J. Phys. Chem. 1083, 87, 3531-3537
3531
Finally, it is worthwhile to note here the stability of FePc with respect to the application of rather large negative potentials close to hydrogen evolution. This is in apparent contrast to the behavior of cobalt tetrasulfonated phthalocyanine which exhibits hysteresis in the Raman spectra upon cathodic p~larization.~ Moreover, in contrast to the iron phthalocyanines, the orientation of the cobalt tetrasulfonated derivative appears to be perpendicular to the electrode surface rather than ~ a r a l l e l . ~
that of FePc; in particular, ita orientation is also parallel to that the metal surface and undergoes distortion with applied field in the same manner as FePc. One feature that is apparently more clearly observed with the watersoluble derivative involves the intensity changes of bands in the 850- and 1100-1200-cm-' region accompanying oxygenation. As mentioned previously the band at 846 cm-' becomes more intense than the 832-cm-' band upon O2 saturation of the solution. Likewise, the bands at 1125 and 1103 cm-' become more prominent than the one at 1139 cm-'. It is interesting to note that bands in the region of 800-900 cm-' are normally associated with 0-0stretching frequency for O2bonded in peroxo-type complexes, while bands at about 1120-1140 cm-' are associated with O2 in superoxo-type compounds.ls I t may not therefore be fortuitous that we see spectral changes in these regions with oxygenation and deoxygenation. We hesitate, however, to assign these bands to 0-0stretches since they appear to be present at all potentials even where O2should be cathodically reduced. They may in effect be FePc modes just perturbed by interaction with oxygen.
Acknowledgment. We gratefully acknowledge the financial support provided by the Division of Materials Science, Office of Basic Energy Sciences of the US.Department of Energy. We thank Dean Wilcox for the synthesis of the tetrasulfonated iron phthalocyanine derivative. C.B.R. and R.McM. thank the Division of Educational Programs of Argonne National Laboratory for financial support through ita student research participation program. Registry No. FePc, 132-16-1; FeTSPc, 59784-64-4; Au, 7440-57-5; CU,1440-50-8.
Photo-Kolbe Reaction at Gas-Solid Interfaces Shinri Sat0 Research Institute tor Cata&sis. HokkaMo Unlverslty, Sapporo 060, Japan (Received: October 13, 1982; In Final Form: January 28, 1983)
Photocatalytic decarboxylation of carboxylic acids, Le., the photo-Kolbe reaction, was found to take place in the gas phase over platinized anatase. Platinized rutile showed much less activity for the gas-phase reaction than platinized anatase. In the reaction of acetic acid the reaction rate and the relative yield of ethane to methane increased with increasing acetic acid pressure, the ethane selectivity being up to 50%. The addition of gas-phase water led to a marked acceleration of the reaction and to an improvement of the ethane selectivity up to 90%, while it had no effect on the reaction over anatase. Water also had an enhancement effect on the photodecomposition of gaseous formic acid and on the liquid-phasereaction of acetic acid. For the gas-phase reaction involving acetic acid and water, the rate was proportional to about the 0.7th power of light intensity and the selectivity for methane increased significantly when illumination intensity was reduced to less than about 3 % of full illumination.
Introduction Photoelectrochemical (PEC) processes at semiconductor electrodes have been intensively studied in view of their potential in solar-tochemical energy conversions and Bard and his co-workers' have shown that the principles of PEC cells can be applied to heterogeneous photocatalysis using powdered semiconductors. For a typical example, though titanium dioxide powder cannot photolyze water, platinized titanium dioxide (Pt/Ti02)does,2in which Pt functions as a cathode while Ti02functions as a photoanode. Thus,Pt/Ti02 is thought to consist of small short-circuited PEC cells. Various photocatalytic reactions using Pt/Ti02 catalyst have been investigated in recent years.' A remarkable feature of Pt/Ti02 and other metalized semiconductor systems is that these make it possible for PEC reactions to occur in the gas phase. For example, the (1) Bard, A. J. J.Phys. Chem. 1982,86, 172 and references therein. (2) (a) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72,83. (b) Sato, S.; White, J. M. J. Catal. 1981,69, 128. (c) Duonghong, D.; Borgarello, E.; GrHtzel, M. J. Am. Chem. SOC.1981, 103, 4685. 0022-3654/83/2087-3531$01 S o l 0
photodecomposition of gaseous water2band the reactions of gaseous water with hydrocarbon^,^ carbon m ~ n o x i d e , ~ and active carbon3i5have been shown to take place by a PEC mechanism. We found recently that the photodecarboxylation of acetic acid, the photo-Kolbe reaction, also occurs in the gas phase over Pt/Ti02 and Ti02.6 Although the liquid-phase photo-Kolbe reaction gives predominantly CH4 and C02irrespective of reaction conditions as reported by Kraeutler and Bard,' the rate and the selectivity of the gas-phase reaction are dependent upon the pressures of acetic acid and water. In particular the presence of gaseous water accelerates the reaction and markedly improves the selectivity for CzHG.This article details the gas-phase photo-Kolbe reaction over Pt/Ti02 and Ti02. (3) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 70, 131. (4) Sato, S.; White, J. M. J. Am. Chem. SOC.1980, 102, 7206. (5) Sato, S.; White, J. M. J. Phys. Chem. 1981, 85, 336. (6) Sato, S. J . Chem. SOC.,Chem. Commun. 1982,26. (7) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978,100,2239,5985. ( 8 ) Sato, S.; White, J. M. J. Phys. Chem. 1981, 85, 592.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983
3532
Sato I
I
0.4
I
1
0.6
- 0.3
-
L
L
-
-r-"
& 0.4
l-
e
?
:0.2
VI VI
f!
?!
a
a 02 0. I
n
40
00 Time (min)
120
Flgure 1. Photo-Kolbe reaction of gaseous acetic acid over F't/TiO, (anatase). The initial pressure of acetic acid is about 1 torr.
Experimental Section Anatase obtained from Kanto Chemical Co. was reduced (doped) in flowing H2 at 700 "C for 6 h and rutile (Furuuchi Chemical Co.) was also reduced at 750 "C for 6 h to enhance photocatalytic activity. The BET area of reduced anatase was 12 m2/g. The H2-doped anatase and rutile were then platinized by photodecomposition of hexachloroplatinic acid as described previ~usly.~ The amount of Pt loaded was about 2 wt %. The catalyst (0.3 g) was spread uniformly on the flat bottom (16 cm2) of a quartz reaction cell. The cell was connected to an evacuable, circulation system (212-mL volume) made of Pyrex glass, and the catalyst was outgassed at 200 "C for 2 h under a vacuum of less than 3 X lo4 torr (1torr = 133.3 N/m2). Carboxylic acids and distilled water were outgassed several times at about -100 "C. Glacial acetic acid thus outgassed was then dried by passing through silica gel which had been outgassed at 200 "C. After the reaction cell temperature was set with a water bath, the vapor of carboxylic acid was introduced into the reaction system at a desired pressure and the reaction was commenced by illuminating the catalyst with a 500-W high-pressure Hg lamp (Ushio UIV-570) that was filtered by a band-pass filter (Toshiba UV-D33S, 240-400 nm). In some experiments, neutral density filters were used to reduce light intensity. Reaction products were sampled at appropriate intervals and, after passage through a cold trap at about -100 "C to remove water and carboxylic acid, were analyzed with a mass spectrometer. The sensitivity of the mass spectrometer for each product was calibrated by using a gas mixture of known composition. The pressure in the system was measured with an ionization gauge, a Pirani gauge, a McLeod gauge, or a Hg manometer. In the reaction involving water, water vapor was added to the system before or during reaction. The pressure of water vapor introduced was usually about 24 torr (saturated vapor pressure at room temperature). All the reactions were carried out at room temperature. Results Decarboxylation of Gaseous Acetic Acid over P t / T i 0 2 . Photodecarboxylation of gaseous acetic acid over Pt/TiOz produces COz, CHI, and small amounts of C2H6 and H2as shown in Figure 1, when the pressure of acetic acid is low.
40
60 80 100 Time (min) Flgure 2. Effect of gaseous H,O on the gas-phase photo-Kolbe reaction of acetic acid over R/Ti02 (anatase). The initial pressure of acetic acid is less than 0.5 torr. Because a fairly large amount of acetic acid adsorbs on the catalyst, the amount of products exceeds that present initially in the form of gaseous acetic acid (dimer). "0
20
The product distribution in Figure 1 shows that two reactions CH3C02H CH4 + C02 (1) 2CH3COzH CzH6 + H2 + 2C02 (2) take place simultaneously. The stoichiometry of reactions 1 and 2 requires that (3) PCH, + 2PC2H6 = pCOz
-
-+
PC2H6
H 'z
(4)
PcH,, etc., being the pressure of relevant species. This requirement is satisfied well in Figure 1,indicating that no side reaction is involved. The reaction rate and the relative yield of product are dependent upon the pressure of acetic acid introduced. In Figure 1the reaction rate is 0.2 torr/h and C2H,selectivity 7 %, while in the reaction shown in the previous papel.6 the former was 0.5 torr/h and the latter 33% at the initial acetic acid pressure of about 4 torr. Thus,the reaction rate and C2H6selectivity increase with increasing acetic acid pressure. Although an exact reaction kinetics was not established, the reaction rate is almost proportional to acetic acid pressure. (See also Figures 2 and 3.) The selectivity for C2H6 as well as the photocatalytic activity depends on the preparation and pretreatment of Pt/Ti02 catalyst. The photocatalytic activity of Pt/Ti02 slightly declined with repeating the reaction at room temperature. When outgassed at 200 "C after the reaction, the catalyst was remarkably deactivated (about 50% 1. The activity was, however, completely restored by heating the catalyst in H2 at 200 "C. The activity loss is therefore due to some poisoning of Pt with organic species. The gas-phase photo-Kolbe reaction is accelerated by the addition of gaseous H 2 0 as reported previously.6 Figure 2 shows the effect of H 2 0 addition on the photodecomposition of acetic acid whose initial pressure was less than 0.5 torr. Before H 2 0 addition products were almost exclusively CHI and cOz, and the formation rate of CzH6 was only 3 X torr/h. After H 2 0 addition the total reaction was more than 70 times accelerated and C2H,
Photo-Koibe Reaction at Gas-Solid Interfaces
12
The Journal of Physical Chemistfy, Vol. 87, No. 18, 1983 3533 TABLE I: Effect of H,O o n the Photodecomposition of Acetic Acid over Pt/TiO,O
-
;
run no.
pretreatment of catalyst
3
outgassed at 2 0 0 "C
4
outgassed at room temp €or 10 min
press. of
H,O, torr
relative rate
0
