Catalytic Performance of Limonite Ores in the Decomposition of Model

Mar 21, 2017 - Department of Chemistry, School of Arts and Sciences, National University of Mongolia, University Street 1, Baga Toiruu,. Ulaanbaatar 1...
0 downloads 0 Views 916KB Size
Download PDF
Article pubs.acs.org/EF

Catalytic Performance of Limonite Ores in the Decomposition of Model Compounds of Biomass-Derived Tar Naoto Tsubouchi,*,† Yuuki Mochizuki,† Enkhsaruul Byambajav,‡ Satoko Takahashi,§ Yuu Hanaoka,§ and Yasuo Ohtsuka§ †

Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 5, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ‡ Department of Chemistry, School of Arts and Sciences, National University of Mongolia, University Street 1, Baga Toiruu, Ulaanbaatar 14201, Mongolia § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, Miyagi 980-8577, Japan ABSTRACT: Catalytic decomposition of toluene (C6H5CH3) or benzene (C6H6) with inexpensive limonite ores, composed mainly of goethite (α-FeOOH), was examined using a vertical, cylindrical flow fixed-bed quartz reactor to develop a novel method of removing biomass-derived tar components. The unsupported limonite catalyst was active for the decomposition of 480 ppm of C6H5CH3 and 1700 ppm of C6H6 in 15 vol % H2O/45 vol % H2/He, leading to C6H5CH3 and C6H6 conversions at 500 °C of nearly 100 and 97%, respectively. When the C6H5CH3 decomposition temperature was increased from 500 to 800 °C, the overall reaction path changed from demethylation to hydrocracking and then to steam reforming. A honeycomb-supported limonite catalyst also was effective and achieved nearly complete C6H5CH3 conversion at 600 °C. In addition, the honeycombsupported catalyst promoted C6H6 conversion of nearly 100% without carbon deposits at 700 °C in 15 vol % H2O/20 vol % H2/ 26 vol % CO/20 vol % CO2/5 vol % CH4 that was designed to simulate raw fuel gas derived from biomass gasification. Powder X-ray diffraction (XRD) measurements after reaction at 700−800 °C revealed the presence of finely dispersed metallic iron (αFe), which is likely responsible for the high catalytic performance.

1. INTRODUCTION The use of woody biomass is expected to be carbon-neutral on a time scale of a few decades. Biomass also offers the possibility of partially replacing fossil fuels, which account for about 80% of the world’s primary energy supply. Consequently, vigorous research is underway to study topics such as biomass-based combined cycle power generation technologies and the manufacturing of liquid fuels, including biodiesel and/or bioethanol. However, although biomass, which has cellulose as the primary component, exhibits high gasification reactivity compared to coal, whose backbone is composed of aromatic rings, raw gas obtained by biomass gasification in fixed-bed or fluidized-bed reactors contains large quantities of tar, which degrades efficiency and creates difficulties with the use of the gas produced. Thus, the development of technologies for efficient tar removal is important. A variety of catalysts for tar removal have been investigated,1 and non-metallic catalyst dolomite [CaMg(CO3)2] exhibited high activity.2 In addition, in studies of C6H5CH3 decomposition in H2O/Ar at 600−650 °C, Ni catalysts supported on olivine [(Mg0.9Fe0.1)2SiO4] or mayenite (12CaO·7Al2O3) exhibited high C6H5CH3 conversion efficiencies of 95− 100%.3,4 Recently, natural limonite, whose primary constituent is α-FeOOH, was reported to nearly completely decompose 2000 ppm of NH3 or 100 ppm of pyridine (C5H5N) at 500 °C in inert gas while exhibiting extremely stable performance at 750 °C in conjunction with fuel gas components produced by high-temperature coal gasification.5−9 © 2017 American Chemical Society

The present study focuses not on Ni catalysts used in previous studies but on inexpensive Fe-containing catalysts. For model compounds to represent biomass tar, decomposition experiments were conducted on C6H5CH3 and C6H6. First, a detailed investigation of the catalytic performance of limonite for decomposing C6H5CH3 in H2O/H2 was conducted, followed by a study of decomposition pathways for the process. Finally, decomposition of C6H6 in H2O/H2/CO/CO2/CH4, which mimics the fuel gas produced by biomass gasification, was performed. The constituents of tar include aromatic compounds, such as C6H5CH3, C6H6, naphthalene (C10H8), and anthracene (C14H10), while the primary constituents of woody biomass are lignin and cellulose, which produce large quantities of C6-based compounds containing a single benzene ring upon gasification.1 Thus, C6H5CH3 and C6H6 were chosen as model compounds because they are stable, even at high temperatures. In addition, catalysts were prepared consisting of limonite particles supported by a honeycomb structure made of cordierite (2MgO·2Al2O3·5SiO2), a type of ceramic, while limiting the agglomeration of particles.

2. EXPERIMENTAL SECTION 2.1. Catalyst Samples. Two types of limonite ores were used as raw materials for the catalysts used in C 6 H 5 CH3 and C 6 H 6 decomposition. The as-received samples were ground and then sieved Received: January 18, 2017 Revised: March 3, 2017 Published: March 21, 2017 3898

DOI: 10.1021/acs.energyfuels.7b00192 Energy Fuels 2017, 31, 3898−3904

Article

Energy & Fuels

2.3. Catalyst Characterization. Fresh H2-reduced and spent limonite samples were characterized by XRD (Shimadzu Corporation) with Mn-filtered Fe Kα radiation and N2 adsorption (Quantachrome Instruments) at −196 °C. To avoid rapid oxidation of α-FeOOHderived α-Fe and, consequently, undesirable changes in the catalyst, reduced or spent limonite was passivated with 2 vol % O2/He at room temperature before exposure to laboratory air for recovery from the reactor.5 The average crystalline size of α-FeOOH or α-Fe was estimated using the Debye−Scherrer method, and the specific surface area was determined by the BET method. The X-ray photoelectron spectroscopy (XPS, Ulvac-Phi) measurements were obtained using a non-monochromatic Mg Kα X-ray source operating at 300 W to determine surface elemental compositions of limonite samples after C6H5CH3 decomposition. Binding energies of XPS spectra referenced an In 3d5/2 peak of In2O3 at 444.9 eV.13,14 Analytical conditions have been described in detail elsewhere.15−17 The number of catalytically active α-Fe sites for decomposition of C6H5CH3 or C6H6 was determined by CO adsorption/temperatureprogrammed desorption (CO-TPD).5,7 Some catalyst materials after H2 reduction at 500 °C were quenched to 100 °C in high-purity He, and the resulting samples were exposed to a stream of pure CO (>99.9999%) at this temperature for 30 min to adsorb CO on the surface of the reduced catalysts. After physically adsorbed CO was purged in a flow of high-purity He, the temperature was raised at 5 °C/min to 950 °C for desorption of chemically adsorbed CO. The concentration of CO evolved was measured online by micro GC.

to obtain limonite particles within a specific size range. The physical and chemical properties of fresh limonite ores (referred to as BL and IL) with a size fraction of 250−500 μm are summarized in Table 1.

Table 1. Physical and Chemical Properties of Fresh Limonite Ores limonite ore surface areaa (m2/g) content (mass %) Mg Al Si Fe Ni α-FeOOH (mass %) crystalline speciesb size of α-FeOOHc (nm)

BL

IL

90

90

0.4 3.1 1.9 57 0.7 90 α-FeOOH (s) SiO2 (w) 13

0.6 1.8 3.2 43 1.1 68 α-FeOOH (m) SiO2 (m) 14

a

Measured by the BET method. bIdentified by XRD, with w being weak, m being medium, and s being strong in intensity. cAverage crystalline size determined by the Debye−Scherrer method. Their Brunauer−Emmett−Teller (BET) surface area was 90 m2/g, and the total Fe content of BL was 57 mass %, while that of IL was 43 mass %. The powder X-ray diffraction (XRD) measurements revealed that BL was richer in α-FeOOH (90 mass %) compared to that of IL (68 mass %). The average crystalline size of α-FeOOH was less than 15 nm, regardless of limonite type. The BL and IL were loaded on a cordierite-constructed honeycomb (1200 cpi, 7 mm outer diameter, and 10 mm length, NGK Insulators) by a dip-coating method, in which 2-propanol (99.9% pure, Wako Pure Chemical Industries) was used as the dispersion solvent for limonite particles. The detailed procedure has been reported previously.10 Commercial hematite (α-Fe2O3) with a purity greater than 99.9% (Wako Pure Chemical Industries) was also used as a reference. 2.2. Catalytic Reactions. All experiments were conducted isothermally with a cylindrical flow quartz reactor (8 mm inner diameter) at atmospheric pressure. Details of the apparatus have been reported previously.11,12 During a typical run, the weight of the catalyst sample was 0.25 g on a dry, honeycomb-free basis and each sample placed into the reactor was exposed to a flow of high-purity He (>99.99995%) until the concentration of N2 in the reaction system was less than 20 ppm. The height of the catalyst bed was 8 mm with an unsupported limonite catalyst and 30 mm with a honeycombsupported catalyst. After precautions against leakage, the reactor was heated electrically in high-purity He at 15 °C/min up to 500 °C, when He was switched to pure H2 (>99.9999%), and each sample was reduced under H2 for 2 h. After H2 reduction, the atmosphere was restored to He and the reactor was heated stepwise to a predetermined temperature to conduct the catalytic reaction. Standard reaction conditions were as follows: feed gas, 480 ppm of C6H5CH3 or 1700 ppm of C6H6 diluted with 15 vol % H2O/45 vol % H2/He; total flow rate, 110 cm3 standard temperature and pressure (STP)/min; decomposition temperature, 500−800 °C, space velocity, 5700− 16 000 h−1; and apparent contact time between gas and catalyst, 0.23− 0.63 s. In some runs, a simulated gas containing 2600 ppm of C6H6, H2O, H2, CO, CO2, and CH4 was passed over the reduced catalyst sample. The amounts of C6H5CH3, C6H6, CO, CO2, and CH4 in the reactor effluent were determined online at intervals of 160 s with high-speed micro gas chromatography (GC, Agilent Technologies), using OV-1, MS-5A, and PP-Q columns for analysis of C6H5CH3/C6H6, CO/CH4, and CO2, respectively. Conversion of C6H5CH3 or C6H6 was calculated using the amounts of the corresponding compound before and after reaction, and the product yield was expressed in percent on a carbon basis (C %).

3. RESULTS AND DISCUSSION 3.1. Catalytic Effect of Limonite Alone. The effects of commercially available α-Fe2O3 and unsupported BL and IL ores on the decomposition of 480 ppm of C6H5CH3 in 15 vol % H2O/45 vol % H2/He are shown in Figure 1, where

Figure 1. Catalytic effects of commercial α-Fe2O3 and BL and IL ores on the decomposition of 480 ppm of C6H5CH3 in 15 vol % H2O/45 vol % H2/He at 500−800 °C.

C6H5CH3 conversion is plotted against time on stream and the reaction temperature was increased stepwise from 500 to 800 °C. Conversion was less than 1% in a blank experiment using quartz wool. α-Fe2O3 was effective as a catalyst precursor for C6H5CH3 decomposition, and the effect tended to be larger at a higher temperature, although conversion at 700 or 800 °C decreased gradually with an increase in time on stream. In contrast, BL possessed very high catalytic activity, with conversion reaching nearly 100% under all conditions. For IL, conversion was nearly 90% at 500 °C and nearly reached 100% at temperatures greater than 600 °C. These observations indicate that α-FeOOH-rich limonite ores are promising as catalysts for C6H5CH3 decomposition. The difference in activity between BL and IL observed at 500 °C (Figure 1) could be attributed to the greater content of α-FeOOH in BL (90 mass %) compared to that in IL (68 mass %) (Table 1). 3899

DOI: 10.1021/acs.energyfuels.7b00192 Energy Fuels 2017, 31, 3898−3904

Article

Energy & Fuels Although BL promoted almost complete conversion, even at 500 °C (Figure 1), about 10 ppm of C6H6 was formed and remained unreacted. Figure 2 presents the change in the

Figure 4. Temperature dependence of standard Gibbs free energy changes for C6H5CH3 + H2 → C6H6 + CH4 (eq 1), C6H5CH3 + 10H2 → 7CH4 (eq 2), C6H5CH3 + 7H2O → 7CO + 11H2 (eq 3), CH4 + H2O → CO + 3H2 (eq 4), CO → 1/2C + 1/2CO2 (eq 5), Fe + 2/ 3CO → 1/3Fe3C + 1/3CO2 (eq 6), and Fe + 1/3C → 1/3Fe3C (eq 7): (a) eqs 1−3 and (b) eqs 4−7.

Figure 2. Yields of CH4, C6H6, and CO observed during BL-catalyzed C6H5CH3 decomposition.

product yield in BL-catalyzed C6H5CH3 decomposition shown in Figure 1. The amount of CO2 was less than 1%. As shown in Figure 2, CH4 was the main product at 500 °C, followed by C6H6; the amount of CO at 500 °C was negligibly small. These observations indicate the occurrence of demethylation (eq 1) and hydrocracking (eq 2). C6H5CH3 + H 2 → C6H6 + CH4

(1)

C6H5CH3 + 10H 2 → 7CH4

(2)

favorable at lower temperatures and eq 3 results in reverse temperature dependence of the Gibbs free energy changes. The data shown in Figures 2 and 3 suggest that C6H6 are more resistant to decomposition compared to C6H5CH3. Determining whether or not the overall reactions (eqs 1−3) take place sequentially also required clarification. Thus, C6H6 decomposition experiments were first conducted, and the results are provided in Figure 5, where 1700 ppm of C6H6 was

At 600 °C, C6H6 formed disappeared and CH4 yield became nearly 100% (Figure 2). Thus, it is likely that the reaction as represented by eq 2 proceeded selectively at 600 °C. When the temperature was increased further to 700−800 °C, as shown in Figure 2, the CH4 yield decreased significantly and the amount of CO increased with the temperature. Therefore, the molar ratio of CH4/CO was lower at a higher temperature. Thus, the predominant reaction is likely steam reforming to produce CO and H2 (eq 3), which is more favorable at higher temperatures. C6H5CH3 + 7H 2O → 7CO + 11H 2

(3)

Overall reaction paths of BL-catalyzed C6H5CH3 decomposition are summarized in Figure 3. As the temperature increased, the predominant reaction shifted from demethylation (eq 1) to hydrocracking (eq 2) and then to steam reforming (eq 3). A similar trend was also observed for IL. According to thermodynamic calculations (Figure 4a), the standard Gibbs free energy changes for eq 1 are approximately −10 kcal/mol, irrespective of the reaction temperature, whereas eq 2 is more

Figure 5. Changes in (a) C6H6 conversion and (b) product yield with BL ore under 1700 ppm of C6H6 and 15 vol % H2O/45 vol % H2/He at 500−800 °C.

added along with 15 vol % H2O/45 vol % H2/He. The C6H6 conversion reached approximately 97% at 500 °C and reached nearly 100% at 600−800 °C (Figure 5a), where no C6H6 was converted without BL. When the particle size of BL was changed from 250−500 to 75−150 μm, no significant difference was found in the conversion, which indicates that the reaction was not controlled by pore diffusion. As shown in Figure 5b, CH4 was the predominant product at 500−600 °C and the yield decreased with an increasing temperature, whereas the CO yield exhibited the reverse temperature dependence and increased significantly with the temperature.

Figure 3. Proposed overall reaction paths of BL-catalyzed C6H5CH3 decomposition. 3900

DOI: 10.1021/acs.energyfuels.7b00192 Energy Fuels 2017, 31, 3898−3904

Article

Energy & Fuels Thus, the trend observed in Figure 5b was similar to that on C6H5CH3 decomposition (Figure 2). Figure 6 shows changes in the CH4 conversion and product yield with BL upon simultaneous feed of 3400 ppm of CH4 and

Figure 7. Catalytic effects of as-prepared IL/H samples on the decomposition of 480 ppm of C6H5CH3 in 15 vol % H2O/45 vol % H2/He at 500−800 °C.

loading of 24 or 46 mass %. In a blank run with the honeycomb support alone, C6H5CH3 conversion was nearly zero, regardless of the reaction temperature. The 46% IL/H contains almost the same IL amount as the corresponding unsupported sample (Figure 1). Conversion at 500 °C was approximately 80% (Figure 7), which was slightly less than that without the support (Figure 1). This phenomenon may be caused by the difference in the amounts of catalytically active sites for C6H5CH3 decomposition. The induction period observed at 500 °C in Figure 7 might also be ascribed to the formation of less reducible Fe species in the process of catalyst preparation. In contrast, at 600−800 °C, 46% IL/H underwent almost complete conversion. When limonite loading was decreased from 46 to 24 mass %, conversion at a steady state was approximately 10% at 500 °C but became nearly 100% at temperatures above 700 °C (Figure 7). These trends were also observed with BL/H. Thus, limonite loading is likely to be an important factor in determining the degree of C6H5CH3 decomposition under the present conditions. The conversion with 24% IL/H at 500 °C decreased with increasing time on stream. The reason is not clear but may be related with the reducibility of loaded limonite and the dispersion of catalytically active iron sites. This point should be confirmed in future work. Figure 8 shows the catalytic effect of 46% IL/H on the decomposition of 2600 ppm of C6H6 in 15 vol % H2O/20 vol % H2/26 vol % CO/20 vol % CO2/5 vol % CH4/He at 500 and 700 °C. The gas composition used simulates typical raw fuel

Figure 6. Changes in (a) CH4 conversion and (b) product yield with BL ore under 3400 ppm of CH4 and 15 vol % H2O/45 vol % H2/He at 500−800 °C.

15 vol % H2O/45 vol % H2/He at 500−800 °C. The conversion was nearly zero at 500−600 °C but increased with the temperature to 20 and 70% at 700 and 800 °C, respectively (Figure 6a). The product yield (Figure 6b) for CO2 was less than 1% under all conditions, whereas significant formation of CO occurred at temperatures greater than 700 °C and was even greater at higher temperatures. These trends agreed well with the results (Figures 2 and 5b) observed for the decomposition of C6H5CH3 and C6H6. The data in Figure 6 also indicate that steam reforming of CH4 (eq 4) occurred selectively at temperatures greater than 700 °C. CH4 + H 2O → CO + 3H 2

(4)

Because the standard Gibbs free energy changes for eq 4 were calculated to be +1.2, − 4.8, and −11 kcal/mol for 600, 700, and 800 °C, respectively (Figure 4b), the driving force is greater at higher temperatures. On the basis of these results, the overall reaction represented by eqs 1−3 appears to occur sequentially under the present conditions. In the limonite-catalyzed C6H5CH3 decomposition, C6H5CH3 appears to undergo a demethylation reaction at 500 °C to provide C6H6, most of which is subsequently converted to CH4, while almost complete conversion occurs at 600 °C. When the temperature was increased to 700−800 °C, it appears that a portion of CH4 formed reacted with H2O to produce CO and the extent of the reaction was larger at the higher temperatures. 3.2. Catalytic Performance of Honeycomb-Supported Limonite. Although no loss of limonite particles occurred by attrition in the experiments shown in Figure 1, even at a very high space velocity of 16 000 h−1, support material for holding 250−500 μm BL or IL may be necessary to improve the mechanical strength. Therefore, BL and IL were loaded on a cordierite honeycomb (referred to as BL/H and IL/H, respectively) (section 2.1), and C6H5CH3 decomposition experiments with BL/H and IL/H samples were conducted. The results for IL/H are shown in Figure 7, with a limonite

Figure 8. Catalytic effect of 46% IL/H on the decomposition of 2600 ppm of C6H6 in a simulated fuel gas of 15 vol % H2O/20 vol % H2/26 vol % CO/20 vol % CO2/5 vol % CH4/He at 500 and 700 °C. 3901

DOI: 10.1021/acs.energyfuels.7b00192 Energy Fuels 2017, 31, 3898−3904

Article

Energy & Fuels Table 2. XRD Results for Active BL and IL Samples after H2 Reduction and C6H5CH3 Decomposition limonite ore

type of reaction

temperature (°C)

time (h)

crystalline Fe phase

size of α-Fea (nm)

BL

H2 reduction C6H5CH3 decomposition H2 reduction C6H5CH3 decomposition

500 500−600−700−800b 500 500−600−700−800b

2.0 5.8 2.0 5.8

α-Fe α-Fe α-Fe α-Fe

24 34 14 28

IL a

Average crystalline size determined by the Debye−Scherrer method. bRaised stepwise.

gas produced during the air gasification of biomass.1 Initially, the C6H6 conversion at 500 °C was about 85% but decreased with increasing time on stream to nearly 0% after 30 min. The total flow rate of the reactant gas also decreased significantly with time, and a measurable amount of carbon was deposited on the IL/H surface and reactor wall. Thus, carbon deposition on the IL/H surface that occurs when a certain amount of CO was present in the reactant gas (eq 5) resulted in catalyst deactivation. CO → 1/2C + 1/2CO2

(5)

According to thermodynamic calculations, standard Gibbs free energy changes for eq 5 were −4.2, − 2.1, and 0 kcal/mol for 500, 600, and 700 °C, respectively (Figure 4b); thus, the driving force was greater at lower temperatures. In contrast, at 700 °C, IL/H achieved nearly complete C6H6 conversion (Figure 8) without a decrease in the flow rate of the reactant gas. In this case, no significant deposition of carbonaceous materials on the catalyst surface occurred. Similar results were observed with 46% BL/H. This high catalytic performance of honeycomb-supported limonite is very interesting from a practical point of view, although long runs with BL/H and IL/ H samples need to be conducted to determine catalyst life and durability. 3.3. Changes in Catalyst States before and after Reaction. Figure 1 revealed that the unsupported BL and IL catalysts were quite active for C6H5CH3 decomposition. The XRD results for the active BL and IL samples are given in Table 2, where they are always passivated with a low concentration of O2 at room temperature upon recovery from the reactor (section 2.3). Because commercial α-Fe2O3 (Figure 1) was agglomerated seriously upon H2 reduction, the reduced and used samples could not be subjected to the XRD measurements. It is likely that small α-Fe crystallites provide catalytically active sites for C6H5CH3 decomposition. Transformation of α-FeOOH in raw BL and IL into α-Fe occurred during H2 reduction, with an average crystalline size of α-Fe of 24 nm for BL and 14 nm for IL. α-Fe was the only iron species after C6H 5CH3 decomposition in H2 O/H 2 (Table 2), regardless of limonite type, with average sizes of 30−35 nm, which were slightly larger than those after H2 reduction. These observations indicate the agglomeration of α-Fe particles. Figure 9 shows XRD profiles for 46% IL/H before and after C6H5CH3 or C6H6 decomposition at 700 °C under different atmospheres. As-prepared IL/H gave XRD signals attributed to cordierite and α-FeOOH (Figure 9a), and the latter species was transformed upon H2 reduction into α-Fe with an average size of 14 nm. IL/H also maintained a metallic form (Figure 9b) and a highly dispersed state (average crystalline size of α-Fe was 15 nm) during C6H5CH3 decomposition at 700 °C in H2O/ H2/He. During C6H6 decomposition in H2O/H2/CO/CO2/ CH4/He, cementite (Fe3C) and α-Fe were identified after reaction at 500 °C, as reported for the limonite-catalyzed NH3 decomposition in syngas.6,8,18 Fe3C may be formed via reaction

Figure 9. XRD profiles for 46% IL/H samples (a) before and (b) after C6H5CH3 decomposition at 700 °C in H2O/H2/He and (c) after C6H6 decomposition at 700 °C in H2O/H2/CO/CO2/CH4/He.

of α-Fe with CO (represented by eqs 6 and 7) in the reactant gas and/or by reaction with the carbon deposited (represented by eq 5). Fe + 2/3CO → 1/3Fe3C + 1/3CO2

(6)

Fe + 1/3C → 1/3Fe3C

(7)

The standard Gibbs free energy changes for eqs 6 and 7 at 500 °C were −2.3 and +0.55 kcal/mol, respectively (Figure 4b). At 700 °C, Fe3C disappeared almost completely (Figure 9c) and α-Fe was present mainly as the crystalline iron species, likely as a result of a decrease in the driving forces for eqs 5 and 6, because the Gibbs free energy changes for these reactions are greater at higher temperatures (Figure 4b). On the basis of the XRD results, the presence of finely dispersed α-Fe accounts for the high performance of the unsupported and honeycombsupported limonite catalysts. Raw BL and IL contained elemental Ni at concentrations of 0.7−1.1 mass % (Table 1). This inherent Ni may also promote decomposition of C6H5CH3 or C6H6 under the conditions applied. To investigate, spent BL after C6H5CH3 decomposition (Figure 1) was examined by XPS. Figure 10 presents the Fe 2p and Ni 2p XPS spectra. BL produced Fe 2p3/2 and 2p1/2 peaks at 707−717 and 721−731 eV, respectively (Figure 10a). Because magnetite (Fe3O4), wustite (Fe1 − xO), and hematite (α-Fe2O3) show Fe 2p3/2 signals at 708.1−711.5, 709.3−710.5, and 710.2−711.6 eV, respectively,13,14 these oxides are likely to be the predominant iron forms on the BL surface. The oxides must be formed upon O2 passivation prior to sample recovery from the reactor, because the XRD analysis showed only the presence of α-Fe as the bulk species (Table 2). With elemental Ni, BL exhibited a very weak 2p3/2 peak and a broad 2p1/2 shoulder signal at approximately 854 and 873 eV, respectively (Figure 10b). In addition, the XPS results showed that the Fe content at the BL surface was about 15 mol %, 3902

DOI: 10.1021/acs.energyfuels.7b00192 Energy Fuels 2017, 31, 3898−3904

Article

Energy & Fuels

position of 1 vol % NH3 at 850 °C or 3000 ppm of H2S at >400 °C in a simulated gas of 5 vol % H2O/50 vol % H2/5 vol % CO/30 vol % CH4/He.21 In addition, we have recently shown that limonite can promote decomposition reactions of coalderived tarry materials and tar nitrogen below 1000 °C.22 Although Ni-based catalysts have provided high C6H5CH3 conversions of >95% for decomposition of C6H5CH3 in H2O/Ar at 600−650 °C,3,4 inexpensive α-FeOOH-rich limonite ores may be practically feasible as additional catalyst materials.

4. CONCLUSION Catalytic decomposition of C6H5CH3 or C6H6 with α-FeOOHrich limonite ores was examined as a hot gas cleanup method to remove tar components formed in biomass gasification. Two limonite ores (BL and IL) demonstrated very high catalytic performance in the decomposition of 480 ppm of C6H5CH3 or 1700 ppm of C6H6 in 15 vol % H2O/45 vol % H2/He at temperatures of ≥500 °C, with C 6 H 5 CH 3 and C 6 H 6 conversions rates at 600−800 °C of nearly 100% in every case. When the temperature for C6H5CH3 decomposition was increased from 500 to 800 °C, the overall reaction process shifted from demethylation to hydrocracking and then to steam reforming. BL and IL supported on a cordierite-made honeycomb at a loading of 46 mass % were also catalytically effective for the decomposition of C6H5CH3 or C6H6, and almost complete decomposition of the two compounds was achieved at ≥600 °C. Furthermore, the honeycomb-supported catalysts exhibited C6H6 conversion of nearly 100% without carbon deposition at 700 °C in 15 vol % H2O/20 vol % H2/26 vol % CO/20 vol % CO2/5 vol % CH4, which simulated typical raw fuel gas produced during air gasification of biomass. Because XRD measurements after reaction at 700−800 °C showed the presence of α-Fe with an average crystalline size of 15−35 nm, such highly dispersed α-Fe is likely to account for the high performance of unsupported and honeycombsupported limonite catalysts.

Figure 10. (a) Fe 2p and (b) Ni 2p XPS spectra for spent BL after C6H5CH3 decomposition.

whereas the amount of Ni was negligibly small (