conversion of Fe rich LSCrF samples (x<0) having more SrCrO4 pre-edge intensity and higher surface area is found higher than Cr-rich samples(x>0). Especially, Fe rich LSCr0.4Fe0.6O3 sample having ~13.14wt.frac.%SrCrO4 impurity phase considered as better catalyst shows much lower carbon balance than other samples unsurprisingly due to its higher surface area with smaller crystallite size and having more catalytically active SrCrO4 pre-edge intensity proved by XANES analysis. Morphological analysis of the LSCrF powders was done by Scanning Electron Microscopy (SEM/EDX). BET measurements were also performed to calculate their surface area.

References:

[1] F. Lefebvre-Joud, G. Gauthier, and J. Mougin, “Current status of proton-conducting solid oxide fuel cells development,” J. Appl. Electrochem. , vol. 39, no. 4, pp. 535–543, 2009.

[2] M. F. Lü et al., “Thermomechanical, transport and anodic properties of perovskite-type (La0.75Sr0.25)0.95CrxFe1-xO3-δ” J.
Power Sources , vol. 206, pp. 59–69, 2012.

[3] N. P. Bansal and B. Wise, “Sol-gel synthesis of La0.6Sr0.4CoO3-x and Sm0.5Sr0.5CoO3-x cathode nanopowders for solid oxide fuel cells,” Ceram. Int., vol. 38, no. 7, pp. 5535–5541, 2012.

[4] Larson A C, “General Structure Analysis System (GSAS),” Los Alamos Lab. Rep., vol. 748, pp. 86–748, 1994.