Redox behaviour of copper(II) ion pairs as surface species on CuTh oxide catalysts. Electron paramagnetic resonance and X-ray photoelectron spectroscopy study

Abstract

EPR and XPS have been used to study the redox reaction on CuTh oxide catalysts with different Cu/Th atomic ratios. Apart from the substitutional (S) sites occupied by Cu2+ ions both in the bulk and on the surface of thoria, other Cu2+ ions, occupying different sites on the surface were susceptible to H, and/or 0, treatment at different temperatures. In particular, copper(ii) ion pairs which can be obtained at high copper contents (Cu/Th > 0.01) are formed from one copper(i1) ion, reducible by hydrogen, and another, corresponding to substitutional surface sites (SJ, which remains stable towards hydrogen. The migration of some Cu2+ ions, from sites very susceptible to the cyclic redox treatment, is responsible for the growth of CuO crystallites on the surface of CuTh oxide catalysts.

Role of different coordinated Cu and reactive oxygen species on the highly active Cu–Ce–Zr mixed oxides in NH3-SCO: a combined in situ EPR and O2-TPD approach

“…It was known that the intensity of the B signal was correlated with the highly dispersed copper oxide on the surface. 59 Particularly, the higher was the dispersion of Catalysis Science & Technology Paper the CuO species, the larger was the fraction of copper oxide detected by EPR. The H 2 -TPR results (Fig.…”

“…It was known that the Cu 2+ ions (A 1 ) were surrounded by fewer oxygen anions than the Cu 2+ ions (A 2 ); from the difference in the coordination number, it was determined that the Cu 2+ ions corresponding to the A 1 signal were more accessible towards oxygen than that corresponding to the A 2 signal. 59 In addition, the Cu 2+ ion monomers (A 1 ) were the precursor of the dimer (K) due to the similar g value for the two signals and the double hyperfine splitting constants of A 1 compared with the K signal. 60,61 So the monomeric Cu 2+ ions of the A 1 signal were more accessible to combine with O 2 to form copperIJII) ion dimers, which could induce a migration of Cu 2+ ions from their sites (A 1 signal) to copperIJII) ion dimers (K signal).…”

O− spin bags formation in (CeO2)0.75(La2CuO4)0.24 catalyst after extended use for reduction of NO by CO

“…Those lines were interpreted as due to Strongly Interacting Dipoles (SID). In fact it is well known [4][5][6][7][8][9] that, when two unpaired spins S ; and SS interact with a dipolar energy Si D S^ = ST D ST (with ST = S, + S^) even overcoming the electron Zeeman energy, SID form, generating low-field EPR lines (in many cases with apparent g -3) moving towards higher fields both at decreasing real g values and at increasing dipolar constant D.…”

“…This work was presented at the occasion of the EMARDIS-97 & Applied EPR workshop organized by Prof. N. D. Yordanov, Sofia-Bojana, June [8][9][10][11][12][13][14][15][16][17] 1997.…”