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Table 1 Morphological properties of bare TiO
2
P25 support and 3 wt% Ru/TiO
2
catalysts: BET surface area (SBET), total pore volume and pore
2 2
diameter (VPT and dPT), average domain size of TiO polymorphs (anatase – d101, rutile – d110), weight fraction of anatase in TiO (wanatase) and
amount of accumulated carbon (w
C
)
2
ꢁ1
3
ꢁ1
Sample
S
BET (m g
)
V
PT (cm g
)
dPT (nm)
d101 (nm)
d110 (nm)
w
anatase (wt%)
C
w (wt%)
Bare TiO
Fresh catalyst
Reduced catalyst
Spent catalyst (test FT10)
Spent catalyst (test FS10)
Spent catalyst (test AT10)
Spent catalyst (test AS10)
2
P25
47.3
47.2
48.3
45.5
26.4
32.4
42.7
0.32
0.30
0.31
0.30
0.23
0.27
0.29
27.0
25.4
25.5
26.2
34.1
33.0
27.4
26.5
27.1
27.4
32.0
30.4
31.5
33.8
46.6
48.3
45.3
54.4
49.8
50.7
55.7
72
70
70
66
71
67
61
0.3
0.1
0.1
0.3
6.1
2.9
0.4
accumulated deposits aer AT10 test (DmTPO ¼ 4.6 wt% vs.
.9 wt% C) indicating their highly oxygenated structure, likely
2
originating from strong adsorption of reactants and interme-
diates, possibly undergoing polymerization during operation at
ꢀ
temperatures below 225 C. When long-term isothermal test at
ꢀ
2
25 C was performed (test AS10), no accumulation of carbo-
naceous species was noticed, as the reaction temperature was
high enough to facilitate their desorption from the catalyst
surface (notice the overlap of both decarboxylation reaction and
desorption temperature windows, Fig. S7†). On the contrary,
carbon deposits containing 91 wt% C were found on the spent
catalyst aer FA decarboxylation (test FS10, DmTPO ¼ 6.7 wt% vs.
6.1 wt% C) indicating their much less oxygenated composition
compared to AT10 experiment with AA. This is in line with less
ꢁ
1
intense IR absorption band at 1686 cm
(C]O bond) in
Fig. 3 DRIFTS spectra of TiO
and after FA and AA decarboxylation reactions.
2
support and Ru/TiO
2
catalysts before
sample FS10 compared to AT10 (Fig. 3). Higher temperature,
required for removal of carbon deposits (247 and 285 C for
ꢀ
AT10 and FS10, respectively, Fig. S7†), resulted in their accu-
ꢀ
mulation during catalytic decarboxylation of FA at 225 C. In
ꢀ
Similar effect was observed for FA, as operation at 240 C in
the last step of the FT10 test run resulted in no accumulation of
carbonaceous deposits. On the other hand, additional peaks at
both cases, only endothermic peaks were identied during
TPO-DSC analysis of spent catalyst samples (Fig. S7†), which
implies that accumulated carbonaceous deposits were essen-
tially removed by desorption instead of combustion.
To conclude, low-temperature catalytic decarboxylation of
2
AA in a trickle-bed reactor over 3 wt% Ru/TiO was demon-
strated (XAA < 98%, SCH4 > 80%). At reaction temperatures below
25 C where incomplete AA conversion is achieved, accumu-
lation of highly oxygenated carbonaceous deposits is observed
over the catalyst, leading to a continuous deactivation. At 225 C
ꢁ
1
2
980, 2940, 2880, 1686, 1473 and 1464 cm can be observed
aer the FS10 and AT10 tests. These can be assigned to
symmetric and asymmetric stretching modes of –CH –, –CH
and C]O bonds, and reveals the structure of carbonaceous
deposits is paraffinic and oxygenated (such as acids, aldehydes
and esters).
The deposits originate from AA and FA and reactions
between such adjacent surface species (dehydration, polymeri-
zation, etc.). Decomposition products of AA (CH , CO ) are
4 2
probably not active precursors for the formation of hydrocar-
bons that remain adsorbed on the catalyst despite the fact that
Ru/TiO is active for hydrogenation and carbon chain growth
through Fischer–Tropsch chemistry, when using syngas to
produce larger hydrocarbons. The reason is twofold: (i) reac-
tion temperature is too low for catalytic C–H bond cleavage in
hydrogenation in case of AA
are exclusive products); (ii)
if Fischer–Tropsch chemistry was occurring, this would be
observed during continuous GC and HPLC analyses.
2
3
ꢀ
2
ꢀ
ꢁ1
and AA concentrations up to 20 g L , complete conversion was
achieved and no carbonaceous residues accumulated over the
catalyst, resulting in stable catalyst operation and roughly
equimolar CH
TOS. In case of FA decarboxylation, lower selectivities (XFA
00%, SCO2 < 60%) compared to AA were achieved. Continuous
deactivation of the catalyst was observed at FA concentration of
4 2
and CO product stream obtained during 70 h
2
<
1
18
ꢁ1
ꢀ
1
0 g L and 180 C due to accumulation of thermally stable
carbonaceous residue which blocks the catalytically active sites.
These deposits were removed by temperature programmed
methane, which would enable CO
2
decarboxylation (where CO and CH
2
4
ꢀ
desorption at 285 C. Besides decarboxylation reaction, simul-
taneous occurrence of RWGS and CO/CO
tions was observed, which shied the product spectrum from
and CO to CH , CO and CO
2
methanation reac-
A combination of C elemental analysis and TPO results
revealed 63 wt% carbon content (the rest is O and H) in
H
2
2
4
2
.
54088 | RSC Adv., 2015, 5, 54085–54089
This journal is © The Royal Society of Chemistry 2015