연료전지 자동차용 고성능 전이금속 탄화물계

Water Gas Shift 촉매 개발

문동주, Jeremy Patt*, Cory Phillips*, Levi Thompson*

한국과학기술연구원, CFC 대체연구센터

*미시간 대학교, 화학공학과

Development of High Performance Transition Metal Carbide

Water Gas Shift Catalyst for Automotive Applications

Dong Ju Moon, Jeremy Patt*, Cory Phillips* and Levi Thompson*

CFC Alternatives Research Center, Korea Institute of Science and Technology,

* Department of Chemical Engineering, University of Michigan, USA.

Introduction

The water gas shift (WGS) is the reaction of CO with H2O to form CO2 and H2.

CO +  H2O  →  CO2  +  H2 ,  □H=-41.1kJ mol-1     (1)

This reaction is used industrially to produce H2 for petroleum refining, chemicals production, and in processes that require synthetic gas with high H2/CO ratios.  Since the reaction is exothermic, the equilibrium CO conversion is the highest at low temperatures.  Consequently, a two-stage process is often used [1].  In industrial reactors, Fe3O4-Cr2O3 catalysts are used for the high temperature shift (HTS) and Cu-Zn/Al2O3 catalysts are used for the low temperature shift (LTS).

An emerging application for the WGS reaction is on-board purification and production of H2 for fuel cell powered vehicles.  Proton exchange membrane (PEM) fuel cells operating with H2 from fuel processor are being increasingly accepted as the most appropriate power source for future generation vehicles.  Recently, it was reported that the fuel processor performance is the critical technical barrier to achieving the Partnership for a New Generation of Vehicles (PNGV) efficiency targets for fuel cell vehicles [2].  The WGS reaction is a critical step during fuel processing since CO severely and irreversibly poisons the PEM electrocatalyst. Shift reactors, charged with currently available commercial catalysts, constitute about a third of the mass, volume, and cost of the fuel processor system [3].  The PNGV has set a goal to reduce the weight of the shift reactors by at least 75% [4].

The goal of the research was to develop better performing WGS catalysts for integration with PEM fuel cells.  Rather than seeking incremental improvements through the modification of known WGS formulations, new catalysts based on transition metal carbides(TMC) were investigated[5-8]. In this presentation, performance characteristics of the carbide catalysts are compared to those of commercial LTS/HTS catalysts.

Experimental

Synthesis of Catalyst

TMC catalysts were prepared via temperature programmed carburization of the corresponding oxides with an equimolar mixture of CH4 and H2.  The oxides were synthesized from ammonium paramolybdate (Alfa, 81-83% as MoO3) and ammonium tungstate (Aldrich, 99.99%).  The oxides calcined in dry air for 3 hrs at 500 □C were carburized in a temperature programmed manner. The synthesis conditions of the carbides are summarized in Table 1.

Characterization of catalyst

BET surface area and pore size distributions were characterized by N2 physisorption using a sorption analyzer(Micromeritics ASAP 2010). The calcination temperature was determined via thermal gravimetric analysis (TGA) using a Cahn Instruments TG-171.  The solid-state reaction pathway and the appropriate final temperature were determined using TGA in conjunction with X-ray diffraction (XRD). The XRD analysis was used to identify the prepared materials.  Temperature programmed reduction (TPR) experiments of the carbides were carried out by using an Altamira AMI-M, retrofitted with a Fisons Sensorlab 200D Mass Spectrometer.  The O2 Uptakes of the catalysts were measured using a Quantasorb Sorption Analyser(Model QS-17).

Evaluation of Catalytic Properties

The WGS reaction rates were measured at an atmospheric pressure and temperatures between 220 and 295 □C in a fixed bed reaction system.  A 0.15 g sample of catalyst was diluted with 0.1 g silica gel (Aldrich, high purity grade, 70-230 mesh) to attain a same bed volume.  The TMC catalyst was reduced at 400 □C for 4.5 hrs in H2.  The Cu-Zn/Al2O3 catalyst was reduced at 200 □C in a mixture of 2% H2 in N2 for 4.5 hrs.  The reactant gas contained 62.5% H2 (99.99% pure), 31.8% deionized H2O and 5.7% CO (99.5% pure).  A gas chromatograph (SRI Model 8610C) equipped with Porapak Q, molecular sieve columns and a thermal conductivity detector was used to determine the effluent compositions.

Results and Discussion

BET surface area and average pore size for the catalysts used in this work were summarized in Table 1.   The XRD patterns of the molybdenum oxides and the molybdenum carbides before and after the WGS reaction at 270℃ is shown in Fig. 1.  The XRD patterns indicate that the prepared molybdenum oxide is pure MoO3 phase, and the carburized molybdenum oxide is pure Mo2C phase.  The proper reduction temperature for the activation of the catalyst was determined via TPR experiment.   A 100 mg sample of the Mo2C catalyst was heated to 600 □C at 600 K hr-1 in a mixture of 5% H2 in Ar while the composition of the effluent was monitored.

Figure 2 shows the desorption rates of CO, CO2, H2, CH4 and H2O as a function of temperature.  The first peak of H2O is due to physisorbed water and the second peak corresponds to the reduction of oxygen deposited during passivation.  The results suggeste that a reduction temperature of 400 □C is adequate.

A blank run of the silica gel bed (0.34 cm3) showed no WGS activity.  There was no evidence of methanation(2) activity for carbide catalyst under the conditions employed.

CO + 3H2  →  CH4  +  H2O ,  □H=-206.2 kJ mol-1  (2)

The gravimetric CO consumption rates at 270℃ and the O2 uptakes are summarized in Table 2.  The Mo2C catalyst was significantly more active than the commercial Cu-Zn/Al2O3 catalyst over the temperature range explored.  An additional advantage associated with the use of the carbide is its high density, allowing for smaller bed volumes as shown Table 1.  The apparent activation energies were similar for the Mo2C (12.6 kcal mol-1) and Cu-Zn/Al2O3(15.4 kcal mol-1) catalysts within experimental error.  There was no decrease in WGS activity over the Mo2C catalyst during 48 hrs.  As shown in Fig. 1, there was no apparent modification of the bulk phase for the Mo carbide before and after the reaction.

The results demonstrate that the transition metal carbides are attractive candidates of WGS catalyst for automotive applications.  In addition to the high activities, these materials are very durable and are tolerant to sulfur.

Acknowledgements

Funding for this work from the Department of Energy CARAT program is greatly appreciated.  We thank Pat Davis (Department of Energy) and Jon Wagner (United Catalysts Inc.) for useful discussions.

References

[1]  K. Kochloefl, in: Handbook of Heterogeneous Catalysis, Vol. 4, eds. G. Ertl, H. Kn□zinger and J. Weitkamp (VCH, Weinheim, 1997) chap. 3.3.

[2]  S.G. Chalk, J. Milliken, J.F. Miller and S.R. Venkateswaran, J. Power Sources 71 (1998) 26.

[3]  Department of Energy CARAT Program Solicitation (1997).

[4]  T.J. Flynn, R.M. Privette, K.E. Kneidel, M.A Perna and D.L. King, Soc. Auto. Eng. Tech. Paper 1999-01-0536 (1999).

[5]  Jeremy Patt, Dong Ju Moon, Cory Phillips and L.T. Thompson, Catal. Lett., in press (1999).

[6]  L.T. Thompson, Jeremy Patt, Dong Ju Moon and Cory Phillips, U.S. Patent, in application (1999).

[7]  Dong Ju Moon, Jeremy Patt, Cory Phillips and L.T. Thompson, in submitted to J. of Catal. (2000).

[8]  Cory Phillips, Jeremy Patt, Dong Ju Moon and L.T. Thompson, in submitted to J. of Catal. (2000).

Table 1.  Synthesis conditions of the carbide catalysts and physical properties

of the catalysts used in this work.

Catalyst	Tf (℃)	BET area (m2/g)	Ave. Pore size (nm)	Density (g/cm3)
Mo2C	615	61	3	1.47
WC	640	27	9.2	2.21
LTS	-	60	14	1.18
HTS	-	57	24	0.93

Table 2. Comparison of gravimetric CO consumption rates at 270 ℃ and

characteristics of the catalysts.

Catalyst	CO consumption rate(□mol/g/sec)	O2 Uptake (□mol/g)	TOF (x 103, s-1)	E.A. (kcal/mol)
Mo2C	8.7	285	55.8	12.6
WC	0.5	-	-	-
LTS	3.1	82	84.3	15.4
HTS	0.6	320	4.3	15.4

Fig. 1. The XRD patterns for the       Fig. 2. Desorption rates of CO, CO2

Mo oxide and the Mo carbides          H2, CH4 and H2O during the

before and after the reaction            TPR of the Mo Carbide.

at 270 ℃.