Tuning Hydrogen and Carbon Nanotube Production from Phenol

Research Article pubs.acs.org/journal/ascecg

Tuning Hydrogen and Carbon Nanotube Production from Phenol Steam Reforming on Ni/Fe-Based Nanocatalysts Qingqing Peng,† Yongwen Tao,‡ Huajuan Ling,‡ Zhenyuan Wu,† Zhanglei Zhu,† Rongli Jiang,*,† Yuemin Zhao,† Yuelun Wang,† Chen Ji,† Xiaozhou Liao,§ Anthony Vassallo,‡ and Jun Huang*,†,‡ †

School of Chemical Engineering & Technology, China University Mining & Technology, Xuzhou 221116, People’s Republic of China ‡ School of Chemical & Biomolecular Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia § School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia S Supporting Information *

ABSTRACT: The development of cost and environmentally efficient catalysts is essential to transferring phenolic compounds to valuable fuels and chemicals. Various nanocatalysts with different amounts of Ni/Fe have been employed in the steam reforming of phenol, which showed high performance in terms of activity and stability. The conversion and hydrogen yield of phenol reforming over Ni/Fe-catalysts can reach 87% and 81%, respectively. The catalyst can keep its high reactivity for more than 200 h at the steam to carbon ratio (S/C) of 13.3, which is much higher than the previous report of 13 h. Using the newly developed Ni/Fe-catalysts in this research, the hydrogen-rich syngas or carbon nanotube (CNT) could be selectively produced via simply tuning the S/C ratio. The influence of the Ni/Fe ratio and S/C ratio on the steam reforming performance was investigated. KEYWORDS: Phenol steam reforming, H2 production, Carbon nanotubes, Ni/Fe, Nanocatalysts, S/C

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INTRODUCTION The production and consumption of fuels and chemicals increases continuously with the development of modern society. Most of them are produced from fossils, including crude oil and coal that shared 33% and 30%, respectively, of the global primary energy consumption in 2013.1 Therefore, the utilization of renewable resources or recycling/reusing the wastes is essential for sustainable development. Phenolic compounds have been largely produced in petro-refining, reforming, and gasification, and in some cases as tar byproducts or wastes.2,3 In addition, they are the main compounds of the pyrolysis bio-oil in the emerging biorefining industry. This has attracted lots of research interest to the development of cost and environment efficient methods to transfer phenolic compounds to valuable fuels and chemicals.2,4,5 It has to be noted that most of the phenolic compounds are produced with a large amount of water, such as ca. 30 wt % of phenolics with 15−30 wt % of water in pyrolysis oil.3,6,7 Removing water from the mixture to obtain purified phenolics by current physical and chemical methods is a cost noneffective process. Moreover, © 2017 American Chemical Society

phenolic compounds are difficult for transformation due to their easy condensation inside devices and pipes accompanying corrosion.8,9 This caused direct deposition of the phenolenriched tar into the environment in most developing countries,10−13 which not only results in the waste of valuable organic carbon resources, but also pollutes the environment seriously. Therefore, it is promising to develop efficient on-site chemical processes to produce useful chemicals from the phenol−water mixture immediately after their production during gasification, reforming, or pyrolysis. Several methods have been developed such as aqueous phase reforming,14−16 hydrodeoxygenation,17−20 and steam reforming.21−25 Among them, on-site steam reforming can use the existing reaction system and waste heat from gasification, reforming, or pyrolysis. As involved in the reforming, the large amount of water in the Received: August 13, 2016 Revised: November 13, 2016 Published: February 11, 2017 2098

DOI: 10.1021/acssuschemeng.6b01936 ACS Sustainable Chem. Eng. 2017, 5, 2098−2108

Research Article

ACS Sustainable Chemistry & Engineering

Catalyst Characterization. The crystal structures of the prepared Ni/Fe-catalysts were investigated by X-ray diffraction (XRD, BRUKER D8 ADVANCE) using Cu Kα radiation (λ = 1.5418 Å). The surface morphology of Ni/Fe-catalysts was examined using a Zeiss Ultra+ scanning electron microscope (SEM). Powdered specimens were spread on the SEM slabs and were sputtered with gold. Fresh and used catalysts were characterized by transmission electron microscopy (TEM) (FEI Tecnai G2 F20 operated at an acceleration voltage of 80 kV). Temperature-programmed reductions (TPRs) of Ni/Fe-catalysts were performed on a ChemBET TPR/TPD instrument (Quantachromes) with 10% H2/Ar and a total flow rate of 20 mL/min for all samples (30 mg of each). Temperature was increased by 10 °C/min from room temperature to 1000 °C for all TPR experiments. The surface composition and electronic properties of Ni/Fecatalysts were analyzed by X-ray photoelectron spectroscopy (XPS). XPS spectra were acquired on a Thermo Fisher ESCALAB 250Xi spectrometer equipped with Al Kα X-ray source and an X-ray spot size of 650 μm. The catalysts were reduced at 800 °C for 2 h and then exposed to air before XPS measurement, and the peak at 284.8 eV corresponding to C 1s was applied as a reference. Raman spectra were obtained for the used catalysts using a Bruker Senterra system at a wavelength of 532 nm. The main purpose of Raman was to investigate the deposited carbons on the reacted Ni/Fe-catalysts. Steam Reforming. The phenol (≥99.0%, solid) as reaction feed was purchased from Sinopharm Chemical Reagent Co. The steam reforming of phenol was carried out in a fixed bed quartz reactor (i.d. 9 mm) containing Ni/Fe-nanocatalysts under continuous-flow and atmospheric pressure. The catalyst (0.15 g) was diluted with quartz sands (30−40 mesh) with a ratio of 1:4. Before reforming, the catalyst was reduced in 50 vol %H2/N2 flow (30 mL min−1) at 800 °C for 2 h. After reduction, the temperature of the reactor was tuned to the reaction temperature, and the gas flow was switched to N2 flow (stabilized in 30 min). Then, an aqueous solution of phenol (preheated at 70 °C) was pumped into a gasifier and evaporated at 200 °C before introduction into the fixed bed reactor via the carrier gas N2 (x% C6H5OH/y% H2O/59.5% N2). In this research, the influence of reaction temperature (600−750 °C) and the ratio of steam to carbon (S/C = 0.3−13.3) were studied on the prepared catalysts. The product gas composition was analyzed by an online gas chromatograph (GC, FuLi 9720) equipped with TDX-01 and HPPLOT-Q columns. The samples were simultaneously analyzed by both TCD and FID detector injected via two manual six-port valves (VICI), respectively. On the basis of literature studies,41,56 the calculation of hydrogen yield and phenol conversion can be described as eqs 1 and 2 while hydrogen yield was calculated from the percentage of the stoichiometric maximum amount of hydrogen that can be produced as shown in eqs 3 and 4. Phenol conversion was defined as the number of moles of carbon in the gaseous products divided by the number of moles of carbon in the feed. In this paper, we assume that steam initially reacts with phenol in eq 3. Also, the excess steam for eq 3 will continue to react with CO as described in eq 4. Therefore, when S/C ≤ 0.8, the calculation of hydrogen yield and phenol conversion is based on eq 3, and when S/C > 0.8, the calculation of hydrogen yield and phenol conversion is based on eqs 3 and 4. When the S/C value was 0.83, there is no excess steam to react with CO in eq 4, and then the CO concentration should theoretically reach the maximum, as described in eq 5. When the S/C ratio was 1.8, based on eqs 3 and 4, all of the CO and steam were converted to H2 in the water gas shift (WGS) reaction, and then the H2 content should reach the maximum in theory at this time, as described in eq 6.

mixture is used to generate steam, and its removal is not necessary, which reduces the investment and operation costs. The produced hydrogen-rich syngas can be purified and transported with the existing system to be used as fuels or building blocks to produce methanol, dimethyl ether (DME), and other hydrocarbons.26 Various supported metal catalysts have been employed in the steam reforming process for higher process efficiency.27−37 Among them, low-cost Ni-catalysts were widely used, but showed two significant challenges, sintering and coke formation.38−40 For better Ni-catalysts, various supports and promoters have been introduced to enhance the dispersion of active sites and generate the support−metal interaction on catalysts, which could strongly enhance the metal activity and stability during the reactions.35,41−43 Another strategy is to introduce small amounts of a second metal to Ni-catalysts to generate bimetallic catalysts.44−46 Noble metals have been added to Ni-based catalysts for high activity and resistance to carbon deposition.47−50 However, introducing a noble metal not only raised the cost of catalysts but also influenced the formation of alloy particles due to the separation of noble metal and Ni during high-temperature calcination. Using a transition metal such as Fe to replace noble metals in Ni-based bimetallic catalysts is an alternative method. Such a way can lead to a homogeneous dispersion of Fe atoms in bulk Ni due to the similar physical and chemical properties of the Fe and Ni metallic atoms. In this study, various amounts of Fe atoms were added to Ni/Al2O3 catalysts during synthesis via the classic coprecipitation method.51,52 The effects of Fe addition on catalytic behavior, especially on activity and carbon deposition, were investigated for the phenol reforming. It has to be mentioned that coke cannot be avoided during the phenol reforming, while the useless coke could be transferred to a valuable carbon nanotube (CNT) or nanofiber with the design of suitable catalysts under optimal operation conditions in the gasification of solids or small alcohol reforming.37,53−55 Using the newly developed Ni/Fe-catalysts in this research, the hydrogen-rich syngas or CNT could be selectively produced via simply tuning the operation conditions. Good utilization of a valuable organic carbon resource for largescale clean energy H2 production with the high-value byproduct CNT would obviously enhance the economic benefits and reduce green gas emission (burning coke during the regeneration of catalysts) for the reforming process and promote the commercial application of this sustainable technique.

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EXPERIMENTAL SECTION

Catalyst Preparation. All chemicals used for the synthesis of Ni/ Fe-nanocatalysts, such as Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, and Al(NO3)3·9H2O, were obtained from Sinopharm Chemical Reagent Co. China. All catalysts were prepared by the coprecipitation method, which contain 80 mol % Al and total 20 mol % Ni/Fe with the Ni:Fe molar ratios of 10:0, 9:1, 8:2, 6:4, 5:5, 4:6, 2:8, 1:9, 0:10. The precursor solution was prepared by dissolving certain amounts of Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, and Al(NO3)3·9H2O salts in the deionized water. NH3·H2O (25−30%) solution was dropped to the precursor solution to adjust the pH of the solution to around 8. Then, the suspension was obtained and further aged in a water bath for an hour. The suspension was filtrated and washed by deionized water until pH reached 7. The solid product was dried at 80 °C for 12 h and finally calcined in static air (58 L/h, 20 vol % oxygen) at 800 °C with a heating rate of 1 °C/min for 4 h.

H 2 yield % = 2099

moles of H 2 obtained × 100% stoichiometric H 2 potential

(1)


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Figure 1. XRD patterns: (a) Ni/Fe-catalyst/1 (nNi:nFe = 10:0), (b) Ni/Fe-catalyst/2 (nNi:nFe = 9:1), (c) Ni/Fe-catalyst/3 (nNi:nFe = 8:2), (d) Ni/Fecatalyst/4 (nNi:nFe = 6:4), (e) Ni/Fe-catalyst/5 (nNi:nFe = 5:5), (f) Ni/Fe-catalyst/6 (nNi:nFe = 4:6), (g) Ni/Fe-catalyst/7 (nNi:nFe = 2:8), (h) Ni/Fecatalyst/8 (nNi:nFe = 1:9), and (i) Ni/Fe-catalyst/9 (nNi:nFe = 0:10).

crystal formed, showing that iron existed as amorphous iron oxide or very fine iron oxide nanoparticles, or was homogeneously dispersed in the NiAl2O4 structure. On the basis of the Scherrer equation and the peak at 37.2°, the particle sizes for Ni/Fe-catalyst/1 to Ni/Fe-catalyst/9 were 7.8, 7.6, 7.3, 6.8, 6.6, 6.7, 6.4, 3.6, and 3.3 nm, respectively, under the assumption that the composition of particles is uniform. As shown in TEM images of the nine prepared catalysts (Figure 2), the very fine particles with uniform size of ca. 3−5 nm were homogeneously dispersed on the surface of the supports (20−50 nm big particles). These nanoparticles were aggregated into big groups with size between 200 and 400 nm as observed in SEM images in Figure S1 (Supporting Information). From both TEM (Figure 2) and SEM (Figure S1) images, no NiO crystal was detected in all samples, which is consistent with the observation from XRD research. There was no obvious Ni and Fe particle aggregation for either catalyst, indicating good dispersion of Ni and Fe on the Al2O3 support. A large number of roughly spherical and stripshaped particles were observed, which was possibly caused by the existence of Al2O3 or NiAl2O4 nanoparticles.56,58−61 TPR under hydrogen has been used to investigate the reduction properties of surface metal particles on all Ni/Fecatalyst/x samples as shown in Figure 3. On the Ni/Fecatalyst/1 (without Fe in the composition), one strong reduction peak at 700−1000 °C was detected, which corresponds to the dominant NiAl2O4 species.62,63 No typical H2-TPR traces of NiO crystal (300−400 °C) were observed on the surface of this catalyst.64 This finding is consistent with the XRD analysis (Figure 1) showing that no crystal NiO was observed and NiAl2O4 was the dominant species in the catalyst. Even upon introduction of Fe precursor during the synthesis, NiAl2O4 species still remained dominant in the catalysts induced by the strong reduction peak at 820 °C (Ni/Fecatalyst/x, x = 1−8), similar to results detected by XRD. For the Ni/Fe-catalyst/9 sample (without Ni in the composition), two reduction peaks at 410 and 435 °C were caused by two types of surface Fe2O3 particles based on different particle sizes or locations.65,66 However, the XRD patterns of Ni/Fe-catalyst/ x do not present clear peaks assigned to Fe2O3 species, which

conversion % moles of carbon (in CO + CO2 + CH4 + C2 − 3) obtained = moles of carbon in the feed

(2)

× 100%

C6H5OH + 5H 2O → 6CO + 8H 2 endothermic CO + H 2O → CO2 + H 2 nmaxCO% =

nmaxH2% =

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(3) −1

ΔH298K = − 41 kJ mol

6nC6H5OH 6nC6H5OH + 8nC6H5OH

14nC6H5OH 14nC6H5OH + 6nC6H5OH

(4)

= 42.86% (5)

= 70.00% (6)

RESULTS AND DISCUSSION Catalyst Characterization. Figure 1 shows the XRD patterns of all catalysts. Three major peaks at 37.0°, 45.0°, and 65.5° (Figure 1a−h) were observed for NiAl2O4 species (JCPDS 10-0339). With the decrease of Ni content in prepared catalysts, the intensity of the NiAl2O4 species decreased as well while it still remains dominant in the XRD patterns of all Ni containing samples, which illustrates that adding Fe changed the particle size and crystal distortion of NiAl2O4. Obviously, the main crystal structure containing Ni in these catalysts is NiAl2O4, which was considered to be the key composition that enhanced the activity and stability in gasification/reforming.57,58 Meanwhile, no clear peak at 2θ = 37.2°, 43.3°, and 62.8° for NiO (JCPDS 78-0423) was observed in all prepared catalysts. The peaks at 37.5°, 45.7°, and 66.6° were assigned to Al2O3 (JCPDS 50-0741) while there is a certain degree of overlap between the characteristic peaks of NiAl2O4 and Al2O3 as shown in Figure 1, notably at 2θ = 35−40° and 65−70°. The diffraction peak summit slightly shifts to a large 2θ value at 2θ = 35−40° and 65−70° from part a to part i of Figure 1a. The possible reason is the NiAl2O4 diffraction peak area decreasing with the Ni loading, while the content of Al2O3 remained unchanged. The diffraction peaks corresponding to Fe2O3 (JCPDS 84-0310) at 24.3°, 33.4°, 35.8°, and 54.5° were very hard to observe in Figure 1, which indicates that no iron oxide 2100


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Figure 2. TEM images: (a) Ni/Fe-catalyst/1 (nNi:nFe = 10:0), (b) Ni/Fe-catalyst/2 (nNi:nFe = 9:1), (c) Ni/Fe-catalyst/3 (nNi:nFe = 8:2), (d) Ni/Fecatalyst/4 (nNi:nFe = 6:4), (e) Ni/Fe-catalyst/5 (nNi:nFe = 5:5), (f) Ni/Fe-catalyst/6 (nNi:nFe = 4:6), (g) Ni/Fe-catalyst/7 (nNi:nFe = 2:8), (h) Ni/Fecatalyst/8 (nNi:nFe = 1:9), (i) Ni/Fe-catalyst/9 (nNi:nFe = 0:10).

are very small nanoparticles (3 nm, based on the Scherrer equation), and XRD patterns only showed broad and very weak peaks. Nearly all Fe containing catalysts have surface Fe2O3 particles as shown in Figure 3b−i. It was reported that the hydrogen reduction of Fe2O3 requires two or three steps via Fe3O4 or Fe1−xO.67,68 On Fe/SiO2, the H2-TPR traces exhibited two peaks at 412 and 644 °C, corresponding to two steps of Fe2O3 reduction.69 Therefore, the reduction peaks at 580−700 °C were assigned to the second step reduction of Fe2O3 particles (reduced at 410 and 435 °C in the first step).70 The two-step reduction was obviously observed for high-Fecontaining catalysts (Figure 3g−i) and hard to find for catalysts with a relatively low Fe amount (Figure 3b−h). A broad shoulder at the temperature higher than 800 °C in high-Fecontaining catalysts (Figure 3g−i) was possibly due to the reduction of Fe in the alumina lattice as reported elsewhere.71 As discussed above, no obvious reducing peak related to Fe has been detected in TPR results for Ni-rich bimetallic catalysts as shown in Figure 3 while XRD results showed some peak

shifting of NiAl2O4, which might be due to the dispersion of Fe atoms into the NiAl2O4 or Al2O3 lattice instead of iron oxides. The main purpose of XPS was to obtain information on the surface state of the Ni/Fe-catalysts. All of the catalysts were reduced in a 20 mL/min 10% H2/He gas flow at 800 °C for 2 h before they were measured by XPS while exposure to air cannot be avoided during the operation of XPS. Figure 4 shows the Ni 2p and Fe 2p spectra for tested catalysts. Monometallic catalysts of Ni/Fe-catalyst/1 and Ni/Fe-catalyst/9 have been tested for comparison. As presented in Figure 4A, the minor peak (852.1 eV) corresponding to Ni0 and the larger peak (856.3 eV) assigned to Ni2+ were observed on Ni/Fe-catalyst/ 1, Ni/Fe-catalyst/2, Ni/Fe-catalyst/5, and Ni/Fe-catalyst/6 while the intensity of these two peaks decreased with a decrease in the Ni content. Even for the samples that have been reduced before measurement, Ni2+ still counted for a majority of Ni due to the fact that NiAl2O4 is hard to reduce below 800 °C, which is consistent with TPR results. In Figure 4B, it can be seen that the intensity of peaks corresponding to Fe2+ (709.5 eV) and Fe3+ (711.4 eV) increased with Ni/Fe ratio from 9:1 to 0:10 2101


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Information. As expected, supported Ni-catalyst (Ni/Fecatalyst/1, 85% conversion) showed much better catalytic activity than did supported Fe-catalyst (Ni/Fe-catalyst/9, 9.5% conversion) during the reforming. Also, Ni/Fe bimetallic catalysts with a dominant amount of Fe showed much lower activity (under 10.1% phenol conversion and 9.5% H2 yield on catalysts with Ni/Fe from 4:6 to 1:9) than the catalysts containing a greater Ni fraction (above 77.4% phenol conversion and 55.0% H2 yield on catalysts with Ni/Fe from 5:5 to 9:1) in phenol steam reforming. This observation is consistent with previously reported work that Ni is more active than Fe in the catalytic reforming process.74,75 However, the phenol conversion and H2 yield did not increase continuously with Ni content in Ni/Fe-catalysts, and pure Ni-catalyst was not the best catalyst in this research. H2 yield and phenol conversion reached 81.5% and 88.3% on Ni/Fe-catalyst/2 (Ni/ Fe = 9:1) at 700 °C, which was the best performance result among results for all prepared catalysts. Obviously, adding a small amount of low-cost Fe for the synthesis of bimetallic catalyst could enhance the catalytic performance of metal catalysts during the catalytic reactions. It is well-known that Nibased-only catalysts are easily deactivated due to serious carbon deposition in the hydrocarbon steam reforming reaction, while some bimetallic Ni-catalysts can dramatically improve catalysts’ resistance to carbon deposition.76,77 Montané et.al. mentioned that Fe remarkably improved catalytic activity in bimetallic Ni/ Fe-catalysts due to the enhanced nickel dispersion and the high surface area.78,79 In this study, adding a proper amount of Fe into Ni-based catalysts has reduced the Ni containing particle size as well as improved Ni dispersion, which further performed higher catalytic activity in phenol steam reforming reaction. After the evaluation of prepared catalysts, Ni/Fe-catalyst/2 showed the best catalytic performance in the research and was selected for the further research. Normally, increasing the amount of water can enhance the hydrocarbon conversion during steam reforming, such as the conversion of ethylene glycol, acetone, m-xylene, and acetol.80,81 In addition, excess steam in the reforming reactor could result in water gas shift (WGS) reaction (eq 4), which will further improve the hydrogen production.82−85 The steam reforming of phenol is an endothermic reaction while WGS is an exothermic reaction.82 The thermal energy requirement for steam reforming could be partially balanced by WGS after overloading the water amount. In this research, we adjusted the steam to carbon molar ratio (S/C) from 0.3 to 13.3 during the catalytic reforming of phenol. Moreover, we calculated the theoretical steam to carbon molar ratio (S/C) based on the reaction stoichiometry for the highest CO production (S/C = 0.8 (7.0% C6H5OH/33.5% H2O/59.5% N2)) and the highest H2 yield (S/C = 1.8 (3.4% C6H5OH/37.1% H2O/59.5% N2)). Then, the experimental results could be compared with the stoichiometric values for a deep understanding of the reforming process on catalysts. As shown in Figure 6a, the phenol conversion was significantly enhanced from 57.1−60.8% to 78.8−85.8% when the S/C increased from 0.3−0.8 to 1.3−1.8. As expected, increasing the water amount could enhance the hydrocarbon conversion during the reforming as the reaction rate should be raised simultaneously with the higher reactant concentration. Under the lower S/C ratios (0.3−1.3), H2 yield barely changed, which indicates that the reaction process was mainly based on phenol reforming. The methanation reaction from CO and H2 occurred as side-reaction, and CH4 was formed as shown in

Figure 3. TPR profiles of the samples: (a) Ni/Fe-catalyst/1 (nNi:nFe = 10:0), (b) Ni/Fe-catalyst/2 (nNi:nFe = 9:1), (c) Ni/Fe-catalyst/3 (nNi:nFe = 8:2), (d) Ni/Fe-catalyst/4 (nNi:nFe = 6:4), (e) Ni/Fecatalyst/5 (nNi:nFe = 5:5), (f) Ni/Fe-catalyst/6 (nNi:nFe = 4:6), (g) Ni/ Fe-catalyst/7 (nNi:nFe = 2:8), (h) Ni/Fe-catalyst/8 (nNi:nFe = 1:9), and (i) Ni/Fe-catalyst/9 (nNi:nFe = 0:10).

Figure 4. XPS spectra in the (A) Ni 2p and (B) Fe 2p regions of the surface of several reduced catalysts. (a) Ni/Fe-catalyst/1 (nNi:nFe = 10:0), (b) Ni/Fe-catalyst/2 (nNi:nFe = 9:1), (c) Ni/Fe-catalyst/5 (nNi:nFe = 5:5), (d) Ni/Fe-catalyst/6 (nNi:nFe = 4:6), (e) Ni/Fecatalyst/9 (nNi:nFe = 0:10).

while Fe3+ is dominant in these samples. As discussed before in TPR results, Fe reduction will require several steps, but the reducibility of Fe is much lower than that of Ni in prepared catalysts in a comparison of the H2 consumption. It can be concluded that only a small part of Fe exists as iron oxides in prepared samples while most of them are participating in the lattice of other particles like NiAl2O4 or Al2O3. Steam Reforming of Phenol. All prepared catalysts were evaluated by the phenol reforming, and their catalytic performance is summarized in Figure 5. The primary reaction conditions such as the temperature at 700 °C and the molar ratio of water steam to the amount of carbon atoms of phenol S/C = 13.3 (0.5% C6H5OH/40% H2O/59.5% N2) were taken from previous reports as the optimal parameters for the same process.72,73 This work also confirmed that the optimal reaction temperature is 700 °C in the temperature range 600−750 °C for phenol reforming as shown in Figure S2 in Supporting 2102


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Figure 5. (a) Influence of various catalysts on H2 yield and phenol conversionand (b) outlet gas composition of phenol reforming. Compositions follow: Ni/Fe-catalyst/1 (nNi:nFe = 10:0), Ni/Fe-catalyst/2 (nNi:nFe = 9:1), Ni/Fe-catalyst/3 (nNi:nFe = 8:2), Ni/Fe-catalyst/4 (nNi:nFe = 6:4), Ni/Fecatalyst/5 (nNi:nFe = 5:5), Ni/Fe-catalyst/6 (nNi:nFe = 4:6), Ni/Fe-catalyst/7 (nNi:nFe = 2:8), Ni/Fe-catalyst/8 (nNi:nFe = 1:9), and Ni/Fe-catalyst/9 (nNi:nFe = 10:0). Reaction conditions: T = 700 °C, GHSV = 60 000 h−1, p = 1 atm, and S/C = 13.3. Catalyst loading: 0.15 g.

Figure 6. Influence of steam to carbon ratio on the catalytic performance of phenol. S/C = 0.8, the theoretical maximum of CO concentration. S/C = 1.8, the theoretical maximum of H2 concentration. Reaction conditions: GHSV = 60 000 h−1, T = 700 °C, p = 1 atm, t = 5 h (for S/C = 0.3, t = 1.5 h). Catalyst loading: Ni/Fe-catalyst/2, 0.15 g.

Figure 7. Stability test of Ni/Fe-catalyst/2 for phenol steam reforming. Reaction condition: T = 700 °C, GHSV = 60 000 h−1, p = 1 atm, and S/C = 13.3. Catalyst loading: 0.15 g.

Figure 6b. Upon an increase of the S/C ratio to a value higher than 0.3, steam reforming of CH4 started, and the CH4 fraction in the gas products decreased. However, phenol conversion remained stable in the range 74.1−88.3% upon a further

increase of the S/C ratios (even 10 times higher). No strong enhancement was observed for phenol conversion, but H2 yield increased dramatically from 30.9% to 81.5% when the S/C ratio increased from 1.8 to 13.3, which might be due to the fact that 2103


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808 N m3/kg phenol with the decrease of water amount. Upon further decrease in the amount of water, more and more carbon was produced as shown in Table 1. Interestingly, TEM images indicated that the formed coke on the surface was valuable carbon nanotubes (CNTs) on the reacted Ni/Fe-catalyst/2 (Figure 8). Obviously, the prepared catalyst had the unique catalytic property, which was able to switch off the formation of filamentous carbons and tune it to the formation of valuable CNTs. CNTs have many widespride promising applications, including hydrogen storage,89−91 sensors,92,93 and electrochemical capacitors.94−96 As shown in TEM, highly purified multiwall CNTs were obtained with diameters around 30 nm. For the confirmation, Raman spectroscopy was used for the characterization of CNTs as shown in Figure 9. The peak around 1345 cm−1 (D band) corresponds to disordered carbonaceous species while the peak at about 1582 cm−1 (G band) is ascribed to tangential vibrations of the graphite carbons.97,98 For the peak at around 2628.3 (G′ band), it should be attributed to the two-photon elastic scattering process.98,99 The degree of graphitization of carbon nanotubes can be evaluated by the intensity of the D band divided by that of the G band (ID/IG). The ID/IG ratio of the CNTs in this research is 0.91, which was lower than the reported value for CNTs of 1.0100 and the value for commercial CNTs of 1.25.101 The ratio IG′/IG for CNTs produced in this research was 0.69, which is higher than the commercial one of 0.64. This indicates that the phenol reforming on the prepared catalyst could produce the high-purity CNTs (better than commercial CNTs) as byproducts from useless cokes. The yield of hydrogen and CNTs can be well-adjusted according to the S/C ratio as shown in Table 1. It will be very promising for industrial applications. Without a change in the catalysts and other operation parameters, the production of hydrogen and CNTs can be controlled according to the energy and materials end-markets only by tuning the water amount for reforming.

phenol conversion is hard to improve further as it is very close to complete conversion. However, increasing the water amount at this stage can still strongly enhance the WGS reaction. As shown in Figure 6b, the CO fraction decreased with the generation of H2 and CO2 when the WGS reaction dominated the process. When the S/C value was set to 0.8, the CO concentration should theoretically reach the maximum value, and our experimental result agreed with the theoretical analysis. The increase of carbon monoxide and the decrease of methane content were mainly attributed to reaction 7 that has a high reaction activity when the S/C value increases from 0.3 to 0.8. When the S/C was 1.8, the H2 content should reach the maximum in theory; however, the experimental result showed that H2 content became higher and CO content dropped sharply with an increasing S/C ratio. CH4 + H 2O → CO + 3H 2

ΔH298K = +206 kJ mol−1 (7)

At 700 °C and S/C = 13.3, the prepared low-cost Ni/Fecatalyst in this research (phenol conversion 88.3% and H2 yield 81.5%) showed better catalytic performance than that on highcost catalysts based on rare-earth and noble metals, such as reported Fe/50Mg-50Ce-O (63.3% conversion, 34.4% H2 yield).72 In addition, this catalyst showed very good stability in the reforming as shown in Figure 7 that phenol conversion was quite stable and H2 yield only decreased slightly in 200 h during the reaction at temperature of 700 °C and water loading of S/C = 13.3. No catalyst deactivation was observed in 200 h, which indicates the extremely good hydrothermal stability of the prepared catalysts and the high potential for industrial application. The fraction of CO was slightly increased after the first 10 h during the reaction. It might be a revised water gas shift reaction of adsorbed surface CO2 and H2 to CO due to the accumulation of CO2 and H2 products inside the reactor. Normally, deposited coke on the surface of catalysts would also deactivate catalysts during the reforming process. In previous reports,56,72,73,86 the residual carbonaceous species were observed on the catalyst bed, which accumulated on the surface as coke and blocked the surface active sites for the reactants. The best performing catalyst in this study did not show any deactivation at 700 °C as shown in Figure 7, and the result confirmed no carbon deposit on the surface after reaction under the condition of S/C = 13.3 as shown in Table 1. It was

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CONCLUSION A series of Ni/Fe-based nanocatalysts with different Ni/Fe ratios were prepared, characterized, and tested on the steam reforming of phenol reaction. A temperature of 700 °C has been tested as the best reaction temperature with the highest reforming activity in this study showing that phenol conversion and hydrogen yield reached 87% and 81%, respectively. The study of influence of the S/C ratio showed that a lower S/C ratio tends to increase carbon monoxide yield, while a higher ratio improves hydrogen yield. In addition, CNTs have been found to be the major deposited carbon form at lower S/C ratio (0.3 and 0.8) after phenol reforming reaction. The maximum production of CNTs was 0.31 kg/kg phenol which corresponded to the hydrogen production being about 247 N m3/kg phenol over the Ni/Fe-catalyst/2 at S/C = 0.3. While at a higher S/C ratio (S/C = 13.3), there were very few carbon nanotubes or other kinds of deposited carbon observed on the reacted catalysts, and the hydrogen production reached the maximum value of about 2483 N m3/kg phenol. With consideration of the influence of the Ni/Fe ratio of catalysts in the phenol reforming reaction, addition of a small amount of Fe has improved the Ni dispersion and modified catalyst reducibility, which revealed that Ni/Fe-catalyst/2 and Ni/Fecatalyst/3 showed excellent hydrogen yield and carbon monoxide selectivity, 81% and 50%, at S/C = 13.3 and 0.8, respectively. In terms of the catalyst stability test, the Ni/Fecatalyst/1 and Ni/Fe-catalyst/2 presented exceedingly high

Table 1. Production of Carbon Nanotubes and Hydrogen with Different S/C Ratio over Ni/Fe-Catalyst/2 at 700 °C S/C

carbon nanotube kg/kg phenol

hydrogen production Nm3/kg phenol

0.3 0.8 1.3 13.3

0.31 0.12 0.02 0.00

247 581 808 2483

observed that slightly reducing the amount of water (S/C from 2.0 to 1.3) could promote the coke generation.87,88 However, the Ni/Fe-catalyst/2 (nNi:nFe = 9:1) prepared in this research showed the excellent property of resistance to coke formation. After a decrease of the water amount of about 10 times (S/C from 13.3 to 1.3), then the coke could be obvisouly observed with 2 wt % of feed phenol as shown in Table 1. As described before, the hydrogen production would decrease from 2483 to 2104


Research Article


Figure 8. TEM of deposited carbons over Ni/Fe-catalyst/2 (nNi:nFe = 9:1) after reaction for 2 h at S/C = 0.3 and 700 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01936. SEM images and details regarding influence of reaction temperature, catalyst, and steam to carbon ratio on the steam reforming of phenol (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rongli Jiang: 0000-0003-0136-3236 Notes

Figure 9. Raman analysis of the carbon nanotube and the reacted Ni/ Fe-catalyst/2.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Natural Science Foundation of China (51674262) for their financial support.

activity and hydrothermal stability after a continuous run for 200 h at S/C = 13.3 and 700 °C. 2105


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