Full Papers
slightly larger than that in Pt/pristine carbon and both catalysts
have a low amount of SOFGs. Thus, the Lewis base, which is
produced by the high-temperature treatment of the support,
should be considered as a major contribution to the increase
in GLY conversion on the Pt/K-AMC-900 catalyst. Furthermore,
the GLY conversion on Pt/K-AMC-900 is lower than that on Pt/
K-AMC-600, which could be because of the larger Pt nanoparti-
cles on the K-AMC-900 support. However, the effect of surface
chemistry should also be considered.
ence of high-desorption-temperature SOFGs such as phenol,
ether, and carbonyl/quinone groups is necessary for carbon-
supported Pt catalysts to achieve a high catalytic activity for
GLY oxidation under base-free conditions.
To further investigate the influence of the surface chemistry
of activated carbon on glycerol oxidation, we performed a tra-
ditional activation experiment using nitric acid. Nitric acid is
used frequently to activate carbon materials and introduce
SOFGs on the surface of carbon supports to improve the cata-
[
53]
To rule out the Pt size effect on the catalytic activity of
carbon-based Pt catalysts, Pt catalysts of comparable sizes on
different carbon supports with different surface chemistries
should be tested. However, in the case of Pt/K-AMC, the sinter-
ing of the Pt nanoparticles cannot be avoided at 9008C under
lytic activities of the carbon-based catalysts. As noted from
the TPD-MS profiles (Tables 2 and 3), the activation was suc-
cessful because a large amount of SOFGs were introduced on
the carbon surface. To exclude the Lewis base factor from the
overall catalytic contributions, HNO -AMC was not annealed at
3
N . Kundu et al. reported that the direct heating of carbon sup-
2
high temperature. After wetness impregnation, TEM and XRD
ports under H flow was more efficient to remove the SOFGs
results show that the size of the Pt particles on Pt/HNO -AMC
2
3
than that under N , and the annealing of the catalyst support
was around 2 nm. With similar amounts of SOFGs on the surfa-
2
at high temperature showed less sintering of the metallic
ces of both HNO -AMC and K-AMC, a lower catalytic conver-
3
[
51]
nanoparticles. Thus, we used H gas for the thermal treat-
sion was observed on the Pt/HNO -AMC catalyst (24.6%) com-
2
3
ment of the Pt/K-AMC catalyst at different temperatures.
pared to Pt/K-AMC catalyst (33.4%). To find an explanation for
this difference in the catalytic activities, we took a closer look
After thermal treatment at 400 and 7008C under H , we ana-
2
lyzed the deconvolution data from the TPD-MS profiles (Fig-
ures S4 and S5) in Tables 2 and 3 and we concluded that the
at the TPD-MS data, more specifically, the CO/CO ratio, which
2
was used as an indication of the carbon surface acidity/basicity
balance (in which a lower ratio represents a stronger surface
surface chemistry of K-AMC-400-H and K-AMC-600 was very
2
[
47]
similar in terms of SOFGs. The K-AMC-700-H and K-AMC-900
acidity). With a CO/CO ratio of 1.4, the surface of K-AMC is
2
2
samples also showed similarities with regards to SOFGs. Impor-
tantly, Pt nanoparticles were still well dispersed and their sizes
more basic in terms of SOFGs than the HNO -AMC, which has
3
a ratio of almost 1. If we compare the two catalysts, the stron-
ger basicity of K-AMC explains its higher catalytic activity well.
In spite of a significant difference in the specific surface area,
were relatively preserved on both Pt/K-AMC-400-H and Pt/K-
2
AMC-700-H compared to the size of Pt particles on Pt/K-AMC
2
according to the TEM and XRD results (Figures S6 and S7). Al-
though we observed slightly larger Pt particles (3–4 nm) in the
the external surface areas of HNO -AMC and K-AMC are still
3
comparable (Table 1 and Table S2).
case of Pt/K-AMC-700-H , they are still smaller than those ob-
After thermal treatment at 6008C under N2 flow, Pt/HNO3-
AMC-600 showed a better catalytic activity (GLY conversion:
2
served on Pt/K-AMC-900 (5–6 nm). The catalytic performance
results listed in Table 5 show that the GLY conversion (64.8%)
46.8%) than Pt/HNO -AMC (24.6%) most likely because of the
3
on Pt/K-AMC-400-H is comparable to that (67.6%) obtained
loss of acidic groups and the creation of Lewis base, however,
its activity is still significantly lower than that of Pt/K-AMC-600
(67.6%). Given the similarities in the Pt particle sizes of both
catalysts (Figure 4 and Figure S6), the lower catalytic activity
2
on Pt/K-AMC-600, which is expected because of the similarity
of these catalysts. Interestingly, the GLY conversion (43.6%) on
Pt/K-AMC-700-H is comparable to that (48.9%) obtained on
2
Pt/K-AMC-900, although the Pt particle size is smaller. This indi-
cates that the Pt nanoparticle size has a minor contribution on
the catalytic performance of the catalyst (at least in this given
size range of Pt nanoparticles). In addition, the GLY conversion
observed on Pt/HNO -AMC-600 could be explained by the
3
lower amount of high-desorption-temperature SOFGs, specifi-
cally ether and carbonyl/quinone groups. These findings show
the higher catalytic performance of Pt supported on KOH-acti-
on Pt/K-AMC-700-H is considerably lower than that on Pt/K-
vated carbon compared to the traditional HNO -activated
2
3
AMC-600 and Pt/K-AMC-400-H . As a result of the minor effect
carbon for the GLY oxidation under base-free conditions.
2
of the Pt particle size on GLY oxidation, these results reveal the
key role that SOFGs play in this catalytic reaction.
Although we still have not confirmed whether the enhance-
ment of the catalytic performance mainly comes from the
presence of high-desorption-temperature SOFGs such as
phenol, ether, and carbonyl/quinone groups or Lewis base, it is
clear that the increase of the surface basicity of the carbon
support induced by one of those two factors or both is benefi-
cial for glycerol oxidation under base-free conditions. Although
recent studies supported the contribution from either the
high-desorption-temperature SOFGs or Lewis base on the
According to mechanistic studies on GLY oxidation under
[
20,52]
+
basic conditions,
NaOH does not only capture H from
one of the primary hydroxyl groups of GLY to initiate the reac-
tion but it also promotes the adsorption of GLY on the Pt sur-
face. Thus, under basic conditions, GLY molecules can adsorb
easily onto metal surfaces with the aid of NaOH. However,
under base-free conditions, the carbon surface should be hy-
drophilic enough to facilitate the adsorption of GLY molecules
on the Pt surface. The carbon surface hydrophilicity originates
from the presence of SOFGs. As carboxylic groups can impede
the adsorption of GLY molecules on carbon surface, the pres-
[
27,28]
carbon surface,
this study shows that the combination of
the high-desorption-temperature SOFGs and Lewis base leads
to a better catalytic activity for AC-supported Pt catalysts
under base-free conditions.
ChemCatChem 2016, 8, 1699 – 1707
1704
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim