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aOH value is considered as a physicochemical constant. A silanol group =Si-OH (one OH group per one surface Si atom) is the most probable compound on the fully hydroxylated surface of amorphous silica. The deuterio-exchange method was used for measuring the silanol number of 16 samples of amorphous silica with different specific surface areas (from 11 to 905 m2 g-’), depending on the temperature of vacuum treatment. The values of aOH at a certain temperature of vacuum treatment are close for all samples under study and lowering
of the concentration of hydroxyl groups on the surfaces of different silicas under similar conditions of the heat treatment also proceeds similarly. Acknowledgment. I express deep gratitude to Prof. A. V. Kiselev for the statement of the problem and constant attention. I also thank A. A. Agzamkhodzhaev, R. L. Gorelik, K. Ya. Shengeliya, and others for their help in this work. Registry No. S O z , 7631-86-9.
Reversible Adsorption of Carbon Monoxide on Copper Oxide? A. V. Larin Institute of Physical Chemistry, Academy of Sciences of the USSR, Leninsky Prospect, 31, 117915 GSP, Moscow, USSR Received September 5, 1986 The reversible adsorption isotherm of CO on active sites of CuO at 293 K was measured by the ideal chromatography method. This isotherm of weak adsorption is satisfactorily approximated by the Langmuir equation with the constants a, = 0.0003 mollkg and b = 370 m3/mol. Introduction The study of catalysts by adsorption techniques enables one to obtain the necessary information on the formation, location, and nature of active sites. It is known that carbon monoxide is irreversibly sorbed on CuO. This is evidenced by the results of the studies carried out by using various methods, mainly spectroscopic’ and adsorption2 methods. At the same time, the reversible physical adsorption or the weak adsorption of CO on CuO has been inadequately studied. The purpose of the present paper was to study the interaction of CO with CuO by employing gas chromatography. Experimental Section In this study we used CuO deposited on activated carbon AC-40 in an amount of 8% by decomposing cupric ammonium carbonate at -130 “C. The geometric micropore area of AC-40 is 455 m2/g, and the mesopore specific area of AC-40 is 100 m2/g. Into glass chromatographic columns with an inside diameter of 6 mm were packed up to 3 g of the prepared catalyst (the layer length was up to 20 cm). As the carrier gas we used air when working with
a thermochemical detector and helium when working with a katharometer. Concordant results were obtained here. The sample volume varied from 5 x up to 5 X cm3 (NTP). The relative error of the measurements was determined mainly by the accuracy of adding the CO dose with a syringe and did not exceed 110%. The experiment was conducted at 293 f 1 K.
Results and Discussion The deposition of copper oxide on activated carbon did not significantly change the adsorption capacity of nonpolar krypton and methane. The values of their specific ‘Presented at the “Kiselev Memorial Symposium”, 60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W.Sing and R. A. Pierotti, Chairmen.
0743-7463/87/2403-0318$01.50/0
retention volumes decreased less than 10% of the mass of the initial activated carbon, and the shape of chromatographic peaks remained almost unchanged and close to Gaussian distribution. When CO was introduced into the carrier-gas flow passing through the catalyst, the irreversible adsorption of its first impulses was observed. The irreversible adsorption of 0.0003 mol/kg stands in quantitative and qualitative correlation with spectroscopic’ and adsorption2 measurements. With more CO added, the complete elution of the samples was observed together with the sharp increase in CO adsorption in the low concentration region as compared with the starting activated carbon. The asymmetrical peaks are due to the nonlinear isotherm of physical adsorption. To evaluate the contributions of the nonlinear isotherm and the adsorption rate to the smearing of the chromatographic peak, the experiments with two size fractions of the catalyst (0.8-1.0 and 0.4-0.5 mm) in the air and helium flow were conducted in accordance with recommendat i o n ~ . ~The , ~ space velocity of the mobile phase varied within 20-80 cm3 (NTP)/min. In all cases the back boundaries of elution curves coincided (Figure 1). This proves that the relative length of the chromatographic column is too big and peak smearing is mainly determined (1) Stobie, R. W.;Dignam, M. J. Can. J. Chem. 1978,56, 1088. (2)Kholjavenko, K. M.; Rubanic, M. Ya.; Barishevskaya, I. M.; Chernukhina, N. A. Catalysis and Catalysts; Naukova Dumka: Kiev, 1965; p 187. (3)Kiselev, A. V.; Yashin, Ya. I. Gas Adsorption Chromatography; Nauka: Moscow, 1967. (4) Larin, A. V.Izu. Akad. N a u k SSSR, Chem. Ser. 1984, 1914.
0 1987 American
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Langmuir,Vol.3,No.3,1987 319
Reversible Adsorption of CO on CuO
91
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Figure 3. Adsorption isotherm for CO on the deposited copper oxide at 293 K as per Langmuir equation. time
Figure 1. Reversible elution of CO samples (differing in size) through the catalyst layer.
0
2
Figure 2. Adsorption isotherm for C O (1)on CuO/AC catalyst, calculated by the method of ideal chromatography; (2) on the support, AC, estimated by the change in the adsorbing capacity of inert gases; (3) directly on CuO, determined by the rule of isotherm addition.
by the adsorption isotherm. The adsorption isotherm for CO on the catalyst (Figure 2) was calculated by means of a standard techniques6 The isotherm for CO adsorption directly onto the deposited copper oxide was singled out by using the rule of isotherm addition. The adsorption isotherm of CO on an activated carbon support was approximated by the Henry equation. It was assumed that, similar to the results with inert gases, the value of the CO specific retention volume on activated carbon was decreased by 10% during copper deposition and amounts to 0.01 m3/kg. mol/m3) In the low concentration region (up to 5 x the adsorption isotherm of CO on copper oxide is satisfactorily approximated by the equation of the localized Langmuir adsorption (Figure 3) with the following constanta: a, = 0.0003 mol/kg; b = 370 m3/mol. The slope of the resultant isotherm in its starting portion, equal to the product amb, was 1 order of magnitude greater than ( 5 ) De Vault, D.J . Am. Chem. SOC.1943, 65, 532.
the slope of the CO adsorption isotherm on the starting AC-40. Since the values of CO chemisorption on the catalyst and a, coincide, it may be assumed that the reversible physical adsorption of CO takes place on the active sites of copper oxide, which were formed by the chemisorbed carbon monoxide. The number of active sites for the catalyst was calculated from a, and amounts to 0.03% of the total deposited copper oxide. The determination of a, from the reversible adsorption of CO on CuO is more reproducible than the number of active sites from the amount of the chemisorbed CO. The influence of the support, the conditions of the catalyst deposition, and other factors of the degree of branching of additives may be studied with the help of the proposed technique.
Conclusion The CO-CuO interaction was studied by gas chromatographic techniques. It was found out that besides chemisorption, the reversible weak physical adsorption of CO on CuO occurs. A coinciding of the back boundaries of the elution curves, obtained when various doses of CO were added, indicates that the smearing of the elution curve is determined mainly by the adsorption isotherm. This isotherm of weak adsorption, calculated according the ideal chromatography equation, is satisfactorily approximated by the Langmuir localized adsorption equation with the following constants: a, = 0.0003 mol/kg; b = 370 m3/moi. The proposed technique may be used for calculation of the number of active sites of catalysts. The number of active sites for the studied catalyst CuO/AC-40 is around 0.03% of the deposited copper oxide. Acknowledgment. The data on reversible adsorption of CO on active sites of copper oxide were submitted by the author to professor A. V. Kiselev in 1981. Professor Kiselev displayed a keen interest in possible use of chromatography for studying active sites of catalysts. This paper is prepared in accordance with professor Kiselev's valuable recommendations. Registry No. CO,630-08-0; CuO, 1317-38-0.
