Isotopic assessment of methanation over molybdenum sulfide catalysts

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Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 214-219

214

Isotopic Assessment of Methanation over Molybdenum Sulfide Catalysts John Happel," Motozo Yoshlklyo, Fushan Yln, Masood Otarod, and H. Y. Cheh Department of Chemical Engineering and Applied Chemistry. Columbia University, New York, New York 10027

Mlguel A. Hnatow and Lalmonls Bajars Catalysis Research Corporation, Palisades Park, New Jersey 07650

Howard S. Meyer Gas Research Institute, Chicago, Illinois 6063 1

A tracer technique was used to study the methanation of carbon monoxide over molybdenum sulfide based catalysts. I t was found that a high concentration of hydrogen is adsorbed on the working catalysts even at low ratios of hydrogen to carbon monoxide. The addition of sulfur and a stability-enhancing element (SEE) further increase hydrogen adsorption and activity over that of pure molybdenum sulfMe. Hydrogendeficient adsorbed carbonaceous radicals also appear to contribute to catalyst activity. Catalysts of this type are not poisoned by sulfur compounds and do not present problems of carbon formation.

Introduction A new process for the more direct methanation of synthesis gas to produce substitute natural gas (SNG) has recently been announced by Meyer et al. (1982). Conventional systems generally employ nickel catalysts, and methanation occurs according to the reaction 3H2 + CO + CH4 + H20 (1) Since most gasifiers do not generate synthesis gas at this high ratio of hydrogen to carbon monoxide, processing is required to alter the composition before the methanation reaction is conducted. Employment of molybdenum sulfide based catalysts causes the methanation to follow the reaction 2HZ + 2CO + CH4 + COZ (2) thus the necessity of using the water-gas-shift reaction CO + H20 + Hz + COZ (3) is avoided. In this way, an expensive process cycle that first requires the use of water in eq 3 and then produces it according to eq 1 is avoided. The employment of these catalysts enables operation at low hydrogen to carbon monoxide ratios that are not possible with nickel catalysts without the use of steam to prevent carbon deposition. Nickel is also extremely sensitive to poisoning by sulfur compounds that are always present in raw synthesis gas. They must be removed before methanation over nickel. We have previously reported kinetic studies for molybdenum sulfide catalyst formulations at the 1983 International Gas Research Conference (Happel et al., 1983). Typically the rate of methanation is approximately proportional to the 0.5 power of hydrogen partial pressure and first power of carbon monoxide partial pressure. In the 0196-4321/86/ 1225-0214$01.50/0

case of nickel the rate is proportional to the first power of hydrogen partial pressure and is zero or negative order in carbon monoxide partial pressure. Over nickel, carbon monoxide is very strongly adsorbed and appears to displace hydrogen from active sites. It has been known for some years that molybdenum sulfide based catalysts exhibit methanation activity, but commercial application has been in hydrogenation and hydrodesulfurization of petroleum fractions (Weisser and Landa, 1973). For this purpose Co-Mo-A1 catalysts have often beem employed, and there have been many studies of the mechanism of desulfurization and of the role of cobalt over such catalysts (Massoth, 1978). Molybdenum-based catalysts have generally exhibited low CO hydrogenation activity (Anderson, 1984). Recently, however, methods for the preparation of improved high surface area molybdenum sulfide catalysts for methanation have been reported by Naumann et al. (1982). A research program was initiated in 1973 at Catalysis Research Corp. (CRC) under the sponsorship of the Gas Research Institute with the object of further developing a high-activity sulfur-insensitive methanation catalyst. A standard evaluation procedure was employed for testing various formulations, using a packed-bed reactor containing 5-10 mL of catalyst and operated as close to isothermal conditions as possible with control by external electric heaters. Several hundred formulations were tested with molybdenum disulfide serving as the point of departure. Table I gives the results of evaluation of four typical catalysts. GRI-C-735 is a pure molybdenum disulfide catalyst made by first preparing the trisulfide by acidification of a solution of ammonium tetrathiomolybdate followed by thermal decomposition in a hydrogen stream at 450 "C for 1h according to the procedure of Furimsky 0 1986 American Chemical Society

Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 215 Table 1. Catalysts Used in Transient Tracing Studies

co convn, catalyst GRI-C-735 GRI-(2-740 GRI-C-763 GRI-C-726

compn

MoSi MoS; + S MoS, + SEE oxide MoS, + S + SEE oxide

temp, "C 548 543 561 569

and Amberg (1975) with slight modifications. In our procedure nitric acid was used for acidification instead of the hydrochloric acid they employed in order to avoid possible contamination by chloride. The precipitate was dried overnight at a temperature less than 110 "C. The dried trisulfide was rapidly reduced in an atmosphere of hydrogen containing 1vol % of hydrogen sulfide in order to avoid possible reduction of MoS2 Reduction temperature was 500 "C. The catalyst GRI-763 in which an SEE oxide hydrate was combined with molybdenum trisulfide was prepared by coprecipitation by acidification of solutions containing ammonium tetrathiomolybdate and SEE nitrate. Nitric acid is added to obtain a solution with pH 4. This acidity is sufficient to precipitate the trisulfide without dissolving the SEE hydrate coprecipitate. The precipitate was dried and reduced in the same manner as for production of the pure molybdenum disulfide catalyst (GRI-C-735). In the cases of the sulfur-containing catalysts GRI-C-740 and GRI-C-726 flowers of sulfur were added to the precipitate and mixed before it was filtered and dried. Drying and reduction followed the same procedure as for the first two catalysts. Other details of catalyst preparation are given by Happel et al. (1985). For the catalysts listed in Table I, the gas feed composition hydrogen to carbon monoxide ratio was 1.2. Operating conditions included a space velocity of 4800 v/v/hr (STP), a pressure of 1.48 MPa (200 psig), and bed temperatures up to a maximum of about 550 "C. H2S (1vol %) was added to the feed gas to simulate the sulfur content of raw synthesis gas. The higher conversion shown for GRI-C-726 represents production of a product stream with a considerably lower CO content than is obtained for GRI-C-735, pure molybdenum sulfide catalyst. Catalysts containing the SEE also exhibit improved thermal stability over a period of time. It was decided to further characterize these catalysts by transient tracing experiments in a program at Columbia University.

Methods Used in Transient Tracing The technique for transient tracer testing of methanation catalysts has been developed over several years in the course of study of a standard Harshaw nickel catalyst, typical of conventional catalysts for methanation. Details are reported in a series of papers (Happel et al., 1980,1982; Otarod et al., 1982). Catalyst evaluation is conducted at atmospheric pressure in a so-called gradientless recirculating reactor. The equipment employs a bellows pump that circulates gas through the system at a high rate, thus minimizing temperature and concentration gradients in the catalyst bed. Reacting gases are introduced into the circulating gas stream by a syringe pump. A separate flow of helium is used as a carrier gas, and the effluent stream is analyzed by a quadrupole mass spectrometer. When steady-state conversion is reached, rapid movement of the carriage on which two syringe pumps are mounted results in the instantaneous replacement of the feed mixture by one of exactly the same chemical composition marked with an

70

46.2 60.3 49.7 76.2

time on stream, min 315 350 180 285

BET initial surface area, m2/g 42.8 56.7 43.3 78.4

Table 11. Transfer of Deuterium to CHI in Absence of CO" deuterated methane produced, % time,min CHI CH3D CHzDz CHD, CD, 0 0 0 0 0 100 30 99+ 1 0 0 0 "Catalyst GRI-C-735 (MoS,). Reaction temp = 410 OC. Inlet deuterium flow rate = 2.53 mL/min (STP). Inlet methane flow rate = 0.63 mL/min (STP). Inlet helium flow rate = 31.33 mL/ min (STP).

appropriate isotope contained in one of the reactant species. A step function is thus introduced into the feed, but the overall reaction continues at steady state. Thus the catalyst can be studied in its working condition. Other types of reactors may be employed using the same technique of a step-function change from unmarked to marked feed. An interesting isotope jump technique described by Tamaru (1978) uses a closed circulating system. In some applications this requires preliminary rapid removal of the unmarked stream from the reactor, which might interfere with the steady-state reaction taking place. Also, in a constant-volumesystem, it is not strictly possible to operate at steady state. The latter is only reached at equilibrium, when no reaction is occurring. The use of a once-through plug-flow reactor for transient tracing instead of a recirculating reactor has the advantage that the dead space in the system is smaller and the response is therefore sharper (Bell and Winslow, 1984; Yang et al., 1984). A disadvantage with this method is the need to operate the reactor at a differential conversion to quantitatively interpret rate data. Molybdenum sulfide based catalysts have a tendency to become oxidized, particularly upon exposure to moist air for a period of time. When heated in the presence of hydrogen, SOz is released, resulting in loss of catalyst activity. This can be avoided by pretreatment with a mixture of CO and Hz, and steady-state methane production is reached after about 6 h at an operating temperature of 400-450 "C. This procedure was generally employed in testing a given catalyst over a period of time, rather than using a reduction treatment with pure hydrogen. In all cases the reacting gas mixture fed from the syringe pumps contained 1 vol 5% of HzS.

Results Exchange Reactions with Methane. In order to determine whether the last step in methanation is reversible, an exchange reaction was conducted at methanation temperature on a feed consisting of methane and deuterium. Results are given in Table 11. Very little exchange takes place, indicating that at least the last step in methanation is irreversible. Another experiment with a feed of equal proportions of 13C0and CHI indicated that no exchange of marked carbon takes place with methane at 410 "C. Studies conducted by Wilson and Kemball(1964) on the decomposition of methyl mercaptan by the reaction CHSSH + Hz * CH4 + H2S (4) over molybdenum disulfide catalyst showed that the rate of exchange of methane and deuterium is 40 times as slow

216

Ind. Eng. Chem. Prod. Res. Dev., Vol. 25,

'

No. 2, 1986

+

I

F-2V

2v

"I

U

+

Figure 1. Model for deuterium tracing.

as the rate of formation of methane from mercaptan at 300 "C. Deuterium Tracing. Preliminary studies using deuterium tracing for molybdenum sulfide methanation have been reported by Happel et al. (1984b). Comparison was made with methanation over nickel, and a model was used in which adsorbed carbon is hydrogenated in a series of unidirectional steps. Concentrations of CH, species were estimated in the present study by this procedure and were found to be much smaller for molybdenum sulfide than for nickel (0.004-0.024 mL/g (STP)). However, because of the high temperatures used in molybdenum sulfide methanation, surface hydrogen atoms are probably very mobile, so this model may not be reliable. Therefore, for results reported here it is assumed that the hydrogen distribution on the deuteriomethane radicals is at equilibrium with both hydrocarbon radicals and gaseous hydrogen. This model gives satisfactory agreement with the data but leaves the exact hydrocarbon radical concentrations unpredicted. The adsorption of hydrogen on molybdenum sulfide catalyst is different than on nickel since a considerable proportion is not readily desorbed. A simple model to describe this behavior is one in which initially hydrogen in the gas phase and an active form on the catalyst are in equilibrium with each other and with an inactive form of hydrogen that exchanges more slowly with either of them. Because of the small concentration of intermediates other than adsorbed hydrogen, it is possible to model the deuterium transient the same as if no reaction were taking place. The system may be depicted schematically in terms of three compartments, as shown in Figure 1, before introduction of deuterium. In Figure 1, the compartment designated as H2 + H a t + CH,C shows the three species in the path of hydrogenation being at equilibrium with each other. The symbol E is used to designate chemisorbed intermediates. These species are assumed to exchange hydrogen among each other very rapidly. When deuterium is introduced, it will also rapidly equilibrate. As shown in the previous section, the final hydrogenation step that converts CHBtto CHI is unidirectional. At a deuterium feed rate introduction of F mL/min if the velocity of methanation is V mL/min, the rate of transfer of molecular hydrogen to methane will be 2 V , the hydrogen leaving the system will be reduced by this amount and equal F - 2V. The inactive hydrogen Hit is also initially at equilibrium with the hydrogen in the pool H2 + Hat + CHJ, but in this case, since the exchange rate u, can be slow, it is not in equilibrium with deuterium after a transient experiment is started. The two differential equations based on material balances for hydrogen-deuterium tracing during methanation are (PCo -t c1w + c3w) dzH2 F = -(I - Z H P ) + UeZH1 - 2% W dt w (5)

Cz-dZ" = dt

Ue(ZHZ

- z "')

at t = 0, Z" = zH2, where Co = CH2+ C H D+ CD2,mL/mL of dead space; C1 = CHaC+ CDae,mL/g,of catalyst in H a t + Dat as diatomic hydrogen; Cz = CHIP+ CDle,mL g of catalyst in H'C D'C as diatomic hydrogen; C, = @dzD*xC (x = 1-3), mL/g in CH,D,,E (x = 1-3) as diatomic hydrogen; F = inlet rate of deuterium at step-up, mL/min; V = rate of methane production, mL/min; u, = exchange velocity, mL/ (min-g of catalyst) (as diatomic hydrogen); W = weight of catalyst, g; zH' = CDIe/C1;zH2 = (CCD 2CD2)/2C0; = dead space, mL (determined by argon injection). This nomenclature follows that in our articles previously referenced for transient tracing. For molybdenum sulfide catalysts, assuming that the contribution due to C, can be neglected, the solution of the differential eq 5 and 6 for the observed deuterium concentration after deuterium has been introduced can then be conveniently written in terms of three constants, k ~k2, , and k3

where

kl = kz =

F PCO + C l W

F+u,W "e + Z(PC0 + c,w) 2cz

(7)

The intermediate concentration Z" is also obtainable, but cannot be checked experimentally. The solution of eq 7 together with known values for P, W , and F enables the parameters u,, C1,and C2 to be computed from observed deuterium transients. The model shown in Figure 1will also enable the transients in effluent concentrations of the five deuterated methane species to be estimated, and they can be compared with observed transients for these species. The observed zH2transient is assumed to be in equilibrium with active hydrogen. The deuterium distribution is statistical in all species except the products CH, D4-x(x = 0-4) since they are produced unidirectionally in the final methanation step. The predicted transients for the modeling of the five hydrocarbon species can then be obtained from the following equations in terms of an observed zH2 transient

,

,

@

dCCHDa

= 4V(1 - z ~ z ) ( z FOCHD3 ~ ~ ) ~

dCCD4 7 = V(zH2)I- FOCD4

0

where FJ is the outlet rate of ith deuteriomethane species, mL/min (STP); CH,D, ( x = 0-4). These models were used to correlate the data for a number of experiments that were conducted with each of the four catalysts listed in Table I. Results for H-D exchange using eq 7 are summarized in Tables 111-VI. A

Ind. Eng. Chem. Prod. Res. Dev.,Vol. 25, No. 2, 1986 217

Table VI. Deuterium Tracing on MoSz + S (GRI-C-726)

Table 111. Deuterium Tracing on MoSz (GRI-C-735) run no. 050783

H2/C0 ratio in feed temp, OC wt of catalyst, g dead space, mL inlet, mL/min (STP) H2

co

He outlet, mL/min (STP) H2

co

CHI COZ HZO CH4 formation rate, mL/(min.g) (STP) exchange velocity, V,, mL/(min.g) (STP) surface species, mL/g C1 (active hydrogen) C2 (inactive hydrogen)

050583

2 450 13.5 170 3.83 1.92 33.85

1.92 3.83 33.85

3.38 1.57 0.20 0.15 0.05 0.015

2.18 2.24 0.34 0.31 0.03 0.025

1.33 3.28 0.29 0.27 0.02 0.021

0.042

0.041

0.048

0.54 0.38

0.39

Table IV. Deuterium Tracing on MoS2

111882

0.5 450 13.5 170

2.88 2.88 33.85

0.50

run no.

050683

1 450 13.5 170

0.38 0.34

+ S (GRI-(3-740)

+ SEE Oxide

HJCO ratio in feed temp, "C wt of catalyst, g dead space, mL inlet, mL/min (STP) HZ

2.0 400 16.5 169 3.84 1.91 88.47

co

He outlet, mL/min (STP) H2

co

CH, COZ HZO CHI formation rate, mL/(min.g) (STP) exchange velocity, ue, mL/(mimg) (STP) surface species, mL/g C1(active hydrogen) C2 (inactive hydrogen)

112482 1

450 16.5 169 2.80 2.80 49.85

111682 0.5 400 16.5 169 1.91 3.84 88.47

3.39 1.47 0.22 0.22 0.00 0.013

1.89 1.93 0.45 0.43 0.02 0.027

1.15 3.09 0.38 0.37 0.01 0.023

0.029

0.034

0.035

3.06 1.12

2.37 1.13

2.50 0.83

run no. 031183

H2/C0 ratio in feed temp, "C wt of catalyst, g dead apace, mL inlet, mL/min (STP) H2

031283

0.5 450 12.6 158

1

450 12.6 158 2.79 2.79 38.78

co

He outlet, mL/min (STP) H2

2.18 2.22 0.30 0.28 0.02 0.024 0.031

co

CHI CO, HzO CH, formation rate. mLl(min.e) (STP) exciange velocity, ie,m i / (mikgj ' surface species, mL/g C1 (active hydrogen) Cz (inactive hydrogen)

Table V. Deuterium Tracing on MoSz (GRI-C-763)

1.87 3.74 38.78 1.31 3.19 0.28 0.27

0.01 0.022 0.033

0.51 0.59

0.44 0.37

+ SEE Oxide run no.

H2/C0 ratio in feed temp, "C wt of catalyst, g dead space, mL inlet, mL/min (STP) H2

co

He outlet, mL/min (STP) H2

co

CH COZ HZO CHI formation rate, mL/(min.g) exchange velocity, ue, mL/(min-g) surface species, mL/g C1(active hydrogen) Cz (inactive hydrogen)

041383

041983

1 450 10 159

450 10 159

2.79 2.79 38.10

0.5

1.87 3.74 37.52

2.53 2.32 0.24 0.23 0.01 0.024 0.034

1.16 3.05 0.35 0.34 0.01 0.035 0.022

1.05 0.59

1.00 0.45

large amount of both active and inactive adsorbed hydrogen is present. The adsorption increases substantially for catalysts containing additives to MoS2 The occupancy of adsorbed hydrogen is not markedly affected by the

.,

0

20

40

60

00

TIME, MIN

Figure 2. Transient of H as fraction of H in total gaseous H observed, 0;correlation, -.

+ D:

H2/C0 ratio, so it appears that CO does not strongly compete for adsorption sites with hydrogen. Values of 1- zH2obtained by using eq 7 can be employed empirically with the constants kl,k2,and k3 for further modeling of the deuteriomethane transients using eq 8. A typical set of data is plotted on Figure 2 comparing the observed (1- zH2) transient with that modeled for a feed ratio of H2/C0 = 1.0 over catalyst GRI-(2-726 (run no. 112482 in Table VI). Equations 8 enable a prediction to be made of the effluent deuteriomethanes for this run. Figure 3 gives calculated and observed values. Good agreement is obtained for all five methanes. The short delay times observed in the appearance of CHD, and CD4 can be explained by the presence of species such as CHC, which do not immediately equilibrate with the gaseous H-D stream. Similar results, showing reasonable agreement, were obtained corresponding to the nine additional runs listed in Tables 111-VI. 13C0 Tracing. To further assess the role of carbon monoxide, the catalysts in Table I were also studied by 13C0tracing. It was not possible to detect any adsorption of undissociated CO. Assuming that CO dissociation to produce carbon is unidirectional, the concentration of CH,C (x = 0-3) intermediates averaged less than mL/g

Ind. Eng. Chern. Prod. Res. Dev., Vol. 25, No. 2, 1986

218 1.0

E

IS

-0

IO

20

30

40 50 TIME, MIN

60

70

80

Figure 3. Transient of methanes as fractions of total methane species: observed CHI, +; observed CH3D, 0;observed CH2D2,0; observed CHD,, A; observed CD4, X; correlation, -. Table VII. Helium Purging on Mo9,-5-SEE Oxide Catalyst (GRI-(3-726)" CHI formation CH!., rate, mL/(min.g) adsorbed, run no. (STP) mL/g (STP) 112382 121582 011483 011983

0.030 0.026 0.022 0.023

0.0255 0.0166 0.0086 0.0137

"Same conditions as Table VI, run 112482.

of catalyst for a series of eight different runs on the four tested catalysts. This concentration is close to the limit of detectability in the present apparatus. Helium Purging. In order to gain more information on the extent of surface coverage by carbonaceous intermediates, experiments were conducted on the desorption by helium of various species present after a steady-state experiment under reaction conditions. Figure 4 shows typical curves of the transients obtained on fresh MoS2S-SEE oxide (GRI-C-726) catalyst. Since they are purged from the system, all species except methane exhibit a rapid decline in concentration. Hydrogen also drops rapidly at the outset but does not fall to zero for a relatively long time because of the presence of inactive hydrogen. It is believed that the inactive hydrogen present on the catalyst continues to react with carbonaceous intermediates during the initial stage of purging, forming methane. An estimate of the extent of these intermediates can be made by taking the area between the COz and CHI curves. Table VI1 gives results for helium purging of the same catalyst after use comparing the amount of intermediate deposit with catalyst activity. There seems to be definite evidence that the concentration of adsorbed hydrocarbon radicals decreases with time, but exact correlation of catalyst performance with CH,C concentration is not possible. Another interesting effect observed in the course of these tests was that upon resumption of the flow of feed gas, following helium purging and hydrogen treatment, methanation activity recovers rapidly but does not exceed the final steady-state activity. In the case of nickel a different situation exists. When feed gas is passed over a clean nickel catalyst, methane production rate is very high at first and then declines to a steady state as carbon deposition builds up. With molybdenum sulfide catalysts, it is

TIME, MIN

Figure 4. Helium purging (same conditions as in Table VI, run 112482).

necessary for intermediates to first form before maximum catalyst activity is attained.

Discussion The most striking result of these studies is the high degree of hydrogen adsorption at high temperatures on the working catalysts. The catalyst based on molybdenum, SEE,and sulfur precursors (GRI-C-726) has a final composition corresponding to 72.2 wt % of MoS,. From the data in Table VI, the total adsorbed hydrogen averaged for the three runs on this catalyst amounts to 3.75 mL/g (STP). In terms of a chemical formula this corresponds to H0.074MoSZ* Material with structural incorporation of hydrogen into MoS, was described by Blake et al. (1981). It was prepared by reducing MoS3in a hydrogen-helium mixture (8 vol 90 H,) at 400 "C for 16 h. The material was evacuated at torr, and the hydrogen content, determined by exhaustive exchange with deuterium at 250 "C, was found to be 0.062 atoms/mol of MoS2. The surface area of Ho.,MoS2 prepared in this manner was found to be 8.4 m2/g, which would be too small to accommodate this much hydrogen by surface chemisorption. In a second paper, these authors (1981) reported the production of H,,,,MoSz by a similar procedure. They also prepared material described BS MoS, by thermal decomposition of ammonium thiomolybdate, or of MoS3 formed by thermal decomposition of thiomolybdate. Surface area of catalysts prepared by this method averaged 16-21 m2/g. Blake et ai. (1981) used their catalysts in a deuterium tracer study of several hydrodesulfurization and hydrogenation reactions. Structural incorporation of hydrogen in MoSz greatly increases activity for butadiene hydrogenation but not for thiophene hydrodesulfurization. Further studies by the same group (Fraser et al., 1981) discussed inelastic neutron scattering spectroscopy to investigate the nature of the hydrogen bonding and found sulfhydryl groups SH on the surface of the catalyst. Since the present study indicates that carbon monoxide adsorption does not compete strongly with hydrogen, it

Ind. Eng. Chem. Prod.Res. Dev., Vol. 25, No. 2, 1986 219

appears that CO may interact with the catalyst by “reactive” chemisorption. Studies by Stiefel(l982) have shown that coordination compounds can exhibit structures with both molybdenum-carbon and sulfur-carbon bonding. The most active catalysts exhibit the highest concentration of adsorbed hydrogen. Both sulfur and SEE increase the extent of hydrogen adsorption. Since sulfhydryl groups are present on the active catalyst, this is perhaps not surprising as regards the effect of sulfur. The effect of SEE seems to be to change the morphology of the molybdenum sulfide, since no evidence exists for complexes that would involve exactly the same stoichiometry of %.,,M0S2 as occurs in the pure MoS2catalysts studied by Blake and Fraser. The surface areas of the catalyst that we used are much larger (Table I) than reported by Blake et al. (1981). Naumann et al. (1982) also found that a rough trend of higher methanation activity was observed with increasing surface area for a number of thiosalt-derived materials with areas in the range of 30-150 m2/g. Unlike nickel, practically no direct formation of water occurs with molybdenum sulfide catalysts. Naumann et al. (1982) report that these catalysts exhibit substantial water gas shift activity. However, since C02 is rapidly produced by the methanation reaction, unless water is present in the feed, water gas shift activity is suppressed until high temperatures, exceeding 450 “C, are reached in a reaction system. At high C02 outlet concentrations, the reverse of the water gas shift reaction then occurs with feeds that are low in water content. The direct production of C02may be due to the fact that sulfhydryl rather than hydroxyl groups predominate on the surface of molybdenum sulfide catalysts, so that the Boudouard reaction is favored rather than reaction of hydroxyl with atomically adsorbed hydrogen to form H,O. To summarize, the present studies indicate that the high activity of a new type of methanation catalysts based on molybdenum sulfide can be ascribed to the formation of high surface area modified MoS2containing considerable structurally incorporated hydrogen. Since these Catalysts are sulfur insensitive and can operate at much lower H2/C0 ratios than is possible with nickel catalysts, they appear to have attractive economic possibilities when combined with currently developing coal gasifiers.

Acknowledgment We are grateful for support of the fundamental studies at Columbia by the National Science Foundation under Research Grant CPE-81-16015. We also appreciate support of catalyst development studies by the Gas Research Institute and especially the encouragement of Dr. Ab Flowers. Thanks are due to Zhen-hong Chen, a visiting scholar from the People’s Republic of China, and to JuoYu Kao for assistance. Motozo Yoshikiyo is grateful for partial support of his research by Ube Industries. Registry No. CO,630-08-0; MOSz, 1317-33-5; CHI, 74-82-8. Literature Cited Anderson, R. 8. The Flscher-Tropsch Synthesis; Academic: New York, 1984; p 161. Bell, A. T.; Wlnslow, P. 8th Infernational Congress on Catalysis, Berlin; Dechema: Frankfurt, 1984; Vol. 111, p 175. Blake, M. R.; Eyre, M.; Moyes, R. B.; Wells, P. B. (Part 1) 7th Infernatlonal Congress on Catalysis, Tokyo;Elsevler: New York, 1981; p 591. (Part 2) Bull. SOC.Chlm. Be/g. 1081, 90, 1293. Fraser. D.; Moyes, R. B.; Wells, P. B.; Wright, C. J.; Sampson, C. F. 7th International Congress on Catalysls. Tokyo;Elsevier: New York, 1981; p 1424. Furlmsky, E.; Amberg, C. H. Can. J . Chem. 1075, 53, 3567. Happel, J.; Suzukl, J.; Kokayeff, P.; Fthenakls, V. J . Catal. 1980, 65, 59. Happel, J.; Cheh, H. Y.; Otarod, M.; Ozawa, S.; Severdla, A. J.; Yoshlda, T.; Fthenakis, V. J . Catal. 1982, 75, 314. Happel, J.; Hnatow, M. A.; Bajars, L.; Otarod, M.; Lee, A. L. R o c . Int. Gas Res. Conf., 1983, London 1083. Happel, J.; Hnatow, M. A.; Bajars, L.; Yin, F.; Lee, A. L. Roc. I n t . Gas Res. Conf ., 1984, Chlcago 1084a. Happel, J.; Cheh, H. Y.; Otarod, M.; Bajars, L.; Hnatow, M. A.; Yln, F. 8th International Congress on Catalysls, Berlin ; Dechema: Frankfurt, 1984b; Vol. 111, p 395. Happel, J.; Hnatow, M. A.; Bajars, L. U S . Patent 4491 639, Jan 1 1985. Massoth, F. E. A&. Catal. 1078, 2 7 , 265. Meyer, H. S.; Hill, V. L.; Flowers, A.; Happel, J.; Hnatow, M. A. Chem. Eng. News 1082, 60(14). Naumann, A. W.; Behan, A. S.; Thorsteinson, E. M. Proceedlings of the Climax Fourfh International Conferenceon Chemistry and Uses of Molybdenum; Barry, H. F.; Mitchell. P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1982; p 313. Otarod, M.; Ozawa, S.; Yin, F.; Chew, M.; Cheh, H. Y.; Happel, J. J . Catal. 1982, 84, 156; Erratum, 1084, 89, 584. Stiefel, E. J. Proceedings of the Climax Fourth International Conference on Chemistry and Uses of Molybdenum; Barry, H. F.; Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1982; p 56. Tamaru, K. Dynamic Heterogeneous Catalysis;Academlc: New York, 1978; p 98. Welsser, 0.; Len&. S. SulfMe Catalysts, Their Properties and Appllcatlons; Pergamon: New York, 1973. Wilson, R. L.; Kemball, C. J . Catal. 1964, 3 , 426. Yang, C. H.; Soong, Y.; Biloen, P. 8th International Congress on Catalysis, Berlln; Dechema: Frankfurt, 1984; Vol. 11, p 3.

Received f o r review May 2, 1985 Accepted December 2, 1985