Stability of Hydroxylated (1̄11) and (1̄01) Surfaces of Monoclinic Zirconia:  A Combined Study by DFT and Infrared Spectroscopy

Abstract

We report the structure and stability of hydroxylated (1 h11) and (1 h01) surfaces for a water coverage ranging from 0.25 to 1. For the (1 h11) surface water dissociated at low coverage (θ ) 0.25), whereas both dissociated and molecular species coexisted at higher coverage. For the (1 h01) surface molecular adsorption was only observed at the highest coverages (θ ) 0.75 and θ ) 1). The calculated adsorption energy for (1 h11) ranged from -1.20 to -0.83 eV for θ ) 0.25 and θ ) 1, respectively. For the (1 h01) surface the adsorption energies ranged from -1.50 to -1.21 eV for θ ) 0.25 and θ ) 1, respectively. The 1-, 2-, and 3-fold coordinated hydroxyl groups were present in our models. Their structure, energetics, and vibrational frequencies were calculated by ab initio techniques and agreed with in situ infrared spectroscopic measurements. The simultaneous presence of hydroxyl groups of different coordination was concluded from both theoretical and experimental results.

Understanding Structure and Stability of Monoclinic Zirconia Surfaces from First-Principles Calculations

“…The m-ZrO 2 bulk was allowed to fully relax until the maximum residual force has been below 0.025 eVÅ −1 , resulting in the structure with a = 5.161Å, b = 5.231Å, c = 5.340Å, and β = 99.6 • , which differs from the experimental data [42,23] by 0.8% at most and is consistent with other computational results. [30,19,18] The m-ZrO 2 surfaces were constructed from the optimized bulk as (1 × 1) two-dimensional slabs of four atomic layers thick (16 ZrO 2 units per computational cell), with the two bottom layers (8 ZrO 2 units) fixed at the ideal bulk positions. The surface slabs were surrounded by the sufficient amount of vacuum along the non-periodic direction to ensure the decay of the wave function within the computational cell.…”

“…The coordination numbers of the surface oxygen atoms are the same for each surface: one [16]; c Ref. [19] two-coordinated and four three-coordinated atoms per computational cell. Fig.…”

“…On the (111) surface, the hydroxyl species showed no preference of the surface zirconium atom, as reflected in the adsorption energies ranging from −0.99 to −1.09 eV, consistent with the earlier computational results. [30,19]. On the two other surfaces, the hydroxyl species clearly favored interaction with the fivecoordinated zirconium atoms, resulting in the dissociative adsorption energies of −1.97 and −1.54 eV for the (011) and (101) surfaces, respectively.…”

“…Zirconia is typically exposed to the water-rich pre-treatment and/or reaction conditions, where adsorption of water on the surfaces of the crystallites may affect their structure [19,33] and stability [41,18] and even alter the properties of the surface sites. [20,44] In particular, zirconia has been extensively studied as a support and a catalyst in reforming processes, [13,12,14,10,9] where the water-gas shift cycle and the formate formation from CO and water are the key steps.…”

“…A few research groups have reported water adsorption on the most abundant low-index m-ZrO 2 surfaces, such as (111), (101), and (111), to be dissociative at low surface coverage. [15,19] Piskorz et al have performed computational studies on surface hydration and equilibria of m-and t-ZrO 2 . [30,31] The authors have reported significant stabilization of the (111), (111), (011), and (001) surfaces of m-ZrO 2 upon adsorption of water.…”