The Low Temperature Conversion of Methane to Methanol on CeO x /Cu 2 O catalysts: Water Controlled Activation of the C H Bond

Transcription

1 The Low Temperature Conversion of Methane to Methanol on CeO x /Cu 2 O catalysts: Water Controlled Activation of the C H Bond Zhijun Zuo, a Pedro J. Ramírez, b Sanjaya Senanayake, a Ping Liu c,* and José A. Rodriguez c,* a Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan , Shanxi, China b Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020 A, Venezuela c Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA Supporting Information: 1. Theoretical and experimental methods A. Computational method The DFT was performed using the Vienna Ab Initio Simulation Package [1]. The generalized gradient approximation (GGA) with the PBE functional and pseudopotential generated in the projector-augmented wave method were employed [2]. A k-point mesh was used to sample the Brillouin zone. A cutoff energy of 400 ev was sufficient to obtain a satisfactory convergence of the total energy. The transition state (TS) was derived by using the nudged elastic band method [3]. The Cu 2 O/Cu(111) surface was consisted of a three-layer 4 4 Cu(111) surface and one monolayer of Cu 2 O [4]. The Cu(111) surface was simulated by a three-layer, and the vacuum thickness is 20 Å. For Ce 3 O 6 /Cu 2 O/Cu(111) surface, Ce 3 O 6 cluster adsorbs on the Cu 2 O/Cu(111) surface, which was similar with our previous works [5]. During the optimization, the bottom two layers of the Cu surface were fixed at the lattice position, while the other atoms including the Cu 2 O, Ce 3 O 6 cluster and adsorbates were fully relaxed. In order to account for the strong localization of the Ce 4f and Cu 3d electrons of Ce 3 O 6 and Cu 2 O, Hubbard-like U term was used, (GGA+U). U eff of 4.5 and 5.2 ev for Ce 3 O 6 and Cu 2 O were used, which were used in our previous works [4b, 5a, 6]. S1

2 The dispersion force was not considered in the present DFT calculations. Including the dispersion force will help to increase the binding energy of methane; yet the highly positive free energy for methane adsorption under reaction conditions may still prevent the molecule sticking to the surface. In addition, using the dispersion force can also lead to the overestimation of the bindings for the strongly bound species, e.g. OH(ads) and CH 3 (ads). B. Experimental method The experiments examining the conversion of methane into methanol over CeO 2 /CuO x /Cu(111) surfaces were performed in a set-up that combined an ultra-high vacuum (UHV) chamber for surface characterization and a micro-reactor for catalytic tests. [7-10] The UHV chamber was equipped with instrumentation for X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), ion-scattering spectroscopy(iss), and thermal-desorption mass spectroscopy (TDS). [7-10] The methodology followed for the preparation of the CeO 2 /CuO x /Cu(111) surfaces is described in detail in refs. [8-10] In the studies of methane activation, the sample was transferred to the batch reactor at ~300 K, then the reactant gas, 1 Torr of pressure, was introduced. In the experiments testing the activity of the CeO 2 /CuO x /Cu(111) catalysts for the conversion of methane to methanol, the samples were exposed to a mixture of 1 Torr of CH 4, 0.5 Torr of O 2 and 0-4 Torr of H 2 O at 300 K and were rapidly heated to the reaction temperature of 450 K in the batch reactor. Product yields were analyzed by mass spectroscopy or gas chromatography. In our experiments data were collected at intervals of 5 min. The amount of molecules (methanol, CO or CO 2 ) produced in the catalytic tests was normalized by the active area exposed by the sample and the total reaction time. The kinetic experiments were done in the limit of low conversion (< 10%). S2

3 2. Results C 1s XPS CH 4 1 Torr C Photoemission intensity (arb units) 450 K CO x CH x C 300 K Binding Energy (ev) Figure S1. C 1s spectra collected after exposing a CeO 2 /Cu 2 O/Cu(111) surface to 1 Torr of methane at 300 or 450 K for 5 minutes. The Cu 2 O film was covered ~ 40% by ceria. S3

4 C 1s XPS C Photoemission intensity (arb units) H 2 O 4 Torr, 450 K CO x C CH 4 1 Torr, 450 K Binding Energy (ev) Figure S2. Sequential dosing of methane and water at 450 K to a CeO 2 /Cu 2 O/ Cu(111) surface in which 40% of the Cu 2 O film covered by ceria. In the first step the exposed in a micro-reactor to 1 Torr of methane for 5 minutes at 450 K. Then, the methane was pumped out from the micro-reactor and the surface was exposed to 4 Torr of water for 5 minutes at 450 K. S4

5 Figure S3. DFT-optimized structures of Cu 2 O/Cu(111) (a) and CeO 2 /Cu 2 O/Cu(111) (b) model surfaces. The blue, brown, and red balls represent Ce, Cu, and O atoms, respectively. Each Ce 3 O 6 cluster preferred to adsorb over a hexagonal ring without chemisorbed oxygen in the center, which allowed the center oxygen of Ce 3 O 6 fitting into the hexagonal cavity. Such model is 1.26 ev more stable than that with Ce 3 O 6 cluster covered over the hexagonal ring with chemisorbed oxygen in the center. In our model, a Ce 3 O 6 occupied 1/3 ML of Cu(111) surface. S5

6 Table S1 The adsorption energies (E ads, ev) of possible intermediates on Cu 2 O/Cu(111) and CeO 2 / Cu 2 O/Cu(111) surfaces E ads Cu 2 O/Cu(111) CeO 2 /Cu 2 O/Cu(111) H(ads) OH(ads) CH 4 (ads) CH 3 (ads) CH 3 OH(ads) H 2 O(ads) a 2H(ads) a dissociative adsorption on Ce adsorption site S6

7 (a) (b) (c) Figure S4. Side and top views of intial (a), transition (b) and final (c) states involved in the reaction between dissociated CH 3 from methane and OH from water to produce methanol on a CeO 2 /Cu 2 O/Cu(111) model surface. The blue, brown, red, grey and white balls represent Ce, Cu, O, C and H atoms, respectively. S7

8 Reaction energy(ev) TS2 Cu2O/Cu(111) TS1 TS3 OH-CeO2/Cu2O/Cu(111) -2.4 CH4(g)+H2O(g) CH4(g)+*H2O CH4(g)+*H+*OH *CH4+*H+*OH *CH3+2*H+*OH *CH3OH+2*H CH3OH(g)+2*H CH3OH(g)+H2(g) Figure S5. Potential energy diagram of methanol synthesis from CH 4 steam reforming on Cu 2 O/ Cu(111) and CeO 2 /Cu 2 O/Cu(111) surfaces. The energy cost for the gas phase reaction (1.32 ev) is close to that from the NIST database (1.20 ev). Up to the point of methanol formation and desorption the reaction is downhill on CeO 2 /Cu 2 O/Cu(111). The final step involves the recombination of H atoms to yield H 2 and requires energy, but it could be overcome by a reaction temperature of 450 K or by reaction of H with the oxygen provided in the reaction feed: CH 4 + H 2 O CH 3 OH + 2H(ads) 0.5O 2 O(ads) 2H(ads) + O(ads) H 2 O CH O 2 CH 3 OH ( G o 298K= ev) a a From reference 11 S8

9 Table S2 Imaginary frequencies for the transition states of key elementary steps involved in methane reforming to methanol on Cu 2 O/Cu(111) and CeO 2 / Cu 2 O/Cu(111) surfaces Surface Reaction Imaginary frequency H 2 O(ads) + * OH(ads) + H(ads) cm -1 Cu 2 O/Cu(111) CH 4 (ads) + * CH 3 (ads) + H(ads) cm -1 CH 3 (ads) + OH(ads) CH 3 OH(ads)+ * cm -1 CH 4 (ads) + * CH 3 (ads) + H(ads) cm -1 CeO 2 /Cu 2 O/Cu(111) CH 3 (ads) + OH(ads) CH 3 OH(ads) + * cm -1 * represents a surface free site. CH 4 (ads) + OH(ads) CH 3 OH(ads) + H(ads) cm -1 S9

10 Figure S6. Optimized configurations of OH(ads) adsorption on the ring center of Cu 2 O/Cu(111) surface. The brown, red and white balls represent Cu, O and H atoms, respectively. Figure S7. Structures of transition states for methanol synthesis involving dissociative adsorption of methane on OH(ads) groups located at bridge (a) and atop (b) sites of Cu ions on Cu 2 O/Cu(111). The brown, red, grey and white balls represent Cu, O, C and H atoms, respectively. S10

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