Interactions of Ni Nanoparticles with Reducible CeO₂(111) Thin Films

Abstract

The interaction between Ni and ceria was investigated under ultrahigh vacuum conditions using model Ni/ceria systems consisting of Ni nanoparticles vapor-deposited on well-ordered CeOx(111) (1.5 < x < 2) thin films grown on Ru(0001). As indicated by X-ray photoelectron spectroscopy studies, metallic Ni is the only species present on the reduced ceria. Ni-0 is the predominate species observed upon deposition of a submonolayer of Ni on CeO2 at 300 K. However, a small amount of Ni is oxidized to Ni2+. A decreased ratio of Ni2+ to Ni-0 was observed with further increase of Ni coverage. Oxidation of Ni on CeO2 can be facilitated by annealing as well as by depositing Ni at 500 K. Scanning tunneling microscopy studies show that Ni forms two-dimensional particles on ceria at room temperature, which suggests a strong Ni-ceria interaction. Upon deposition at 500 K, metallic Ni particles as well as NiO particles can be formed on the reduced and oxidized ceria, respectively. NiO particles exhibit a flatter particle shape than that of Ni particles. Our results can be explained by thermodynamics as well as by previous computational studies.U.S. Department of Energy Office of Fossil Energy (FE); U.S. Department of Energy by Battelle Memorial Institute [DE-AC05-76RL01830

Mechanistic Insights of Ethanol Steam Reforming over Ni–CeOx(111): The Importance of Hydroxyl Groups for Suppressing Coke Formation

“…Heating the surface to 700 K causes extensive Ni oxidation to NiO due to the existence of strong metal−support interaction between Ni and ceria. 9,23,24 Ni forms two-dimensional particles with an average height of 0.3 nm on CeO 2 film at 300 K as shown in the scanning tunneling microscopy (STM) images ( Figure 1). After heating to 700 K, Ni sinters into larger particles that are 2.7 nm wide and 0.5 nm high at the cost of particle density.…”

Growth and Structure of Ni–Au Bimetallic Particles on Reducible CeO2(111)

“…The surface free energy of Ni (*1.7 J/m 2 ) is larger than that of Au (1.1 J/m 2 ) and ceria (0.7 J/m 2 ) [20,[28][29][30][31][32][33][34]. However, the fact that Ni tends to wet the ceria surface more than Au is the result of a stronger interaction between Ni and ceria [11,13,22]. On the CeO 1.8 (111) surface, smaller particles with a larger particle density were observed for both Au and Ni.…”

“…2. Detailed discussion of their growth and sintering on ceria can be found elsewhere [13,19,20,[22][23][24][25][26]. At 300 K, Au particles show preferential nucleation at the step edges on CeO 2 .…”

“…The surface was subsequently annealed to 1,150 K for 2 min. Reduced CeO 1.8 thin films were grown using an oxygen pressure of 2 9 10 -8 Torr [13]. The stoichiometric value x for CeO x was determined by XPS analysis of Ce 3d spectra as demonstrated in our previous study [19].The grown films are well-ordered and exhibit p(1.4 9 1.4) LEED patterns with respect to that of the Ru substrate.…”

“…However, both Au and Ni can sinter at high temperatures and thus lose their activity. Furthermore, Ni can experience encapsulation by reducible oxide supports like ceria and titania [11][12][13]. During the steam reforming process, it can deactivate due to the coke formation [14,15].…”