Spontaneous Reactivity of La₂CuO₄ toward CO₂ Diagnosed Using Variable Temperature Raman Spectroscopy

Abstract

Raman spectroscopic measurements on a series of perovskites as a function of the temperature and gaseous atmosphere reveal chemical reactions between CO 2 and La 2 CuO 4 that alter the thermal phase transitions and inhibit the synthesis of pristine La 2 CuO 4 . Variable temperature Raman spectra acquired under O 2 environments show that La 2 CuO 4 synthesized under ambient conditions undergoes not only the two previously documented phase transitions but also a loss of Raman vibrations at ca. 200 °C that we attribute to desorption of chemisorbed CO 2 . Samples heated to 700 °C and then cooled to room temperature under O 2 show a Raman spectrum with many fewer peaks; subsequent exposure to CO 2 at room temperature reveals a spontaneous surface reaction through an increased number of peaks in the spectrum. A subsequent heat ramp under CO 2 shows desorption of the additional CO 2 -related peaks, a ca. 50 °C change in temperature for the two thermal phase transitions, and signs of La 2 O 2 (CO 3 ) formation at temperatures above 538 °C. The original La 2 CuO 4 spectrum is obtained only after heating in the presence of CO 2 , which suggests that synthesis under ambient conditions leads to incorporation of CO 2 into the structure. Partial substitution of Ni for Cu to obtain La 2 Cu 0.9 Ni 0.1 O 4 facilitates manipulation of oxygen vacancies in the lattice by heating in oxygen deficient or rich atmospheres but also prevents thermal phase transitions and inhibits direct reactions with CO 2 . We attribute both features to strengthened bonds between B-site cations and oxygen, which significantly alters chemical reactivity. We anticipate that in situ variable temperature Raman spectroscopy will be an effective tool for exploring chemical reactivity and guide synthetic protocol development for solid state materials.

Tuning the Oxygen Reduction Reactivity of Layered Perovskites Using the Jahn–Teller Effect

“…Layered perovskite oxides of the A 2 BX 4 type offer an intriguing ability to interface structural regions with redox activity to regions that are dominated by ionic bonding. This combination introduces properties that have been shown to introduce properties such as oxide conductivity, , superconductivity, , magnetism, and catalysis. ,,− The diversity of properties that can be installed into this family of materials has led to the identification of many examples, including La 2– x A x CoO 4 (A = Sm 3+ , Gd 3+ , Dy 3+ ), Nd 2– x Sr x Ni 1– y Co y O4, La 2 Co 1– y Cu y O 4 , La 2 Ni 1– y Co y O 4 , and La 2– x Sr x Ni 1– y Co y O 4 . Their electrochemical behavior, however, has been understudied compared to their classic perovskite analogues, and we find no previous reports on the ability of La 2– x Sr x Ni 1– y Co y O 4 to catalyze ORR.…”

“…Such behavior demonstrates a complex electronic structure in this family of materials, where variable oxidation states, ionic defects, and spin states combine to yield the observed electronic structure and properties. Electrocatalysis shows distinctive sensitivity to the type and concentration of defects in solid state materials, as seen by the demonstration that B-site substitution alters the electronic structure of La 2– x Sr x Ni 1– y Fe y O 4 to facilitate the electrocatalytic oxygen reduction reaction (ORR), but that tuning oxygen vacancies by A-site doping is required to drive electrocatalytic CO 2 reduction on La 2– x Sr x CuO 4 . ,, We therefore analyze the ORR capabilities of a subset of La 2– x Sr x Ni 1– y Co y O 4 to gain insight into how structure and reactivity correlate in this family of materials.…”