E L S E V I E R Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 47-50

JOURNAL OF ELECTRON SPECTROSCOPY

and Related Phenomena

An XPS study of nitrogen-sulfur compounds

D a v i d L i t t l e j o h n * , S h i h - G e r C h a n g

Energy and Environment Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA

First received 13 June 1994; in final form 5 July 1994

Abstract

An X-ray photoelectron spectroscopy study has been conducted on a series of nitrogen-sulfur compounds to char- acterize the charge shifts on the nitrogen and sulfur atoms in the molecules. In all of the compounds, the sulfur was found to have a ls binding energy equivalent to a formal charge of +6. The nitrogen ls binding energies varied in a systematic manner that reflected the change in oxidation state. The XPS results have been used to provide insight into the formation of these compounds in two reaction systems.

Keywords: Nitrogen; Sulphur; XPS

1. Introduction

A number of nitrogen-sulfur oxides can be formed by the reaction of nitrogen oxides and sul- fur dioxide in aqueous solution [1]. These com- pounds have been found in aqueous scrubbing solutions used for pollution control at coal-fired power plants [2]. There is the possibility that these compounds may also be formed in aqueous aerosols in polluted environments. The ni t rogen- sulfur oxides and some of the reactions associated with their formation and destruction are shown in Fig. 1. The compounds include hydroxyimidodisul- fate (HIDS), hydroxysulfamic acid (HSA), nitrido- trisulfate (NTS) and imidodisulfate (IDS). These compounds are known by several different names and abbreviations, but will be referred to by the acronyms listed above. Hydroxylamine and sul- famic acid are also included in the reaction scheme in Fig. 1 because they are the hydrolysis products

*Corresponding author.

of HSA and IDS, respectively. Most of the materials are moderately stable in aqueous solu- tion and undergo acid hydrolysis [1]. Although many of the reactions involving these compounds have been investigated [1,3-5], the oxidation states of nitrogen and sulfur in the compounds have not been well defined. We have made X-ray photo- electron spectroscopy (XPS) measurements on a number of solid nitrogen-sulfur oxides in order to understand better the formation and decom- position of these compounds.

2. Experimental

The potassium salts of the nitrogen-sulfur oxi- des were prepared by the methods described in the literature using reagent grade compounds (HIDS [6], HSA [7,8], NTS and IDS [9]). Two forms of IDS were prepared. The dipotassium salt was pre- cipitated at pH 6-7 and the more soluble tri- potassium salt was precipitated at pH 11. The

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48 D. Littlejohn, S.-G. Chang~Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 47-50

HSO3-

NOT + HSO3- w -ONSO3 HSO3"

[HON(SO3)2] 2- ~ [N(SO3)3] 3-

(HIPS) (NTS)

HSOy NONH(SO3)]" ~ [~(SO3)2] 2

(HSA) (IDS)

H+ S O , / . , , ~ H+

H3N+OH ~ H3N+SO3 -

Fig. 1. A diagram of some of reactions involved with the formation and conversion of the nitrogen-sulfur compounds.

tripotassium salt contains no hydrogen atoms. Commercial salts of hydroxylamine hydrochloride (B & A), hydroxylamine sulfate (MCB) and sul- famic acid (Eastman) were also used. Addition- ally, potassium sulfamate was prepared by adding KOH to a warm, saturated sulfamic acid solution until the pH was ,,~ 8. The solution was then cooled in an ice bath until a precipitate formed. The solid was filtered off, washed with ethanol and dried under vacuum.

The samples for XPS were prepared by grinding the salts to a fine powder with a mortar and pestle. Small amounts of the powders were placed on

Table 1

Compound Nls binding Sis binding energy/eV energy/eV

HIDS 400.6 168.9 HSA 400.8 169.0 Hydroxylamine sulfate 401.9 168.8 Hydroxylamine hydrochloride 402.1 NTS 399.6 169.0 IDS 2- 399.8 169.0 Sulfamic acid 401.5 168.9 IDS 3- 398.4 168.8 Potassium sulfamate 399.5 168.6 Na2SO3 166.7 NaHSO 3 167.4 Na2S206 168.6 NaNO 2 [12] 403.2 Typical Fe2+(L)NO [18] 401-402

adhesive tape mounted on the sample holder of the XPS instrument. A Perkin-Elmer ~ 5300 spec- trometer was used to acquire the XPS spectra. Mg Kc~ X-radiation (1253.6 eV) was used to excite the samples. The N ls and S ls peak positions were corrected using adventitious carbon as a reference (corrected to 284.6 eV).

3. Results and discussion

The XPS measurements for the compounds stu- died are compiled in Table 1. The relationship between the core electron binding energy and the oxidation state on the atom has been well estab- lished [10-12]. It can be seen that the sulfur atoms in the R-SO 3 groups of all the nitrogen- sulfur compounds have essentially the same bind- ing energy. The value of the sulfur binding energy (168.8-169.0 eV) is about the same as that for sulfate ion (168.8 eV) in hydoxylamine sulfate and noticeably higher than that of the dithionate ion (168.6 eV). This suggests that the sulfur may be assigned a formal charge of + 6.

The binding energies for nitrogen in the nitro- gen-sulfur compounds vary from 399.6 to 402 eV. The relative positions of the nitrogen ls bind- ing energies for the compounds of interest are shown in Fig. 2. The nitrogen in HIDS is substan- tially more reduced than that of nitrite ion, and the nitrogen in NTS is even more reduced. Using a + 6

D. Littlejohn, S.-G. Chang/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 47-50 49

nitrite nltrososulfonate (est.) HIDS NTS

hydroxylamine HSA HIDS • • w

sulfamic acid IDS NTS

I I I I i

4 0 4 403 4 0 2 401 4 0 0 399

Nitrogen l s Binding Energy (eV)

Fig. 2. A comparison of the nitrogen l s binding energies for the compounds in the top row of Fig. 1 (upper line), and the compounds in the two columns of Fig. 1 (middle and lower lines).

charge on the sulfur atoms in the compounds, we can assign formal charges of - 1 for the nitrogen in HIDS and - 3 for the nitrogen in NTS. As the sulfur atoms in the hydrolysis products of these com- pounds have a + 6 formal charge, the hydrolysis reactions of HIDS and NTS do not involve oxida- t ion-reduction. The 1 s binding energies of nitrogen in the hydrolysis products HSA and IDS are slightly more positive than those of the parent compounds. We attribute this to the presence of the hydrogen bound to the nitrogen, creating a more positive environment. The nitrogen atoms in HSA and IDS are still expected to have formal charges of - 1 and - 3 , respectively. The hydrolysis products of HSA and IDS show a substantial shift in the nitrogen 1 s binding energies. This is because the products, hydroxylamine and sulfamic acid, form an H3 N+ structure and create a much more positive environment for the nitrogen. The nitro- gen ls binding energy shifts by 1.4-2 eV to less positive values when the hydrogens in IDS and sulfamic acid are replaced by potassiums, as indi- cated by the values for IDS 3- and potassium sul- famate in the table.

HIDS is formed from nitrite ion and hydrogen sulfite ion in a two-step process, with nitrososul- fonate as the inferred intermediate [13,14]. Once formed, nitrososulfonate reacts rapidly with a second hydrogen sulfite ion to form HIDS. In the

process, the sulfur in each hydrogen sulfite ion is oxidized from + 4 to + 6, while the nitrogen under- goes stepwise reduction from + 3 in nitrite ion to + 1 in the intermediate to - 1 in HIDS. From this, we estimate that the nitrogen ls binding energy for the nitrososulfonate intermediate would be ~ 402 eV. Earlier studies had given nitrososulfonate the formula ONSO; , although it would be more appropriate to write it as -O N S O 3 [14]. No oxi- dation or reduction would be required to form hyponitrous acid or nitrous oxide, which was observed as a product in an earlier study [14]. The overall hydrolysis reaction for nitrososulfo- nate would be

2H20 + 2 -ONSO 3 ---+ H2N202 + 2HSO4 (1)

Hyponitrous acid undergoes pH-dependent decomposition to form nitrous oxide. It was found in an earlier study that the compounds in which nitrogen has a - 3 formal charge were more resistant to attack by oxidants than com- pounds in which nitrogen has a - 1 formal charge [151.

HIDS is also formed by the reaction of ferrous nitrosyls with hydrogen sulfite ion [16]. Earlier, we proposed a mechanism for the formation of HIDS in this system [16]. The reaction begins by the for- mation of a complex between the ferrous nitrosyl

50 D. Littlejohn S.-G. Chang/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 47-50

and hydrogen sulfite ion

Fe 2+ (L)NO + HSO3 - ~- Fe 2+ (L) (NOHSO3)

or Fe 2+ (L) (NOSO3)2

(2) This complex then interacts with a ferrous nitrosyl complex, resulting in oxidation of the sulfur to + 6 and reduction of the nitrogens to + 1

FeZ+(L)(NOHSO3) - + FeZ+(L)NO --~

intermediate ~ 2Fe2+(L) + ONSO3

+NO + H + (3)

A somewhat similar process has been discussed by Pearsall and Bonnet in their studies of aqueous nitrosyliron(II) chemistry [17]. The nitrososul- fonate formed can react with hydrogen sulfite ion to produce HIDS

HSO3 + - ONSO3 --~ HIDS (4)

The nitrosyl hydride can dimerize to hyponitrous acid. HNO has a pK a of 4.7 [18].

N O - + H + ~ HNO --~ 0.5H2N202 (5)

XPS studies on 6-coordinate metal nitrosyls have shown a linear relationship between the nitrosyl infrared band position and the nitrogen 1 s binding energy [19]. The ferrous nitrosyls used in the studies of HIDS formation have nitrosyl Raman bands in the range 1760-1800 cm I [20], which correlate with a nitrogen ls binding energy of ~ 402 eV, a value similar to that estimated for the nitrososul- fonate intermediate. This implies NO- character in the complex.

4. Conclusions

This study better defines the oxidation states of the nitrogen-sulfur compounds and clarifies some of the reactions involved in the formation and con- version of these materials. Additionally, we have gained insight into the structure and formal charges of the nitrososulfonate intermediate. The nitrogens in HiDS and HSA appear to have a - 1 formal charge. In NTS and IDS, the nitrogens appear to have a - 3 formal charge. In all of these compounds sulfur has a + 6 formal charge, as the sulfur in hydrogen sulfite ion is oxidized on for- mation of the compounds.

Acknowledgment

This work was supported by the Assistant Secretary for Fossil Energy, Office of Coal Utili- zation Systems, US Department of Energy under Contract No. DE-AC03-76SF00098 through the Pittsburgh Energy Technology Center, Pitts- burgh, PA.

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