Journal of the Less-Common
HYDROGENATION
TOSHIYA
Metals, 136 (1988)
217
217 - 225
OF CO OVER FeTi,.,,O,,,
AFTER ACTIVATION
HIRATA
National Research Institute for Metals, 2-3-12, Nakameguro, Tokyo, 153 (Japan) (Received
February
23,
1987;
in revised form
Meguro-ku,
April 29, 1987)
Summary
The hydrogenation’ of CO over the oxygen-stabilized hydrogen-storage catalytically activated by thermal treatment in pure alloy FeTii. 1400.03~ hydrogen, was investigated by gas chromatography, X-ray diffractometry, X-ray photoelectron spectroscopy and pressure differential scanning calorimetry, paying attention to active sites and changes in the alloy before and after hydrogenation. It is concluded that hydrogen atoms available when the cysolid solution is formed or when the /3hydride is formed and decomposed, play a governing role in the hydrogenation, producing hydrocarbons such as CH4 (predominant), C2H4 and n-C4Hlo above the catalytically active alloy. However, it was also found that the catalytically active alloy is deactivated on repeated hydrogenation. This is caused by the re-formation at the alloy surface of an oxide layer(s) which progressively blocks the more active sites for hydrogenmolecule dissociation.
1. Introduction
In the reaction between CO and Hz over a catalyst, it is considered that adsorption of Hz as well as CO is a prerequisite step for reaction [l, 21. Previous work [ 3,4] has demonstrated that the oxygen-stabilized hydrogenstorage alloy FeTi,.1400_,03 has some potential as a catalyst for the production of hydrocarbons by the reaction of H, with CO. It has also been shown that the hydrogenation of CO occurs only above an alloy surface which can dissociate hydrogen molecules. Thus, it is important to identify active sites for the dissociation of hydrogen molecules, and this is still a matter of controversy in FeTi and its related alloys [ 5 - 81. Furthermore, it is essential to examine changes in the alloy before and after hydrogenation, in order to control the catalytic activity and selectivity towards specified hydrocarbons. The present work is a continuation of the previous research [ 3,4], and its objective is to investigate the hydrogenation of CO over a catalytically @ Elsevier
Sequoia/Printed
in The Netherlands
218
active alloy FeTi,.,,O,,-,,, paying attention to active sites and changes in the alloy before and after the hydrogenation. 2. Experimental details The alloy FeTi,.1400.03 was prepared by arc-melting the constituents (99.9 wt.% iron, 99.9 wt.% titanium and FezOs powders) in a graphite crucible under an argon atmosphere. X-ray diffractometry confirmed that the alloy is a three-phase mixture of FeTi (the dominant phase), /3-Ti and an oxygen-stabilized compound whose composition is close to Fe,Ti,,Os or Fe5Ti,02 [9, lo]. This ternary alloy was developed to react with hydrogen more readily than FeTi [ 111. For catalytic studies, the powdered alloy (16 - 32 mesh) was used and impure hydrogen containing 410 ppm CO was used as a synthesis gas. The synthesis gas contained no gaseous impurities other than CO and Hz0 (about 50 ppm). The exit gases from the reactor were analysed during the hydrogenation by gas chromatographs equipped with thermal conductivity and flame ionization detectors (Shimazu GC 9A and GC 7A). X-ray diffraction and X-ray photoelectron spectroscopy (XPS) measurements were made in order to examine changes in the alloy before and after the hydrogenation. Pressure differential scanning calorimetry (PDSC) [ 121, was also used to obtain supplementary information concerning the hydrogenation. 3. Results 3.1. Preparation of the catalytically active alloy It was found previously [3, 41 that no hydrogenation of CO occurs over because the alloy is unreactive with the as-powdered alloy FeTi,_,,O,,s, hydrogen. Thermal treatments in hydrogen were necessary to prepare a catalytically active alloy that would allow the hydrogenation of CO to proceed (hereafter we refer to this series of treatments as activation). The changes in pressure are shown in Fig. 1 as the reactor charged with the as-powdered alloy was heated from room temperature to 320 “C at 2 “C min-’ under 40 bar of pure hydrogen, was furnace cooled to room temperature, i.e. the heating element was switched off, and then was reheated at 2 “C min-‘. The dissociation-pressure curve for the P-hydride phase in the FeTi-Hz system is also shown in Fig. 1 to indicate the domains in which the (x and/or fl phases can exist. It is clear that the pressure drops abruptly at 240 “C. The abrupt pressure drop has been ascribed to the adsorption and subsequent dissolution of hydrogen in FeTi1_i400.a3, which is preceded’by cracking caused by the differential expansion of the constituent phases in the alloy [ 13 3. This abrupt pressure drop was indispensable for the preparation of the catalytically active alloy as described previously [ 3, 4 3.
219
No significant disintegration of the alloy was found after the activation. The decrease and increase in the hydrogen pressure during the furnace cooling and/or reheating is responsible for the formation and desorption respectively of two hydride phases, /3and y, in the FeTi-H, system [ 141.
1
d OO Temperature,
‘C
100 200 Temperature
300
, ‘C
Fig. 1. Changes in the hydrogen pressure while the reactor was heated from room temperature to 320 “C at 2 “C mine1 under 40 bar of pure hydrogen and then was furnace cooled (0); the curve of dissociation pressure for the b-hydride phase in the FeTi-Hz system (chain line); and the pressure changes during reheating at 2 “C min-’ after the furnace cooling (0). Fig. 2. Plots of the concentration of CO, CH4 and CaH4 in the reactor exit gases us. the alloy temperature in the first hydrogenation after activation.
3.2. Hydrogenation of CO During hydrogenation above the catalytically active alloy, hydrocarbons such as CH4 (in predominance), CzH4 and n-C4H,, are produced and the concentration of CO decreases. The concentration of CO, CH4 and C&H4 in the exit gases from the reactor are plotted in Fig. 2 against the alloy temperature in the’first hydrogenation after activation; the hydrogenation was performed for a sample mass of 6 g with 4 bar of the synthesis gas. The synthesis gas was continuously introduced to the reactor to maintain a constant pressure after some synthesis gas has been used for analysis as well as purging of the reactor. It is clear that some amounts of CH4 and CzH4 are produced at room temperature after the activation, but they appear at an increasing rate around 150 “C. It is of particular interest to note that the concentration of CO exhibits a substantial decrease at the same time as exaggerated production of CH4 and CzH4 is observed at 100 “C; this phenomenon has been noted previously [3,4] and will be discussed later. The hydrogenation of CO was repeated over the used alloy under the same conditions as before after the reactor had been cooled from 340 “C to room temperature in a dynamic vacuum. On repeating the hydrogenation, the yield of hydrocarbons
220
decreased whereas the concentration of CO in the exit gases gradually approached the level of CO in the synthesis gas. The concentration ratio (CH,):(CO) is plotted against the alloy temperature for the first, second, third and fourth hydrogenations in Fig. 3. It is considered that the ratio (CH,):(CO) indicates an extent of the catalytic activity of the alloy for producing hydrocarbons. The ratio decreases as the hydrogenation was repeated, suggesting some deactivation of the alloy while the hydrogenation proceeds. With regard to the product distribution of hydrocarbons, it was found that the product selectivity shifts towards CH,, as the alloy temperature is raised, whereas other hydrocarbons tend to be produced below 120 “C in the first hydrogenation. Also, CH4 tended to be a predominant product below 120 “C when hydrogenation was repeated. 50
Temperature
,
‘C
Fig. 3. The concentration ratio (CH,)/(CO) third and fourth hydrogenations.
Temperature,
‘C
us. the alloy temperature in the first, second,
Fig. 4. PDSC curves for the catalytically active alloy (measured at 10 “C min-’ and 4 bar of synthesis gas), for (a) the first and (b) the second reactions.
3.3. Exothermic peak due to the hydrogenation PDSC curves for the catalytically active alloy measured at 10 “C min-’ and 4 bar of the synthesis gas are shown in Fig. 4. Curve (a) corresponds to the first reaction after the alloy was activated in the calorimeter by heating to 360 “C at 30 bar of pure hydrogen and then cooled to room temperature in a dynamic vacuum; curve(b) corresponds to the second reaction after the first reaction was stopped at 360 “C and the calorimeter was cooled to room temperature in a dynamic vacuum. An exothermic peak is observed over a temperature range between 220 “C and 340 “C, and this corresponds to the hydrogenation of CO, as seen in Fig. 2. No exothermic peak was observed when the catalytically active alloy was heated in pure hydrogen. In addition, the peak area became smaller with repeated hydrogenation; this is consistent with the observation that the
221
productivity of hydrocarbons decreases with repeated hydrogenation (see Fig. 3). Therefore, it is possible to attribute the observed exothermic peak to the hydrogenation of CO to produce hydrocarbons. In the present work, it should be noted that the exothermic peak is observed even when the alloy activated in the synthesis gas or the alloy used for the hydrogenation is then heated in pure hydrogen. This suggests carbon deposition on the alIoy surface during the activation in the synthesis gas as well as during hydrogenation. It is likely that these deposited carbon atoms cause the exothermic peak in question, by reacting with hydrogen atoms above the alloy surface. The gas chromatography confirmed that hydrocarbons are produced while the alloy activated in the synthesis gas or the used alloy was heated in pure hydrogen. 3.4. Changes in the alloyA.before and after the hydrogenation X-ray diffractometry revealed that no carbides are formed in the alloy after the hydrogenation of CO [ 151. X-ray diffraction patterns (Cu Kcwradiation; 34” - 50” for 28) were recorded for the as-powdered alloy (Fig. 5(a)), the catalytically active alloy (Fig. 5(b)) and the used alloy after four hydrogenation cycles (as shown in Fig. 3) (Fig. 5(c)). The catalytically active alloy was prepared by heating the as-powdered alloy to 330 “C under 31 bar of pure hydrogen and cooling to room temperature in a dynamic vacuum. It is evident that the X-ray diffraction pattern for the as-powdered alloy contains the reflection due to &Ti, FeTi and the oxygen-stabilized compound denoted by *O*. It is also clear that &Ti transforms to TiHz with
36 -Scattering
angle,.
34
28
Fig. 5. X-ray diffraction patterns for (a) the as-powdered alloy, (b) the catalytically active alloy and (c) the used alloy after four hydrogenation cycles had been performed (as shown in Fig. 3 (c)) (Cu KCVradiation; 34 - 50” for 26).
222 an f.c.c. structure and the oxygen-stabilized compound would disappear on activation. It should be noted that the X-ray diffraction pattern for the used alloy (Fig. 5(c)) is different from that for the catalytically active alloy (Fig. 5(b)), but rather it is the same as that for the as-powdered alloy. The X-ray diffraction pattern for the used alloy again contains reflections due to the oxygenstabilized compound whose lattice parameter (1.143 nm) is slightly larger than the 1.132 nm for the as-powdered alloy [9, lo]. No doubt, the oxygen-stabilized compound was re-formed by a reaction of the alloy with oxygen atoms dissociated from CO, and the re-formation of the oxide layers at the alloy surface is responsible for deactivation of the catalytically active alloy when hydrogenation was repeated. The peak of the FeTi (110) reflection in Fig. 5(b) or (c) is shifted to a smaller scattering angle compared with that in Fig. 5(a), indicating that small amounts of hydrogen atoms are dissolved into the FeTi during the activation and hydrogenation, The XPS spectra, over the binding energy range 450 - 720 eV, are shown in Fig. 6 for the as-powdered alloy (a), the catalytically active alloy (b) and the alloy used for the hydrogenation (c). Each alloy was pressed to a disk and then sputtered (to about 3 nm) by argon etching before XPS measurements (Mg Ko X-radiation). In Fig. 6, the ratio of the number of iron and titanium atoms is also indicated; the number of iron and titanium atoms was evaluated using values of atomic sensitivity factors based on peak area measurements for the titanium 2PJ,2 and iron 2P3,* lines.
FelTi
I1
720
1
I
I
I
-
I
710 Binding
*
.
..a..
.
5 1.0
.
.
700
460
Energy,
eV
4.I50
Fig. 6. XPS spectra over the binding energy range 450 - 720 eV for (a) the as-powdered alloy, (b) the catalytically active alloy, and (c) the used alloy.
223
The XPS spectra were also measured over the binding energy range of carbon. It was found that carbon is present in the alloy used for the hydrogenation as well as in the alloy activated in impure hydrogen containing CO, and that carbon almost disappears in the alloy after activation in pure hydrogen. It is clear that the XPS spectrum in Fig. 6(a) is almost the same as that in Fig. 6(c), whereas the spectrum in Fig. 6(b) is quite different. The photoelectron peak in Fig. 6(b) corresponds to that for the iron 2P3,2 .. line (706.75 eV) after a small correction to the binding energy. It seems that the peaks in Figs. 6(a) and (c) are convolutions of two photoelectron lines: the Fe,O, 2P,,* line and the iron 2P3,* line. The peak position of each line is indicated by an arrow in the figure. The peak position of each photoelectron line in Figs. 6(a) and (c) does not correspond exactly to that of the iron 2P3,2 ahd Fe,Os 2Ps,2 lines. However, the difference in the peak position betti-een each line is the same as the difference in the binding energy between the iron 2P3,2 and Fez03 2P3,2 lines. Thus, it is reasonable to attribute each photoelectron line in Figs. 6(a) and (c) to the iron 2P3,* and Fez03 2P3,2 lines. As far as titanium and its oxides are concerned, the photoelectron lines of the titanium 2Ps,2 and 2P1,2 are observed in Fig. 6(b), whereas the photoelectron lines corresponding to TiO, 2P,,* and TiO, 2P3,2 were observed in Figs. 6(a) and (c). Thus, it is evident that some ternary oxide phase exists in Fig. 6(a), is removed in Fig. 6(b) and then re-formed in Fig. 6(c), in accord with the results in Fig. 5.
4. Discussion Small amounts of CH4 and CzH4 are produced below 120 “C in the first hydrogenation after activation; on raising the temperature, however, their production increases. The production of these hydrocarbons was not significant below 120 “C in the suceeding reactions after the first hydrogenation. Curves of log,,C(CH,) are plotted in Fig. 7 for the first hydrogenation together with P(H,) and logi& as a function of temperature, where P(H,) and Sn are the dissociation pressure of the P-hydride phase and the solubility of hydrogen in the FeTi-H, system [16,17]. The solubility of hydrogen is given by the ratio of hydrogen atoms to metal atoms: Sn = nn/ (nn + nFe), where nn, nn and nre indicate the number of moles of hydrogen, titanium and iron atoms. The values of P(H,) and Sn were evaluated using the formulae log,, P(H,) = 4.8 - 1257/T [16], valid between 0 and 100 “C and log&& = +log&H,) -1.86 - 556/T [17] valid between 127 and 527 “C at P(H,) = 4 with P(H,) in bar and T in kelvin. No formation of the P-hydride phase can be expected if the applied hydrogen pressure P(H,) is lower than a certain critical value as then the
224
r
’
F
120
-2.5.
;: t; J0”-3.0,
-3.5. Temperature,
‘C
Fig. 7. The curves of logIOC(CH4) as a function of temperature in the first hydrogenation together with P(H2) and logI&(
hydride decomposes. At the pressure 4 bar at which the hydrogenation was carried out in this work, therefore, the /3hydride can be formed at T < 30 “C after activation, but it decomposes at T > 30 “C and with increasing speed around 100 “C (see Fig. 1); the applied hydrogen pressure is shown as PHI (exp.) in Fig. 7. The plot in Fig. 7 seems to indicate that the hydrogen atoms available when the p hydride is formed at T < 30 “C after activation and/or when it is decomposed when the temperature is raised (T > 30 “C), play an important role in the production of CH4 and CzH4 at temperatures below 120 “C (Fig. 2). The concentration decrease of CO and the pronounced production of CH4 and C&H4at 100 “C seen in Fig. 2 was misinterpreted in a previous work [ 41. It is now explained by the accelerated hydrogenation resulting from the hydrogen atoms which become availabIe when the p hydride is decomposed. Furthermore, it is of interest to note that the curve of log&(CHa) us. T above 120 “C agrees with that of logi,C(Sn) us. T. Particularly, it is noteworthy that the production curve of CH4 becomes level at about 250 “C, where the (x phase is completely saturated with dissolved hydrogen. The maximum concentration of hydrogen dissolved in FeTi is given by logn,S, (max.) = 1.1- 1340/T [17]. This equation gives T = 230 “C for Sn(max.) = 0.03 (the highest obtainable concentration of hydrogen in the cr phase of FeTi [16]). T = 230 “C is nearly equal to the temperature where production of CH4 ceases. Thus, we can conclude that hydrogen atoms available when the a! solid solution is formed or the fl hydride is formed and decomposed, play a governing role in the production of hydrocarbons by hydrogenation above the catalytically active alloy.
225
On repeating the hydrogenation, the productivity of hydrocarbons gradually decreased, suggesting some kind of deactivation of the alloy while the hydrogenation proceeds. The deactivation is responsible for the re-formation of the oxide layer(s) at the alloy surface which progressively blocks more active sites for the dissociation of hydrogen molecules. It is also of interest to identify active sites for the dissociation of hydrogen molecules. Schlapbach and coworkers [ 5,8] have demonstrated by activation treatment of FeTi and its related alloys that hydrogen molecules dissociate on surface-segregated iron clusters. However, some results [6,7, 131 have thrown doubt on the suggestion that iron clusters are active sites indispensable for the dissociation of hydrogen molecules. Khatamian et al. [18] emphasized the important role of surface oxides in the activation of FeTi, providing experimental evidence that the surface segregation of iron particles does not occur. ’ The present work-‘demonstrates that a reduction in the oxide phase present on the alloy surface is responsible for the activation. It is possible for hydrogen molecules to dissociate on clean subsurfaces which are exposed by the removal of the oxide layers. The ratio of the number of iron to titanium atoms on the catalytically active alloy surface was Fe:Ti x 1.0, indicating that no surface segregation of iron particles takes place during the activation of the alloy. Acknowledgments The author is grateful to Mr. H. Saito for assistance during experiments. He also thanks Drs. M. Tosa and I. Yoshirara for their XPS measurements. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R. C. Baetzold and J. R. Monnier, J. Phys. Chem., 90 (1986) 2944. F. Fischer and H. Tropsch, Brennst. Chem., 7 (1926) 97. T. Hirata, J. Mater. Sci. Lett., 5 (1986) 528. T. Hirata, J. Less-Common Met., 124 (1986) 11. L. Schlapbach and T. Riesterer, Appl. Phys., 32 (1983) 169. P. S. Rudman, J. Less-Common Met., 89 (1983) 93. T. Schober, J. Less-Common Met., 89 (1983) 63. G. Bush, L. Schlapbach and F. Stucki, Znt. J. Hydrogen Energy, 4 (1978) 29. W. Rostoker, Trans. AZME, 203 (1959) 113. T. Matsumoto and M. Amano, Ser. Metall., 15 (1981) 879. M. Amano, Y. Sasaki and T. Yoshioka, J. Jpn. Inst. Met., 45 (1981) 957. T. Hirata, Znt. J. Hydrogen Energy, 9 (1984) 855. T. Hirata, J. Less-Common Met., 130 (1987) 497. J. J. Reilly and R. H. Wiswall, Jr., Znorg. Chem., 13 (1974) 218. J. Fournier, L. Larreiro, Y.-T. Qian, S. Soled, R. Kershaw, K. Dwight and A. Void, J. Solid State Chem., 58 (1985) 211. 16 H. Wenzl and E. Lebsanft, J. Phys. F, 10 (1980) 2147. 17 J.-M. Welter, G. Arnold and H. Wenzl, J. Phys. F, 13 (1983) 1773. 18 D. Khatamian, G. C. Weatherly and F. D. Manchester, Acta Metall., 31 (1982) 1773.