- Email: [email protected]
Catalytic oxidation of sulfide ions over NiPS3
Applied Catalysis A: General, 107 (1994)
189
189-199
Elsevier Science B.V., Amsterdam APCAT
A2652
Catalytic oxidation of sulfide ions over Nip& A. Andreev, V. Ivanova, K. Kirilov and G. Passage Institute
of Kinetics
and Catalysis, Bulgarian Academy
of Sciences,
Sofia 1113
(Bulgaria)
(Received 26 March 1993, revised manuscript received 29 August 1993)
Abstract Sulfide ion oxidation to elementary sulfur over NiPS,, which is an efficient catalyst for that process, has been studied by means of electrochemical measurements, electron spin resonance spectroscopy and X-ray photoelectron spectroscopy. Based on the results obtained by these physical methods and on reaction kinetics, a mechanism of the process is suggested. Reduction and oxidation steps of the catalytic cycle are modelled by the topotactic redox reaction of intercalation-deintercalation of hydrated alkali ions. Key words: catalytic oxidation;
NiPSs; S2- oxidation
INTRODUCTION
Sulfide ions are among the most hazardous pollutants in waste water [ 11. Their oxidation by oxygen from the air: S2-+1/202+H20~-&+20H-
(I)
leads to the formation of non-toxic elementary sulfur which is readily removed by biological treatment. This determines the great importance of the process for environmental protection. The catalyst used in this work, Nip&, is a chalcogenide with interesting physicochemical properties. For example, it can form intercalation compounds
121. This study deals Iwiththe oxidation of sulfide ions by gaseous oxygen in an alkaline medium. An attempt is made to explain the experimental results on the basis of a mechanism related to intercalation processes. In particular, the redox process (1) over the catalyst is modelled by means of intercalationdeintercalation of alkali ions in the chalcogenide catalyst. Correspondence to: Dr. A. Andreev, Sciences, Sofia 1113, Bulgaria.
Institute of Kinetics and Catalysis, Bulgarian Academy of Tel. (+35-92)724901, fax. (+35-92)756116, e-mail:
[email protected].
0926-860X/94/$07.00
@ 1994 Elsevier Science B.V.
SSD10926-860X(93)E:0189-J
All rights reserved.
190
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
EXPERIMENTAL,
Sample preparation Polycrystalline Nip& was prepared by annealing a mixture of powdered nickel, sulfur, a:nd red phosphorus in an evacuated quartz tube. The mixture was thoroughly ground before hand in an agate mortar. Heating was carried out at 700’ C for 120 h followed by cooling to ambient temperature at the rate of 5°C h-l [2]. Phase purity was examined through X-ray diffraction. The specific surface area of powdered samples was about 2 m2 g-l. The physical and chemical prsopertiesof NiPS3 are described elsewhere [ 21. Catalytic activity The catalytic iactivity in reaction ( 1) was measured in a static system under continuous stirring by monitoring the volume of oxygen consumed at 20°C. Results were also checked with a chemical method by determining the converted amount of sulfide ions. A Na,S aqueous solution, 15 ml for each run, was used at varying S2- concentration and catalyst loading. In experiments concerned with the effect of the nature of the alkali ions on the reaction rate, a 15-ml solution of Na2S, 22.28 g/l, was used together with a three-fold alkali hydroxide-to-sodium sulfide molar ratio of LiOH*H,O, KOH, or NaOH. The amount of Nip& was 0.01 g. Electrochemical measurements A NiPS,-containing electrode was prepared by pressing the powdered material in an insulated platinum holder. Further, the potential difference between the NiPS:, electrode and a calomel electrode was measured. A special glass cell was used to carry out the measurements in which a Na,S aqueous solution, 2 g/l, was inserted. The cell could be purged with both argon and air. Electron spin resonance (ESR) spectroscopy ESR spectra were recorded at room temperature on a Bruker 200 D spectrometer in the X band at ca. 9.756 Hz and 100 kHz modulation frequency. X-ray photoelectron spectroscopy (XPS) The XPS spectra were acquired on an ESCALAB MK II instrument using Al Ka radiation. Calibration was performed by the C 1s peak at 284.6 eV. Exposure to air did not affect the XPS and ESR experiments because the catalytic activity measurements were performed in the presence of air.
191
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199 RESULTS
Catalytic activity Figure 1 shows typical kinetic curves of catalytic oxidation of sulfide ions to elementary sulfur over Nip&. A pronounced induction period can be seen, followed by a linear increase in conversion. Figure 2 shows the dependence of the reaction rate, in the linear part of the kinetic curve, on catalyst loading at constant concentration of the sulfide ion, 22.65 g 1-l. A linear relationship was observed. Results of the study of the relationship between the rate of reaction (1) and the concentration of the sulfide ion at constant catalyst loading (0.01 g NiPS, ) are presented in Fig. 3. It is seen that the reaction rate is directly proportional to the concentration of the sulfide ion. Investigation of the effect of the type of alkali ion on the rate of reaction (1) gave the following results: K+ 0.35
Alkali ion Rate (mol S2-/min*10W4)
Na+ 0.30
Li+ 0.24
L-I-
9
12
15
I8
21
21
27 30
Fig. 1. Kinetic curve of the sulfide ion catalytic oxidation over NiPS,. Na,S concentration (g/l): (1) 22.65, (2) 5.53, (3) 1.53; the NiP& sample weighed 0.01 g.
A. Andreeu et al. /Appl.
Catal. A 107 (1994) 189-199
Fig. 2. Dependence o’fthe rate of S2- oxidation, linear part of the kinetic curve (Fig. 1) , on catalyst loading.
Fig. 3. Dependence of the rate of reaction (1) on the Sloading (NiPSs amount was 0.01 g).
ion concentration at constant catalyst
In previous work from this laboratory [ 31, it was found that MoS, is active in the oxidation of sulfide ions. A parallel study of the rate of reaction (1) over MoS, and NiP& demonstrated that the rate per gram of catalyst was 5.5 times higher over NiPS3. In addition, the rate over NiP& was 8.7 times higher if the surface area is considered. Electrochemical measurements Figure 4 shows the variation of the potential difference between the NiPSB electrode and this calomel electrode dipped in a Na,S aqueous solution con-
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
193
taining 2.2 g/l S2- and 0.5 g/l NaOH. Purging the solution in the electrochemical cell with argon gave rise to a negative shift of the NiPS3 electrode potential. On the other hand, purging the system with oxygen moved the electrode potential to the anodic direction. The potential changes of the NiPSB electrode were reversible and could be repeated many times (over 10 runs ) . ESR spectroscopy The ESR spectrum of NiPS3 demonstrated an intense broad (H z 1450 Gs) signal at about g= 215 which is characteristic of nickel (II) ions in a distorted octahedral field. After operation under reaction conditions, 20 g/l Na,S solution and air bubbling, a considerable decrease in intensity of the ESR signal (about 80% ) was observed, without changes of the other parameters. X-ray photoelectron spectroscopy Figure 5 presents Ni (2p,,,) XPS spectra of “pure” NiPS3 (Fig. 5 (1) ) and after treatment unlder working conditions (Fig. 5 (2) ). The main peak of Ni(2p,,,) in the fresh sample was observed at 855.1 eV which is ascribed to octahedral nickel ions in the field of (P2S,)4- anions in the lattice of NiPS,. However, changes occurred in the spectrum after treatment under operating conditions (Fig. 5 (~2) ). The main peak arising from Ni (2~,,~ ) was broadened. It is suggested that this is due to the appearance of novel oxidation states of nickel. Using computer simulation, the experimental curve could be fitted as a sum of two basic states of nickel with the following quantitative distribution: Ni’, 88%; and Ni2+, 12%. In addition, after operation the XPS spectrum exhibited an intensity loss of the satellite peaks.
Fig. 4. Variation of the potential difference between the NiP& electrode and a calomel electrode, dipped in a Na,S aqueous solution, during consecutive purging with argon and air.
194
I,
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
au.
Ni 2p
?
312
:
1
:
(’c\ :*’ .:
Fig. 5. Ni(Sp,,,) X-ray photoelectron spectra of Nip&: (1) fresh sample, (2) after operation under reaction conditions. DISCUSSION
One of the most interesting properties of metal phosphorous trichalcogenides is their ability to form intercalation compounds [2, 41, with the alkali metal ions in particular [ 51. The compound capable of forming intercalates contains stacks of layer units, with partially covalent interlayer bonding, which are held together in the crystal lattice by van der Waals forces. The van der Waals gap between the two-dimensional matrix units provides empty lattice sites that can be occupied by guest species [ 51. In the present work the oxidation of sulfide ions is performed in aqueous solution at a comparatively high concentration of the alkali ion (Na+ ) which provides favouralble conditions for their intercalation in the van der Waals gap of NiPS,: xA++y
H,O+x
e-+
[Nip&]
a
.
(A+),(H,O),[NiPS,]“-
(2)
Process (2) is a reversible topotactic redox reaction of intercalation-deintercalation which is related to a rapid ion exchange observed in aqueous electrolytes. An intercalation compound with the stoichiometric composition Na,& (H,O), [ NiPS, ] o.5- has been successfully isolated [ 51. Figure 6 displa;ysthe suggested scheme of the mechanism of catalytic occurrence of reaction (1) over NiPS3 involving the formation of intercalation compounds. It is proposed that the catalytic process is composed of a chemical reduction by intercalation in which the S- ion acts as the reducing agent:
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
x:Na++x/2S2-+yHaO+ +r/2
195
[NiPS,]-+(Na+),(H,O),[NiPS,]“-
so
(3)
and an oxidative deintercalation reaction, in which oxygen is the oxidizing agent: (Na+),(H,0),[NiPS3]“-+x/2
02+x Na++ (y-x)
Hz0
+ [Nip&] +2x OH-
(4)
Reaction (4) restores the initial state of the catalyst, thus closing the catalytic cycle. The kinetic investigation and the data from the physical methods enabled us to advance a detailed scheme of the catalytic process (Fig. 6). Electrochemical Iprocessesare among the most efficient means to investigate intercalation phenomena [ 61. They can be successfully applied to discriminate between the oxidation and reduction cycles of the catalytic process [ 31, in our case reactions (3) and (4). Figure 4 indicates the variation of the potential difference between the NiP$ electrode and the calomel electrode, dipped in Na,S aqueous solution under conditions that model reactions (3) and (4) of the catalytic process. Whilst purging the solution with argon, an oxygen-free atmosphere is created which favors reaction (3). This is accompanied by a negative shift of the NiP& electrode potential. Blowing air through the system leads to the realization of reaction (4). As a result the negative charge is re[Ni2fPS*],
TINa+ n/2
.!P-
I
d&O n/2
so 3’
Fig. 6. Scheme of the S2- ion catalytic oxidation by gaseous oxygen over Nip&.
196
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
duced and the el.ectrode potential moves in the anodic direction. Repeated reversible iterati0.n of the observed effects confirmed the possibility that processes (3) and (4) might accomplish the catalytic cycle. The time dependence of the rate of reaction (1) at variable S2- concentration indicates the existence of an induction period (about 3-4 min). After this period the rate was constant (Fig. 1) . Under experimental conditions, when the amount of dissolved oxygen is under the control of the rate of its dissolution in the liquid phase at constant catalyst loading, the reaction rate obeys pseudofirst order kinetics in [ S2- ] (Fig. 3 ) . A study of the dependence of the reaction rate on catalyst loading under steady-state conditions (Fig. 2) showed a direct proportionality. These are good grounds to direct our attention to the state of the catalyst under operating conditions. Very often the appearance of an induction period in a kinetic curve is associated with the occurrence of an initial step with slow attainment of steady state [ 71. This step can be step I in the scheme of the process presented in Fig. 6. Upon the first contact of the NiPS, catalyst with the Na,S solution, a comparatively slow intercalation process starts to proceed which occurs until an equilibrium, related to the concentration of the solution, is attained. The intercalation compound, formed during the induction period, is denoted in the scheme (Fig. 6). It maintains its steady-state composition during the realization of the catalytic intercalation-deintercalation cycle (cycle II). As is shown in the scheme (Fig. 6), the redox transition is accomplished by varying the oxidation state of the nickel ions between zero and two ( NiO-Ni2+ ) . Results of the ESR study indicate that only about 17% of the initial amount of Ni2+ ions remained in the sample after treatment under working conditions. This can be possibly explained by a transition of Ni2+ to Ni+ or Ni’. However, no ESR signal characteristic of Ni+ was observed [ 81. As was stated, analysis of the XPS data on Ni indicated the following distribution: Ni2+, 12%; and Ni’, 88%. The occurrence of nickel in a zero oxidation state in a sulfur environment during alkali ion intercalation in NiP& was confirmed by room temperature magnetic susceptibility measurements [ 111. Based on these results, one can draw the conclusion that the nickel most probably takes part in the catalytic act through a Ni 2+-Nio transition. The intensity loss of the satellite peaks in the XPS spectra after operation under reaction conditions also suggests that Ni” is ,present. However, expansion of the lattice, associated with intercalation [5], can also be considered. This effect was observed in X-ray diffraction experiments made in our laboratory. The formation of mixed valence transition metal phosphorous trisulfides is not a rare phenomenon [ 21. Usually the negative charge is assumed to be delocalized, i.e. [PS,,]+ is described as a quasi macroanion with delocalized electron charge. According to extended Huckel calculations [9, lo], the band structure of NiP& shows that the low-energy partially filled levels are eg subband levels of the Ni2+ d orbitals. This explains the formation of Ni” ions by
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
197
accepting an electron into these levels. It should be noted that despite a certain localization of the nickel levels, it is possible that the electrons become mobile via bonding with th.e valence band of P2 S;- ions, namely, through overlap of the nickel d orbitals with the sulfur lone pair orbitals. In particular, ep levels are more concentrated on sulfur [lo]. As essential part of the discussed mechanism is the probable activation of water. Scission of a stable molecule like water requires preliminary activation. The intercalation offers an elegant method to do this. As can be seen from eqn. (2 ), the polar water molecules are introduced into the host lattice by cointercalation with an alk.aliion. In this way a phase of solvated alkali ions is formed, which can be considered as a metastable compound that cannot be obtained by methods other than intercalation [5]. The [NiP&]“- layers represent a quasi-two-dimensional macroanion with delocalized negative charges [ 51. The activation of the water molecules can be explained by polarization as a result of interaction with the positively charged alkali ion and with the negative charge of the chalcogenide layer (Fig. 7a). This type of interaction was observed with alkali ion intercalates of transition metal chalcogenides in NMR studies [ 12, 131. Evidence for the crucial role of water activation in the reaction mechanism can be derived from the observed influence of the type of alkali ion on the reaction rate. Depending on the charge : radius ratio of the cation, monolayer or bilayer hydrates are formed in the van der Waals gap (Figs. 7b and 7~). For some chalcogenides investigated in the series Cs-Rb-K-Na-Li, the change from monolayer to bilayer hydrates takes place between potassium and sodium [5, 14, 151. Bearing in mind the structure of the hydrated layers (Fig. 7), it can easily be seen that the water molecule activation would be much more effective in monolayer hydrates. One should thus expect a higher activity in the presence of K+ ions than in the presence of Na+ and Li+ ions, as confirmed by our experimental results. The above leads to the following proposal for the main processes of oxygen and water activation. Oxygen activation takes place by means of an electron transfer from zero-valent nickel. On the other hand, the water molecule is
b Fig. 7. Hydrated phases in the van der Waals gap: (a) polarization of intercalated water, (b) monolayer hydration, (c) bilayer hydration.
198
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
activated through heterolytic the van der Waals gap:
decomposition,
facilitated
by its intercalation
in
S2-+Ni2+-+S+Nio l/21 0, +o 0+Ni0+02-+Ni2+ 02-+H+OH--t2 S2-+1/2
0, +H,O+S+2
OHOH-
Taking into consideration the structure of the discussed catalysts (transition metal chalcogenides) and the experience gained from catalysis over transition metal sulfides, the nickel atoms which participate in the discussed mechanism are most likely located on “edge sites” [ 161, i.e. the sites exposed at the edge of the stacking layers in the NiPS3 crystal. This facilitates the access of oxygen and the contact with products of water decomposition. The high mobility of the Na+ ions is also important. It should be noted that the polarization of the intercalated water molecules also takes place at the beginning of the van der Waals gap, in close proximity to the nickel edge atoms. These conside:rations emphasize the following merits of NiPS3 as a catalyst for the oxidation of sulfide ions to elementary sulfur, related to its structure and the possibility of forming intercalation compounds of alkali ions: (i ) an effective implementation of the catalytic redox cycle by means of the intercalation-deintercalation cycle; (ii) the unique possibility of heterolytic activation of the water molecule under the polarizing influence of the alkali ion and the chalcogenide layer in the van der Waals gap; (iii) the presence of nickel ions of sufficient electron localization, located on “edge sites”, which are able to activate oxygen by changing the oxidation state; (iv) facilitated1 electron transfer within one layer of the crystal giving the possibility of electron transfers between the localized metal levels and the chalcogenide zone. Tlhis favors the effective “concert” proceeding of the catalytic reaction with a high degree of interstage energy compensation between oxidation-reduction cycles [ 171. ACKNOWLEDGEMENT
The authors gratefully acknowledge financial National Foundation for Scientific Research.
support
from the Bulgarian
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
199
REFERENCES
2 3
4 5 6 7 8 9 10 11 12 13 14 15 16 17
T. Almgren and I. ‘Hagstram, Water Res., 8 (1974) 395. R. Brec, Solid State Ionics, 22 (1986) 3. A. Andreev, K. Kirilov, V. Ivanova, L. Prahov and E. Manova in Y.E. Fischer (Editor), NATO AS1 Chemical Physics of Intercalation II, Series B, Vol. 305, Plenum, New York, 1993, p. 375. Y.W. Johnson in M.S. Whittingham and A.Y. Yacobson (Editors), Intercalation Chemistry, Academic Press, New York, 1982, p. 267. R. Schollhorn in M.S. Whittingham and A.Y. Yacobson (Editors), Intercalation Chemistry, Academic Press, New York, 1982, p. 315. W. Schramm, E. Glocke and R. Schollhorn, Mat. Res. Bull., 21 (1986) 929. S.W. Benson, The Foundation of Chemical Kinetics, McGraw-Hill, New York, 1960, p. 102. W. Hayes and J. Wilkens, Proc. Roy. Sot. A, 281 (1964) 340. H. Mercier, Y. Mathey and E. Candell, Inorg. Chem., 26 (1987) 963. M.H. Whangbo, R. Brec, G. Ouvrard and Y. Rouxel, Inorg. Chem., 24 (1985) 2459. G. Ouvrard, Ph.D. Thesis, Nantes, France (1980). U. Roder, W. Muller-Warmuth and R. Schollhorn, J. Chem. Phys.,70 (1979) 2861. A. Lerf, P. Burke& Y.O. Bersenhard and R. Schollhorn, Z. Naturforsch., B32 (1977) 1033. A. Lerf and R. Schollhorn, Inorg. Chem., 16 (1977) 2950. R. Schollhorn and W. Schmucker, Z. Naturforsch., B30 (1975) 975. H. Topsoe, R. Canldia,N.-Y. Topsoe and B.S. Clausen, Bull. Sot. Chim. Belg., 53 (1984) 783. A. Andreev, V. Kafedjiiski, T. Halachev, B. Kunev and M. Kaltchev, Appl. Catal., 78 (1991) 199.
189
189-199
Elsevier Science B.V., Amsterdam APCAT
A2652
Catalytic oxidation of sulfide ions over Nip& A. Andreev, V. Ivanova, K. Kirilov and G. Passage Institute
of Kinetics
and Catalysis, Bulgarian Academy
of Sciences,
Sofia 1113
(Bulgaria)
(Received 26 March 1993, revised manuscript received 29 August 1993)
Abstract Sulfide ion oxidation to elementary sulfur over NiPS,, which is an efficient catalyst for that process, has been studied by means of electrochemical measurements, electron spin resonance spectroscopy and X-ray photoelectron spectroscopy. Based on the results obtained by these physical methods and on reaction kinetics, a mechanism of the process is suggested. Reduction and oxidation steps of the catalytic cycle are modelled by the topotactic redox reaction of intercalation-deintercalation of hydrated alkali ions. Key words: catalytic oxidation;
NiPSs; S2- oxidation
INTRODUCTION
Sulfide ions are among the most hazardous pollutants in waste water [ 11. Their oxidation by oxygen from the air: S2-+1/202+H20~-&+20H-
(I)
leads to the formation of non-toxic elementary sulfur which is readily removed by biological treatment. This determines the great importance of the process for environmental protection. The catalyst used in this work, Nip&, is a chalcogenide with interesting physicochemical properties. For example, it can form intercalation compounds
121. This study deals Iwiththe oxidation of sulfide ions by gaseous oxygen in an alkaline medium. An attempt is made to explain the experimental results on the basis of a mechanism related to intercalation processes. In particular, the redox process (1) over the catalyst is modelled by means of intercalationdeintercalation of alkali ions in the chalcogenide catalyst. Correspondence to: Dr. A. Andreev, Sciences, Sofia 1113, Bulgaria.
Institute of Kinetics and Catalysis, Bulgarian Academy of Tel. (+35-92)724901, fax. (+35-92)756116, e-mail:
[email protected].
0926-860X/94/$07.00
@ 1994 Elsevier Science B.V.
SSD10926-860X(93)E:0189-J
All rights reserved.
190
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
EXPERIMENTAL,
Sample preparation Polycrystalline Nip& was prepared by annealing a mixture of powdered nickel, sulfur, a:nd red phosphorus in an evacuated quartz tube. The mixture was thoroughly ground before hand in an agate mortar. Heating was carried out at 700’ C for 120 h followed by cooling to ambient temperature at the rate of 5°C h-l [2]. Phase purity was examined through X-ray diffraction. The specific surface area of powdered samples was about 2 m2 g-l. The physical and chemical prsopertiesof NiPS3 are described elsewhere [ 21. Catalytic activity The catalytic iactivity in reaction ( 1) was measured in a static system under continuous stirring by monitoring the volume of oxygen consumed at 20°C. Results were also checked with a chemical method by determining the converted amount of sulfide ions. A Na,S aqueous solution, 15 ml for each run, was used at varying S2- concentration and catalyst loading. In experiments concerned with the effect of the nature of the alkali ions on the reaction rate, a 15-ml solution of Na2S, 22.28 g/l, was used together with a three-fold alkali hydroxide-to-sodium sulfide molar ratio of LiOH*H,O, KOH, or NaOH. The amount of Nip& was 0.01 g. Electrochemical measurements A NiPS,-containing electrode was prepared by pressing the powdered material in an insulated platinum holder. Further, the potential difference between the NiPS:, electrode and a calomel electrode was measured. A special glass cell was used to carry out the measurements in which a Na,S aqueous solution, 2 g/l, was inserted. The cell could be purged with both argon and air. Electron spin resonance (ESR) spectroscopy ESR spectra were recorded at room temperature on a Bruker 200 D spectrometer in the X band at ca. 9.756 Hz and 100 kHz modulation frequency. X-ray photoelectron spectroscopy (XPS) The XPS spectra were acquired on an ESCALAB MK II instrument using Al Ka radiation. Calibration was performed by the C 1s peak at 284.6 eV. Exposure to air did not affect the XPS and ESR experiments because the catalytic activity measurements were performed in the presence of air.
191
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199 RESULTS
Catalytic activity Figure 1 shows typical kinetic curves of catalytic oxidation of sulfide ions to elementary sulfur over Nip&. A pronounced induction period can be seen, followed by a linear increase in conversion. Figure 2 shows the dependence of the reaction rate, in the linear part of the kinetic curve, on catalyst loading at constant concentration of the sulfide ion, 22.65 g 1-l. A linear relationship was observed. Results of the study of the relationship between the rate of reaction (1) and the concentration of the sulfide ion at constant catalyst loading (0.01 g NiPS, ) are presented in Fig. 3. It is seen that the reaction rate is directly proportional to the concentration of the sulfide ion. Investigation of the effect of the type of alkali ion on the rate of reaction (1) gave the following results: K+ 0.35
Alkali ion Rate (mol S2-/min*10W4)
Na+ 0.30
Li+ 0.24
L-I-
9
12
15
I8
21
21
27 30
Fig. 1. Kinetic curve of the sulfide ion catalytic oxidation over NiPS,. Na,S concentration (g/l): (1) 22.65, (2) 5.53, (3) 1.53; the NiP& sample weighed 0.01 g.
A. Andreeu et al. /Appl.
Catal. A 107 (1994) 189-199
Fig. 2. Dependence o’fthe rate of S2- oxidation, linear part of the kinetic curve (Fig. 1) , on catalyst loading.
Fig. 3. Dependence of the rate of reaction (1) on the Sloading (NiPSs amount was 0.01 g).
ion concentration at constant catalyst
In previous work from this laboratory [ 31, it was found that MoS, is active in the oxidation of sulfide ions. A parallel study of the rate of reaction (1) over MoS, and NiP& demonstrated that the rate per gram of catalyst was 5.5 times higher over NiPS3. In addition, the rate over NiP& was 8.7 times higher if the surface area is considered. Electrochemical measurements Figure 4 shows the variation of the potential difference between the NiPSB electrode and this calomel electrode dipped in a Na,S aqueous solution con-
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
193
taining 2.2 g/l S2- and 0.5 g/l NaOH. Purging the solution in the electrochemical cell with argon gave rise to a negative shift of the NiPS3 electrode potential. On the other hand, purging the system with oxygen moved the electrode potential to the anodic direction. The potential changes of the NiPSB electrode were reversible and could be repeated many times (over 10 runs ) . ESR spectroscopy The ESR spectrum of NiPS3 demonstrated an intense broad (H z 1450 Gs) signal at about g= 215 which is characteristic of nickel (II) ions in a distorted octahedral field. After operation under reaction conditions, 20 g/l Na,S solution and air bubbling, a considerable decrease in intensity of the ESR signal (about 80% ) was observed, without changes of the other parameters. X-ray photoelectron spectroscopy Figure 5 presents Ni (2p,,,) XPS spectra of “pure” NiPS3 (Fig. 5 (1) ) and after treatment unlder working conditions (Fig. 5 (2) ). The main peak of Ni(2p,,,) in the fresh sample was observed at 855.1 eV which is ascribed to octahedral nickel ions in the field of (P2S,)4- anions in the lattice of NiPS,. However, changes occurred in the spectrum after treatment under operating conditions (Fig. 5 (~2) ). The main peak arising from Ni (2~,,~ ) was broadened. It is suggested that this is due to the appearance of novel oxidation states of nickel. Using computer simulation, the experimental curve could be fitted as a sum of two basic states of nickel with the following quantitative distribution: Ni’, 88%; and Ni2+, 12%. In addition, after operation the XPS spectrum exhibited an intensity loss of the satellite peaks.
Fig. 4. Variation of the potential difference between the NiP& electrode and a calomel electrode, dipped in a Na,S aqueous solution, during consecutive purging with argon and air.
194
I,
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
au.
Ni 2p
?
312
:
1
:
(’c\ :*’ .:
Fig. 5. Ni(Sp,,,) X-ray photoelectron spectra of Nip&: (1) fresh sample, (2) after operation under reaction conditions. DISCUSSION
One of the most interesting properties of metal phosphorous trichalcogenides is their ability to form intercalation compounds [2, 41, with the alkali metal ions in particular [ 51. The compound capable of forming intercalates contains stacks of layer units, with partially covalent interlayer bonding, which are held together in the crystal lattice by van der Waals forces. The van der Waals gap between the two-dimensional matrix units provides empty lattice sites that can be occupied by guest species [ 51. In the present work the oxidation of sulfide ions is performed in aqueous solution at a comparatively high concentration of the alkali ion (Na+ ) which provides favouralble conditions for their intercalation in the van der Waals gap of NiPS,: xA++y
H,O+x
e-+
[Nip&]
a
.
(A+),(H,O),[NiPS,]“-
(2)
Process (2) is a reversible topotactic redox reaction of intercalation-deintercalation which is related to a rapid ion exchange observed in aqueous electrolytes. An intercalation compound with the stoichiometric composition Na,& (H,O), [ NiPS, ] o.5- has been successfully isolated [ 51. Figure 6 displa;ysthe suggested scheme of the mechanism of catalytic occurrence of reaction (1) over NiPS3 involving the formation of intercalation compounds. It is proposed that the catalytic process is composed of a chemical reduction by intercalation in which the S- ion acts as the reducing agent:
A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
x:Na++x/2S2-+yHaO+ +r/2
195
[NiPS,]-+(Na+),(H,O),[NiPS,]“-
so
(3)
and an oxidative deintercalation reaction, in which oxygen is the oxidizing agent: (Na+),(H,0),[NiPS3]“-+x/2
02+x Na++ (y-x)
Hz0
+ [Nip&] +2x OH-
(4)
Reaction (4) restores the initial state of the catalyst, thus closing the catalytic cycle. The kinetic investigation and the data from the physical methods enabled us to advance a detailed scheme of the catalytic process (Fig. 6). Electrochemical Iprocessesare among the most efficient means to investigate intercalation phenomena [ 61. They can be successfully applied to discriminate between the oxidation and reduction cycles of the catalytic process [ 31, in our case reactions (3) and (4). Figure 4 indicates the variation of the potential difference between the NiP$ electrode and the calomel electrode, dipped in Na,S aqueous solution under conditions that model reactions (3) and (4) of the catalytic process. Whilst purging the solution with argon, an oxygen-free atmosphere is created which favors reaction (3). This is accompanied by a negative shift of the NiP& electrode potential. Blowing air through the system leads to the realization of reaction (4). As a result the negative charge is re[Ni2fPS*],
TINa+ n/2
.!P-
I
d&O n/2
so 3’
Fig. 6. Scheme of the S2- ion catalytic oxidation by gaseous oxygen over Nip&.
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duced and the el.ectrode potential moves in the anodic direction. Repeated reversible iterati0.n of the observed effects confirmed the possibility that processes (3) and (4) might accomplish the catalytic cycle. The time dependence of the rate of reaction (1) at variable S2- concentration indicates the existence of an induction period (about 3-4 min). After this period the rate was constant (Fig. 1) . Under experimental conditions, when the amount of dissolved oxygen is under the control of the rate of its dissolution in the liquid phase at constant catalyst loading, the reaction rate obeys pseudofirst order kinetics in [ S2- ] (Fig. 3 ) . A study of the dependence of the reaction rate on catalyst loading under steady-state conditions (Fig. 2) showed a direct proportionality. These are good grounds to direct our attention to the state of the catalyst under operating conditions. Very often the appearance of an induction period in a kinetic curve is associated with the occurrence of an initial step with slow attainment of steady state [ 71. This step can be step I in the scheme of the process presented in Fig. 6. Upon the first contact of the NiPS, catalyst with the Na,S solution, a comparatively slow intercalation process starts to proceed which occurs until an equilibrium, related to the concentration of the solution, is attained. The intercalation compound, formed during the induction period, is denoted in the scheme (Fig. 6). It maintains its steady-state composition during the realization of the catalytic intercalation-deintercalation cycle (cycle II). As is shown in the scheme (Fig. 6), the redox transition is accomplished by varying the oxidation state of the nickel ions between zero and two ( NiO-Ni2+ ) . Results of the ESR study indicate that only about 17% of the initial amount of Ni2+ ions remained in the sample after treatment under working conditions. This can be possibly explained by a transition of Ni2+ to Ni+ or Ni’. However, no ESR signal characteristic of Ni+ was observed [ 81. As was stated, analysis of the XPS data on Ni indicated the following distribution: Ni2+, 12%; and Ni’, 88%. The occurrence of nickel in a zero oxidation state in a sulfur environment during alkali ion intercalation in NiP& was confirmed by room temperature magnetic susceptibility measurements [ 111. Based on these results, one can draw the conclusion that the nickel most probably takes part in the catalytic act through a Ni 2+-Nio transition. The intensity loss of the satellite peaks in the XPS spectra after operation under reaction conditions also suggests that Ni” is ,present. However, expansion of the lattice, associated with intercalation [5], can also be considered. This effect was observed in X-ray diffraction experiments made in our laboratory. The formation of mixed valence transition metal phosphorous trisulfides is not a rare phenomenon [ 21. Usually the negative charge is assumed to be delocalized, i.e. [PS,,]+ is described as a quasi macroanion with delocalized electron charge. According to extended Huckel calculations [9, lo], the band structure of NiP& shows that the low-energy partially filled levels are eg subband levels of the Ni2+ d orbitals. This explains the formation of Ni” ions by
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197
accepting an electron into these levels. It should be noted that despite a certain localization of the nickel levels, it is possible that the electrons become mobile via bonding with th.e valence band of P2 S;- ions, namely, through overlap of the nickel d orbitals with the sulfur lone pair orbitals. In particular, ep levels are more concentrated on sulfur [lo]. As essential part of the discussed mechanism is the probable activation of water. Scission of a stable molecule like water requires preliminary activation. The intercalation offers an elegant method to do this. As can be seen from eqn. (2 ), the polar water molecules are introduced into the host lattice by cointercalation with an alk.aliion. In this way a phase of solvated alkali ions is formed, which can be considered as a metastable compound that cannot be obtained by methods other than intercalation [5]. The [NiP&]“- layers represent a quasi-two-dimensional macroanion with delocalized negative charges [ 51. The activation of the water molecules can be explained by polarization as a result of interaction with the positively charged alkali ion and with the negative charge of the chalcogenide layer (Fig. 7a). This type of interaction was observed with alkali ion intercalates of transition metal chalcogenides in NMR studies [ 12, 131. Evidence for the crucial role of water activation in the reaction mechanism can be derived from the observed influence of the type of alkali ion on the reaction rate. Depending on the charge : radius ratio of the cation, monolayer or bilayer hydrates are formed in the van der Waals gap (Figs. 7b and 7~). For some chalcogenides investigated in the series Cs-Rb-K-Na-Li, the change from monolayer to bilayer hydrates takes place between potassium and sodium [5, 14, 151. Bearing in mind the structure of the hydrated layers (Fig. 7), it can easily be seen that the water molecule activation would be much more effective in monolayer hydrates. One should thus expect a higher activity in the presence of K+ ions than in the presence of Na+ and Li+ ions, as confirmed by our experimental results. The above leads to the following proposal for the main processes of oxygen and water activation. Oxygen activation takes place by means of an electron transfer from zero-valent nickel. On the other hand, the water molecule is
b Fig. 7. Hydrated phases in the van der Waals gap: (a) polarization of intercalated water, (b) monolayer hydration, (c) bilayer hydration.
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A. Andreev et al. / Appl. Catal. A 107 (1994) 189-199
activated through heterolytic the van der Waals gap:
decomposition,
facilitated
by its intercalation
in
S2-+Ni2+-+S+Nio l/21 0, +o 0+Ni0+02-+Ni2+ 02-+H+OH--t2 S2-+1/2
0, +H,O+S+2
OHOH-
Taking into consideration the structure of the discussed catalysts (transition metal chalcogenides) and the experience gained from catalysis over transition metal sulfides, the nickel atoms which participate in the discussed mechanism are most likely located on “edge sites” [ 161, i.e. the sites exposed at the edge of the stacking layers in the NiPS3 crystal. This facilitates the access of oxygen and the contact with products of water decomposition. The high mobility of the Na+ ions is also important. It should be noted that the polarization of the intercalated water molecules also takes place at the beginning of the van der Waals gap, in close proximity to the nickel edge atoms. These conside:rations emphasize the following merits of NiPS3 as a catalyst for the oxidation of sulfide ions to elementary sulfur, related to its structure and the possibility of forming intercalation compounds of alkali ions: (i ) an effective implementation of the catalytic redox cycle by means of the intercalation-deintercalation cycle; (ii) the unique possibility of heterolytic activation of the water molecule under the polarizing influence of the alkali ion and the chalcogenide layer in the van der Waals gap; (iii) the presence of nickel ions of sufficient electron localization, located on “edge sites”, which are able to activate oxygen by changing the oxidation state; (iv) facilitated1 electron transfer within one layer of the crystal giving the possibility of electron transfers between the localized metal levels and the chalcogenide zone. Tlhis favors the effective “concert” proceeding of the catalytic reaction with a high degree of interstage energy compensation between oxidation-reduction cycles [ 171. ACKNOWLEDGEMENT
The authors gratefully acknowledge financial National Foundation for Scientific Research.
support
from the Bulgarian
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