I. Tankov, H. Kolev and G. Avdeev
Journal of Molecular Liquids 340 (2021) 117192
vibrations in the pure ionic liquid should be hindered in some
extent after PHS immobilization AC. In other words, lower S–O
bond strength is expected in xPHS/AC samples in comparison with
the S–O bond strength in bulk PHS. However, the small difference
in the electronegativity of hydrogen and carbon proposes that the
S–O bond in PHS and xPHS/AC possesses similar strength. Indeed,
the FT-IR spectrum of PHS clearly exposed that the bands related
to S–O bond vibrations do not change their position when the
xPHS/AC are obtained.
at.%) and 292.6 eV (5 at.%). In light of this, contributions values of
1.5, 2.5 and 0.7 are established to characterize the O 1 s compo-
nents at 531.7, 533.5 and 535.6 eV, respectively. Based on the
above, carbon and oxygen concentrations of 95 and 5 at.%, respec-
tively denote the activated carbon surface. Notably lower contribu-
tion of surface oxygen in comparison with the surface carbon in the
XPS spectrum of AC agrees well with FT-IR profile of the carrier,
where the presence of surface oxygen-containing species is pre-
sented as bands of low intensity.
Except more intensive, the infrared band at 1113 cmꢁ1 in the
FT-IR spectra of 8PHS/AC, 17PHS/AC and 33PHS/AC is found to be
considerably broaden (ranges from 950 to 1350 cmꢁ1) with respect
to that in the vibrational spectrum of AC. It implies that a surface
PHS–AC interaction via formation of (H–C)PHSꢃꢃꢃ(O–C)AC bonds
should not be excluded as well. The (H–C)PHS component abbrevi-
ates the H–C bonds in the aromatic ring, while (O–C)AC one – AC
surface. The existence of (H–C)PHSꢃꢃꢃ(O–C)AC surface interactions
can explain notably lower intensity of the H–C bands in the infra-
red spectra of xPHS/AC in comparison with the intensity of the
same bands in pure PHS. It worth noted that the intensity of the
band at 1113 cmꢁ1 in the infrared profile of 66PHS/AC is remark-
ably less pronounced in comparison with that in the case of
8PHS/AC, 17PHS/AC and 33PHS/AC. This could be attributed to
the presence of a great amount of agglomerated PHS particles on
the AC surface for 66PHS/AC, which hinders the FT-IR detection
[64–66]. Analyzing the vibrational modes in the infrared region
above 3400 cmꢁ1, it is seen that the peak due O–H vibrations in
xPHS/AC slightly increases in comparison with the FT-IR profile
of AC. It shows the presence of: (i) additional O–H groups on the
AC surface in the form of [HSO4]– fragments and/or (ii) surface
hydrogen bonds with participation of the aromatic ring, namely
Impregnation of the activated carbon carrier with small
amounts (8 wt%) of pyridinium hydrogen sulfate slightly
decreases/increases the surface carbon/oxygen concentration on
AC to 92/7 at.%, which could be attributed to a poorly covered AC
surface by ionic liquid particles. It is in accordance with the FT-IR
data, where the PHS phase on AC surface is presented as infrared
peaks of low-intensity. XPS C 1 s line of 8PHS/AC sample revealed
components at 285.0, 286.0 and 287.6 eV, which do not differ (as
contribution and position) from those (285.0, 286.2 and
287.9 eV) observed previously for AC. Hence, it is reasonable to
assume that the components within 285–288 eV in the in the
XPS spectrum of 8PHS/AC describe the support surface. It is inter-
esting to point out that the component at 290.4 eV in the C 1 s
spectrum of AC is characterizes with a notably lower BE value
(289.5 eV) when 8PHS/AC is obtained. This could be correlated to
the presence of a reductive atmosphere in the form of C–H and
N–H fragments in pyridine ring. As a result, hydrogen bonds such
as (N–H)PHSꢃꢃꢃ(O–C)AC and (C–H)PHSꢃꢃꢃ(O–C)AC are present on the
carrier surface, being already proposed by FT-IR analysis. However,
this component can be interpreted as carbon attached to a less
electronegative atom such as hydrogen in the environment of
nitrogen one. In other words, the component at 289.5 eV in the C
1 s spectrum of 8PHS/AC may describe the aromatic H –C–N bonds
in the PHS structure. The existence of PHS particles on the AC sur-
face is confirmed when the highest binding energy components
(N–H)PHSꢃꢃꢃ(O–C)AC and (C–H)PHSꢃꢃꢃ(O–C)AC
.
In summary, the infrared bands registered in bulk PHS and AC
do not change its position after the xPHS/AC samples synthesis
which allows the surface PHS–AC interactions (identified as (H–
N)PHSꢃꢃꢃ(C–C)AC, (S–O)PHSꢃꢃꢃ(C–C)AC and (H–C)PHSꢃꢃꢃ(O–C)AC) to be
referred as primary electrostatic in nature.
(shake-up satellite peak due to
p ? p conjugation) in the C 1 s line
of AC and 8HS/AC samples are studied. It is revealed that the AC
impregnation with PHS ionic liquid causes BE variety from 292.6
(AC) to 291.6 eV (8PHS/AC), which corresponds to a change in
the activated carbon structure after PHS immobilization. The latter
agrees well with the XRD data, where a reduced graphite crystal-
lites size due to a surface PHS–AC interaction is observed.
Comparing the O 1 s lines of AC and 8PHS/AC samples, it is
found that the components at 531.7, 533.5 eV in AC spectrum
raised with 50 and 17 %, respectively after PHS deposition. It sug-
gests that additional amounts of C = O and O–C fragments (conse-
quence of surface PHS–AC interaction) on the AC surface in the
presence of PHS are formed, supporting the FT-IR data. On the
other hand, components with BE values of 531.7 and 533.5 eV
are found to characterize oxygen in the form of HSO–4 [73,74] and
SO24ꢁ [65,75] groups. Hence, notably promoted participation of
the components at 531.7 and 533.5 eV in the O 1 s line of 8PHS/
AC system is manly attributed to oxygen in the heterogenized ionic
liquid. Similar to the components between 531 and 534 eV in O 1 s
line of AC, the contribution of this at 535.6 eV is discovered to
increase notably (30%) in the case of 8PHS/AC as well. Moreover,
its BE shifts from 535.6 (AC) to 536.4 eV (8PHS/AC). These observa-
tions propose that a large amount of weakly bonded oxygen-
containing species (such as O–H groups) populate the 8PHS/AC
surface.
3.2.2. Core-electron levels description
To confirm the FT-IR results, XPS analysis of AC and xPHS/AC
systems is conducted. XPS of O 1 s and C 1 s core-electron levels
for activated carbon, and O 1 s, C 1 s, N 1 s and S 2p lines for hetero-
genized ionic liquid are illustrated in Fig. 4. The surface atomic
concentrations of the elements in the studied samples are depicted
in Table 2.
Results exposed that components with binding energy (BE) val-
ues of 285.0, 286.2, 287.9, 290.4 and 292.6 eV present in the C 1 s
spectrum of non-modified AC. The first of them describes sp3 bulk
bonded carbon (C–C) in graphite structure [67], while the second
one (286.2 eV) is recognized as C–O chemical bonds [68]. In light
of this, the components with BE values of 287.9 and 290.4 eV are
due to C = O [69] and O–C = O [70] groups, respectively. The com-
ponent with the highest binding energy (292.6 eV) is referred as a
shake-up satellite peak due to
p ? p conjugation [71]. Studying
the XPS O 1 s line of AC, the presence of above noted functional
groups on the support surface is undoubtedly confirmed. In details,
components at 531.7 (C = O) [69], 533.5 (C–O) [51] and 535.6 eV
(O–C = O) [72] are detected. C 1 s and O 1 s effects for AC com-
pletely coincide with the corresponding phase composition analy-
sis (Fig. 1A) and FT-IR spectrum (Fig. 3), where: (i) a peak at
2h = 26.5° due to crystal graphite and (ii) infrared bands (within
1000–1120 and 1722 cmꢁ1) related to different types of surface
oxygen-containing species are registered.
An indication for the presence of PHS phase on the AC surface is
also obtained evaluating the S 2p and N 1 s core-electron levels
(Fig. 4, Table 2). The S 2p line of 8PHS/AC showed two characteris-
tic doublets at BE values of 169.9 (S 2p1=2) and 168.8 eV (S 2p3=2),
which are typical for SO24ꢁ groups [76]. In this regard, the N 1 s
core-electron level of 8PHS/AC is presented as a single peak at
It is established that the sp3 bulk bonded carbon (53 at.%) is the
most pronounced component in the XPS spectrum of AC, followed
by the C 1 s components at 286.2 (20 at.%), 287.9 (9 at.%), 290.4 (8
7