Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
27
Oxidation of Carbon Monoxide on Vanadia Mireya R. Goldwasser and David L. Trirnrn" Department of Chemical Engineering and Chemical Technology, Imperial College, London S W 7, England
Studies have been made of the effect of support on the activity of vanadia used to catalyze the oxidation of carbon monoxide. Changes in the coordination of vanadia caused by t h e support affect both the adsorption of oxygen and the kinetics of the oxidation of carbon monoxide. The investigation falls into two parts. In the first, the development of a method of measuring the surface area of vanadia on a support is described: this is based on the reoxidation of reduced vanadia at 78 K. The second part describes measurements of the kinetics of oxidation of carbon monoxide on supported catalysts in which the coordination of vanadia is found to affect both the adsorption of reactants and the rate of t h e catalytic oxidation.
Introduction T h e choice of a catalyst support is generally based on the optimization of surface area, porosity, and mechanical strength. However, there is increasing evidence that catalyst-support interactions can affect the chemical nature of the solid and, possibly, the course of the reaction being catalyzed. Such interactions have been identified in a variety of supported metals (Sterba and Haensel, 1976; Kakati and Wilman, 1973), metal oxides (Andreikov et al., 1971; Asmolov and Krylov, 1972), and metal sulfides (Mitchel and Trifiro, 1974). Interaction can take several forms. In addition to catalytic interaction (bifunctional catalysis), a support may react with a catalyst to form a new chemical compound of different catalytic activity (Asmolov and Krylov, 1972) or may induce a particular structure in the catalyst without chemical reaction (Kakati and Wilman, 1973): this, in turn, may affect the adsorption and reaction of gases. One such example was reported by Shvets and Kazanskii (1971, 1972), who studied the adsorption of oxygen on a range of supported catalysts containing Ti, V, and Mo ions. Oxygen was found to be capable of adsorbing as the 02anion radical on vanadia supported on silica, as a result of electron transfer from tetrahedrally coordinated V4+ions to oxygen molecules (Van Reijen and Cossee, 1966; Vorotinzev et al., 1971). No 02 could be detected on vanadia supported on y-alumina, since the V4' coordination is either square pyramidal or octahedral, shortened by the formation of a vanadyl bond. Thus the support is inducing a given geometry on the catalyst and this is affecting the adsorptive properties of the vanadia. The present studies were initiated in order to study the effect of such changes on catalytic activity. Coordination changes can be expected to be most pronounced in a monolayer, and the investigation fell into two parts: measurements of surface area to establish that a monolayer is formed during preparation, and measurements of the kinetics of a catalytic reaction. For simplicity, the oxidation of carbon monoxide on supported vanadia was chosen as a test example. The catalytic oxidation of carbon monoxide on metal oxides has been extensively reviewed (Thomas and Thomas, 1967; Shelef et al., 1968), and the general mechanism is suggested to be
O2 + e-
-
02-
(1)
* T o whom inquiries should be made at the Laboratory of Industrial Chemistry, The University of Trondheim, N-7034 Trondheim-NTH, Norway. 0019-7890/79/1218-0027$01.00/0
02-+ e0- + CO
-
-
20-
(2)
COS+ e-
(3)
The catalyst may participate in the reaction by charge transfer to the adsorbate or by transfer of oxygen through a reduction-oxidation mechanism. The reaction mechanism over vanadia is less certain. Use of labeled oxygen suggests that oxygen originating from the catalyst is the active species (Hirota et al., 1955; Tarama et al., 1965). On the other hand, Marshneva et al. (1972) and Kon et al. (1971, 1972, 1973, 1974) prefer to describe the reaction in terms of adsorbed oxygen species reacting with adsorbed carbon monoxide. At least part of this disagreement may result from the fact that various forms of adsorbed oxygen may be produced (Dowden, 1967; Shvets et al., 1968 1969): Kon et al. (1972) have suggested a general sequence
O2 + eO2 + e0-
+ e-
-
-
02-
(1)
20-
(2)
02-
(4)
with increasing temperature favoring the conversion of 02and 02-. The adsorption of carbon monoxide on vanadia has been less well studied but, again, there appears to be the possibility of more than one adsorbed state (Gandhi and Shelef, 1972; Stone, 1962). The kinetics of the oxidation of carbon monoxide on vanadia have also received little attention. The reaction is reported to be zero order in oxygen (McHughes and Hill, 1955),and the catalytic activity passes through a maximum a t a catalyst involving 1 wt % vanadia on y-alumina. Rate constants and activation energies for reaction 3 have been reported by Kon et al. (1974) for a vanadia/silica catalyst.
Experimental Section Unsupported vanadium pentoxide was obtained by decomposing AnalaR ammonium metavanadate in air at 550 "C. Supported vanadium pentoxide was prepared by impregnating ammonium metavanadate on pure silica (L. Light & Co), AnalaR alumina (B.D.H.) and pure zirconia (B.D.H.). A slurry was prepared, dried at 120 "C for 6 h, and calcinated a t 550 "C for 9 h to give a 1wt 70supported vanadia. Gas adsorption experiments were carried out using a standard volumetric gas adsorption system (Gregg and Sing, 1967), based on the constant volume principle. Krypton adsorption was carried out using a slightly
0 1979 American
Chemical Society
28
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
Table I . Surface Area Measurements substance 'lo,
A1203 ZrO, SiO, V,O,/SiO, V,O,/ZrO, V,O ,/A1,0 a
S N ~m'k ,
16.8 i 1.2 3.13 i 0.32 110 i 30 10.8 i 2 45 i la 3 4 9 i 14 524 ? 420 i 1 6 51 * 3.0 143 * 4
S K ~m,' k 16.3 * 1.5 3.2 * 0.5
St, m ' k
Sa,m'/g
11.1* 1.2
419.5 52.0 140.6
460.7 44.6 111.9
vo (78 K), mLISTP)/g 0.84 i 0.02 0.31 t 0.01
vo (823 K),
3.73
2.77
i
0.3
56.9 t 2.3
0.56
t
0.1
96 i 4 5.21 i 1.56 3 5 . 7 t 1.0
7.9 i 0.6 16.4 t 1.6 13.0 i 1
t
0.47
mt(STP)/g 32.2 i 1.3 0.57 * 0.04
Samples treated in a similar way as the supported catalysts, but with no catalyst deposition.
modified form of the apparatus used by Aylmore and Jepsom (1961): 85Kr was used as the radioactive tracer. Catalytic studies were based on a flow reactor maintained at a pre-set temperature (3~0.5"C). The reactor was normally loaded with 5 g of catalyst prepared on a 20-30 mesh support. Gases were mixed and passed through the reactor to an on-line gas chromatograph. Carbon dioxide was separated on a silica gel column (40-60 mesh; 20 cm; 87 "C), while carbon monoxide, oxygen and nitrogen were separated on a molecular sieve 5A column (30-60 mesh; 1 m; 87 "C). Results a n d Discussion Gas Adsorption. Adsorption measurements for the determination of surface areas by the BET method are generally made within the range of relative pressure PIPo = 0.05-0.34 (Gregg and Sing, 1967). For V2O5/Al2O3, monomolecular coverage was completed at low relative pressures, and the BET equation gave a linear plot over the range PIPo = 0.02-0.1 for nitrogen adsorption a t 78 K. For V205/Si02and V2O5/ZrO2,the BET plots were linear over the range PIPo = 0.06-0.3. Surface areas calculated on the basis of the results are given in Table 1 (SNJ. If both monolayer adsorption and micropore filling are important at relative pressures
