- Email: [email protected]
Novel low-temperature synthesis of titanium nitride: proposal for cyanonitridation mechanism
SOLID STATE ELSEVIER
Solid State Ionics 101-103 (1997) 171-174
IONI
Novel low-temperature synthesis of titanium nitride: proposal for cyanonitridation mechanism Franqois C16ment, Philippe Bastians, Paul Grange* Universitd catholique de Louvain, Unitd de Catalyse et Chimie des matdriaux divisds, Place Croix du Sud, 2/17, B-1348 Louvain-laNeuve, Belgium
Abstract
Low temperature nitridation of titanium dioxide using monomethylamine as a reactant is obtained. The first reaction is the decomposition of monomethylamine into HCN and atomic hydrogen. The second step is the partial reduction of titanium dioxide. The third step is the nitridation of the TiO by HCN. Keywords: Nitridation Materials." TiO2; TiN
1. Introduction
Carbothermal nitridation of titanium dioxide powders is one of the most studied routes for the preparation of titanium nitride. It has been proposed that it may be described by two main pathways. The first one consists in titanium dioxide reduction with carbon, leading to different sub-oxide phases such as T i 4 0 7 or T i 3 0 5. The second one is the nitridation of the reduced oxide with formation of CO. We demonstrated that, in the case of aluminium nitride synthesis, the nitridation reaction can be described as gas-solid reaction. The first step is the formation of HCN (or CN ) by reaction of C + NH 3 followed by cyanonitridation of the oxide by this activated intermediate [1 ]. Based on this finding, we also proposed that the CN intermediate could Corresponding author. Fax: grange @cata.ucl.ac.be
+32-10 47 36 49; e-mail:
0167-2738/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 2 7 3 8 ( 9 7 ) 0 0 2 2 0 - 8
also be produced by thermal decomposition of nitrogen-containing organic molecules such as acetonitrile or monomethylamine [2]. Nitriles, and particularly acetonitrile, are relatively stable molecules and decompose into HCN at high temperature. This explains why the starting temperatures for nitridation of oxide using CH3NH 3 instead of a mixture of C H 4 and N H 3 or C + NH 3 are quite similar. On the contrary, amines present lower thermal stability than the corresponding nitriles. Consequently, the monomethylamine, amine with the lower molecular weight, gives cyanidric acid and H 2 at lower temperature than the nitriles. The monomethylamine is a gaseous product at ambient temperature and thus the flow rate can be easily controlled. In addition, monoethylamine is an important industrial feedstock and is sometimes used as a source of HCN. It decomposes into HCN, H a, C H 4 , NH 3 and N 2 between 500 and 550°C.
172
F. Clement et al. / Solid State tonics 1 0 1 - 1 0 3 (1997) 1 7 1 - 1 7 4
This paper describes the nitridation of titanium dioxide powder with monomethylamine. The reaction was studied in a fixed bed reactor.
2. Experimental Commercial TiO 2 anatase (Janssen) was used as starting material. Gaseous monomethylamine (99%, 20 ml/min) was diluted with N 2 on He (180 ml min-l). Two kinds of experiments are reported: 1. Isothermal nitridation between 600 and 1400°C. 2. Heating at constant rate 10°C min-J up to 800°C followed by isothermal nitridation. In the first case, X-ray diffraction (XRD) analyses of the solids allow to estimate the nitridation level. In the second case, on line gas chromatography (GC) and mass spectrometry (MS) were used to analyse the gaseous products of the reaction.
in this paper, we shall only illustrate the MS analyses. Fig. 1 illustrates the results in function of the reaction temperature. Methylamine (m/e = 31) starts to decompose at 520°C. Although this decomposition is weak between 520 and 620°C, it is completed at 750°C. The most abundant product of the C H 3 - N H 2 decomposition is hydrogen. However, it is necessary to mention that the plateau obtained from 650°C is not representative of the hydrogen concentration since, for this gas, the saturation of the detector is obtained and further increase of the hydrogen content in the flow cannot be analysed. The second gas analysed is HCN. Three parameters influenced the HCN content detected: the splitting of the methylamine in MS, the decomposition of the methylamine into HCN and the consumption of this gas during the nitridation. The HCN content increases from 670°C up to 740°C. At this tempera-
3. Results and discussion • H20
3.1. Isothermal nitridation between 600 and 1400°C
•
XRD analysis of the solid after reaction at 600°C clearly indicates the presence of TiO. At 1400°C, the X-ray diffraction lines correspond almost perfectly to TiN. However, it is not possible to completely discard the presence of traces of TiC at this temperature. Between 600 and 1400°C, a progressive shift of the diffraction lines attributed to TiO and TiN is evidenced, suggesting a progressive substitution of oxygen by nitrogen in the TiON phase. In addition, chemical analysis allows to estimate the nitridation levels in the following way: 73% at 660°C, 86% at 800°C and 90% at 1000°C.
i.
CH4
=H2
C
I ............
.
-
[
.... ".
•
CH3NH2
+
NH3
.
.
-....
i
xx • ~ x X X X X x x x X ~ X ~ x ~ x xXx
x x:K X~
xx xxxx X x
x:~xxx ~
~
°e ° °°Ooooeoooooeooooooooo +++++++++++++++++++
3.2. Nitridation at constant heating rate
GC and MS analyses of the gas at the outlet of the reactors allow to identify the gaseous products. Both techniques identify the same gases evolved, which is a good confirmation of the analyses. For this reason,
++++++++++++++
++++++++++
........................................ 500 550 600 650 700 750 800
Temperature ( ° C )
Fig. 1. Mass spectrometry analyses of the gas evolved during the nitridation reaction.
F. Climent et al. / Solid State lonics 101-103 (1997) 171-174
ture, in spite of the total decomposition of the CH3NH 2 molecule, the HCN content decreases, clearly due to the nitridation of the titanium oxide. The further increase of the HCN content at 830°C may be interpreted as follows. At this temperature, the upper limit of nitridation (86%) is obtained and the reaction stops while monoethylamine decomposition goes on, indicating that all the HCN formed is not converted and remains in the exhaust gas. It is difficult to evaluate the amount of water formed during the reaction. The concentration of this molecule, due to the reduction of titanium dioxide, seems constant up to 700°C and then decreases. The evolution of the concentration of the CO molecule (m/e = 28) brings new arguments. This evolution varies in the opposite way to that of HCN. This is logical if we consider that the formation of this molecule depends on the nitridation of TiO by HCN following the reaction: TiO + HCN -4 TiN + CO + 1/2H 2. The last molecules detected are C H 4 and N H 3. Their concentrations are always quite low and probably connected to the secondary hydrogenation of the methylamine. The concentration of both molecules increases up to 710°C. However, as C H 4 is less influenced by thermal decomposition than ammonia, its concentration at high temperature is higher than that of ammonia. It is worthwhile to mention that neither CO 2 (m/ e = 4 4 ) nor C2N 2 (m/e=52) are detected. These results are consistent and may be analysed as follows. The thermal decomposition of methylamine: CH3NH 2 -4 HCN + H 2 is thermodynamically possible at 396°C, while the hydrogenation reaction CH3NH 2 + H 2 -4 CH 4 + NH 3 is always possible whatever the temperature. It should be recalled that we detected experimentally the beginning of the decomposition at 520°C, which indicates that the kinetic parameters are quite important. The two other decomposition reactions, NH 3 and CH 4, are possible at 183 and 551°C, respectively. Experimentally, these gases are detected at 700
173
and 800°C, which confirms the higher stability of CH 4 as compared with ammonia. The secondary reaction CH 4 + NH 3 -4 HCN + 3 H 2 is possible at 783°C. This is another confirmation of the experimental results. We have to note that synthesis of HCN from CH 4 and NH 3 is an industrial reaction catalysed by A I 2 0 3 . Considering the reduction reaction, we have to consider first the reaction TiO 2 + CH 4 ---)TiO + CO + H 2. Theoretically, this reaction is possible at 1066°C. So, in these conditions, the only way to explain the reduction of titanium dioxide is by hydrogen. There are three different sources of hydrogen: from the decomposition of methylamine, ammonia or methane. Thermodynamic calculations indicate that the last two possibilities are possible only at 1407 and 1401°C, respectively. The hydrogen coming from the reduction of the methylamine is then the only source. However, the reaction TiO 2 + H e -4TiO + H20
is not thermodynamically possible, while for the reaction TiO 2 + 2H ° -~ TiO + H20, AG is always negative. This suggests that the reduction should take place thanks to atomic hydrogen. Consequently, the presence of water detected during the nitridation is due to the reduction of TiO 2 by atomic hydrogen. This supports some previous findings [3-6]. We then have to consider the nitridation reaction. Two molecules may act as nitridation reactant: HCN and NH 3. Thermodynamically, both are possible and, in addition, NH 3 decomposition becomes important at 700°C. It is possible to propose that HCN is, at least at low temperature, the main nitridation reactant for two reasons. First, nitridations either by carbonitruration: T i O 2 + C + N H 3 - 4 T I N + CO + H 2 0 + 1 / 2 0 2
or pure ammonia: T i O 2 + 4 / 3 N H 3 - 4 T I N + 2H~_O + l / 6 N 2
174
F. Cldment et al. / Solid State lonics 101-103 (1997) 171-174
or ammonia and methane: T i O 2 + NH 3 + C H 4 -->TIN + CO + H20 + 5/2H 2
are theoretically possible at 991, 1095 and 852°C, respectively [7-10]. Secondly, the evolution of CO in the stream is opposite to that of HCN. In these conditions, at low temperature, the main reaction should be the following: TiO + HCN--~ TiN + CO + 1/2H 2 or, more precisely, the progressive substitution of oxygen by nitrogen gives intermediates TiO~N,. oxynitrides. In parallel, at temperatures higher than 800°C, partial nitridation of TiO with NH 3 TiO + NH 3 ---)TIN + H20 + 1/2H 2 should occur in the same time as methane deposition and coke formation C H 4 ---->C + 2H 2.
At temperatures higher than 1000°C, conventional carbonitridation of TiO 2 could also be observed, and at temperatures higher than 1400°C, a small amount of TiC is also produced.
4. Conclusions The monomethylamine as source of cyanidric acid is a very effective nitridation reactant of titanium dioxide.
At temperature as low as 600°C, after a first reduction step of titanium dioxide into titanium oxide, a partial substitution of oxygen by nitrogen leading to the formation of titanium oxynitride is obtained. The level of nitridation mainly depends on the temperature of reaction. At higher temperatures, nitridation with ammonia or carbonitridation are complementary reactions.
Acknowledgements We acknowledge the 'R6gion Walionne', Belgium, for supporting this research.
References [1] S. Colque, P. Grange, Solid State Ionics 63 (1993) 122. [2] S. Colque, P. Bastians, R Grange, B. Delmon, Patent Fr. 9209872 and Patent DE 4326-446. [3] S. Shin, G. Bugh, G. Djega-Mariadasson, J. Solid State Chem. 95 (1991) 145. [4] S. Umezu, Proc. Imp. Acad. (Tokyo) 7 (1931) 353. [5] S.T. Oyama, J.C. Schlatter, J.E. Metcalfe, J.M. Lambert, Ind. Eng. Chem. Res. 24 (1988) 1639. [6] K. Kamiya, T. Yoko, M. Bessho, J. Mater. Sci. 22 (1987) 937. [7] W.-Y. Li, F.L. Riley, J. Eur. Ceram. Soc. 8 (1991) 345. [8] G.V. White, K.J.D. Mackenzie, J.H. Johnston, J. Mater. Sci. 27 (1992) 4287. [9] G. V White, K.J.D. Mackenzie, J.H. Johnston, J. Mater. Sci. 27 (1992) 4294. [10] GN. White, K.J.D. Mackenzie, J.H. Johnston, J. Mater. Sci. 27 (1992) 4300.
Solid State Ionics 101-103 (1997) 171-174
IONI
Novel low-temperature synthesis of titanium nitride: proposal for cyanonitridation mechanism Franqois C16ment, Philippe Bastians, Paul Grange* Universitd catholique de Louvain, Unitd de Catalyse et Chimie des matdriaux divisds, Place Croix du Sud, 2/17, B-1348 Louvain-laNeuve, Belgium
Abstract
Low temperature nitridation of titanium dioxide using monomethylamine as a reactant is obtained. The first reaction is the decomposition of monomethylamine into HCN and atomic hydrogen. The second step is the partial reduction of titanium dioxide. The third step is the nitridation of the TiO by HCN. Keywords: Nitridation Materials." TiO2; TiN
1. Introduction
Carbothermal nitridation of titanium dioxide powders is one of the most studied routes for the preparation of titanium nitride. It has been proposed that it may be described by two main pathways. The first one consists in titanium dioxide reduction with carbon, leading to different sub-oxide phases such as T i 4 0 7 or T i 3 0 5. The second one is the nitridation of the reduced oxide with formation of CO. We demonstrated that, in the case of aluminium nitride synthesis, the nitridation reaction can be described as gas-solid reaction. The first step is the formation of HCN (or CN ) by reaction of C + NH 3 followed by cyanonitridation of the oxide by this activated intermediate [1 ]. Based on this finding, we also proposed that the CN intermediate could Corresponding author. Fax: grange @cata.ucl.ac.be
+32-10 47 36 49; e-mail:
0167-2738/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 2 7 3 8 ( 9 7 ) 0 0 2 2 0 - 8
also be produced by thermal decomposition of nitrogen-containing organic molecules such as acetonitrile or monomethylamine [2]. Nitriles, and particularly acetonitrile, are relatively stable molecules and decompose into HCN at high temperature. This explains why the starting temperatures for nitridation of oxide using CH3NH 3 instead of a mixture of C H 4 and N H 3 or C + NH 3 are quite similar. On the contrary, amines present lower thermal stability than the corresponding nitriles. Consequently, the monomethylamine, amine with the lower molecular weight, gives cyanidric acid and H 2 at lower temperature than the nitriles. The monomethylamine is a gaseous product at ambient temperature and thus the flow rate can be easily controlled. In addition, monoethylamine is an important industrial feedstock and is sometimes used as a source of HCN. It decomposes into HCN, H a, C H 4 , NH 3 and N 2 between 500 and 550°C.
172
F. Clement et al. / Solid State tonics 1 0 1 - 1 0 3 (1997) 1 7 1 - 1 7 4
This paper describes the nitridation of titanium dioxide powder with monomethylamine. The reaction was studied in a fixed bed reactor.
2. Experimental Commercial TiO 2 anatase (Janssen) was used as starting material. Gaseous monomethylamine (99%, 20 ml/min) was diluted with N 2 on He (180 ml min-l). Two kinds of experiments are reported: 1. Isothermal nitridation between 600 and 1400°C. 2. Heating at constant rate 10°C min-J up to 800°C followed by isothermal nitridation. In the first case, X-ray diffraction (XRD) analyses of the solids allow to estimate the nitridation level. In the second case, on line gas chromatography (GC) and mass spectrometry (MS) were used to analyse the gaseous products of the reaction.
in this paper, we shall only illustrate the MS analyses. Fig. 1 illustrates the results in function of the reaction temperature. Methylamine (m/e = 31) starts to decompose at 520°C. Although this decomposition is weak between 520 and 620°C, it is completed at 750°C. The most abundant product of the C H 3 - N H 2 decomposition is hydrogen. However, it is necessary to mention that the plateau obtained from 650°C is not representative of the hydrogen concentration since, for this gas, the saturation of the detector is obtained and further increase of the hydrogen content in the flow cannot be analysed. The second gas analysed is HCN. Three parameters influenced the HCN content detected: the splitting of the methylamine in MS, the decomposition of the methylamine into HCN and the consumption of this gas during the nitridation. The HCN content increases from 670°C up to 740°C. At this tempera-
3. Results and discussion • H20
3.1. Isothermal nitridation between 600 and 1400°C
•
XRD analysis of the solid after reaction at 600°C clearly indicates the presence of TiO. At 1400°C, the X-ray diffraction lines correspond almost perfectly to TiN. However, it is not possible to completely discard the presence of traces of TiC at this temperature. Between 600 and 1400°C, a progressive shift of the diffraction lines attributed to TiO and TiN is evidenced, suggesting a progressive substitution of oxygen by nitrogen in the TiON phase. In addition, chemical analysis allows to estimate the nitridation levels in the following way: 73% at 660°C, 86% at 800°C and 90% at 1000°C.
i.
CH4
=H2
C
I ............
.
-
[
.... ".
•
CH3NH2
+
NH3
.
.
-....
i
xx • ~ x X X X X x x x X ~ X ~ x ~ x xXx
x x:K X~
xx xxxx X x
x:~xxx ~
~
°e ° °°Ooooeoooooeooooooooo +++++++++++++++++++
3.2. Nitridation at constant heating rate
GC and MS analyses of the gas at the outlet of the reactors allow to identify the gaseous products. Both techniques identify the same gases evolved, which is a good confirmation of the analyses. For this reason,
++++++++++++++
++++++++++
........................................ 500 550 600 650 700 750 800
Temperature ( ° C )
Fig. 1. Mass spectrometry analyses of the gas evolved during the nitridation reaction.
F. Climent et al. / Solid State lonics 101-103 (1997) 171-174
ture, in spite of the total decomposition of the CH3NH 2 molecule, the HCN content decreases, clearly due to the nitridation of the titanium oxide. The further increase of the HCN content at 830°C may be interpreted as follows. At this temperature, the upper limit of nitridation (86%) is obtained and the reaction stops while monoethylamine decomposition goes on, indicating that all the HCN formed is not converted and remains in the exhaust gas. It is difficult to evaluate the amount of water formed during the reaction. The concentration of this molecule, due to the reduction of titanium dioxide, seems constant up to 700°C and then decreases. The evolution of the concentration of the CO molecule (m/e = 28) brings new arguments. This evolution varies in the opposite way to that of HCN. This is logical if we consider that the formation of this molecule depends on the nitridation of TiO by HCN following the reaction: TiO + HCN -4 TiN + CO + 1/2H 2. The last molecules detected are C H 4 and N H 3. Their concentrations are always quite low and probably connected to the secondary hydrogenation of the methylamine. The concentration of both molecules increases up to 710°C. However, as C H 4 is less influenced by thermal decomposition than ammonia, its concentration at high temperature is higher than that of ammonia. It is worthwhile to mention that neither CO 2 (m/ e = 4 4 ) nor C2N 2 (m/e=52) are detected. These results are consistent and may be analysed as follows. The thermal decomposition of methylamine: CH3NH 2 -4 HCN + H 2 is thermodynamically possible at 396°C, while the hydrogenation reaction CH3NH 2 + H 2 -4 CH 4 + NH 3 is always possible whatever the temperature. It should be recalled that we detected experimentally the beginning of the decomposition at 520°C, which indicates that the kinetic parameters are quite important. The two other decomposition reactions, NH 3 and CH 4, are possible at 183 and 551°C, respectively. Experimentally, these gases are detected at 700
173
and 800°C, which confirms the higher stability of CH 4 as compared with ammonia. The secondary reaction CH 4 + NH 3 -4 HCN + 3 H 2 is possible at 783°C. This is another confirmation of the experimental results. We have to note that synthesis of HCN from CH 4 and NH 3 is an industrial reaction catalysed by A I 2 0 3 . Considering the reduction reaction, we have to consider first the reaction TiO 2 + CH 4 ---)TiO + CO + H 2. Theoretically, this reaction is possible at 1066°C. So, in these conditions, the only way to explain the reduction of titanium dioxide is by hydrogen. There are three different sources of hydrogen: from the decomposition of methylamine, ammonia or methane. Thermodynamic calculations indicate that the last two possibilities are possible only at 1407 and 1401°C, respectively. The hydrogen coming from the reduction of the methylamine is then the only source. However, the reaction TiO 2 + H e -4TiO + H20
is not thermodynamically possible, while for the reaction TiO 2 + 2H ° -~ TiO + H20, AG is always negative. This suggests that the reduction should take place thanks to atomic hydrogen. Consequently, the presence of water detected during the nitridation is due to the reduction of TiO 2 by atomic hydrogen. This supports some previous findings [3-6]. We then have to consider the nitridation reaction. Two molecules may act as nitridation reactant: HCN and NH 3. Thermodynamically, both are possible and, in addition, NH 3 decomposition becomes important at 700°C. It is possible to propose that HCN is, at least at low temperature, the main nitridation reactant for two reasons. First, nitridations either by carbonitruration: T i O 2 + C + N H 3 - 4 T I N + CO + H 2 0 + 1 / 2 0 2
or pure ammonia: T i O 2 + 4 / 3 N H 3 - 4 T I N + 2H~_O + l / 6 N 2
174
F. Cldment et al. / Solid State lonics 101-103 (1997) 171-174
or ammonia and methane: T i O 2 + NH 3 + C H 4 -->TIN + CO + H20 + 5/2H 2
are theoretically possible at 991, 1095 and 852°C, respectively [7-10]. Secondly, the evolution of CO in the stream is opposite to that of HCN. In these conditions, at low temperature, the main reaction should be the following: TiO + HCN--~ TiN + CO + 1/2H 2 or, more precisely, the progressive substitution of oxygen by nitrogen gives intermediates TiO~N,. oxynitrides. In parallel, at temperatures higher than 800°C, partial nitridation of TiO with NH 3 TiO + NH 3 ---)TIN + H20 + 1/2H 2 should occur in the same time as methane deposition and coke formation C H 4 ---->C + 2H 2.
At temperatures higher than 1000°C, conventional carbonitridation of TiO 2 could also be observed, and at temperatures higher than 1400°C, a small amount of TiC is also produced.
4. Conclusions The monomethylamine as source of cyanidric acid is a very effective nitridation reactant of titanium dioxide.
At temperature as low as 600°C, after a first reduction step of titanium dioxide into titanium oxide, a partial substitution of oxygen by nitrogen leading to the formation of titanium oxynitride is obtained. The level of nitridation mainly depends on the temperature of reaction. At higher temperatures, nitridation with ammonia or carbonitridation are complementary reactions.
Acknowledgements We acknowledge the 'R6gion Walionne', Belgium, for supporting this research.
References [1] S. Colque, P. Grange, Solid State Ionics 63 (1993) 122. [2] S. Colque, P. Bastians, R Grange, B. Delmon, Patent Fr. 9209872 and Patent DE 4326-446. [3] S. Shin, G. Bugh, G. Djega-Mariadasson, J. Solid State Chem. 95 (1991) 145. [4] S. Umezu, Proc. Imp. Acad. (Tokyo) 7 (1931) 353. [5] S.T. Oyama, J.C. Schlatter, J.E. Metcalfe, J.M. Lambert, Ind. Eng. Chem. Res. 24 (1988) 1639. [6] K. Kamiya, T. Yoko, M. Bessho, J. Mater. Sci. 22 (1987) 937. [7] W.-Y. Li, F.L. Riley, J. Eur. Ceram. Soc. 8 (1991) 345. [8] G.V. White, K.J.D. Mackenzie, J.H. Johnston, J. Mater. Sci. 27 (1992) 4287. [9] G. V White, K.J.D. Mackenzie, J.H. Johnston, J. Mater. Sci. 27 (1992) 4294. [10] GN. White, K.J.D. Mackenzie, J.H. Johnston, J. Mater. Sci. 27 (1992) 4300.