G Model
CATTOD-9196; No. of Pages10
ARTICLE IN PRESS
S. Wang et al. / Catalysis Today xxx (2014) xxx–xxx
7
of d-glucose hydrogenation on the Ru–B/NH &CH -MSNSs catalyst
2
3
−
1
was determined to be 19.4 kJ mol , much lower than that obtained
−
1
over Raney Ni (52.8 kJ mol ), further confirming that the Ru active
sites in the former were more active than those in the latter. The
enhanced intrinsic activity of Ru had been also demonstrated by
comparing the activities of Ru- and Ni-based catalysts during the
d-glucose hydrogenation in other groups [9,40].
Even if d-glucose hydrogenation was performed under a lower
hydrogen pressure, Ru–B/NH &CH -MSNSs delivered far better ini-
2
3
−
1
−1
at 373 K under 3 MPa of
tial specific activity (3.3 mol h gRu
hydrogen) than that over the Ru/C (1.1 mol h gRu at 373 K under
MPa of hydrogen) reported by Gallezot et al. [42]. For comparison,
−
1
−1
8
a commercial Ru/C catalyst (5 wt.% Ru loading) was measured under
the same reaction conditions, and only 23% conversion of d-glucose
was achieved using Ru/C within 80 min (Table 2). Apparently, the
superior activities of the as-prepared Ru–B amorphous alloys to
the commercial Ru/C can be attributed to both the higher Sact and
the enhanced intrinsic activity. The superior RHS of the as-prepared
Ru–B catalysts could be linked to their unique amorphous alloy
structure, which had proven to be favorable for hydrogenation pro-
cesses [15]. This could be also supported by the fact that an abrupt
decrease in activity was observed for the Ru–B/NH &CH -MSNSs
2
3
treated at 873 K for 2 h in N flow (Table 2), which was firstly due to
2
the significant agglomeration of Ru–B NPs at elevated temperatures
S
and, thus, decreasing Sact and, secondly, the decrease in RH resulted
S
from the crystallization of Ru–B amorphous alloy. The larger RH
of Ru–B/NH &CH -MSNSs amorphous alloy catalyst compared to
2
3
its corresponding crystalline counterpart and the commercial Ru/C
can be interpreted in terms of both the structural and the electronic
effects: (i) Structural effect: the unique short-range ordering but
long-range disordering amorphous structure endowed Ru–B amor-
phous alloy with a stronger synergistic effect between Ru active
sites and more highly unsaturated Ru active sites than the crys-
talline catalyst, which may promote the adsorption of reactants and
favor hydrogenation activity [15]. (ii) Electronic effect: The afore-
mentioned XPS experiments demonstrated the strong electronic
interaction between Ru and B in the Ru–B amorphous alloy, mak-
ing Ru electron-enriched. The higher electron density on Ru active
Fig. 8. Low-angle XRD patterns of (a) MSNSs and Ru–B/MSNSs, (b) NH2-MSNSs and
Ru–B/NH2-MSNSs, (c) CH3-MSNSs and Ru–B/CH3-MSNSs, and (d) NH2&CH3-MSNSs
and Ru–B/NH2&CH3-MSNSs.
d-glucose on Ru–B/NH &CH -MSNSs catalyst followed the classic
2
3
Langmuir-Hinshelwood model as observed by Crezee et al. [39].
At low d-glucose concentration the reaction showed an apparent
first order dependency, while at high concentration this changed
−
sites might facilitate the formation of H species [41], which could
promote d-glucose hydrogenation activity taking into account that
the d-glucose hydrogenation was first-order with respect to hydro-
gen while it was zero-order to the d-glucose. However, no electron
donation-acceptance was present in the commercial Ru/C catalyst
and the Ru-based crystalline catalyst due to the decomposition
of Ru–B during the crystallization process [16,38], which could
account for the lower RHS than the corresponding Ru–B amorphous
alloy catalyst.
m
to zero order behavior. Meanwhile, RH increased almost linearly
with an increase in hydrogen pressure (Fig. 11b), implying the
first-order kinetics with respect to hydrogen. The kinetic behav-
iors in Fig. 11 could be understood by considering the difference in
the adsorption strength between d-glucose and hydrogen on the
Ru–B amorphous alloys. Due to its stronger adsorption, d-glucose
can reach to saturated adsorption rapidly even at low concentra-
tion. Accordingly, the change in the d-glucose concentration has
no influence on either the adsorption amount of d-glucose on
the catalyst or the hydrogenation rate. At low d-glucose concen-
tration, the hydrogenation rate increased with the enhancement
of d-glucose concentration as its surface adsorption was unsatu-
rated. However, the adsorption for hydrogen on Ru–B is relatively
weak and cannot reach surface saturation under the present con-
ditions. As a result, the d-glucose hydrogenation exhibited a first
order dependency with respect to hydrogen in the operating regime
studied.
m
Based on the RH and d-glucose conversion data presented
in Table 2, one can see that the reactivity of all the supported Ru–B
amorphous alloy catalysts changed in the following sequence:
Ru–B/NH &CH -MSNSs > Ru–B/NH -MSNSs ≈ Ru–B/CH -MSNSs >
2
3
2
3
Ru–B/MSNSs. On one hand, the different hydrogenation activities
should be related to the difference in the dispersion degree of Ru
active sites (Sact) resulted from the diverse promotional effect of
surface organic groups, as discussed above. On the other hand, the
different activities of all the supported Ru–B catalysts apparently
S
arose from the different intrinsic activity, since the RH values
m
Table 2 summarized some of the catalytic parameters over the
different catalysts during liquid-phase d-glucose hydrogenation.
Although Raney Ni possessed a much higher Sact than all the Ru-
changed in a same trend as that of the RH and d-glucose con-
version. Because the Ru–B deposited on different nanocarriers
possessed similar compositions, amorphous alloy structure, as
well as surface electronic characteristics, the district location of
m
based catalysts, it still exhibited a much lower RH and d-glucose
S
conversion within 80 min because of its extremely lower intrinsic
Ru–B should be considered to be responsible for the diverse RH .
S
activity (RH ). This suggests that Ru was much more active than
Maybe the pore channels could enrich the d-glucose molecules in
to effectively increase the collision frequency between reactants
Ni in nature for d-glucose hydrogenation [40,41]. On the basis of
the dependence of the logarithm of RHm on the reciprocal of the
reaction temperature (Fig. 12), the apparent activation energy (Ea)
Please cite this article in press as: S. Wang, et al., Ru–B amorphous alloy deposited on mesoporous silica nanospheres: An efficient