In Situ and Theoretical Studies for the Dissociation of Water on an Active Ni/CeO₂ Catalyst: Importance of Strong Metal–Support Interactions for the Cleavage of O–H Bonds

Abstract

Water dissociation is crucial in many catalytic reactions on oxide-supported transition-metal catalysts. Supported by experimental and density-functional theory results, the effect of the support on OH bond cleavage activity is elucidated for nickel/ceria systems. Ambient-pressure O 1s photoemission spectra at low Ni loadings on CeO2 (111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO2 (111) compared with pyramidal Ni4 particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni(2+) species by accommodating electrons in localized f-states. The fast dissociation of water on Ni/CeO2 has a dramatic effect on the activity and stability of this system as a catalyst for the water-gas shift and ethanol steam reforming reactions.

Ambient pressure XPS and IRRAS investigation of ethanol steam reforming on Ni–CeO2(111) catalysts: an in situ study of C–C and O–H bond scission

“…This reduction is likely due to several effects, including the ability of Ni and ceria to dissociate H2O, the strong interaction between Ni and ceria, and a possible strain in the ceria due to doping with Ni, which can make oxide lattice favor the formation of Ce 3+ and O vacancies. 16,21 For the surface species evolution, figure 4 depicts the C 1s/O 1s AP-XPS results for the heated surface. In the C 1s spectra, starting from 300 K, a small amount of acetate (289.8 eV) as well as dioxyethylene species (288.2 eV) are formed along with the ethoxy species (287.2 eV and 286.1 eV), which matches the results from the bare ceria film.…”

“…53 Additionally, Ni/Ni 2+ on the CeOx(111) substrate can also readily dissociate water to form hydroxyls on the surface. 21 Furthermore, our earlier isotope study of the reaction of ethanol on the Ni-CeOx(111) surface under UHV showed that there is oxygen transfer between ceria and surface carbon on Ni nanoparticles, and a significant amount of D2 and CO are produced in the temperature programmed desorption (TPD) data after introducing D2O into the reaction. 26 Herewith, considering all the factors mentioned above and also noting that top few layers of the ceria film was fully reduced and hydroxylated at 700 K, this in situ observation of On the other hand, the reappearance of nickel carbide species (Ni3C/NixC) after introducing water in the C 1s spectra of figure 7 points at a possible re-oxidation pathway of surface carbon through a strong interaction with Ni: OH(ad) + xNi 0 + coke → HO-NixC → xNi 0 + CO/CO2 + H(ad) 22 Thus, under steady state reaction conditions, it is likely that two competing processes will take place on the surface: 1. the accumulation of coke due to C-C/C-H bond cleavage by metallic Ni and 2. the re-oxidation of surface carbon by hydroxyls provided by the interaction between water and ceria in the form of Ce 3+ (OH)x.…”

“…16,[20][21] Hydroxyl groups generated from these strong metal-support interactions could play an important role in redox chemistry such as in the water-gas shift (WGS) reaction and for suppressing coke formation with subsequent deactivation. 21 …”

Water–Gas Shift Reaction over Ni/CeO2 Catalysts

“…The fast dissociation of water on Ni/CeO 2 would have a dramatic effect on the activity and stability of this system as a catalyst for the water-gas shift [32]. The catalysts in this set of experiments were actually tested under forward WGS feed conditions because of the large excess of water.…”

The Catalytic Hydrogenation of Maleic Anhydride on CeO2−δ-Supported Transition Metal Catalysts

“…In theory, the electron-rich O vac in CeO 2−δ may modify the geometric and electronic structure of the active metal [18,19], as well as enforce the metal-support interaction [20]. It is thought that the O vac defects could synergistically enhance the catalytic performance of M/CeO 2−δ catalysts, through coordination with the oxygen-containing group in OH cleavage [21], NO 2 reduction [22] and CO 2 methanation [23]. Moreover, the ionic TM (M δ+ ) species can diffuse into the CeO 2 lattice and modulate the population of O vac in mixed oxides with dependence of the ionic radius and the valence state of the M [24,25].…”