Ind. Eng. Chem. Res. 2008, 47, 4063–4070
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Ammonia Decomposition on Tungsten-Based Catalysts in the Absence and Presence of Syngas Sourabh S. Pansare and James G. Goodwin, Jr.* Department of Chemical and Biomolecular Engineering, Clemson UniVersity, Clemson, South Carolina 29634-0909
Synthesis gas produced from biomass gasification can serve as a starting point for producing electricity, low-to-medium energy fuels, and even hydrogen for fuel cells. The major barrier in commercialization of biomass gasification is the presence of impurities such as NH3, tars, and H2S in the gas products that are detrimental to downstream processes. This paper reports the results of a study of NH3 decomposition to N2 and H2 on tungsten-based catalysts, tungsten carbide (WC) and tungstated zirconia (WZ), for a gasification gas cleanup strategy that involves removal of tars first followed by NH3 decomposition. The effects of the presence of H2 and CO on the behavior of these catalysts are also reported. At the NH3 decomposition reaction conditions used in the present study (1 atm, 465-650 °C, 4000 ppm), both WC and WZ showed an induction period. The main reason for this induction period is hypothesized to be a restructuring of the catalyst surface by NH3 so that the surface is more favorable for reaction. Both WC and WZ displayed superior activity compared to a commercial Fe-based NH3 synthesis catalyst (Amomax-10). At 600 °C and in the presence of syngas, no conversion was observed on WC while ca. 20% and 10% conversions were observed on WZ and Amomax-10, respectively, at steady state for the reaction conditions used. This corresponds to intrinsic rates of 1.2 µmol/g cat./s and 0.53 µmol/g cat./s (0.021 µmol/m2 cat./s and 3.1 µmol/m2 cat./s), respectively, for WZ and Amomax-10. 1. Introduction Environmental problems, the rising price, and anticipated scarcity of petroleum have triggered an increased interest in renewable energy sources such as biomass, defined as any organic material of plant origin.1 Among other renewable energy resources, biomass is the only source for liquid transportation fuels and probably the only economical source of H2. Currently, biomass contributes ca. 10-14% of the world’s energy needs.2 Solar energy is stored as chemical energy by photosynthesis in various biomass resources such as forestry products, agricultural byproduct, and municipal wastes.3 This chemical energy can be transformed through gasification to produce low-to-medium energy fuel gases, synthesis gas for production of a variety of chemicals and liquid fuels, and hydrogen for fuel cell applications. An inherent problem and hence a major barrier in commercialization of biomass gasification technology is the presence of impurities such as NH3, tars, and H2S in the syngas produced. Organic nitrogen is the major source of formation of NH3 with ca. 60-80% getting converted to NH3 during the gasification process.4 When the gasification gas stream is fed to turbines for power generation, NH3 gets converted to NOx with conversion levels as high as 50%.4 The biomass gasification stream can also be considered as a potential source of H2 for proton exchange membrane fuel cells (PEMFC). But NH3 present in the stream can block acid sites of Nafion membranes used for proton transfer in PEMFC resulting in poor performance. The presence of NH3 in the biomass gasification stream may also interfere with the direct conversion of the H2 and CO to liquid transportation fuels via the Fischer-Tropsch synthesis. Hence, it is very important to be able to decompose NH3, preferably to N2 and H2, especially in the presence of syngas (H2 + CO) before any downstream utilization of the gasification gas. Thus, the development of catalysts that can decompose NH3 in the * To whom correspondence should be addressed. E-mail: jgoodwi@ clemson.edu. Tel.: (864)-656-6614. Fax: (864)-656-0784.
presence of these gases represents a key step in commercialization of biomass gasification. NH3 decomposition has been widely studied, mainly due to its relevance to NH3 synthesis. The conventional catalysts for NH3 decomposition include catalysts based on Ru,5 Ni,5,6 Fe, W,7–9 and V.10 After the pioneering work of Levy and Boudart11 that demonstrated Pt-like behavior of tungsten carbide (WC), the interest in WC-catalyzed reactions increased tremendously. Reactions studied using WC include ethylene hydrogenation,12 methane reforming,13 NH3 synthesis,14 NOx reduction,15 and electrocatalytic hydrogen oxidation.16 WC has been found to be able to act as a bifunctional catalyst.17,18 Boudart and coworkers have shown that WC with small amounts of adsorbed O can behave similar to Pt supported on SiO2 or Al2O3 and is active for isomerization and hydrogenolysis reactions.19–21 The chemisorbed oxygen introduces WOx acid sites similar to those present on supported WO3 catalysts that catalyzes these reactions through formation of carbenium ions.17–19 Besides the bifunctional character, WC also offers other excellent properties such as extreme hardness, attrition resistance, and sulfur tolerance.22–24 Recently, we have reported that WC is highly active for the decomposition of NH3.25 Since the original work of Hino and Arata,26 there has been much interest in the use of tungstated zirconia (WZ) for various acid-catalyzed reactions. Tungstated zirconia had been shown to be active for isomerization of n-butane, 26 n-pentane,27 and o-xylene.28,29 Recently, WZ had also been shown to be active for esterification, transesterification, and biodiesel synthesis.30,31 WZ also has the capability to form WC sites and, hence, it has the potential to function as a bifunctional catalyst. The current paper reports the results of an investigation into NH3 decomposition on W-based catalysts (WC and WZ). The behaviors of these catalysts are also compared with a commercial Fe-based NH3 synthesis catalyst. A critical requirement for catalysts that are active for NH3 decomposition and that could be used in biomass gas cleanup is satisfactory performance
10.1021/ie800077p CCC: $40.75 2008 American Chemical Society Published on Web 05/23/2008
4064 Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 Table 1. Physicochemical Properties of WC, WZ, and Amomax-10 surface acidity
catalyst
calcination temp (°C)
W surface density (W atoms/nm2)
surface area (m2/g)a
WC WZ WZ WZ WZ Amomax-10
450 700 800 900 -
2.3 5.2 6.6 7.7 -
1.5 ( 0.3 173 86 68 57 0.17 ( 0.03
a
XRD phases
NH3-TPD (µmol NH3/g)b
exchange titration (µmol/g)b
hexagonal WC t-ZrO2 t-ZrO2, WO3 t-ZrO2, m-ZrO2,WO3 -
0 100 54 44 18 -
137 161 72 -
Maximum error ) (4% unless otherwise indicated.b Maximum error ) (8%.
in the presence of syngas. In the current study, the behavior of these W-based catalysts in the absence and presence of syngas (H2 and CO) is reported. 2. Experimental Section 2.1. Materials and Gases. The WC catalyst was obtained from Alfa Aesar while the WZ catalyst was provided by Magnesium Electron, Inc. A commercial Fe-based NH3 synthesis catalyst (Amomax-10) was obtained from Su¨d-Chemie. All pure gases used in the current study (H2, He, CO) including a standard gas mixture of 10% NH3 in He were UHP grade from National Specialty Gases. 2.2. Catalyst Characterization. The BET surface areas of WC, WZ, and Amomax-10 were determined using a Micromeritics ASAP 2010 analyzer. The samples were degassed at 300 °C for 3 h followed by N2 adsorption at 77 K. XRD analyses of WC and WZ were performed using a Philips PW3050 X’Pert X-ray diffractometer with monochromatized Cu KR radiation and a Ni filter. NH3-temperature programmed desorption experiments were performed using an Altamira AMI-1 system. The catalyst (0.2-0.3 g) was heated from room temperature (RT) to 315 °C in the presence of 30 sccm of He for 1 h. The sample was cooled to RT followed by saturation with 100 sccm of 10% NH3/He for 2 h. The catalyst was then flushed with He at RT and the temperature then raised to 60 °C and held for 4 h in He to remove any physisorbed NH3. Next, the temperature was raised to 600 °C at a ramp rate of 10 °C/min and the desorbed NH3 was measured using a TCD detector. Ion exchange titrations to determine the acid strength of catalysts were performed as described elsewhere. 32 Briefly, the method involved an exchange between H+ ions of the catalyst and Na+ ions of the NaCl solution in which the catalyst was added at 28 °C. The resulting ion exchange liquid was then titrated with aqueous NaOH solution and the total acidity of catalyst was determined. Scanning electron micrographs of fresh WC and WZ were collected using a Hitachi FESEM-4700 and a Hitachi HD-2000, respectively. The accelerating voltages were 5 and 200 kV for WC and WZ, respectively. WC was coated with Pt to prevent charging effects. The use of the Hitachi HD-2000 did not require coating of WZ with Pt. 2.3. Reaction Studies. The TOS behavior of WC and WZ for NH3 decomposition was followed at several temperatures in the range of 465-650 °C at 1 atm in a stainless-steel plug flow microreactor (8 mm i.d.). About 50-55 mg of catalyst was placed at the center of the reactor sandwiched between quartz wool with a thermocouple at the bottom of the catalyst bed. Samples of WC were pretreated at 650 °C for 1 h (after a ramp of 5 °C/min from 30 °C) in a 100 sccm flow of an 80/20 mixture of H2/CO as preliminary experiments indicated this as
the best pretreatment. After pretreatment, the H2/CO flow was turned off, a flow of He was turned on, and the reactor was allowed to cool to the desired reaction temperature. For WZ, the pretreatment was carried in a flow of He at 650 °C for 1 h and then the catalyst was cooled down to the desired reaction temperature. Amomax-10 was pretreated in the flow of pure H2 at 650 °C for 1 h. A stream of 4000 ppm of NH3 in He (total flow rate: 100 sccm) was fed to the reactor after the reaction temperature was reached. For reaction runs in the presence of syngas, flows of 10% H2 and 15% CO were used to replace some of the He while keeping the total flow rate at 100 sccm and the concentration of NH3 4000 ppm. The effluent from the reactor was analyzed using a Varian CP-3800 GC equipped with three columns (Poraplot, CPSil5CB, and CP-Molsieve 5A) and two parallel detectors (one TCD and one FID). Mears’ criterion for external diffusion indicated no external mass transfer effects existed at the reaction conditions used for both the catalysts. Calculation of Weisz-Prater parameters for WC and WZ indicated that there were no internal mass transfer effects at the reaction conditions used in the present study. 3. Results and Discussion 3.1. Catalyst Characterization. The physicochemical properties of WC, WZ, and Amomax-10 are shown in Table 1. Fresh Amomax-10 showed a very low surface area of 0.17 ( 0.03 m2/g, typical of bulk Fe catalysts. WC only showed the hexagonal phase of WC while features related to W, W2C, and WO3 were absent. The highly crystalline nature of WC was further confirmed by SEM (Figure 1a). The W loading in the WZ based on elemental analysis was 13.6 ( 0.4 wt % W.31 Lo´pez et al.31 have observed that the calcination temperature of WZ has a significant effect on esterification and transesterification reaction rates in both liquid and gas phase. In order to investigate whether the calcination temperature of WZ had any effect on NH3 decomposition, WZ was calcined at 450, 700, 800, and 900 °C. The BET surface areas, W-surface densities, XRD phases, and surface acidities of the calcined samples are reported in Table 1. The SEM micrograph of fresh WZ calcined at 900 °C is shown in Figure 1b. It was observed that there was a drop in the surface area with increasing calcination temperature. The original dehydrated WZ had a surface area of ca. 325 m2/g (not shown in Table 1). While below a calcination temperature of 800 °C only the tetragonal phase of ZrO2 was observed, at 800 °C and above, WO3 and the monoclinic ZrO2 phase were also detectable. The W-surface density was estimated to have increased monotonic from 2.3-W atoms/nm2 at 450 °C to 7.7-W atoms/nm2 at 900 °C since W and Zr do not form mixed oxides under these conditions. On the other hand, the acidity values obtained from NH3-TPD and ion-exchange titrations showed contrasting values. Acidity values from NH3TPD showed a decrease with increase in temperature while those
Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 4065
Figure 2. Time-on-stream behavior of WC at different temperatures in the absence of H2 and CO (pretreatment in the presence of a 80/20 mixture of H2/CO at 650 °C for 1 h).25
Figure 1. SEM micrographs (a) fresh WC; (b) WZ calcined at 900 °C.
from ion-exchange titrations went through a maximum at 800 °C. Although the two profiles did not match perfectly with each other, both of them showed lower acidity values for WZ calcined at 900 °C. One main difference between the two measurements is the presence of H2O during ion-exchange titrations which could be responsible for generation of additional acid sites. Generation of acid sites due to H2O pretreatment of WZ has been shown using NH3-TPD by Naito et al.33 A question can be asked whether the adsorbed NH3 decomposed during TPD on WZ. The NH3-TPD peak for WZ was centered ca. 180 °C while WZ was active for NH3 decomposition only above 465 °C. Hence, it is highly unlikely that the adsorbed NH3 decomposed during NH3-TPD. 3.2. Reaction Studies of NH3 Decomposition in the Absence of Syngas. 3.2.1. WC. The TOS behavior of WC in the absence of syngas is shown in Figure 2. The catalyst showed an induction period which may have decreased in time with increasing temperature.25 Complete conversion of NH3 was observed at 5 min TOS at 550 °C and a space velocity of 4 900 000 h-1. A possible reason for the observed induction period could be that NH3 initially partially restructured the WC surface and made it more favorable for reaction resulting in an increase in activity.25,34 As stated earlier, WC was pretreated in the presence of an 80/20 mixture of H2/CO for all the reaction runs. Previous experiments showed that there was deposition of carbonaceous material during this pretreatment (as confirmed by energydispersive X-ray spectroscopy) on surface irregular sites such as pits and edges of WC crystals and that this pretreatment made the WC surface more uniform.25 SEM after ca. 5 h of NH3 decomposition at 550 °C showed similar carbonaceous deposits
Figure 3. SEM micrograph of WC after 5 h TOS of reaction at 550 °C (pretreatment in the presence of a 80/20 mixture of H2/CO at 650 °C for 1 h).
remaining, as shown in Figure 3. The effect, if any, of these carbonaceous deposits on NH3 decomposition is still unclear. 3.2.2. WZ: Effect of Calcination Temperature. The steadystate rates of NH3 decomposition at 500 °C and 1 atm on WZ calcined at various temperatures are shown in Figure 4a. The surface acidity values obtained from NH3-TPD are also shown in Figure 4a. As seen from Table 1, the surface area of WZ decreased significantly with an increase in the calcination temperature. In order to account for this effect, the rate of NH3 decomposition was plotted on a “per surface area” basis and is shown in Figure 4b. The W-surface density of WZ (there is no evidence that W can be incorporated into the bulk of ZrO2) as a function of calcination temperature is also shown in Figure 4b. From the figure, it is observed that the steady-state rate of reaction (on both g of catalyst and m2 of catalyst bases) increased with an increase in calcination temperature. The highest rate of 0.013 µmol/m2 cat./s was observed for WZ calcined at 900 °C. The activity curve did not follow the total acidity (per g of catalyst) profile obtained from either ionexchange titrations or NH3-TPD, but it did follow the W-surface density profile (W-atoms/nm2). The question can be asked as to why the total acidity does not correlate with the activity for NH3 decomposition. There are at least two possible explanations for this question. The first
4066 Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008
Figure 5. Time-on-stream NH3 decomposition behavior of WZ calcined at 900 °C at different temperatures (pretreatment in the presence of He at 650 °C for 1 h).
Figure 4. Effect of WZ calcination temperature on steady-state rate of NH3 decomposition at 500 °C: (a) per g basis; (b) per m2 basis. For comparison, the variations in WZ surface acidity and W surface density with the calcination temperature is also shown.
logical explanation could be the dependence of NH3 decomposition on support acidity. NH3 decomposition is known to be dependent on support acidity35,36 with more acidic supports being less active. As seen from Figure 4a, the total acidity values obtained from NH3-TPD decreased with increase in calcination temperature. The lowest value of acidity was 18 µmol NH3/g cat. for WZ calcined at 900 °C. The same catalyst showed the highest steady-state activity of 0.7 µmol/g cat./s. Thus, the catalyst with the least apparent acidity showed the highest activity and vice versa, in accordance with the literature. The second logical explanation could be the possibility of an increase in the number of active sites at higher temperatures. According to Kim et al.,37 above a monolayer coverage of W (ca. 5 W-atoms/nm2), the presence of bulk WO3 becomes more dominant supposedly on the top of WOx species. In the present study, only WO3 was evident from XRD analysis. When WZ was calcined at temperatures less than or equal to 700 °C, there was no XRD evidence for significant formation of WO3 particles >4.0 nm and the active sites for NH3 decomposition were potentially the WOx clusters or perhaps very small WO3 particles. However, for WZ calcined at 800 °C or above, WO3 particles >4 nm were present and possibly contributed the most active sites, along with WOx clusters. This increase in the number of active sites may have been the cause for the increase in rate/g and rate/m2 at the higher calcination temperature. An increase in TOF for methanol dehydration at 230 °C on WZ due to the high activity of WO3 nanoparticles present was observed by Kim et al.37 As WZ calcined at 900 °C showed the best performance for NH3 decomposition, it was chosen for further investigation. The catalyst is henceforth referred to as WZ900. 3.2.3. WZ900: TOS Behavior. The TOS behavior of WZ900 in the absence of syngas is shown in Figure 5. From the figure
it is observed that WZ900, similar to WC, showed an induction period. At 465 °C, it took almost 45 min for WZ900 to attain a steady-state conversion of ca. 9%. As the temperature of the reaction was increased, a smaller change in conversion during the induction period was observed but the induction period appeared to be longer. No induction period was observed at 600 °C and a complete conversion of NH3 was obtained at 5 min TOS and at a space velocity of 346 000 h-1. As discussed earlier, the possible reason for the observed induction period in the case of WC may be the time required for NH3 molecules to reconstruct the surface to be more favorable for the reaction. A similar reason can be hypothesized also in the case of WZ900. The surface of WZ900 consisted of acid sites in the form of polyoxotungstate clusters and WO3 nanoparticles. There could be a possibility that at lower reaction temperatures (
