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Electronic and structural features of Pd3 cluster on MgO(100) surface cluster
Applied Surface Science 130–132 Ž1998. 572–575
Electronic and structural features of Pd 3 cluster on MgO ž100/ surface cluster Ryo Yamauchi, Isao Gunji, Akira Endou, Xilin Yin, Momoji Kubo, Abhijit Chatterjee, Akira Miyamoto ) Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku UniÕersity, Aoba-ku, Sendai, 980-77, Japan Received 1 October 1997; accepted 12 December 1997
Abstract The structural characters and the electronic features of Pd 3 cluster on the MgOŽ100. surface cluster were investigated by performing density functional calculations. The geometric features of the cluster shape of the Pd 3 cluster depended on the kinds of neighboring atoms interacting with the Pd atoms. The metal–support interatomic distance was compared with experimental results and the quantitative consistency was found. The dissociation of the Pd 3 cluster occurred, indicating the way of the bond breaking of metal cluster. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Pd; Metal cluster; MgO; Interaction site; Interatomic distance
1. Introduction Ultrafine metal particles on metal oxides function as catalysts and have been used in the industrial production process. Since the ultrafine metal particles undergo the conditions occurring in reactions such as methane combustion w1–4x and deoxidization of nitrogen oxides at higher temperature, the small metal particles coalesce into the larger particles. This phenomenon of the sintering on the small metal particles is one of the important problems that should be solved and improved. In order to enhance the stability of the ultrafine metal particles on the metal oxides, knowledge on the fine structure and the electronic states of the metal particles on supports is
)
Corresponding author. Tel.: q81-2-217-7233; fax: q81-2217-7235; e-mail: [email protected].
indispensable. Quantum chemical calculations can possibly pave the way for the theoretical understanding of the electronic and structural features of the metal particle on the metal oxides in the level of electrons. In the present report, the fine structure and the electronic states of Pd 3 cluster supported on the MgOŽ100. surface cluster were investigated by using density functional theory. The electronic characters represented as geometrical features were evaluated. The energetical characters were also evaluated, and were discussed, taking structural characteristics into consideration.
2. Method The total energy of the system was represented by the functions of the charge density in the density
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 1 7 - 2
R. Yamauchi et al.r Applied Surface Science 130–132 (1998) 572–575
573
functional theory w5,6x. The element terms of the total energy were expressed as the addition of the kinetic energy, Coulombic interactions, the exchange energy, and the correlation energy. Quantum chemical calculations based on density functional theory were performed by using the DMol program ŽMSI.. The optimization of the geometry was performed in local approximation, and the single-point energy calculation utilizing the structure obtained from the local level calculation was performed in non-local approximation w7,8x. Interaction energy was estimated from the subtraction of the energy of the isolated system from the combined system.
3. Results and discussion Ultrafine Pd particle was represented as the trimer of Pd atom ŽPd 3 cluster., and MgO support was modeled utilizing bulk MgO crystal ŽNaCl-like structure.. Fig. 1 shows the schematic representation of the possibility of the geometrical relationship between metal cluster composed of a few atoms and metal oxide surface. The standing type displays the model having the spatial Ž2- or 3-dimensional. character of the metal cluster, and the lying type shows the model having the plane character. If the case of Pd trimer is considered, and the kinds of interaction sites of the Pd 3 cluster on the Mg 8 O 8 cluster are also taken into consideration, four models can be formed on the Mg 8 O 8 cluster surface. Categorizing the possible cases based on the further consideration of the spatial equivalence of the relation between Pd atom and support atom, the models are the standing Pd 3 cluster interacting with two Mg atoms Žmodel 1., interacting with two O atoms Žmodel 2., lying Pd 3 cluster interacting with one Mg and two O atoms Žmodel 3., and interacting with one O and two Mg atoms Žmodel 4..
Fig. 1. Schematic view of the geometrical relation between Pd 3 cluster and MgOŽ100. surface: standing type Ža. and lying type Žb..
Fig. 2. The structural characteristics of the Pd 3 cluster neighboring on the different kinds of interaction sites of the surface of the Mg 8 O 8 cluster; occupying two Mg atoms Ža., occupying two O atoms Žb., occupying two O and one Mg atoms Žc. and occupying two Mg and one O atoms at the first stage interacting with the support.
The optimized structures of the Pd 3 clusters having the spatial feature at the first stage of the interaction with the support Žmodel 1 and model 2. are shown in Fig. 2. O and Mg atoms are represented as a ball reflecting the ionic radius. The Pd atom is shown as a ball smaller than the ball expressing Mg atom. The common character of the supported Pd 3 clusters was the longer interatomic distance between bottom Pd atoms than the bond length between bottom Pd and top Pd atoms. This findings mean that the geometrical difference of the metal cluster is recognized as the small difference of Pd–Pd distances. These remarkable difference on the geometrical feature of the Pd 3 cluster was found in the interatomic distance between Pd atom and the support atom neighboring the Pd atom. The metal–support distance of the Pd 3 cluster interacting with two O atoms Ž r Pd – O . was shorter than that of the Pd 3 cluster interacting with two Mg atoms Ž r Pd – Mg .; the shorter Pd–O distance was consistent with the results
574
R. Yamauchi et al.r Applied Surface Science 130–132 (1998) 572–575
estimated from the experiments w9x, and the quantitative similarity on the metal–O distance was found in ˚ for Rh–O. w10x, the system of RhrAl 2 O 3 Ž2.06 A ˚ for Pt–O. w11x, and RerMgO PtrMgO Ž2.19 A ˚ for Re–O. w12x. The degree of the difference Ž2.06 A found on the interatomic distance in comparing the difference of the metal–metal bond length occupying the different interaction sites, with the difference of the metal–support interatomic distance interacting with the different atoms of the support, indicates that the degree of the dependence of the metal–support distance on the interaction site is relatively larger than that of the dependence of the metal–metal distance on the site. The Pd 3 cluster having the plane character was optimized on the Mg 8 O 8 cluster surface, using Cs symmetry. The geometrical structure of the Pd 3 clusters is shown in Fig. 3. The Pd 3 cluster neighboring one Mg and two O atoms at the first stage of the metal–support interaction Žmodel 3. formed the tilted structure. The shortest Pd–O distance of the tilted Pd 3 cluster was longer than the Pd–O distance of the standing Pd 3 cluster neighboring two O atoms, and the shortest Pd–Mg distance of the tilted Pd 3 cluster was shorter than the Pd–Mg distance of the standing Pd 3 cluster neighboring two Mg atoms. These findings on the metal–support distance indicate that the relation between model 1 and the model 2 is complementary, and suggest that model 3 is located in the mixture of model 1 and model 2. The bond length between Pd atoms interacting with the O atom was longer than the bond length between the Pd atom neighboring Mg atom and the Pd atom interacting with O atom. Since this kind of anisotropy on the Pd–Pd distance of the Pd 3 cluster does not depend on the difference in the geometrical relation between metal cluster and MgO support, it can be noted that the anisotropic structure of the Pd cluster is independent of the kinds of geometrical states on the support, although the retention of metal–metal bonds is needed for the assumption. The optimized structure of the Pd 3 cluster neighboring two Mg and one O atoms at the first stage Žmodel 4. was different from other models. The Pd 3 cluster was dissociated into one atom and dimer on the Mg 8 O 8 cluster surface, owing to the larger Pd–O interaction than the Pd–Pd interaction acting in the Pd 3 cluster. The isolated Pd atom interacting with
one O atom formed the shortest bond length. The Pd atoms forming a pair such as dimer occupied the bridge site composed of two O atoms. These results on the formation of the atomistic distribution of the Pd atoms on the Mg 8 O 8 cluster surface suggest that the components of the interaction occurring from supports induce the dissociation of small metal clusters in the case when the components of metal–support interactions heading opposite directions work on the metal cluster, although it is a necessary condition that the degree of the metal–support interaction is larger than the degree of the energy needed in extracting metal atom from metal cluster. In addition, it is suggested that the dissociation of the small metal cluster is enhanced by the occupation of the interaction site neighboring two O and one Mg atoms. The interaction energy of the Pd 3 cluster on the MgO surface cluster was evaluated from Eq. Ž1., which is described as follows: Einter s EAB y Ž EA q EB .
Ž 1.
where Einter is the interaction energy, EAB is the total energy of the system including both cluster A and cluster B, EA is the total energy of cluster A in the isolated system, and EB is the total energy of cluster B in the isolated system. The energetical feature of the Pd 3 cluster of each model is listed in Table 1. The negativity of the value of the interaction energy indicates that the formed structural state is a possible case. The interaction energy of the Pd 3 cluster of the model 3 was lower than other models energetically. This suggests that the tilted structure of the Pd 3 cluster neighboring two O and one Mg atoms is favorable as structural states on metal oxide surface exposing cationic and anionic atoms. The standing Pd 3 cluster neighboring two O atoms was also a favorable structural state energetically. The
Table 1 Total energy Ž Etot . and interaction energy Ž Einter . of Pd 3 cluster on Mg 8 O 8 cluster
Model 1 Model 2 Model 3 Model 4
Etot, BLYP Ža.u..
Einter, BLYP Ža.u..
Einter, BLYP ŽeVratom.
y17023.9944153 y17024.0996654 y17024.1829366 y17024.127163
y0.0268612 y0.1321113 y0.2153825 y0.1596089
y0.244 y1.198 y1.954 y1.448
R. Yamauchi et al.r Applied Surface Science 130–132 (1998) 572–575
smaller energetical benefit was found in the case of the model 1. These relative energetical character reflects the feature found in the interatomic distance between metal cluster and support; the shorter interatomic distance between metal atom and substrate atom corresponds to the larger benefit of the interaction.
575
tion to the opposite directions acted on the cluster; this indicates one aspect of the way of the dissociation of the metal cluster. The negativity of the value of the interaction energy indicates the possible cases of the interaction sites that should be taken into consideration.
References 4. Summary The findings on both the atomistic structural features and the electronic properties provide the direction of the research, and enhance the understanding of incidents that should be considered. In this report, the structural and electronic characteristics of the Pd 3 cluster on the MgOŽ100. surface cluster were evaluated from the quantum chemical calculations based on density functional theory. The difference in the cluster shape on the different interaction sites appeared as the small difference in the Pd–Pd distances, and was recognized as the larger difference in the metal–support distances. The anisotropy of the structure of the supported cluster is independent of the kinds of interaction sites. The dissociation of the metal cluster occurred in the case when the interac-
w1x B. Beguin, E. Garbowski, S.D. Peter, M. Primet, Reaction Kinetics Catal. Lett. 59 Ž1996. 253. w2x A.M. Pisanu, C.E. Gigola, Appl. Catal. B Env. 11 Ž1996. L37. w3x H. Widjaja, K. Sekizawa, K. Eguchi, H. Arai, Catal. Today 35 Ž1997. 1. w4x K. Muto, N. Katada, M. Niwa, Catal. Today 35 Ž1997. 145. w5x P. Hohenbeg, W. Kohn, Phys. Rev. 136 Ž1964. B864. w6x W. Kohn, L.J. Sham, Phys. Rev. 140 Ž1965. A1133. w7x A.D. Becke, J. Chem. Phys. 88 Ž1988. 2547. w8x C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 Ž1988. 785. w9x S. Kawi, O. Alexeev, M. Shelef, B.C. Gates, J. Phys. Chem. 99 Ž1995. 6926. w10x J.H.A. Martens, R. Prins, D.C. Koningsberger, J. Phys. Chem. 93 Ž1989. 3179. w11x J.R. Chang, D.C. Koningsberger, B.C. Gates, J. Am. Chem. Soc. 114 Ž1992. 6460. w12x P.S. Kirlin, F.B.M. van Zon, D.C. Koningsberger, B.C. Gates, J. Phys. Chem. 94 Ž1990. 8439.
Electronic and structural features of Pd 3 cluster on MgO ž100/ surface cluster Ryo Yamauchi, Isao Gunji, Akira Endou, Xilin Yin, Momoji Kubo, Abhijit Chatterjee, Akira Miyamoto ) Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku UniÕersity, Aoba-ku, Sendai, 980-77, Japan Received 1 October 1997; accepted 12 December 1997
Abstract The structural characters and the electronic features of Pd 3 cluster on the MgOŽ100. surface cluster were investigated by performing density functional calculations. The geometric features of the cluster shape of the Pd 3 cluster depended on the kinds of neighboring atoms interacting with the Pd atoms. The metal–support interatomic distance was compared with experimental results and the quantitative consistency was found. The dissociation of the Pd 3 cluster occurred, indicating the way of the bond breaking of metal cluster. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Pd; Metal cluster; MgO; Interaction site; Interatomic distance
1. Introduction Ultrafine metal particles on metal oxides function as catalysts and have been used in the industrial production process. Since the ultrafine metal particles undergo the conditions occurring in reactions such as methane combustion w1–4x and deoxidization of nitrogen oxides at higher temperature, the small metal particles coalesce into the larger particles. This phenomenon of the sintering on the small metal particles is one of the important problems that should be solved and improved. In order to enhance the stability of the ultrafine metal particles on the metal oxides, knowledge on the fine structure and the electronic states of the metal particles on supports is
)
Corresponding author. Tel.: q81-2-217-7233; fax: q81-2217-7235; e-mail: [email protected].
indispensable. Quantum chemical calculations can possibly pave the way for the theoretical understanding of the electronic and structural features of the metal particle on the metal oxides in the level of electrons. In the present report, the fine structure and the electronic states of Pd 3 cluster supported on the MgOŽ100. surface cluster were investigated by using density functional theory. The electronic characters represented as geometrical features were evaluated. The energetical characters were also evaluated, and were discussed, taking structural characteristics into consideration.
2. Method The total energy of the system was represented by the functions of the charge density in the density
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 1 7 - 2
R. Yamauchi et al.r Applied Surface Science 130–132 (1998) 572–575
573
functional theory w5,6x. The element terms of the total energy were expressed as the addition of the kinetic energy, Coulombic interactions, the exchange energy, and the correlation energy. Quantum chemical calculations based on density functional theory were performed by using the DMol program ŽMSI.. The optimization of the geometry was performed in local approximation, and the single-point energy calculation utilizing the structure obtained from the local level calculation was performed in non-local approximation w7,8x. Interaction energy was estimated from the subtraction of the energy of the isolated system from the combined system.
3. Results and discussion Ultrafine Pd particle was represented as the trimer of Pd atom ŽPd 3 cluster., and MgO support was modeled utilizing bulk MgO crystal ŽNaCl-like structure.. Fig. 1 shows the schematic representation of the possibility of the geometrical relationship between metal cluster composed of a few atoms and metal oxide surface. The standing type displays the model having the spatial Ž2- or 3-dimensional. character of the metal cluster, and the lying type shows the model having the plane character. If the case of Pd trimer is considered, and the kinds of interaction sites of the Pd 3 cluster on the Mg 8 O 8 cluster are also taken into consideration, four models can be formed on the Mg 8 O 8 cluster surface. Categorizing the possible cases based on the further consideration of the spatial equivalence of the relation between Pd atom and support atom, the models are the standing Pd 3 cluster interacting with two Mg atoms Žmodel 1., interacting with two O atoms Žmodel 2., lying Pd 3 cluster interacting with one Mg and two O atoms Žmodel 3., and interacting with one O and two Mg atoms Žmodel 4..
Fig. 1. Schematic view of the geometrical relation between Pd 3 cluster and MgOŽ100. surface: standing type Ža. and lying type Žb..
Fig. 2. The structural characteristics of the Pd 3 cluster neighboring on the different kinds of interaction sites of the surface of the Mg 8 O 8 cluster; occupying two Mg atoms Ža., occupying two O atoms Žb., occupying two O and one Mg atoms Žc. and occupying two Mg and one O atoms at the first stage interacting with the support.
The optimized structures of the Pd 3 clusters having the spatial feature at the first stage of the interaction with the support Žmodel 1 and model 2. are shown in Fig. 2. O and Mg atoms are represented as a ball reflecting the ionic radius. The Pd atom is shown as a ball smaller than the ball expressing Mg atom. The common character of the supported Pd 3 clusters was the longer interatomic distance between bottom Pd atoms than the bond length between bottom Pd and top Pd atoms. This findings mean that the geometrical difference of the metal cluster is recognized as the small difference of Pd–Pd distances. These remarkable difference on the geometrical feature of the Pd 3 cluster was found in the interatomic distance between Pd atom and the support atom neighboring the Pd atom. The metal–support distance of the Pd 3 cluster interacting with two O atoms Ž r Pd – O . was shorter than that of the Pd 3 cluster interacting with two Mg atoms Ž r Pd – Mg .; the shorter Pd–O distance was consistent with the results
574
R. Yamauchi et al.r Applied Surface Science 130–132 (1998) 572–575
estimated from the experiments w9x, and the quantitative similarity on the metal–O distance was found in ˚ for Rh–O. w10x, the system of RhrAl 2 O 3 Ž2.06 A ˚ for Pt–O. w11x, and RerMgO PtrMgO Ž2.19 A ˚ for Re–O. w12x. The degree of the difference Ž2.06 A found on the interatomic distance in comparing the difference of the metal–metal bond length occupying the different interaction sites, with the difference of the metal–support interatomic distance interacting with the different atoms of the support, indicates that the degree of the dependence of the metal–support distance on the interaction site is relatively larger than that of the dependence of the metal–metal distance on the site. The Pd 3 cluster having the plane character was optimized on the Mg 8 O 8 cluster surface, using Cs symmetry. The geometrical structure of the Pd 3 clusters is shown in Fig. 3. The Pd 3 cluster neighboring one Mg and two O atoms at the first stage of the metal–support interaction Žmodel 3. formed the tilted structure. The shortest Pd–O distance of the tilted Pd 3 cluster was longer than the Pd–O distance of the standing Pd 3 cluster neighboring two O atoms, and the shortest Pd–Mg distance of the tilted Pd 3 cluster was shorter than the Pd–Mg distance of the standing Pd 3 cluster neighboring two Mg atoms. These findings on the metal–support distance indicate that the relation between model 1 and the model 2 is complementary, and suggest that model 3 is located in the mixture of model 1 and model 2. The bond length between Pd atoms interacting with the O atom was longer than the bond length between the Pd atom neighboring Mg atom and the Pd atom interacting with O atom. Since this kind of anisotropy on the Pd–Pd distance of the Pd 3 cluster does not depend on the difference in the geometrical relation between metal cluster and MgO support, it can be noted that the anisotropic structure of the Pd cluster is independent of the kinds of geometrical states on the support, although the retention of metal–metal bonds is needed for the assumption. The optimized structure of the Pd 3 cluster neighboring two Mg and one O atoms at the first stage Žmodel 4. was different from other models. The Pd 3 cluster was dissociated into one atom and dimer on the Mg 8 O 8 cluster surface, owing to the larger Pd–O interaction than the Pd–Pd interaction acting in the Pd 3 cluster. The isolated Pd atom interacting with
one O atom formed the shortest bond length. The Pd atoms forming a pair such as dimer occupied the bridge site composed of two O atoms. These results on the formation of the atomistic distribution of the Pd atoms on the Mg 8 O 8 cluster surface suggest that the components of the interaction occurring from supports induce the dissociation of small metal clusters in the case when the components of metal–support interactions heading opposite directions work on the metal cluster, although it is a necessary condition that the degree of the metal–support interaction is larger than the degree of the energy needed in extracting metal atom from metal cluster. In addition, it is suggested that the dissociation of the small metal cluster is enhanced by the occupation of the interaction site neighboring two O and one Mg atoms. The interaction energy of the Pd 3 cluster on the MgO surface cluster was evaluated from Eq. Ž1., which is described as follows: Einter s EAB y Ž EA q EB .
Ž 1.
where Einter is the interaction energy, EAB is the total energy of the system including both cluster A and cluster B, EA is the total energy of cluster A in the isolated system, and EB is the total energy of cluster B in the isolated system. The energetical feature of the Pd 3 cluster of each model is listed in Table 1. The negativity of the value of the interaction energy indicates that the formed structural state is a possible case. The interaction energy of the Pd 3 cluster of the model 3 was lower than other models energetically. This suggests that the tilted structure of the Pd 3 cluster neighboring two O and one Mg atoms is favorable as structural states on metal oxide surface exposing cationic and anionic atoms. The standing Pd 3 cluster neighboring two O atoms was also a favorable structural state energetically. The
Table 1 Total energy Ž Etot . and interaction energy Ž Einter . of Pd 3 cluster on Mg 8 O 8 cluster
Model 1 Model 2 Model 3 Model 4
Etot, BLYP Ža.u..
Einter, BLYP Ža.u..
Einter, BLYP ŽeVratom.
y17023.9944153 y17024.0996654 y17024.1829366 y17024.127163
y0.0268612 y0.1321113 y0.2153825 y0.1596089
y0.244 y1.198 y1.954 y1.448
R. Yamauchi et al.r Applied Surface Science 130–132 (1998) 572–575
smaller energetical benefit was found in the case of the model 1. These relative energetical character reflects the feature found in the interatomic distance between metal cluster and support; the shorter interatomic distance between metal atom and substrate atom corresponds to the larger benefit of the interaction.
575
tion to the opposite directions acted on the cluster; this indicates one aspect of the way of the dissociation of the metal cluster. The negativity of the value of the interaction energy indicates the possible cases of the interaction sites that should be taken into consideration.
References 4. Summary The findings on both the atomistic structural features and the electronic properties provide the direction of the research, and enhance the understanding of incidents that should be considered. In this report, the structural and electronic characteristics of the Pd 3 cluster on the MgOŽ100. surface cluster were evaluated from the quantum chemical calculations based on density functional theory. The difference in the cluster shape on the different interaction sites appeared as the small difference in the Pd–Pd distances, and was recognized as the larger difference in the metal–support distances. The anisotropy of the structure of the supported cluster is independent of the kinds of interaction sites. The dissociation of the metal cluster occurred in the case when the interac-
w1x B. Beguin, E. Garbowski, S.D. Peter, M. Primet, Reaction Kinetics Catal. Lett. 59 Ž1996. 253. w2x A.M. Pisanu, C.E. Gigola, Appl. Catal. B Env. 11 Ž1996. L37. w3x H. Widjaja, K. Sekizawa, K. Eguchi, H. Arai, Catal. Today 35 Ž1997. 1. w4x K. Muto, N. Katada, M. Niwa, Catal. Today 35 Ž1997. 145. w5x P. Hohenbeg, W. Kohn, Phys. Rev. 136 Ž1964. B864. w6x W. Kohn, L.J. Sham, Phys. Rev. 140 Ž1965. A1133. w7x A.D. Becke, J. Chem. Phys. 88 Ž1988. 2547. w8x C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 Ž1988. 785. w9x S. Kawi, O. Alexeev, M. Shelef, B.C. Gates, J. Phys. Chem. 99 Ž1995. 6926. w10x J.H.A. Martens, R. Prins, D.C. Koningsberger, J. Phys. Chem. 93 Ž1989. 3179. w11x J.R. Chang, D.C. Koningsberger, B.C. Gates, J. Am. Chem. Soc. 114 Ž1992. 6460. w12x P.S. Kirlin, F.B.M. van Zon, D.C. Koningsberger, B.C. Gates, J. Phys. Chem. 94 Ž1990. 8439.