Volume 2,
Issue 9
2008.1
Theoretical Study of Methanol Decomposition and CO Oxidation Mechanisms on Pt(111)
Surface
蔡順如、 江志強
Method : Density functional theory
Package : VASP 4.6.27
Potential : PAW_GGA
parameter :
spin polarized calculations : ISPIN = 1
energy cutoff : ENCUT= 350 eV
K-point: 3x3x3 (bulk)
3x3x1 (Geometry optimization)
3x3x1 (scan)
Vacuum slab =12 angstrom
The surfaces have been modeled by a
periodic slab composed of six metal layers
and top four layers of atom was allowed to
relax .
binding
configuration
bond length
methanol(CH
3OH)
-0.27
-0.33
top-bound
through oxygen
1.45a
1.44
2.44b
2.43
methoxy(CH3
O)
-1.49
-1.54
top-bound
through oxygen
1.41a
1.40
2.04b
2.03
formaldehyde
(CH2O)
-0.44
-0.50
top-bound
through oxygen
1.34a
1.34
2.12b
2.10
formyl(CHO)
-2.02
-2.36
top-bound
through carbon
1.21a
1.21
1.99d
2.02
carbon monoxide(CO)
-1.65
top-bound
through carbon
1.16a
HCP-bound
through carbon
1.20a
FCC-bound
through carbon
1.20a
bridge-bound
through carbon
1.18a
top-bound
through carbon
1.39a
binding energy (eV)
Ref
-1.84
-1.83
-1.82
-1.79
hydroxymethyl(CH2O
H)
-1.89
hydroxyl(OH)
aO-C
BL
-1.98
-2.04
bO-Pt
BL
cO-H
top-bound
through oxygen
-2.12
BL
dC-Pt
BL
Reaction pathway
1b
1a
e
reactant
d
2a
2b
c
Ref
1.38
2.15d
TS2-c(0.73)
0 .6
2.11d
2.12d
TS2-a (0.81)
0 .8
2.13
1.95d
2.14
(0.62)
(0.58)
TS2-e(0.23)
0 .4
0 .2
0 .0
2.02b
0.99
2.02
(0.00)
TS2-d(0.06)
(-0.24)
5
10
15
20
25
Reaction pathway
1.0
0.8
2-a
0.81
+0.58
881.7 i
2-b
0.27
+0.04
IMF
Barrier
rxn energy
pathway 3.1-1( COtop + OH --> COOHcis )
250.9 i
0.43
-0.44
pathway 3.1-2( COOHcis --> COOHtrans )
405.9 i
0.4
-0.07
pathway 3.2-1( CObridge + OH --> COOHcis )
230.4 i
0.78
-0.39
TS-4
pathway 3.3-1( COhcp + OH--> COOHcis )
221.3 i
0.98
-0.25
pathway 3.4-1( COfcc + OH --> COOHcis )
223.2 i
0.96
-0.23
pathway 4.1( COtop + OH --> COOHtrans )
217.5 i
1.11
-0.51
0.6
TS-2
0.4
TS-1
TS-3
0.2
pathway 4.2( CObridge + OH --> COOHtrans )
211.7 i
0.43
-0.31
0.0
pathway 4.3( COhcp + OH --> COOHtrans )
207.9 i
0.58
-0.39
-0.2
pathway 4.4( COfcc + OH --> COOHtrans )
209.5 i
0.56
-0.37
-0.4
pathway 5.1( COhcp + OH --> COtop +OH )
110.1 i
0.28
0.18
pathway 5.2( COhcp+OH --> CObridge + OH )
119.9 i
0.24
0.14
pathway 6( COOH --> CO2+ Hads )
709.7 i
0.94
-0.03
TS-1
0
2
4
6
8
10
12
R e a ctio n co o rdin a te
14
16
18
+0.72
0.81
+0.62
842.1 i
-0.48
0.25
-0.32
-0.70
<0.10
-0.61
-0.87
0.23
-0.85
c
0.11
-0.86
788.5 i
0.3
+0.01
602.1 i
e
0.46
-0.40
788.5 i
1.55
1-a
0.91
-0.36
828.5 i
1.02
-0.17
0.67
-0.16
1-b
1.07
+0.89
748.4 i
1.33
+0.42
0.75
+0.46
Table.4 The reaction energy and barrier for water dissociation
Model
barrier
reaction energy
Pt(111)
0.98
+0.68
1.57
+0.65
1.16
+0.58
Pt(111)
PtRu
[3]
[3]
Ru(0001)
Pt(111)
-2
rxn energy
Barrier
d
My result
TS-2
DACAPO result [2]
rxn energy
Barrier
[1] J. P.C.B 2002,106,2559-2568 : 748.4
2x2 unit
i cell K-Point : 4X4X1 Energy cutoff :350eV
[2] J.A.C.S 2004,126,3910-3919 : 3 layers 3x3 unit cel
Energy cutoff : 350eV
R e a c tio n c o o r d in a t e
TS-3
IMF
(0.2)
-0 .4
0
1.2
rxn energy
-0 .2
(-0.23)
0.98c
Barrier
(1.06)
TS2-b (0.85)
1.88d
1.20
VASP result [1]
My result
Rxn
pathway
1 .0
Ref. J.A.C.S 2004,126,3910-3919
R e a ctio n p a th w ay 1
R e a ctio n p a th w ay 2
product
X-Pt bond length
Ref
Relative energy
specie
Relative energy
USER REPORTS
HIGH PERFORMANCE COMPUTING
Periodic, self-consistent, Density Functional Theory (PAW-GGA) calculations are used to
study competitive paths for the decomposition of methanol on Pt(111).
Pathway proceeding through initial O-H scission event in methanol is considered to compare
pathway proceeding through an opening C-H scission event.
Our calculated results suggest that methanol decomposition via CH3O and either CH2O
intermediates is an energetically feasible pathway; C-H scission to CH2OH, followed by sequential
dehydrogenation, may be another realistic route
[3]
[4]
1.02
+0.51
1.35
+0.71
[3] Electrochimica 2003,: 3x3 unit cell 4 layers K-Point : 4X4X1Energy cutoff : 340eV
[4] J.C.P 2006,124,184704: 3x3 unit cell
K-Point : 2X2X1
Energy cutoff : 340eV
Conclusion
First principles periodic DFT calculations has been used to study thermochemistry and reaction barriers of methanol
decomposition on Pt(111).
The rate-limiting step in this pathway is the initial methanol O-H scission reaction (Table 1), and the end product of pathway, CO,
is shown to be strongly bond that it could poison the Pt(111) surface. Three intermediates in the pathway( methoxy, formaldehyde,
and formyl ) have very low barriers to decomposition. The second step in CO oxidation mechanism, occurs via the
disproportionation of CO and OH. In my work, consider many kind of reaction pathway to found the lowest activation barriers,
and check the transition state imaginary frequencies.