ACS Catalysis
Research Article
Table 5. Calculated Reaction Energies (ΔH), Energy
Supporting Information). The overall reaction CO* + 6H* →
Barriers (E ), and Bond Lengths of TS on a Ru (0001) Flat
CH + H O is highly exothermic, indicating that the formation
a
4
2
Surface
of CH is thermodynamically favorable.
4
It is interesting to note that the hydrogenation of adsorbed
CO to formyl species is also the rate-controlling step for the 1-
hexadecanol hydrogenolysis to n-pentadecane, due to its greater
barrier over other elementary steps. The hydrogenation of
adsorbed CO species on Ru (0001) is not involved in the
elementary reaction steps for the 1-hexadecanol hydrogenolysis
to n-hexadecane; however, it has a huge effect on n-hexadecane
production, considering that sufficient free sites on Ru (0001)
surface are required for the reaction. That is, the active sites for
n-hexadecane production during the reaction are poisoned by
the strongly adsorbed CO species from its parallel reaction, n-
pentadecane production. In this context, adequate external
energy is required during the 1-hexadecanol hydrogenolysis to
overcome the energy barrier for the removal of adsorbed CO
and the recovery of a clean Ru (0001) surface.
reaction step
ΔH (eV) Ea (eV) d (Å)
−0.42
RCH CH OH + * → RCH CH OH*
2
2
2
2
RCH CH OH* → RCH CHOH* + H*
0.13
−0.57
0.21
0.86
0.79
1.08
0.73
1.00
0.28
0.50
0.54
1.21
1.11
0.71
0.68
0.27
0.51
0.71
0.68
1.60
1.41
1.73
1.56
1.65
1.20
1.64
1.57
1.87
2.02
1.69
1.72
1.66
1.65
1.69
1.72
2
2
2
RCH CH OH* → RCH CH O* + H*
2
2
2
2
RCH CH OH* → RCHCH OH* + H*
2
2
2
RCH CH O* → RCH CHO* + H*
0.02
2
2
2
RCH CH O* → RCHCH O + H*
0.27
2
2
2
RCH CHO* → RCH CO* + H*
−0.30
−0.43
−0.10
−0.36
−0.41
0.39
2
2
RCH CHO* → RCHCHO* + H*
RCH CO → RCHCO + H*
2
2
RCHCO* → RCHC* + O*
RCHCO* → RCH* + CO*
RCH* + H* → RCH2*
RCH * + H* → RCH
−0.18
0.01
2
3
RCHC* + H* → RCHCH*
The complete energy profile of the 1-hexadecanol hydro-
genolysis on flat Ru (0001) is shown in Figure 4. The intrinsic
overall barriers are observed to be, as it happens, 1.21 eV both
for the production of n-hexadecane and for the production of n-
pentadecane, in line with the similar experimental apparent
barriers of 101.3 and 97.0 kJ/mol, respectively (Figure 2). The
RCHCH* + H* → RCH CH*
0.22
2
RCH CH* + H* → RCH CH *
0.39
2
2
2
RCH CH * + H* → RCH CH
3
CO* → C* + O*
CO* → CO + *
−0.18
0.42
2
2
2
>2
1.98
CO* + H* → HCO*
24.4
1.21
0.31
0.78
0.55
0.67
1.14
1.52
2.01
1.64
1.57
removal of adsorbed CO (CO* + 6H* → CH + H O) on the
HCO* + H* → H CO*
0.39
4
2
2
H CO* → CH * + O*
−0.68
0.06
Ru (0001) surface is acknowledged as a crucial step for the 1-
hexadecanol hydrogenolysis. On the one hand, it contributes to
the largest energy barrier of 1.21 eV and is the rate-controlling
step for n-pentadecane production. On the other hand, it also
retards the rate for n-hexadecane production due to the
covering of Ru (0001) sites by strongly adsorbed CO species.
As a result, the 1-hexadecanol hydrogenolysis on Ru (0001)
undergoes two parallel pathways, and n-pentadecane and n-
hexadecane are produced simultaneously. Considering that the
carbon−carbon bond scission carries an energy barrier smaller
than carbon−oxygen bond scission, n-pentadecane is observed
as the dominant product from the 1-hexadecanol hydro-
genolysis on Ru (0001). On the basis of the theoretical
calculations, it can be expected that C−O versus C−C bond
scissions are key factors determining the product distribution
from aliphatic alcohol hydrogenolysis. We further calculated the
energy barriers for these two steps on other possible exposed
Supporting Information). The results are summarized in Table
2
2
CH * + H* → CH *
2
3
CH * + H* → CH4
−0.18
3
barriers are low enough for reaction under conditions employed
in this study. The carbon−carbon bond scission in
C H CHCO species appears to be the preferred step, due
to its smaller energy barrier in comparison with that of carbon−
oxygen bond scission.
ii). Hydrogenation of the Resulting Species. The
C H CH species undergo two successive hydrogenation
steps to generate the final product C H CH . The first
1
4
29
(
4
1
29
1
4
29
3
hydrogenation is endothermic, while the second hydrogenation
is exothermic. However, both hydrogenation steps carry a
similarly small barrier of ∼0.7 eV.
C H CHC species need four steps for hydrogenation to
1
4
29
generate the final product C H CH CH . The hydrogenation
may take place on an α-carbon or β-carbon, and the most
feasible hydrogenation pathway is established as C H CHC
1
4
29
2
3
1
4
29
→
1
C H CHCH → C H CH CH → C H CH CH →
14 29 14 29 2 14 29 2 2
6
. It is clearly seen that C−O bond scission is preferred on
C H CH CH . All of the hydrogenation steps carry small
4
29
2
3
stepped Ru (0001) and Ru (100), while C−C bond scission is
preferred on flat Ru (0001). Since flat Ru (0001) constitutes
barriers of below 0.8 eV, indicating that the hydrogenation is
facile.
the dominating exposed facets in 1.62% Ru/TiO , the selective
(
iii). CO Hydrogenation into Methane. When CO is formed
2
scission of C−C bonds is observed in 1-hexadecanol hydro-
genolysis. Regardless, it can be stated that the selective catalytic
bond fission during aliphatic alcohol hydrogenolysis is not only
controlled by the types of active metals but also controlled by
their exposed facets.
These results can explain the effects of supports and metal
loadings on the catalytic behaviors of ruthenium catalysts well
(Table 1) and can also provide a potential strategy to adjust the
product distribution from aliphatic alcohol hydrogenolysis.
Preliminary results indicate that the introduction of 0.5%
on the surface from carbon−carbon bond scission, it should
desorb or react further. Since the CO desorption energy (1.98
eV) or CO dissociation energy (>2 eV) is much larger than the
barrier for hydrogenation (1.21 eV) on Ru (0001), the
adsorbed CO prefers to undergo further hydrogenation
processes. The detailed pathway for CO hydrogenation is
established as CO* → HCO* → H CO* → CH * → CH * →
CH . The first hydrogenation (CO* + H* → HCO* + *) is
denoted as the rate-controlling step with the largest reaction
barrier, similar to previous reports on Rh (111). Once CH is
2
2
3
4
36
4
formed, it does not stick to the Ru (0001) surface and desorbs
methane as the exclusive gaseous product (Figure S6 in the
potassium or calcium to Ru/TiO can increase the n-
pentadecane selectivity, while the introduction of 0.5%
vanadium can significantly increase the n-pentadecane
2
7
204
ACS Catal. 2015, 5, 7199−7207