Influence of Surface Reactivity of Lanthanum Oxide on the Activation of Methane and Oxygen†

doi:10.7503/cjcu20190063

Abstract

Abstract:

The properties of lanthanum oxide facets[(001),(110) and (100)] and the adsorption, activation and dissociation of methane and oxygen on them were studied by means of periodical density functional theory(DFT), based on the previous experimental investigation upon structure sensitivity of lanthanum oxide in Oxidative coupling of methane(OCM). The result shows that surface energy of lanthanum oxide facets decreases in the order of (110)>(100)>(001), and the energy gap between valence and conduction bands follows the order of (110)<(100)<(001), meaning that the facet (001) is most stable and (110) is most reactive among the three facets. The associative adsorption of methane on lanthanum oxide is very weak(ca. 0.03 eV), and the barriers for H—CH₃ on lanthanum oxide (001), (110) and (100) surfaces are 2.16, 0.68 and 0.90 eV, respectively, showing the reactivity of the facets is matched with the dissociation of methane. On the other hand, the associative adsorption energy of oxygen on lanthanum oxide (001), (110) and (100) surfaces is -0.04, -0.31 and -0.12 eV, respectively, and the dissociation barrier is 1.22, 0.53 and 1.52 eV, respectively. The oxygen adsorption is greatly influenced by the varied structures of lanthanum oxide. The interaction of oxygen with the facet (001) is the weakest, but the strongest with the facet (110) among the three facets of lanthanum oxide, which promotes O—O bond cleavage at lower barrier(0.53 eV) on the facet (110) to lead to the formation of electrophilic peroxide species. As oxygen adsorption on lanthanum oxide is stronger than methane, the structure sensitivity of lanthanum oxide in OCM should be closely associated with oxygen adsorption and activation. In addition, the mechanism of methane and oxygen activation on lanthanum oxide can be ascribed to the transfer of electrons from the surface to their antibonding orbitals, which implies that varied structures of the lanthanum oxide surfaces can change the driving forces on the electron transfer.

Key words: Lanthanum oxide, Methane, Oxygen, Structure sensitivity, Density functional theory

CLC Number:

O643

TrendMD:

CHENG Wenmin,XIA Wensheng,WAN Huilin. Influence of Surface Reactivity of Lanthanum Oxide on the Activation of Methane and Oxygen^†[J]. Chem. J. Chinese Universities, 2019, 40(5): 940.

Figures/Tables 8

Fig.1 Structures for bulk La2O3 and the experimental and calculated(in parentheses) values for the three unique La-O bond lengths in nmO4c and O6 crepresent 4- and 6-coordinated oxygen atoms, respectively.

Fig.2 Side(A1—D1) and top(A2—D2) views of La2O3 surfaces(A1, A2)(2×2)(001);(B1, B2)(1×1)(110);(C1, C2)(2×2)(100), before dipole correction;(D1, D2)(2×2)(100), after dipole correction; O4c and O6c represent 4- and 6-coordinated oxygen atoms, respectively.

Fig.3 Density of states(DOS) of La2O3 surfaces (001)(A), (110)(B) and (100)(C)The densities of states above 0 are for orbitals with spin up, and those below 0 are for orbitals with spin down.

Fig.4 Structures, energies and related bond length of associative(vdw) and dissociative(dis) adsorption states and corresponding transition states(TS) for CH4 on La2O3 surfaces(001), (110) and (100)

Fig.5 Diagram of potential energy surface for methane C—H activation on La2O3 surfaces(A) (001), H is bound to O4c; (B) (110), H is bound to O6c; (C) (100), H is bound to O6c; (D) (100), H is bound to O4c.The energy is in eV.

Fig.6 Structures, energies and related bond length of O2 associative adsorption(vdw), dissociative(dis) adsorption modes and corresponding transition states(TS) on La2O3 surfaces (001), (110) and (100)

Fig.7 Diagram of potential energy surface for O—O activation on La2O3 surfaces(A) (001), O is bound to two O4c; (B) (110), O is bound to two O4c; (C) (110), O is bound to O6c/O4c;(D) (100), O is bound to O6c/O4c. The energy is in eV.

Table 1 Energy of the preferred associative adsorption states, dissociative adsorption states and transition states relative to reactants(Ead(AB), Ead(A-B), ETS(A-B)) for CH4 and O2 on La2O3 surfaces, and an analysis on Bader charges(qAB, qTS(A-B), qA-B) of the corresponding species

Crystal plane of La₂O₃	E_ad(AB)/eV		E_ad(A-B)/eV		E_TS(A-B)/eV		q_AB/e		q_TS(A-B)/e		q_A-B/e
Crystal plane of La₂O₃	CH₄	O₂	H—CH₃	O—O	H—CH₃	O—O	CH₄	O₂	H—CH₃	O—O	H—CH₃	O—O
(001)	-0.03	-0.04	1.56	0.57	2.16	1.22	-0.01	-0.02	-0.03	-1.02	-0.10	-1.19
(110)	-0.04	-0.31	0.45	-0.43	0.68	0.53	-0.02	-0.24	-0.10	-1.05	-0.10	-1.34
(100)	-0.03	-0.12	-0.22	-0.13	0.90	1.52	-0.02	-0.10	-0.09	-1.04	-0.12	-1.30

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