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Surface stabilized reaction intermediate: Formic anhydride
Surface Science 51 (1975) 546-548 0 North-Holland Publishing Company
SURFACE STABILIZED
REACTION
INTERMEDIATE:
FORMIC ANHYDRIDE Received 27 May 1975
The kinetics and mechanism of formic acid decomposition were studied on both clean and carburized Ni(1 IO) surfaces using flash decomposition spectroscopy and Auger electron spectroscopy [ 1,2] . Striking differences in selectivity and activity for the decomposition were observed for the two surfaces, though the reaction probability in both cases was near unity. The flash decomposition spectra for the reaction products following the adsorption of deuterated formic acid below room temperature on the carbided nickel surface indicated that both the carbon-bound hydrogen and the acid hydrogen reacted intermolecularly to form di-hydrogen [3]. No di-hydrogen was formed as a result of reactions of these two distinguishable hydrogen atoms with one another to form a mixed isotope. In this work similar experiments were performed for adsorption of HCOOD at low temperatures on the clean Ni(ll0) surface in order to determine the origin of the water formation. The apparatus and techniques used in this investigation were described previously [ 1] . The sample was heated by radiation from a tungsten filament and could be cooled to -90 “C by liquid nitrogen. The sample was cleaned by bombardment with 300 V argon ions and annealed at 500 “C until a clean Auger spectrum was obtained. Formic acid was exposed to the surface through a hypodermic syringe directed at the crystal face. The HCOOD was purchased from International Chemical and Nuclear as 98% pure and 98% deuterated. The freezing point of -13 “C was the same as that obtained for a mixture of 2% H20 in HCOOH and indicated the water impurity in HCOOD was 2% or less. NMR spectra showed 95% deuteration of the acid hydrogen. The HCOOD was further purified by many cycles of thawing, freezing and pumping until higher vapor pressure impurities were removed so that the vapor pressure at -45 “C did not vary with time. Before the surface was exposed to HCOOD the acid was flowed through the stainless steel syringe for thirty minutes at circa 80 pm to exchange any H20 or HCOOH on the walls. The mass spectrum of the purified gas was similar to that of Ropp and Melton [4] ; additional peaks were observed at mass 48,30 and 20, which indicated approximately 10 per cent of the formic acid was in the form of DCOOD. Flash decomposition of DCOOH adsorbed on clean Ni(ll0) at 37 ‘C resulted in CO,, D2 and CO products [2] ; the acid hydrogen desorbed immediately upon adsorption at this temperature and was not observed. Likewise, flash decomposition
R.J. Madix, J.L. FalconerjSurface stabilizedreaction intermediate
I
225
I
250
275
300
325 350 TEMF’ERATURE
375 (“K)
400
425
450
541
J 475
Fig. 1. Flash decomposition spectrum following HCOOD adsorption at -55°C on a clean nickel (110) surface. Coverage corresponds to 0.7 of saturation. Heating rate was 10 K s-l.
of adsorbed HCOOD yielded HZ, CO, and CO as the only products. Both the CO : CO, ratio and the D,: CO, ratio from DCOOH decomposition were observed to be 1 .O + 0.2, suggestive of an intermediate with the composition of formic anhydride. Fig. 1 shows the flash decomposition spectra for all observed products following adsorption of HCOOD at -55 “C. As on the carbide, low temperature adsorption on the clean surface allowed observation of the product containing the acid hydrogen; the acid hydrogen (D atom) desorbed entirely in the form of D,O. No HZ0 was detected; all the product observed at mass 18 was due to D,O cracking in the mass spectrometer ionizer to form DO +. Repeating the experiments with DCOOH yielded no D20/DCOOD(-55). The small D2 peaks at 365 and 310 K in fig. 1 were probably the result of D#ICOOD(-55) from the DCOOD impurity and D,/D,(-55) from background D, formed by cracking of the formic acid in the mass spectrometer and the ion pump, respectively [2]. The large HZ peak was observed at 32’“C corresponding identically to Hz/Hz(-55). This peak was due to the presence of background Hz in the system which exhibited a sticking probability near unity at -55 “C. The selective evolution of water via an intermolecular reaction and the stoichiometric quantities of hydrogen, carbon monoxide and carbon dioxide evolved in the decomposition on the clean surface strongly suggest the formation of formic acid anhydride as the stable reaction intermediate following room temperature adsorption. The presence of only a single D#C!OOH(37) [5] peak also precludes the existence of separate DCO, DC00 intermediates. Department of Chemical Engineering, Stanford University, Stanford, California 94305,
R.J. MADIX and J.L. FALCONER* U.S.A.
* Current address: Stanford Research Institute, 333 Ravenswood Avenue, Menlo Park, California 94025, U.S.A.
R. J. Madix, J. L. Falconm/Surface
548
stabilized reaction intermediate
References [l] [2] [3] [4] [5]
J. McCarty, J. Falconer and R.J. Madix, J. Catalysis 30 (1973) 235. J.L. Falconer and R.J. Madix, Surface Sci. 46 (1974) 473. J. McCarty and R.J. Madix, J. Catalysis, in press. G.A. Ropp and G.E. Melton, J. Am. Chem. Sot. 80 (1958) 3504. For the flash desorption/decomposition reaction the notation A(or)/B(T) was used to denote desorption of the OLstate of gas A following adsorption of gas B at T’C.
SURFACE STABILIZED
REACTION
INTERMEDIATE:
FORMIC ANHYDRIDE Received 27 May 1975
The kinetics and mechanism of formic acid decomposition were studied on both clean and carburized Ni(1 IO) surfaces using flash decomposition spectroscopy and Auger electron spectroscopy [ 1,2] . Striking differences in selectivity and activity for the decomposition were observed for the two surfaces, though the reaction probability in both cases was near unity. The flash decomposition spectra for the reaction products following the adsorption of deuterated formic acid below room temperature on the carbided nickel surface indicated that both the carbon-bound hydrogen and the acid hydrogen reacted intermolecularly to form di-hydrogen [3]. No di-hydrogen was formed as a result of reactions of these two distinguishable hydrogen atoms with one another to form a mixed isotope. In this work similar experiments were performed for adsorption of HCOOD at low temperatures on the clean Ni(ll0) surface in order to determine the origin of the water formation. The apparatus and techniques used in this investigation were described previously [ 1] . The sample was heated by radiation from a tungsten filament and could be cooled to -90 “C by liquid nitrogen. The sample was cleaned by bombardment with 300 V argon ions and annealed at 500 “C until a clean Auger spectrum was obtained. Formic acid was exposed to the surface through a hypodermic syringe directed at the crystal face. The HCOOD was purchased from International Chemical and Nuclear as 98% pure and 98% deuterated. The freezing point of -13 “C was the same as that obtained for a mixture of 2% H20 in HCOOH and indicated the water impurity in HCOOD was 2% or less. NMR spectra showed 95% deuteration of the acid hydrogen. The HCOOD was further purified by many cycles of thawing, freezing and pumping until higher vapor pressure impurities were removed so that the vapor pressure at -45 “C did not vary with time. Before the surface was exposed to HCOOD the acid was flowed through the stainless steel syringe for thirty minutes at circa 80 pm to exchange any H20 or HCOOH on the walls. The mass spectrum of the purified gas was similar to that of Ropp and Melton [4] ; additional peaks were observed at mass 48,30 and 20, which indicated approximately 10 per cent of the formic acid was in the form of DCOOD. Flash decomposition of DCOOH adsorbed on clean Ni(ll0) at 37 ‘C resulted in CO,, D2 and CO products [2] ; the acid hydrogen desorbed immediately upon adsorption at this temperature and was not observed. Likewise, flash decomposition
R.J. Madix, J.L. FalconerjSurface stabilizedreaction intermediate
I
225
I
250
275
300
325 350 TEMF’ERATURE
375 (“K)
400
425
450
541
J 475
Fig. 1. Flash decomposition spectrum following HCOOD adsorption at -55°C on a clean nickel (110) surface. Coverage corresponds to 0.7 of saturation. Heating rate was 10 K s-l.
of adsorbed HCOOD yielded HZ, CO, and CO as the only products. Both the CO : CO, ratio and the D,: CO, ratio from DCOOH decomposition were observed to be 1 .O + 0.2, suggestive of an intermediate with the composition of formic anhydride. Fig. 1 shows the flash decomposition spectra for all observed products following adsorption of HCOOD at -55 “C. As on the carbide, low temperature adsorption on the clean surface allowed observation of the product containing the acid hydrogen; the acid hydrogen (D atom) desorbed entirely in the form of D,O. No HZ0 was detected; all the product observed at mass 18 was due to D,O cracking in the mass spectrometer ionizer to form DO +. Repeating the experiments with DCOOH yielded no D20/DCOOD(-55). The small D2 peaks at 365 and 310 K in fig. 1 were probably the result of D#ICOOD(-55) from the DCOOD impurity and D,/D,(-55) from background D, formed by cracking of the formic acid in the mass spectrometer and the ion pump, respectively [2]. The large HZ peak was observed at 32’“C corresponding identically to Hz/Hz(-55). This peak was due to the presence of background Hz in the system which exhibited a sticking probability near unity at -55 “C. The selective evolution of water via an intermolecular reaction and the stoichiometric quantities of hydrogen, carbon monoxide and carbon dioxide evolved in the decomposition on the clean surface strongly suggest the formation of formic acid anhydride as the stable reaction intermediate following room temperature adsorption. The presence of only a single D#C!OOH(37) [5] peak also precludes the existence of separate DCO, DC00 intermediates. Department of Chemical Engineering, Stanford University, Stanford, California 94305,
R.J. MADIX and J.L. FALCONER* U.S.A.
* Current address: Stanford Research Institute, 333 Ravenswood Avenue, Menlo Park, California 94025, U.S.A.
R. J. Madix, J. L. Falconm/Surface
548
stabilized reaction intermediate
References [l] [2] [3] [4] [5]
J. McCarty, J. Falconer and R.J. Madix, J. Catalysis 30 (1973) 235. J.L. Falconer and R.J. Madix, Surface Sci. 46 (1974) 473. J. McCarty and R.J. Madix, J. Catalysis, in press. G.A. Ropp and G.E. Melton, J. Am. Chem. Sot. 80 (1958) 3504. For the flash desorption/decomposition reaction the notation A(or)/B(T) was used to denote desorption of the OLstate of gas A following adsorption of gas B at T’C.