Article pubs.acs.org/IECR
H2 Production by Autothermal Reforming of n‑Dodecane over Highly Active Ru−Ni−Ce−Al2O3 Catalyst Vidya Sagar Guggilla,*,† Venkata Phanikrishna Sharma Mangalampalli, Jale F. Akyurtlu, and Ates Akyurtlu Chemical Engineering Department, Hampton University, Hampton, Virginia 23668, United States ABSTRACT: Autothermal reforming of n-dodecane, a main constituent of jet fuel, over Ru-promoted nickel−ceria−aluminabased catalyst was carried out in a fixed-bed microreactor operating at atmospheric pressure. The temperature was varied from 873 to 1073 K, space velocity range was 96000−480000 h−1, and H2O/C molar ratio in the feed was between 1.5 and 4.0. The monometallic and bimetallic catalysts were prepared by the sol−gel method. X-ray diffraction (XRD), BET surface area, temperature-programmed reduction (TPR), and temperature-programmed desorption of H2 (TPD of H2) were used to characterize the prepared catalysts. Characterization results reveal that the presence of Ru and CeO2 enhances the catalyst reducibility. H2-TPD data indicate that Ru-promoted Ni−Ce−Al2O3 catalyst exhibits larger nickel surface area compared to monometallic Ni and Ru catalysts. The activity and hydrogen yield on the Ru-promoted Ni−Ce−Al2O3 catalyst was observed to be significantly higher than those on the monometallic catalysts under the same experimental conditions.
1. INTRODUCTION The high energy density and existing refueling infrastructure of petroleum-derived liquid hydrocarbon fuels, such as gasoline, diesel, and JP-8 (used as military fuel) have made them popular in all areas of industrial applications.1 They are considered to be excellent candidate fuels for the production of hydrogen for fuel cells.2 Fuel cells, where hydrogen functions as the prominent energy carrier, are currently the most efficient devices for converting chemical energy to electricity.3 When they are introduced to the energy supply system in the transportation sector, emissions can be reduced. Thus far, infrastructure for H2 storage and production is lacking; this is a critical issue for establishing a hydrogen fuel distribution system. An attractive option is on-site or on-board reforming of high density liquid hydrocarbons to provide hydrogen for fuel cells.4,5 The conversion of hydrocarbon fuels to hydrogen can be carried out by several different reforming approaches including steam reforming, partial oxidation, and autothermal reforming.6−10 Partial oxidation (POX) is exothermic, while a steam reforming (SR) reaction is endothermic and energy must be provided to the reforming process. Autothermal reforming (ATR) is a hybrid of POX and SR. The energy generated by the exothermic POX directly provides the energy needed by the endothermic SR. Therefore, the ATR reaction can start quickly and stand-alone without an additional energy supply, which makes it possible to construct a compact reformer.11−13 Also, ATR conditions lead to better sulfur tolerance compared to steam reforming (SR). Hence, ATR is regarded as the best option for the reforming of higher hydrocarbons. However, there are also several disadvantages of the ATR of liquid hydrocarbons. Hot spots can form easily due to the relative difference of reaction rates between POX and SR,14,15 which may cause degradation of the reforming catalyst. Heavier hydrocarbons, such as iso-octane and hexadecane, are easily decomposed by thermal cracking during ATR and have a high possibility of coke formation on the catalyst.16 Therefore, the development of very active and stable catalysts for ATR is an important challenge in devising fuel processors with small © 2012 American Chemical Society
volumes, that have high durability and low cost and thus, for the fast commercialization of fuel cells. Autothermal reforming catalysts must be active for both SR and POX, and they must have resistance to coking during operation at low steam to carbon ratios. They must not sinter at the high temperatures associated with partial oxidation, must be tolerant to sulfur (in the case, for example, of gasoline, jet fuel or diesel reforming), and must have high activity, high hydrogen selectivity, high water gas shift activity (no need for CO cleanup for PEM fuel cells), and durability. The catalyst should be able to handle oxidizing-reducing atmospheres without loss in activity. So it is crucial to develop highly effective catalysts for ATR especially for the fast commercialization of fuel cells. ATR catalyst formulations typically comprise metals such as Pt, Rh, Ru, and Ni deposited or incorporated into carefully engineered supports, doped with other elements to improve thermal robustness or to achieve better activity.17 ATR of C8− C16 range liquid hydrocarbons has been investigated over various supported metals, including Ni,18,19 Pt,1,11,18−20 Ru,18 Rh,20 Pd,18 Fe,18 and Co.18 Precious metals (Pt, Pd, Ru, Rh, etc.), particularly platinum supported on ceria, seem to be the materials of choice for the reforming of heavier hydrocarbons;1,21 and unlike conventional nickel catalysts, they tolerate sulfur22 and are not as prone to coke deposition.23 However, the cost of noble metals is a significant drawback. Because of this reason, recently, there was a lot of interest in the development of bimetallic catalysts of Ni and precious metals, improving the performance of Ni owing to the synergistic noble metal−Ni interaction, as well as suppressing the cost of the noble metal, simultaneously. Hou and Yashima24 reported that Ni/α-Al2O3, doped with only a small amount of Rh, showed higher activity than Ni/α-Al2O3 catalyst in the CO2-reforming Received: Revised: Accepted: Published: 338
March 18, 2012 November 10, 2012 November 29, 2012 November 29, 2012 dx.doi.org/10.1021/ie300726k | Ind. Eng. Chem. Res. 2013, 52, 338−345
Industrial & Engineering Chemistry Research
Article
temperature was raised from 298 to 1273 K at a rate of 10 K/min. The desorbed H2 (uptake) was detected using a TCD. 2.3. Reforming Experiments. Autothermal reforming reactions were performed in a quartz flow reactor at a low oxygen to carbon ratio (O/C) of 0.35, steam to carbon ratio (H2O/C) in the range of 1.5 to 4.0, and temperature in the range of 873 to 1073 K. The details of the quartz reactor setup, dimensions, experimental start-up procedure, catalyst pretreatment, and product analysis have been provided previously.29 The catalytic bed, 75 mg of catalyst diluted with quartz chips to avoid preferential gas flow paths and hot spots, was placed in a tubular reactor with two coaxially centered thermocouples in contact with the top and bottom of the catalytic bed. Prior to the reaction, the catalysts were reduced in situ with a flow rate of 50 sccm of 10% H2/Ar mixture at 873 K for 2 h. The pretreatment gases were flushed from the reactor with N2 prior to the addition of feed mixture. The flow rates of the liquid hydrocarbon feeds of n-dodecane and water were controlled by liquid pumps; and the liquid feed streams were preheated in an evaporator before passing through the catalyst bed in the reactor. Nitrogen (carrier gas) and argon (internal standard) gases were also fed to the evaporator to facilitate the evaporation and passage of both the hydrocarbon and water streams. The total gas flow rate was kept at 300 sccm, and the n-dodecane concentration in the feed gas mixture was fixed at 1% by mole. Activity was measured at atmospheric pressure and 873−1073 K maintaining the reactor for 5 h at the selected reaction temperature. The products were analyzed periodically (every hour) by two online gas chromatographs (Varian 3800) equipped with TCD and FID.29 The extent of reforming (eq 5) is the conversion of carbon in n-dodecane to reforming products, which is quantified by carbon monoxide and carbon dioxide formation due to reforming and WGS/RWGS reactions (eqs 6−8). The extent of reforming (Xref) and the product yields of H2, CO, and CO2 were calculated as follows:
of methane. According to the studies performed on the CO2reforming of methane over Ni−Ru and Ni−Pd bimetallic catalysts, the strong improvement in the activity and the stability observed on the silica-supported Ni−Ru catalysts was attributed to the formation of Ni−Ru bimetallic clusters, which led to an increase in the metallic dispersion of Ni and favored the formation of a more reactive intermediate carbonaceous species.25,26 Nano catalysts (ultrafine particles of metals, metal oxides and composites) are expected to have higher surface areas than conventional catalysts, and thereby higher activities. In particular, alumina materials prepared by a sol−gel method have high surface areas and controllable textural and chemical properties. It was reported that an alumina aerogel with a welldeveloped pore structure inhibited carbon deposition in the CO2-reforming of methane, and this resulted in an enhancement of both methane conversion and coke resistance.27,28 The role and effect of xerogel catalysts on the catalytic performance in the ATR of higher hydrocarbons have not yet been clearly investigated. Therefore, developing sol−gel-derived active catalysts for the autothermal reforming of heavier fuels like diesel and jet fuel is of great interest for fuel processors for SOFC applications. In this paper, the performance of a Ni−Ru−Ce-Al2O3 catalyst in the autothermal reforming of n-dodecane is investigated. The main objective of this paper is to report the results of a study on the effect of operating conditions and the promoting effect of Ru on the hydrogen yield during the ATR of n-dodecane.
2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. Quaternary RuO−NiO−CeO2− Al2O3 xerogel catalysts were prepared by the sol−gel method, processing ruthenium chloride, nickel acetate, ammonium ceric nitrate, and aluminum trisec-butoxide in ethanol. Subsequently, the gel was aged for 72 h at room temperature followed by drying at 353 K for 48 h. The resulting xerogel was finally calcined at 773 K for 5 h in static air to produce the desired oxide species. The detailed synthetic procedure has been described in the previous work.29 The catalysts prepared in this work were Ce (3 wt %) O2−Al2O3, 1 wt % Ru/Ce (3 wt %) O2−Al2O3, 10 wt % Ni/Ce (3 wt %) O2−Al2O3 and 1 wt % Ru/10 wt % Ni/Ce(3 wt %) O2−Al2O3, which are denoted as 3CA, 1R3CA, 10N3CA, and 1R10N3CA, respectively. 2.2. Catalyst Characterization. X-ray powder diffraction patterns were obtained with an XPERT diffractometer (Phillips), using Cu Kα radiation (1.5406 Å) at a scanning rate of 5.4°/min. The BET surface areas of catalysts were measured by a Pulse Chemisorb 2705 (Micromeritics, USA) instrument using dynamic adsorption procedures. H2-TPR for the fresh catalysts was conducted in 10% H2/Ar using a Pulse Chemisorb 2705 instrument to study the metal dispersion and reducibility. The sample (100 mg) was pretreated at 423 K for 30 min in Ar flow to remove water. At room temperature, Ar was replaced with 10% H2/Ar (50 mL/min) and the cell temperature was raised from 298 to 1273 K at a rate of 10 K/ min. TPD of H2 was carried out in the same system as described for TPR. After reduction at 873 K for 2 h, the sample was heated up to 1073 K under argon flow, then cooled down to room temperature in the same argon flow. The adsorption of H2 took place for 45 min at room temperature. Subsequently the flow was switched from H2 to argon and the cell
YH2 =
FH2 , out 13FC12H26 , in
YCO2 =
YCO = YCH4 =
X ref =
(1)
FCO2 , out 12FC12H26 , in
(2)
FCO , out 12FC12H26 , in
(3)
FCH4 , out 12FC12H26 , in
(4)
FCO + FCO2 12FC12H26 , in
(5)
3. RESULTS AND DISCUSSION 3.1. Physical Properties of Catalysts. The BET specific surface areas determined by nitrogen physisorption of monometallic and bimetallic catalysts are presented in Table 1. The specific surface areas of pure alumina and 3CA support are also presented and are found to be 310 and 370 m2/g, respectively. By the addition of ruthenium or nickel to the 3CA substrate, the BET surface area increased considerably. This might be due to the homogeneous distribution of metal in the support matrix due to the use of the sol−gel method in catalyst 339
dx.doi.org/10.1021/ie300726k | Ind. Eng. Chem. Res. 2013, 52, 338−345
Industrial & Engineering Chemistry Research
Article
solution, variation in the pH of the solution may occur leading to the dissolution of small crystals and the growth of ceria.30 XRD reflections due to crystalline CeO2 and γ-Al2O3 were observed at 2θ = 28.5° and 43.3°, respectively. The diffraction line at 43.3° is difficult to distinguish since the (012) reflection of NiO may be overlapped with the (113) reflection of αAl2O3.19 The XRD patterns also indicate that no characteristic peaks resulting from the formation of a mixed oxide phase between NiO, RuO, CeO2, and Al2O3 were found. 3.3. Temperature-Programmed Reduction (TPR). The TPR profiles of the 1R3CA, 10N3CA, and 1R10N3CA catalysts as well as the 3CA are shown in Figure 2. A broad reduction
Table 1. Specific Surface Area and H2 TPD of Calcined Catalysts sample no.
catalyst ID
BET specific surface areaa of calcined catalysts (m2/g)
1 2 3 4 5
Al2O3 3CA 1R3CA 10N3CA 1R10N3CA
310 370 380 428 360
a
Hydrogen uptake from H2 TPD (μmol/g)
% dispersionb of Ni
69 51 110
3.9 4.8
Calculated from N2 physisorption at 77 K. bCalculated from H2 TPD.
preparation.29 It is also probable that the active metal precursors may have caused surface roughening on the support matrix. In Ru-promoted NiO−CeO2−Al2O3 catalyst, the BET surface area is slightly low when compared to base case 3CA support. This might be due to surface hydroxyl groups of the support being consumed by the reaction with the active phase precursor. Such a surface reaction may have caused the decrease of available surface area of the substrate, probably by closure of the pores. 3.2. X-ray Diffraction (XRD). The XRD patterns of calcined catalysts are presented in Figure 1. The support
Figure 2. Temperature programmed reduction profiles of various Ru− Ni catalysts: (a) 3CA, (b) 1R3CA, (c) 10N3CA, (d) 1R10N3CA.
peak in the range 653−1073 K can be observed for the 3CA support. This very broad peak is due to the ongoing reduction of the ceria particles or conglomerates with partial interaction with Al2O3. 1R3CA catalyst exhibited a main reduction peak centered at 393 K, with a shoulder on the high temperature side at about 418 K, which likely denotes a bimodal dispersion of the RuO2 phase.31,32 The high temperature peak at 418 K shifted toward higher temperatures (423 K) in the 1R10N3CA catalyst. A small hump in the range 653−1073 K also appeared which could be assigned to the reduction of segregated CeO2 particles or CeO2 conglomerates. The 10N3CA catalyst showed one reduction peak centered at 848 K, which could be assigned to the reduction of a well-dispersed NiO phase. According to literature, pure NiO shows a sharp reduction peak at about 693 K followed by a small hump.33 The high temperature reduction peak in 10N3CA catalyst compared to pure NiO suggests a stronger interaction with ceria-alumina matrix. 1R10N3CA catalyst showed two main reduction peaks centered at 393 and 748 K along with shoulder peaks. The first low temperature peak at 393 K along with the shoulder peak, which was assigned to Ru associated with ceria, shift back to the higher temperature compared to monometallic 1R3CA catalyst. The shift indicates that the reduction of both RuO2 and CeO2 became difficult with the addition of Ni. The second high temperature peak at 748 K, along with the shoulder at 948 K, indicates the reduction of the NiO species. Thus, these two peaks at 748 and 948 K can be attributed, respectively, to the reduction of relatively free NiO species, and complex NiO species, which strongly interact with Ce−Al2O3, like NiAl2O4, and it seems to be concurrent with the reduction of highly
Figure 1. X-ray diffraction patterns of calcined support and various catalysts: (a) Al2O3 pure support, (b) 3CA, (c) 1R3CA, (d) 10N3CA, (e)1R10N3CA (■ due to γ-Al2O3, ● due to CeO2).
calcined at 773 K is relatively amorphous as evidenced by broad peaks that correspond to γ-Al2O3 (Figure 1a). The possible crystalline phases in the catalysts calcined at 773 K are RuOx, NiO, NiAl2O4, CeO2, and γ-Al2O3. However, XRD results are not conclusive about the presence of any Ru or/and Nicontaining crystalline phases, suggesting a high-level of dispersion of Ni and Ru. The results seem to indicate that nanoparticles of NiO and RuOx would be highly dispersed on the 3 wt % CeO2−Al2O3 surface as smaller crystallites of
