Technology Development for Iron and Cobalt Fischer-Tropsch Catalysts. Quarterly Report. April 1, 2001 to June 30, Burtron H.

Transcription

1 Technology Development for Iron and Cobalt Fischer-Tropsch Catalysts Quarterly Report April 1, 2001 to June 30, 2001 Burtron H. Davis Enrique Iglesia (UC/B Subcontract) DE-FC26-98FT40308 University of Kentucky Research Foundation 201 Kinkead Hall Lexington, KY University of California-Berkeley (Subcontract) Laboratory for the Science and Application of Catalysis Department of Chemical Engineering University of California at Berkeley Berkeley, CA 94720

2 Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 1

3 Abstract CAER A series of Fe-Al (0-25% Al) and Fe-Si (0-8% Si) catalyst were prepared to study the effect of structural promoters on iron FTS catalysts. BET analysis showed that as the quantity of the promoters is increased, the surface area increased. In addition, as the calcination temperature increases, the surface area decreases. XRD analyses of the catalysts are also reported. Iron based FTS catalyst promoted with K or Be showed a superior deactivation rate. The K promoted catalyst showed a deactivation rate of 0.59% CO conversion per week after passing an initial conditioning period of 300 hours. The Be promoted catalyst showed a deactivation rate of 0.43% CO conversion per week. FTS activity and selectivity are also reported. We are continuing the study on the impact or reducibility of cobalt oxides by the use of different supports and by incorporation of different promoters and the order of addition to the supported cobalt catalysts. The reduction of Co 3 O 4 is a two-step process which passes through an intermediate CoO phase before reduction to the metal. The promotion of the supported Co catalysts with Pt and Ru noble metal promoters had a similar effect on catalyzing both reduction steps. Promotion with Re only aided in catalyzing the second step when a significant interaction of the Co species with the support was present, such as found on the Al 2 O 3 supported catalysts. The effect of water on Co FTS catalysts was studied using a Co catalyst. The results obtained for conversion, selectivity and deactivation using this catalyst were compared to the previous data using a Pt promoted Co/SiO 2 catalyst. 2

4 UC/B Most of the efforts during this reporting period were focused on the preparation of manuscripts covering the work done the past several months. A new transient experimental setup was developed in order to perform transient isotopic switch experiments. In addition, modifications to the high-pressure reactor unit were done to improve the sample collection procedure for the 13 CO 2 addition experiments, which are in progress. Switching experiments were carried out on the Co-FT unit under typical reaction conditions in order to estimate the number of active sites on the catalyst during the reaction. The experimental technique for collecting data and analyzing them was modified in order to take into account active sites only. 3

5 Table of Contents Page Disclaimer... 1 Abstract... 2 Table of Contents... 4 Executive Summary... 5 Task 1. Iron Catalyst Preparation A. Effect of Structural Promoters on Iron Catalysts Task 2. Catalyst Testing A. Deactivation and Regeneration of Alkali Metal Promoted Iron Fischer-Tropsch Synthesis Catalysts Task 3. Catalyst Characterization A. Characterization of Pt-Re Promoted Co/Al 2 O 3 Catalysts Task 4. Wax/Catalyst Separation A. Slurry Bubble Column Reactor (SBCR) Activities Task 5. Oxygenates Task 6. Literature Review of Prior Fischer-Tropsch Synthesis with Co Catalysts Task 7. Co Catalyst Preparation Task 8. Co Catalyst Testing for Activity and Kinetic Rate Correlations A. Effect of Water on the Catalytic Properties of Co/SiO 2 Fischer-Tropsch Catalyst Task 9. Co Catalyst Life Testing A. Cobalt Catalyst Life Testing and Deactivation Task 10. Co Catalyst Mechanism Study Task 11. University of California, Berkeley (Subcontract) Task 12. Reporting and Management

6 Executive Summary CAER A series of Fe-Al (0-25% Al) and Fe-Si (0-8% Si) catalysts were prepared to study the effect of structure promoters on iron FTS catalysts. BET data show that as the quantity of the promoter is increased, the surface area increased. In addition, as the calcination temperature increases, the surface area decreases. XRD analysis shows that Al promoted catalysts are not very crystalline at loadings greater than approximately 7 atom %. In contrast, the Si promoted catalysts are amorphous, with crystallinity being evident only for the small atomic fraction. It appears that Si retards the crystallization of Fe 2 O 5 to a much greater degree than Al. In order to make accurate lattice parameter determinations, the catalysts were calcined at 600 o C for 24 hours prior to additional XRD determinations. In all cases, Fe 2 O 3 was identified as the only species present. At the higher levels of Al, 17.9% and 24.22%, there is evidence of another species that has not been identified. The XRD analysis of the catalysts showed that as the quantity of the promoter increased, there was a shift in all 22 to higher values indicating that the promoter had entered the hematite lattice and was influencing its dimensions. Iron Fischer-Tropsch synthesis (FTS) catalysts promoted with potassium or beryllium showed a superior deactivation property. Potassium promoted iron FTS catalyst showed a low deactivation rate of 0.59% CO conversion/week after passing an initial conditioning period of 300 hours. Beryllium promoted iron catalyst produced an even more stable activity than potassium. A deactivation rate as low as 0.43% CO conversion/week was obtained from beryllium promoted catalyst. Higher temperature generated a shorter conditioning period for potassium promoted catalyst. The FTS activity began to decrease when the bulk phase of the catalyst changed from 100% carbide to iron oxides. Although the initial activity changed rapidly, little change occurred in the bulk phase of the catalyst. For potassium promoted catalyst, FTS activity, CO 2 selectivity, hydrocarbon productivity and water gas shift activity 5

7 showed the same conditioning period of 300 hours of time on stream after passing a peak value at about 120 hours of reaction time. A higher water gas shift activity, higher CO 2 and CH 4 selectivity by beryllium promoted catalyst than those by potassium were also found in an FTS process. Beryllium promoted catalyst also showed a superior regenerability when either H 2 or CO was utilized. When rejuvenated with CO a better activity recovery was achieved than that with H 2. We are continuing to study the impact on reducibility of cobalt oxides by the use of different supports and by the incorporation of different promoters and additives to supported cobalt catalysts. The reduction of Co 3 O 4, which is present on the catalyst after calcination, is a two step process which passes through an intermediate CoO phase before reduction to the metal. In our previous investigations, we found by temperature programmed reduction that a 15%Co/Al 2 O 3 displayed a broad second peak attributed to the reduction of cobalt species interacting with the support. In agreement, hydrogen chemisorption with pulse reoxidation revealed that the unpromoted catalyst displayed poor percentage reduction of only 30% after reduction at 623K, the standard activation temperature of Co FTS catalysts. While promotion of supported Co catalysts with the noble metal promoters Pt and Ru had a similar effect on catalyzing both reduction steps of Co 3 O 4 to Co metal, Re only aided in catalyzing the second step when a significant interaction of the Co species with the support was present, such as found on Al 2 O 3 supported catalysts. After reduction at 623K, therefore, the number of Co active sites increases remarkably with the addition of noble metal promoter, thereby increasing the initial activity under reaction testing. We have investigated the possibility of a synergism between Pt and Re on the reducibility of supported Co oxides, and are testing the catalysts in the CSTR to draw conclusions as to the effect of Pt-Re promotion and synergism on the catalyst stability. 6

8 In this report we describe SBCR pilot plant results and operating experiences using an improved catalyst/slurry filtration system. The following improvements to the filtration system were included: an enlarged let-down valve trim, an automatic differential pressure controller for the filter media and a by-pass for the let-down valve. Further filtration tests were conducted using a high alpha iron-based F-T catalyst. During the activation, problems were encountered with the gas sparger in several pilot runs. Catalyst had infiltrated the sintered metal sparger causing an excessive pressure drop. A new activation procedure was developed to minimize sparger plugging. As in previous reported pilot-scale experiments, conversion results of the high alpha catalyst in the SBCR were compared to that of CSTR experiments. A method for estimating the SBCR recirculation rate is also described. The effect of water on iron FTS catalysts has been widely studied and it is well known that water may reoxidize these catalysts during synthesis. The effect of water on Co FTS catalysts is less well understood. In a previous study the effect of water on the catalytic properties of a Pt promoted Co/Al 2 O 3 was studied. These previous results were compared to the results of this study using a Co/SiO 2 catalyst. In order to determine the effect of water addition time on the catalytic activity, the smaller or larger amount (5 or 25 vol%) was continuously added for 96 hours. It was found that the longer time addition of the smaller amount of water did not show a significant effect on the CO conversion and the catalyst deactivation rate. However, for the addition of the larger amount of water, CO conversion increased for the initial 24 hours and for the long time addition resulted in the irreversible deactivation of the catalyst. The effect of the water partial pressure was also studied. Methane selectivity decreased and CO 2 and olefin ratios increased with increasing water partial pressure. 7

9 UC/B During this reporting period, 13 CO 2 addition experiments were initiated on a Fe-Zn-Cu-K catalyst in order to determine the extent of participation of CO 2 in chain initiation and growth. The experiments are carried out at 508 K and 0.8 MPa, i.e., at conditions where the water gasshift reaction is far from equilibrium. Prior to these experiments, the sampling system at the outlet of the reactor system (after the gas chromatograph) was modified in order to allow the separation of light hydrocarbons (>C 4 ) from CO, N 2 and H 2, and hence enhance the sensitivity of analysis of the former. The preparation and revision of three manuscripts describing the site requirements and the effect of promoters (K, Cu and Ru) on the structure, reduction/carburization behavior and the catalytic performance of Fe- or Fe-Zn-based catalysts, begun during the previous reporting period, were completed and submitted to peer-reviewed journals. One manuscript titled "Spectroscopic and transient kinetic studies of site requirements in iron-catalyzed Fischer- Tropsch Synthesis" was submitted to the Journal of Physical Chemistry B. Another manuscript titled Promoted iron-based catalysts for the Fischer-Tropsch Synthesis: Design, synthesis, site densities, and catalytic properties, prepared during the previous reporting period, was submitted to the Journal of Catalysis, and a third manuscript titled Effects of Zn, Cu and K promoters on the structure, reduction/carburization behavior, and performance of Fe-based Fischer-Tropsch Synthesis catalysts was submitted to Catalysis Letters. Another manuscript describing the effects of K and Cu promoters and the effect of CO2 addition on CO 2 formation during Fe-based FTS reactions is currently under preparation. In addition, a manuscript titled "Structure and site evolution of iron oxide catalyst precursors during the Fischer-Tropsch Synthesis" was accepted for publication in the Journal of Physical Chemistry B, and a manuscript titled "Structural analysis of unpromoted Fe-based 8

10 Fischer-Tropsch catalysts using X-ray absorption spectroscopy, submitted jointly with the University of Kentucky to Applied Catalysis, has also been accepted for publication. In this reporting period, a new transient experimental setup was developed. An improved gas inlet manifold and a new micro fluidized reactor were also constructed. This system is capable of performing isotopic switch experiments at steady-state conditions. It will be used to study the surface carbon capacities at working conditions by the help of 12 CO/ 13 CO isotopic switch. Switching experiments have been performed on Co/SiO 2 under FTS reaction during this quarter. These studies involved switches between synthesis gas and hydrogen at 453 K and 0.5 MPa (FT conditions) in order to quantify the number of active sites during the reaction. A set of experiments were conducted in order to investigate the effect of CO conversion, at constant pressure and temperature, on the number of active sites on the catalyst, identified by the active carbon adsorbed on the surface. The data showed a slight increase in the number of active adsorbed carbon; however this increase was not significant. 9

11 Task 1. Iron Catalyst Preparation The objective of this task is to produce robust intermediate- and high-" catalysts. A. Effect of Structural Promoters on Iron Catalysts Synthesis of the Catalysts The Fe-Al samples were prepared by co-precipitation of iron and aluminum from nitrate solutions at constant ph of 8.5. Iron nitrate (1.3M) and aluminum nitrate (1.43M) solutions were mixed to yield a given Al/Fe ratio. Ammonium hydroxide (30%NH 3 ) was used as the precipitant. About 100 cm 3 deionized water was placed in a large beaker and heated to 85ºC using a magnetic stirrer/hotplate. The ph was adjusted to 8.5 by addition of an appropriate quantity of NH 4 OH. The temperature was held at 85 ºC throughout the precipitation procedure. The solution of Fe 3+ and Al 3+ ions was added slowly to the beaker with continuous stirring. Simultaneously, the NH 4 OH was added at a rate such that the ph was maintained at a constant value of 8.5. The resulting precipitate was filtered using vacuum filtration and washed with deionized water until the ph of the filtrate was measured to be in the range 7.0 to 7.5. The precipitate was subsequently dried in an oven overnight at 110ºC followed by calcination in static air at 400ºC for 10 hours. Samples were prepared that contained from 0% to ~25% Al. The samples were analyzed for iron content following calcination using ICP. The iron content by weight in the unpromoted samples was found to contain 70% Fe (average of 3 analyses) which agrees very well with the theoretical value of % in Fe 2 O 3, confirming that the sample was hematite. XRD analysis also provided further proof that the samples contained hematite. Table 1 lists the compositions of the promoted iron catalysts. The Fe-Si samples were already available in the lab. They were prepared by hydrolyzing an appropriate quantity of tetraorthosilicate (TEOS) in the acidic Fe(NO 3 ) 3 solution followed by precipitation using NH 4 OH. They had silicon contents of up to ~8% Si, had been calcined at 10

12 400ºC and were used without any further treatment. Their compositions are summarized in Table 2. XRD analysis of the samples showed that as the degree of loading of the promoter increased the samples became more amorphous. The samples were then treated in an oven at 600ºC for 24 hours to improve crystallinity. BET Surface Area Measurements Nitrogen sorption isotherms were measured using a Micromeritics TriStar 3000 instrument. Prior to analysis each sample was outgassed at 160ºC for at least 12 hours to a pressure less than 100 mtorr. The results of the analyses for the samples calcined at 400ºC and 600ºC are presented in Tables 3 and 4. It can readily be seen that as the quantity of the promoter is increased the surface area also increases. Also, as the temperature of the calcination is increased the surface area decreases. Representative plots illustrating the trend in change of surface area with increasing atomic percent of structural promoter for both the Al and Si additions are shown in Figures 1 and 2, respectively. XRD Analysis The samples containing varying atom percentages of the structural promoters Al or Si were prepared in order to investigate whether they are capable of entering into solid solution with the hematite and hence be well dispersed throughout the oxide mass. Whether or not these promoters had entered into solid solution was determined from their effect on the lattice parameters of the magnetite. Powder X-Ray Diffraction was used to determine the unit cell dimensions of the hematite. Hematite has a hexagonal structure and thus two cell dimensions, a 0 and c 0, had to be determined. X-Ray Diffractograms were obtained for all samples. Figures 3 and 4 show the diffractograms obtained for the Al and Si promoted iron oxide samples calcined at 400ºC. It can be seen that the Al promoted samples are not very crystalline at loadings greater 11

13 than about 7 atom%. In contrast, the Si promoted samples are quite amorphous to X-Rays, crystallinity only evident for the very small atomic fractions. It appears that the Si retards the crystallization of the Fe 2 O 3 to a much greater degree than the presence of the Al. In order to make accurate lattice parameter determinations a greater degree of crystallinity is required so all the samples were heated in an oven to 600ºC for 24 hours and the resulting diffractograms are shown in Figures 5 and 6. In all cases Fe 2 O 3 was identified as the only species present. At the higher levels of Al addition, 17.9% and 24.22%, there is evidence of another species present but this has yet to be identified. The ten most intense peaks of the diffractograms were each analyzed individually using the fitting program WINFIT to determine accurate peak locations in terms of 22. These 22 values together with their corresponding Miller indices were input to the program UnitCell to calculate the lattice parameters for the hexagonal Fe 2 O 3. It was found that as the quantity of the promoter increased there was a shift in all 22 to higher values indicating that the promoter had entered the hematite lattice and was influencing its dimensions. These shifts in the lattice dimensions are shown as a function of the promoter concentration in Figures 7 to 10. The shift to higher 22 values resulted in a decrease in both a 0 and c 0 lattice parameters. This is to be expected as the ionic radii of both Al 3+ and Si 4+, 0.5Å and 0.41Å, respectively, are both less than that of Fe 3+ (0.64Å), and would lead to a contraction of the unit cell. The shifts in the lattice dimensions are taken as evidence that the cations have entered into the hematite lattice. 12

14 Table 1 Al Content of the Samples Sample Al/Fe Atom % Al 8A A A A A A A A

15 Table 2 Si Content of the Samples Sample Si/Fe Atom % Si FeSiO FeSi FeSi FeSi

16 Table 3 Results of BET Analysis of Samples Treated at 400 o C Sample BET S.A. m 2 /g Pore Volume cm 3 /g Average Pore Radius nm 8A A A A A A A A FeSiO FeSi FeSi FeSi

17 Table 4 Results of BET Analysis of Samples Treated at 600 o C Sample BET S.A. m 2 /g Pore Volume cm 3 /g Average Pore Radius nm 8B B B B B B B B FeSiOB FeSi 2 B FeSi 4 B FeSi 8 B

18 Figure 1. Variation of BET surface area with increasing atomc percent of Al. All samples were calcined at 600 o C. 17

19 Figure 2. Variation of BET surface area with increasing atomic percent of Si. All samples were calcined at 600 o C. 18

20 Figure 3. X-ray diffractograms of Al promoted hematite samples calcined at 400 o C. 19

21 Figure 4. X-ray diffractograms of Si promoted hematite samples calcined at 400 o C. 20

22 Figure 5. X-ray diffractograms of Al promoted hematite samples calcined at 600 o C. 21

23 Figure 6. X-ray diffractograms of Si promoted hematite samples calcined at 600 o C. 22

24 Figure 7. Variation of unit cell parameter a 0 with concentration of Al promoter. 23

25 Figure 8. Variation of unit cell parameter c 0 with concentration of Al promoter. 24

26 Figure 9. Variation of unit cell parameter a 0 with concentration of Si promoter. 25

27 Figure 10. Variation of unit cell parameter c 0 with concentration of Si promoter. 26

28 Task 2. Catalyst Testing The objective of this task is to obtain catalyst performance on the catalysts prepared in Task 1. A. Deactivation and Regeneration of Alkali Metal Promoted Iron Fischer-Tropsch Synthesis Catalysts Abstract Iron Fischer-Tropsch synthesis (FTS) catalysts promoted with potassium or beryllium showed a superior deactivation property. Potassium promoted iron FTS catalyst showed a low deactivation rate of 0.59% CO conversion/week after passing an initial conditioning period of 300 hours. Beryllium promoted iron catalyst produced an even more stable activity than potassium. A deactivation rate as low as 0.43% CO conversion/week was obtained from beryllium promoted catalyst. Higher temperature generated a shorter conditioning period for potassium promoted catalyst. The FTS activity began to decrease when the bulk phase of the catalyst changed from 100% carbide to iron oxides. Although the initial activity changed rapidly, little change occurred in the bulk phase of the catalyst. For potassium promoted catalyst, FTS activity, CO 2 selectivity, hydrocarbon productivity and water gas shift activity showed the same conditioning period of 300 hours of time on stream after passing a peak value at about 120 hours of reaction time. A higher water gas shift activity, higher CO 2 and CH 4 selectivity by beryllium promoted catalyst than those by potassium were also found in an FTS process. Beryllium promoted catalyst also showed a superior regenerability when either H 2 or CO was utilized. When rejuvenated with CO a better activity recovery was achieved than that with H 2. Introduction Hydrocarbons of various chain length can be produced from CO and H 2 in Fischer- Tropsch synthesis process, which can be expressed as 27

29 (2n+1) H 2 + n CO 6 C n H 2n +2 + n H 2 O [1] When an iron catalyst is used for FTS reactions, the water gas shift (WGS) reaction can also occur. This reaction consumes CO and water formed by the FTS reaction and produces additional hydrogen and carbon dioxide. H 2 O + CO 6 CO 2 + H 2 [2] Potassium has long been used as a promoter for iron catalysts. It provides an increase in the alkene yield and a decrease in the CH 4 selectivity (1,2,3). Potassium can also increase the catalytic activity for FTS and water-gas shift (WGS) reactions (4). The influence of potassium on iron catalysts has been investigated by other researchers (e.g., 5-8). It is believed that alkali metals have significant effects on both FTS activity and product selectivity. As the most effective promoter, potassium salts are widely used in iron catalysts; however, the readiness to form an alkali compound with common catalyst supports, or structural promoters such as alumina or silica, complicates the situation. Although potassium enhanced the FTS activity and heavy fraction product distribution, high potassium loadings may cover too large of a fraction of the surface of the iron catalyst, resulting in a limited promotion effect or even a decrease in FTS conversions. Bonzel and Kerbs (9) claimed that potassium lowered the methane formation rate and increased the carbon deposition rate. It was also found that the deposited carbon was covered by potassium compounds rather than carbon sitting on top of the promoter. Huang et al. (6) studied the potassium promoted iron catalysts with XPS and found that two-thirds of the catalyst surface was covered by K 2 O and SiO 2. Wang, et al. (7) applied the temperature programmed reduction (TPR) technique to study the effects of potassium. They suggested that potassium facilitates the desorption process of carbon monoxide and strengthens the Fe-C bond. Thus, potassium enhanced the selectivity of long-chain products; i.e., it resulted in a high alpha product distribution. Copper, as a secondary promoter, can facilitate the activation process. 28

30 Copper can therefore minimize the sintering of iron catalysts by lowering the reduction temperature (3). Experimental Catalyst Two iron FTS catalysts with an atomic ratio of K:Fe=10:100 and Be:Fe=1.44:100 were prepared and utilized in this study. Precipitated iron catalysts were prepared using Fe(NO 3 ) 2 O tetraethyl orthosilicate, Cu(NO 3 ) 2 O, and K 2 CO 3 or Be(NO 3 ) 2 was used as the promoter precursor. Details of the preparation procedure was given elsewhere (5). The iron catalyst needs to be activated with H 2, CO or synthesis gas. Activation procedures can have a significant effect on the selectivity and activity of iron catalyst (8,10). It was reported that catalysts activated with CO yielded higher amounts of long-chain hydrocarbons than catalysts activated with syngas or with H 2. In addition, activation conditions may also influence the performance of the iron catalyst during the course of the run. In this study, the potassium promoted iron catalysts were pretreated with CO at 270 C and 1.2 MPa for 24 hours. The CO flowed through a catalyst slurry in 300 ml of Ethylflow oil. The reduction of Fe 2 O 3 with CO occurs in two steps: 3Fe 2 O 3 + CO 6 2Fe 3 O 4 + CO 2 [3] 5Fe 3 O CO 6 3Fe 5 C CO 2 [4] Reactor System A one-liter continuous stirred tank reactor (CSTR) was used in this study. A sintered metal filter was installed to remove the wax samples from the catalyst slurry. The wax sample was extracted through the internal filter and collected in the hot trap held at 200 C. A warm trap (100 C) and cold trap (0 C) were used to collect light wax and the water plus oil samples, respectively, by condensing from the vapor phase that was continuously withdrawn from the reactor vapor space. CO and H 2 mass flow controllers were used to provide a simulated 29

31 synthesis gas of the desired composition. After the catalysts was activated with CO, syngas was introduced into the CSTR with a stirrer speed of 750 rpm. Reaction conditions were 1.2 Mpa, H 2 /CO = 0.67, 230 C for potassium promoted catalyst and 270/C for beryllium promoted catalyst. Product Sampling and Analysis Daily gas, water, oil, light and heavy wax samples were collected and analyzed. A heavy wax sample was taken from the 200 C hot trap connected to the filter. The vapor phase above the slurry passed continuously to the warm (100 C) and the cold (0 C) traps outside the reactor. The light wax and water mixture was collected from the warm trap and an oil plus water sample from the cold trap. Tail gas from the cold trap was analyzed with an online HP Quad Series Micro GC, providing molar compositions of C 1 -C 7 olefins and paraffins as well as for H 2, CO and CO 2. Hydrogen and carbon monoxide conversions were calculated based on the gas product GC analysis results and the gas flow measured at reactor outlet. The oil and light wax samples were mixed before analysis with an HP 5790A GC. The heavy wax was analyzed with an HP5890 Series II Plus GC while the water sample was analyzed using an HP5890 GC. Results and Discussion Activation, conditioning and deactivation of potassium promoted catalyst In this study, a potassium promoted iron FTS catalysts with an atomic ratio of Fe:K=100:10 was used. Reaction was carried out at 230/C, 1.2 MPa at a space velocity of 3.1 sl/h/g-iron. The results (Figure 1) show that a conditioning period, in which CO conversion increased from below 10% to a peak value of 45% at 120 hours of time on stream and then gradually decreased to a stable level at 300 hours of reaction time. As indicated in the figure, a deactivation rate of 0.827% per day was observed from the peak CO conversion during the conditioning period. Then a very low deactivation rate of % per day was obtained following the initial conditioning period. Although CO conversion increased rapidly during the 30

32 initial conditioning phase, Mossbauer spectroscopy analysis shows that the bulk phase iron carbide did not change. Almost all iron was converted during the in-situ catalyst activation prior to the FTS reaction. As carbon monoxide conversion decreased from the maxima, Fe 3 O 4 started to appear in the bulk phase of the catalyst. This result suggests that as both the FTS and WGS reactions proceeds, increasing carbon dioxide and water partial pressure changed the reaction sytem to a more oxidizing environment and thus cause the formation of iron oxide, which subsequently cause the CO conversion decrease. A stabilized activity may obtained after the iron oxides-carbides phase or the three-phase reaction system thermodynamic equilibrium was reached. Figure 1 also shows that a temperature increase caused a new conditioning period, in which CO conversion reached a maxima of 75% before it stabilized at about 57%. This new conditioning period (250/C) appeared more sharply than that over the fresh catalyst at 230/C, suggesting that at a higher temperature, the new equilibrium on the catalyst surface is easier to established than that at a lower temperature. Selectivity and productivity of potassium promoted iron catalyst Figure 2 gives the CO 2 and CH 4 selectivity over the potassium promoted catalyst. Methane selectivity decreased from 3.5% at 24 hours of reaction time to a stable level of 1.6% in 300 hours of time on stream. While methane selectivity decreased monotonously, carbon dioxide went through a conditioning period similar to that of CO conversion. The CO2 selectivity started from 23% at 24 hours of time on stream and decreased from its maxima of 43% to a stabilized level of 39% in 300 hours of time on stream. Hydrocarbon rate showed a similar trend to that of CO conversion and CO2 selectivity, as indicated in Figure 3. The rate rapidly increased from below 0.1g/h/g-iron to a peak value of 0.32 g/h/g-iron before reaching a stabilized level of 0.28 g/h/g-iron in 300 hours of time on stream. Water gas shift activity over potassium promoted catalyst 31

33 When iron catalyst was utilized in an FTS process, water gas shift as expressed by equation [2] is always an important reaction involved. This property makes the iron catalyst more suitable for a carbon-rich syngas produced from coal. A water gas shift coefficient was calculated by the following equation to evaluate the WGS activity: K WGS = P P P P [ H2] [ CO2] [ CO] [ H 2O] [5] Figure 4. gives the results of WGS coefficient and an initial conditioning period similar to the ones for CO conversion and CO 2 selectivity was found. The water gas shift activity rapidly increased from an initial value of 1.2 and then gradually decreased from it maxima of 2.8 to a stable level of 2.3. It indicates that potassium promoted iron catalyst showed a similar FTS and WGS activity trend. Deactivation and Regeneration of beryllium promoted iron catalyst A beryllium promoted iron catalyst with an atomic ratio of Fe:Be=100:1.44 was prepared and tested in this study. FTS was carried our at 270/C, 1.2MPa and 10 sl/h/g-iron. Table 1 gives the summary of results from the FTS reaction over beryllium promoted iron catalyst. At 270/C, a CO conversion of 51% and a CO2 selectivity of 45% were obtained; however, a higher methane selectivity of 8.14% than that over the potassium promoted catalyst at 230/C. In addition, a higher hydrocarbon rate and higher water gas shift activity were produced over beryllium promoted catalyst at 270/C. Figure 5 gives the results of CO conversion and deactivation rate from the FTS reaction over the beryllium promoted iron catalyst. After passing an initial conditioning period, CO conversion stabilized at 50% in the first 500 hours of time on stream. As indicated in Figure 4, a deactivation rate as low as % during the first 500 hours of time on stream, suggesting that a superior stability was achieved over beryllium promoted iron FTS catalyst. 32

34 The reaction was operated at a series of low space velocity at 500 hours and thus the catalyst deactivated significantly. Regeneration with hydrogen was carried out at the same temperature as the FTS but zero gauge pressure for 24 hours. As shown in the figure, CO conversion increased from 37.03% to 45.35% after hydrogen regeneration. Another regeneration with CO further improved the activity by almost 10% to 55.20%. This rejuvenated activity was maintained in the next 150 hours of time on stream. These results indicated that beryllium promoted catalyst showed a good regenerability and CO is a more efficient regeneration reagent than H 2. Conclusion Potassium promoted iron FTS catalyst showed a low deactivation rate of 0.083% per day after passing an initial conditioning period of 300 hours. Higher temperature generated a shorter conditioning period. The FTS activity began to decrease when the bulk phase of the catalyst changed from 100% carbide to iron oxides. Beryllium promoted iron catalyst produced an even more stable activity than potassium. A deactivation rate as low as % per day was obtained from beryllium promoted catalyst. Although the initial activity changed rapidly, little change occurred in the bulk phase of the catalyst, suggesting only the surface change may attribute to the activity change in the initial conditioning period. For potassium promoted catalyst, FTS activity, CO 2 selectivity, hydrocarbon productivity and water gas shift activity showed the same conditioning period of 300 hours of time on stream after passing a peak value at about 120 hours of reaction time. Methane selectivity, however, showed a monotonous decrease from an initial maxima to a stable level of 1.6%. Beryllium promoted iron catalyst showed a better deactivation property than potassium promoted catalyst, even operated at a higher reaction temperature than that over the potassium promoted catalyst. A higher water gas shift activity, higher CO2 and CH4 selectivity by beryllium promoted catalyst than those by potassium were also found in an FTS process. 33

35 Beryllium promoted catalyst also showed a superior regenerability when either H 2 or CO was utilized. Carbon monoxide rejuvenation generated a better CO conversion recovery than hydrogen. Acknowledgment Funding from the Department of Energy (DE-FC-26-98FT40308) and the Commonwealth of Kentucky are acknowledged. 34

36 References 1. X. Zhan, B. H. Davis, 1999 Spring Symp. of the Tri-State Catalysis Society, April 20-21, Louisville, KY, R. J. O Brien, L. Xu, R. L. Spicer and B. Davis, Symposium on Syngas Conversion to High Value Chemcials, , Presented at the 211 th ACS Annual Meeting, New Orleans, March 24-29, (1996). 3. M. E. Dry, in Catalysis Science and Technology, Vol. 1, , (1981). 4. D. B. Bukur, D. Mukesh, and S. A. Patel, Ind. Eng. Chem. Res., 29, 194 (1990). 5. R. J. O Brien, L. Xu, R. L. Spicer and B. H. Davis, Energy & Fuels, 10, 921 (1996). 6. Z. E. Huang, Ran Liao Hua Xue Xue Bao, (1990). 7. X. Z. Wang, Ran Liao Hua Xue Xue Bao, (1990). 8. D. B. Bukur, L. Nowichi and X. Lang, Energy & Fuels, 9, 620 (1995). 9. H. P. Bonzel, H. J. Kerbs, Surface Science, 109, L527 (1981). 10. R. J. O'Brien, Y. Zhang, H. H. Hamdeh, B. H. Davis, "Mossbauer study of precipitated unpromoted iron Fischer-Tropsch catalyst," Preprints, 44(1) ACS, Division of Petroleum Chemistry, Mar , Anaheim, CA, , (1999). 11. A. P. Raje, R. J. O Brien and B. H. Davis, J. Catal., 180, 36 (1998). 12. M. E. Dry, Appl. Catal. A: General, 189, 1850 (1999). 13. A. J. Forney, W. P. Haynes, J. J. Elliot, Zarochak, Am. Chem. Soc., Div. Fuel, 20, 3 (1975). 35

37 Table 1 Productivity and Selectivity for Beryllium Promoted Catalysts Conversion (%) CO H 2 Syngas Product distribution (%) C 1 C 2 -C 4 C 5 -C 11 C 12 -C 18 C 19 + Hydrocarbon rate, g/hr/g-iron Water-Gas Shift K p a 1.76 H 2 :CO usage b 0.81 Selectivity (%) CO CH a. Kp = [P H2 P CO2 ]/[P H2O P CO ]. b. Usage = r H2 / r CO. 36

38 Figure 1. Deactivation rate over potassium promoted iron FTS catalyst. 37

39 Figure 2. Methane and carbon monoxide selectivity over potassium promoted iron FTS catalyst. 38

40 Figure 3. Hydrocarbon rate over potassium promoted iron catalyst. 39

41 Figure 4. Water-gas shift coefficient over potassium promoted iron FTS catalyst. 40

42 Figure 5. CO conversion over beryllium promoted iron catalyst. 41

43 Task 3. Catalyst Characterization The objective of this task is to obtain characterization data of the prepared catalysts using routine and selected techniques. A. Characterization of Pt-Re Promoted Co/Al 2 O 3 Catalysts Introduction We are continuing to study the impact on reducibility of cobalt oxides by the use of different supports and by the incorporation of different promoters and additives to supported cobalt catalysts. The reduction of Co 3 O 4, which is present on the catalyst after calcination, is a two step process which passes through an intermediate CoO phase before reduction to the metal. In our prevoius investigations, we found by temperature programmed reduction that a 15%Co/Al 2 O 3 displayed a broad second peak attributed to the reduction of cobalt species interacting with the support. In agreement, hydrogen chemisorption with pulse reoxidation revealed that the unpromoted catalyst displayed poor percentage reduction of only 30% after reduction at 623K, the standard activation temperature of Co FTS catalysts. While promotion of supported Co catalysts with the noble metal promoters Pt and Ru had a similar effect on catalyzing both reduction steps of Co 3 O 4 to Co metal, Re only aided in catalyzing the second step when a significant interaction of the Co species with the support was present, such as found on Al 2 O 3 supported catalysts. After reduction at 623K, therefore, the number of Co active sites increases remarkably with the addition of noble metal promoter, thereby increasing the initial activity under reaction testing. We have investigated the possibility of a synergism between Pt and Re on the reducibility of supported Co oxides, and are testing the catalysts in the CSTR to draw conclusions as to the effect of Pt-Re promotion and synergism on the catalyst stability. 42

44 Catalyst Preparation Condea Vista Catalox B γ-alumina ( mesh, 200 m 2 /g, pore volume 0.4 cm 3 /g) was used as support material for the preparation of 15% loaded cobalt FTS catalysts. A three-step incipient wetness impregnation method was used to add 15 wt % cobalt to the supports with a drying procedure at 353K in a rotary evaporator following each impregnation. Noble metal promoted catalysts were prepared with different loadings of platinum and/or rhenium after cobalt addition and prior to calcination. Where both promoters were added, the Pt was loaded first. Catalysts were calcined only one time in air at 673K for 4hrs following the final impregnation step. Catalyst Characterization BET Surface Area BET measurements were conducted using a Micromeritics Tri-Star system for all catalysts to determine the loss of surface area, if any, following loading of the metal. Prior to testing, samples were slowly ramped to 433K and evacuated for 4hrs to approximately 50mTorr. Temperature Programmed Reduction Temperature programmed reduction (TPR) profiles of catalysts were recorded using a Zeton Altamira AMI-200 unit. Calcined fresh samples were first purged in flowing inert gas at 623K to remove traces of water. TPR was performed using a 10%H 2 /Ar mixture referenced to Ar at a flowrate of 30 ccm. The sample was heated from 323K to 1073K using a heating ramp of 10K/min and the H 2 consumption measured using a thermal conductivity detector (TCD).. H 2 Chemisorption by TPD and % Reducibility by Pulse Reoxidation The amount of chemisorbed hydrogen was measured using the Zeton Altamira AMI-200 unit. The sample weight was always g. The catalyst was activated using hydrogen at 623K for 10hrs and cooled under flowing hydrogen to 373K. The sample was held at 373K under flowing argon to prevent adsorption of physisorbed and weakly bound species, prior to 43

45 increasing the temperature slowly to the activation temperature. At that temperature, the catalyst was held under flowing argon to desorb the remaining chemisorbed hydrogen until the TCD signal returned to the baseline. The TPD spectrum was integrated and the number of moles of desorbed hydrogen determined by comparing to the areas of calibration pulses of hydrogen in argon. Prior to experiments, the sample loop was calibrated with pulses of N 2 in a helium flow and compared against a calibration line produced from using gas tight syringe injections of N 2 into a helium flow. The volume of the loop was found to be 52 µl. After TPD of H 2, the sample was reoxidized at 623K by pulses of pure O 2 in helium carrier referenced to helium gas. After oxidation of the cobalt metal clusters (where the entire O 2 pulse was observed by the TCD), the number of moles of O 2 consumed was determined, and the percent reducibility was calculated assuming that Co 0 reoxidized to Co 3 O 4. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) of adsorbed CO Infrared spectroscopy of adsorbed CO was performed on a Nicolet Nexus 870 FT-IR which is equipped with a DTGS detector. The catalyst was mixed with KBr and placed into a Spectra-Tech High Temperature/High Pressure DRIFTS cell with ZnSe windows that allowed us to perform in situ thermal pretreatments. For each IR spectrum, taken at a resolution of 8 cm -1, 128 scans were added. Samples were pre-reduced ex-situ under H 2 :He (2:1) flow of 100 ccm/g catalyst at 623 K and passivated with 1%O 2 in helium at room temperature for 24 hours. Prior to each spectrum, the catalyst was re-reduced in situ in a flow of 33%H 2 in helium for one hour, held in helium for 30 minuted and cooled under helium flow (note: oxygen scrubbers were employed) to room temperature. The background was recorded at this time. Then, the catalyst was exposed to 1%CO in helium for 30 minutes at room temperature, scanned, purged in helium for 30 minutes, and scanned again. The latter step was employed to remove the contributions of gas phase and weakly adsorbed CO. Results and Discussion 44

46 Results of surface area measurements by physisorption of nitrogen are reported in Table 1. Results show that the BET surface area for the 15% loaded Co catalysts on 200 m 2 /g (-Al 2 O 3 were all close to 160 m 2 /g. A weight % loading of 15% metal is equivalent to 20% by weight Co 3 O 4. If the Al 2 O 3 is the main contributor to the area, then the area of the Co/Al 2 O 3 catalysts should be m 2 /g = 160 m 2 /g, which matches the measured value. As shown in Table 1, addition of small quantities of noble metal promoters did not measurably impact the BET area. Figure 1 shows the results of TPR. The unpromoted catalyst exhibits two peaks, and remarkably, the second peak is much broader than observed for similarly loaded TiO 2 and SiO 2 supported Co catalysts. Note that in the TPR, we did not ramp the temperature high enough to observe the reduction of bulk cobalt aluminate species, which has been shown to occur above 1073 K with up to 30% loading of cobalt. Therefore, the broad peak on the unpromoted catalyst (ca. 700 to 1000 K) is attributed to the reduction of Co surface species interacting with the support, and the different shoulders are likely due to varying degrees of interaction with the support as a function of cluster size. The smallest Co surface species, with the greatest interaction with the support, are therefore likely represented by the 950 K shoulder. The precise identity of these species is not clear, although it is surmised that the species are either the result of a strong interaction between very small cobalt oxide clusters and the support (deviating from bulklike cobalt oxides and reducing at higher temperatures than the bulk oxides) or small surface Co species which include support atoms in the structure (reducing at temperatures below that of bulk Co-aluminate). Hereafter, the species responsible for this peak will be referred to loosely as Co surface support species. The addition of 0.5%Pt caused the peaks to shift markedly to lower temperatures, presumably due to spillover of H 2 from the metallic promoter to reduce the Co oxide and Co surface support species. The reduction of Re oxide occurs at higher temperatures than Pt oxide. Figure 1 shows that although there appears to be no improvement in the reduction of the low temperature peak 45

47 assigned to the reduction of cobalt oxides, Re still plays a valuable role in decreasing the reduction temperature of Co species for which there is a significant surface interaction with the support. Our previous work shows that Re oxide reduces at 620K, which may explain the lack of effect on the low temperature peak responsible for reduction of cobalt oxides. TPR profiles in Figure 1 show that bulk cobalt oxide has essentially been reduced before the Re oxide is reduced, so no spillover effect can operate to aid in reducing those species. However, H 2 spillover from the reduced Re metal occurs to facilitate reduction of Co species interacting with the support only after the reduction peak of Re oxide to Re metal is achieved. By keeping the same number of moles of noble metal promoter but by varying the ratio of Pt and Re, interesting effects on reduction are observed in TPR. The best molar ratio appears to be 50%Pt and 50%Re. Relative to Pt promoted catalyst, the addition of Re appears to sharpen the peaks somewhat, while relative to Re, both peaks are shifted to lower temperatures. Since our preliminary findings show that Re promotion results in a more stable catalyst than Pt promotion of Co/Al 2 O 3, addition of Pt to a Re-promoted catalyst may aid in catalyzing Re reduction. Results of H 2 chemisorption by TPD (Table 2) after reduction at 623K indicate that the number of surface sites increases with addition of either Pt or Re promoter. By performing pulse reoxidation, it is clear that the remarkable gain in Co 0 site density is mainly due to an enhancement in the reducibility of the clusters, and not to improvements in the actual dispersion (cluster size) of the reduced cobalt. Addition of Pt or Re causes a fraction of the smaller Co surface species that interact with the support to be reduced in this temperature range. Results of H 2 chemisorption by TPD and pulse reoxidation (Table 2) revealed that, for both unpromoted and promoted catalysts, there were significant increases (from 30% to approximately 60% with addition of 0.5% noble metal promoter). Maintaining the 50%Pt/50%Re molar ratio but 46

48 doubling the loading increased the percentage further to 72%. This is in line with the further observed shifts of both peaks in the TPR. DRIFTS of adsorbed CO is useful for probing the electronic and geometric effects of promoters on supported metal catalysts. Figure 2 shows the resulting spectra after purging the gas phase and weakly bound species with helium at room temperature. The linear stretch vibration of CO adsorbed on Co occurs at approximately 1980 cm -1, while the bridged species are observed at lower wavenumbers. An interesting trend emerges where increasing Re concentration results in a decrease in the linear to bridge-bonded CO ratio (Table 3). Typically, an increase in bridge bonded CO is the result of larger clusters present on the catalyst. The possibility also exists that the increase in linear bonded CO for the Pt promoted catalyst may be the result of a geometric alloying effect, whereby Pt might break up the geometry to Co sites. Clearly, adsorption of CO also occurs on the promoter. For CO on Pt, the band occurs at approximately 2080 cm -1. However, one cannot rule out the possibility of an electronic effect. To test for the possibility of alloy formation, we are continuing to study these catalysts by High Resolution Transmission Electron Microscopy. With this technique, we may be able to determine if there are changes in the lattice spacing due to alloy formation. At this time, important differences were observed for Pt and Re promoted catalysts in terms of reducibility and catalyst stability. Future catalytic testing results combined with characterization should provide insight into the nature of these differences. 47

49 Table 1 BET surface areas Support/Catalyst BET SA Ave Pore Size (nm) Calcination (m 2 /g) T (K) Condea Vista Catalox B γ-al 2 O Unpromoted 15%Co/ γ-al 2 O , flow 0.5%Pt/15%Co/Al 2 O , flow 0.477%Re/15%Co/Al 2 O , flow 0.239%Re/0.25%Pt/15%Co/Al 2 O , flow 0.477%Re/0.5%Pt/15%Co/Al 2 O , flow 0.358%Re/0.125%Pt/15%Co/Al 2 O , flow %Re/0.375%Pt/15%Co/Al 2 O , flow 3 48