6.5. CATALYTIC OSR OF HYDROCARBONS
6.5.4. Oxygenated Compounds
Oxygenated compounds are another class of fuels which can also be considered to be major H2 resources along with NG, and petroleum-based resources. Increased interest in these fuels has emerged for the sustainable production of H2, because they are generally derived from renewable biomass resources. The physical properties of these fuels make them attractive for reforming. These include low boiling point, low processing temperature, and minimal amounts of contaminants.
6.5.4.1. Methanol
OSR of methanol (OSRM) has been suggested for hydrogen production as an alternative to energy intensive steam reforming. The reaction can be expressed by the following equation[223].
CH3OHþ aO2þ ð1 2aÞH2O/
ð3 2aÞH2þ CO2
(6.42)
6.5.4.1.1. COPPER-BASED CATALYSTS
Variations of Cu-based catalysts are the most commonly reported for the OSRM, especially derivates of CuO/ZnO/Al2O3 systems which originated as a catalyst for commercial low-temperature WGS and methanol synthesis processes. It is highly desirable for Cu-based catalysts to have high dispersion and metal surface area, since its reforming activity depends greatly on these properties. The reducibility of Cu is also an important factor in determining
the efficiency of the Cu catalysts in OSRM when redox properties of the catalyst play a part in the mechanism for H2formation[224].
PROMOTERS FOR COPPER Promoters are added to Cu to improve its dispersion and reduc-ibility. Zn is the most common promoter for commercial Cu-based catalysts. The role of Zn, and other promoting metals, may also lead to improved adsorption of methanol [223,225]. It has also been suggested that the promoters may lead to spillover effects of hydrogen (from Cu to promoter) and oxygen (from promoter to Cu), which may affect the reaction mechanism[225].
Alumina is commonly used as a structural modifier to improve mechanical strength and dispersion of a Cu catalyst. However high load-ings of Al2O3 promote a strong interaction between Cu and Al2O3 (>10 wt% in the case for Chang et al. [223]), which can lead to increased reduction temperature, lower Cu surface area, and lower conversions.
Other promoters added to Cu-Al solid solu-tions include Cr, Zn, Zr, Ce [226,227], and their combinations [223,228]. The promotional effect of some of these components when added to a Cu-Al system can be exemplified by the study by Lindstrom et al.[229], which evaluated a series of Cu/M/Al2O3catalysts (M¼ Cr, Zn, and Zr) with various weight loadings of Cu (3e12 wt%) and (Cu/M weight ratios; 4e0.25) for the OSRM (O/C ¼ 0.15 and S/C ¼ 1.3). Over all Cu : M loadings tested for activity, Zn-promoted catalysts had the highest conversions, H2
production rate (mmol/kgcat/s), and turnover frequency than the Cr and Zr catalysts (seeTable 6.16). Characterization techniques showed Zn-promoted catalysts had consistently higher Cu metal surface area, lower reduction peak maxima, and more reducible Cu. No strong trends in performance are observed with Cu/
Zn ratio in Table 6.16, which is consistent with the literature. Values between 4 and 0.7 have been reported to be optimal for activity and H2
production from methanol reforming[230e233].
CATALYTIC OSR OF HYDROCARBONS 175
EFFECT OF SYNTHESIS METHOD Since the activity of Cu for OSRM is known to be sensitive to its physicochemical properties (i.e., disper-sion, surface area, and particle size), the method used to synthesize the catalyst greatly influ-ences reforming activity. Many studies have focused on the effect of preparation method to produce an active and selective Cu-based cata-lyst for the OSRM. While the best synthesis method for a desired composition may not be obvious, generally one can be guided based on which components (mainly alumina) are present in the catalyst.
The co-precipitation (CP) method appears to be suitable for producing highly active and selective Cu-based catalysts containing alumina [223,227,228,231]. The catalysts synthesized by this method have adequate BET SA, Cu SA, and low reducibility for high methanol conver-sions and H2selectivities. Shen and Song[231]
investigated several synthesis methods for
a Cu/Zn/Al catalyst (33/43/24 by weight), including hydrotalcite-type, impregnation, and CP on activity for steam reforming and OSRM at 230C. The catalyst produced by CP was found to have the highest conversion and yields of H2 for both reactions compared with the catalysts synthesized by the other methods.
In cases where Cu-Zr or Cu-Ce solid solutions are used, other methods have been shown to be more effective for synthesis. This is likely due to the fact that CP may not provide adequate mixing to form a uniform solid solution, and/
or the required SA to obtain high Cu dispersion, small particle sizes, and lower reducibility. A study by Shan et al. [234] investigated three synthesis methods (deposition-precipitation (DP), CP, and complexation-combustion (CC)) on activity for OSRM of a Ce0.9Cu0.1Oycatalyst (O/C ¼ 0.6, S/C ¼ 1.3). The catalyst produced by combustion-complexation was found to have the highest methanol conversions and selectivities to H2(seeFig. 6.37) over all temper-ature ranges. The catalysts synthesized by the CP and DP methods were found to have a poor interaction between Cu and CeO2, and produced a greater amount of CuO dispersed on CeO2. The catalyst produced by the CC method had more uniformly distributed Cu2þ within the CeO2structure, which therefore led to more lattice defects. The resulting solid solu-tion was found to have lower Cu reducsolu-tion temperatures, and better redox properties due to a synergistic function between Cu2þ/ Cuþ and Ce4þ/Ce3þ, which was attributed to its higher activity. Some alternate synthesis methods are summarized in Table 6.17, along with important properties of the resulting Cu-based catalysts.
6.5.4.2. Ethanol
Ethanol reforming has been gaining atten-tion as a source of H2 for fuel cells. It is considered to be a promising renewable fuel supply, because significant amounts can be TABLE 6.16 Effect of Promoting Metal on H2
Production Rate, Conversion and TOF for Cu Catalysts during OSRM at (O/C ¼ 0.15, S/C ¼ 1.3 and 300C; O/C and S/C do not include O from fuel)[229]
Catalyst* H2production
rate (mmol/kgcat/s) Conversion
(%) TOF
(103sL1)y
Cu15 135 73.0 154
Zn15 15 8.2 0
Cr15 14 7.4 0
Zr15 16 8.6 0
Cu9/Zn6 133 72.0 114
Cu6Zn9 161 87.0 153
Cu9Cr6 81 44.0 71
Cu6Cr9 109 59.0 138
Cu9Zr6 76 41.0 86
Cu6Zr9 91 49.0 136
* Subscript of catalyst is weight percent.
yTOF is defined as methanol molecules converted per surface copper atom per second.
6. OXIDATIVE STEAM REFORMING
176
produced from the fermentation of a wide variety of abundant starch-rich materials (i.e., sugar cane, corn, potatoes, etc.). Also, ethanol is less toxic than methanol and other liquid hydrocarbons.
The oxidative steam reforming of ethanol (OSRE) and higher alcohols is different than methanol reforming since a cleavage of at least
one CeC bond is involved. Therefore, reactions require comparatively higher temperatures than methanol, typically between 450e600 C, and use metals like Ni, Rh, Ru, Pt, and Ir which are known to be active for the scission of CeC bonds.
Using Ni (20 wt%), Youn et al. [239] evalu-ated the effect of support for the OSRE at TABLE 6.17 Additional Synthesis Methods and the Physicochemical Properties of Cu-based Catalysts
Catalyst Cu
(wt%) Preparation
Method* BET SA
(m2/g) Cu SA
(m2/g) Cu dispersion
(%) XMeOH
(%) Conditions
Cu-ZrO2[235] 4.5 Sol-gel 131 24.4 85 72 T¼ 400C
O/C¼ 0.12 S/C¼ 1.1
Cu-ZrO2[236] 75 GCOP 72 18.4 e 100 T¼ 260C
S/C¼ 1.3
Cu-ZrO2[237] 50 SRGOA-N 45 14.8 e 71 T¼ 240C
S/C¼ 1.3
Cu-Zn-Cr[238] 2.5 UNC 68 e e >85 T¼ 250C
O/C¼ 0.32 S/C¼ 1.2
* Abbreviations: GCOP-coprecipitated oxalate precursor method; SASG-Surfactant assisted sol-gel; SRGOA-N- soft reactive grinding of oxalic acid with nitrate precursors; UNC-urea-nitrate combustion method.
FIGURE 6.37 Effect of synthesis method [deposition-precipitation (DP), coprecipitation (CP) and complexation-combustion (CC)] on methanol conversion, CO2selectivity and H2selectivity for a Ce0.9Cu0.1Oyduring OSRM (O/C¼ 0.6, S/C¼ 1.3); Reprinted from Shan et al.[234], Copyright (2004), with permission from Elsevier.
CATALYTIC OSR OF HYDROCARBONS 177
500C (O/C¼ 0.5; S/C ¼ 1.5). After 3 h time on stream, their results, (see Fig. 6.38) shown in terms of H2 composition as a function of support acidity, suggest that TiO2 and ZrO2
supports with more moderate acidity produce higher amounts of hydrogen. The relative acidity is defined as the ratio of support acidity/acidity Ni/Al2O3. TiO2 and ZrO2 are also known to have some redox capabilities, which helps prevent carbon formation during reaction. More basic supports, MgO and ZnO, were determined to be less active because of the formation of a solid solution which limited the amount of exposed active Ni particles. The highly acidic alumina was found to promote undesirable side reactions, likely due to the formation of carbon, which caused the rapid deactivation of the Ni.
FIGURE 6.38 Comparison of support in terms of H2
composition vs. relative support activity for 20 wt% Ni after 3 h TOS for OSRE (O/C¼ 0.5, S/C ¼ 1.5, and 500 C).
Oxygen from ethanol not included in O/C or S/C ratios;
Reprinted from Youn et al. [239], Copyright (2008), with permission from Springer.
FIGURE 6.39 Comparison of conversion and product selectivities for 3 wt%M/La2O3(M¼ Ir, Pd, Rh, or Ru) for OSRM (O/C¼ 0.83 and S/C ¼ 1.3) at different temperatures. Oxygen from ethanol not included in O/C or S/C ratios; Reprinted from Chen et al.[240], Copyright (2009), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
178
Chen et al.[240]performed an extensive study of OSRE using noble metals (3 wt% Ir, Pd, Rh, or Ru) supported over various oxides (g-Al2O3, CeO2, ZrO2, and La2O3). For all metal catalysts, La2O3 was found to be the best functional support, because of its considerable activity and selectivity for the OSRE (650C; O/C¼ 1; S/C ¼ 2) in the absence of a catalytic metal. Conversions and product selectivities for the different metals supported over La2O3 are shown in Fig. 6.39.
All metals reached conversion near 100% around 600 C. Pd was recognized to have the lowest selectivity to H2 and highest selectivity to CH4
and other carbon by-products. Consistent with the literature, Rh was observed to be the most active and selective to H2 [241], especially at temperatures <500C. Rh also had the lowest selectivity to acetaldehyde and ethylene compared with the other metals. Ir and Ru proved to be cheaper alternatives to Rh, as they produced similar selectivities to H2and CO2. It is worth mentioning that the Ir catalyst had the lowest selectivity to CH4over all temperatures.
This correlates to the activation and conversion of CH4 not being a major pathway over Ir for H2production, which is especially advantageous at lower temperatures where such a process is limited by its energetics.
6.5.4.3. Biodiesel
Biodiesel is mainly produced through the transesterification of vegetable oil or animal fats into long-chain methyl esters. While biodie-sel fuels are currently synthesized to supplant diesel for transportation purposes, they can also be reformulated into large quantities of H2
and CO through OSR for fuel cells. Structurally they are similar to the n-paraffin compounds contained in traditional diesel, yet they are much cleaner and easier to reform catalytically because they do not contain aromatic and sulfur compounds.
Despite recent interest in biodiesel synthesis, studies involving the reforming of biodiesel are not yet commonly reported.
From the available literature, a study by researchers at the NETL [242] found biodiesel to be highly reactive and easily converted into H2 and CO. Using a structured monolith consisting of Rh-based pyrochlore/zirconia-doped ceria/Al2O3, a reforming unit was run on biodiesel (O/C ¼ 0.6, S/C ¼ 0.5, 900 C and 25,000 h1) for over 100 h. The average dry gas composition from the run is shown in Fig. 6.40. Based on these results, it can be surmised from this work that catalysts active for biodiesel reforming are similar to those used for commercial diesel reforming.