Low-Temperature Dissociation of CO₂ on a Ni/CeO₂(111)/Ru(0001) Model Catalyst

Abstract

The adsorption of CO2 on CeO2‑x (111) and Ni/CeO2‑x (111)/Ru­(0001) surfaces has been studied with reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). On the maximal-oxidized CeO2(111) surface physisorbed linear CO2 and a CO2 – species are identified at 97 K. The reduced CeO2‑x (111) surface exhibits higher reactivity toward adsorbed CO2, which leads to higher coverages of CO2 – and promotes CO2 dissociating into CO and an active oxygen species at higher temperature, reoxidizing the reduced CeO2‑x (111) films. Deposition of Ni on the maximal-oxidized CeO2 thin films leads to slight reduction of ceria films. Adsorption of CO2 on Ni/CeO2‑x (111) films causes dissociation at 97 K and leads to Ni-CO adsorbates plus partial oxidation of Ni nanoparticles. This process is inhibited when Ni nanoparticles on CeO2 are fully oxidized. In contrast to the results reported for CO2 adsorption on Ni single-crystals, where the dissociation temperature was found to be higher than 240 K, the much lower dissociation temperature (∼97 K) for CO2 on Ni nanoparticles supported on CeO2(111) suggests that the Ni/CeO2 catalyst exhibits high activity toward CO2 activation.

Dry Reforming of Methane on a Highly‐Active Ni‐CeO2 Catalyst: Effects of Metal‐Support Interactions on C−H Bond Breaking

“…[12,13] To model nickel in close contact with stoichiometric and reduced ceria, we have considered aquarter of amonolayer of Ni atoms deposited on CeO 2 (111) and Ce 2 O 3 (0001) as ideal catalyst surfaces,aiming to elucidate the effects of the two supports in the Ni electronic and chemical properties towards CH 4 adsorption and dissociation as compared to Ni(111). [12,13] To model nickel in close contact with stoichiometric and reduced ceria, we have considered aquarter of amonolayer of Ni atoms deposited on CeO 2 (111) and Ce 2 O 3 (0001) as ideal catalyst surfaces,aiming to elucidate the effects of the two supports in the Ni electronic and chemical properties towards CH 4 adsorption and dissociation as compared to Ni(111).…”

“…We found that Ni and Oatoms work in acooperative way to dissociate CH 4 molecules in accordance with previous theoretical works. [12,13] Once CH 3 is formed, sequential decomposition of this intermediate into Cshould occur very fast. [19] Adsorption sites with adjacent Ni and Oa toms exist in our catalysts since Ni forms solid solutions with ceria, Ce 1Àx Ni x O 2Ày .…”

“…Zuschriften of the Ni sintering and other part migrating as single atoms into the bulk of ceria. [12,13] To model nickel in close contact with stoichiometric and reduced ceria, we have considered aquarter of amonolayer of Ni atoms deposited on CeO 2 (111) and Ce 2 O 3 (0001) as ideal catalyst surfaces,aiming to elucidate the effects of the two supports in the Ni electronic and chemical properties towards CH 4 adsorption and dissociation as compared to Ni(111). Figures 6a,b as how the Ni/ CeO 2 -(111) and Ni/Ce 2 O 3 (0001) structures,r espectively.O nC eO 2 -(111), the nickel oxidation state is 2 + ,while on Ce 2 O 3 (0001) it is 0( Figure S3).…”

“…[19] Adsorption sites with adjacent Ni and Oa toms exist in our catalysts since Ni forms solid solutions with ceria, Ce 1Àx Ni x O 2Ày . [12,13] Once CH 3 is formed, sequential decomposition of this intermediate into Cshould occur very fast. [18] Indeed, after exposing Ni/CeO 2 (111) to CH 4 at 300 K, aCO x species is detected in XPS as ap roduct of the reaction between the Cdeposited by CH 4 decomposition and Oatoms present in the catalyst surface ( Figure 3).…”

“…[10,11] Annealing to 500 Kinduced some sintering of the Ni particles and large fraction of the Ni migrated into the ceria substrate to form aCe 1Àx Ni x O 2Ày solid solution. [12,13] Figure 3shows aseries of XPS spectra collected for aNi/ CeO 2 (111) surface under 100 mTorr of methane.I nt he C1s region, at 300 K, there is as harp peak for gaseous methane near 287 eV,astrong peak near 290.2 eV that can be attributed to adsorbed CO x species, [14] and weak features at 285-286 eV that probably come from CH x on the surface.This result implies that methane dissociates on the Ni/CeO 2 surface at room temperature.S ome of the CH 4 molecules fully dissociate producing Catoms that eventually react with oxygen atoms of the ceria to yield CO x species.I nt he corresponding spectrum for the Ce 4d and Ni 3p regions no sign is evident for achange in the oxidation state of Ce 4+ and Ni 2+ cations.At700 K, no features for CO x or CH x are seen in the in the C1sr egion, but ac lear change is seen in the line shape of the Ce 4d region (formation of Ce 3+ )a nd on the position of the Ni 3p peaks (Ni 2+ !Ni 0 transformation). We also detected the evolution of CO into gas phase during the exposure to CH 4 at this elevated temperature ( Figure S2).…”