Study of Bulk and Surface Reduction by Hydrogen of Ce_xZr_1-xO₂ Mixed Oxides Followed by FTIR Spectroscopy and Magnetic Balance

Abstract

The CeO2 and Ce x Zr1 - x O2 reduction by hydrogen was studied using IR spectroscopy to follow the evolution of the ν(OH) vibrational modes and a magnetic balance to estimate the global reduction percentage from the magnetic susceptibility. H2 was introduced between 298 and 873 K on activated samples. On ceria, different OH groups exist depending on cerium unsaturation. In particular, the band due to OH type II shifts toward higher frequencies when ceria is reduced. It is therefore possible to monitor the surface oxidation state of ceria or mixed oxides through the ν(OH) band wavenumber. For ceria, the surface reduction begins at around 573 K. That leads to the formation of OH(I) species and adsorbed H2O which are observed at the beginning of the reduction. Their elimination leads to the creation of surface O-vacancies. At higher temperatures, there is a surface/subsurface reorganization through the reverse migration of O-vacancies and O-ions from the bulk. However, the adsorption sites are conserved during the evacuation step, the bonded OH, OH(II−B), and OH(II-A) species disappearing by evacuation in the range 773−873 K through H2 (and not H2O) evolution. For mixed Ce x Zr1 - x O2 compounds, the most interesting OH species are those adsorbed on cerium ions. The results lead to the conclusion that the mechanism of reduction is the same as in the case of pure ceria but that an increased mobility of bulk oxygen (in relation with the Zr content) for mixed compounds allows surface oxygen vacancies to be faster filled from O migration in subsurface underlayers. The hydrogen reduction was also followed in a magnetic balance in the case of Ce0.5Zr0.5O2 mixed oxide. The Ce3+ content obtained at different temperatures confirms that the surface reduction is easier for the mixed oxide, the hydrogen chemisorption occurring for T > 373 K and the reduction of Ce4+ into Ce3+ ions beginning at 473 K. Moreover, the better reducibility of the bulk that is observed (76% of Ce3+ at 873 K for the mixed oxide instead of 17% for ceria) evidences the higher oxygen mobility in the bulk, in good agreement with the FTIR conclusions.

FTIR and density functional study of NO interaction with reduced ceria: Identification of N3− and NO2− as new intermediates in NO conversion

“…S1 from the Supplementary Content for more details) are consistent with earlier reports [55][56][57][58][59][60]. In particular, the weak bands at 3712 and 3687 cm -1 are assigned to type I hydroxyls (i.e., the oxygen atom is bound to one cerium cation) [34,51,[57][58][59]. The most intense band at 3654 cm -1 is due to hydroxyls of type II-A and the satellite at 3636 cm -1 , to type II-B bridging OH groups (i.e., the oxygen atom is bound to two cerium cation) [55][56][57][58][59][60].…”

“…The bands registered in the OH region (see Fig. S1 from the Supplementary Content for more details) are consistent with earlier reports [55][56][57][58][59][60]. In particular, the weak bands at 3712 and 3687 cm -1 are assigned to type I hydroxyls (i.e., the oxygen atom is bound to one cerium cation) [34,51,[57][58][59].…”

“…In particular, the weak bands at 3712 and 3687 cm -1 are assigned to type I hydroxyls (i.e., the oxygen atom is bound to one cerium cation) [34,51,[57][58][59]. The most intense band at 3654 cm -1 is due to hydroxyls of type II-A and the satellite at 3636 cm -1 , to type II-B bridging OH groups (i.e., the oxygen atom is bound to two cerium cation) [55][56][57][58][59][60]. The weak band at 3495 cm -1 has been attributed to cerium hydroxide microphase [57] or to type III hydroxyls [59,60].…”

Structural investigation of Ce2Zr2O8 after redox treatments which lead to low temperature reduction

“…Four types of -OH and -OCH 3 species have been identified on the surface of CZMOs: linear (Ce 4+ and Zr 4+ ), doubly bridged (homocationic-Ce 4+ , Ce 4+ and heterocationic-Ce 4+ , Zr 4+ ), doubly bridged (vacancy-Ce 4+ ) and triply bridged [17,18]. In particular, methanol adsorption is a relatively sensitive technique, which may be used to quantify the surface composition of CZMOs [18].…”

NO Heterogeneous Catalysis Viewed from the Angle of Nanoparticles

“…A mechanism for the redox process in ceria -based compounds was proposed [86,91,92] . For the fi rst time it was also evidenced that the high temperature redox cycling can reversibly enhance/depress the redox behavior of the mixed oxides through a peculiar nanostructural modifi cation of the outer surface layers of the samples [93] .…”