이산화탄소 수소화 반응에 사용된 Fe 촉매의 분광학적 분석 연구

홍지숙, 황진수, 이규완, 최명재

한국화학연구소 화학기술연구부 환경자원기술연구팀

Spectroscopic Analysis of Fe catalyst for CO2 Hydrogenation

Ji Sook Hong, Jin Soo Hwang, Kyu Wan Lee and Myoung Jae Choi

E & R Team, Korea Research Institute of Chemical Technology,

P.O Box 107, Yusong, Taejon 305-600, Korea

1. Introduction

The coprecipitated Fe based catalyst showed significant changes in the CO2 conversion and the hydrocarbon selectivity and the activity decreased after being used for CO2 hydrogenation[1]. In case of CO hydrogenation, catalyst aging is believed to be a result of factors  such as phase change (Fe3O4), sintering, fouling (coke/carbon deposit) and sulfur poisoning[2]. However, in case of CO2 hydrogenation, a few report has been appeared on the catalyst deactivation. In this study, to elucidate spectroscopically how the catalyst is deactivated during CO2 hydrogenation, the characterizations and reactions are being carried out over fresh catalyst, activated, deactivated catalyst and post-treated catalyst. Here, deactivated catalyst and activated catalyst mean the Fe-Cu based catalyst used in CO2 hydrogenation at 573K for 2150h and 100h. And post-treated catalyst was treated oxidation and H2 reduction after CO2 hydrogenation for 2150h in-situ state.

2. Experimental

2.1 Catalyst preparation and Characterization

The catalyst preparation involved two steps : preparation of the 100Fe : 6.6Cu : 15.7Al (wt. ratio) precursor followed by potassium impregnation. The catalyst precursor used in this study was prepared in one-batch by continuous precipitation from an aqueous solution of metal nitrates with ammonium hydroxide at pH value 6.8. The dried sample was loaded potassium with Fe/K ratio of 100/6 by incipient wetness technique. The composition of calcined catalyst was determined by using ICP-AES before using. After pelletizing at 40000 psi in CIP(cold isostatic pressure), the sample obtained was crushed and sieved to the required particle size(50∼100mesh). And the prepared and used catalysts are characterization by means of X-ray Deffraction(XRD), M□ssbauer spectroscopy, X-ray photoelectron spectroscopy(XPS).

2.2 Reaction scheme and procedures

CO2 hydrogenation reactions was conducted in a 1/2" fixed bed reactor. In experiment, 10g of the catalyst(50∼100mesh) was loaded into the reactor. Prior to reaction the catalyst was reduced in-situ with 10% H2/N2 mixture gas at 673 K and 81 psig (5.5 atm) for 6hrs. For the reactions, the flows rates of the reactant gases (H2 and CO2) as well as of internal standard gas (Ar) were controlled by thermal mass flow controllers (Brooks 5850E). For the hydrogenation of CO2, the following conditions were used : H2/CO2=3.0, pressure=10atm, space velocity= 1800ml/g-cat.h, temp.=573K and the reactions were carried out for 100hrs and 2150hrs. After 2100hrs, hydrogenation of CO2 were carried out continuously in-situ which the reduction for 24h at 673K, oxidation for 16h at 673K and then reduction for 24h at 673K, oxidation for at 16h at 723K and then reduction for 24h at 673K for the purpose of regeneration the deactivated catalyst.

3. Results

3.1 CO2 Hydrogenation

The CO2 hydrogenation was carried out under 300℃, 10atm and space velocity 1800ml/g-cat.h. In the course of CO2 hydrogenation, the catalytic activity for the production of hydrocarbons is changed in a similar way to the behavior of iron catalysts which is known in CO hydrogenation as the Fischer-Tropsch reaction.  Initially the activity increases slowly, next it reaches a maximum point, and then decreases slowly according to increasing time on stream. Because CO2 conversion and hydrocarbon yield reached the steady state at around 100h of time on stream, the catalyst was taken as the activated catalyst sample. After the catalyst had been employed in the reactin for 2150h, CO2 conversion decreased from 41.79% to 34.50%. Hydrocarbons yield decreased to 30.24% and instead of that CO increased to 3.78%. This catalyst was taken as the deactivated catalyst sample.

Fig. 1. Results of CO2 hydrogenation

3.2 XPS

A spectrum was synthesized by using means of mixed Lorentzian/Gaussian function. The quality of the curve fit obtained could be evaluated by value of the weighed □2. The Fe 2p3/2 peak at 712.2eV in fresh catalyst corresponds to Fe3+. The 1eV shift to lower binding energies after CO2 hydrogenation is consistent with a lowering of the oxidation state[3]. During the CO2 hydrogenation, the surface composition of C increases drastically time on stream. These can be explained that the surface C species including hydrocarbon and carbonaceous deposits is growing as the reaction going on. As the result of Table 1, on the layers of same ething times, compositions of Al and Cu were increased as the reaction going on. But K was decreased, reversely. From these results, the surface compositions of activated and deactivated catalyst do not like same. It seems possible that the rearrangement of surface is one of the reasons of deactivation.

Table 1. The weight ratio for catalyst surface

Element	States	Etching Times
		1	2	3	4	5	6	7	8	9	10
Fe (100)	Fresh	100	100	100	100	100	100	100	100	100	100
	Activated	100	100	100	100	100	100	100	100	100	100
	Deactivated	100	100	100	100	100	100	100	100	100	100
Al (15.7)	Fresh	143	119	112	102	88	95	98	95	90	85
	Activated	127	119	98	89	84	81	75	89	84	70
	Deactivated	142	185	171	117	154	131	140	99	122	77
Cu (6.7)	Fresh	9	8	6	7	6	4	5	4	5	5
	Activated	11	9	8	4	6	4	4	7	4	3
	Deactivated	6	13	9	10	14	12	16	9	12	18
K (6)	Fresh	72	49	34	28	22	21	19	18	16	14
	Activated	90	56	38	29	25	28	23	23	23	20
	Deactivated	61	38	21	15	13	15	7	0	0	0

3.3 M□ssbauer spectroscopy

Fig. 2 is shown real spectrum during computer fitting. A computer program utilizing a non-linear least squares method was used to fit a number of correlated Lorentzian peaks to the data. Seven sextets were taken into consideration, in which two sextets correspond to the tetrahedral (t) and octahedral (o) sites in Fe3O4, Three sextets to three different sites in □-Fe5C2, and □-Fe lead to a single sextet. Iron bulk phase is not changed significantly during the course of the deactivation. But area of the post treated catalyst is deferent from that of activated and deactivated catalyst. It noted that this post treatment method was not suitable for the regeneration of deactivated Fe-Cu catalyst.

(a) Reduced Catalyst    (b) Activated Catalyst

(c) Deactivated Catalyst  (d) Post treated Catalyst

Fig. 2. M□ssbauer spectrums

3.4 XRD (X-ray diffraction)

The Fe bulk phase of catalysts were donfirmed by means of the X-ray diffraction. The Fe phase of deactivated catalyst which had been employed over 2150h consists of the H□gg carbide(□-Fe5C2) and magnetite(Fe3O4), as can be seen in Fig. 3. The diffraction peak aroun 36o corresponds to Fe3O4 and the diffractions between 39-50o are due to □-Fe5C2, which has been known as an active phase in F-T reaction. In Table 2, the crystal size of deactivated catalyst is larger than that of activated catalyst. These results like the proposal by Dry[4]. But post-treated catalyst does not returned to activated catalyst. From this result, the changing of crystal size is one of deactivation reasons.

Fig. 3. XRD patterns

Table 2. Crystal sizes of activated catalyst and deactivated catalyst.

	2 □	radian	degree	radian	size (Å)
Activated	35.64	0.62	0.89	0.02	94.44
	43.31	0.76	0.66	0.01	124.58
Deactivated	35.57	0.62	0.59	0.01	142.85
	43.34	0.76	0.51	0.01	162.25
Post treated	35.45	0.62	0.75	0.01	111.00
	43.26	0.76	0.90	0.02	91.00

5. Conclusion

From the results, it has been concluded that the growing of crystal having been acitivity in CO2 hydrogenation is major reason of deactivation on the Fe-Cu catalyst surface. The crystal size and composition of surface are changed but iron bulk phase is not changed significantly during the course of the deactivation. The post treatment method in this study, was not suitable for the regeneration of deactivated Fe-Cu catalyst.

Reference

1. J. S. Hwang et al., Applied Catalysis A: General, 1-6, 5316, 2000.

2. M. E. Dry, Catalysis science and Technology, Springer-Verlag, 195, 1981.

3. E. S. Lox, G. B. Marin, E. De Grave, P. Bussiere, Appl. Catal., 40, 197, 1988.

4. Mark E. Dry, Catalysis Letters, 7, 241, 1990.