May, 1962
PRESSURE OF GaaO OVER G A L L I U M - G ~MIXTURES ~O~
the chromia in such a way as to stabilize the Cr+6 electron configuration; this is consistent with the observation of Volta and Weller3 that potassium increases the surface oxidation state of a chromiaalumina catal;rst, possibly via the formation of potassium chromate or dichromate. The potassium-chromium complex, however, does not appear to be stable against reduction since, as shown in Table 11, treatment with hydrogen restored the effective moments of the promoted samples to that of the unpromoted catalyst. Since activities were determined with the catalysts in the reduced state it would not appear that the decrease in activity came about simply because of a change in average oxidation state of the chromia. However, despite the fact that the potassium-chromium complex is reduced, the po1,assium presumably remains at least physically associated with the chromia in some way since reoxidation of a promoted catalyst gave the same effective moment as was observed prior to the reduction. This potassium may exist in some form such as 820 on the surface of the reduced chromia and in this way block some of the dehydrogenation sites. In line with this it will be observed from the data of Table V that treating pure chromia with potassium decreased the reaction rate per. unit area, suggesting that! the same effect is possible when the chromia is supported on an alumina surface. Another possible explanation for the deactivation is that the potassium causes a sintering or clumping of the chromia crystallites on the alumina. It is interesting to note that pure chromia decreased in area when treated with potassium (Table I) and tha’. the Weiss constants of the reduced potassium promoted chromia-alumina catalysts increased slightly with potassium (Table 11), as would be expected if the chromia was becoming less well dispersed over the alumina surface. I n any case, it appeared that the deactivation could best be explained on the basis of a decrease in available chromia area either by blocking or sintering.
a77
Earlier work by Bridges, MacIver, and Tobin’l has shown that the amount of oxygen chemisorbed by a reduced chromia-alumina catalyst a t - 195’ is an approximate measure of the extent of chromia surface, that is, the portion of the total surface which is contributed by the chromia component. Similar oxygen chemisorption measurements made on the present series of promoted chromia-alumina catalysts are reported in Table I, where it may be seen from the data in column 7 that as the potassium concentration increases, the amount of oxygen chemisorbed per unit catalyst area decreases. This would seem to suggest that the potassium causes a decrease in the available chromia area which, as discussed above, is suflicieiit to explain the decline in dehydrogenation activity of the catalysts a t high potassium concentrations. It is of interest to note that the activation energy for the cyclohexane reaction was independent of both the chromium concentration and the level of potassium promotion (Table 111), suggesting that there is no change in the energetics of the reactantcatalyst interaction as these two factors are varied and that the promotion does not take place via some “electronic” factor entering into the activation energy. Using pure chromia, however, the activation energy was significantly greater (10 kcal./mole) than that over the supported chromia catalysts, which may be indicative of a basic chemical or physical difference between supported and unsupported chromia. A change in the lattice constants of the chromia when it is placed on a support, for example, possibly would be significant since the dehydrogenation reaction is thought to involve a dual site inechanism,21 which could be energetically dependent upon the distance between active sites on the catalyst ~urface.22~23 (21) E. F. G. Herington and E. K. Rideal, Proc. Roy. Soc. (London) 190A,289 (1947). 122) A. Sherman and H. Eyring, J. Am. Chem. Soc., 54, 2061 (1932). (23) A. M. Rubinshtein, S. G. Kulikov, and N. A. Pribytkova, Doklady Akad. N a u k S.S.S.R., 85, 121 (1952).
THE PRESSURE OF GazO OVER GALLIUM-Ga203 MIXTURES BY C. J. FROSCH AJSD C. D. THURJ~OND Bell Telephone Laboratories, Inc., Murray Hill,New Jersey Received October 88, 1961
The pressure of GaeO over a mixture of Ga(1) and GazOs(s)has been measured over the temperature range of 800 to 1000° by the transport method. A heat of formation at 298°K. for GasO(g) of -20.7 kcal. has been obtained.
We have measured the pressure of Ga20 over mixtures of Ca and Ca20sin the temperature range 800 t o 1000’ b y the transport method. The loss in weight of the mixture was measured as a function of time, teimperature, and flow rate. A pestled mixture of 5 moles of Ga to 1 mole of Gaz08 was placed in a fused silica boat which was heated in a fused silica tube under a helium gas stream. When this mixture was heated for a sufficient length of time it reached constant weight and all of the oxide had disappeared. The weight of Ga left was that calculated for the evaporation of GaaO.
Brukl’ has shown that the solid phase condensed from the vapor phase obtained by heating GaGa203mixtures has the composition GazO. Mass spectrometric evidence for the vapor species GaaO has been obtained by hiitkin and Dibelerz and others.3 The experimental results are shown in Fig. 1 where the logarithm of the ratio W / V hits been (1) D. A. Brukl. Z. anorg. allgem. Chem., 203,23 (1932) (2) S. Antkin and V. H. Dibeler, J . Chem. Phys., 21, 1890 (1953). (3) W. S. Chupka, J. Berkonitz, C . F. Giese, and M. G Inghram, J. Phys. Chem., 62, 611 (1958).
C. J. FROSCH ASD C. D. THCEMOND
878
ten.4 We propose that eq. 1 be used to extrapolate undersaturat'ed pressures a t finite flow rates to zero flow rate in order to obtain a good estimation of the equilibrium pressure under conditions where diffusional losses can be shown t o occur at lower and small flow rates. The pressures of GazO obtained a t the four temperatures are given in Table I. Included in the
- 1.25 -150
- I75
TABLE I
-2.00 To,K.
1073 1173 1223 1273
-2.25 -2.50 - 2 75
0
P G ~ Patm. O,
AHOzss
x 1.49 x 3.48 x 9.90 x 1.56
10-4 65.7 10-3 66.3 10-3 66.7 10-8 66.6 ( A.H02y8) = 66.3 kcal./mole ( A.HaGaZO)288 = -20.7 kcal./mole
table are the calculated values of AHoZg8 for the reaction
- 3.00 -3.25
Vol. 66
25
50
75
100 125 Y CCIMIN.
150
175 200
34 Ga(1) + 31 Gad&(s)
=
GazO(g)
(3)
I n evaluating AH0298, the free energy function for GanO(g) recommended by Cochran and Foster5 was used. They assumed that (S0298)Ga20(g) was plotted as a function of the flow rate u and at 69.54 e.u. from a comparison with other triatomic various temperatures to which the samples were gaseous molecules of comparable molecular weights heated: W is the weight loss of the sample when a and t,hat the heat capacity of GazO(g) was the same volume V of helium is passed over it. The curves as for CTeS(g).6v7 The free energy function used drawn through the points have been obtained from for Ga(1) came from Stull and Sinke.8 The free the two-constant equation energy function for GazOs(s) is given by Cochran and Fosterf5 who used aHTfand AFTf values by CoughlinJg and the free energy functions for the given by Stull and Sinke.8 A plot was made of log (1 - e--OlIu) us. u for various elements The value of 66.3 kcal. for the heat content values of CY. These curves were superimposed on the experimental results plotted as log W / V change of reaction 3 at 298'K., and the heat of formation of Ga203(s)at 298'K. of -261 kcal.,10 us. u and a curve found which fit the measurements. give a heat of formation of Ga20(g) at 298'K. This curve was used to extrapolate to zero flow rate to obtain (W/V)O. The pressure of GazO was of -20.7 kcal. This is in good agreement with a value of -20.4 kcal. obtained by Cochran and obtained from the ideal gas law equation Fost'er5 from a study of the reaction of SiOz(s) with Ga(l), and a value of -19.7 kcal. obtained by them from tlhe reaction of MgO(s) with Ga(1). We estimate the uncertainty in AHf298 or GazO(g) Equation 1 has not been used previously. We obtained this equation from a model which as- to be k2.5 kcal., t,his uncertainty arising from a 2 sumed that there was a uniform diffusion layer e.u. uncertainty in the estimated value of Sozg8 over the surface of the sample and that a steady- for GazO(g) . (4) U. Nerten, J . Phys. Chem., 63,443 (1959). state condition was reached between each incre( 5 ) C. N. Cochran and C. M. Foster, J . ~ ~ e c t r o c h e m Soc., . 109, 144 mental area of the sample surface perpendicular to the gas flow stream and the element of gas volume (1962). (6) S e K is probably a closer analogy to GazO than CTeS. Howabove this area. The constant a then equals ever, the heat capacity differs from t h a t of CTeS by such a small AD/26 where A is the sample area, D the dif- amount that a maximum of less than 0.2 e.u. occurs in the free energy fusion constant of GazO, and 6 the diffusion layer function over t h e temperature range of interest. This is negligible to an estimated error of rt2 e.u. arising from uncertainties in thickness. Reasonable values of CY were obtained. compared S%W. At flow rates less than 25 cc./min., it was found (7) K. K. Kelley, Bulletin 584, U. S. Bureau of Mines, 1960. (8) D. R. Stull and G. C. Sinke, "Thermodynamic Properties of the t'hat the diffusion of GazO to the cooler portions of the tube became the more important transport Elements," Amer. Chem. Sac., Washington, D. C., 1956. (9) J. P. Coughlin, U. S. Bureau of Mines, Bulletin 542, 1954. mechanism. L4tflow rates smaller than this, the (10) Private communication from K. K. Kelley. The value weight loss per unit volume of gas began to rise -261 & 0.3 kcal. has been measured recently by Mrs. Alla D. Mah. = I z 3 ked.' This process has been discussed by MerThe prior literature value was -258 rapidly. Figure 1.
