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4.19 The Relationship between the Absolute Acidities of Monoatomic Cations and their Catalytic Activities in a Few Heterogeneous Systems
4.19 The Relationshp between the Absolute Acidties of Monoatomic Cations and their Catalytic Activities in a Few Heterogeneous Systems R.Notoya” and A.Matsudab aCayalysis Research Center,Hokkaido University,Sapporo 060 Japan bLate Emeritus Professor,Hokkaido University,Sapporo 060 Japan
Abstract The absolute acidity E~ of a monoatomic cation i which had been expressed numerically in terms of the ionization energy of Ii of cation i on the basis of the empirical rules for solvation of monoatomic ions, was found to give a measure of the catalytic activity for many catalysis systems in both cases of acid and base catalysts. The present paper is concerned with the elucidation of the selectivity of catalyst in terms of this acidity in a few cases of heterogeneous systems. 1.Introduction Since the absolute acidity E~ of a monoatomic cation i was determined on the basis of the electron donor-acceptor concept for solvation[l], of E~ proved to be a measure of catalytic activity various catalysisC2-31. It was revealed that catalytic reactions were divided into two groups, one of which was accelerated with increasing E~ and on the contrary the other, retarded with increasing E i. In both groups, the relationships between E and the catalytic activity in the cases of the homogeneous catalysis, catalysis of metallo-porphyrines and zeolites showed more quantitativle than that in the case of heterogeneous one. ‘The present paper aims for revealing such relatioships as described above concerning not overall reaction but elementary acts or .partially grouped elementary steps even in a few cases of heterogeneous catalysisl4-51. We shall start by the basic concept for 49 1
492
R. NOTOYA and A. MATSUDA
the evaluation of the absolute acidity c i of cation i. 2. The Empirical Rules for Solvation of Monoatomic Cation
Two empirical rules were found to be present for the relationships, between chemical free energy AG"i,g+s of solvation of ion i for solvent s and that AG"i,g4w for water w , and between chemical free energy AG"i,gts of solvation of ion i for solvent s and the total ionization energy for ion i Ii, i.e.: the rule I ; A G " i , g t w = P AG"i,g+s and the rule 11; AG'i,g+s = p:.ci, where p equal to the ration p : / p t and p denote the propotional constant and the absolute basicity of solvent 6. All monoatomic cations on the periodic table are divided by the electronic configuration of ion i into three groups which are characterized by the electronic configuration of ion i, i.e. a-, b- and c-groups' ions which configurations can be specifyed by the rare gas type X, X+d3"lo and X+5d10, respectively. Each c i is given by the total ionization energy I i and the valence zi of ion i, as follows: for a-group's ion; c i = Ii/zi, eV for b-group's ion; c i = 0.936{(Ii/zi) - 0.94}, eV and f o r c-group's ion; c i = 0.862{(Ii/zi) 1.43}, eV.
-
(3a) (3b) (3c)
3. The Selectivities of Some Metal Pyrophosphates for Oxidation I so-butane
of
The oxidation of iso-butane was studied by use of various metal pyrophosphates[4]. As the products of this reaction, iso-butene ('C4), propylene('C3), CO and CO2(CO,) were observed. The relations between the selectivities for the formations of 'Cq+'C3 and for that of CO+CO2, and c i are shown in Figure 1. It is found that the oxidation reaction of iso-butane is including the two types of catalytic reactions, one of which is accelarated with increasing ci('C4 and 'C3 formation) and anothor is retarded with increasing ci(CO, formation). The correlation coefficients of these linear relations were found to be 0.985 for the former reaction and 0.981
Absolute Acidities of Monoatomic Cations in Heterogeneous Systems
493
for the l a t t e r , respectively, if the valencies of copper and chromium ions were assumed to be + 1 and +2. Such assumption may be reasonable because some kinds of cations often can be reduced in the conditions of lack of oxygen and excess of hydrocarbon.
ei
, eV
Fici.1 Relations between selectivities and the absolute acidity E~ of cation i of catalyst, for formations of 'C4+'C3(0) and of COX ( A ) in iso-buthane oxidation on metal-pyrophosphates;(O) and ( A ) i f Cr3+ and Cu2+ remain to be not reduced.
4.
The Effect of Component in Mixed Metallo-Cation Oxygen Coverage of Cu Surface in Methanol Synthesis
Catalyst
on
Methanol synthesis from C02 and H2 was investigated on the mixed catalysts of Cu, ZnO and A1203, or Cu, ZnO and GaaOj with the variety of the component ratio[5]. The absolute acidity C E of ~ a mixed cation catalyst was given by the mean value of E i t a k i n g account of the molar ratios and the valencies of the component cations. The relation &€wen oxygen coverage of Cu surface on the various catalysts and C c i is shown in Figure 2. The correlation coefficient of this line was found to be 0.979.
494
R. NOTOYA and A. MATSUDA
Q)
F 20 r
8 c
a Q)
10
5 0
10
20
30
40
Fig.2 Relation between oxygen coverage of Cu surface and mean value of the absolute acidities C s i of cations of oxides, on mixed oxide catalysts(Cu-ZnO-A1203 and Cu-ZnO-Ga203) in methanol synthesis. The slope of the line is understandable by reason that a catalyst having larger C c i can induce a negatively charged particle more strongly from Cu to himself. Conclusion
It is proved that catalysis actions in considerably complicated systems as like as heterogeneous one can be clarified by means of the absolute acidity as the measure of the catalytic activity. [l] R.Notoya and A.Matsuda,J.Phys.Chem.,l985,89,3922. [ 2 ] R.Notoya and A.Matsuda, Fundamental Problems of Interface Structure and Electrochemical Kinetics, Ed. A.Matsuda, S.Haruyama and R.Notoya, Hokkaido University Press,1988,p.41. [3] R.Notoya and A.Matsuda, Acid-Base Catalysis,Ed. K.Tanabe, H.Hattori, T.Yamaguchi and T.Tanaka, Kodansha Ltd.,1989,p.505. [ 4 ] Y.Takita, K.Kurosaki, S.Ichimaru, Y.Mizuhara and T.Ishihara, 71t:h CATSJ Meeting Abstracts:NolA2,Shokubai(Catalyst),l993,35,62. [5] T.Fujiya,M.Saito, Y.Kanai, M.Takeuchi,K.Moriya and T.Watanabe, 71t:h CATSJ Meeting Abstracts:No2A2,Shokubai(Catalyst),l993,35,92.
Abstract The absolute acidity E~ of a monoatomic cation i which had been expressed numerically in terms of the ionization energy of Ii of cation i on the basis of the empirical rules for solvation of monoatomic ions, was found to give a measure of the catalytic activity for many catalysis systems in both cases of acid and base catalysts. The present paper is concerned with the elucidation of the selectivity of catalyst in terms of this acidity in a few cases of heterogeneous systems. 1.Introduction Since the absolute acidity E~ of a monoatomic cation i was determined on the basis of the electron donor-acceptor concept for solvation[l], of E~ proved to be a measure of catalytic activity various catalysisC2-31. It was revealed that catalytic reactions were divided into two groups, one of which was accelerated with increasing E~ and on the contrary the other, retarded with increasing E i. In both groups, the relationships between E and the catalytic activity in the cases of the homogeneous catalysis, catalysis of metallo-porphyrines and zeolites showed more quantitativle than that in the case of heterogeneous one. ‘The present paper aims for revealing such relatioships as described above concerning not overall reaction but elementary acts or .partially grouped elementary steps even in a few cases of heterogeneous catalysisl4-51. We shall start by the basic concept for 49 1
492
R. NOTOYA and A. MATSUDA
the evaluation of the absolute acidity c i of cation i. 2. The Empirical Rules for Solvation of Monoatomic Cation
Two empirical rules were found to be present for the relationships, between chemical free energy AG"i,g+s of solvation of ion i for solvent s and that AG"i,g4w for water w , and between chemical free energy AG"i,gts of solvation of ion i for solvent s and the total ionization energy for ion i Ii, i.e.: the rule I ; A G " i , g t w = P AG"i,g+s and the rule 11; AG'i,g+s = p:.ci, where p equal to the ration p : / p t and p denote the propotional constant and the absolute basicity of solvent 6. All monoatomic cations on the periodic table are divided by the electronic configuration of ion i into three groups which are characterized by the electronic configuration of ion i, i.e. a-, b- and c-groups' ions which configurations can be specifyed by the rare gas type X, X+d3"lo and X+5d10, respectively. Each c i is given by the total ionization energy I i and the valence zi of ion i, as follows: for a-group's ion; c i = Ii/zi, eV for b-group's ion; c i = 0.936{(Ii/zi) - 0.94}, eV and f o r c-group's ion; c i = 0.862{(Ii/zi) 1.43}, eV.
-
(3a) (3b) (3c)
3. The Selectivities of Some Metal Pyrophosphates for Oxidation I so-butane
of
The oxidation of iso-butane was studied by use of various metal pyrophosphates[4]. As the products of this reaction, iso-butene ('C4), propylene('C3), CO and CO2(CO,) were observed. The relations between the selectivities for the formations of 'Cq+'C3 and for that of CO+CO2, and c i are shown in Figure 1. It is found that the oxidation reaction of iso-butane is including the two types of catalytic reactions, one of which is accelarated with increasing ci('C4 and 'C3 formation) and anothor is retarded with increasing ci(CO, formation). The correlation coefficients of these linear relations were found to be 0.985 for the former reaction and 0.981
Absolute Acidities of Monoatomic Cations in Heterogeneous Systems
493
for the l a t t e r , respectively, if the valencies of copper and chromium ions were assumed to be + 1 and +2. Such assumption may be reasonable because some kinds of cations often can be reduced in the conditions of lack of oxygen and excess of hydrocarbon.
ei
, eV
Fici.1 Relations between selectivities and the absolute acidity E~ of cation i of catalyst, for formations of 'C4+'C3(0) and of COX ( A ) in iso-buthane oxidation on metal-pyrophosphates;(O) and ( A ) i f Cr3+ and Cu2+ remain to be not reduced.
4.
The Effect of Component in Mixed Metallo-Cation Oxygen Coverage of Cu Surface in Methanol Synthesis
Catalyst
on
Methanol synthesis from C02 and H2 was investigated on the mixed catalysts of Cu, ZnO and A1203, or Cu, ZnO and GaaOj with the variety of the component ratio[5]. The absolute acidity C E of ~ a mixed cation catalyst was given by the mean value of E i t a k i n g account of the molar ratios and the valencies of the component cations. The relation &€wen oxygen coverage of Cu surface on the various catalysts and C c i is shown in Figure 2. The correlation coefficient of this line was found to be 0.979.
494
R. NOTOYA and A. MATSUDA
Q)
F 20 r
8 c
a Q)
10
5 0
10
20
30
40
Fig.2 Relation between oxygen coverage of Cu surface and mean value of the absolute acidities C s i of cations of oxides, on mixed oxide catalysts(Cu-ZnO-A1203 and Cu-ZnO-Ga203) in methanol synthesis. The slope of the line is understandable by reason that a catalyst having larger C c i can induce a negatively charged particle more strongly from Cu to himself. Conclusion
It is proved that catalysis actions in considerably complicated systems as like as heterogeneous one can be clarified by means of the absolute acidity as the measure of the catalytic activity. [l] R.Notoya and A.Matsuda,J.Phys.Chem.,l985,89,3922. [ 2 ] R.Notoya and A.Matsuda, Fundamental Problems of Interface Structure and Electrochemical Kinetics, Ed. A.Matsuda, S.Haruyama and R.Notoya, Hokkaido University Press,1988,p.41. [3] R.Notoya and A.Matsuda, Acid-Base Catalysis,Ed. K.Tanabe, H.Hattori, T.Yamaguchi and T.Tanaka, Kodansha Ltd.,1989,p.505. [ 4 ] Y.Takita, K.Kurosaki, S.Ichimaru, Y.Mizuhara and T.Ishihara, 71t:h CATSJ Meeting Abstracts:NolA2,Shokubai(Catalyst),l993,35,62. [5] T.Fujiya,M.Saito, Y.Kanai, M.Takeuchi,K.Moriya and T.Watanabe, 71t:h CATSJ Meeting Abstracts:No2A2,Shokubai(Catalyst),l993,35,92.