Comparative in Situ DRIFTS-MS Study of¹²CO- and¹³CO-TPR on CuO/CeO₂Catalyst

Abstract

A ceria-supported copper oxide catalyst has been examined with respect to interaction with CO by means of CO-TPR tests employing a DRIFTS cell as reactor. Parallel analysis of isotopic shifts in the DRIFTS spectra obtained upon interaction of the catalysts with either 12CO or 13CO allows unambiguous assignment of the various carbonyl and carbonate-type species formed. The study is complemented by separate examination of the stability of carbonyl species focused to get hints on the nature of CO adsorption centers at the catalyst surface. Joint analysis of gases evolution during the CO-TPR tests and DRIFTS spectra is employed to determine the nature of processes taking place during such tests. Additionally, analysis of the various CO2 isotopes evolving during the course of a TPO run subsequent to 13CO-TPR test is employed to discard the occurrence of CO disproportionation upon interaction of the catalyst with CO up to 400 °C.

Characterization of Active Sites/Entities and Redox/Catalytic Correlations in Copper-Ceria-Based Catalysts for Preferential Oxidation of CO in H2-Rich Streams

“…Since such carbonyl species must be formed as a consequence of a support-promoted reduction process of the copper and taking also into account that the relatively low frequency of this band, with respect to those expected for this type of carbonyls [60], has been related to the interaction between the partially reduced copper centers and the underlying ceria [27,29,40,41,51], the correlation evidenced by Figure 7 reveals that the active species for CO oxidation under CO-PROX conditions must be related to surface-dispersed partially reduced copper oxide species interacting with the support, i.e., at CuO-support interfacial positions. This is consistent with the aforementioned results collected in previous reports [27,29,40,49], which show that these copper species are the most reducible upon contact with CO at low temperatures within a process whereby the ceria support that is in contact with the copper species can also become reduced [27,29,61,62]. Thus, differences between the CO oxidation activities under CO-PROX conditions for this type of catalyst is related to the extent of support promotion of such partial (to Cu + ) CuO reduction at interfacial sites which is attained in each case.…”

“…In this respect, amounts of CO consumed at 20 °C prior to launching the ramp are estimated as 230, 210 and 175 μmol/g for CuC, CuCT4 and CuCT1 samples, in good qualitative agreement with the aforementioned decrease of CO oxidation activity upon increasing the amount of terbium in the support (Figure 3). The respective CO oxidation activity is, in turn, reflected by the respective intensity of a Cu + -carbonyl species (attribution in accordance with previous works, including 12 CO- 13 CO isotopic substitution experiments and analysis of carbonyls thermal stability, where full details can be found [27,29,48,49,51]) formed upon interaction of the calcined samples with CO at room temperature, Figure 5. Since the catalysts were shown to display a fully oxidized state of copper in the initial calcined sample, formation of such carbonyl species upon room temperature interaction evidences the existence of a low temperature copper oxide reduction process, also in agreement with concomitant formation of carbonate-type species upon such room temperature interaction with CO [49,51].…”

“…The third spectral zone (at intermediate frequencies) shows the formation of CO 2 (g), evolving in accordance with CO oxidation activity, and a carbonyl species (as discussed earlier, a Cu + -carbonyl giving rise to a band at ca. 2120-2110 cm −1 [27,29,49,51]. As discussed previously, the presence of these Cu + -CO species already upon initial contact at 303 K with the reactant mixture is consistent with the easy reduction of copper in the catalysts, considering that copper is fully oxidized in the initial calcined catalysts [40].…”

“…Concerning CuCT1, the reduction starts around 50 °C and an intense peak at 155 °C is observed. There is also a hump at 250 °C in this case, which could in part be related to the existence of residual WGS activity (as a consequence of interaction of CO with surface hydroxyls) [49]. Similar results can be found in reports by Wang et al [38,50], in which the formation of a low temperature α peak in case of CuO on samarium-doped ceria has been attributed to the effect of the presence of surface oxygen vacancies within the oxygen ion conducting support.…”

Investigation of strong shock wave interactions with CeO2 ceramic

“…Peak around 3700 cm 1 as shown in Fig. 6(b) is due to bi-or tri-coordinated O-H species formed in presence of huge O 2 pressure during shock wave treatment [27]. The results demonstrate that as-prepared sample has absorbed H 2 O molecules on the surface of CeO 2 .…”