J . Phys. Chem. 1986, 90, 4217-4218
Potassium Promotlon of Supported V,O, Catalysts
TABLE I: Molecular Hardness Calculated by Eq 3 and the Correspondence with the Experimental Valueso molecule I2
IBr s 2
Br2 Cl2 p2
so c2
CH CN 0 2
OH NH F2
cs2 cos
SO2 0 3 "2
N20 PBr3 PC13 POC1, CHJ
so3
CF31 C2H2 CF3Br CH3 HNO,
a,. eV
3.70 3.96 4.14 4.24 4.70 4.87 5.01 4.99 5.66 6.02 6.08 6.24 6.83 7.01 4.40 5.01 5.34 6.07 6.69 6.85 4.38 4.74 4.98 5.47 5.52 5.76 5.66 5.92 6.03 6.37
a.. eV 3.49 3.62 3.87 3.98 4.54 4.47 4.43 4.53 4.70 5.34 5.81 5.67 6.37 6.31 4.54 5.36 5.64 5.42 6.01 5.71 4.12 4.55 5.00 4.67 4.65 4.41 5.49 5.45 4.88 5.23
9J9.
1.06 1.09 1.07 1.06 1.03 1.09 1.13 1.10 1.20 1.13 1.05 1.10 1.07 1.11 0.97 0.93 0.95 1.12 1.11 1.20 1.06 1.04 1 .oo 1.17 1.19 1.31 1.03 1.09 1.24 1.22
For the meaning of the symbols, see text.
Weinstein.Io We want to derive a similar principle in terms of hardness. A very significant recent observation is that qo a xo. The proportionality holds for atomsl*8and molecules' as well, and the proportionality constant y' takes the same valuelJ1 for atoms and molecules. With this finding, Yang et al. have described the molecular softness as the average of atomic softnesses. We start with eq 2, and by substituting xM by qM/y', we obtain (3)
The implications of eq 3 are same as those of eq 2. The hardness of all the atoms is equalized in a molecule and equilibrium value is given by their geometric mean. From eq 3, the hardnesses ( q c )of various molecules have been calculated from the knowledgeI2 of atomic hardnesses. These are given in Table I. The correspondence with the experimental values (qe)I3is reflected in the ratio qc/qe. The calculated values, in general, were found to be very close to what was found by Yang et al. It should be mentioned here that the average softness principle is essentially what can be called a harmonic mean principle for hardness equalization. Extension of Nalewajski's works to hardness shows that the geometric mean principle is valid under the conditions where the harmonic mean principle becomes valid. Anyway, it is realized that, because of the proportionality between hardness and electronegativity of a neutral species, on the formation of a molecule an equalization of the hardnesses of the constituent atoms is also achieved. (10) Politzer, P.; Weinstein, H. J . Chem. Phys. 1979, 71, 4218. (11) What is y in ref 1 is 27' in our case because of the factor of 2 incorporated in the definition of 9. (12) All experimental data are taken from ref 1 . (13) ae = (I, - A,)/2 where I, and A, are experimental ionization potential and electron affinity, respectively.
Department of Chemistry Manipur University Irnphal 795 003, India
4217
Sir: The bond strength of lattice oxygen is an important parameter influencing the activity and selectivity of oxidation catalysts.' In a recent investigation2 we successfully prepared monolayers and double layers of V205supported on various carriers. Complementing other characterization techniques,2 ESR spectroscopy has been applied to study these catalyst system^.^ From the ESR results, a correlation between the catalytic selectivity for methanol oxidation and the vanadyl ( V 4 ) bond strength was establi~hed.~ This strength was strongly influenced by the support material and was found to parallel the selectivity to formaldehyde: a stronger V = O bond results in a higher yield for partial oxidation and less total oxidation products. Here we extend the previous studies2v3performed on unpromoted samples to potassium-promoted catalysts. The aim is to investigate the influence of K+ promotion on the selectivity and to interpret the observed trends in terms of reducibility and ESR parameters. Preparation of the supported V205catalysts has been described previ~usly.~.~ Potassium was introduced into the system by subsequent impregnation with a methanolic solution of KOCH3. This reactant was immobilized by reaction with the OH groups on the catalyst surface. Following impregnation samples were calcined in a stream of dry air at 573 K. Vanadium(V) was determined by iodine titration, and the potassium content was measured by acid titration. Results given in Table I indicate that some of the V5+ was dissolved off the surface during the K+ impregnation procedure. Textural properties of the carriers and the impregnated samples, the apparatus and conditions used in temperature-programmed reduction (TPR), and methanol oxidation experiments have been reported p r e v i o ~ s l y . ~ ~ ~ ESR equipment and measurements have been described in ref 3. From the analysis of the powder spectra principal values of the g and A tensors have been determined. The deviations from the free electron g value, Agll= gll- g, and Agl = g, - g,, have been used to calculate the ratio Agll/Agl.This ratio was found3 to be a measure of tetragonal distortion of the local coordination polyhedron. An increase in Agl,/Agl was established to correspond to a strengthening of the V=O bond. Results from unpromoted and K+-promoted V2OSlayers supported on A1203and MgO carriers are presented in Table I. A comparison of the ratio Ag,,/Ag, determined for the various types of catalysts demonstrates that potassium promotion increases the V=O bond strength in the A1203-supportedsystems, while on MgO a less significant decrease of the bond strength is observed. The TPR results parallel these findings: the temperature T , of maximum hydrogen consumption is markedly increased by the K+ promotion on A1203and is slightly decreased by promotion on MgO. It is most interesting to compare these characteristic properties with the performance of the catalysts in methanol oxidation. Selectivities have been measured at less than 10% conversion. Note that the increase in the V=O bond strength on A1203results in an increase from 3% to 40% in the selectivity to formaldehyde. Magnesia-supported catalysts are considerably more selective than the A1203-supportedsystems. Potassium promotion on MgO does not result in marked changes of the selectivity. This behavior is not surprising in view of the comparatively small coocentration of K+ on these samples, which is reflected in the weak changes of T,,,and Agll/Aglresulting from promotion. The slight decrease in selectivity from 87% to 81% parallels the slight weakening of the V=O bond strength, which is indicated by both the TPR and ESR parameters.
Dipankar Datta
Received: February 10, 1986 0022-3654/86/2090-4217$01.50/0
(1) Sachtler, W. H. M.; Dorgelo, G. J. W.; Fahrenfort, J.; Voorhoeve, R. J. H.Proc.'Int. Congr. Catal., 4th, 1971, 1968, 454. ( 2 ) Kijenski, J.; Baiker, A.; Glinski, M.; Sharma, V. K.; Dollenmeier, P.; Wokaun, A. J . Catal., in press.
0 1986 American Chemical Society
4218
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Additions and Corrections
TABLE I: Rmults Obtained with Unpromoted and K+-PromotedV,O, Layers Supported on Alumina and Magnesia catalyst' A19OrV AI;O;-2V A1203-V-Kt A120,-2V-Kt MOO-V MgO-2V MgO-V-Kt MgO-2V-Kt
[Vst], mmol/(g cat.) 0.34 0.63 0.25 0.47 0.22 0.29 0.15 0.22
[K'], mmol/(g cat.)
4.8 3.6 0.44 0.72
Tm,bK
780 770 840 f 5 840 f 5 840 860 825 825
Agll/Ag, after TPR 1.1 1.1 1.7 1.5 1.3 1.2 1.4
selectivity to formaldehyde, % 7 1538 K) 3 (553 K j 41 (553 K) 81 (568 K) 87 (560 K) 81 (560 K)
T,,, represents the temperature of maximum hydrogen cona Monolayers and double-layer catalysts are denoted by -V and -2V, respectively. sumption in the TPR profiles. TPR profiles of promoted and unpromoted catalysts exhibited a similar shape.2
Our results extend the previously established correlation between the vanadyl bond strength and the partial oxidation selectivity. In a comparative study of Vz05 supported on various carriers we had found (i) that both AgulAg, and T,,, can be used as a measure for the V=O bond strength and (ii) that an increase of these quantities corresponded to an increase in the selectivity to formaldehyde. Here we have shown that this correlation is equally valid in the comparison of promoted and unpromoted V,05 systems.
(3) Shanna,V.K.;Wohun, A.; Baiker, A. J. Phys. Chem. 1986,90,2715. (4) Monti, D.; Baiker, A. J. Catal. 1985, 83, 323.
Acknowledgment. Financial support by the Swiss National Science Foundation is gratefully acknowledged. Department of Industrial and Engineering Chemistry Swiss Federal Institute of Technology ETH-Zentrum, CH-8092 Ziirich, Switzerland Institute of Physical Chemistry University of Bayreuth 0-8580 Bayreuth. FRG Received: February 5, I986
A. Baiker*+ M. GLinski* J. Kijenski' V. K. Sharmat A. Wokaud
Swiss Federal Institute of Technology. 'On leave from the Technical University of Warsaw, Poland. f University of Bayreuth.
ADDITIONS AND CORRECTIONS 1985, Volume 89
Jamey K. Hovey* and Peter R. Tremaine*: Thermodynamics of the Complexes of Aqueous Iron(III), Aluminum, and Several Divalent Cations with EDTA: Heat Capacities, Volumes, and Variations in Stability with Temperature. Pages 5541-5549. The ionic scale used in this work was C;"(H+,aq) = -63 J K-' mol-', not -68 J K-' mol-' as reported. Equations 20, 22, and 23 should read
GgUl = -16.2Q2 J K-'mol-'
G&nmul = cpo- 63Q + 16.2Q2 Moreover, some values of ~$m,,, are incorrectly plotted in Figure 4. The discussion was based on the correct equations and the conclusions are unchanged. The corrected Figure is shown elsewhere' with additional data. (1) Hovey, J. K.; Hepler, L. G.; Tremaine, P. R., submitted to Solution Chem.
J.
