Insights into the Redox Behavior of Pr_0.5Ba_0.5MnO_3−δ-Derived Perovskites for CO₂ Valorization Technologies

In situ temperature-programmed (TP) analyses in a multianalytical approach including X-ray diffractometry (XRD), temperature-programmed reduction (TPR), thermogravimetry (TGA), near-edge X-ray absorption fine structure spectroscopy (NEXAFS) are used to study the relationship between redox properties and structural changes in Pr0.5Ba0.5MnO3−δ (m-PBM), PrBaMn2O5+δ (r-PBM), and PrBaMn2O6−δ (o-PBM) when exposed to reduction/oxidation cycles. TP-XRD analysis shows that under reducing conditions, between 300 and 850 °C, the biphase perovskite m-PBM turns into the monolayered perovskite r-PBM. Stabilization of the latter phase at room temperature requires early oxidation in air at a high temperature (850 °C) to avoid segregation, resulting in the formation of the oxidized layered phase (o-PBM). The o-PBM layered perovskite is characterized by the H2-TPR profile, showing two reduction peaks at temperatures below 500 °C. TP-NEXAFS characterization reveals the copresence of Mn­(IV) (60%), Mn­(III) (30%), and Mn­(II) (10%) and helps to interpret the reduction profile: Mn­(IV) converts to Mn­(III) at ∼300 °C (I pk), Mn­(III) to Mn­(II) at ∼450 °C (II pk). The TGA characterization confirms the reversibility of the o-PBM ↔ r-PBM process at 800 °C; in addition, it shows that the r-PBM can be oxidized almost completely (∼99%) also by CO2 without accumulation of carbonates. This study sheds light on the peculiar redox behavior of PBM-based materials and paves the way for their application as oxygen carriers and catalytic promoters in different CO2 enhancement technologies. Here, we discuss the results obtained to develop versatile and redox-resistant electrodes for solid oxide electrochemical cell/solid oxide fuel cell applications

Key Properties and Parameters of Pd/CeO₂ Passive NO<i>_x</i> Adsorbers

In this paper, a series of Pd/CeO2 catalysts prepared by different synthesis routes and showing different morphological and textural properties have been investigated for passive NOx adsorption (PNA) applications. The results obtained by NOx adsorption/desorption tests demonstrated that NOx storage capacity and NOx storage efficiency of Pd/CeO2 materials depend strictly on their surface area, whereas the morphology of the support and the Pd deposition method do not seem to play a key role. In contrast, the Pd deposition method does impact the dynamics of NOx desorption by affecting the amount of NOx desorbed at different temperatures. This seems to be connected to Pd–Ce interactions at the nanoscale that favor NOx desorption at higher temperatures suitable for PNA application. These findings are relevant in designing and optimizing the properties of Pd/CeO2 materials for their function as passive NOx adsorbers

Mechanism of Ethylene Oxychlorination on Ceria

Ethylene oxychlorination on CeO2 provides ethylene dichloride (EDC) and the desired vinyl chloride (VCM) in a single operation, in contrast to the traditional process that requires two separate units. The origin of this outstanding performance is unclear, and the mechanism has not been discussed in detail. In the present work, we combine density functional theory (DFT) with steady-state experiments and temporal analysis of products (TAP) to close this gap. The catalyst surface is found to contain CeOCl, while the bulk phase is CeO2, regardless of the starting materials CeCl3, CeOCl, or CeO2. Catalysis by different nanostructures highlights that the CeO2(111) surface is more active than the (100) surface due to the poisoning of the latter, while the selectivities are comparable. In any case, the degree of oxygen removal from CeO2 and the replenishment of the accordingly formed oxygen vacancies by Cl and its replenishment by Cl species lead to increased selectivity to chlorinated products and decreased selectivity to carbon oxides. DFT and TAP studies reveal that the most likely pathway of VCM formation takes place by a cascade reaction. First, EDC appears and then HCl is extracted in a concerted step to lead to VCM. Such steps are a key characteristic of ceria. Other paths leading to minor products such as 1,2-dichloroethene (DCE) are found possible by starting from VCM or EDC. CO is formed by combustion of chlorinated species, whereas CO2 can either stem from further oxidation of CO or directly from ethylene. In summary, our work points out a rich complex behavior of the chemistry of chlorinated compounds on the oxide surface, indicating that concerted steps and cascade reactions are possible for these materials

Tuning Chemical and Morphological Properties of Ceria Nanopowders by Mechanochemistry

Cerium oxide powders are widely used and are of fundamental importance in catalytic pollution control and energy production due to the unique chemical properties of CeO2. Processing steps involved in catalyst preparation, such as high-temperature calcination or mechanical milling processes, can alter the morphological and chemical properties of ceria, heavily affecting its final properties. Here, we focus on the tuning of CeO2 nanopowder properties by mild- and high-energy milling processes, as the mechanochemical synthesis is gaining increasing attention as a green synthesis method for catalyst production. The textural and redox properties were analyzed by an array of techniques to follow the aggregation and comminution mechanisms induced by mechanical stresses, which are more prominent under high-energy conditions but strongly depend on the starting properties of the ceria powders. Simultaneously, the evolution of surface defects and chemical properties was followed by Raman spectroscopy and H2 reduction tests, ultimately revealing a trade-off effect between structural and redox properties induced by the mechanochemical action. The mild-energy process appears to induce the largest enhancement in surface properties while maintaining bulk properties of the starting materials, hence confirming its effectiveness for its exploitation in catalysis

Identification of Highly Selective Surface Pathways for Methane Dry Reforming Using Mechanochemical Synthesis of Pd–CeO₂

The methane dry reforming (DRM) reaction mechanism was explored via mechanochemically prepared Pd/CeO2 catalysts (PdAcCeO2M), which yield unique Pd–Ce interfaces, where PdAcCeO2M has a distinct reaction mechanism and higher reactivity for DRM relative to traditionally synthesized impregnated Pd/CeO2 (PdCeO2IW). In situ characterization and density functional theory calculations revealed that the enhanced chemistry of PdAcCeO2M can be attributed to the presence of a carbon-modified Pd0 and Ce4+/3+ surface arrangement, where distinct Pd–CO intermediate species and strong Pd–CeO2 interactions are activated and sustained exclusively under reaction conditions. This unique arrangement leads to highly selective and distinct surface reaction pathways that prefer the direct oxidation of CHx to CO, identified on PdAcCeO2M using isotope labeled diffuse reflectance infrared Fourier transform spectroscopy and highlighting linear Pd–CO species bound on metallic and C-modified Pd, leading to adsorbed HCOO [1595 cm–1] species as key DRM intermediates, stemming from associative CO2 reduction. The milled materials contrast strikingly with surface processes observed on IW samples (PdCeO2IW) where the competing reverse water gas shift reaction predominates