C. Ban, et al.
Applied Catalysis A, General 578 (2019) 98–104
the active surface of Ru for hydrogen spillover, resulting in lowered
hydrolysis activity and unconverted oligomeric compounds when the
amount of Cu was increased. As summarized in Table 1, acid densities
of catalysts measured by back titration under atmospheric condition
exhibited an inverse correlation with the amount of Cu loaded. This
might arise partly from the deposition of Cu on surface acidic oxyge-
nates formed after oxidation of activated carbon with nitric acid. It has
been previously reported that surface oxygenates of an oxidized carbon
material could act as anchoring sites for a metal which eventually af-
fects its dispersion [27].
As shown in Fig. 3, Ru catalysts added with varied amounts of Cu
exhibited different activities. The yield of target C6 sugar alcohols,
sorbitol and mannitol, was greatly improved from 33.3% to 47.4%
when 1 wt% of Cu was added. However, further increase of Cu content
to 3 wt% and 5 wt% reduced yields of the target product to 15.2% and
5
1
.0%, respectively. Surprisingly, the yield was partially recovered to
8.0% when the loading amount of Cu was increased to 10 wt%. In
addition, it was found that pure Cu itself, Cu(10)/AC-N-13, was inactive
in the hydrogenation of alginic acid. To investigate such unusual cat-
alytic behavior with the addition of Cu, physicochemical properties of
these catalysts were analyzed as follows.
Fig. 1. Product distribution over various bimetallic carbon catalysts at 150 °C
for 3 h under 50 bar of H . C6 sugar alcohols: sorbitol (Sor), mannitol (Mann),
2
and galactitol (Gal); C5 sugar alcohols: xylitol, arabitol, and ribitol; C4 sugar
alcohols: threitol and erythritol; Aldonolactones: glucono-1,5-lactone and
mannono-1,4-lactone.
As shown in Fig. S1, all ruthenium-based catalysts displayed type IV
isotherm with H4 hysteresis, a representative of micro-mesoporous
carbons [28]. As summarized in Table 1, when the amount of Cu was
increased, surface area and pore volume of catalysts were decreased
Ultimate 3000 calibrated with Pullulan (molecular weight,
2
3
from 642.4 to 510.6 m /g and from 0.46 to 0.39 cm /g, respectively,
indicating partial pore blocking of the support.
342–80500).
As shown in Fig. 4, diffraction lines for Cu metal were observed for
samples loaded with Cu at 5 wt% or higher. It was worth noting that Cu
existed mainly in the metallic form in bimetallic catalysts in the bulk
3. Results and discussion
Various Ru-based bimetallic catalysts were synthesized and applied
2
state while Cu O crystallite and metallic Cu co-existed in pure Cu cat-
to hydrolytic hydrogenation of alginic acid for the production of sugar
alcohols, mainly sorbitol and mannitol. The support material used was
nitric acid-treated activated carbon to facilitate the hydrolysis of gly-
cosidic bonds in alginic acid since the acidity of activated carbon could
be enhanced by oxidation of carbon materials with nitric acid [22]. It
has been previously reported that the acid density of various carbon
materials is increased when the concentration of nitric acid is increased,
resulting in improved hydrolysis of sodium alginate into uronic acids
alyst. This suggests that the noble metal, Ru, can inhibit metallic Cu
from being oxidized upon exposure to air during passivation [29]. On
the other hand, in all samples, no diffraction lines for Ru was detected.
This might suggest that Ru was well dispersed with average particle size
below the detection limit of the instrument (< 5 nm). This might also
2 2
suggest the formation of amorphous RuO ·xH O upon exposure to the
air after the reduction of catalysts [30]. As shown in Fig. 5, this oxidized
Ru species was further evidenced by positive shifts of binding energy of
Ru 3p3/2 compared to metallic Ru (ca. 461 eV) [30,31].
[
1
16]. As shown in Fig. 1, a screening experiment was carried out at
50 °C for 3 h. It was found that Cu-promoted catalyst exhibited the
Surface oxidation states of Ru and Cu on the activated carbon was
further investigated by XPS. As shown in Fig. 5, a positive shift (ca.
0.4 eV) for Cu° (932.5 eV) in Cu 2p spectra was observed along with a
negative shift (ca. 0.8 eV) for oxidized Ru species (462.8 eV) in Ru 3p
spectra when the loading amount of Cu was increased compared to each
monometallic catalyst. This result is inconsistent with previous studies
concerning a direction of electron transfer between Ru and Cu. Previous
XPS studies have proposed that an electron transfer from Ru to Cu can
occur for silica supported RuCu bimetallic catalysts [32,33]. However,
electron transfer from Cu to Ru has also been suggested based on in-
frared spectroscopy of CO adsorbed on RuCu supported on silica [34].
The result obtained in the present study indicates electron transfer is
more likely to occur from Cu to Ru. Since the formation of galactitol, C4
epimer of sorbitol, was well-reported to be catalyzed by metallic sites
highest sugar alcohol ratio of C6/(C4 + C5) and the lowest galactitol
formation. The formation of byproducts such as C4-C5 sugar alcohols
and galactitol, would decrease the selectivity to desired products,
namely sorbitol and mannitol. Thus, Cu was chosen for further studies
shown below. Since the cleavage of CeC bonds and the isomerization of
sugar alcohols could be expedited under harsher reaction condition, the
reaction temperature was further elevated to 180 °C to better under-
stand a synergistic effect of Cu addition on Ru catalyst [23,24].
To investigate the effect of Cu addition, catalysts were applied to the
reaction after various amounts of Cu were loaded to 5 wt% Ru.
Conversion of alginic acid over the catalysts was indirectly measured by
GPC due to difficulty in separating unreacted alginic acid from the re-
action mixture [16,20]. As shown in GPC chromatograms (Fig. 2), the
reactant was fully converted to smaller molecules over bimetallic cat-
alysts in all cases. However, a peak corresponding to a compound
having molecular weight higher than a C6 sugar alcohol was also ob-
served. For comparison, a sugar alcohol with 12 carbon atoms, namely
maltitol, was analyzed. The general trend of the increase in the in-
tensity of such higher MW compound was in line with the increase in
2
under pressurized H , [35]. Hence, the suppression of galactitol for-
mation, as shown in Fig. 3, might be due to non-zero valent Ru species
formed by the electronic effect caused by Cu addition. Copper in its
oxidation state of Cu2+ was also observed, which could be character-
ized by broad satellite peaks (939–946 eV and 960–965 eV) on the side
of main peaks partially screened by large Cu° peak [36]. Unfortunately,
+
Cu content. It has been previously suggested that, under pressurized H
2
the oxidation state of Cu could not be characterized since photo-
atmosphere, spilled-over hydrogens by a metallic site can result in the
formation of protonic acid sites that are able to catalyze the hydrolysis
of cellulose [25]. Similarly, the ability of Ru to catalyze the hydrolysis
of CeC bonds in cellobiose into glucose has been also reported [26].
Thus, the above result implies that the addition of Cu could partly cover
electron peaks for Cu+ overlapped with those of Cu°. The effect of Cu
addition was further investigated by chemisorption analysis as follows.
Results of H
indicated in the last row of Table 2, Cu was unable to chemisorb H
CO under the condition studied [37]. This corresponds well to the
2
- and CO-chemisorption are summarized in Table 2. As
2
or
100