Ceria-Praseodymia Mixed Oxides: Relationships Between Redox Properties and Catalytic Activities Towards NO Oxidation to NO2 and CO-PROX Reactions

Abstract

A series of CexPr1-xO2-δ catalysts was prepared by co-precipitation method in alkali media. These catalysts were characterized by N2 adsorption-desorption isotherms at -196°C, X-ray diffraction (XRD), thermogravimetry combined with mass spectrometry (TG-MS), and temperature-programmed reduction with H2 and CO (H2-TPR and CO-TPR, respectively). Catalytic tests were performed for temperature programmed NO oxidation to NO2 (from 25 to 750ºC) and for the preferential oxidation of CO in H2 rich stream (CO-PROX reaction) in the range of 150-500ºC. The trends in the order of catalytic activities towards NO oxidation and CO-PROX are correlated with the redox properties of the catalysts and their composition.CexPr1-xO2-δ mixed oxides present very different catalytic behaviours towards NO oxidation and CO-PROX reactions. These experimental trends might be explained by the balance of several factors: the acid character of the NO and CO molecules, the different lattice oxygen mobility of the catalysts, the presence of surface carbonates species in the samples, and the catalysts' reducibility under H2 and CO. The understanding of the features that govern the activity towards these environmentally relevant oxidation reactions is important in the designing of effective catalysts.

Insights into the relationship between the catalytic oxidation performances of Ce-Pr mixed oxides and their semiconductive and redox properties

“…The present study is complementary to the characterization and catalytic performance studies of the Ce-Pr mixed oxides catalysts reported earlier [11,14] and has been undertaken to investigate their semiconductive and redox properties in correlation with their catalytic behavior by means of in situ electrical conductivity measurements. Thus, DC-electrical conductivity of the Ce-Pr mixed oxides, namely Ce0.8Pr0.2O2, Ce0.5Pr0.5O2 and Ce0.2Pr0.8O2, as well as of pure CeO2 and…”

“…Notably, the evolution of the electrical conductivity of the Ce1-xPrxO2-δ oxides is in agreement with the trends in reducibility measured by means of H2-TPR experiments. Indeed, with gradually increasing the Pr content in the catalysts, the overall hydrogen consumption is also increased [11].…”

“…Catalytic activity tests in the CO-PROX process were carried out in a stepwise manner under isothermal conditions at selected temperatures in the 150-500 °C range as described in Ref. [11].…”

“…Furthermore, in ceria-praseodymia solid solutions, both elements can adopt 3+ and 4+ oxidation states, and the anionic vacancies are highly mobile in this system. Thus, the insertion of praseodymium within the fluorite host lattice of CeO2 provides new materials with improved catalytic properties for environmental applications, such as automotive three-way catalysts [6,7], NOx trap catalysts [8], N2O decomposition [9,10], (preferential) CO oxidation [11][12][13], volatile organic compounds combustion [13][14][15] and soot removal [16][17][18]. These studies showed that the CexPr1-xO2-δ mixed oxides can undergo more oxygen exchange than pure ceria at lower temperatures and that they are more promising catalysts than other trivalent rare-earth substituted ceria formulations.…”

“…These studies showed that the CexPr1-xO2-δ mixed oxides can undergo more oxygen exchange than pure ceria at lower temperatures and that they are more promising catalysts than other trivalent rare-earth substituted ceria formulations. The Ce1-xPrxO2-δ mixed oxides exhibit intermediate and close to optimal combination of capacity, stability and reactivity with respect to reversible oxygen evolution/uptake [11], and the establishment of redox propertiescatalytic performances relationships in different environmental reactions could be of special interest.…”