xps study of group ia carbonates · a1 ks source operated at 225 w and a low-energy electron gun...

Central European Science Journals

w w w . c e s j . c o m

Central European Journal of Chemistry

CEJC 2(2) 2004 347 362

XPS Study of Group IA Carbonates

A.V. Shchukarev 1 *, D.V. Korolkov 1

Chemical Faculty, St.Petersburg State University,

Universitetsky pr. 2, 198 504 Petergof, St.Petersburg, Russia

Received 18 September 2003; accepted 22 December 2003

Abstrac t : The results of systematic XPS measurements of all alkali metal carbonates (Li, Na, K, Rb and Cs) are presented. The first set of experiments was performed with "as received" commercial carbonate powders under liquid nitrogen conditions using a precooling procedure. A second set of experiments was performed under similar experimental conditions after a preliminary grinding (mechanical activation) of the carbonates. In addition, Na2COa "1H20, NaHCOa and KHCOa powders were studied. It was found that sample pre-cooling allows distinction between hydrocarbonates and carbonate hydrates. Storage in air leads to formation of hydrocarbonates at the surface of Li2COa and Na2COa. This phenomenon being more pronounced in the former. In contrast, K2COa forms a hydrate with one H20 molecule. Rb2COa and Cs2COa have hydrocarbonates as well as hydrates at the surface and this is more pronounced for Cs2COa. Grinding of the carbonates results in the formation of hydrocarbonates at the surface, the tendency to do so was found to increase down the group IA, namely, K < Rb << Cs. For the most part, the hydrocarbonates formed were unstable in vacuum even under liquid nitrogen conditions. Chemical trends in C ls and O ls binding energies in carbonates and hydrocarbonates of the Group IA are discussed and related to the nature of the anion and alkali cation. @ Central European Science Journals. All rights reserved.

Keywords: Carbonates, hydrocarbonates, XPS

1 I n t r o d u c t i o n

Group IA carbonates are widely used in industry, medicine, and play an i m p o r t a n t role in

nature . Thei r chemical proper t ies are well known and they differ in the rmal stabil i ty f rom

other carbonates . The Group IA carbonates exhibit the effect of increasing size and mass

on chemical and physical propert ies . Due to this effect the lat t ice energy of carbonates

* E-mail: [email protected]

348 A.V. Shchukarev, D.V. Korolkov / Central European Journal of Chemistry 2(2) 2004 347 362

and the ease of their thermal decomposition decrease down the group [l]. From a chemical

point of view, such behaviour is connected to the decrease in first ionisation potential (I1)

of alkali metal atoms and determines the ionicity of the metal-carbonate bonds.

Alkaline metal carbonates are easily formed at oxide/hydroxide surfaces. Owing to

their high chemical stability, they can be used as reference compounds for analytical

purposes. The availability of XPS re%fence data on alkali metal carbonates is however

limited to carbonates of Li and Na [2, 3].The objective of this work was to obtain the bind-

ing energies re%fence data for all Group IA carbonates and to understand the chemical

changes which can be introduced by sample preparation techniques.

2 Exper imenta l

All XPS spectra were recorded with a Kratos Axis Ultra electron spectrometer using mono

A1 Ks source operated at 225 W and a low-energy electron gun for charge compensation.

Survey scan spectra were recorded in 5 rain using pass energy of 160 eV and a 1 eV step.

No elements other than those of bulk composition were detected, except in the case of

lithium carbonate containing 1 at. % of Na and 0.3 at. % of sulphate. Narrow scans

of the C ls, O ls and the main metal line were acquired using a pass energy of 20 eV

and a 0.1 eV step. The hybrid lens mode was used for all measurements. The samples

were hand-pressed in a molybdenum holder with a 5 mm diameter through of variable

volume. To avoid loss of water and to minimize possible decomposition of the surface,

a liquid nitrogen precooling procedure was carried out before the measurements. This

procedure includes precooling the end of the sample trans%r rod (20 rain at 170~

and then waiting 30 s after sample loading before pumping of the introducing chamber.

After pumping to 4-5 x 10 -5 Pa, the sample was trans%rred to the precooled (-160~

manipulator where it was kept until a base vacuum of 4-6 x 10 -7 Pa in the analysis

chmnber was reached. No frost layer was detected at the surface, despite not using a

glove box for sample introduction. After the measurements under liquid nitrogen, the

samples were kept overnight at the analysis position allowing warming in vacuum to

room T, and the XPS spectra were recorded the following day. Processing of the spectra

was accomplished with Kratos software. The binding energy (BE) scale was referenced

to the C ls line of aliphatic carbon contamination, set at 285.0 eV.

Li, Na and K carbonates used in this study were powders from Merck and had pro analysi purity. Cs2COa powder (eztra pure) was also from Merck. Rb2COa powder

from Aldrich had 99.8 % purity. All carbonates were investigated ~%s received" and

measurements were repeated with samples ground in an agate mortar for 3 rain.

3 Resul t s and discuss ion

C ls and O ls spectra of all carbonates are presented in Fig.1 - 7. BE and FWHM values

are given in Table 1.

A.V. Shchukarev, D.V. Korolkov / Central European Journal of Chemistry 2(2) 2004 34~362 349

3.1 Li2CO3

The Li + ion has an exceptionally high charge-radius ratio comparable with Mg 2+. Li2CO3

is thermally much less stable than other alkali metal carbonates owing to poor packing

of relatively large carbonate anions and small lithium ions [1]. The Ia of 5.39 eV is the

highest among the Group [4].

The C ls spectrum (Fig. 1) contains a main narrow component at 290.1 eV corre-

sponding to carbon in the carbonate ion. The O ls spectrum consists of two lines, the

main at 531.8 eV, and the minor at 533.2 eV. The intensity of the minor component

decreases after sample warming. This line can be assigned to an adsorbed water molecule

or an H-O-C bond in the hydrocarbonate ion. The Li/carbonate atomic ratio is 1.6 and

is less than a theoretical value of 2 calculated from the chemical %rmula and indicates

the presence of Li hydrocarbonate at the surface.

3.2 Na2CO3

Anhydrous sodium carbonate has BE values of both C ls and O ls lines 0.8 eV less than

those %r Li. Presence of hydrocarbonate traces at the surface can be detected %r the

sample measured under liquid nitrogen conditions (Fig. 2). Sodium hydrogen carbonate

(NaHCO3) has binding energies of main C ls and O ls lines 0.6 eV higher than the

carbonate (Table 1). In addition, a second component in the O ls spectrum appears

at 532.9 eV (H-O-C bond) (Fig. 3). This component slightly decreases in intensity

during the sample warming. The atomic ratio of two main oxygen lines is approx. 2:1

reflecting the structure of the HCO~ ion. Na2COa.IH20 (a hydrate) shows an additional

component in the O ls spectrum at 532.3 eV, which disappears during warming the

sample in vacuum. Again, the Na/carbonate atomic ratio is less than 2 (1.75, Table 1),

and seems to be due to some hydrolysis of the surface in air. In the case of NaHCO3 this

ratio is 1 in accordance with the chemical %rmula.

3.3 K2COa

Comparing with Li and Na, the surface chemistry of K2COa differs. The C ls and O ls

binding energies of the carbonate ion continue to decrease. Potassium carbonate has a

main C ls line at 289.0 eV and two O ls components at 530.7 eV and 532.0 eV. The

last one nearly disappears after warming in vacuum, whereas the first one and the C ls

component decrease in BE (Table 1, Fig. 4). In parallel, the K/carbonate atomic ratio

(1.9) does not change, indicating that the surface of potassium carbonate is significantly

hydrated in air. KHCO3 has much higher binding energies than both K2CO3 samples and

in contrast with hydrated carbonate does not lose the second O ls component responsible

%r the H-O-C bond in hydrocarbonate (Table 1, Fig. 5). It is interesting to note that

an increase in BE %r the K 2p3/2 line %r KHCO3 seems to be connected with increased

polarisibility of the K + ion.


3.4 Rb2CO3

Rb carbonate is very hygroscopic. The interaction with water is clearly seen in corre-

sponding O ls spectra (Fig. 6), where part of the third component at 532.3 eV corre-

sponds to a H20 molecule in the carbonate structure. Moreover, this interaction results

in additional formation of RbHCO3 appearing as the high BE component in the C ls

spectrum at 289.2 eV and the second O ls component at 530.7 eV (Fig. 6). This hydro-

carbonate phase is rather stable in vacuum and the corresponding XPS lines can be found

in the spectra even after three days of keeping the sample in vacuum at room T, whereas

water molecules are nearly completely removed during warming. A slow decomposition

of the R b H C Q is confirmed by the increase in Rb/carbonate atomic ratio from 1.75 for

a pre-cooled sample to 2.0 after 3 days in vacuum. Both C ls and O ls BE values of

Rb2CO3 are comparable to potassium carbonate (Table 1) which may be a consequence

of the close ion crystal radii of K (1.44 A) and Rb (1.58 A) [1] and corresponding lattice

energies.

3.5 Cs2CO3

In the case of Cs carbonate, the formation of the hydrocarbonate phase due to storage

in air becomes more pronounced (Fig. 7). Within the Group, Cs has the highest crystal

radius and the lowest I1 (3.893 eV) which leads to the highest ionicity for the Cs-O(CQ)

bonds. This, in turn, results in the lowest BE values for C ls and O ls lines of the

carbonate ion (Table 1).

3.6 Influence of grinding on the surface composition

In XPS practice, grinding is often used to produce a fresh surface more or less free

of possible ea ~itu "artefacts" like hydrocarbon contamination, oxidation products, etc.

On the other hand, the surface of oxygen and moisture sensitive samples can change in

chemical composition during such a treatment.

As an example, C ls and O ls spectra of Cs carbonate subjected to grinding are

given in Fig. 8. A remarkable increase in the surface hydrocarbonate phase was detected

also for Rb2CO3. Most part of the hydrocarbonate %rmed is unstable in spectrometer

vacuum even under liquid nitrogen conditions, as was in the case of Cs carbonate (Fig.

8), and the surface composition aspires to that of unground sample. In contrast with

the initial powders, some hydrocarbonate formed by sample grinding is stable enough to

be detected at the surface even after briefly heating the sample up to 100~ in vacuum.

Na2CO3 and K2CO3 powders are much less sensitive to the grinding and give hydrates

as the main product.

A.V. Shchukarev, D.V. Korolkov / Central European Journal of Chemistry 2(2) 2004 34~362 351

3.7 Chemical bonding and trends in C Is and O Is binding energies

The ls and 0 ls binding energy dependence on the first ionization potential of free alkali

metals for carbonates and hydrocarbonates is given in Fig. 9.

All O ls values consistently decrease with the decreasing 11. This effect is caused

by increasing delocalisation of valence electron density from the atom M to the oxy-

gen with decreasing I1. Due to the mutual screening, core O ls levels are subjected to

destabilisation. This decreases their BE in accordance with an increase in their energy.

BE of O ls in solid CO2 (536.3 eV) [5] displays tile same trend. In tile case of

carbon dioxide, an effective negative charge at oxygen atoms is determined by valence

electron density displacement from carbon only, while in the carbonates this is associated

with an additional delocalisation along the M--+O bonds, increasing the negative charge

significantly.

The %rrnation of ~external" metal-carbonate bonds with considerable ionic character

influences the C--+O electron density displacement within the anion. The lower I~ of alkali

metal, the more negative the charge of oxygen, which in turn shifts the electron density

back to carbon decreasing its C ls BE value. This is clearly illustrated in Fig. 9.

The effect is diminished %r the corresponding hydrocarbonates where one metal atom

is replaced by the exceptionally small hydrogen atom. This H-O-C bond introduces an

additional ~constant" delocalisation of valence electron density. In this case, the hydro-

carbonate anion undergoes alkali metal attack through one oxygen atom only.

Considering tile first ionisation potentials of H (13.595 eV), O (13.614 eV) and C

(11.256 eV) [4], we can suppose that formation of carbon dioxide will decrease tile I~ of

oxygen making it less than one %r hydrogen. In the case of hypothetical carbonic acid

%rrnation, electron density would be delocalised along the O--+H bonds causing a small

negative charge at the hydrogen atom. Such a structure cannot exist due to the huge

difference in corresponding electron affinities. Moreover, the formation of H2CO3 was not

confirmed by XPS even in the case of solid CO2/H20 stoichiometric mixture under liquid

nitrogen temperature [6].

4 Conclus ions

The reference XPS data obtained in this study may be useful for identification of Group

IA carbonates forming in different industrial and natural processes.

Using the sample-precooling technique developed here, the surface chemistry of the

carbonate powders was elucidated. Two types of surface chemical reactions with atmo-

spheric water occur, resulting in hydrocarbonate and/or carbonate hydrate phases, and

a specific behaviour of K2COa was noted.

The influence of grinding (mechanical activation) of the carbonates on the %rrnation

of surface hydrocarbonates phase was %und. This effect increases down the Group IA

K<Rb<<Cs and should be taken into account in sample preparation and treatment.

XPS spectra of C ls and O ls core levels and their chemical shifts in solid M2CO3 and


MHCO3 are in agreement with basic concepts of valence electron density distribution in chemical bonds for the investigated compounds.

R e f e r e n c e s

[1]

[2]

[3]

[4] [5]

[6]

F.A. Cotton and G. Wilkinson: Advanced Inorganic Chemistry, Fifth Edition, John Wiley &; Sons, 1988.

NIST X-Ray Photoelectron Spectroscopy Database, NIST SRD 20, Version 3.2 (http: / /srdata.nist .gov/xps/) .

J. Chastain and R.G. King, Jr. (Ed.): Handbook on X-Ray Photoelectron Spectroscopy, Physical Electronics, Inc., 1995.

Handbook of Chemistry and Physics, 59th Ed., CRC Press, Inc., 1979.

T.R. Dillingham, D.M. Cornelison, K. Galle et al.: "A Study of Solid CO2 by XPS", Surface Science Spectra, Vol. 4, No. 2, (1997), pp. 15~160.

T.R. Dillingham and D.M Cornelison: "A Study of Solid CO2/H20 and CO2/CHaOH by XPS", Surface Science Spectra, Vol. 6, N 2 (1999), pp. 146-152.

A.V. S h c h u k a r e v , D.V. K o r o l k o v / C e n t r a l E u r o p e a n J o u r n a l of C h e m i s t r y 2(2) 2004 347 362 353

8 o

8 o

o

8 o

,..@

8 ,..@

8 o

4 4

c~ ~ ~ c~

0

c~ ~

LO

C~ LO LO

o

o

G"

G" G"

z ~ z �9

O

o].

,d

O

o].

o

8

d S

�9

2

,z=

�9

L)

2 c5

�9

a.


Fig. 1 C ls and O ls spectra of Li2COa �9 upper taken under liquid nitrogen, bot tom after warming.

A.V. Shchukarev, D.V. Korolkov / Central European Journal of Chemistry 2(2) 2004 347 362 355

Fig. 2 C ls and O ls spectra of Na2COa �9 upper taken under liquid nitrogen, bot tom after warming.


Fig. 3 C ls and O ls spectra of NaHCOa �9 upper taken under liquid nitrogen, bot tom after warming.


Fig. 4 C ls and O ls spectra of K2COa �9 upper taken under liquid nitrogen, bot tom after warming.


Fig. 5 C ls and O ls spectra of KHCOa �9 upper taken under liquid nitrogen, bot tom after warming.


Fig. 6 C ls and O ls spectra of Rb2COa �9 upper taken under liquid nitrogen, bot tom after warming.


Fig. 7 C ls and O ls spectra of Cs2COa �9 upper taken under liquid nitrogen, bot tom after warming.


Fig. 8 C ls and 0 ls spectra of Cs2C03 after grinding: upper taken under liquid nitrogen, bottom after warming.


m

290.5

290.0

289.5

289.0

288.5

288.0

287.5

5.50

Li

R ' ~ ~ C s

I I I

5.00 4.50 4.00

I1, eV

A

3.50

532.0

531.5

531.0

"~ 530.5

530.0

Li

K

529.5 , , ,

5.50 5.00 4.50 4.00

ll, e V

B

3.50

Fig. 9 C ls (A) and O ls (B) BE dependence on first ionisation potential of alkali metals: o carbonates, �9 hydrocarbonates.

xps study of group ia carbonates · a1 ks source operated at 225 w and a low-energy electron gun...

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