Ab initio thermodynamic evaluation of Pd atom interaction with CeO2 surfaces

Abstract

Palladium supported on ceria is an effective catalytic material for three-way automotive catalysis, catalytic combustion, and solid-oxide fuel cell (SOFC) anodes. The morphology, oxidation state, and particle size of Pd on ceria affect catalytic activity and are a function of experimental conditions. This work utilizes ab initio thermodynamics using density functional theory (DFT) (DFT+U) methods to evaluate the stability of Pd atoms, PdO(x) species, and small Pd particles in varying configurations on CeO(2) (111), (110), and (100) single crystal surfaces. Over specific oxygen partial pressure and temperature ranges, palladium incorporation to form a mixed surface oxide is thermodynamically favorable versus other single Pd atom states on each ceria surface. For example, Pd atoms may incorporate into Ce fluorite lattice positions in a Pd(4+) oxidation state on the CeO(2) (111) surface. The ceria support shifts the transition between formal Pd oxidation states (Pd(0), Pd(2+), Pd(4+)) relative to bulk palladium and stabilizes certain oxidized palladium species on each surface. We show that temperature, oxygen pressure, and cell potential in a SOFC can influence the stable states of palladium supported on ceria surfaces, providing insight into structural stability during catalytic operation.

Periodic trends of oxygen vacancy formation and C–H bond activation over transition metal-doped CeO2 (1 1 1) surfaces

“…We introduced a U term on the 4f orbitals of ceria to properly describe the localization of electrons in reduced cerium oxide (CeO 2Àx ). We used a U value of 5 eV, which is consistent with values used in previous DFT studies of ceria [1,20,25,[27][28][29]35,[49][50][51]. Our previous study examined the dependence of oxygen vacancy and methane activation on the Hubbard U term on CeO 2 and Pd x Ce 1Àx O 2 .…”

“…Similarly to the experimental studies, some of the dopants that have been thoroughly examined are Au [20,22,23], Cu [3,24,26], Pd [3,21,25,[27][28][29], and Zr [1,16,34,35], with concentration on surface reducibility as well as CO, CH 4 , H 2 , or H 2 O interactions with the surface. Using Au as a dopant in the CeO 2 lattice for CO oxidation allows the surface oxygen atoms to become reactive, but not too reactive that the oxygen vacancies will not heal.…”

“…Several experimental [8][9][10][11][12][13]18,19] and theoretical studies [1][2][3][20][21][22][23][24][25][26][27][28][29] have examined how one or two transition metal dopants can affect the activity of the ceria surface; however, an electronic level explanation remains incomplete. For example, density functional theory (DFT) studies have demonstrated that Pd becomes the reduction center for O-vacancy formation or methane conversion when doped into CeO 2 [27][28][29][30]. Herein, density functional theory (DFT + U) is used to examine the energetic and electronic structure effects of all groups IV-XII transition metal dopants on CeO 2 (1 1 1) reducibility and C-H bond activation.…”

Catalytic propane reforming mechanism over Mn-Doped CeO2 (111)

“…This approach introduces an on-site Coulombic interaction (U-term) which penalizes non-integer occupation of localized orbitals, effectively penalizing delocalization of electrons. A U-value of 5 eV was utilized on the f-states of cerium, consistent with previous studies of ceria [2,3,13,[29][30][31][32][33]. The oxygen vacancy formation and methane adsorption energies depend upon the U-value of the f-states of cerium [29].…”

“…The fluorite crystal structure of pure CeO 2 is the same as used in our previous studies, with a bulk lattice parameter of (5.466 Å) which is within 1% of experimental values [2,3,13,[29][30][31]. The (1 1 1) surface was utilized as it is the lowest energy surface of ceria and will, therefore, represent a large portion of the surface of polycrystalline ceria [29,32,36].…”