Catalytic Oxidation of Sulfide Ions Over Nickel

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ELSEVIER

Applied Catalysis B: Environmental 7 ( 1996) 225-235

Catalytic oxidation of sulfide ions over nickel

hydroxides

A. Andreev a,*, P. Khristov a, A. Losev b

’ Institute of Catalysis, Bulgarian Academy of Sciences. 1 I13 Sojia, Bulgaria

’ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, I1 I3 Sofia, Bulgaria

Received 17 February 1995; revised 10 August 1995; accepted 14 August 1995

Abstract

The catalytic sulfide ion oxidation by oxygen to elemental sulfur over P-Ni( OH) , and LiNiO Z has

been studied. As a result of experimental investigation performed, a reaction mechanism is suggested

which involves heterogeneous and homogeneous processes. Dioxygen activation in the heterogeneous

process proceeds via a redox Ni2+ @ Ni’+

transition and participation of OH- groups. Th e active

HO; species thus formed carries on the reaction in homogeneous phase. Nickel hydroxides are

promising catalysts for practical application.

Keywords:

Oxidation;

Nickel hydroxide; S’- oxidation

1 Introduction

The process of sulfide ion catalytic oxidation to elemental sulfur by oxygen from

air is important for environmental protection. Due to the high toxicity of sulfide

ions, water containing these ions is hardly purified throu gh biological treatment.

However, elemental sulfur is readily removed from waste and natural w ater by

biological treatment. Sulfide ion oxidation in aqueous medium can be successfully

used to manufacture colloidal sulfur on a large scale.

The oxidation process in alkali medium can be represented by the following

equations:

S*- + ;O,+H,O -+ S” +20H-

(1)

or

*

Corresponding author. Tel. ( + 35-92) 7249 01, fax. ( + 35-92) 756 116, e-mail [email protected].

0926 -3373 /96/ 15.0 0 0 199 6 Elsevier Science B.V. All rights reserved

.SSOIO926-3373(95)00045-3



226

A. Andrew et al. /Applied Catalysis B: Environmental 7 (1996) 225-235

HS- + 0, -+ S’+OH-

(2)

Transition metal complexes and some inorganic salts [ l] and transition metal

oxides

[

2,3] have been reported to manifest catalytic activity for that process. A

high catalytic activity for this reaction was found for NiP& and a reaction mecha-

nism has been proposed

[

41. Catalysts based on iron chelate compoun ds

[

5,6] and

cobalt phthalocyanines [ 7-101 have found practical application.

This work p resents results of a study of sulfide ion oxidation to elemental sulfur

in aqueous alkali solution by using a novel type of heterogeneous catalysts: nickel

hydroxides. By means of a set of experimental methods we aimed a t gaining

information about the catalytic reaction mechan ism and the possibilities for appli-

cation.

2. Experimental

2 I Sample preparation

A sample denoted as NH was prepared by precipitation of nickel from an aqueous

solution of Ni( N03)* *6H20 (p.a. grade, 450 g/l) and NaOH (250 g/l), aqueous

solution at 80°C an d pH = 9. After aging for 1 h the slurry was filtered and washed

until the NO , ions were absent and dried at 110°C . The dried sample contained

77.68 wt.-% nickel as NiO and had a BET area of ca. 110 m2/g. The X-ray

diffraction pattern of that sample indicated reflections at 0.46, 0.271, 0.233 and

0.156 nm, specific of /3-Ni( 0H)2, as well as the two most intense reflections at

0.175 and 0.148 nm for NiO

[

111. The NiO content in the sample could be evaluated

at no more than 20%.

Samp le NH/C was prepared by impregnation of activated charcoal (CEC A-

ACLH , BET area = 750 m2/g) with an aqueous solution of Ni( N03) 2. 6H2O , p.a.

grade. Further, NaO H (aqueous solution, 250 g/l) was added at 80°C to attain

pH = 9. After ag ing for 1 h at the same temperature the product was wash ed with

distilled water an d dried at 110°C . The dried sam ple co ntained 3 1.23 wt.-% n ickel

as NiO . We ak a nd very broad reflections for /3-Ni( OH), were observed in the

diffraction pattern which are consistent with a high dispersion of the deposited

phase.

Samp les NH and NH/C were calcined at 400°C for 2 h under inert atmosp here.

They are denoted as NH’ and NH/C ”, respectively. The BET surface area of NH’

was 87 m2/g.

A sample denoted as LN was LiNi02 prepared by calcination of a mixture of

Li20 and NiO and had a BET area of about 2 m2/g. The procedure is described in

detail elsewhere [ 121. The phase identity was verified by means of X-ray diffraction

(reflection at 0.467,0.245,0.235,0.203 and 0.144 nm) [ 131.



A. Andrew et al. /Applied Curalysis B: Environmental 7 (1996) 225-235 227

2.2. Catalytic activity measurements

The catalytic activity in S2- oxidation by oxygen was measured in a static system

under continuous stirring by monitoring the volume of oxygen consumed at 20°C.

Results were checked with a chemical method by determining the sulfide ion

concentration. For this purpose EDTA titration of excess C u2+ ions with respect

to the amount of S2- ions was carried out, the Cu2 + ions being a dded as Cu( Clod) 2

aqueous solution. A Na,S aqueo us solution of 19.47 g/l concentration, 10 ml for

each run, was used. The amoun t of catalyst (very fine powder) was 0.06 g for NH

and NH/C and 0.1 g for LN.

Catalyst activity was expressed as mol S*- converted per gram atom of nickel

(mol S2- /g,, Ni) on comparing supported with unsupported samples . Comparison

between unsupported samples was made by the productivity per 1 m*( mol S2- /

m2). On deducing the temperature dependence of the reaction rate, rate values were

determined as the first derivative of the time dependence of productivity in the

linear part of the curve.

2.3. X-ray diffraction

X-ray diffraction measurem ents were carried out by means of a conventional

powder diffractometer using Cu Ka radiation.

2.4. X-ray photoelectron spectroscopy XPS)

X-ray photoelectron spectra were recorded on an ESCA LAB MK II instrument

using Al Ka excitation source. Corrections related to a charge on the samples were

made with respect to the position of the C 1s peak at 284.6 eV.

2.5. Electrochemical measurements

A nickel hydroxide (NH) containing electrode was prepared by pressing the

powdered material in an insulated platinum holder. The potential difference

between the NH electrode and a calomel electrode was measured. A special glass

cell was used to conduct the measurem ents in which a 10% NaO H a queous solution

was introduced. The cell could be purged with both argon a nd air.

3. Results

3 I Catalytic activity

Fig. 1 shows data on the catalytic activity of the NH and LN samp les. Both

samples exhibit high catalytic activity in reaction ( 1) . The activity of sample LN

was one order of magnitude higher than that of sample NH.



228

A. Andreev et al. /Applied Catalysis B: Environmental 7 (1996) 225-235

1

“E

10

\

T

0

1

x

0.01 ~II:IlIl1’ ‘111,11,ij

0 10

20 30

time [min]

Fig. 1.Catalytic activity of samples NH and LN. (Final conversions: NH-38.5% and LN-16.6%).

These results were confirmed by studies of the temperature dependence of the

reaction rate. The following values were obtained: 8200, 107 00 and 3600 cal/mol

for NH, NH/C and LN, respectively.

Experimental results presented in Fig. 2 shows that calcination of the samples at

400°C caused a considerable decrease in catalytic activity.

A study of the effect of catalyst amoun t on the productivity demonstrated a

striking dependence. Samp les NH and NH /C manifested decreased amoun ts of

converted S2- ions as the amoun t of catalyst was increased (Fig. 3a and Fig. 3b).

3.2. X-ray photoelectron spectroscopy

XPS spectra of the investigated samples are described by stable charging which

allowed the acquisition of narrow and well resolved peaks. Three ranges w ere

scanned: 0 1 s (520-550 eV), Ni 2p (830-880 eV) and S 2p (140-190 eV).

Fig. 4 and Fig. 5 present the 0 1s peaks of fresh and used N H sam ple after

operation under the working conditions of reaction ( 1) . Substantial changes in the

spectrum of the fresh sample are observed after the treatment under the working

conditions, namely, considerable peak broadening and clearly resolved asymm etry.

These findings are good grounds to suggest the occurrence of several surface species

10.0

0.0

time [min]

Fig. 2. Effect of calcination on the catalytic activity of samples NH and NH/C. Catalyst productivity was measured

at 50°C. (Final conversions: NH 38.5%, NH’4.5%, NH/C 31.9% and NH/C’ 6.4%).




229

6.0

N 4.0

:

0

‘;; 2 0

“m

s

E 0 0

15.0

z

s 10.0

\”

A

2 5.0

z

0.0

0

10

20 30

time [min]

Fig. 3. Dependence of the catalytic activity on the catalyst amount with samples NH (a) and NH/C (b) at 50°C.

t

530.00 540.00 55i.00

binding energy [eV]

Fig. 4. 0 Is X-ray photoelectron spectra of fresh NH sample.

524.20

529.20

534.20

539.:

binding energy [eV]

Fig. 5.0 1s X-ray photoelectron spectra of the NH sample after the operation under reaction conditions

from oxygen. By means of computer simulation the experimental curve was pre-

sented as a sum of three comp onents. The first peak, hav ing 530.0 eV binding

energy, can be interpreted as due to nickel oxide ad mixtures

[

14,151 formed on

drying the sample. The second peak w ith 532.3 eV binding energy is determined

by the basic phase, P-Ni( OH), [ 14,161. This result is in agreement with X-ray



230 A. Andrew et al. /Applied Catalysis B: Environmental 7 (1996) 225-23.5

I,, 1/,,,/,,,,,,,,,,,

840.00 850.00 860.00 870.00 880.00

binding energy [eV]

Fig. 6. Ni 2p,,, X-ray photoelectron spectra of sample NH: (a) fresh; (b) after the operation under reaction

conditions.

diffraction data. The third peak, 535.0 eV binding energy, can be attributed to the

presence of oxygen-containing compound of trivalent nickel

[

171. M ost likely, this

is a surface species with NiOOH like structure. Similar b inding energies have been

found in the spectra of the active pha se in nickel batteries where redox transitions

are realized, con ditionally, between Ni( OH), and NiOO H [ 181. The quantitative

ratio between the three components can be evaluated as 30:60: 10.

Argum ents in favour of Ni”’ could be found on recording spectra in the 830-880

eV range. Fig. 6 shows spectra of sample NH in the region of Ni 2p. The relative

decrease in intensity of the satellite pea k and its broadening are consistent with the

presence of Ni”’

[

191.

The XPS spectra of the used LN sample are compatible with considerable

amou nts of Ni”‘.

Fig. 7 shows experimental data in the S 2p region ( 140-190 eV) on a NH sample

treated under working conditions and washe d w ith distilled w ater. The observed

peak at 163.5 eV is assigned to elemental sulfur, S8 [20], which is a reaction

product. Another peak at 168.9 eV is due to the surface SOi- groups [ 211. Small

amou nts of sulfates, b eing also the product of the oxidation, are strongly adsorbed

onto the catalyst surface. It is interesting to note that no emission of sulfide ion

from a surface metal sulfide w as observed at around 162.0 eV

[

191. Weak emission

in that region could not be registered because of the strong peak of elemental sulfur.

150.00 160.00 170.00

binding energy [ eV]

Fig. 7. S 2p,,, X-ray photoelectron spectra of the NH sample after operation under reaction conditions.



A. Andr eev et al. Appl ied Catal ysis B: Envi ronmental 7 1996) 225-235

231

3.3. Electrochemical measurements

The potential difference between a nickel hydroxide containing electrode and a

calomel electrode was measured under controlled atmosphere. Upon purging the

electrochemical cell with argon, a potential difference of about 63 mV was attained.

Adm ission of air caused a shift of the potential to more neg ative values. The air-

argon ‘cycles’ were reversible. They are related to interaction between oxygen and

the nickel hydroxide surface an d the occurrence of electron transition.

4. Discussion

Results of the catalytic activity measurem ents (Fig. 1 and Fig. 2) indicate that

the three samples studied (NH, NH/C and LN) exhibited high catalytic activity in

the oxidation of sulfide ions. The higher activity of sample NH/C , compared to

that of NH, is explained by the higher dispersion of the deposited active com ponent

which was verified by X-ray diffraction.

As was shown by X-ray diffraction phase analysis, P-Ni(O H), was the basic

component in the NH and NH/C samples. The presence of NiO admixtures did not

substantially affect the catalytic activity. Results in Fig. 2 show that thermal treat-

ment of samples NH and NH /C, causing the formation of nickel oxide phase, is

accompanied by a drastic fall in the catalytic activity. The low activity exhibited

by the nickel oxide samples can be interpreted in terms of a partial hydroxylation

of their surface at high alkalinity (pH = 14) of the working medium . This alkalinity

originates from a strong hydrolysis of the sodium sulfide and accumulation of OH-

ions, being the product of reaction ( 1) .

It

is worth noting that according to the XPS study under the reaction conditions

no significant amoun t of sulfide phase is formed on the catalyst surface. Mo st

probably, this is due to a shifted equilibrium to the hydroxide of the surface sulfide

hydrolysis in a strong alka li solution. In this connection an active nickel hydroxide

phase occurs on the surface under reaction conditions, despite the presence of sulfide

ions in the solution.

Based on the above mentioned, one can arrive at the conclusion that the nickel

hydroxide manifests high and stable catalytic activity in sulfide ion oxidation by

oxygen in aqueous solution.

Detailed notion about the active nickel hydroxide phase can be obtained from

XPS data. It is essential that, along with nickel hydroxide, NiOO H like structures

also occur in the working catalyst. This allows to model the catalytic redox process

with a reversible redox transfer between the active species, Ni” t) Ni”‘, like in the

anode phase of nickel batteries

[

221.

The higher catalytic activity of sample LN, compared to that of NH, can be

explained in the following way. Prior to any contact with the reaction medium ,

only trivalent nickel occurs on the surface of that sample. Under the influence of



232


Fig. 8. Absorption spectrum

catalyst bed at 25°C.

18

240

300 360 420

Wavelength [nm]

in the 200-400 nm range of Na,S solution after passing it with air through a NH

the reaction med ium, however, the surface undergoes reduction hydrolysis with the

formation of NiOO H and Ni( OH), like structures

[

231. An optimum ratio between

Ni” and Ni”’ creates favourable conditions for the participation of high amount of

surface nickel ions in the redox transition Ni” ti Ni”‘. A detailed study of LN

samples related to the promising prospect of practical application is now in progress.

The XPS study is consistent with the conclusion that redox Ni” ++ Ni”’ transitions

proceed on the catalyst surface w hich are associated with hydroxide, Ni*+ (OH)*,

and oxyhydroxide, Ni3+O OH, structures.

Electrochemical studies indicated that the catalyst electrode was sensitive to

oxygen from the air. Summ arizing these studies one can draw the conclusion that

a reversible interaction between oxygen and the surface of the nickel hydroxide

electrode in an alkali solution was found w hich is related to the electron transfer

between oxygen and the catalyst.

Special attention should be given to the observed experimental finding of the fall

of the reaction rate on increasing the catalyst amoun t. It is assum ed that sulfide ion

oxidation by oxygen from the air in aqueous solution

[

241 as well as in the presence

of homogeneous catalyst [ 251 proceeds via a chain-radical mechan ism. A similar

mechan ism can operate in the oxidation of sulfide ions in aqueous solution in the

presence of heterogeneous catalysts. If the oxidation reaction proceeds both over

the catalyst surface and with the participation of active species from the solution,

the decrease in the reaction rate upon increasing catalyst amount should be consis-

tent with the destruction of the active sites on the solid catalyst [ 261.

For that reason an attempt was made to determine the active species in the liquid

medium under the conditions of reaction ( 1) . A solution of sodium sulfide w as

circulated with air through a specially designed cell containing the NH sample.

Fig. 8 shows the absorbency in the range 200-400 nm. The observed maxim um

around 290 nm can be ascribed to the HO; ion radical. This species has been

identified upon H202 dissociation in alkali solution

[

271.

Thus the observed antibate dependence of the catalytic activity in reaction ( 1)

on the amount of heterogeneous catalyst is a good reason to propose that the reaction

proceeds both on the surface and with the participation of active species in the



A. Andr eev et al. Appl ied Catal ysis B: Envi ronmental 7 1996) 225-235

233

HETEROGENEOUS PROCESS

(HS- + 02 - s + IiO2_)

HOMOGENEOUS PROCESS

KS- + H02-

- S + 20H-

Scheme 1

solution. Mo st likely, the HO; ion radical is the active species for that reaction.

Based on the experimental results and the conclusions made, a reaction mecha-

nism of sulfide ion oxidation over nickel hydroxide catalysts can be proposed

( Scheme 1) .

According to the experimental data, a steady state of the Ni” hydroxide catalyst

is assum ed, containing certain am ount of Nirn ions bonded to NiOO H-like struc-

tures. The mechan ism involves two processes: heterogeneous and homogeneous.

Considering the heterogeneous process, along with elemental sulfur active HO,

species are formed which carries on the reaction in homogeneous phase. Interaction

between the acid HS and the base O*- of the catalyst leads to an initially reduced

state of the active site.

The heterogeneous mechan ism envisages dioxygen activation through electron

transfer onto the oxygen species via the Ni*+ ++ Ni3+ transition.

Of essential importance is the formation of intermediate species from oxygen, a

hydroxyl group a nd the metal ion (Scheme 2). Such a type of dioxygen activation

with the participation of hydroxyl group has been described for oxidation processes

in aqueous solution with transition metal complex catalysts [ 241.

The Ni( OH), catalyst considered has a layered structure and can be represented

as a package of lamellas

no.

HO\

-

Ni,-

OH

) n.

Trivalent nickel ions

( . . .Ni. . ) are

0

build in the plane of divalent nickel as defects. In this case Na+ ions and water

molecules can intercalate into the interlamellar space thus increasing the interla-



234

A. Andreev et al. /Applied Catalysis B: Environmental 7 (1996) 225-235

Scheme 2.

mellar distance and promoting a highly defective structure. The nickel ions which

participate in catalysis are probably located on ‘edge sites’ at the end of the lamellae

(Scheme 2). W ater intercalation increases the number of defects and ‘edge sites’.

The presence of intercalated Na + ions into the interlamellar space is substantial.

Its considerable potential facilitates the formation of HO; ions.

Recently, a high catalytic activity of the nickel hydroxide catalysts, discussed in

the present work, ha s been reported in the water-gas shift reaction

[

281. Due to

their specific structure, the nickel hydroxides are potential catalysts for a wide range

of oxidative and other processes.

Acknowledgements

The authors are grateful to the Bulgarian National Scientific Research Foundation

for financial support.

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Documents

Catalytic Oxidation of Sulfide Ions Over Nickel