Promoting Molecular Exchange on Rare-Earth Oxycarbonate Surfaces to Catalyze the Water-Gas Shift Reaction

Abstract

It is highly desirable to fabricate an accessible catalyst surface that can efficiently activate reactants and desorb products to promote the local surface reaction equilibrium in heterogeneous catalysis. Herein, rare-earth oxycarbonates (Ln₂O₂CO₃, where Ln = La and Sm), which have molecular-exchangeable (H₂O and CO₂) surface structures according to the ordered layered arrangement of Ln₂O₂²⁺ and CO₃^2- ions, are unearthed. On this basis, a series of Ln₂O₂CO₃-supported Cu catalysts are prepared through the deposition precipitation method, which provides excellent catalytic activity and stability for the water-gas shift (WGS) reaction. Density functional theory calculations combined with systematic experimental characterizations verify that H₂O spontaneously dissociates on the surface of Ln₂O₂CO₃ to form hydroxyl by eliminating the carbonate through the release of CO₂. This interchange efficiently promotes the WGS reaction equilibrium shift on the local surface and prevents the carbonate accumulation from hindering the active sites. The discovery of the unique layered structure provides a so-called "self-cleaning" active surface for the WGS reaction and opens new perspectives about the application of rare-earth oxycarbonate nanomaterials in C1 chemistry.

Figure 1

Structure of Sm₂O₂CO₃ calculated by DFT. (a) Layer structure of the Sm₂O₂CO₃ supercell. The Sm–O bonds are hidden in order to see the layering principle better. The Sm₂O₂²⁺ and CO₃^2– layers are parallel to the {001} plane. The calculations suggest that section II is the advantaged position when exposed crystal surfaces are generated from the crystal Sm₂O₂CO₃. (b) Top view of the hydroxylated {001} Sm₂O₂CO₃ surface (the inner atoms are concealed). (c) Main view of the hydroxylated {001} Sm₂O₂CO₃ surface, which contains two complete Sm₂O₂CO₃ layers and one hydroxylated Sm₂O₂²⁺ layer.

Figure 2

Substitution process of carbonate and hydroxyl species on the surface of Sm₂O₂CO₃. (a) XRD patterns of pristine Sm₂O₂CO₃; Spectral variation of (b) carbonate and (c) OH regions during the switching experiment of CO₂ and H₂O; Mass spectrometry signal variation during isotopic gas exchanges of (d) H₂O and CO₂ and (e) C¹³O₂ and H₂O.

Figure 3

Morphology and catalytic performance over catalysts. (a) HRTEM and (b,c) aberration-corrected HAADF–STEM images of used 5Cu/Sm₂O₂CO₃ catalysts; (d) catalytic performances test (100 mg, GHSV = 42,000 mL g_cat^–1 h^–1), (e) comparison of WGS reaction rates, (f) apparent activation energy value, and (g) stability test for about 100 h of the 5Cu/Sm₂O₂CO₃, 5Cu/CeO₂, and 5Cu/Al₂O₃ catalysts.

Figure 4

Identification of the active site of the Cu/Sm₂O₂CO₃ catalyst. (a) Reaction rates normalized by Cu weight for CuO and 20Cu/Sm₂O₂CO₃ samples at 250 °C; (b) reaction rates normalized by Cu weight at various temperatures and (c) TCD signals of H₂-TPR profiles over xCu/Sm₂O₂CO₃ (x = 5, 10 and 20) catalysts; (d) reaction rates normalized by Cu weight at 250 °C and calculation of hydrogen consumption normalized by Cu weight from H₂-TPR for xCu/Sm₂O₂CO₃ samples; (e) ex situ Cu 2p XPS spectra of fresh 5Cu/Sm₂O₂CO₃ catalyst and quasi in situ Cu 2p XPS spectra of used 5Cu/Sm₂O₂CO₃ catalyst; (f) in situ infrared spectra of 5Cu/Sm₂O₂CO₃ exposed to different CO pressures at −143 °C after H₂ pretreatment. (g) MS signals of H₂-TPR profiles of fresh and used 5Cu/Sm₂O₂CO₃ samples.

Figure 5

WGS reaction mechanism study of the 5Cu/Sm₂O₂CO₃ catalyst. (a) In situ WGS reaction at 300 °C, (b) experiment of H₂O dissociation, (c) CO-TPSR of the catalyst, and (d) structure of catalyst surface (top view). (e) Reaction mechanism scheme of the WGS reaction. The structure diagrams with red frames are the main views of intermediate structures, and the diagrams with black frames are top views (the inner atoms are concealed). The Gibbs free energies are calculated at 250 °C, with the partial pressure of CO of 2 kPa and the partial pressure of H₂O of 10 kPa.