Inorg. Phys. Theor. 2985

Decomposition of Formic Acid on Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt, Nickel, and Copper

By H. S. lnglis and Duncan Taylor," Chemistry Department, University of Edinburgh, Edinburgh EH9 3JJ

The catalytic decomposition of formic acid has been investigated on evaporated films of the first transition-series elements from titanium to copper. Zero-order rates were observed for the first 15-20% of the decomposition with acid pressures of 30 mm Hg at temperatures, depending on the metal, in the range 100-300". The product ratio CO,/H, was always unity, but the CO,/CO ratio varied in a twin-peaked manner across the series, with maxima at chromium and nickel. The latter product ratio w a s constant for a given metal irrespective of reaction temperature and percentage decomposition. Titanium was unique in that only dehydration of the acid was observed. The results are consistent with the occurrence, except with titanium, of a bimolecular transition state intermediate which breaks down to give both CO, and CO in proportions determined by the nature of the metal catalyst.

WHILE there is general agreement ld3 that the mechanism of the metal-catalysed deconiposition of formic acid in- volves adsorbed formate-ion species, it is not yet clear which, or how many, of the species formate ions, formic acid molecules, and protons derived froin the acid are involved in the rate-determining stage. The four kinetic isotope effects which have been reported for deuteriated formic acids and studies with 14C-labelled acid suggest that the transition state is most probably formed in a bimolecular process from a formate ion and a formic acid molecule. Furthermore, there is disagreement 2 9 3 9 5 9 7 as to the ratio CO,/CO in the initial products: the ratio appears to be influenced by the nature and cleanliness of the surface and by the occurrence of the water-gas shift reaction. To provide further data for a wide range of metals, the decomposition kinetics together with analysis of products have been investigated on evaporated films of the first transition-series elements from titanium to copper.

EXPERIMENTAL

The high-vacuum system previously described was modified by the addition of an ionisation gauge to monitor pressures during the preparation of metal films, and a calibrated thermistor pressure gauge (Experimental Ther- mistor Manometer, S.T.C. Ltd.) for use in metal-film surface- area measurements by the B.E.T. method at -196°C with krypton. For pressure measurements in decomposition runs, the Bourdon gauge was replaced by a glass spiral gauge.8 Metal films were prepared in a cylindrical reaction vessel, volume ca. 120 cm 3, fitted by means of a B19 joint with two glass-sheathed tungsten electrodes to which was attached either a hairpin filament made from 0.5 mm. diam. wire of the required metal (Ti, V, Fe, Co, Ni, Cu) or a tungsten spiral supporting chips of metal (Cr,Mn). After rigorous baking out of the reaction vessel and degassing of the metal, the latter was evaporated with the reaction vessel cooled in liquid nitrogen to give films of weight 10-60 mg. in about

1 P. Mars, J. J. F. Scholten, and P. Zwietering, Adv. Catalysis, 1963, 14, 35.

J. Fahrenfort, L. L. Van Keijen, and y. M. H. Sachtler, 'The Mechanism of Heterogeneous Catalysis, ed. J. H. de Boer, Elsevier, London, 1960, p. 23.

D. F. Quinn and D. Taylor, J. Chem. SOC., 1965, 5248. 4 T. Otaki, J . Chern. SOC. Japan, 1959, 80, 255; W. M. H,

Sachtler and J. H. de Boer, J . Phys. Chem., 1960, 64, 1579; K. Hirota, K. Kuwata, T. Otaki, and S. Asai, ' Actes 2me Congres Internationale de Catalyse (Paris 1960) ,' Technip, Paris, 1961, vol. 1, p. 809; W. M. H. Sachtler and J. Fahrenfort, ibid., p. 831; K. Tamaru, Trans. Faraduy SOC., 1968, 64, 522.

30 inin. During evaporation, the pressure did not exceed 10-6 mm Hg. Titanium of purity better than 99.7% was obtained from Imperial Metal Industries (Kynoch) Ltd ; all the other metals were obtained from Johnson and R!tattliey & Co. Ltd., as spectrographically standardised samples except vanadium which was 99.7% pure.

Immediately after preparation of a film, decomposition runs with purified formic acid were carried out in the manner previously described at temperatures, depending on the metal, in the range 100-300". Normally only one run, but occasionally two, could be carried out with a given film before marked deactivation and a significant change in the CO,/CO ratio in the products occurred. In some cases, e.g. Ti and Mn, after admission of the formic acid, a few per- cent decrcase in pressure was observed due to adsorption of the acid on the metal film. The effect was more pronounced the lower the temperature and with fresh than with used films. B.E.T. area measurements were carried out after the decomposition runs had been completed and after the rc- action vessel had been pumped out overnight. Film areas were typically 0.2 m per 20 mg. weight.

For all the metals except iron, with an initial formic acid pressure of ca. 30 mm Hg, the pressure (after any adsorption effect) increased linearly with time for the first 15-20% of the reaction, thus allowing zero-order rate constants to be calculated ; with lower initial pressures the reaction order was fractional. Rates were reproducible from film to film within & 5%. Rate constants and pre-exponential factors were calculated in units of molecules site-l s-l, assuming 1.6 x 1015 sites per cm2 of B.E.T. area for Cr, Mn, Co, and Ni, 1.5 x 1015 for Cu, 1.4 x 1015 for V and 1.0 x 10l5 for Ti. These site densities are average values calculated on the assumption that the films were randomly orientatcd and presented only low index planes to the gas phase and that each metal atom on the film surface constituted one site. Activation energies were calculated by the method of least squares from zero-order rate constants determined at a minimum of twelve temperatures.

With iron films, zero-order rates were not found under any conditions : at the higher temperatures the rate decreased continuously with time, while at lower temperatures occur- rence of the adsorption effect mentioned above, followed by irregular pressure increases and decreases, made it impossible to determine reliable rates for the first 15-20% of the re- action. However, if after admission of the formic acid, the

5 G.-M. Schwab and A. M. Watson, Trans. Fnraday SOC., 1964, 60, 1833.

6 A. Lawson, J. Catalysis, 1968, 11, 295. 7 D. K. Walton and F. H. Verhoek, Adu. (Catalysis, 1956, 9,

* S. G. Yorke, J . Sci. Instr., 1945, 22, 196. 682.

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2986 J. Chem. SOC. (A), 1969 reaction vessel temperature was raised slowly and the pres- sure and temperature read alternately at short intervals of time, rates could be obtained from two consecutive pressure readings for the mean temperature between the two. These rates gave good reproducible Arrhenius plots for fresh films from which activation energies were calculated by the method of least squares. Pre-exponential factors in mole- cules site-l s-l were calculated on the basis of 1.6 x 1015 sites per cm2 of B.E.T. area.

The ratios CO,/CO and CO,/H, in samples of the de- composition products were determined with a mass spectro- meter after the freezing out of water and residual formic acid in the sampling tube at - 79'. Measurements were made a t sevcral reaction temperatures and percentage decomposi- tions. The mass spectrometer was calibrated by use of synthetic mixtures of the gases. The CO,: CO ratio was also determined by cooling the reaction vessel to -196' as quickly as possible after the desired percentage decomposi- tion had occurred and noting the total pressure; the pressure was then read at room temperature and again at -79". After correction of all pressures to room temperature, the value from the -196' reading gave (CO + H,), and that from the -79' reading gave (CO + CO, + H,). Analysis of synthetic mixtures by this method showed that a correc- tion to the CO, value, obtained by difference, was necessary to allow for the preferential adsorption at -79" C of a monolayer of the gas on the metal film. The CO,/CO ratio could then be calculated since the mass spectrometer results showed that the C0,/H2 ratio in all cases was unity within &5yo. Both methods gave results for the CO2/CO ratio agreeing within & 5%.

RESULTS AND DISCUSSIOK

Activation energies and pre-exponential factors are shown in Figure 1. Two compensation effects are

l A

FIGURE 1 Compensation effects for the transition series metals Pre-exponential factors, A, are in units titanium to copper.

of molecules site-1 s.-l

indicated, one for the metals Ti, V, Cr, Mn, and Fe and a second for Co, Xi, and Cu. Relative catalytic activities,

I<. Tamaru, Trans. Favaday SOL, 1959, 55, 824, 1191. 10 D. A. Dowden and D. Wells, ' Actes 2me Congres Inter-

nationale de Catalyse (Paris 1960),' Technip, Paris, 1961, vol. 2, p. 1499.

expressed 2 as 103K/T,, where T, is the temperature a t which the zero-order rate has an arbitrarily selected value of 0.16 molecules site-l s-l, are as follows: Ti 1.29, V 1.34, Cr 1.61, Mn 1.44, Fe 1.66, Co 2.15, Ni2.20, and CU 2.16.

These values fit well onto the right-hand branch of a volcano-shaped curve, similar to that obtained by Fahrenfort et al. who plotted relative activities for a wide range of metals 2*9 against calculated values of the heats

T 'r

I

I \

< V Cr Mn Fe Co N i Cu FIGURE 2 Variation of product mole ratio CO, : CO across the

transition series from titanium to copper

of formation of the metal formates. Correlation between activity and heats of formation may therefore be inferred. On the other hand, a twin-peaked activity pattern with maximum activities a t chromium and nickel and a minimum at manganese is evident on passing along the series. This behaviour is roughly in accord with the pre- dictions of Dowden lo based on crystal-field theory. If smaller values of the rate, and hence of Ts, are used to calculate activities, while the maximum at nickel per- sists, the peak a t chromium is progressively depressed as T, decreases and high activity appears at titanium. The minimum a t manganese, however, is always found irre- spective of the value of T,. Similar twin-peaked activity patterns have been reported for para-hydrogen conver- sionll on the same series of metals and for hydrogen- deuterium exchange l2 on oxides of the metals.

For each metal, the CO2/CO ratios given in Figure 2 were independent, within the limits shown, both of re- action temperature and percentage decomposition. Since the ratios differ from those expected for the water- gas equilibrium, and since they are virtually constant for a given metal, indicating absence of the shift reaction, they are regarded as true values for the decomposition on fresh films. The constancy of each ratio also indicates the absence of two parallel overall reactions such as HC0,H _j_ CO, + H, and H-CO,H CO + H,O, for the latter reactions have different activation energies 2

l1 D. D. Eley and D. Shooter, Proc. Chewz. Soc., 1959, 315. l2 D. A. Dowden, N. MacKenzie, and B. M. W. Trapnell, Puoc.

Roy. Soc., 1956, A237, 245.

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Inorg. Phys. Theor.

and the CO,/CO ratios would not then be independent of temperature. The observations can best be explained on the basis of a bimolecular transition-state complex con- sisting of a formate ion and a formic acid molecule,5r6 and linked to one or more metal catalyst atoms. De- composition of the complex is believed to occur by a single path to give carbon dioxide, carbon monoxide, hydrogen, and water, the CO,/CO ratio being uniquely determined by the nature of the metal. The overall reaction can be represented by, 2HC02H--t x(C0, + H,) + (2 - x)(CO + H,O), where the value of x is characteristic of the catalyst. Figure 2 clearly shows twin-peaked dehydrogenation activity across the trans- ition series. In third and later runs on a given film, except for titanium, the CO,/CO ratio increased con- siderably, most probably due to contamination, and approached values reported for metal powders and alloys prepared by reduction of oxides.

On titanium, dehydration of the acid without any de- hydrogenation was always found. That this represents the primary reaction on the metal was confirmed by the observation that, when a 1 : 1 mixture of carbon dioxide and hydrogen was admitted to a fresh film a t a typical reaction temperature, a slow reaction occurred in which carbon dioxide was replaced by an equal quantity of the

2987

monoxide while hydrogen was absorbed by the titanium. No water appeared in the gas phase at any stage. Water, and hence carbon monoxide must, therefore be primary products of the decomposition on this metal, and it is therefore unnecessary to postulate the formation of a bimolecular intermediate in this case. Owing to the high affinity of titanium for oxygen, the rupture of a carbon- oxygen bond to give a CHO species may be the initial step in the mechanism rather than rupture of an oxygen- hydrogen bond to give a formate ion or radical.

Entropies of activation, calculated a t the Ts temper- atures from pre-exponential factors, are as follows in JK-l mol-l: Ti -217, V -163, Cr -25, hln -92, Fe -84, Co -92, Ni -117, and Cu -29. No correlation with the trend of activities or CO, : CO ratios across the series is evident. With the exception of the titanium and vanadium values, the entropy decreases are appreciably less than those reported for palladium l3 and gold.,

Acknowledgements are made to the S.R.C. for the award of a maintenance grant (to H. S. I.) and to Imperial Metal Industries (Kynoch) Ltd., for the gift of titanium wire samples.

[9/1001 Received, June 13th, 19691

l3 D. D. Eley and P. Luetic, Trans. Faraduy Soc., 1957, 53, 1483.

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