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Active centres of magnesium oxide surface and calculations of dissociative chemisorption of methane on modified MgO
Catalysis To&y, 13 (1992) 517-522 Elsevier Science Publishera B.V., Amsterdam
ACTIVE CWl’RE
OF MAGNESIUN OXIDE
517
SURFAC8 AND CALCULATIONS
OF DISSOCIATIVE CHBlISORPTION OFMETHANEONMODIFIEDM&l G.Y.ZHIDMIROV and N.U.ZHANPEISOV* Institute of Catalysis,Novosibirsk 630090, USSR ABSTRACT In the framework of supermolecular approaoh using MIND0/3 method the various channels of dissooiativechemisorption of methane molecule on pure and lithium doped magnesium oxide surfaoes are oonsidered. On the basis of the obtained oaloulation results the possible ways of methane activation on MgO and Li/MgO are disoussed. INTRODUCTION Chemical activity of magnesium oxide in catalytic reactions is often connected with surface defects suoh as low-coordinated magnesium and oxygen ions (Mgz and O&j of various surfaoe irregularities- faces, edges, oorners, eto. (refs. 1-3). Probably the magnesium or oxygen vaoanoies (loos1 oondensation 0*of Ngz and Ic 1 can play an important role in the processes of ohemisorptionon MgO. Oxygen vacanoy oan trap an eleotron forming well-known FL oentre showing a oonsiderable ohemioal activity and leading to the formation of oxygen-containingion-radical at dioxygen adsorption , for example, O-, 02 ,eto. (ref. 4). At present in literature two main hypotheses about methane aotivation on the oxidative dimeriaation catalysts are disoussed. According to the first (ref. 5) the methane aotivation proceeds via radioal abstraction of methane hydrogen atom by surface 0; ion radioal whioh is formed under substitutionof MgE on Li$ and is in equilibriumwith [IX'- 0-l oentre. According to the second (ref.6) this process proceeds via heterolytio dissociation of methane on the acid - base oentres of oatalysts. Both the relatively high values of observed
0920~53Q1/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved.
518
aotivation energy of methane oxidative dimerizationreactions the formation of superoxide ion - radical (ref. 7) and (ref. 8) under processing the magnesium oxide with adsorbed methane by dioxygen upon low temperaturescan serve in favour of the last hypothesis. The present paper when studying the methane molecule chemisorptionon MgO and IL/&O disousses the semiempiricalquantum-chemioal MINDO/3 method whioh is speoially parameterized for studying these systems. One oan give two main reasons that from our point of view prove the use of this method. First, the espeoially parameterized b!INDo/3 method allows to calculate more exactly geometry and energy of the formation of various surface oomplexes. Moreover, MINDO/3 method is better than many other semiempirioal methods when calculating hydrooarbons and their derivatives (ref. 9). Seoond, in oontrast to nonempirioalmethods .(refs. 10-12) in the framework of MINDO/3 method there is the possibility of quantum-ohemioalcalculation of the large surfaoe fragments of the lattice of MgO and Li/MgO. It is of great importance at the study of the active centres (ACj structure and their rearrangementsin the process of chemisorptional interaotion. The present work within the limits of this method oaloulates geometry and energy of the formation of various aurfaoe struotures their realization being possible upon methane molecule ohemisorptionon MgO and Li/MgO. On the basis of the oaloulated results some possible ways of methane activation on the studied oxides are discussed. CALCUIATIONMETHOD AND SURFACE MODEL The quantum-ohemioaloluster oaloulationswere performed in the framework of MIND0/3 method whose paremeterization was extended for studying lithium (ref. 13) and magnesium (ref. 14) compounds.Magnesium oxide surface was modelled by the clusters of Mg909 and Yg12012 9 Such choice of models allows first, to analyse the participation in the ohemisorption interaction coordinated (three-, four- and low all types of and 0:; ions. Second, we consider five - coordinated) Mgz
519
that the olusters size is large enough so that the further does not influenoe the optimal growth of their sizes geometry and ohemisorptionproperties of various surface parts. Naturally,the latter is based on the supposition about the significant localizationof the ohemical interactions in the studied oxide system. As it was shown (ref. 14) the growth of oluster size does not lead to the large change of geometry and oharge state of the similar surfaoe parts - the oorners, the edges, the faoes. The oharaoter of the charge ohanges on the and O& with their ooordinationnumber variations is in *E aooordanoe with this one oaloulated in the framework of nonempirioal method taking into aooount Madelung field the framework of such (ref. 15). Note also that in supermoleoularapproaoh both the relaxation of various Mg:; and their influenoe on energetios of the considered and 02, ohemisorptionalprocesses are correctly described (ref. 14). The oaloulations of molecular clusters oorresponding to IX&O promoted by lithium were oarried out by isomorphio substitutionof one of Mg$$ ions on oation with I& further compensation of the excessive oharge by proton oonneoted with 0;;. Here also full optimization of looal struoture of this substitutionwas performed. FEEXGTS AND DISCUSSIONS It is a generally accepted idea that at the interactionwith the dehydroxylated MgO surfaoe many molecules chemisorb dissooiatively (refs. 2,3,10). Taking this into aooount various forms of dissociative ohemisorptionof methane molecule both on are studied. In the oaloulations full W and Li/MgO optimizationof geometry of the ohemisorption complexes was aarried out up to the minimum of total energy taking into account AC relaxation of oxide surface. Calculationresults of dissociativechemisorptionof methane molecule on are given in Table 1. In case Z? :: WI 2012 five-coordinated 0;; or 025C and ions are the QG: looalizationoentres of the dissociated moleoule fragments the
520
oaloulations are carried out for the extended %2O12 oluster. We note that the transition from Mgs09 cluster to more extended Mgl2O,2 ones praotioallg does not influenoe the struotural and energetical characteristicsof ohemisorption complexes realized on the similar surface parts. As it was
TABLE1 AE$ KJ/mol AC pair
0;;
0% 0;; *
2 @3'c
@G
62
-14
-42
-124
-147
-114
-198
-235
Mg25; -48
2 03; 121
0%
0;; ,
-3
- 55
- 3
-112
-174
-55
-174
-231
Sign (+) corresponds to stabilizationunder chemisorption.
expected with increasingLC both magnesium cations and oxygen anions the energetic effeot of dissooiative ohemisorption decreases.Maximum gain in chemisorption energy takes place only when two O',, ions sot as the oentres of localization of dissociatedmolecule fragments. But it is naturally proved by direot caloulations (ref. 10) that heterolytio dissociation of adsorbate on the neighbour O& and I&$ oentres of lattioe must be the initial sot of dissociative chemisorption. OnlY after this one can expect the migration of adsorbate fragments along adsorption sites of the surface. In accordance with the oaloulationsof stabilizationenergy of ohemisorption states (Table 1) the ohemisorptionwith partioipationof a pair of 0;; and Mg$ ions is more preferable. In ease of methane this is the only state of heterolytio adsorption with the gain of energy (Table 1). Evidently, the quantity of such pair oentres on the surface must be very small. Adsorption on the statisticallypreferable ion pairs 0% - Mg24: or 0:i - $'c only in the case of H2 leads to stabilization (see ref. 14). Perhaps the conclusion of the paper (ref. IO) is oonneoted with the ciroumstancesthat the dissociativeadsorption of methane
521
in the contrast with H2 proceeds only at high temperatures.For instance, the latter could proceed through adsorption of CH4 on ion pair Mg:A - OS$ with further migration of the fragment to the another nearest O",,(see Table 1). OH3 from h!g$ As it was noted above for the dissociativechemiaorption of methane molecule on Li/MgO the clusters were used where the ions were isomorphicallysubstitutedby L& one and QE proton connected with O$ . It is worth noting that in our case we use the molecular model which by its nature is neutral but not radical. This approach is in contrast to Lunsford et al (ref. 5) who proposed the existence of [Iis - 0-l centres in lithium promoted IX/b&O . As it is shown below by using such unradical molecular model for Li/MgO one can rather fully describe the observed sharp increasing of activity of Li/MgO in comparison with pure MgO . In the calculations llbgg08LiOH and Mg,,O,,LiOH clusters were used . Note, that in this case the number of potentially possible acid-base pair centres for methane activation is rather high : these may be both Lib - O& and Mgz - O& pairs. Moreover, the number of combination increases both with changing LC and with possibility to form either metal -CH3 or metal -H bonds. But some of these combinations may be set aside in regards with energeticsof these processes. Thus, preliminary calculations using even more active three-coordinated Li& pair - 0;; centres showed that the latter two channels are strongly unprofitable energetically. For example, the energy of formation for Li - CH3 / 0 - H and Li - H / 0 - CH3 ohemisorptionalcomplexes are equal to - 215 and - 143 kJ/mol, correspondingly.Taking into account these circumstances the 'belowcalculation results for dissociative chemisorption of methane molecule on statistically more active pairs of acid-base centres are given in Table 2. In contrast to the analogous results for dehydroxylated MgO surface (Table 1) dissociative chemisorptionof CH4 on Li/MgO can proceed not only with participationof three - coordinated acid - base a pair of centres, but also with participation of ‘coordinated AC (placed on the edges of .oxide lattice). four -
522
TABLE 2 AC pair
5
4
3
*SC 2 - e4'c 2
34
50
21
O',, - Ng;+G
71
75
38
- 38
- 75
- 63
O',, - Ng$
=
This result oorresponds to numerous experimental data proving that promotion of magnesium oxide by lithium leads to sharp increase of activity of the first in comparison with non-promoted NgO. The qualitativepioture of the aotivation of =2 and CH4 on Li/NgO is the same as for NgO one, i.e. dihydrogenmolecule is easier aotivated than the methane one. Thus, the quantum-chemicaloaloulation results represented in this paper are in our opinion one more argument in favour of the importanoe of heterolytio ohannel (ref. 6) of oxidative dimeriaationof methane reaotions.
T. Ito, T. Nurakami and T. Tokuda, J. Chem. Sot., Faraday Trans. 1, 79 (1983) 913. F.S. Stone, E. Garrone and A. Zeoohina, Mater. Chemistry Phys., 13 (1985) 331. C.F. Jones, R.A. Reeve, R. Rigg,.R.L. Segal, R.St.C. Smart and P.S. Turner, J. Chsm. SOD., Faraday Trans. I, 80 (1984) 2609. M. Che and A.J. Tenoh, Adv. Catal., 32 (1983) 1. T. Ito and J.H. Lunsford, Nature (London) , 314 (1985) 721. V.D. Sokolovskii,Reaot. Kinet. Catal. Lett., 35 (1987) 337. O.V. Buyevskaya,Ph. D. Thesis, Novosibirsk,USSR, 1991. 1. Ito, T. Tashiro, T. Watanabe, K. Toi and I. Ikemoto, Chem. Lett., (1987) 1723. N.J.S. Dewar, J. Noleo. Struct., 100 (1983) 41. H. Kobayashi, N. Y ohi and T. Ito, Acid - Base Catalysis, Abstracts Sapporo, 1988) p. 96. ama?u E.A. Colbourn, J. Kendriok and W.C. Mackrodt, Surf. Sci., 126 (1983) 550. H. Kawakami and S. Yoshida. J. Chem. Soo.. Faraday Trans. 2, 80 (1984) 921. G.N. Zhidomirov,A.G. Pelmenschikov,N.U.Zhanpeisov and A.G. Grebenyuk, Kinet. Catal., 28 (1987) 86 (in Russian). N.U. Zhanpeisov,A.G. Pelmenschikov and G.N. Zhidomirov, Kinet. Catal., 31 (1990) 563 (in Russian). P.W. Fowler and P. Tole, Surf. Soi., 197 (1988) 457.
ACTIVE CWl’RE
OF MAGNESIUN OXIDE
517
SURFAC8 AND CALCULATIONS
OF DISSOCIATIVE CHBlISORPTION OFMETHANEONMODIFIEDM&l G.Y.ZHIDMIROV and N.U.ZHANPEISOV* Institute of Catalysis,Novosibirsk 630090, USSR ABSTRACT In the framework of supermolecular approaoh using MIND0/3 method the various channels of dissooiativechemisorption of methane molecule on pure and lithium doped magnesium oxide surfaoes are oonsidered. On the basis of the obtained oaloulation results the possible ways of methane activation on MgO and Li/MgO are disoussed. INTRODUCTION Chemical activity of magnesium oxide in catalytic reactions is often connected with surface defects suoh as low-coordinated magnesium and oxygen ions (Mgz and O&j of various surfaoe irregularities- faces, edges, oorners, eto. (refs. 1-3). Probably the magnesium or oxygen vaoanoies (loos1 oondensation 0*of Ngz and Ic 1 can play an important role in the processes of ohemisorptionon MgO. Oxygen vacanoy oan trap an eleotron forming well-known FL oentre showing a oonsiderable ohemioal activity and leading to the formation of oxygen-containingion-radical at dioxygen adsorption , for example, O-, 02 ,eto. (ref. 4). At present in literature two main hypotheses about methane aotivation on the oxidative dimeriaation catalysts are disoussed. According to the first (ref. 5) the methane aotivation proceeds via radioal abstraction of methane hydrogen atom by surface 0; ion radioal whioh is formed under substitutionof MgE on Li$ and is in equilibriumwith [IX'- 0-l oentre. According to the second (ref.6) this process proceeds via heterolytio dissociation of methane on the acid - base oentres of oatalysts. Both the relatively high values of observed
0920~53Q1/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved.
518
aotivation energy of methane oxidative dimerizationreactions the formation of superoxide ion - radical (ref. 7) and (ref. 8) under processing the magnesium oxide with adsorbed methane by dioxygen upon low temperaturescan serve in favour of the last hypothesis. The present paper when studying the methane molecule chemisorptionon MgO and IL/&O disousses the semiempiricalquantum-chemioal MINDO/3 method whioh is speoially parameterized for studying these systems. One oan give two main reasons that from our point of view prove the use of this method. First, the espeoially parameterized b!INDo/3 method allows to calculate more exactly geometry and energy of the formation of various surface oomplexes. Moreover, MINDO/3 method is better than many other semiempirioal methods when calculating hydrooarbons and their derivatives (ref. 9). Seoond, in oontrast to nonempirioalmethods .(refs. 10-12) in the framework of MINDO/3 method there is the possibility of quantum-ohemioalcalculation of the large surfaoe fragments of the lattice of MgO and Li/MgO. It is of great importance at the study of the active centres (ACj structure and their rearrangementsin the process of chemisorptional interaotion. The present work within the limits of this method oaloulates geometry and energy of the formation of various aurfaoe struotures their realization being possible upon methane molecule ohemisorptionon MgO and Li/MgO. On the basis of the oaloulated results some possible ways of methane activation on the studied oxides are discussed. CALCUIATIONMETHOD AND SURFACE MODEL The quantum-ohemioaloluster oaloulationswere performed in the framework of MIND0/3 method whose paremeterization was extended for studying lithium (ref. 13) and magnesium (ref. 14) compounds.Magnesium oxide surface was modelled by the clusters of Mg909 and Yg12012 9 Such choice of models allows first, to analyse the participation in the ohemisorption interaction coordinated (three-, four- and low all types of and 0:; ions. Second, we consider five - coordinated) Mgz
519
that the olusters size is large enough so that the further does not influenoe the optimal growth of their sizes geometry and ohemisorptionproperties of various surface parts. Naturally,the latter is based on the supposition about the significant localizationof the ohemical interactions in the studied oxide system. As it was shown (ref. 14) the growth of oluster size does not lead to the large change of geometry and oharge state of the similar surfaoe parts - the oorners, the edges, the faoes. The oharaoter of the charge ohanges on the and O& with their ooordinationnumber variations is in *E aooordanoe with this one oaloulated in the framework of nonempirioal method taking into aooount Madelung field the framework of such (ref. 15). Note also that in supermoleoularapproaoh both the relaxation of various Mg:; and their influenoe on energetios of the considered and 02, ohemisorptionalprocesses are correctly described (ref. 14). The oaloulations of molecular clusters oorresponding to IX&O promoted by lithium were oarried out by isomorphio substitutionof one of Mg$$ ions on oation with I& further compensation of the excessive oharge by proton oonneoted with 0;;. Here also full optimization of looal struoture of this substitutionwas performed. FEEXGTS AND DISCUSSIONS It is a generally accepted idea that at the interactionwith the dehydroxylated MgO surfaoe many molecules chemisorb dissooiatively (refs. 2,3,10). Taking this into aooount various forms of dissociative ohemisorptionof methane molecule both on are studied. In the oaloulations full W and Li/MgO optimizationof geometry of the ohemisorption complexes was aarried out up to the minimum of total energy taking into account AC relaxation of oxide surface. Calculationresults of dissociativechemisorptionof methane molecule on are given in Table 1. In case Z? :: WI 2012 five-coordinated 0;; or 025C and ions are the QG: looalizationoentres of the dissociated moleoule fragments the
520
oaloulations are carried out for the extended %2O12 oluster. We note that the transition from Mgs09 cluster to more extended Mgl2O,2 ones praotioallg does not influenoe the struotural and energetical characteristicsof ohemisorption complexes realized on the similar surface parts. As it was
TABLE1 AE$ KJ/mol AC pair
0;;
0% 0;; *
2 @3'c
@G
62
-14
-42
-124
-147
-114
-198
-235
Mg25; -48
2 03; 121
0%
0;; ,
-3
- 55
- 3
-112
-174
-55
-174
-231
Sign (+) corresponds to stabilizationunder chemisorption.
expected with increasingLC both magnesium cations and oxygen anions the energetic effeot of dissooiative ohemisorption decreases.Maximum gain in chemisorption energy takes place only when two O',, ions sot as the oentres of localization of dissociatedmolecule fragments. But it is naturally proved by direot caloulations (ref. 10) that heterolytio dissociation of adsorbate on the neighbour O& and I&$ oentres of lattioe must be the initial sot of dissociative chemisorption. OnlY after this one can expect the migration of adsorbate fragments along adsorption sites of the surface. In accordance with the oaloulationsof stabilizationenergy of ohemisorption states (Table 1) the ohemisorptionwith partioipationof a pair of 0;; and Mg$ ions is more preferable. In ease of methane this is the only state of heterolytio adsorption with the gain of energy (Table 1). Evidently, the quantity of such pair oentres on the surface must be very small. Adsorption on the statisticallypreferable ion pairs 0% - Mg24: or 0:i - $'c only in the case of H2 leads to stabilization (see ref. 14). Perhaps the conclusion of the paper (ref. IO) is oonneoted with the ciroumstancesthat the dissociativeadsorption of methane
521
in the contrast with H2 proceeds only at high temperatures.For instance, the latter could proceed through adsorption of CH4 on ion pair Mg:A - OS$ with further migration of the fragment to the another nearest O",,(see Table 1). OH3 from h!g$ As it was noted above for the dissociativechemiaorption of methane molecule on Li/MgO the clusters were used where the ions were isomorphicallysubstitutedby L& one and QE proton connected with O$ . It is worth noting that in our case we use the molecular model which by its nature is neutral but not radical. This approach is in contrast to Lunsford et al (ref. 5) who proposed the existence of [Iis - 0-l centres in lithium promoted IX/b&O . As it is shown below by using such unradical molecular model for Li/MgO one can rather fully describe the observed sharp increasing of activity of Li/MgO in comparison with pure MgO . In the calculations llbgg08LiOH and Mg,,O,,LiOH clusters were used . Note, that in this case the number of potentially possible acid-base pair centres for methane activation is rather high : these may be both Lib - O& and Mgz - O& pairs. Moreover, the number of combination increases both with changing LC and with possibility to form either metal -CH3 or metal -H bonds. But some of these combinations may be set aside in regards with energeticsof these processes. Thus, preliminary calculations using even more active three-coordinated Li& pair - 0;; centres showed that the latter two channels are strongly unprofitable energetically. For example, the energy of formation for Li - CH3 / 0 - H and Li - H / 0 - CH3 ohemisorptionalcomplexes are equal to - 215 and - 143 kJ/mol, correspondingly.Taking into account these circumstances the 'belowcalculation results for dissociative chemisorption of methane molecule on statistically more active pairs of acid-base centres are given in Table 2. In contrast to the analogous results for dehydroxylated MgO surface (Table 1) dissociative chemisorptionof CH4 on Li/MgO can proceed not only with participationof three - coordinated acid - base a pair of centres, but also with participation of ‘coordinated AC (placed on the edges of .oxide lattice). four -
522
TABLE 2 AC pair
5
4
3
*SC 2 - e4'c 2
34
50
21
O',, - Ng;+G
71
75
38
- 38
- 75
- 63
O',, - Ng$
=
This result oorresponds to numerous experimental data proving that promotion of magnesium oxide by lithium leads to sharp increase of activity of the first in comparison with non-promoted NgO. The qualitativepioture of the aotivation of =2 and CH4 on Li/NgO is the same as for NgO one, i.e. dihydrogenmolecule is easier aotivated than the methane one. Thus, the quantum-chemicaloaloulation results represented in this paper are in our opinion one more argument in favour of the importanoe of heterolytio ohannel (ref. 6) of oxidative dimeriaationof methane reaotions.
T. Ito, T. Nurakami and T. Tokuda, J. Chem. Sot., Faraday Trans. 1, 79 (1983) 913. F.S. Stone, E. Garrone and A. Zeoohina, Mater. Chemistry Phys., 13 (1985) 331. C.F. Jones, R.A. Reeve, R. Rigg,.R.L. Segal, R.St.C. Smart and P.S. Turner, J. Chsm. SOD., Faraday Trans. I, 80 (1984) 2609. M. Che and A.J. Tenoh, Adv. Catal., 32 (1983) 1. T. Ito and J.H. Lunsford, Nature (London) , 314 (1985) 721. V.D. Sokolovskii,Reaot. Kinet. Catal. Lett., 35 (1987) 337. O.V. Buyevskaya,Ph. D. Thesis, Novosibirsk,USSR, 1991. 1. Ito, T. Tashiro, T. Watanabe, K. Toi and I. Ikemoto, Chem. Lett., (1987) 1723. N.J.S. Dewar, J. Noleo. Struct., 100 (1983) 41. H. Kobayashi, N. Y ohi and T. Ito, Acid - Base Catalysis, Abstracts Sapporo, 1988) p. 96. ama?u E.A. Colbourn, J. Kendriok and W.C. Mackrodt, Surf. Sci., 126 (1983) 550. H. Kawakami and S. Yoshida. J. Chem. Soo.. Faraday Trans. 2, 80 (1984) 921. G.N. Zhidomirov,A.G. Pelmenschikov,N.U.Zhanpeisov and A.G. Grebenyuk, Kinet. Catal., 28 (1987) 86 (in Russian). N.U. Zhanpeisov,A.G. Pelmenschikov and G.N. Zhidomirov, Kinet. Catal., 31 (1990) 563 (in Russian). P.W. Fowler and P. Tole, Surf. Soi., 197 (1988) 457.