Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts

Abstract

Reducible oxides have been shown to greatly improve the activity of water gas shift (WGS) catalysts. The precise mechanism for this effect is a matter of intense debate, but the dissociation of water is generally considered to be the key step in the reaction. We present here a study of the water activation on oxygen vacancies at the support as part of the mechanism of the WGS reaction on Pt supported on pure and gallium-doped ceria. Doping the ceria with gallium allows tuning the vacancies in the support while maintaining constant the metal dispersion. An inverse relationship was found between the catalytic activity to WGS and the amount of oxygen vacancies. In situ time-resolved X-ray diffraction, mass spectrometry, and diffuse reflectance infrared spectroscopy (DRIFT) showed that the oxygen vacancy filling by water is always fast in either Pt/CeO2 or Pt/CeGa. DFT calculation provides molecular insights to understand the pathway of water reaction with vacancies at the metal–oxide interface sites. Our results suggest that the activation of the water molecule in the WGS mechanism is not the rate-limiting step in these systems. Concentration-modulation spectroscopy in DRIFT mode under WGS reaction conditions allows the selective detection of key reaction intermediates, a monodentate formate (HCOO) and carboxylate (CO2 δ−) species, which suggests the prevalence of a carboxyl (HOCO) mechanism activated at the oxide–metal interface of the catalyst.

Promotion of the Water‐Gas‐Shift Reaction by Nickel Hydroxyl Species in Partially Reduced Nickel‐Containing Phyllosilicate Catalysts

“…[6][7][8] To circumvent the methanation reaction, numerous methods have been studied, such as the incorporation of alkali metals, [9][10][11][12][13][14] formation of alloys with as econdary metal, [5,[15][16][17][18][19] addition of noble metals [18,19] and tuning the structural configuration. [20][21][22][23] Saw et al reported that Ni-Cu alloy could enhance the adsorption of CO on the metals, which can prevent CO dis-sociation. [5] Watanabee tal.…”

CO2 Reduction by Hydrogen Pre‐Reduced Acceptor‐Doped Ceria

“…An archetypical example encompasses the oxygen vacancy‐mediated route of the water‐gas shift reaction on metal‐doped CeO 2 materials, as it requires water splitting on oxygen vacancies. The reactive nature of oxygen vacancies in CeO 2 was therefore tested abundantly …”

“…The reactive nature of oxygen vacancies in CeO 2 was therefore tested abundantly. [13][14][15] Aside from oxygen vacancy formation, bulk incorporation/ dissolution of hydrogen also occurred upon heating in hydrogen atmospheres. [16] Using doped ceria materials, such as Gd-(GDC) or Sm-doped CeO 2 (SDC), the effect of the doping level on the reactivity of oxygen vacancies and/ or the role of dissolved hydrogen can be assessed.…”

Catalytic behavior of Ru supported on Ce0.8Zr0.2O2 for hydrogen production

“…In particular, when ZrO 2 to CeO 2 lattice is added, the ceria redox properties are enhanced: CeO 2 basicity is reduced as the ZrO 2 content is increased, CeO 2 thermal resistance is improved and metal dispersion is increased. Ceria based materials are largely present in Three-Way Catalysts [39,40], and are active for the selective oxidation of carbon monoxide and for the promotion of the WGSR [41][42][43][44][45]. During oxidation of carbon monoxide, mobile oxygen present in ceria can react with CO, as oxygen re-oxidizes the reduced ceria.…”

“…During oxidation of carbon monoxide, mobile oxygen present in ceria can react with CO, as oxygen re-oxidizes the reduced ceria. As CeO 2 can enhance the rate of one of the main steps (water dissociation) [27,[42][43][44][45] due to the existence of O vacancies and Ce 3+ , it is considered to be the key in WGSR pathways.…”