6.5. CATALYTIC OSR OF HYDROCARBONS
6.5.3. Transportation Fuels
Heavier liquid hydrocarbon fuels, like gaso-line and diesel, have higher gravimetric and volumetric H2 densities compared with lighter gaseous hydrocarbon fuels in a convenient, transportable form. OSR of these fuels is there-fore desirable in transportation applications where H2storage is currently limited and cannot adequately meet the demands of the fuel cell load. Compared to lighter molecular weight hydrocarbons, gasoline and diesel are more chal-lenging to reform. Rates of carbon formation are much higher since longer chain alkanes will undergo pyrolysis upon vaporization. The carbon chains linked to the C-radicals formed during the reaction will readily deposit onto the catalyst surface and easily polymerize into graphitic carbon. The presence of aromatics and FIGURE 6.25 Comparison of activity for OSR of propane
(O/C¼ 1.35, S/C ¼ 3.0, 9600 ml (STP) gcat1 h1) for Ni/
MgAl-hydrotalcite like oxide and traditional Ni/g-Al2O3; Reprinted from Lee et al. [178], Copyright (2009), with permis-sion from Elsevier.
6. OXIDATIVE STEAM REFORMING
164
polyaromatic compounds, which are not found in other fuels, further contributes to carbon formation as they are not only less reactive, but also chemically similar to graphitic carbon.
Sulfur compounds are also commonly found in these fuels at higher levels than other fuels.
Levels of sulfur can reach up to 3000 ppmw for military fuels, however, most fuels like pump gasoline and diesel will have substan-tially less (9e15 ppmw). For fuels containing sulfur, reforming temperatures must be kept high to reduce the irreversible formation of metal sulfides. At the expense of obtaining maximum H2 yields, which occurs around 700C (seeFig. 6.2), temperatures for reforming are usually maintained above 800 C to mini-mize sulfur poisoning, although higher oper-ating temperatures also reduces carbon formation. Researchers from Argonne National Lab (ANL)[182]show the performance benefits, in terms of reforming efficiency (likely defined by HHV of H2/HHV fuel, but not mentioned), for increasing reaction temperature for OSR of dodecane (O/C¼ 1.0; S/C ¼ 1.0) from 700 to
800C (Fig. 6.26). Catalyst tests were performed over a Ru-substituted perovskite catalyst.
Figure 6.26shows three sequential steps in their catalyst test: (1) establish baseline activity of OSR ofn-dodecane, (2) examine effect of adding 50 ppmw sulfur for an extended time, and (3) remove sulfur to establish recovery to baseline.
It can be seen that the efficiency of the catalyst in each part of the test was higher at 800 C, especially in the presence of sulfur. The recovery was also greater indicating the effects of sulfur were reduced at the elevated temperature.
6.5.3.1. Nickel-Based Catalysts 6.5.3.1.1. ALUMINA
Ni-based catalysts studied for the OSR of transportation fuels primarily consist of a varia-tion of Ni/Al2O3, which is used for commercial CH4 SR applications. These are considered benchmark catalysts for reforming due to their widespread use and well-characterized physical and chemical properties, as well as their low cost.
When used as a catalyst for OSR of larger MW hydrocarbon fuels, the Ni-based SR catalysts suffer rapid deactivation from carbon formation, sulfur poisoning, and thermal degradation.
Deactivation of these catalysts is likely due to the large clusters of Ni on the surface. Despite the presence of promoters, these clusters can be easily poisoned by carbon and sulfur. Moon et al. [183]performed an initial screening study on a commercial Ni-based catalyst (Ni-SiO2 -MgAl2O4) to determine the effects of gasoline constituents (either aromatics or sulfur) on carbon formation. Based on the catalyst used, it was established that the accumulation of carbon could be reduced at temperatures >640 C.
Meanwhile, the presence of sulfur was found to not only promote carbon formation, but also deactivate the catalyst through irreversible sulfur poisoning. Consistent with the work by Liu et al.
[182], it was recommended that an elevated oper-ating temperature (>770C) was needed to mini-mize the effects of sulfur.
FIGURE 6.26 Effect of increasing operating temperature on reforming efficiency of a Ru-substituted perovskite (LaCr0.95Ru0.05O3) in the presence of sulfur (O/C = 1.0, S/C
= 1.0, and 100,000 h1). Reprinted from Liu et al. [182], Copyright (2004), with permission from authors.
CATALYTIC OSR OF HYDROCARBONS 165
Continuing their work on the development of carbon and sulfur tolerant Ni-based catalysts, Moon and coworkers [184,185] evaluated numerous supported transition metal catalyst formulations for the OSR of isooctane with and without sulfur. The addition of small amounts of either Fe, Co, or Mo (2.94 wt%) to Ni (11.76 wt%)/MgO/Al2O3 improved H2
yields compared with a 14.7 wt% Ni/MgO/
Al2O3catalyst, possibly by enhancing the WGS reaction[184]. Promoted catalysts also showed tolerance to small levels of sulfur in the feed.
The Ni/Fe/MgO/Al2O3 catalyst produced near-equilibrium amounts of H2for over 700 h during OSR of isooctane (700C; O/C¼ 1; S/
C¼ 3) with less than 5 ppmw sulfur [184]. In the presence of higher levels of sulfur (100 ppmw), the Fe-promoted Ni catalyst had a slower rate of deactivation compared with the commercial catalyst (Ni-SiO2-MgAl2O4) when tested at the same conditions (700 C;
O/C ¼ 1; S/C ¼ 3) for 25 h (see Fig. 6.27)
[183,184]. Activity loss could likely be attributed to morphological changes on the catalysts which were more pronounced for the commercial cata-lyst. Increasing levels of sulfur were found to accelerate the BET surface area loss, as well as active metal surface area reduction, even in the Fe-promoted Ni metal catalysts. High levels of sulfur promoted severe carbon formation on each catalyst. Despite showing more stable activity, the Fe-promoted Ni catalyst had 20 wt% carbon formed after the 25-h time on stream [183,184]. While this value was lower than the commercial catalyst, the amount is still limiting, and given sufficient time would even-tually deactivate the catalyst completely.
6.5.3.1.2. OXYGEN-CONDUCTING SUPPORTS Krumpelt et al. [6] from ANL developed a series of new reforming catalysts using concepts of material properties from solid oxide fuel cell-based technologies for supports.
Commonly used transition metals were sup-ported on oxygen-conducting substrates such as Ce, Zr, or LaGaO3doped with small amounts of non-reducible oxides (Gd, Sm, or Ze) for the OSR of i-C8 (O/C ¼ 0.46 and S/C ¼ 1.14).
Although specific weight loadings for each metal were not provided, the supported non-precious metals were shown to have comparably activity, in terms of conversion, to noble metals at temperatures above 700C. However, for most catalysts, the selectivity to synthesis gas was much lower than the precious metals, which generally precludes their use for reforming.
Their results, shown inFig. 6.28, confirm Ni to be the most active and selective non-precious metal. Co also shows favorable activity and selectivity, but is again lower than Ni at all condi-tions tested. Activity improvements are believed to be a result of an interaction between the metal and oxygen vacancies in the support, which provide an oxygen supply to the metal-reducing carbon formation around the active metal.
Schwank’s group from the University of Michigan have investigated the interaction FIGURE 6.27 Activity test of commercial CH4-reforming
catalyst and 2.94 wt% Fe/11.76 wt% Ni/MgO/Al2O3 cata-lyst for OSR of isooctane containing 100 ppmw S (O/C¼ 1, S/C ¼ 3, 700 C and 8776 h1); shaded symbols: Fe-promoted catalyst and open symbols: commercial catalyst;
Reprinted from Kim et al. [184], Copyright (2008), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
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between Ni and Ce0.75Zr0.25O2(CZO) on activity and carbon formation for the OSR of gasoline and diesel surrogates. High conversions and selectivities to reforming products have been observed over various forms (either powder or foam) and weight loadings of Ni [186,187].
Despite the oxygen-conducting properties of the support, carbon formation, especially varia-tions of that with a filament nature, was still observed to be a major hindrance for Ni during reaction, even at weight loadings as low as 5 wt
%[98,188,189]. OSR experiments on engineered foam catalysts (23 wt% CZO/cordierite) contain-ing Ni loadcontain-ings between 1 and 16 wt% suggest a weight loading of 2 wt% was a compromise between optimal synthesis gas yields and carbon formation [98,186]. The formation of filament carbon was found to require a critical particle size for growth and a minimum number of Ni atoms per unit support area[189]. They deter-mined that by maintaining Ni particles below 26 nm during reaction, carbon formation over Ni/CZO could be minimized[98].
PEROVSKITES To avoid the thermal degra-dation (sintering) and corresponding deactiva-tion by carbon and sulfur of supported metals, researchers have employed mixed-metal oxides
containing Ni or Co to improve size control of active metal particles during the OSR of heavier hydrocarbons[190e194]. Perovskite-type mate-rials have been widely studied as anode and cathode materials for solid oxide fuel cells, and the same properties utilized for these applica-tions are advantageous for reforming: thermal stability, OSC, and oxygen-ion mobility. Perov-skites have the general formula ABO3where A is a trivalent lanthanide series element, and B is a trivalent transition metal. In general, the element in the A-site provides thermal stability and B-site is responsible for catalytic activity.
Both the A-site and the B-site in the perovskite structure can be doped with other elements to improve catalyst stability and performance giving the general formula A1xA0xB1yB0yO3z. The partial substitution of the A-site element with one of a different valence has been shown to create structural defects in the lattice, which may improve oxygen mobility, and help reduce carbon formation. The acid-base nature of the dopant in the A-site may also play a role in limiting carbon formation. Alkaline earth metals are often used as A-site dopants for perovskites, as they provide improved Lewis basicity, a prop-erty linked to suppression of carbon [193,195], and oxygen vacancies due to their lower valence.
FIGURE 6.28 Conversion and H2selectivity as a function of temperature for the OSR of i-C8(O/C¼ 0.46, S/C ¼ 1.14, and 3000 h1) over various transition metals supported on Sm-Gd-doped ceria;Reprinted from Krumpelt et al.[6], Copyright (2002), with permission from Elsevier.
CATALYTIC OSR OF HYDROCARBONS 167
As shown inFig. 6.29, undoped LaNiO3and LaCoO3 perovskites were found to produce near-equilibrium conversion and selectivity of H2 during OSR of i-C8 at 650 C [194].
However, these materials were not structurally stable in the OSR atmospheres and both cata-lysts were reduced under the reaction condi-tions and decomposed into Ni/NiO or Co/
CoO and La2O3, which are essentially sup-ported metal catalysts [191,193]. While high yields of H2 were produced over these cata-lysts, the benefits of dispersion of the perov-skites diminished and the metal particles
were susceptible to carbon formation and sulfur poisoning.
Mawdsley et al.[193]found that the structural stability of perovskite materials could be improved by the substitution of a majority of Ni or Co with less active, but more stable metals (Fe, Cr, and Mn) with minimal decrease in H2
yield for OSR of i-C8 at 700C. Cr was shown to be the best in terms of H2yield and conver-sion. However, when these catalysts (LaM0.9
-Ni0.1O3) were then further substituted with Sr in the A-site and tested for activity in the OSR of i-C8 with various levels of sulfur, all were FIGURE 6.29 Isooctane conversion and H2selectivity of undoped perovskite materials (O/C¼ 0.76, S/C ¼ 2.0, 650C, and 8000 h1);Reprinted from Qi et al.[194], Copyright (2005), with permission from Elsevier.
FIGURE 6.30 The effect of sulfur (5 and 50 ppmw) on the OSR of i-C8 gasoline surrogate fuel.
(La0.8Sr0.2Cr0.9Ni0.1O3, La0.8Sr0.2
Mn0.9Ni0.1O3, and La0.8Sr0.2Fe0.9
Ni0.1O3); (O/C ¼ 0.9, S/C ¼ 1.6, 700 C, and 25,000 h1); Reprinted from Mawdsley et al.[193], Copyright (2008), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
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shown to have a low tolerance to sulfur, even levels as low asw5ppmw. Of their compositions tested the La0.8Sr0.2Cr0.9Ni0.1O3had the highest sulfur tolerance (Fig. 6.30). SEM images showed existence of whisker carbon on the surface of both La0.8Sr0.2Fe0.9Ni0.1O3 and La0.8Sr0.2Cr0.9 -Ni0.1O3 catalysts, suggesting that Ni metal still enriched the surface and formed clusters large enough to promote this type of carbon. In light of these results, it is likely that deactivation is due to the combination of both sulfur poisoning and carbon formation.
The addition of Ce into the A-site of the perovskite structure has been shown to improve catalyst activity. Ce is believed to promote local-ized oxygen transfer from the bulk to the active metal and help reduce the rate of carbon forma-tion. Despite improved oxygen mobility, Ce was found to have limited solubility in the perov-skite lattice. According to Qi et al.[194] when the Ce substitution level for a La1xCexMeO3 catalyst (Me ¼ Ni, Co, Fe, etc.) exceeds x >
0.05, segregation of both Ce and Me will occur.
They found yields of H2 and CO to increase up to Ce substitution level of x ¼ 0.2, but decreased upon further addition. At substitu-tion level of x ¼ 0.5 no perovskite phase was observed at all and yields were lowest (seeTable 6.13). Erri et al.[190]found that while some Ce segregation was observed, its substitution
significantly improved carbon resistance com-pared with perovskites without Ce during the OSR of a JP-8 surrogate fuel (Table 6.14).
Although the Ce-substituted perovskite may have greater carbon resistance, little benefit in terms of sulfur tolerance is obtained. Qi et al.
[194]found sulfur concentrations higher than 5 ppmw would severely deactivate the Ce-con-taining perovskite catalysts.
To improve sulfur tolerance of perovskites, Dinka and Mukasyan [196] investigated the substitution of 2 wt% non-noble metal (Me ¼ K, Na, Li, Cs, Co, Mo) or noble metal (Me¼ Pt, Pd, Ru, Re) dopant into the structure of a La0.6Ce0.4 Fe0.8ezNi0.2MezO3 perovskite. All dopants led to an increase in conversion and H2content compared with the catalyst with no additive for OSR of a JP-8 surrogate containing 50 ppmw sulfur (800 C; O/C ¼ 0.7, S/C ¼ 3, and GHSV ¼ 130,000 h1). Interestingly, almost all non-noble metals had undetectable levels of carbon during this reaction. However, the noble metals showed considerable carbon formation, with the exception of Ru.
Sulfur and carbon tolerance of alkali metals may be explained by a modification of the elec-tronic properties of the metal by improving surface basicity and electron donor properties [197], which may hinder adsorption of electron-rich C and S molecules through steric effects.
TABLE 6.13 Effect of Partial Substitution of La with Ce on Perovskite (La1-xCexNiO3) Activity (O/C ¼ 0.76, S/C ¼ 2.0, 800C, and 8000 h1)[194]
x
Yield of
(COD H2), mol/mol C n-Octane conv. (%)
Initial 8 h Initial 8 h
0 2.34 2.20 100 98
0.1 2.30 2.30 100 100
0.2 2.36 2.36 100 100
0.5 2.12 2.12 98 98
TABLE 6.14 Effect of Ce Addition on Carbon Formation on Perovskite Catalysts after OSR of JP-8 Surrogate (O/C ¼ 0.75, S/C ¼ 3.0, 650C, WHSV ¼ 130,000 h1)[190]
Catalyst Carbon (wt%)
LaFe0.8Ni0.2O3 0.6
LaFe0.6Ni0.4O3 0.5
LaFe0.4Ni0.6O3 1.8
La0.6Ce0.4Fe0.8Ni0.2O3 0.1 La0.6Ce0.4Fe0.6Ni0.4O3 0.2 La0.6Ce0.4Fe0.4Ni0.6O3 0.1
CATALYTIC OSR OF HYDROCARBONS 169
Large amounts of carbon formation on noble metals Pt, Pd, and Re may be a result of their segregation to the surface and forming metal clusters, which are active for the decomposition of hydrocarbons but also large enough to easily accumulate carbon and be deactivated by sulfur.
Without further characterization it is not clear why Ru has higher resistance to carbon forma-tion. However, Ru may remain well dispersed at the surface in clusters small enough to not be deactivated by carbon and sulfur. The presence of Ru may also improve the stability of the perov-skite by hindering the diffusion of oxygen from the lattice while under reducing atmospheres [191]. Further testing of perovskite La0.6Ce0.4
-Fe0.8ezNi0.2MezO3 catalysts with optimized Ni loading doped with Ru (1 wt%) or K (2 wt%) has shown these materials to exhibit tolerance of sulfur levels up to 220 ppmw during OSR of a JP-8 surrogate fuel as shown inFig. 6.31.
6.5.3.2. Noble Metals 6.5.3.2.1. RHODIUM
Catalyst development efforts at ANL deter-mined Rh metal to be the most active and
selective catalyst for the OSR of higher hydrocar-bons[198,199]. A Rh catalyst supported onto an oxygen-conducting oxide, gadolinium-doped ceria (GDC), was found to be more active than corresponding Ni or Pt catalyst on the same support, and had product yields close to equilib-rium (seeFig. 6.32)[198]. Their results are consis-tent with those obtained by Ayabe et al. [13]
shown in Fig. 6.15, suggesting the activity of metals for OSR of higher hydrocarbons may also follow similar trends which were observed for OSR of CH4and steam reforming.
When selecting a substrate carrier for a Rh catalyst, dispersion and reducibility of the metal must be considered. Ferrandon et al.[133] evalu-ated five supports, Gd-CeO2, Y-ZrO2, g-Al2O3, La-Al2O3, and CaAl12O19each containing 2 wt%
Rh, for the OSR of isobutane. Isobutane was selected as a surrogate to represent the C4
compounds formed during the cracking of isooc-tane at the bed entrance. It was found that the order of activity, dispersion, and reducibility all followed the ranking Rh/La-Al2O3 > Rh/
Y-ZrO2 > Rh/Gd-CeO2 > Rh/g-Al2O3 > Rh/
CaAl12O19 suggesting these properties may be useful for evaluating a support material. Their
FIGURE 6.31 Sulfur tolerance of modified perovskite catalysts during OSR of JP-8 surrogate fuel. Catalyst is La0.6Ce0.4Fe0.8Ni0.2O3modified with either 2 wt% K or 1 wt
% Ru (O/C ¼ 0.7, S/C ¼ 3.0, 800 C, and 130,000 h1);
Reprinted from Dinka and Mukasyan[196], Copyright (2007), with permission from Elsevier.
FIGURE 6.32 Yields of H2, CO, CO2, and CH4produced over Rh-, Pt-, or Ni-CGO supported on a cordierite mono-lith during OSR of i-C8(O/C¼ 1.0, S/C ¼ 1.2, 700C, and 11,000 h1); Reprinted from Krause et al. [198], Copyright (2002), with permission from ANL.
6. OXIDATIVE STEAM REFORMING
170
results also concluded that the supports only served to disperse the Rh and did not play a role in the reaction.
Rh on a calcium-impregnated alumina sup-port (0.5 wt% Rh on 15 wt% calcium, balance alumina) was also found to limit carbon forma-tion from OSR of No. 2 fuel oil at a low O/C ratio of 0.72 [200,201]. Therefore, pyrolytic carbon formation, which occurs at high temper-atures, can therefore also be minimized. Greater than 98% conversion of No. 2 fuel oil was obtained at an O/C ratio of 0.72.
Wieland et al. [202] observed that a small amount of Pt metal present in the Rh-based catalyst could significantly improve the cata-lyst activity for OSR of gasoline range fuels.
They claimed that the role of Pt is to enhance oxidation activity, whereas Rh provides high SR activity. The Rh-Pt/alumina catalyst used in the study was supported on monolithic honeycombs and had a Rh:Pt ratio of 3:10 by weight. The geometry (metal monolith, ceramic monolith, or ceramic foam) of the support did not affect the product composition [203].
The differences in reactions at different reactor position was studied by Springmann et al.[204], who reported product compositions for OSR of model compounds as a function of reactor length in a metal monolith coated with a proprietary noble metal containing Rh. As expected, the oxidation reactions take place at the reactor inlet, followed by the SR, shift, and methanation reactions. Figure 6.33 shows the product concentration profiles for a 1-hexene feed, which are typical results for all the fuels tested. These results show that steam, formed from the oxidation reactions, reaches a maximum shortly after the reactor inlet, after which it is consumed in shift and reforming reactions. This corresponds to the combustion-reforming mechanism described in Section 6.3.1. H2, CO and CO2 concentrations increase with reactor length and temperature. In this reactor, shift equilibrium is not reached, and the increase in CO with distance from the inlet is the net result of the shift and SR reactions.
The use of Rh supported onto washcoated alumina monoliths has attracted interest for OSR of higher hydrocarbons [205,206]. Reyes
FIGURE 6.33 Product profiles for 1-hexene OSR as a function of reactor length (S/C¼ 2.3, 0.3 MPa, 600e650C, and ðnair=nfuelÞactual=ðnair=nfuelÞstoichiometric ¼ l ¼ 0.32); Reprinted from Springmann et al.[204], Copyright (2002), with permission from Elsevier.
CATALYTIC OSR OF HYDROCARBONS 171
et al.[206]carried out OSR ofn-C6in monolithic catalysts containing Rh as an active component.
A maximum H2selectivity of 170% (due to H2
from water) was obtained from the reforming ofn-C6at an O/C ratio of 1, a S/C of 1, preheat temperature of 700C, and GHSV of 68,000 h1. Brandmair et al.[205]also carried out OSR of n-C6over Rh supported onto ceramic monoliths at similar conditions, and reported that the Rh catalyst provided better performance over time.
6.5.3.2.2. OTHER NOBLE METALS
A comparison of Al2O3-supported Pd and Pt showed that their activity is comparable to or greater than Ni catalysts up to 600C, but they were less active at higher temperatures [207].
Interestingly, bimetallic Ni-Pd catalyst had much greater activity than either the Pd/Al2O3
or Ni/Al2O3(Fig. 6.34a), though the difference in reaction conditions among the studies does not allow a direct comparison of the results.
Moreover, the metal loadings of the catalysts were not provided. However, this result is in agreement with Zhang et al. [208,209] who studied a Ni-Pd bimetallic catalyst and compared it to a Ni catalyst on an identical alumina support.
They observed a similar improvement in activity and stability for the Ni-Pd catalyst, but the
difference was not as dramatic as shown in Fig. 6.34a. As expected, selectivity to CO þ H2
depended on the O/C and S/C ratios, but was independent of temperature (Fig. 6.34b).
However, the Ni-Pd bimetallic catalyst showed a better performance than the Ni-Pt or individual metals over 500 h on stream for OSR of n-C8
[208,209].
A direct comparison of Al2O3-supported Pt, Pd, and Ru suggests that Ru is the most active metal for diesel OSR, at least on this support.
Berry et al.[210]studied diesel OSR at a temper-ature range of 750e850 C and GHSVs of 25,000e200,000 h1. Activity increased in the order: Pd < Pt < Ru. Complete conversion of diesel was obtained at 850C and space velocity of 50,000 h1from the OSR of diesel over the g-alumina-supported Ru catalyst.
Encouraging results have been reported on other noble metal-based bimetallic catalysts.
Researchers at Engelhard Corporation[211e213]
reported reforming of a No. 2 fuel oil containing 1200 ppm sulfur in a dual-bed autothermal process. A preheated stream of steam, air, and No. 2 fuel oil (S/C¼ 2.57 and O/C ¼ 0.82) was introduced into the first catalytic oxidation zone, which comprised Pt group metal (PGM) catalysts (Pt/Pd in equal portions by weight) dispersed on
FIGURE 6.34 OSR of n-C8 (a) Activity comparison of reforming catalysts (note: Ni-Pd catalysts tested under “non-reduction” conditions; Ni and Pd under ““non-reduction” conditions) and (b) Selectivity of Ni-Pd catalysts;Reprinted from Yanhui and Diyong[207], Copyright (2001), with permission from International Association for Hydrogen Energy.
6. OXIDATIVE STEAM REFORMING
172
a lanthana-baria stabilized alumina washcoat.
The conversion of O2 was complete, giving a temperature high enough for SR. The first-stage effluent was then introduced into a second cata-lyst zone which contained a PGM SR catacata-lyst (Pt-Rh/Al2O3). The hydrocarbon conversion was greater than 96% with a maximum H2 composition of 63% (N2-free basis) in the product gas stream. Over the same catalyst bed configura-tion, a JP-4 hydrocarbon produced a H2 composi-tion of 62% (N2 free basis) with a complete conversion of the fuel[214].
Similar improvement in Pt-based catalyst activity by the addition of a second metal has been shown for Pd-Pt/alumina and Ni-Pt/ceria [215,216]. High H2 yields from OSR of diesel,
compared with monometallic catalysts, were shown at S/C ¼ 3, O/C ¼ 1, preheat tempera-ture ¼ 400 C, and space velocity¼ 17,000 h1 (Fig. 6.35). Based on TPR and XPS studies, they attributed this superior performance of bimetallic catalysts to synergistic effects from strong metale metal interaction in the bimetallic sample. The order of impregnation had no affect on the performance of Pt-Pd catalysts (Fig. 6.35a), but interestingly, the impregnation order in a Pt-Ni bimetallic catalyst significantly affected H2 selec-tivity (Fig. 6.35b). Higher H2yields were always observed when the Pt was impregnated second.
Also, the Pt-Ni/ceria catalyst showed better sulfur resistance capabilities over 50 h in the pres-ence of a sulfur-laden JP-8 fuel.
6.5.3.3. Mixed Metal Oxides (substituted oxides)
6.5.3.3.1. PEROVSKITES
In the effort to develop catalysts with compa-rable activity and selectivity to benchmark Rh catalysts, researchers at ANL have investigated perovskite-type oxides substituted with other noble metals as cheaper alternatives. Liu and Krumpelt [217] observed Ru-doped lanthanum chromite and lanthanum aluminate catalysts to produce high yields of synthesis gas during OSR of n-dodecane (O/C ¼ 1.0, S/C ¼ 1.5, 800C, and 50,000 h1). As shown inTable 6.15, H2yields were greater for the Ru catalysts than Rh- and Ni-substituted oxides. Further testing of the Ru-substituted perovskite, LaCr0.95
-Ru0.05O3, for the OSR of dodecane containing 50 ppmw sulfur (as dibenzothiophene) showed the catalyst to have a high sulfur tolerance, with
>75% reforming efficiency (calculation not given, but likely HHV H2formed/HHV fuel) and COx selectivity (>95%) for 100 h. Characterization of the material suggested Ru enrichment at the surface during reaction conditions, permitting a greater amount of Ru to be exposed to the reac-tant gases, as opposed to residing in the bulk.
However, Ru still remained atomically dispersed FIGURE 6.35 H2 yields from OSR of diesel (a) over
alumina-supported Pd and/or Pt catalysts (b) over ceria supported Pt and/or Ni catalysts (O/C¼ 1.0, S/C ¼ 3.0, preheat temperature¼ 400C, 17,000 h1);Reprinted from Cheekatamarla and Lane [216], Copyright (2005), with permission from International Association for Hydrogen Energy.
CATALYTIC OSR OF HYDROCARBONS 173
in the B-site, avoiding large crystallite sizes, which are deactivated by sulfur and carbon.
6.5.3.3.2. PYROCHLORES
Researchers at the National Energy Techno-logy Laboratory (NETL) have found pyrochlore materials to be viable catalysts for the reforming of logistic fuels into synthesis gas [218e221].
A pyrochlore has a general formula A2B2O7,
which differs slightly from the perovskite.
High thermal and chemical stability makes pyrochlores attractive for high-temperature and redox atmospheres. Pyrochlores can also be partially substituted with multiple cations in both A- and B-sites to tailor their catalytic properties for high activity and resistance to deactivation by carbon and sulfur.
A lanthanum zirconate (La2Zr2O7) pyrochlore catalyst modified with Sr in A-site, and either Ni, Ru, or Rh in the B-site was observed to be less susceptible to deactivation by sulfur and carbon during CPOX of 5 wt% 1-methlynaphthalene (MN) and 50 ppmw sulfur in n-tetradecane compared with the same metals supported on g-Al2O3 [218e221]. Resistance to deactivation was believed to be a result of not only stabiliza-tion of the metal particles in the pyrochlore struc-ture, but also oxygen-ion mobility which occurs after the replacement of La3þ with Sr2þ. Much like perovskites, alkaline properties of Sr prob-ably also assisted with carbon gasification and prevention of sulfur accumulation.
The sulfur and carbon resistance of the pyro-chlores has promoted the continued develop-ment of the pyrochlore formulation and synthesis at NETL. Through systematic testing TABLE 6.15 H2yield (mol H2/mol fuel) and COx
selectivity of produced over doped perovskites during OSR of dodecane (O/C ¼ 1.0, S/C¼1.5, 800C, and 50,000 h1)[217].
Catalyst H2yield
(mol H2/mol fuel) COx selectivity La0.8Sr0.2Cr0.95Ru0.05O3 16 >90
LaCr0.95Ru0.05O3 22 >95
La0.8Sr0.2Cr0.95Rh0.05O3 19 >85 La0.8Sr0.2Al0.95Ru0.05O3 20 >90
LaAl0.95Ru0.05O3 21 >95
La0.8Sr0.2CrO3 9 >65
La0.8Sr0.2Cr0.9Ni0.1O3 8 >65
0 100 200 300 400 500 600 700 800 900
0 5 10 15 20 25 30
0 200 400 600 800 1000 1200
Olefins produced (ppm)
Composition (%)
Time on stream (hrs)
Hydrogen Carbon Monoxide Carbon Dioxide Methane Olefins H2
CO
CO2 Olefins= ethylene + propylene + C4-ene + benzene Water
pump off Water
pump on
FIGURE 6.36 OSR of commercial diesel fuel (9 ppmw S) using NETL developed pyrochlore (O/C ¼ 1.0, S/C ¼ 0.5, 900 C, and 25,000 h1) [222].
6. OXIDATIVE STEAM REFORMING
174