A⁺¹Nb⁵⁺O₃ (A = Li, Na, K) Perovskites with Different Fine Structures for Oxidative Coupling of Methane: Tracing the Crystalline Phase Effect on the Surface Active Sites

Abstract

Targeting to unravel the catalytic sites of some specific perovskites for OCM reaction, and elucidate the influences of crystal phase changes on the active sites, three A +1 Nb 5+ O 3 (A = Li, Na, K) complex oxides were synthesized in the pure phase. The reaction performance is ranked as NaNbO 3 > KNbO 3 > LiNbO 3 for the catalysts. It is revealed that LiNbO 3 possesses a hexagonal ilmenite-type phase, and NaNbO 3 and KNbO 3 possess orthorhombic and tetragonal perovskite phases, respectively. Due to the fine structure difference, NaNbO 3 and KNbO 3 have higher oxygen mobility than LiNbO 3 , as testified by the electric conductivity measurements. Moreover, the Nb−O bond strength of NaNbO 3 and KNbO 3 is much weaker than that of LiNbO 3 , which promotes also the oxygen mobility and phase transformation of the two samples, especially in the high temperature OCM process. As a result, more oxygen vacancies/defects are generated on NaNbO 3 and KNbO 3 , which facilitates the efficient formation of OCM active and selective surface O 2 − species and basic sites. Subsequently, the OCM performance of NaNbO 3 and KNbO 3 can be evidently improved. Combining the above advantages, NaNbO 3 exhibits the optimal OCM performance in all the samples.

Fabrication of Ln2Zr2O7 Fluorite and LnAlO3 Perovskite (Ln = La, Nd, Sm) Compounds to Catalyze the OCM Reaction: On the Temperature-Induced Phase Transformation and Oxygen Vacancy

“…To further analyze the basic properties of the catalysts and their effect on the OCM performance, CO 2 -TPD tests were conducted. As shown in Figure , the CO 2 desorption peak around 50–250 °C is assigned to weak basic sites, the peak around 250–600 °C is related to the moderate basic sites, and the peak around 600–800 °C is related to strong basic sites . Moderate basic sites can affect the OCM reaction performance more, due to their suitable interaction with CH 4 molecules, which is favorable to the cleavage of C–H bonds.…”

“…As shown in Figure 7, the CO 2 desorption peak around 50−250 °C is assigned to weak basic sites, the peak around 250−600 °C is related to the moderate basic sites, and the peak around 600−800 °C is related to strong basic sites. 56 Moderate basic sites can affect the OCM reaction performance more, due to their suitable interaction with CH 4 molecules, which is favorable to the cleavage of C−H bonds. In Table 4, the content of moderate basic sites obeys the sequence of La 2 Zr 2 O 7 > Nd 2 Zr 2 O 7 > Sm 2 2 O 7 > LaAlO 3 > NdAlO 3 > SmAlO 3 , which is in good agreement with the OCM performance.…”

“…where a is the theoretical side length of the cubic unit cell, r A is the radius of the 8-coordinated A-site cation, r B is the radius of the 6-coordinated B-site cation, m i is the content of A- compound is structurally stable at high temperatures, while perovskite can distort to a cubic phase by increasing the temperature. 44,56 In situ Raman experiments in the OCM reaction feed were thus performed to investigate the crystal structure changes with increasing temperature. First, the catalysts were pretreated at 800 °C in a 30 mL•min −1 O 2 stream for 1 h, then purged in He for 0.5 h, and cooled to ambient temperature.…”

“…The FTIR technique has been used to measure the strength of the chemical bonds in the compounds and to identify different kinds of surface carbonates. 27,56 Usually, the bond force constant (k) is adopted to quantify the bond strength, which is calculated as follows 61 v c…”

The Journal of Physical Chemistry C Virtual Special Issue on “Energy and Catalysis in China”

“…These papers can be roughly classified as follows. (1) Energy catalysis: conversion of syngas to higher alcohols on CoMn-modified Cu-based mixed oxides, direct methane conversion to methanol on PdAu nanowires, hydrogenation of CO 2 to ethanol on Na-promoted Rh embedded in S-1 zeolites, oxidative coupling of methane on A +1 Nb 5+ O 3 (A = Li, Na, K) perovskites, and synthesis of jet fuel from glycerol and tert -butyl alcohol on organic solid acid catalysts . (2) Environmental catalysis: catalytic elimination of soot and NO x on Mn-based perovskites, catalytic dehydrochlorination of dichloroethane on porous carbon, oxidation of propane and CO on reducible oxide-supported Pt or PtCu nanocatalysts, , complete oxidation of benzene on Pt SACs, preferential oxidation of CO on Cu x Ce 1– x O 2 nanorods, CO oxidation on Pd/ZnO catalysts or Au–Fe 2 O 3 interfaces, and selective catalytic reduction of nitrogen oxide with methane on Co-exchanged SSZ-13 zeolite catalysts .…”