C-H bond cleavage in small alkanes
catalyzed by dimers of group 8B
transition metal elements:
A B3LYP study
Fan Yi, Phuong Nguyen, Ben Livingston, Yingbin Ge*
Department of Chemistry
Central Washington University
The 244th ACS National Meeting & Exposition, Philadelphia, Pennsylvania, August 19-23, 2012
Contents
• Introduction
• Computational method
• R−H + TM2  H−TM2−R, where
o R−H = CH4, C2H6, C3H8
o TM = Rh, Ir, Ni, Pd, Pt
•
•
•
•
Charge effect: neutral vs. +1 charged TM2
Ligand effect: TM2 vs. Cl−TM2 vs. CH3−TM2
Modeling metal-oxide surface supported TM2
Conclusions
2
Ptn catalyzed oxidative
dehydrogenation of propane
Ptn
C3H8 + ½O2  C3H6 + H2O
Polypropylene
Epoxypropane
Acetone
Isopropanol
Acrylonitrile
3
Introduction
[catalyst]
C3H8 + ½ O2  C3H6 + H2O (~$90B/yr)
The rate limiting step is to break the first C-H bond.
Traditional catalysts: energy barrier = ~100 kJ/mol
Pt8-10 nanocatalysts: energy barrier = ~20 kJ/mola
Our goals:
1. Further enhance the activity of TM nanocatalysts
2. Reduce the cost of the nanocatalysts.
aVajda
S, Curtiss LA, et al (2009) Nat Mater 8:213-216
4
5
Co
27Co
Cobalt
[Ar]3d74s2
45Rh Rhodium [Kr]4d85s1
77Ir
Iridium
$0.9/Oz
$1230/Oz
Rh
[Xe]4f145d76s2 $1080/Oz
Ir
Ni
28Ni
Pd
Nickel
46Pd Palladium
78Pt Platinum
[Ar]3d84s2
$0.45/Oz
[Kr]4d105s0
$570/Oz
[Xe]4f145d96s1 $1400/Oz
Pt
[18Ar]=[Ne]3s23p6
[36Kr]=[Ar]3d104s24p6
[54Xe]=[Kr]4d105s25p6
6
Computational Method
•
•
•
•
•
B3LYP
6-31G(d) on main group elements
LANL2DZ(f) on transition metals
LANL2 effective core potential (ECP)
Minima verified by vibrational frequency
calculations
• Transition states verified by intrinsic reaction
coordinate (IRC) calculations
7
Major sources of errors
•
•
•
•
•
•
•
Single-reference DFT method
Moderate double-ζ basis set
Effective core potential for inner electrons
Spin-orbit coupling not included
Thermal corrections not included
Exploratory study to obtain qualitative insights
How about numerical accuracy?
8
Calculated ionization energy & electron affinity
vs. experimental data on Rh, Ir, Ni, Pd, Pt
mean abs. error of I.E. & E.A.
Error (eV)
0.6
0.4
0.2
0.0
B3LYP B3PW91
PBE
PW91
MP2
9
Errors of calculated bond enthalpy (kJ/mol)
RhO
PtO
PdO
NiO
IrO
MP2
RhH
PW91
PdH
PBE
Rh2
B3PW91
Pt2
B3LYP
Pd2
Ni2
-600
-400
-200
0
6-31G(d) on H & O; LANL2DZ(f) basis set & LANL2 ECP on TM
200
10
Mean abs. error (kJ/mol):
B3LYP: 51 (12 kcal/mol)
B3PW91: 54
PBE:
82
PW91: 83
MP2:
154
Mean abs. %error:
B3LYP: 17%
B3PW91: 18%
PBE:
27%
PW91: 28%
MP2:
64%
Experimental bond
enthalpy at 298 Ka
PdH
234 ± 25
RhH
247 ± 21
Ni2
203 ± 1
Pd2
100 ± 15
Pt2
357 ± 15
Rh2
285 ± 21
IrO
415 ± 42
NiO
382 ± 17
PdO
381 ± 84
PtO
392 ± 42
RhO 405 ± 42
aCRC
handbook of chemistry and physics 76th ed.
11
Comparison against
experimental data
on Pt compounds
BE: bond energy
IE: ionization energy
EA: electron affinity
Mean abs. %error:
B3LYP: 5%
B3PW91: 9%
PBE: 16%
PW91: 17%
MP2: 22%
BE PtO2
BE PtO
BE PtC
BE Pt2
IE PtO2
B3LYP
IE PtO
B3PW91
IE PtC
PBE
IE Pt2
PW91
MP2
IE Pt
EA Pt2
EA Pt
-75% -50% -25%
0%
25% 50% 75%
Percent Error
12
Inclusion of f-type functions
B3LYP data compared
to experiments
LANL2DZ
LANL2DZ(f)
BE PtO2
BE PtO
BE PtC
BE Pt2
IE PtO2
IE PtO
IE PtC
IE Pt2
Mean abs. error :
LANL2DZ(f): 5%
LANL2DZ: 7%
IE Pt
EA Pt2
EA Pt
-20%
-10%
0%
10%
Percent Error
20%
13
Methods for group 8B elements
• DFT methods are more accurate than MP2.
• Hybrid DFT methods are more accurate than
the pure DFT ones.
• B3LYP with 6-31G(d) on main group elements
and LANL2DZ(f) on transition metals achieves
a reasonable level of accuracy.
• Higher accuracy requires multi-configuration
calculations, inclusion of spin-orbit coupling,
and a larger basis set.
14
Relative B3LYP energy (in kJ/mol) of
various electronic states
Rh2
Ir2
Ni2
Pd2
Pt2
Multiplicity
1
3
5
62 51
0
54
40
36
92
0
31
Rh2+
Ir2+
Ni2+
0 248
0 127
Pd2+
Pt2+
50
0
Multiplicity
2
4
6
51
0 31
102
6
68
0
0 190
0 119 899
44
0 239
All numbers are obtained from B3LYP/LANL2DZ(f) calculations.
15
Diagram of potential energy surface
TM = Rh, Ir, Ni, Pd, Pt
CxHy = CH4, C2H6, C3H8
16
Pt2 + CH4  H−Pt2−CH3
Rh2 + C3H8  H−Rh2−C3H7
17
Pt2 + CH4  Pt2---CH4  H−Pt2−CH3
E (kJ/mol)
100
Pt2 (M=1)
Pt2 (M=3)
TS
0
Pt2 + CH4
Pt2---CH4
H−Pt2−CH3
-100
18
Pt2 + CxHy  Pt2---CxHy  H−Pt2−CxHy-1
CH4
C2H6
38
100
34
0
0
0
-8
33
-20
Pt2 (M=1)
-21
Pt2 (M=1)
Pt2 (M=1)
Pt2 (M=3)
-100
E (kJ/mol)
100
E (kJ/mol)
E (kJ/mol)
100
C3H8
-100
Pt2 (M=3)
-100
Pt2 (M=3)
Both energy barrier and reaction energy decreases as the size of
alkane increases.
• The Pt2 catalyzed dehydrogenation reaction involves electron
transfer from alkane to Pt2; the electron transfer can be facilitated
by the additional -CH3 e- pushing group(s).
• -CH3 stabilizes its neighboring C radical through hyperconjugation.
19
Neutral transition metal dimers
Energy barrier (kJ/mol)
120
80
CH4
C2H6
C3H8
40
0
Pt2
Pd2
Ni2
Ir2
Rh2
20
Crossing of potential energy surfaces
E (kJ/mol)
120
80
Pd2 (M=1)
Pd2 (M=3)
40
0
-40
separated
reactants
reactanct
complex
transition
state
product
Pd2 + CH4  Pd2---CH4  H−Pd2−CH3
21
Pd2 + C2H6
E (kJ/mol)
120
80
Pd2 (M=1)
Pd2 (M=3)
40
0
-40
separated
reactants
Pd2 + C3H8
E (kJ/mol)
120
80
reactanct
complex
transition
state
product
transition
state
product
Pd2 (M=1)
Pd2 (M=3)
40
0
-40
separated
reactants
reactanct
complex
22
Neutral TM2 clusters
Energy barrier (kJ/mol)
80
CH4
C2H6
C3H8
40
0
Pt2
Pd2*
Ni2
Ir2
Rh2*
* Crossing of PES taken into account.
23
When will energy barrier be negative?
TM = Rh, Ir, Ni, Pd, Pt
CxHy = CH4, C2H6, C3H8
24
+1 charged TM2
Energy barrier (kJ/mol)
50
0
CH4
C2H6
C3H8
-50
-100
Pt2* Pd2 Ni2* Ir2*
Rh2
* Crossing of PES taken into account.
25
Neutral vs. +1 charged dimers
Energy barrier (kJ/mol)
100
neutral
positively charged
50
0
-50
Pt2
Pd2
Ni2
Ir2
Rh2
The Pt2 catalyzed dehydrogenation of CH4 involves electron
transfer from C-H bond to Pt2. +1 charge facilitates the e- transfer.
26
Ligand (Cl or CH3) effects on Pt2 + CH4
The Pt2 catalyzed dehydrogenation reaction involves electron
transfer from alkane to Pt2. Ligands such as Cl or CH3 on Pt2 may
facilitate or hinder the electron transfer.
27
Ligand effects on Pt2 + CH4
Energy barrier in kJ/mol
120
60
0
-60
Pt2
Pt2(+)
Cl-Pt2
CH3-Pt2
The electron transfer from alkane to Pt2 is facilitated by the Cl e-withdrawing ligand and hindered by the CH3 e- pushing group.
28
Energy barrier (kJ/mol)
Ligand (Cl) effect on group 8B
transition metal dimers (TM2)
TM2
100
Cl-TM2
60
20
-20
Pt2
Pd2
Ni2
Ir2
Rh2
29
Energy barrier (kJ/mol)
Ligand (CH3) effect on group 8B
transition metal dimers (TM2)
TM2
100
CH3-TM2
60
20
-20
Pt2
Pd2
Ni2
Ir2
Rh2
30
To be more practical…
• Heterogeneous catalysis: interaction between
metal oxide and the supported transition metal
dimers is important.
• Thermodynamics: it’s not just about reaction rate
constants; it’s also about equilibrium constants.
• Cost per mole:
28Ni
45Rh
$4100/mol
77Ir
$6700/mol
$0.85/mol
46Pd $1900/mol
78Pt $8800/mol
31
Modeling Pd2 + C3H8 on a MgO surface
32
(S)Pd2 + C3H8 → H-(S)Pd2-C3H7
120
OMg-Pd2
E(kJ/mol)
80
40
MgO-Pd2
O(HO)Mg-Pd2
(HO)MgO-Pd2
endothermic
0
-40
separated
reactants
reactant
complex
transition
state
product
The energy barrier for Pd2 + C3H8 → H-Pd2-C3H7 is 16 kJ/mol.
33
(S)Ni2 + C3H8 → H-(S)Ni2-C3H7
120
OMg-Ni2
E(kJ/mol)
80
MgO-Ni2
40
0
-40
-80
separated
reactants
reactant
complex
transition
state
product
The energy barrier for Ni2 + C3H8 → H-Ni2-C3H7 is 43 kJ/mol.
34
Conclusions
• Ir2 ≈ Pd2 > Pt2 > Rh2 > Ni2 in their ability to break
C-H bonds.
• +1 charged TM2+ are significantly more active
than their neutral counterparts.
• Electron-withdrawing Cl bonded to TM2 enhances
the activity of Pt2, Ir2, and Rh2. CH3 groups
bonded to TM2 generally lower its activity.
• A MgO support may hinder the C-H activation by
Pd2 and Ni2. Making the MgO surface oxygen-rich
may alleviate the hindrance.
35
Acknowledgements
• Central Washington University (CWU) SEED
grant
• CWU college of the sciences faculty grant
• CWU Department of Chemistry
Questions and Comments?
36
Supplemental Information
•
•
•
•
•
•
•
•
•
LANL2 ECP
Exponents for the f-type polarization function
B3LYP bond distances
IE and EA of Rh, Ir, Ni, Pd, Pt
Errors of calculated bond enthalpy at 298 K.
BE, IE, and EA of Pt compounds
Pd2 and Ni2 potential energy surfaces (PES)
Neutral and +1 charged TM2 PES
MgO supported Ni2, Pt2, and Pt4 IRC movies
37
LANL2 ECP for inner electrons
27Co
28Ni
1s22s22p6
45Rh 46Pd 1s22s22p63s23p63d10
77Ir
78Pt 1s22s22p63s23p63d104s24p64d104f14
QM wavefunction for outer electrons
27Co
3s23p63d74s2
45Rh 4s24p64d85s1
77Ir
5s25p65d76s2
28Ni
46Pd
78Pt
3s23p63d84s2
4s24p64d10
5s25p65d96s1
38
LANL2DZ(f) f-type
polarization functions
27Co
45Rh
77Ir
ζ
2.78
1.35
0.938
28Ni
46Pd
78Pt
ζ
3.13
1.472
0.993
39
Bond distances
5Rh
2
5Ir
2
3Ni
2
3Pd
2
3Pt
2
B3LYP
2.30
2.24
Exp.
2.08
4Rh +
2
6Ir +
2
4Ni +
2
2.52
2.36
2Pd +
2
4Pt +
2
B3LYP
2.73
2.30
Exp.
2.24
2.65
2.44
Theoretical data are obtained from B3LYP/LANL2DZ(f) calculations.
40
B3LYP B3PW91 PBE
EA Ir 1.57 1.46 1.60
EA Ni 0.72 0.55 0.63
EA Pd 0.26 0.09 0.34
EA Pt 1.96 1.90 2.05
EA Rh 0.88 0.70 0.84
IE Ir 9.22 9.66 9.41
IE Ni 7.74 7.63 7.81
IE Pd 8.55 8.50 8.69
IE Pt 9.33 9.28 9.46
IE Rh 7.86 7.82 7.70
aCRC
PW91
1.65
0.67
0.41
2.10
0.89
9.48
7.89
8.72
9.50
7.76
MP2
-1.58
-0.16
-0.78
0.19
-1.33
8.84
6.82
7.73
8.46
6.99
Exp
1.565
1.156
0.562
2.128
1.137
9.1
7.6398
8.3369
8.959
7.4589
handbook of chemistry and physics 76th ed.
41
Calculated ionization energy (IE) vs.
experimental data on Rh, Ir, Ni, Pd, Pt
mean abs. error of ionization energy
Error (eV)
0.6
0.4
0.2
0
B3LYP B3PW91
PBE
PW91
MP2
LANL2DZ(f) basis set & LANL2 ECP
42
Calculated electron affinity (EA) vs.
experimental data on Rh, Ir, Ni, Pd, Pt
mean abs. error of electron affinity
Error (eV)
0.6
0.4
0.2
0.0
B3LYP B3PW91
PBE
PW91
MP2
43
Exp. Data at 0 K
EA Pt
2.125
EA Pt2
1.898
IE Pt
8.959
IE Pt2
8.68
IE PtC
9.45
IE PtO
10
IE PtO2
11.35
BE Pt2
3.14
BE PtC
5.95
BE PtO
4.3
BE PtO2
4.41
R (Pt2)
2.333
R(PtC)
1.679
R(PtO)
1.727
dipole(PtC)
0.99
dipole(PtO)
2.77
Energies are in eV, bond distances
in Å, dipole moment in Debye.
Airola MB, Morse MD, J Chem
Phys 116:1313, 2002.
Steimle TC, Jung KY, Li BZ, J Chem
Phys 103:1767, 1995.
Bilodeau RC, Scheer M, Haugen
HK, Brooks RL, Phys Rev A
61:Art. No. 012505, 2000.
Ho J, Polak ML, Ervin KM,
Lineberger WC, J Chem Phys
99:8542, 1993.
Jakubek ZJ, Simard B, J Phys B
33:1827, 2000.
Taylor S, Lemire GW, Hamrick YM,
Fu Z, Morese MD, J Chem Phys
89:5517, 1988.
Citir M, Metz RB, Belau L, Ahmed
M, J Phys Chem A 112:9584,
2008.
44
Error in bond enthalpy (kJ/mol)
RhO
PtO
PdO
NiO
PW91
IrO
RhH
PBE
PdH
B3PW91
Rh2
B3LYP
Pt2
Pd2
Ni2
-200
-100
0
100
200
45
% error in bond enthalpy
RhO
PtO
PdO
NiO
IrO
PW91
RhH
PBE
PdH
B3PW91
Rh2
B3LYP
Pt2
Pd2
Ni2
-100%
-50%
0%
50%
100%
46
Pd2 + CxHy  Pd2---CxHy  H−Pd2−CxHy-1
CH4
C2H6
0
-200
100
0
-100
-200
Pd2 (M=2)
Pd2 (M=4)
0
-200
0
-200
Pd2 (M=1)
Pd2 (M=3)
200
100
-100
100
-100
Pd2 (M=1)
Pd2 (M=3)
200
E (kJ/mol)
E (kJ/mol)
200
+1
0
E (kJ/mol)
-200
100
-100
Pd2 (M=1)
Pd2 (M=3)
E (kJ/mol)
100
-100
200
200
E (kJ/mol)
E (kJ/mol)
200
C3H8
Pd2 (M=2)
Pd2 (M=4)
100
0
-100
-200
Pd2 (M=2)
Pd2 (M=4)
47
Ni2 + CxHy  Ni2---CxHy  H−Ni2−CxHy-1
CH4
C2H6
100
0
-200
-200
E (kJ/mol)
E (kJ/mol)
0
-200
Ni2 (M=2)
Ni2 (M=4)
0
-200
0
-200
Ni2 (M=1)
Ni2 (M=3)
200
100
-100
100
-100
Ni2 (M=1)
Ni2 (M=3)
200
100
-100
0
-100
Ni2 (M=1)
Ni2 (M=3)
200
+1
100
E (kJ/mol)
-100
200
E (kJ/mol)
200
E (kJ/mol)
E (kJ/mol)
200
C3H8
Ni2 (M=2)
Ni2 (M=4)
100
0
-100
-200
Ni2 (M=2)
Ni2 (M=4)
48
Pt2 + CxHy  Pt2---CxHy  H−Pt2−CxHy-1
Relative Energy (kJ/mol)
50
Pt2
CH4
C2H6
C3H8
CH4(+)
C2H6(+)
C3H8(+)
0
-50
-100
separated
reactants
reactant
complex
transition
state
product
49
Relative Energy (kJ/mol)
150
100
Pd2 + CxHy  Pd2---CxHy  H−Pd2−CxHy-1
Pd2
CH4
C2H6
C3H8
CH4(+)
C2H6(+)
C3H8(+)
50
0
-50
-100
separated
reactants
reactant
complex
transition
state
product
50
Relative Energy (kJ/mol)
100
50
Ni2 + CxHy  Ni2---CxHy  H−Ni2−CxHy-1
Ni2
CH4
C2H6
C3H8
CH4(+)
C2H6(+)
C3H8(+)
0
-50
-100
separated
reactants
reactant
complex
transition
state
product
51
Relative Energy (kJ/mol)
100
50
Ir2 + CxHy  Ir2---CxHy  H−Ir2−CxHy-1
Ir2
CH4
C2H6
C3H8
CH4(+)
C2H6(+)
C3H8(+)
0
-50
-100
separated
reactants
reactant
complex
transition
state
product
52
Relative Energy (kJ/mol)
100
50
Rh2 + CxHy  Rh2---CxHy  H−Rh2−CxHy-1
Rh2
CH4
C2H6
C3H8
CH4(+)
C2H6(+)
C3H8(+)
0
-50
-100
-150
separated
reactants
reactant
complex
transition
state
product
53
+1 charged dimers
Energy barrier (kJ/mol)
100
50
CH4
C2H6
C3H8
0
-50
-100
Pt2
Pd2
Ni2
Ir2
Rh2
* Crossing of PES not taken into account.
54
Modeling Ni2 + C3H8 on a MgO surface:
the active Ni bonded to O
55
Modeling Ni2 + C3H8 on a MgO surface:
the active Ni bonded to Mg
56
(S)Ni2 + C3H8 → H-(S)Ni2-C3H7
120
E(kJ/mol)
80
40
OMg-Ni2
MgO-Ni2
O(HO)Mg-Ni2
(HO)MgO-Ni2
0
-40
-80
separated
reactants
reactant
complex
transition
state
product
The energy barrier for Ni2 + C3H8 → H-Ni2-C3H7 is 43 kJ/mol.
57
Modeling Pt2 + C3H8 on a MgO surface
58
Pt2 + C3H8 on an O-rich MgO surface
59
Modeling Pt4 + C3H8 on a MgO surface
60