In Situ EPR Study of the Redox Properties of CuO–CeO₂ Catalysts for Preferential CO Oxidation (PROX)

Abstract

Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ Electron Paramagnetic Resonance (EPR) spectroscopy is capable to monitor the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), mass spectrometry (MS) studies was utilized to study the redox behavior of CuOCeO2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce 4+ /Ce 3+ during oxidation of Cu 0 /Cu + species to Cu 2+ , and (ii) a direct mechanism that bypasses the redox pair Ce 4+ /Ce 3+ . In addition, EPR experiments with isotopically enriched 17 O2 established the synergetic mechanism as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. Guided by these results an optimized CuO-CeO2 catalyst could be designed. A rather wide temperature operation window of 11 degrees (from 377 K to 388 K), with 99% conversion efficiency and 99% selectivity was achieved for the preferential oxidation of CO in a H2 feed.

Modulating Cu+ distribution on the surface of Ce-doped CuO composite oxides for SO2-resistant NH3-selective catalytic reduction of NO

“…With careful deconvolution measurements, besides Cu 2p 3/2 and Cu 2p 1/2 peaks of CuO, additional peaks located around 933.4 eV and 953.3 eV were also resolved for the (CeO x ) m CuO samples. These peaks, with lower BE compared to those of CuO, were close to the BE value of Cu + in the composite oxides reported in the literature, 26,27 suggesting the existence of Cu + on the (CeO x ) m CuO samples. Among the species on the surface, Ce(III) and Cu + could be more attractive since they are believed to be helpful for creating charge imbalance or promoting active species mobility to facilitate the reaction; 21,29 thus, the contents or distributions of these two species were worthy of further investigation.…”

“…The v (881.5 eV)-v 0 (885.1 eV) and u (898.8 eV)-u 0 (903.4 eV), refer to 3d 5/2 and 3d 3/2 of Ce(III), respectively, suggesting the co-presence of Ce(III) and Ce(IV) on (CeO x ) m -CuO samples. 21,26,27 For a CuO reference sample, BEs of 933.7 eV and 953.7 eV (Fig. 2B) for the two main peaks are close to the standard values of Cu 2p 3/2 and Cu 2p 1/2 of CuO, respectively, 28,29 which were accompanied by two shakeup satellite peaks in the range of 938.2-946.3 eV and a isolated satellite peak around 962.1 eV.…”

“…These interactive changes among active components were always accepted as a hint or consequence of the presence of a synergistic effect in the composite samples. 26,27 Note that the synergistic effect in the present samples was distinguished with that of the reported Cu-based NH 3 -SCR-NO x catalysts (especially with ternary- component samples) since one of the main features of the synergistic effect in these catalysts was embodied by the presence of isolated Cu 2+ (with higher BE compared to that of CuO). 27,29 In addition, the synergistic effect in composite structures would also inuence the distribution of other active species in the samples such as active oxygen species required for redox reactions.…”

Forty years of temporal analysis of products

“…For instance, TAP has been used to quantify the number of active sites present on catalysts, 36 and these were found to be in agreement with previously published steady-state isotopic transient kinetic analysis diffuse reflectance infrared Fourier transform spectroscopy (SSITKA DRIFTS) results. 43 Other examples of correlation/complementation of TAP experiments with other methods include electron paramagnetic resonance (EPR) spectroscopy, 44,45 X-ray photoelectron spectroscopy (XPS), 44,46,47 X-ray diffraction (XRD), 44,46,48 X-ray absorption spectroscopy (XAS), 44,47 Raman, 46 energy dispersive X-ray scanning transmission electron microscopy (EDX-STEM), 46 high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), 48,49 Fourier transfer Infrared (FTIR), 49 high resolution scanning transmission electron microscopy (HRTEM) 47,50,51 and prompt gamma-ray activation analysis (PGAA). 52 As such, while many reports have been dedicated solely to TAP, it should be noted that TAP is not a necessarily standalone characterization technique and can/should be used in a complimentary way with other methodologies.…”

Highly stable and Active Cu1/CeO2 Single‐Atom Catalyst for CO Oxidation: A DFT Study

“…As an alternative and cheap metal, copper is extensively used in three-way catalysts, especially Cu/CeO 2 system. [13][14][15][16][17] Experimentally, single-metal atoms on iron oxide display an remarkable catalytic performance of for low-temperature CO oxidation. [1,[18][19][20] Pt single atoms dispersed on Al 2 O 3 , TiO 2 and CeO 2 recently have been observed with an excellent activity for CO oxidation.…”