science.sciencemag.org/content/368/6490/513/suppl/DC1

Supplementary Materials for

Water-promoted interfacial pathways in methane oxidation to methanol

on a CeO2-Cu2O catalyst

Zongyuan Liu*, Erwei Huang*, Ivan Orozco, Wenjie Liao, Robert M. Palomino,

Ning Rui, Thomas Duchoň, Slavomir Nemšák, David C. Grinter, Mausumi Mahapatra,

Ping Liu†, José A. Rodriguez†, Sanjaya D. Senanayake†

*These authors contributed equally to this work.

†Corresponding author. Email: pingliu3@bnl.gov (P.L.);

rodrigez@bnl.gov (J.A.R.); ssenanay@bnl.gov (S.D.S.)

Published 1 May 2020, Science 368, 513 (2020)

DOI: 10.1126/science.aba5005

This PDF file includes:

Materials and Methods

Figs. S1 to S14

Tables S1 and S2

References

mailto:pingliu3@bnl.govmailto:rodrigez@bnl.govmailto:ssenanay@bnl.gov

2

Materials and Methods

Experimental Methods

The ambient-pressure XPS experiments for the catalysts were performed at the Advanced

Light Source in Lawrence Berkeley National Laboratory (beamline 9.3.2). The model catalyst was

generated in a preparation chamber connected to the analysis chamber by vapor evaporating Ce

metal onto a Cu(111) single crystal held at 600 K in the presence of 5 × 10−7 torr O2 and

subsequentially post-annealing at the same conditions for 5 minutes, which led to the formation of

CeO2/Cu2O/Cu(111) surfaces (16). The ceria coverages presented in this work are estimated to be

0.3-0.5 ML where a maximum of methane to methanol conversion has been observed (16). The

prepared surfaces were then exposed to different reactant feeds (CH4, CH4/O2, CH4/O2/H2O) at a

range of temperatures (300-550 K) while performing the AP-XPS measurements. The spectra were

recorded using a VG-Scienta R4000 HiPP analyzer. A photon energy of 650 eV was used to excite

the O 1s region. The C 1s, Cu 3p, and Ce 4d regions were probed with a photon energy of 490 eV.

The total energy resolution in the photoemission experiments was ∼0.2 eV. Similar experiments were also performed in a commercial SPECS AP-XPS chamber equipped with a PHOIBOS 150

EP MCD-9 analyzer at the Chemistry Division of Brookhaven National Laboratory (BNL) to

acquire the Cu 2p, Cu LMM and Ce 3d spectra using a Mg K-α source.

Density Functional Theory (DFT) Calculations

In these calculations, we used Ce3O6 cluster which was employed before to model the

CeO2/Cu2O/Cu(111) system (16), reproducing well its chemical properties. Images of scanning

tunneling microscopy for CeO2/Cu2O/Cu(111) surfaces show two types of ceria nanoparticles (22).

The first type, has a size of 30-50 nm and the essential chemistry occurs on the borders which can

be represented by a Ce3O6 cluster where the atoms have a low coordination (16). The second type

of ceria nanoparticle is much smaller (ranging from 0.5 to 5 nm) and is well represented by the

Ce3O6 cluster embedded on a copper oxide film supported on Cu(111) (16). All the calculations

were performed by Vienna Ab Initio Simulation Package (VASP) (16, 21, 22 27) based on spin-

polarized DFT. A 400eV kinetic energy cutoff (16, 22) and the projector augmented wave method

(PAW) together with GGA exchange correlation functional with the PBE functional (16) were

employed. The Brillouin zone of all the surface calculations was sampled by Monkhorst-Pack )

meshes with 3×3×1 while 9×9×9 was applied for all gas-phase species (16). Geometry

optimizations were converged with criteria that total energy change and the maximum forces on

all atoms were less than 10−5 eV and 0.02 eV Å−1, respectively. A Gaussian smearing with width

0.05 eV was used to improve the convergence. The Cu (3p, 3d, 4s), Ce (4f, 5s, 5p, 5d, 6s), C (2s,

2p), O (2s, 2p) and H (1s) electrons were treated as valence states, while the remaining electrons

were kept frozen as core states. For each reaction intermediates, calculations with nudged elastic

band method (NEB) (16, 21) were performed to derive their transition states. The dispersion forces

were not considered here due to the overestimation the strong interactions, e.g. water and oxygen,

which represent the major part of the reaction network and thus the activity/selectivity.

CeO2/Cu2O/Cu(111) catalyst was modeled with 3 layer of 4×4 Cu(111) surface and one

monolayer of CuxO (x 1.13) with Ce3O6 cluster deposited on top (Fig. S5), according to our

previous works (16, 22). The Hubbard-like U term was also considered to address Cu 3d and Ce

4f electrons for the CuxO surface (Ueff = 5.2) and Ce3O6 cluster (Ueff = 4.5), in which the Coulomb

U and exchange J parameters were combined into a single parameter Ueff = U - J.

The adsorption energy obtained directly from DFT calculations for adsorbate on the

CeO2/Cu2O/Cu(111) model surface was calculated as:

3

ΔEads = E(Adsorbate/Surface) − E(Adsorbate) − E(Surface)

where E is the total energy calculated using DFT. The zero-point energy (ZPE) correction

(EZPE) was included as:

𝐸𝑍𝑃𝐸 = ∑1

2ℎ𝑣𝑖

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑑𝑒𝑠

𝑖=1

where vi is DFT obtained vibrational frequency for each intermediate.

Kinetic Monte Carlo (KMC) Simulations

In KMC simulations, to describe CeO2/Cu2O/Cu(111), CeO2 cluster were randomly

distributed on a Cu2O layer supported by a 128 128 matrix of Cu(111), corresponding to 0.1

monolayer (ML) of coverage with respect to Cu on Cu(111) (Fig. S9). Only the CeO2-

Cu2O/Cu(111) interface site were considered as the active sites for selective CH4 oxidation

according to the DFT calculations. All elementary steps involved in the KMC simulations were

listed in Fig. S14 and Table S1. The experimental conditions (pressure ratio: CH4:O2 = 2:1 or

CH4:O2:H2O = 2:1:8; T = 450K) were considered. The elementary reactions rate constant k was

calculated by Arrhenius equation given by:

𝑘 = 𝐴0 ∗ 𝑒𝑥𝑝 (−𝐸𝑎

𝑘𝐵𝑇)

in which, Ea is the activation energy with ZPE-correction from DFT calculations for each

elementary reaction; kB is the Boltzmann constant and the T stands for the absolute temperature

implemented in KMC simulations. A0 represents pre-exponential factor, a constant for each

elementary reaction which may vary by several orders of magnitude for different elementary

reactions (28). In the present work, A0 was calculated by (29):

𝐴0 =𝑘𝐵𝑇

ℎ𝑒𝑥𝑝 (

∆𝑆

𝑘𝐵)

in which, h is Planck’s constant. ∆S is the entropy change from initial state to transition state

for each elementary reaction. The entropy (S) for each intermediate was estimated with their

corresponding DFT calculated vibrational frequency (29):

𝑆 = 𝑘𝐵 ∑ {

ℎ𝑣𝑖𝑘𝐵𝑇

⁄

𝑒𝑥𝑝 (ℎ𝑣𝑖

𝑘𝐵𝑇⁄ ) − 1

− ln [1 − 𝑒𝑥𝑝 (−ℎ𝑣𝑖

𝑘𝐵𝑇⁄ )]}

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑑𝑒𝑠

𝑖=1

where vi is the above DFT obtained vibrational frequency for each intermediate during the

vibrational frequency calculations.

The entropy for gas phase molecules involving in the elementary steps was taken from NIST

Computational Chemistry Comparison and Benchmark DataBase (CCCBDB) (30). The calculated

pre-exponential factor for each step was listed in Table S2.

Furthermore, the adsorption coefficient for gas phase molecules was calculated as (29):

𝑘𝑎𝑑𝑠 = 𝑃𝐴𝑠𝑖𝑡𝑒𝜎

√2𝜋𝑚𝑘𝐵𝑇

where P is the pressure of the adsorption gas, Asite is the area of a single site, 𝜎 is the sticking coefficient, and m is the mass of adsorption gas. The sticking coefficients for O2, CH4 and H2O

are all set to 1 in our KMC model, the coverage of adsorbed molecules on the surface, which is

catalytically interesting, strongly depends on the desorption rate or the binding energy of each

molecule.

4

Fig. S1. O 1s region of the AP-XPS spectra after exposing the CeO2/Cu2O/Cu(111) surface (θCeO2~

0.5 ML) to 80 mTorr of H2O from 300 K to 450 K. Two types of hydroxyl species were observed,

-OH(1) and -OH(2), and they are assigned to hydroxyls bound to Cu2O (~ 531.1 eV) and CeO2 sites

(~ 532.1 eV), respectively.

5

Fig. S2. Deconvoluted C 1s region of the AP-XPS spectra shown in Fig. 2A collected while

exposing a CeO2-Cu2O-Cu(111) surface to 20 mTorr of CH4 + 80 mTorr of H2O + 10 mTorr of O2

at 300 K.

6

Fig. S3. Deconvoluted C 1s region of the AP-XPS spectra of the CeO2-Cu2O-Cu(111) surface at

450 K under a 20 mTorr CH4 + 80 mTorr H2O + 10 mTorr O2 (top) and 20 mTorr CH4 + 80 mTorr

H2O (bottom) of gas mixture.

7

Fig. S4. Ce 3d region in AP-XPS for a CeO2-Cu2O-Cu(111) surface exposed to a 20 mTorr of

CH4 at different temperatures (θ ≈ 0.5 ML).

8

Fig. S5. Ce 3d region in AP-XPS for a CeO2-Cu2O-Cu(111) surface exposed to a 20 mTorr of

CH4 + 80 mTorr of H2O + 10 mTorr of O2 at different temperatures (θ ≈ 0.5 ML).

9

Fig. S6. (A) Top and side view of DFT-optimized structure for CeO2/Cu2O/Cu(111); (B) Partial

density of states (PDOS) of Ce 4f for CeO2/Cu2O/Cu(111); Yellow: Ce; brown: Cu; red: O.

10

Fig. S7. DFT-calculated potential energy diagram of CH4 oxidation by O2 on CeO2/Cu2O/Cu(111).

Inset: Structures for selected intermediates. Yellow: Ce; brown: Cu; red: O in

CeO2/Cu2O/Cu(111); green: O in O2 gray: C; white: H.

11

*CH4 + *O TS of *CH4 + *O to *CH3OH *O

A B C

TS of *CH4 + *O to *CH3O + *H *CH3O + *H*CH3OH

D E F

G H I

TS of *CH2O + 2*H

to *HCO + *H2O + *H + *Ov*CH2O + 2*H

TS of *CH3O + *H

to *CH2O + 2*H

12

Fig. S8. Top and side view for the structures of reaction intermediates and transition states (TS)

involved in CH4 oxidation by O2 on CeO2/Cu2O/Cu(111), where the corresponding energies

were shown in Fig. S7. Yellow: Ce; brown: Cu; red: O in CeO2/Cu2O/Cu(111); green: O in O2;

gray: C; white: H.

*HCO + *H2O + *H + *Ov *HCO + *H + *Ov *HCOO + *H + *Ov

J K L

M N O

*CO2 + *H2O + 2*OvTS of *HCOO + *H + *Ov

to *COO + *H2O + 2*Ov*CO2 + 2*Ov

*CO2 + *Ov *CO2 Model Catalyst

P Q R

13

Fig. S9. KMC snapshot of CeO2/Cu2O/Cu(111) matrix under steady states when exposure to

CH4/O2 with pressure ratio of 2:1 and 450K. Yellow: Ce; Big red: O in CeO2: Small Red: O in

Cu2O; small brown: Cu in in Cu2O; small gray (inset): Cu in Cu(111).

14

Fig. S10. DFT-calculated potential energy diagram of CH4 oxidation by H2O and O2 on

CeO2/Cu2O/Cu(111). Inset: Structures for selected intermediates. Yellow: Ce; brown: Cu; red: O

in CeO2/Cu2O/Cu(111); green: O in O2 gray: C; white: H.

15

*H2O *OH +*H *CH4 + *OH + *H

A B C

2*H*CH3OH + 2*HTS of *CH4 + *OH + *H

to *CH3OH + 2*H

D E F

16

Fig. S11. Top and side view for the structures of reaction intermediates and transition states (TS)

involved in CH4 oxidation by O2 and H2O on CeO2/Cu2O/Cu(111), where the corresponding

energies were shown in Fig. S10. Yellow: Ce; brown: Cu; red: O in CeO2/Cu2O/Cu(111); green:

O in O2; purple: O in H2O gray: C; white: H.

*H2O + *OH + *H + *OvTS of *H2O + 2*H to

*H2O + *OH + *H + *Ov*H2O + 2*H

G H I

*OH + *H + *Ov *H2O*OH + *H

J K L

17

Fig. S12. Top and side view for the structures of reaction intermediates and transition states (TS)

involved in the conversion process from *CH3O to CH3OH by water extraction on

CeO2/Cu2O/Cu(111), where the corresponding energies were shown in Fig. 3C. Yellow: Ce;

brown: Cu; red: O in CHxO and CeO2/Cu2O/Cu(111); purple: O in H2O gray: C; white: H.

A B C

D E

*OH *CH3O

18

Fig. S13. KMC screenshot of CeO2/Cu2O/Cu(111) matrix under steady states when exposure to

CH4/O2/H2O with pressure ratio of 2:1:8 and 450K. Yellow: Ce; Big red: O in CeO2: Small Red:

O in Cu2O; small brown: Cu in in Cu2O; small gray: Cu in Cu(111); pink: *OH; blue: *CH3O.

19

Fig. S14. KMC-extracted major operating pathway on CeO2/Cu2O/Cu(111) on exposure to

CH4/O2 (A) and CH4/O2/H2O (B).

20

Table S1. ZPE-corrected reaction barrier, Ea (kcal/mol), and reaction energy, E (kcal/mol), for

each elementary steps involved in selective methane oxidation over CeO2/Cu2O/Cu(111) by O2

or O2/H2O.

Elementary Reactions Ea E

CH4 (g) + * → *CH4 -- -2.54

O2 (g)+ * → *O2 -- -14.53

H2O (g) + * → *H2O -- -15.91

*CH3OH → CH3OH (g) + * 20.06 20.06

*CO2 → CO2 (g) + * 13.84 13.84

*CH4 + *H + *OH → *CH3OH + 2*H 22.37 -18.68

*CH3OH + 2*H → *CH4 + *H + *OH 41.05 18.68

*H2O + 2*H + * → *H + *OH + *H2O + *Ov 2.31 -8.30

*H + *OH + *H2O + *Ov → *H2O + 2*H + * 10.61 8.30

*OH + *H → *H2O + * 18.91 18.91

*H2O + * → *OH + *H 0.00 -18.91

*O2 + * → 2*O 5.54 -24.44

2*O → *O2 + * 29.98 24.44

*CH4 + *O → *CH3O + *H 18.45 -37.82

*CH3O + *H → *CH4 + *O 56.27 37.82

*CH4 + *O → *CH3OH + * 16.37 -23.29

*CH3OH + * → *CH4 + *O 39.66 23.29

*CH3OH + * → *CH3O+ *H 0.00 -14.53

*CH3O + *H → *CH3OH + * 14.53 14.53

*CH3O + *H + * → *CH2O + 2*H 7.15 -14.53

*CH2O + 2*H → *CH3O + *H + * 21.68 14.53

*CH2O + 2*H + * → *HCO + *H2O + *H + *Ov 20.52 -51.89

*HCO + *H2O + *H + *Ov → *CH2O + 2*H + * 72.41 51.89

*HCOO + *H + *Ov + *→ *CO2 + *H2O + 2*Ov 7.61 -32.51

*CO2 + *H2O + 2*Ov → *HCOO + *H + *Ov + * 40.12 32.51

21

*CH3O + *H2O → *CH3OH + *OH 6.69 4.85

*CH3OH + *OH → *CH3O + *H2O 1.84 -4.85

22

Table S2. DFT-calculated pre-exponential factors for each elementary step involved in selective

methane oxidation over CeO2/Cu2O/Cu(111) by O2 or O2/H2O.

Elementary Reactions Pre-exponential Factors(s-1)

*CH4 + *H + *OH → *CH3OH + 2*H 2.74E+12

*CH3OH + 2*H → *CH4 + *H + *OH 1.15E+15

*H2O + 2*H + * → *H + *OH + *H2O + *Ov 1.17E+13

*H + *OH + *H2O + *Ov → *H2O + 2*H + * 4.26E+12

*OH + *H → *H2O + * 4.05E+12

*H2O + *→ *OH + *H 2.17E+13

*O2 + * → 2*O 1.58E+12

2*O → *O2 + * 3.92E+13

*CH4 + *O → *CH3O + *H 2.16E+11

*CH3O + *H → *CH4 + *O 4.73E+14

*CH4 + *O → *CH3OH + * 2.36E+12

*CH3OH + * → *CH4 + *O 3.08E+13

*CH3OH + * → *CH3O + *H 2.63E+13

*CH3O + *H → *CH3OH + * 3.35E+12

*CH3O + *H + * → *CH2O + 2*H 1.70E+12

*CH2O + 2*H → *CH3O + *H + * 6.13E+10

*CH2O + 2*H + *→ *HCO + *H2O + *H + *Ov 3.81E+15

*HCO + *H2O + *H + *Ov → *CH2O + 2*H + * 1.04E+16

*HCOO + *H + *Ov + *→ *CO2 + *H2O + 2*Ov 8.15E+12

*CO2 + *H2O + 2*Ov → *HCOO + *H + *Ov + * 1.37E+14

*CH3O + *H2O → *CH3OH + *OH 4.16E+13

*CH3OH + *OH → *CH3O + *H2O 3.20E+14

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