ELSEVIER Applied Surface Science 119 (1997) 253-259

aoo~o surface science

Magnesium salts and oxide: an XPS overview

S. Ardizzone, C.L. Bianchi *, M. Fadoni, B. Vercelli Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, 20133 Milan, Italy

Received 10 December 1996; accepted 2 March 1997

Abstract

XPS measurements have been performed on a series of Mg II salts (NO 3, CO 2-, SO42-, CI-, CH3COO-), hydroxides and oxides, these latter compounds are both commercial and prepared in laboratory. The binding energy (BE) of the anionic partner in Mg H salts compares well in any case with literature data for the same anions in other alkaline and alkaline earth compounds. The BE of Mg n appears to be affected by the electroattractive nature of the parent anion. The Mg n spectra, in the case of MgO samples, are not influenced either by the nature of the oxide precursor salt or by the temperature of the oxide preparation (873 K, 1073 K, 1253 K). The BE of Mg 2p ranges between 49.9 eV and 50.2 eV. Oxygen spectra are regular and show the presence of the surface chemisorbed -OH component even at heating temperatures of 1473 K. The choice, as internal reference, of the hydrocarbon contaminant carbon peak is discussed, specifically, in the case of MgO samples with reference to specific lattice and surface properties of the oxide itself. 1997 Elsevier Science B.V.

Keywords: Mg II salts; MgO; XPS data

1. Introduction

In recent years considerable effort has been de- voted to the oxidative coupling of methane (OCM) both on theoretical and experimental grounds, due to its applicative relevance as an alternative to the conventional steam cracking [1]. The conversion of methane to give various hydrocarbons is supported by simple oxide catalysts. The main oxide material used for this process is magnesium oxide promoted with either alkali metals [2,3,4a,4b,5,6] or rare earth metals [7-9], Pb [10,11], Zn [12], Mn [13], metal chlorides [14], etc. Since MgO is the base component of the promoted MgO catalysts, the catalytic activ- ity/selectivity in the OCM process is expected to be

* Corresponding author. Tel.: +39-2-26603266; fax: +39-2- 70638129; e-mail: claudia@rsl4.csrsrc.mi.cnr.it.

strongly influenced by intrinsic bulk and surface properties of MgO itself.

While the literature is rich in data concerning the catalytic performance of variably doped MgO cata- lysts [15-18], there is no parallel information on the features of the material. Often, some data concerning surface area [19,20], X-ray photoelectron diffraction (XPD) [21] and X-ray diffraction (XRD) [22] are presented, while acid/base and surface chemical state characterizations are very seldom reported. More specifically, XPS results are few and random: Choundhary et al. [19] report XPS data on the Mg 2p binding energy (BE) varying in a very broad range from 49.1 eV to 50.3 eV, while Zhen et al. [22] give XPS values between 49.1 eV and 49.3 eV for the same cation.

Further, the data concerning precursor salts of MgO are random or non-existent.

0169-4332/97/$17.00 1997 Elsevier Science B.V. All rights reserved. PII S0 169-4332(97)001 80-3

254 S. Ardizzone et al. /Applied Surface Science 119 (1997) 253-259

Owing to this randomization about XPS data on MgO and its precursor salts, the present work is aimed at making clarity in the XPS data of Mg 2p and giving a collection of XPS data of the commer- cial precursor salts (characterized by anions of dif- ferent nature and electron attraction), commercial MgO, MgO samples prepared in the laboratory and Mg(OH) z both commercial and obtained from aque- ous precipitation.

In particular, the choice of the 1 s level of contam- inant carbon as internal reference is discussed in connection with specific bulk and surface features of MgO.

2. Experimental

2.1. Sample preparation and characterization

All the commercial reagents were of high grade purity and were used as received without further purification.

The following commercial samples have been used: (1) Mg(NO3)2.6H20 (Carlo Erba), (2) (MgCO3) 4 Mg(OH) 2 5H20 (Carlo Erba), (3) Mg(OH) 2 (Fluka), (4) Mg(OOCCH3) 2 4H20 (Carlo Erba), (5) MgSO 4 7H20 (Baker), (6) MgC12 6H20 (Riedel-de HaSn/Seelze-Hannover) and (7) MgO (Carlo Erba).

A sample of Mg(OH) 2 (sample 8) was prepared in the laboratory by precipitating it from an aqueous solution of magnesium nitrate by an ammonium hydroxide solution at pH 10-11, in order to compare the data obtained by the commercial one. The precip- itate was purified by centrifugation and resuspension cycles with Milli-Q water, then dried at 373 K and stored as a powder. All solutions and suspensions were prepared with doubly distilled water passed through a Milli-Q apparatus.

MgO samples have been prepared from different precursors (1, 2, 3, 8) by thermal decomposition at variables temperatures.

Owing to the melting of the precursor (1) at a temperature (about T = 573 K) lower than the tem- perature of its conversion to the oxide, the calcina- tion was performed in two steps. First the precursor was calcined at T = 703 K for 3 h in flowing 0 2.

The resulting solid MgO compound was finely crushed in an alumina mortar. The powder was then heated at the desired temperatures in flowing 02 .

The precursors (3) and (8) have also been cal- cined in two steps, due to the significant loss of water, which boiled violently at high temperatures.

The precursor (2) has been calcined in two steps, only for coherence with the other salts.

The first calcination steps were executed using a pyrex pipe cell, while the second steps were per- formed using a quartz cell with porous sect.

MgO samples have been prepared as follows: MgO A 1, A 2, A 3, have been prepared by

treating precursor (1) at T = 703 K for 3 h in an 02 stream and then by its calcination at T= 873 K, T= 1073 K, T= 1253 K for 5 h in an 0 2 stream, respectively.

MgO B l, B 2, B3, have been prepared by treating precursor (2) at T = 703 K for 3 h in an 02 stream and then by its calcination at T= 873 K, T= 1073 K, T= 1253 K for 5 h in an O 2 stream, respectively.

MgO C 1, C 2, C 3, have been prepared by treating precursor (3) at T = 473 K for 5 h and then by its calcination at T = 873 K, T = 1073 K, T = 1253 K for 5 h, respectively.

MgO D~, D 2, D 3, have been prepared by treating precursor (8) at T = 473 K for 2 h and then at T= 573 K for 3 h, and finally calcining it at T = 873 K, T = 1073 K, T = 1253 K for 5 h, respec- tively.

The phase composition of MgO samples has been verified by XRD measurements (here not reported).

2.2. XPS determinations

XPS spectra were obtained using an M-probe apparatus (Surface Science Instruments). The source was monochromatic A1 K s radiation (1486.6 eV). A spot size of 200 750 /xm and a pass energy of 25 eV were used. The energy scale was calibrated with reference to the 4f7/2 level of freshly evaporated gold sample, at 84.00_ 0.1 eV, and with reference to 2P3/2 and 3s levels of copper at 932.47 _ 0.1 eV and 122.39 _ 0.15, respectively.

With a monochromatic source, an electron flood gun is required to compensate the build up of posi- tive charge on the samples during the analyses, when

S. Ardizzone et al. / Applied Surface Science 119 (1997) 253-259 255

insulating samples are analyzed: a value of 3 eV has been selected.

For the precursor salts, the 1 s level hydrocarbon- contaminant carbon was taken as the internal refer- ence at 284.6 eV [23]; while for the MgO samples the choice of the internal reference will be discussed in Section 3.

The powders were softly pressed into a copper sample holder and directly introduced into the spec- trometer prechamber, and here left at a pressure of 1.0 )< 10 -7 Torr for 4 h before performing the XPS analyses.

The accuracy of the BE can be estimated to be _+0.1 eV.

3. Results and discussion

3.1. Precursor salts

The data on the precursor salts are collected in Table 1.

In the case of the data in the table, the I s level hydrocarbon contaminant carbon has been taken as the internal reference at 284.6 eV.

In general, the BE of the parent anions are in agreement with the data reported for the same anions in other alkaline and alkaline earth compounds [24].

The BE of Mg 2p appears to vary between 49.2 eV and 51.4 eV, relating to the electron attraction of the parent anion. In fact, electronegative groups,

such as NO 3 and CI-, render Mg 2+ cation more positive, so the BE falls at the higher extreme of the range. On the other side, electron donor groups, such as OH-, render Mg 2 cation less positive and shift the BE to the lower values. For this reason, samples (3) and (8) have Mg 2p BE values lower than the others. In these samples, the BE of the O 1 s relative to the structural -OH groups is lower, in agreement with literature [25] and different from the values of the surface -OH groups found on all the MgO samples (Section 3.2).

In Fig. 1, the fitting XPS peaks of samples (3), (2), (1) have been interpreted on the basis of the following reasons:

I. Sample (1) Mg 2p peak presents two compo- nents at 50.5 eV and 51.4 eV, respectively. The value at 51.4 eV has been attributed to the nitrate and the choice is in agreement with the quantitative ratio between the N at 407.3 eV and Mg 2p at 51.4 eV, which is 1.9, while the theoretical one is 2. The Mg 2p BE at 50.5 eV is comparable to the one at 50.4 eV of the carbonate, in sample (2).

In the same manner, the O ls shows two compo- nents at 533.2 and 531.8 eV. The former BE can be attributed to the oxygen in the nitrate; in fact, the quantitative ratio between this oxygen component and N at 407.3 eV is 3 and with Mg 2p at 51.4 eV is 5.7, against the theoretical values of 3 and of 6, respectively.

The O ls component at 531.8 eV has been at- tributed to the oxygen in the carbonate. In fact, the

Table 1 Binding energy values (in eV) for Mg n salts and hydroxides

Sample C ls Mg 2p O ls Anion

(1) Mg(NO3) 2 . 6H20 284.6 " 50.5 b 531.8 407.3 (nitrogen) 289.1 51.4 533.2

(2) (MgCO3) 4 Mg(OH)2 . 5H20 284.6 ~ 49.3 ~ 531.4 - - 289.5 50.4 532.8

(3) Mg(OH) 2 284.6 a 49.2 530.9 - - (4) Mg(OOCCH3) 2 4H20 284.6 ~ 50.6 531.7 - -

288.6 (5) MgSO 4 - 7H20 284.6 a 50.6 531.7 168.9 (sulphur) (6) MgCI 2 - 6H~O 284.6 a 51.2 532.2 198.9 (chlorine) (8) Mg(OH) 2 284.6 a 49.2 530.8 - -

a Internal reference. b Relative to the carbonate, present in traces.

Relative to the hydroxide, present in structure.

256 S. Ardizzone et aL /Applied Surface Science 119 (1997) 253-259

peak of the C ls BE at 289.1 eV, in agreement with the values reported in literature for other carbonate compounds [24] is, also, present.

II. Sample (2) is a magnesium basic carbonate, so in the compound Mg-OH bonds are present, which justify the BE value at 49.3 eV of Mg 2p, compara- ble to that of samples (3) and (8).

The O Is BE at 531.4 eV has been attributed to the oxygen in the carbonate and the choice is in agreement with the quantitative ratio between this component O ls and Mg 2p at 50.4 eV, which is 2.8, while the theoretical one is 3.

The quantitative ratio between O ls at 531.4 and C ls at 289.5 eV is 3.33, while the theoretical is 3, so the C 1 s BE has been confirmed to be relative to carbon in carbonates.

Table 2 Binding energies (in eV) for commercial MgO and for the same sample evacuated up to 1473 K

Sample Mg 2p O ls O b/os C ls

C Is c C ls d

(7) 50.3 530.5 a 3.7 284.5 286.1 532.7

(7) b 50.3 530.5 a 5.0 283.7 286.2 532.8

" The new internal reference. b MgO comm. after evacuation till T = 1473 K. c The BE of C Is, as directly obtained by XPS measurements. d The BE of C ls corrected with the new internal reference ob /o S quantitative ration between bulk and surface oxygen.

3.2. MgO samples

i

(a)

i

(b)

i

(c)

56 531.6 511.2 4818 461.4 44 B.E. (eV)

Fig. l. XPS spectra of the Mg 2p peak of: (a) Mg(NO3) 2 .6H~O, (b) (MgCO3) 4 Mg(OH) 2 - 5H20, (c) Mg(OH) 2. DoUed lines: peak used for curve fitting; full lines: resulting peak; vertical lines: peaks corresponding to the hydroxide and carbonate BE, respec- tively.

In a first approach, XPS measurements have been executed on sample (7), by using C ls at 284.6 eV as the internal reference. The obtained BE of Mg 2p at 48.8 eV was very anomalous, because it was signifi- cantly lower even than the BE pertaining to metallic magnesium (about 50 eV), reported in literature [24].

Consequently, the value of the ls level of the bulk oxygen, at 530.5 eV, has been taken as the internal reference (530.5 eV is taken as a media of the bulk O ls XPS data reported in literature) [26-28].

In Table 2 the BE of sample (7) and of the same sample after evacuation up to T= 1473 K are listed together with a column reporting the quantitative ratios between the bulk and surface oxygen of the samples (ob /os) .

For each sample, in fact, two values of the O ls BE are present: the highest peak is relative to the oxygen of the surface chemisorbed -OH groups of hydration, while the lower value is relative to the bulk oxygen.

Fig. 2 shows the XPS fitting peaks, relative to the BE of O ls of the two samples: the intensity of the higher BE component appears to decrease signifi- cantly upon the heating treatment, while the lattice oxygen component remains virtually unchanged. The affinity, between the surface chemisorbed -OH and MgO, is so strong that a fraction of them remains even after a treatment at a temperature as high as 1473 K.

S. Ardizzone et al. / Applied Surface Science 119 (1997) 253-259 257

I

(A)

[B]

t~

536.0 532.8 529.6 526.4 523.2

B.E (eV)

Fig. 2. XPS spectra of the O I s peak of: (a) commercial MgO; (b) commercial MgO after evacuation up to 1473 K. Dotted lines: peak used for curve fitting; full lines: resulting peak.

Table 3 lists the XPS data of MgO obtained at different temperatures from different precursors.

The values of BE present no substantial differ- ences from sample to sample. So, there are some characteristic features of MgO itself that do not change in varying the salts and the temperatures of preparation.

There are two O 1 s BE, which indicate the con- stant presence of surface chemisorbed -OH groups, besides the use of different precursors and the calci- nation at various temperatures.

3.3. Notes about hydrocarbon-contaminant carbon

The C ls BE relative to MgO samples were observed to fluctuate in a broad range (between 274.9 eV and 284.6 eV) (see, for example, Tables 2 and 4), notwithstanding all the analyses had been performed fixing at 3 eV the charge neutralizer.

Table 4 reports the experimental C I s BE of MgO samples at T= 873 K from different precursors, together with Ahy p (obtained by shifting the experi- mental value to the 284.6 eV 'classical' reference) and Areal (obtained by adopting, as internal refer- ence, the O Is level at 530.5 eV and shifting the C ls peak consequently). Results showing a fully com- parable trend have been obtained at 1073 K and 1253 K respectively, and are not reported for reasons of space.

Table 3 Binding energies (in eV) for MgO calcined at different tempera- ture and from different precursors

Sample Mg 2p O I s

T = 873 K

A i 50.2 530.5 "

532.6

B I 49.9 530.5 a

532.8

C t 50.1 530.5 "

532.6

D I 50.1 530.5 ~

532.4

T = 1073 K

A 2 50.1 530.5 a

532.6

B e 50.2 530.5 ~

532.7

C 2 50.1 530.5 ~

532.4

D 2 50.1 530.5 a

532.7

T = 1253 K

A 3 50.0 530.5 "

532.6

B 3 50.0 530.5 ~

532.6

C, 49.9 530.5 ~

532.6

D 3 50.2 530.5 ~

532.6

a Taken as internal reference.

The variations between Ahy p and Are,l are random and Areal is systematically higher. This occurrence might suggest the presence of a sort of 'electron- pump' which reduces the shifts of the C l s BE, in

Table 4 Binding energies for C ls of MgO samples calcined at T = 873 K and relative A associated to the 'classical' and the new internal reference

Sample C ls Ahy p Areal

A I 280.9 3.6 5.05

B ~ 274.9 9.7 10.7

C I 277.5 7.1 7.9

D I 277.8 6.8 7.9

Ahy p obtained by shifting the experimental value to the 284.6 eV, 'classical reference'. 3re~t obtained by adopting, as internal reference, the ls oxygen level at 530.5 eV and shifting the C ls peak consequently.

258 s. Ardizzone et al. / Applied Surface Science 119 (1997) 253-259

respect to those of Mg and O. Probably, this aspect may be explained by the existence of F-centers (anionic vacancies with a trapped electron), whose formation and density is related to the temperature of the oxide preparation and its crystal faceting [29,30]. These defects are also considered to be responsible for the activation step in the methane coupling reac- tion, catalyzed by MgO. In fact, the rate determining step of the process is the formation, at the oxide surface, of a methyl radical, due to the electronic exchange with the catalyst [1].

In the light of what exposed above, considering again the problem of an internal reference in XPS MgO data, the BE of carbon 1 s could be the result of a kind of electron-exchange process between the F-centers and hydrocarbon carbon.

Owing to this specific interaction, the BE value of the C Is hydrocarbon carbon component could not be chosen as the internal reference of the system. Thus, the choice of O l s , as internal reference, has been needed and is proposed as general alternative to carbon in the case of MgO.

4. Conclusions

On the basis of the experimental XPS measure- ments, the BE of the anionic partner in Mg H salts compares well in any case with literature data for the same anions in other alkaline and alkaline earth compounds. Further, the BE of Mg 2p appears to be affected by the electroattractive nature of the partner anion, giving rise to a congruent sequence.

In the case of MgO samples, the Mg II spectra are not influenced either by the nature of the oxide precursor salts or by the temperature of the oxide preparation.

The BE of Mg 2p ranges between 49.9 eV and 50.2 eV, without the random variations of the data reported in the literature.

The choice of the bulk oxygen BE, as the internal reference, in the case of MgO samples, is made in relation both with the coherence of the obtained Mg 2p data between each other and with the existence of F-centers in the bulk and surface of the oxides, which, probably, influences the shifts of the C ls BE.

These defects are considered to be responsible for

the activation step in the methane coupling reaction, being the source of electrons for the formation of the methyl radical; consequently, they, might interact with the hydrocarbon contaminant carbon, through a kind of electron-exchange process, thus affecting its BE. In the light of these considerations, the BE of the C ls hydrocarbon carbon component could not be chosen as the internal reference of the system and the O ls BE is proposed as a general alternative to carbon in the case of MgO.

Acknowledgements

Financial support from the MURST (40 and 60% Research Founds) is gratefully acknowledged.

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