CARBON
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
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XPS and NMR studies of phosphoric acid activated carbons
A.M. Puziya,*, O.I. Poddubnayaa, R.P. Sochab, J. Gurgulb, M. Wisniewskic
a
Institute for Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Naumov Street 13, 03164 Kyiv, Ukraine
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30–239 Kraków, Poland
c
Physicochemistry of Carbon Materials Research Group, Department of Chemistry, Nicholas Copernicus University Gagarin Street 7,
87–100 Torun, Poland
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Chemical structure of phosphorus species in two series of polymer-based and fruit-stone-
Received 6 May 2008
based carbons obtained by phosphoric acid activation at 400–1000 C were investigated by
Accepted 3 September 2008
XPS and solid state 31P-NMR and 13C-NMR. It has been shown that the most abundant and
Available online 11 September 2008
thus thermally stable phosphorus species in all investigated carbons is phosphate-like
structure bound to carbon lattice via C-O-P bonding. Small contribution of phosphonates
(C-P-O linkage) was observed by
31
P NMR in carbons obtained at temperature range of
500–700 C, phosphorus oxide was evidenced by XPS in carbon prepared at 900 C and elemental phosphorus in carbon activated at 1000 C.
2008 Elsevier Ltd. All rights reserved.
1.
Introduction
It is common knowledge that the properties of carbon adsorbents are determined by porous structure and surface chemistry [1]. Pore size and pore volume are important factors for
physical adsorption [2–4], while surface chemistry plays a
key role in specific adsorption and surface reactions [4–7].
Surface chemistry of carbon materials depends on the presence of heteroatoms like hydrogen, oxygen, nitrogen, phosphorus, chlorine etc. [4,5,8] that may come from carbon
precursor or activating agent [9]. Heteroatoms affect acid–
base characteristics of carbons and modify their electrochemical and catalytic properties [4,10–12]. Oxygen-containing surface groups confer hydrophilic and cation exchange
properties [13–15]. Nitrogen-containing carbons show enhanced anion exchange properties [15,16] and catalytic activity in red–ox reactions [11,17,18]. Phosphorus-containing
carbons show a number of specific characteristics that range
from acid surface groups and cation exchange properties [19–
23] to enhanced oxidation stability [24].
Recent studies have shown that phosphoric acid activation
results not only in developing porosity of carbons but also
leads to inclusion of significant amount of phosphorus into
carbon structure [19–23]. It has been shown that phosphorus
compounds are responsible for enhanced cation exchange
properties of phosphoric acid activated carbons. The studies
of phosphoric acid activated carbons by chemical analysis,
FTIR spectroscopy, XPS and pH-titration have shown that surface groups of phosphoric acid activated carbons may be classified as phosphorus-containing, carboxylic, lactone and
phenol like groups [19–23,25]. Phosphorus-containing surface
groups were ascribed to condensed phosphates formed during pyrolysis of carbonaceous precursor in presence of phosphoric acid [19–22]. However, above studies were unable to
show explicitly how phosphorus species are bound to phosphoric acid activated carbons.
To shed light on the chemical structure of phosphorus species and the way they are bound to carbon matrix two series
of phosphoric acid activated carbons obtained at different
temperatures from polymer and lignocellulosic precursors
were examined by XPS and solid-state NMR methods.
* Corresponding author: Fax: + 380 44 4529328.
E-mail address: [email protected] (A.M. Puziy).
0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2008.09.010
2114
CARBON
2.
Experimental
2.1.
Carbons
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
Preparation of carbons was described in previous studies
[19,21]. Briefly, carbonaceous precursor was impregnated with
phosphoric acid, dried in air and then carbonized at different
temperatures in argon flow. After carbonization, carbons were
extensively washed with hot water until neutral pH and dried
at 105–110 C. Two series of carbons were obtained – polymerbased carbons and fruit-stone-based carbons.
2.2.
Methods
The X-ray Photoelectron Spectroscopy (XPS) measurements
were performed in the ultrahigh vacuum (1 · 10 7 Pa) system
equipped with hemispherical analyzer (SES R 4000, Gammadata Scienta). The samples (granules of 0.3–0.5 mm) were
mounted on a carbon tape then degassed and transferred into
analysis chamber.
a
The unmonochromatized Mg Ka X-ray source of incident
energy of 1253.6 eV was applied to generate core excitation.
The spectrometer was calibrated according to ISO
15472:2001. The energy resolution of the system, measured
as a full width at half maximum (FWHM) for Ag 3d5/2 excitation line, was 0.8 eV. The energy step at the survey spectra
was 0.25 eV and the step at the detailed spectra was
0.025 eV. The spectra were calibrated for a carbon C 1s excitation at binding energy of 285 eV. The XPS analysis depth, in
case of carbon matrix, is about 10–12 nm. This value takes
into account 95% of the photons leaving the surface. The
accuracy of the XPS analysis is approximately 3%. The spectra
were analyzed and processed with the use of CasaXPS 2.3.10
software. The background was approximated by Shirley algorithm and the detailed spectra were fitted with Voigt function.
The P 2p curves were fitted taking into account the spin-orbit
splitting of 0.84 eV and ratio of 2p1/2:2p3/2 components as 0.5.
All NMR spectra have been carried out using a Bruker AMX
300 spectrometer at a 13C resonance frequency of 75.47 MHz,
1
H and 31P frequency of 300.13 MHz and 121.49 MHz,
20
Oxygen content, Wt%
18
16
14
12
10
8
surface
bulk
6
4
2
0
400
500
600
700
800
900
1000
900
1000
Temperature, °C
Phosphorus content, Wt%
b
12
10
8
6
4
surface
bulk
2
0
400
500
600
700
800
Temperature, °C
Fig. 1 – Comparison of surface and bulk concentration of oxygen (a) and phosphorus (b) in polymer-based carbons
obtained by phosphoric acid activation at different temperatures.
CARBON
respectively. The high-resolution spectra have been measured
using cross-polarization (CP), magic angle spinning (MAS) –
for 13C experiments and high power decoupling in the case
of 31P measurements. The 13C spectra were also recorded
using the total suppression of sidebands (TOSS-B) pulse sequence that removes rotational sidebands arising from inhomogeneous interactions larger than the spinning frequency.
The MAS spinning frequency was 7 kHz.
3.
Results and discussion
The XPS spectra of polymer-based phosphoric acid activated
carbons indicate the presence of carbon, oxygen, phosphorus,
sulfur and silicon in samples obtained at all temperatures.
Comparison between surface and bulk [19] concentrations
shows that surface of the samples is enriched with oxygen
for carbons obtained at 400–600 C (Fig. 1a) and with phosphorus for carbons obtained at all temperatures (Fig. 1b). Similar
trend was observed for carbons obtained from fruit stones
(Fig. 2).
a
2115
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
The enrichment of surface with oxygen for the samples
prepared at low temperatures as compared to the bulk suggests that phosphoric acid acts as oxidizing agent resulting
in formation of oxygen–carbon structures (Fig. 1a). Surface
concentration of phosphorus is higher than in the bulk signifying that chemical reaction of both precursors with H3PO4
occurs superficially (Fig. 1b). The difference between surface
and bulk concentration of oxygen progressively decreases as
carbonization temperature increases implying progressive
involvement of deeper layers of carbon into oxidation
process.
High-resolution XPS spectra of C 1s excitation showed
complicated envelope that indicated several carbon species
at the carbons’ surface (Fig. 3). The spectra were deconvoluted
into six components adopted in the analysis of carbon materials [26–30]. The components (Table 1) represent carbide carbon (peak A); graphitic carbon (peak B); carbon species in
alcohol, ether groups and/or C-O-P linkage (peak C); carbon
in carbonyl groups (peak D); carboxyl and/or ester groups
(peak E); and shake-up satellite due to p-p* transitions in
18
Oxygen content, Wt%
16
14
12
10
8
surface
bulk
6
4
2
0
400
500
600
700
800
900
1000
900
1000
Temperature, °C
Phosphorus content, Wt%
b
12
10
8
6
4
surface
bulk
2
0
400
500
600
700
800
Temperature, °C
Fig. 2 – Comparison of surface and bulk concentration of oxygen (a) and phosphorus (b) in fruit-stone-based carbons
obtained by phosphoric acid activation at different temperatures.
2116
Intensity (arb. units)
CARBON
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
B
C
D
E
F
294
292
A
290
288
286
284
282
280
Binding Energy (eV)
Fig. 3 – High-resolution X-ray photoelectron spectrum of C
1s peak of polymer-based carbon obtained by phosphoric
acid activation at 800 C. For details see Table 1.
aromatic rings (peak F). With the rise of carbonization temperature the content of graphite carbon (area of peak B) decreases while the content of oxygenated carbon (sum of
peaks C + D + E) increases indicating enhancing degree of
advancement of the reaction between carbon and phosphoric
acid (Fig. 4). It is worth mentioning that the amount of oxygenate carbon in fruit-stone-based carbons obtained from initially reach in oxygen lignocellulosic precursor is lower (28–
30%) that that in polymer-based carbons (35–55%). It should
be noted that phosphorus compounds cannot be clearly
determined from C 1s region because binding energy of CO-P bonding is similar to binding energy in alcohol and ether
groups (peak C). Moreover, the C 1s electron binding energy of
phosphonates (compounds with direct C-P bonding) is between graphitic (peak B) and oxygenate (peak C) carbon [31].
O 1s peaks are broad indicating the presence of different
chemical states of oxygen (Fig. 5). Since O 1s line shows
weakly developed structure, it is not possible to separate contributions of organic oxygen (O in carboxyl, carbonyl, alcoxyl
or ether groups) and inorganic oxygen (O in phosphates) [32].
The O 1s peaks were deconvoluted into four components for
all carbons (Table 1). The peak A of BE = 530.6–530.9 eV is
attributed to oxygen double bonded to carbon (C=O) and
non-bridging oxygen in the phosphate group (P=O) [33]. The
peak B at BE = 532.5–532.9 eV can be assigned to combined effects of singly bonded oxygen (-O-) in C-O and in C-O-P
groups. The peak C at BE = 535.0–535.9 eV is ascribed to chemisorbed oxygen and water [31]. The peak D at BE = 537.4–
538.8 eV is due to OH groups like in cyclohexanol or phenol
[34]. The singly bonded oxygen is main component in all carbons (-O-, peak B) while the second most abundant form is
double bonded oxygen (=O, peak A) (Fig. 6). The contribution
of other forms of oxygen (peaks C and D) is less than 10% except chemisorbed water and oxygen (peak C) for polymerbased carbon obtained at 900 C. This can be explained by for1
2
mation of phosphorus pentoxide (see next paragraph) which
is well known as powerful desiccant (Fig. 7).
P 2p peaks were deconvoluted into four components. The
components do not all present in each carbon (Table 1). The
evolution of binding energy and relative intensity of P 2p line
components with carbonization temperature are shown in
Fig. 8 and Fig. 9. The main component (peak B) constitutes
87–100% of total phosphorus content for both series of carbons prepared at all temperatures. The main component with
BE = 132.9–133.1 eV is attributed to pentavalent tetra coordinated phosphorus (PO4 tetrahedra) surrounded by different
chemical environment (phosphate-like structure) [35–37]. It
should be noted that phosphonates (compounds with C-P
bonding) also fall in this region1. P 2p core excitation of polymer-based carbons obtained at temperatures 400–800 C contains a single component (peak B) (Fig. 8b). An additional
contribution (about 5%) is observed in P 2p spectrum of carbon obtained at 900 C (component D, BE = 136.0 eV), which
can be attributed to phosphorus oxide P2O5 [35]. The phosphorus oxide signal (peak D) disappears in carbon prepared
at 1000 C and a small contribution (5%) of elemental phosphorus (peak A with BE = 129.6 eV [35]) was observed (Fig. 8).
It may appear that the existence of elemental phosphorus
in carbon obtained at so high temperature conflicts with
physical properties of phosphorus. Indeed, white phosphorus
boiling point is 280 C, red phosphorus sublimes at 416 C
[38,39] and critical temperature of phosphorus is 721 C2. Nevertheless, formation of elemental phosphorus in carbon
materials obtained at high temperatures must be considered
as real possibility. Imamura et al. using XPS and P-NMR observed red phosphorus in carbon fibre obtained by carbonization of phosphorus-containing phenol–formaldehyde resin at
1000–2000 C [40]. Ðurkić et al. using XRD found a separate
phase of phosphorus in carbon obtained by carbonizing mixture of phenol–formaldehyde resin with P2O5 [41]. The separate phosphorus phase was presumed to be in the voids
between carbon crystallites. This appears to be the case also
in our study since easily accessible elemental phosphorus
cannot survive in ultra-high vacuum, which is necessary for
XPS measurements.
For fruit-stone-based carbons, in parallel with the main
phosphate component B, there is also observed a small contribution (8–13%) of metaphosphates (component C [36]) at
600 C and 800 C (Fig. 9).
It is significant that the main phosphate component B is
slightly shifted to lower binding energy (from 133.7–133.9 eV
to 132.7–132.9 eV) as the carbonization temperature increased
from 400 C to 500–600 C. Gradual decrease of P 2p binding
energy with increasing carbonization temperature was also
reported for phosphoric acid activated carbon fibres from sisal [42]. This trend was explained by two reasons: (i) gradual
dehydration and condensation of phosphoric acid into polyphosphates and (ii) interaction of phosphorus with enlarged
aromatic carbon ring system.
Solid-state 31P-NMR spectra of polymer-based carbons are
shown in Fig. 10. The dominant signal (around 0 ppm) is
attributed to phosphate-like structure, i.e. phosphorus bound
NIST X-ray Photoelectron Spectroscopy Database, http://srdata.nist.gov/xps/.
Web Elements, http://www.webelements.com/phosphorus/.
CARBON
2117
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
Table 1 – Deconvolution results of XPS spectra of polymer-based carbons obtained by phosphoric acid activation
Region
Peak
Polymer-based carbons
Position (eV)
Fruit-stone-based carbons
FWHM (eV)
Position (eV)
FWHM (eV)
Assignment
C 1s
A
B
C
D
E
F
283.1 ± 0.2
284.3 ± 0.2
285.0 ± 0.5
286.5 ± 0.4
288.5 ± 0.4
290.4 ± 0.3
1.6 ± 0.2
1.2 ± 0.1
2.0 ± 0.3
2.3 ± 0.2
2.1 ± 0.5
2.6 ± 0.3
283.1 ± 0.1
284.4 ± 0.2
285.5 ± 0.5
287.2 ± 1.0
289.4 ± 0.8
291.6 ± 0.5
1.4 ± 0.3
1.3 ± 0.1
2.0 ± 0.3
2.3 ± 0.2
2.9 ± 0.1
3.2 ± 0.2
Carbide
Graphite
R-OH + C–O–C + C–O–P
C=O + >C=O
COOH + C(O)–O–C
p–p*
O 1s
A
530.6 ± 0.5
1.8 ± 0.3
530.9 ± 0.5
2.0 ± 0.2
B
C
D
532.5 ± 0.4
535.0 ± 0.4
537.4 ± 0.3
2.8 ± 0.2
2.8 ± 0.8
3.2 ± 0.3
532.9 ± 0.4
535.9 ± 0.2
538.8 ± 0.2
2.4 ± 0.2
2.5 ± 0.4
3.5 ± 0.0
=O in carbonyl, carboxyl
and phosphates
-OChemisorbed O + H2O
A
B
129.6 ± 0.0
132.9 ± 0.4
2.0 ± 0.0
2.1 ± 0.1
–
133.1 ± 0.5
–
2.1 ± 0.1
C
D
–
136.0 ± 0.0
–
2.8 ± 0.0
134.6 ± 0.3
–
2.3 ± 0.0
–
P 2p
a
P
Phosphates and
pyrophosphates
Metaphospates
P2O5
70
60
%Area
50
40
30
20
graphite carbon
oxygenate carbon
10
0
400
500
600
700
800
900
1000
Temperature, °C
b
70
60
%Area
50
40
30
20
graphite carbon
oxygenate carbon
10
0
400
500
600
700
800
900
1000
Temperature, °C
Fig. 4 – Relative amount of graphite carbon atoms (area of peak B) and oxygenate carbon atoms (sum of area of peaks C + D + E)
in polymer-based carbons (a) and fruit-stone-based carbons (b) obtained by phosphoric acid activation at different
temperatures.
2118
CARBON
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
Intensity (arb. units)
Intensity (arb. units)
B
A
C
D
542
540
538
536
534
532
530
528 526
138
2p3/2
2p1/2
136
Binding Energy (eV)
Fig. 5 – High-resolution X-ray photoelectron spectrum of O
1s peak of polymer-based carbon obtained by phosphoric
acid activation at 800 C. For details see Table 1.
a
134
132
130
Fig. 7 – High-resolution X-ray photoelectron spectrum of P
2p peak of polymer-based carbon obtained by phosphoric
acid activation at 800 C. For details see Table 1.
90
80
70
%Area
60
A
B
C
D
50
40
30
20
10
0
400
500
600
700
800
900
1000
Temperature, °C
b
70
60
%Area
50
A
B
C
D
40
30
20
10
0
400
500
128
Binding Energy (eV)
600
700
800
900
1000
Temperature, °C
Fig. 6 – Relative intensities of deconvoluted peaks in O 1s region in polymer-based carbons (a) and fruit-stone-based
carbons (b) obtained by phosphoric acid activation at different temperatures.
CARBON
a
137
A
B
D
136
Peak position, eV
2119
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
135
134
133
132
131
130
129
400
500
600
700
800
900
1000
800
900
1000
Temperature, °C
b
120
100
%Area
80
A
B
D
60
40
20
0
400
500
600
700
Temperature, °C
Fig. 8 – Position (a) and relative intensity (b) of deconvoluted peaks in P 2p region of polymer-based carbons obtained by
phosphoric acid activation at different temperatures.
to four oxygen atoms [43]. The phosphate peak is shifted to
higher magnetic field with increasing carbonization temperature, most probably due to increasing positive shielding from
p-electrons of enlarged graphene layers. The shift mechanism
is supported by the fact that the most significant peak shift
occurs in the same temperature range (between 600 C and
700 C) where drastic decrease of electrical resistance associated with increasing size of graphene layers was observed for
the same polymer precursor [44]. Upfield shift of phosphate
peak with increasing carbonization temperature was also reported in phosphoric acid activated carbon fibres [42].
The second peak at 7–14 ppm is most likely due to phosphonates, i.e. compounds with C-P bonding. It appears at
500 C, gradually decreases with increasing temperature and
vanishes at 800 C. The occurrence of phosphonates at
31 ppm was also reported for carbon obtained from phosphorylated phenol–formaldehyde resin [45].
Interesting is an analysis of changes in relative intensity of
main peaks together with their spinning side (SS) bands
resulting from anisotropy of the carbon. The spectrum of carbon obtained at 400 C shows only phosphate structures with-
out SS bands indicating homogeneous distribution of the
phosphate species. SS bands appear in spectrum of carbons
obtained at 500 C, their intensity increase up to 700 C and
decrease at 800 C.
Carbon molecular structure was investigated using 13C solid-state NMR measurement. 13C-NMR spectra of polymerbased carbons show (Fig. 11) bands from carbonyl or carboxyl
carbon (190 ppm), sp2-hybridized carbon in condensed aromatic rings (125–143 ppm), alcohol carbon (C-O 55–70 ppm)
and aliphatic carbon (9–44 ppm). Aromatic carbon dominates
the spectra for carbons prepared at all temperatures indicating that carbons are composed of condensed aromatic ring
system. The relative intensity of aliphatic carbon gradually
decreases as carbonization temperature increases. An
increasing carbonization temperature from 400 C to 600 C
resulted also in an increase in anisotropy of chemical bonds
that produces an increase of rotational spinning sidebands
(absent earlier) in spite of the TOSS pulse sequence used.
The spectrum of the sample heated at 800 C shows only
one weak resonance, about 124 ppm, issuing from polyaromatic hydrocarbons. Thus, one can conclude that the degree
2120
CARBON
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
a 135.0
Peak position, eV
134.5
134.0
B
C
133.5
133.0
132.5
400
500
600
700
800
900
1000
Temperature, °C
b 120
100
%Area
80
B
C
60
40
20
0
400
500
600
700
800
900
1000
Temperature, °C
Fig. 9 – Position (a) and relative intensity (b) of deconvoluted peaks in P 2p region of fruit-stone-based carbons obtained by
phosphoric acid activation at different temperatures.
of aromaticity here is considerable because of the fact that
graphite in CP methods does not give any bands. The broad
resonances in the spectrum of this sample demonstrate the
amorphous nature, the presence of free radicals and the complex structure of this highly dipolar coupled materials.
The relative intensity of carbon bound to oxygen (carbonyl
and ether) drastically increases with increasing carbonization
temperature from 400 C to 600 C with leveling off at higher
temperature.
4.
Concluding remarks
There is controversy in literature as to the chemical state of P
in phosphorus-containing carbons. Some authors based on
both experimental data and theoretical calculations proposed
that C-O-P bonding is more stable [46,47], the others argued
that this bond would not be strong enough to survive at high
temperature and proposed alternative structure C-P-O bonding [48,49]. The present study using XPS and 31P-NMR methods revealed that the most abundant phosphorus specie in
carbons obtained at 400–1000 C by phosphoric acid activation
is phosphate-like structure – pentavalent tetra coordinated
phosphorus bound to four oxygen atoms. This finding is in
line with our previous FTIR [19–22] and XPS studies [25]. The
fact that phosphate-like structure is of greatest abundance
in carbons obtained at all investigated temperatures signifies
its thermal stability.
31
P-NMR revealed a small contribution of phosphonates at
500–700 C, which implies that phosphate-like structure may
be only partly bonded by C-P bonding. The presence of phosphonates at higher temperature was not observed showing
higher thermal stability of C-O-P over C-P-O bonding.
The resolution of XPS and NMR methods did not allowed
clearly distinguish between phosphates and polyphosphates.
Nevertheless, taking into account that phosphoric acid readily produces polyphosphates at high temperature [50], condensed phosphates must be considered as most probable
form of phosphate-like structure.
Another chemical reaction that could be expected to take
part during phosphoric acid activation is reduction of pentavalent phosphorus with carbon. However, none of the applied
methods (FTIR [19–22], XPS [[25], this study], NMR [this study])
CARBON
2121
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
-0.4
SP400
0.4
14.3
SP500
**
*
** *
-2.6
12.5
SP600
**
*
** *
-4.6
7.0
SP700
*
*
*
*
-4.9
SP800
200
150
*
100
50
*
0
-50
-100
-150
-200
Fig. 10 – 31P-NMR spectra of polymer-based carbons obtained by phosphoric acid activation at different temperatures.
Spinning side bands are marked with asterisk.
Fig. 11 –
13
C-NMR spectra of polymer-based carbons obtained by phosphoric acid activation at different temperatures.
did not reveal reduced forms of phosphorus at temperatures
up to 900 C. Elemental phosphorus was observed in the ma-
trix of polymer-based carbon obtained at temperature as high
as 1000 C. Taking into account the volatility of elemental
2122
CARBON
4 6 ( 2 0 0 8 ) 2 1 1 3 –2 1 2 3
phosphorus at this temperature the location of it should be in
closed pores, between carbon crystallites or at least in very
fine pores that hinder evaporation.
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