Mechanism of SRM
TOF s
-1
T= 773 K; P = 1 atm; CH
4
conversion 10
• Experiment show that TOF decreases in the order Ru ~ Rh
Ni
~ Ir ~ Pt ~ Pd •
Theory shows that TOF decreases in the order Ru Rh
Ni
Ir Pt ~ Pd
G. Jones et al., J. Catal., 259, 147, 2008
CH
4g
+ H
2
O
g
⇋ CO
g
+ 3 H
2g
E. D. German, M. Sheintuch, J. Phys. Chem. C, 107, 10229, 2013
Relationship of TOF s
-1
and H and CH
3
binding energies for T = 500 K
• TOF for CH
4
dissociation decrease in the order Rh Ru ~ Ir
Ni
~ Pd ~ Pt • For Ni111, CO is formed from CHO
Dissociation of CH to C and H is disfavored on Ni111
S. G. Wang et al., Surf. Sci. 601, 1271, 2007
Ni111
CH
4g
+ CO
2g
⇋ 2 CO
g
+ 2 H
2g
4
Kinetics for the steam
reforming of CH
4
at 873 K on Ni
MgO
j. Wei and E. Igelsia, J. Catal., 224, 370, 2004
CH
4g
+ H
2
O
g
⇋ CO
g
+ 3 H
2g
4
Kinetics for the dry
reforming of CH
4
at 873 K on Ni
MgO
CH
4g
+ CO
2g
⇋ 2 CO
g
+ 2 H
2g
4
Kinetics for the dry
reforming of CH
4
at 873 K on Ni
MgO
• The kinetics for the forward reaction in steam and dry reforming are identical
4
R
f
= k
f
P
CH4
• The rate expression of steam and dry reforming and for CH
4
decomposition on Ni are the same • The rate coefficient for all three reactions is the
same • The process controlling all three reactions is the
dissociative adsorption of CH
4
Ni MgO
Steam and Dry Reforming of CH
4
• The kinetics of carbon accumulation are the same for steam
and
dry
reforming of CH
4
on Coke Deposition on Ni
CH
4g
→ CH
3s
+ H
s
• CH
4
dissociative adsorption occurs preferentially at
Ni211 steps
• Graphene sheets nucleate at Ni211 steps and then
grow over the nanoparticle
J. Sehested, Catal. Today, 111, 103, 2006 F. Abild-Pedersen et al., Surf. Sci, 590, 127, 2005
1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010
Energy-driven Carbon Growth
1
:
ΔG = total free energy change for a graphene island
N
tot
= total atoms in graphene island
Δμ
C
= carbon chemical potential E
edge
= energyC atom on edge of island
E
stretch
= energy cost for stretching graphene layer to
match Pt lattice
Lattice mismatch strain cost
Surface cost
Graphene growth
Bulk energy
Step edge Graphene
nucleus
∆� = −�
���
∆�
�
+ 3 ��
���
�
����
+ 2 ��
���
�
������
• Graphene growth nucleates at steps
• To nucleate the step width has to be greater than a critical value
Ni NiAu
• Introduction of Au into Ni introduces
additional strain and raises N
tot
required to nucleate the growth of graphene
Ni Bulk Energy
Edge Energy Strain Energy
∆� = −�
���
∆�
�
+ 3 ��
���
�
����
+ 2 ��
���
�
������
E
st ra
in
e V
ato m
Au
1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010
• Thermodynamics predicts that the preferred products should be Cs C
6
H
6g
C
2
H
4g
• Carbon deposition along with C
6
H
6
and C
2
H
4
is observed for MoC
x
ZSM-5, FeSiO
2
• Only FeSiO
2
produces ethene, benzene, and naphthalene but not coke
• A CH
4
conversion of 48 is achieved at 1363 K and a space velocity of 21,400 mlg h
FeSiO
2
X. Bao and coworkers, Science, 344, 616, 2014
2
• CH
4
pyrolysis at 1363 K over FeSiO
2
achieves 48 conversion and selectivity of 48.4 to C
2
H
4
and the rest to benzene and naphthalene • FeSiO
2
is stable to for 60 h • The high stability is attributed to isolated FeC
2
sites
2
• Graphite is the thermodynamically preferred product of methane pyrolysis • The absence of soot or coke is attributable to the very rapid quenching of the
product gases, which inhibits the kinetics of soot formation • Coke is not formed on FeSiO
2
because the sites are too small to nucleate coke
Active site for FeSiO
2
X. Bao and coworkers, Sci., 344, 616, 2014
• The active center is taken to be a [Cu
2
µ-O
2
]
2+
core based on UV-Vis observations and comparison with compounds of known structure
• CH
4
is activated on [Cu
2
µ-O
2
]
2+
cores to produce CH
3
O species that can then be hydrolyzed to form CH
3
OH • Catalyst reactivation in O
2
at elevated temperature is required
CH
4
+ [Cu
2
µ-O
2
]
2+
[Cu
2
CH
3
OOH]
2+
[Cu
2
CH
3
OOH]
2+
+ H
2
O [Cu
2
µ-OH
2
]
2+
+ CH
3
OH [Cu
2
µ-OH
2
]
2+
[Cu
2
µ-O
2
]
2+
+ H
2
O CH
4g
+ ½ O
2g
⇋ CH
3
OH
g
M. H. Groothaert et al., J. Am. Chem. Soc. 127, 1394 2005
