Thermal reduction of ceria nanostructures on rhodium(111) and re-oxidation by CO₂

Abstract

The thermal reduction of cerium oxide nanostructures deposited on a rhodium(111) single crystal surface and the re-oxidation of the structures by exposure to CO2 were investigated. Two samples are compared: a rhodium surface covered to ≈60% by one to two O-Ce-O trilayer high islands and a surface covered to ≈65% by islands of four O-Ce-O trilayer thickness. Two main results stand out: (1) the thin islands reduce at a lower temperature (870-890 K) and very close to Ce2O3, while the thicker islands need higher temperature for reduction and only reduce to about CeO1.63 at a maximum temperature of 920 K. (2) Ceria is re-oxidized by CO2. The rhodium surface promotes the re-oxidation by splitting the CO2 and thus providing atomic oxygen. The process shows a clear temperature dependence. The maximum oxidation state of the oxide reached by re-oxidation with CO2 differs for the two samples, showing that the thinner structures require a higher temperature for re-oxidation with CO2. Adsorbed carbon species, potentially blocking reactive sites, desorb from both samples at the same temperature and cannot be the sole origin for the observed differences. Instead, an intrinsic property of the differently sized CeOx islands must be at the origin of the observed temperature dependence of the re-oxidation by CO2.

How the surface state of nickel/gadolinium-doped ceria cathodes influences the electrochemical performance in direct CO2 electrolysis

“…2, the electrocatalytic performance of the pre-oxidized sample is slightly better than that of the pre-reduced one. Binding Energy /eV terminate with Ce 3+ ceria species typically accompanied by a high population of oxygen vacancies [40]. A comparable surface arrangement of NiGDC was found previously for steam electrolysis reaction, indicating that Ni 0 /Ce 3+ is a surface configuration favoured in electrolysis reactions [28,36].…”

“…In general CO 2 is considered capable to oxidize Ni in cermet electrodes [3,40], but contrary to the expected trend under the employed CO 2 conditions, nickel remains metallic. In order to clarify if the resistance of Ni to oxidation by CO 2 is related to the low pressure used in NAP-XPS cell, the dependence of the electrode oxidation state to the CO 2 pressure was examined in a separate experiment.…”

“…Overall, nickel oxidation state is not affected under the employed CO 2 treatments, while ceria undergoes either reduction or oxidation depending on its prior oxidation state. Being an oxidant [40,41], CO 2 cannot be accountable for ceria reduction, leaving thermal decomposition of the oxide according to the reaction: 2CeO 2 → Ce 2 O 3 + ½ O 2 as the most likely explanation of the results shown in Figure 1b and 1c.…”

Dynamics of the Interaction Between Ceria and Platinum During Redox Processes

“…The continuous cerium oxide films provide essential information to interpret the more complex results obtained on cerium oxide islands. On the other hand, the islands allow for a direct observation of the interactions between the oxide and the metal at the edge sites (see, e.g., Schaefer et al, 2018).…”

Structure–function relationship for CO2 methanation over ceria supported Rh and Ni catalysts under atmospheric pressure conditions

“…We used the W‴ and X‴ signals, located at binding energies of 126.1 and 122.8 eV, respectively, to estimate the stoichiometry (x in CeO x ) of the oxide. 35,36 The "degree of oxidation" refers to the amount of Ce 4+ and Ce 3+ in the oxide: "100% oxidised" refers to all Ce present as Ce 4+ (CeO 2 ) and "0% oxidised" refers to all Ce present as Ce 3+ (CeO 1.5 ), i.e., reduced ceria. The presence of Ce 3+ is usually associated with the formation of oxygen vacancies on the ceria surface.…”