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Ni-encapsulated graphene chainmail catalyst for ethanol steam reforming
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Ni-encapsulated graphene chainmail catalyst for ethanol steam reforming Dong Chen, Wenju Wang*, Chenlong Liu School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
article info
abstract
Article history:
Nickel nanoparticles encapsulated with multilayered graphene were fabricated through
Received 26 May 2018
segregation of the dissolved carbon atoms from nickel. The catalyst, vividly described as
Received in revised form
“chainmail catalyst”, was demonstrated to be promising for ethanol steam reforming (ESR)
21 November 2018
to produce H2. Chainmail catalysts with different content of Ni were denoted as Ni@G2,
Accepted 21 January 2019
Ni@G4, Ni@G8 and Ni@G12, respectively. Fresh and spent catalysts were characterized to
Available online 15 February 2019
analyze the role of graphene shell using various techniques (e.g., XRD, Raman, TEM). The graphene shell can protect the Ni core away from sintering, oxidation, or corrosion. ESR
Keywords:
was investigated with a focus on the characterization of the catalysts, reaction conditions,
Ethanol steam reforming (ESR)
and reaction mechanism. The ESR tests showed that the Ni@G4 exhibited better activity,
Chainmail catalyst
stability and lower byproducts with no deactivation phenomena for 4 h. During ESR,
Graphene
ethanol was preferentially adsorbed on the external surface of the chainmail catalyst by
Nickel
forming pep conjugated system. The spent catalyst could be separated easily by an
Core-shell structure
external magnetic field due to the ferromagnetism of nickel core. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, an alternative fuel, has received special attention due to global warming, energy crisis and urgent need for sustainable development [1e3]. There are various ways of producing H2 such as water electrolysis, plasma arc decomposition of methane, thermochemical water splitting, photolysis and thermolysis of water, fossil fuel and biofuel reforming, and steam reforming and oxy-reforming of hydrocarbons [4e7]. Photocatalytic hydrogen production has been recently studied a lot, but still suffers from high costs due to the requirement of expensive electrodes [8]. Biological methods have also aroused great interest. However, the main bottleneck for the commercialization is lower system efficiency [9].
Therefore, thermochemical method could be a better choice for H2 production [10,11]. In industrial processes, steam reforming of natural gas, which consists mainly of methane, is not a sustainable strategy [12]. Other feeds such as ethanol, glycerol and methanol have also been used for the steam reforming to produce H2. Particularly, ethanol is considered as a promising hydrogen carrier because of its renewable nature, relatively high hydrogen content, increasing availability, low toxicity, and ease of transport. Moreover, absence of sulfur in ethanol does not cause poisoning of catalysts in the ethanol steam reforming (ESR) [10,13]. The products of ESR are mainly governed by the equilibrium between decomposition of ethanol [Eq. (1)], water gas shift [WGS, Eq. (2)] and methane steam reforming [MSR, Eqs. (3) and (4)] reactions. C2H5OH¼CH4þCO þ H2
* Corresponding author. E-mail addresses: [email protected], [email protected] (W. Wang). https://doi.org/10.1016/j.ijhydene.2019.01.204 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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CO þ H2O¼CO2þH2
(2)
CH4þH2O¼COþ3H2
(3)
CH4þ2H2O¼CO2þ4H2
(4)
Theoretically, ESR could generate up to 6 mol of H2 per mole ethanol via the reaction as represented by the overall stoichiometric equation [Eq. (5)]. C2H5OHþ3H2O ¼ 2CO2þ6H2
(5)
Great endeavors have been made to search for an appropriate catalyst for ESR, which can produce hydrogen-rich gas with few undesired products (e.g. CH3CHO, C2H6, C2H4, CH4 and CO) [14,15]. The undesired products are easily generated at moderate temperatures due to highly intense CeC, CeH and OeH bonds [14,16]. Therefore, in ESR, the catalyst must be effective in the cleavage of these bonds. Precious metal catalysts [17,18] (Pt, Pd, Rh, Au and Ru) would be the most promising catalyst candidates for ESR reaction due to their greater ability to break CeC bonds, and, in fact, precious metal catalysts show high activity and selectivity to H2 production with negligible or no coke formation. However, precious metals are expensive for large-scale industrial application. It is necessary to seek for low-cost, rich-reserve, and highly efficient alternatives. Transition metals (Cu, Co and Ni) as well as the combinations of both have been used to catalyze the ESR reaction [19,20]. Nevertheless, these catalysts suffer from their poor stability in ESR due to carbonaceous deposits and metal sintering. Several simultaneous reactions [Eqs. (6)e(9)] along with ESR could generate carbon. At low temperatures, Boudouard reaction [Eq. (6)] is the dominant reaction facilitating the carbon formation [10]. The hydrogenation reactions of CO [Eq. (7)] and CO2 [Eq. (8)], and the thermal decomposition of CH4 [Eq. (9)] are favored at high temperatures [21,22]. These problems could be relieved by improving the dispersion of metal particles, doping with alkali earth and rare earth metals, and modifying the properties of support [23e26]. 2CO ¼ CO2 þ C
(6)
CO þ H2 ¼ H2O þ C
(7)
CO2 þ 2H2 ¼ 2H2O þ C
(8)
CH4 ¼ 2H2 þ C
(9)
In conventional catalysis, active phases of catalysts could be damaged due to direct contact with reactants and the reaction medium [27,28]. In ESR, nanometals as active phases are easily corroded, oxidation, or sintering. Many research have focused on structural modulation to protect active phases [29e31]. Recently, Bao et al. [32] described a novel approach to protect nonprecious metals with multilayered graphene coating them. The active sites were primarily located at the carbon atoms on the outermost surface of the graphene shell [31]. The graphene shell and the nickel core
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delivered unique electrons to modulate electronic state density of these sites to conduct catalytic reactions [33,34]. The catalysts exhibit high catalytic activity and outstanding stability with graphene shell completely coating nonprecious metals. It has been vividly described as “chainmail catalyst” [32]. Their extensive applications in oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, metaleair batteries, and syngas conversion have shown great potential of the unique chainmail catalysts for catalysis [29e31,35,36]. However, there is little research on their application in ESR reaction. Herein, we described a novel pathway for the fabrication of Ni-based chainmail catalyst. For nickel has significant carbon solubility, carbon segregates from the bulk Ni upon annealing, leading to graphene growth [37,38]. In this work, we tested its performance over ESR for H2 production while varying steam to ethanol (S/E) molar ratio, nickel content and reaction temperature. We particularly selected different conditions to favor the formation of H2, and to limit generation of byproducts, such as CO and CH4. Furthermore, the function of chainmail catalyst for ESR was discussed on the atomic level.
Experimental Catalysts preparation Glucose (AR) was used to reduce Ni2þ or NiO, and also served as the source of carbonaceous shell. Carbon rings, which play an important role in the formation of graphene shell, are easily generated from glucose after heat treatment [39,40]. Growth of carbon rings is shown schematically in Fig. 1. Nickel acetate tetrahydrate (AR) was used as an organic precursor, and improved dispersion of metallic Ni due to the generation of volatile vapors in the pyrolysis stage [41]. The general procedure of catalyst preparation was illustrated in Fig. 2. The glucose char has abundant micropores, which improve dispersion of Ni ions in the char. The char was fabricated by slow pyrolysis of glucose from 25 to 500 C in N2 (50 mL min1) at a heating rate of 3 C$min1. After being held for 30 min, the carbon materials were ground to sizes between 100 and 120 mesh. A given quality of nickel acetate tetrahydrate was added to a 100-mL beaker, which contained 2.0 g glucose char and 50 mL deionized water. The beaker containing the mixture was dipped in an ultrasonic generator (25 kHz and 400 W) for 30 min. After ultrasonic impregnation, the residues obtained were dried in an oven at 110 C overnight, and subsequently ground to sizes between 100 and 120 mesh. The nickelbased chainmail catalysts were obtained by thermal treatment in argon flow (50 mL min1) before storage and further use. Different mass ratios of Ni ions to glucose char were 1:2, 1:4, 1:8, 1:12, denoted as Ni@G2, Ni@G4, Ni@G8, Ni@G12, respectively. The reference Ni/Al2O3 catalyst was prepared by the impregnation method. The g-Al2O3 support was impregnated with an aqueous solution of nickel acetate tetrahydrate. The resulting mixture was ultrasonic impregnated for 20 min followed by drying at 110 C overnight. Then the obtained material was calcined in air at 900 C for 4 h. Finally, the prepared catalyst was washed at 200 C for 30 min in N2 flow and then reduced at 650 C in H2 flow.
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between 10 and 80 . The number of graphene layer was calculated by N¼(L002 þ d002)/d002. The size of Ni particles was calculated by the Scherrer equation, D ¼ Kl/(b2qcosqmax), where K was a dimensionless shape factor fixed at 0.89, l was the X-ray wavelength, b2q was the width of the peak at halfheight, and qmax was the Bragg angle at the peak maximum position. The microstructure of the catalysts was determined by means of transmission electron microscopy (TEM). The TEM analysis was conducted on a JEM-2100F operating at 100 kV. The sample powders were dispersed as a suspension in ethanol solutions and sonicated for 3 min. After being well dispersed, a droplet of suspension was deposited on copper grids and followed by drying in air. Fourier transform infrared (FTIR) spectroscopy for the identification of the bonds and quantification of their relative abundance were performed using a Shimadzu IRAffinity-1 FTIR spectrophotometer. The samples analyzed were a pressed pellet consisting of the catalyst (1 mg) in a KBr support (200 mg). Thermogravimetric analysis (TGA) was performed on a NETZSCH STA409 C/PC equipment. The sample was heated in air from room temperature to 1000 C at a heating rate of 10 C$min1. Nickel loading in the catalysts was also measured by TGA, burning the sample at 850 C in pure air (50 mL min1) for 30 min and weighing the residual product as NiO. Raman spectra was carried out to assess graphene. Raman spectra of the fresh and spent catalysts were recorded on an iHR550 Raman microscope from HORIBA scientific with 532 nm solid laser as an excitation source.
Catalytic tests Fig. 1 e Schematic growth of carbon rings (the carbon rings formed after chain scission, polycondensation, and nucleation are schematically represented by aromatic rings).
Catalytic characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2500 instrument (40 kV, 100 mA) using Cu/Ka X-ray source at a scanning rate of 3 $min1 over the range
The catalytic activity of the catalysts was tested isothermally in ESR reaction using a fixed-bed quartz reactor (20 mm inner diameter and 600 mm length) at atmospheric pressure. Fig. 3 presents a schematic diagram of the experimental apparatus, composed of feeding system, ESR reaction system, condensation system, drying system and gas product analysis system. Argon was used as the carrier gas at a flow rate of 200 mL min1. The aqueous solution of ethanol was injected into a vaporizer (150 C) at 20 mL min1 by a syringe pump. The
Fig. 2 e The general procedure of the catalysts preparation.
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Fig. 3 e Schematic diagram of the experimental apparatus for ESR. liquid hourly space velocity (LHSV) and catalyst mass were fixed at 2.4 mL g1 h1 and 0.5 g, respectively. During the experiment, the reactor was heated electrically, and a K-type thermocouple monitored the actual temperature of the catalyst bed. The gas cleaning system includes a series of condensers and a calcium chloride drying tube. Non-condensable products (mainly H2, CO, CO2 and CH4) were periodically analyzed by a gas chromatograph (GC-9890A) with a thermal conductivity detector (TCD) equipped with Porapak Q and TDX-01 packed columns, although some trace condensable products (e.g., CH3CHO, C2H5OC2H5, CH3COCH3) were not detected because of the limitation of the instrument in this experiment. The selectivity to the gas products denoted as SCO , SCO2 , SCH4 and SH2 was calculated as: Fi Si ¼ P *100 Fi
(10)
where Fi is the molar flow of the gas products containing CO, CO2, CH4 and H2. The H2 yield was calculated as: YH2 ¼
FH2 Fethanol
(11)
where Fethanol is the ethanol molar flow in the feedstock. The carbon conversion was calculated as: X¼
FCO2 þ FCO þ FCH4 100 Fethanol
sample exhibited a rapid weight loss at about 440 C, corresponding to the formation of NiO or Ni [42]. With increasing temperature, a broad peak centered at 600 C appeared, ascribed to the reduction of residual NiO by carbon. Therefore, the Ni/GC was held at 330 C, 440 C, 600 C for 10 min, respectively, during the thermal treatment. The complete release of volatile components improved the dispersion of nickel nanoparticles. Graphene growth was promoted on Ni after being held at 950 C for 90 min. Low temperatures tend to give amorphous or poorly crystalline carbon [38,43]. However, longer retention time, in the carbonization stage, could cause the excessive thickness of graphene shell. Thicker graphene shell would limit electron penetration, which affects the catalytic activity. The XRD results of all the catalysts are presented in Fig. 5. There is only slight difference in 2q values among Ni@G2, Ni@G4, Ni@G8 and Ni@G12 chainmail catalysts. All samples exhibited three peaks at 44.5 , 51.8 , and 76.4 , corresponding to (111), (200) and (220) of metal phase (JCPDS #04-0850), respectively. The diffraction peaks of were sharp and intense, indicating the highly crystalline nature of Ni. No peaks of NiO were observed, which confirmed that NiO was completely reduced to Ni in the calcination stage. With decreasing Ni content, the peaks of Ni were severely broadened and reduced in intensity, illustrating that crystallite size of Ni core was reduced [44]. According the Scherrer formula, the Ni core size
(12)
Result and discussion Catalysts characterization The results in Fig. 4 show TGA of the nickel-impregnated glucose char (Ni/GC) under nitrogen atmosphere. Weight loss of Ni/GC was observed as the temperature increased. Specifically, there was a broad and weak peak at 130 C registered in the DTG curve, assigned with the dehydration of crystallized water in nickel acetate tetrahydrate and acetic acid generation concurrently. A sharp peak was clearly observed at 330 C, which was attributed to the further decomposition of dehydrated intermediate. Subsequently, the
Fig. 4 e TGA of Ni/GC in N2 at a heating rate of 5 C·min¡1.
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Fig. 5 e XRD patterns of Ni@G2, Ni@G4, Ni@G8 and Ni@G12 from the peak at 2q ¼ 44.5 for Ni@G2 is 56 nm, for Ni@G4 is 38 nm, for Ni@G8 is 30 nm and for Ni@G12 is 26 nm. In addition, the broad and weak (002) peaks at 2q angle of 26.4 were seen for all samples, indicating the distance between stacked graphene layers [45].average stacking number of a graphene layer was estimated via the d-spacing and size of the crystallite [46]. The detailed X-ray crystalline parameters are given in Table 1. Graphene interlayer spacing of all samples were larger than that of graphite (3.35 A) [47]. And there were no obvious changes in the number of graphene layers among catalysts, which was attributed to the consistency of retention time during catalyst preparation. The existence of graphene was further confirmed by FTIR spectra. In Fig. 6, both the peak at 1397 cm1 and broad peak centered at 3500 cm1 are related to vibrations of adsorbed moisture. The characteristic at 1627 cm1 could be ascribed to C¼C bending vibrations [48]. In addition, Raman analysis (Fig. 7) was carried out to identify the carbon structure of fresh catalysts. The three characteristic peaks at around 1345, 1580, and 2690 cm1 were in good agreement with the typical Raman modes of the D, G, and 2D bands of graphene, respectively, confirming the existence of graphene in samples. The D peak originated from the presence of disordered amorphous carbon while the G peak referred to the tangential vibrations of the graphitic carbon sp2 bonds [49]. The larger ID/ IG apparent in the Raman spectra indicated that more amorphous carbon was present in Ni@G12. Transmission electron microscopy (TEM) micrograph in Fig. 8a shows thatNi@G4 exhibits a coreeshell structure in the range of 35e43 nm. The high-resolution TEM (HRTEM) image
Fig. 6 e FTIR spectras of Ni@G2, Ni@G4, Ni@G8 and Ni@G12. shown in Fig. 8b reveals the graphene shell with an interlayer of about 3.452 A, corresponding to the graphene (002) plane. As depicted in Fig. 8c, the thickness of graphene shell is about 8 nm. These multilayered graphene could protect Ni core from oxidation and sintering. In the core, lattice distances of 1.792 A and 2.065 A in Fig. 8d are indexed to the (200) and (111) crystal planes of Ni. According to the above results of XRD, FTIR, Raman and TEM analyses, the chainmail catalysts are heterogeneous with metallic nickel for and graphene for shell. The graphene shell can protect the inner nickel well enough from oxidation in air at high temperatures. The TGA data of all catalysts in Fig. 9 show an interesting trend of weight change during the thermal treatment procedure in air. Different from common metal nanoparticles, the encapsulated particles started to gain weight after a slight weight loss below 400 C. This indicated that the metallic Ni was completely coated by graphene because of no oxidation of Ni. A dramatic weight loss was noticed in the range of 600e750 C, corresponding to the combustion of the carbon skeleton of graphene [50]. Above 750 C, the rate of weight loss slowed
Table 1 e X-ray structural parameters of catalysts from (002) band. Sample Ni@G2 Ni@G4 Ni@G8 Ni@G12
2q, Plane (002)
Value of d ( A)
Value of L002 ( A)
No. of layers
26.2 26.4 26.3 26.2
3.432 3.445 3.426 3.438
87 76 81 74
26 23 24 22
Fig. 7 e Raman spectras of fresh Ni@G4 and Ni@G12.
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Fig. 8 e HRTEM images of the Ni@G4 chainmail catalyst at different magnification. down due to the oxidation of the exposed Ni core while the carbon skeleton of graphene was burnt. Ni@G8 and Ni@G12 began to lose weight obviously previous to Ni@G2 and Ni@G4, attributed to more amorphous carbon existing on the surface
of catalysts with lower nickel content, which coincided with Raman analysis of fresh Ni@G4 and Ni@G12.
Catalytic activity Effect of steam to ethanol molar ratio
Fig. 9 e Thermal stability curves of Ni@G2, Ni@G4, Ni@G8 and Ni@G12 catalysts. The content of Ni in all samples estimated from the thermal analysis were 38.52 wt%, 22.80 wt%, 13.29 wt% and 8.61 wt% respectively.
S/E molar ratio plays an important role in ESR, influencing the yield, selectivity and deactivation of catalysts. Overstoichiometric mixtures are commonly used in ESR reaction to facilitate the WGS reaction via shifting the equilibrium conversion. In addition, it intensifies coke gasification by steam and consequently, renews catalysts to obtain activity and stability [51]. Recently, diluted ethanol, characterized by lower purification cost, proved effective in improving efficiency of steam reforming [52]. However, higher S/E molar ratio associated with steam reforming requires more energy consumption to vaporize the feedings. In this work, the S/E molar ratio began with the stoichiometric value. All the experiments were conducted in argon (50 mL min1) at 550 C and atmospheric pressure. Typical experimental results are presented in Fig. 10 in terms of H2 yield, selectivity to products, and weight changes of catalysts. Data reports for each catalyst were collected under steady-state conditions. Fig. 10a shows the H2 yields of Ni@G4 catalyst at various S/E molar ratios (varying from 3 to 7). Fig. 10b shows products selectivity for the first 4 h of operation.
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5, because high concentration of ethanol had beneficial influence on the adsorption of ethanol on the external graphene shell. In the catalytic reaction, ethanol was preferentially adsorbed on the surface of the Ni@G4 catalyst, which can be verified by another reference experiment as follows. The Ni@G4 catalyst was tested under similar conditions but feeding deionized water (13 mL min1). In Fig. 11, we plot the H2 yield at 550 C as a function of the time on stream. The Ni@G4 catalyst suffered a strong gasification. A weight loss of 45.32% was estimated at the end of the test, while weight loss of the same catalyst with the corresponding S/E ratio of 6 was 6.28%. The results indicated that ethanol was preferentially adsorbed on the external surface of the chainmail catalyst. As to S/E molar ratio of 7, it showed higher H2 yield than that with S/E ratio of 6. Combined with weight loss after 4 h of reaction in Fig. 11c and products selectivity in Fig. 11b, the selectivities to CO and CH4 accounted more, and the spent catalyst weight lost more. As listed in Table 2, the carbon conversion at S/E molar ratio of 7 was 128.9%. Higher S/E molar ratio induced excessive carbon gasification, and increased H2 yield to some extent. Therefore, the S/E molar ratio for ESR was fixed at 6 in the following tests.
Effect of nickel content The catalytic performance of all samples were tested for ESR at 550 C and S/E molar ratio of 6, aiming to study the effect of Ni content on each catalyst. As shown in Fig. 12a, the H2 yield was high up to 3.4 mol H2/mol ethanol for the first 150 min with the Ni@G2 catalyst, and then decreased quickly. This was because multilayered graphene could not protect the large Ni core (56 nm) sufficiently enough with time on stream. Jens et al. [54] reported that smaller Ni particles conduced to an increase in catalytic activity of steam reforming. Ni@G4 catalyst showed the best activity and stability in the ESR reaction, with the H2 yield ranging from 3.33 to 3.47 mol H2/mol ethanol. As Fig. 12b shows, the selectivity of Ni@G4 to H2 (65.34%) and CO2 (24.56%) were significantly higher, and the selectivity to CH4 was relatively low (0.66%). It indicated that the methanation reaction was not dominant in ESR on Ni@G4. For the Ni@G8 and Ni@G12 catalysts, the initial H2 yields were
Fig. 10 e Variation of H2 yield (a), products selectivity (b) and weight changes of spent catalysts (c) as a function of S/E molar ratio on Ni@G4 catalyst at 550 C.
In general, increasing S/E molar ratio from 4 to 6 increased H2 yield and reduced CO selectivity, due to the promotion of the WGS reaction according to the thermodynamic equilibrium [53]. Most surprisingly, higher H2 yield was obtained at the S/E molar ratio of 3, compared to S/E molar ratios of 4 and
Fig. 11 e Comparison between S/E ¼ 6 and pure water on Ni@G4 catalyst at 550 C.
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Table 2 e Carbon conversion (%) at various S/E molar ratios. 3:1
4:1
5:1
6:1
7:1
86.7
70.5
80.9
97.2
128.9
at low level, and then increased continuously. The existence of amorphous carbon (based on previous Raman analysis of fresh catalyst in Fig. 7) inhibited electron transmission from Ni core to the catalyst surface reducing the catalytic activity. With the gasification of amorphous carbon, graphene was exposed to facilitate catalysis, coinciding with the more weight loss of spent catalyst (see Fig. 12c). Table 3 lists the carbon conversion of chainmail catalysts with different nickel content. Carbon balance occurred on Ni@G4 catalyst, indicating that the amount of condensed products could be very small. The carbon imbalance occurred on Ni@G8 and Ni@G12 catalysts, attributed to the gasification of more amorphous carbon on the surface of chainmail catalysts. In general, Ni@G4 was more suitable for ESR.
Effect of reaction temperature The effect of reaction temperature on ESR was investigated from 400 to 600 C on Ni@G4 catalyst at S/E molar ratio of 6. As can be seen from Fig. 13, the reaction temperature significantly affected the catalytic activity. Ni@G4 showed the lowest catalytic activity at 400 C, and the H2 yield gradually decreased. Combined with the weight gain of spent catalyst in Fig. 13c, carbon deposition could be the primary reason for catalyst deactivation. Compared to fresh Ni@G4, the ID/IG value of the Ni@G4 after reaction at 400 C in the Raman spectroscopy (Fig. 14) increased to 0.99, indicating that there were a variety of amorphous carbon in the spent catalyst caused by carbon deposition. Moreover, Ni could not efficiently split CeC bond at low temperatures [55]. With the temperature up to 550 C, the H2 yield achieved 3.46 mol H2/mol ethanol and remained stable in the test. As the temperature further increased to 600 C, the initial H2 yield was over 4.77 mol H2/mol ethanol, which was quite higher than that at 550 C. It was attributed to extensive steam gasification of the carbon on the surface of the catalyst, coinciding with the more weight loss of spent catalyst (see Fig. 13c). Nevertheless, the H2 yield dramatically decreased after 75 min and fluctuated later. As shown in Fig. 13b, from 400 to 550 C, a certain quantity of CH4 and CO could be presented in the gas mixture and their variation followed an identical pattern, the selectivity to which decreased with increasing temperature. On the contrary, CO2 showed the opposite tendency for selectivity. At 400 C, the H2 selectivity was about 49.75%, along with a certain amount of CO (18.04%), CO2(15.83%) and CH4 (16.38%) as products. Combined with the weight gain of spent catalyst, the phenomena were probably related to MSR [Eqs. (3) and (4)], and Boudouard reaction [Eq. (6)]. It also implied that ethanol decomposition [Eq. (1)] was the first step in the ESR reaction. At higher steam-reforming temperatures, the CH4 selectivity decreased while the H2 selectivity increased concurrently,
Table 3 e Carbon conversion (%) of chainmail catalysts with different nickel content. Fig. 12 e Variation of H2 yield (a), products selectivity (b) and weight changes of spent catalysts (c) as a function of nickel content on different catalysts.
Ni@G2 95.5
Ni@G4
Ni@G8
Ni@G12
97.2
102.9
108.2
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Fig. 14 e Raman spectras of fresh Ni@G4 and spent Ni@G4. Spent catalyst was collected after reaction at 400 C (S/E molar ratio: 6).
surface gasified by steam and the thermodynamics of the WGS, a moderately exothermic reaction. It was coinciding with the results of Gerd et al. [53]. The carbon conversion listed in Table 4 also indicated that the more favorable reaction temperature for ESR on Ni@G4 catalyst is 550 C.
Special function of chainmail catalyst for ESR
Fig. 13 e Variation of H2 yield (a), products selectivity (b) and weight changes of spent catalyst (c) as a function of reaction temperature on Ni@G4 catalyst at S/E ratio of 6.
showing that the MSR favored high temperature. Reasonable H2 selectivity (64.9%) and negligible CH4 selectivity (0.62%) were achieved at 550 C. Additionally, the H2 selectivity strongly decreased and the CO selectivity strongly increased at 600 C. This was because of the carbon on the catalyst
Several types of catalysts such as calcined rocks, iron ores, alkali metals, transition metals, and noble metals have been employed for ESR. In consideration of catalytic activity and economic reasons, the nickel catalysts have been widely used in ESR. Nickel catalysts are usually supported by metal oxides, zeolites or minerals [56e58]. These supports are rather expensive, and the carbon deposition and metal sintering are relatively obvious. As an alternative, char has been employed as a low-cost catalyst carrier in ESR due to its inert chemical property and high hydrothermal stability [59]. Hence, in this work, glucose was chosen as the carbon precursor due to the absence of metal elements. The unique chainmail catalyst was fabricated via dissolved carbon segregating from nickel. Monolayered graphene is a zero-overlap semimetal with extremely low density of states (DOS) close to the Fermi level, which is actually very inert in catalysis [32,60]. In this sense, it is necessary to increase the DOS near the Fermi level to improve graphene catalytic performance. Metal-anchored graphene exhibits highly catalytic activity due to the interaction of the metal species with graphene [61]. The nickel core donates electrons to the p-band of
Table 4 e Carbon conversion (%) on Ni@G4 at different temperatures. 400 C 45.4
450 C
500 C
550 C
600 C
51.7
60.2
98.9
144.3
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graphene to tune the electronic state of graphene surface. Kozlov et al. [34] proved that graphene also back-donated electrons from its s-band. Therefore, a chemical bond was formed via a donation/back-donation mechanism. The graphene shell of the chainmail catalyst can completely prevent reaction molecules and medium contacting the Ni core, protecting the Ni core from sintering, oxidation, or corrosion. To demonstrate the role of graphene shell, Ni/Al2O3 was used as the reference catalyst under the same condition. As shown in Fig. 15a, the reference Ni/Al2O3 catalyst significantly deactivated in terms of H2 yield with time on stream. XRD profiles of fresh and spent Ni/Al2O3 (see Fig. 15b) show that NiO and graphitic carbon were generated after reaction. Structural analysis also revealed that Ni0 sites were much more active for CeC bond cleavage and for the WGS reaction than Ni2þ species [62,63]. According the Scherrer formula, the Ni particle size from 2q ¼ 44.5 for fresh Ni/ Al2O3 is 29 nm, for spent Ni/Al2O3 is 48 nm. Carbon deposition, oxidation of Ni0 and metal sintering were the primary reasons for catalyst deactivation. Whereas the XRD profiles of Ni@G4 chainmail catalyst in Fig. 15c revealed no NiO peak after reaction, even after reaction at 600 C. And the Ni particle sizes were in the range of 33e38 nm without apparent metal sintering. TEM micrograph of Ni@G4 after reaction at 550 C (see Fig. 15d) shows that there were no obvious changes in nanoparticle's size and morphology, and the graphene shell still
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survived after ESR. The core-shell structure of the chainmail catalyst not only prevented the sintering and oxidation of Ni, but also improved the stability of chainmail catalysts. In addition, the spent catalyst could be separated easily by an external magnetic field due to the ferromagnetism of nickel core. Ethanol undergoes a dissociative adsorption on the catalyst surface [64]. The strong electron donating ability of the hydroxyl group makes ethanol a p-electron rich system [65]. Thus, it induces an interaction between the external graphene shell and hydroxyl group via the formation of pep conjugated system, which is the primary driving force in adsorption of graphene shell. Furthermore, electrons donated by the hydroxyl group also increase the DOS of graphene at the Fermi level, thus improve catalytic performance. In conclusion, ethanol is preferentially adsorbed on the external surface of the Ni@G4 catalyst, as the results of the reference test depicted in Section Effect of stream to ethanol molar ratio. Thermodynamic equilibrium calculation for ESR reveals that H2, CO, CO2, and CH4 are the main products [53]. At higher temperatures, the formations of H2 and CO are thermodynamically favored [51]. Therefore, in this work, taking into account the products selectivity, the first step of ESR on chainmail catalyst could be regarded as ethanol decomposition, and then the reactive CO and CH4 were reformed by steam, as illustrated in Fig. 16.
Fig. 15 e (a) H2 yields of ESR on Ni/Al2O3 and Ni@G4 chainmail catalyst (Temperature: 550 C, S/E molar ratio: 6); (b) XRD patterns of fresh and spent Ni/Al2O3 catalysts; (c) XRD patterns of Ni@G4 catalysts after reaction at different temperatures; (d) TEM images of the Ni@G4 chainmail catalyst after reaction at 550 C (S/E molar ratio: 6). The inset in (d) shows the HRTEM image of graphene shell.
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Fig. 16 e Schematic diagrams of the ESR reaction on chainmail catalyst.
Conclusion [3]
A novel nickel-based chainmail catalyst was successfully fabricated for H2 production from ESR. Compared to Ni@G2, Ni@G8 and Ni@G12, the Ni@G4 as a promising catalyst displayed superb catalytic activity with a high H2 selectivity (65.34%) as well as a high stability at 550 C. At the S/E molar ratio of 6, the H2 yield was high up to 3.47 mol H2/mol ethanol, and the selectivity to CH4 was rather low (0.66%). According to the product distribution, a relatively reasonable ESR reaction pathway on the catalyst was proposed. Ethanol was firstly decomposed to H2, CO and CH4. Then the CO and CH4 were converted via WGS and MSR, respectively. From the results of XRD, Raman and TEM of fresh and spent catalysts, the coreshell structure of the chainmail catalyst not only prevented the sintering and oxidation of Ni, but also effectively improved the H2 production and reduced the undesired products. In ESR, ethanol was preferentially adsorbed on the surface of the Ni@G4 catalyst. Furthermore, the spent catalyst could be separated easily by an external magnetic field owing to the ferromagnetism of nickel core.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Acknowledgments This work is financially supported by National Natural Science Foundation of China with Grant No. 21676148 and Qing Lan Project.
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Ni-encapsulated graphene chainmail catalyst for ethanol steam reforming Dong Chen, Wenju Wang*, Chenlong Liu School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
article info
abstract
Article history:
Nickel nanoparticles encapsulated with multilayered graphene were fabricated through
Received 26 May 2018
segregation of the dissolved carbon atoms from nickel. The catalyst, vividly described as
Received in revised form
“chainmail catalyst”, was demonstrated to be promising for ethanol steam reforming (ESR)
21 November 2018
to produce H2. Chainmail catalysts with different content of Ni were denoted as Ni@G2,
Accepted 21 January 2019
Ni@G4, Ni@G8 and Ni@G12, respectively. Fresh and spent catalysts were characterized to
Available online 15 February 2019
analyze the role of graphene shell using various techniques (e.g., XRD, Raman, TEM). The graphene shell can protect the Ni core away from sintering, oxidation, or corrosion. ESR
Keywords:
was investigated with a focus on the characterization of the catalysts, reaction conditions,
Ethanol steam reforming (ESR)
and reaction mechanism. The ESR tests showed that the Ni@G4 exhibited better activity,
Chainmail catalyst
stability and lower byproducts with no deactivation phenomena for 4 h. During ESR,
Graphene
ethanol was preferentially adsorbed on the external surface of the chainmail catalyst by
Nickel
forming pep conjugated system. The spent catalyst could be separated easily by an
Core-shell structure
external magnetic field due to the ferromagnetism of nickel core. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, an alternative fuel, has received special attention due to global warming, energy crisis and urgent need for sustainable development [1e3]. There are various ways of producing H2 such as water electrolysis, plasma arc decomposition of methane, thermochemical water splitting, photolysis and thermolysis of water, fossil fuel and biofuel reforming, and steam reforming and oxy-reforming of hydrocarbons [4e7]. Photocatalytic hydrogen production has been recently studied a lot, but still suffers from high costs due to the requirement of expensive electrodes [8]. Biological methods have also aroused great interest. However, the main bottleneck for the commercialization is lower system efficiency [9].
Therefore, thermochemical method could be a better choice for H2 production [10,11]. In industrial processes, steam reforming of natural gas, which consists mainly of methane, is not a sustainable strategy [12]. Other feeds such as ethanol, glycerol and methanol have also been used for the steam reforming to produce H2. Particularly, ethanol is considered as a promising hydrogen carrier because of its renewable nature, relatively high hydrogen content, increasing availability, low toxicity, and ease of transport. Moreover, absence of sulfur in ethanol does not cause poisoning of catalysts in the ethanol steam reforming (ESR) [10,13]. The products of ESR are mainly governed by the equilibrium between decomposition of ethanol [Eq. (1)], water gas shift [WGS, Eq. (2)] and methane steam reforming [MSR, Eqs. (3) and (4)] reactions. C2H5OH¼CH4þCO þ H2
* Corresponding author. E-mail addresses: [email protected], [email protected] (W. Wang). https://doi.org/10.1016/j.ijhydene.2019.01.204 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
(1)
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CO þ H2O¼CO2þH2
(2)
CH4þH2O¼COþ3H2
(3)
CH4þ2H2O¼CO2þ4H2
(4)
Theoretically, ESR could generate up to 6 mol of H2 per mole ethanol via the reaction as represented by the overall stoichiometric equation [Eq. (5)]. C2H5OHþ3H2O ¼ 2CO2þ6H2
(5)
Great endeavors have been made to search for an appropriate catalyst for ESR, which can produce hydrogen-rich gas with few undesired products (e.g. CH3CHO, C2H6, C2H4, CH4 and CO) [14,15]. The undesired products are easily generated at moderate temperatures due to highly intense CeC, CeH and OeH bonds [14,16]. Therefore, in ESR, the catalyst must be effective in the cleavage of these bonds. Precious metal catalysts [17,18] (Pt, Pd, Rh, Au and Ru) would be the most promising catalyst candidates for ESR reaction due to their greater ability to break CeC bonds, and, in fact, precious metal catalysts show high activity and selectivity to H2 production with negligible or no coke formation. However, precious metals are expensive for large-scale industrial application. It is necessary to seek for low-cost, rich-reserve, and highly efficient alternatives. Transition metals (Cu, Co and Ni) as well as the combinations of both have been used to catalyze the ESR reaction [19,20]. Nevertheless, these catalysts suffer from their poor stability in ESR due to carbonaceous deposits and metal sintering. Several simultaneous reactions [Eqs. (6)e(9)] along with ESR could generate carbon. At low temperatures, Boudouard reaction [Eq. (6)] is the dominant reaction facilitating the carbon formation [10]. The hydrogenation reactions of CO [Eq. (7)] and CO2 [Eq. (8)], and the thermal decomposition of CH4 [Eq. (9)] are favored at high temperatures [21,22]. These problems could be relieved by improving the dispersion of metal particles, doping with alkali earth and rare earth metals, and modifying the properties of support [23e26]. 2CO ¼ CO2 þ C
(6)
CO þ H2 ¼ H2O þ C
(7)
CO2 þ 2H2 ¼ 2H2O þ C
(8)
CH4 ¼ 2H2 þ C
(9)
In conventional catalysis, active phases of catalysts could be damaged due to direct contact with reactants and the reaction medium [27,28]. In ESR, nanometals as active phases are easily corroded, oxidation, or sintering. Many research have focused on structural modulation to protect active phases [29e31]. Recently, Bao et al. [32] described a novel approach to protect nonprecious metals with multilayered graphene coating them. The active sites were primarily located at the carbon atoms on the outermost surface of the graphene shell [31]. The graphene shell and the nickel core
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delivered unique electrons to modulate electronic state density of these sites to conduct catalytic reactions [33,34]. The catalysts exhibit high catalytic activity and outstanding stability with graphene shell completely coating nonprecious metals. It has been vividly described as “chainmail catalyst” [32]. Their extensive applications in oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, metaleair batteries, and syngas conversion have shown great potential of the unique chainmail catalysts for catalysis [29e31,35,36]. However, there is little research on their application in ESR reaction. Herein, we described a novel pathway for the fabrication of Ni-based chainmail catalyst. For nickel has significant carbon solubility, carbon segregates from the bulk Ni upon annealing, leading to graphene growth [37,38]. In this work, we tested its performance over ESR for H2 production while varying steam to ethanol (S/E) molar ratio, nickel content and reaction temperature. We particularly selected different conditions to favor the formation of H2, and to limit generation of byproducts, such as CO and CH4. Furthermore, the function of chainmail catalyst for ESR was discussed on the atomic level.
Experimental Catalysts preparation Glucose (AR) was used to reduce Ni2þ or NiO, and also served as the source of carbonaceous shell. Carbon rings, which play an important role in the formation of graphene shell, are easily generated from glucose after heat treatment [39,40]. Growth of carbon rings is shown schematically in Fig. 1. Nickel acetate tetrahydrate (AR) was used as an organic precursor, and improved dispersion of metallic Ni due to the generation of volatile vapors in the pyrolysis stage [41]. The general procedure of catalyst preparation was illustrated in Fig. 2. The glucose char has abundant micropores, which improve dispersion of Ni ions in the char. The char was fabricated by slow pyrolysis of glucose from 25 to 500 C in N2 (50 mL min1) at a heating rate of 3 C$min1. After being held for 30 min, the carbon materials were ground to sizes between 100 and 120 mesh. A given quality of nickel acetate tetrahydrate was added to a 100-mL beaker, which contained 2.0 g glucose char and 50 mL deionized water. The beaker containing the mixture was dipped in an ultrasonic generator (25 kHz and 400 W) for 30 min. After ultrasonic impregnation, the residues obtained were dried in an oven at 110 C overnight, and subsequently ground to sizes between 100 and 120 mesh. The nickelbased chainmail catalysts were obtained by thermal treatment in argon flow (50 mL min1) before storage and further use. Different mass ratios of Ni ions to glucose char were 1:2, 1:4, 1:8, 1:12, denoted as Ni@G2, Ni@G4, Ni@G8, Ni@G12, respectively. The reference Ni/Al2O3 catalyst was prepared by the impregnation method. The g-Al2O3 support was impregnated with an aqueous solution of nickel acetate tetrahydrate. The resulting mixture was ultrasonic impregnated for 20 min followed by drying at 110 C overnight. Then the obtained material was calcined in air at 900 C for 4 h. Finally, the prepared catalyst was washed at 200 C for 30 min in N2 flow and then reduced at 650 C in H2 flow.
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between 10 and 80 . The number of graphene layer was calculated by N¼(L002 þ d002)/d002. The size of Ni particles was calculated by the Scherrer equation, D ¼ Kl/(b2qcosqmax), where K was a dimensionless shape factor fixed at 0.89, l was the X-ray wavelength, b2q was the width of the peak at halfheight, and qmax was the Bragg angle at the peak maximum position. The microstructure of the catalysts was determined by means of transmission electron microscopy (TEM). The TEM analysis was conducted on a JEM-2100F operating at 100 kV. The sample powders were dispersed as a suspension in ethanol solutions and sonicated for 3 min. After being well dispersed, a droplet of suspension was deposited on copper grids and followed by drying in air. Fourier transform infrared (FTIR) spectroscopy for the identification of the bonds and quantification of their relative abundance were performed using a Shimadzu IRAffinity-1 FTIR spectrophotometer. The samples analyzed were a pressed pellet consisting of the catalyst (1 mg) in a KBr support (200 mg). Thermogravimetric analysis (TGA) was performed on a NETZSCH STA409 C/PC equipment. The sample was heated in air from room temperature to 1000 C at a heating rate of 10 C$min1. Nickel loading in the catalysts was also measured by TGA, burning the sample at 850 C in pure air (50 mL min1) for 30 min and weighing the residual product as NiO. Raman spectra was carried out to assess graphene. Raman spectra of the fresh and spent catalysts were recorded on an iHR550 Raman microscope from HORIBA scientific with 532 nm solid laser as an excitation source.
Catalytic tests Fig. 1 e Schematic growth of carbon rings (the carbon rings formed after chain scission, polycondensation, and nucleation are schematically represented by aromatic rings).
Catalytic characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2500 instrument (40 kV, 100 mA) using Cu/Ka X-ray source at a scanning rate of 3 $min1 over the range
The catalytic activity of the catalysts was tested isothermally in ESR reaction using a fixed-bed quartz reactor (20 mm inner diameter and 600 mm length) at atmospheric pressure. Fig. 3 presents a schematic diagram of the experimental apparatus, composed of feeding system, ESR reaction system, condensation system, drying system and gas product analysis system. Argon was used as the carrier gas at a flow rate of 200 mL min1. The aqueous solution of ethanol was injected into a vaporizer (150 C) at 20 mL min1 by a syringe pump. The
Fig. 2 e The general procedure of the catalysts preparation.
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Fig. 3 e Schematic diagram of the experimental apparatus for ESR. liquid hourly space velocity (LHSV) and catalyst mass were fixed at 2.4 mL g1 h1 and 0.5 g, respectively. During the experiment, the reactor was heated electrically, and a K-type thermocouple monitored the actual temperature of the catalyst bed. The gas cleaning system includes a series of condensers and a calcium chloride drying tube. Non-condensable products (mainly H2, CO, CO2 and CH4) were periodically analyzed by a gas chromatograph (GC-9890A) with a thermal conductivity detector (TCD) equipped with Porapak Q and TDX-01 packed columns, although some trace condensable products (e.g., CH3CHO, C2H5OC2H5, CH3COCH3) were not detected because of the limitation of the instrument in this experiment. The selectivity to the gas products denoted as SCO , SCO2 , SCH4 and SH2 was calculated as: Fi Si ¼ P *100 Fi
(10)
where Fi is the molar flow of the gas products containing CO, CO2, CH4 and H2. The H2 yield was calculated as: YH2 ¼
FH2 Fethanol
(11)
where Fethanol is the ethanol molar flow in the feedstock. The carbon conversion was calculated as: X¼
FCO2 þ FCO þ FCH4 100 Fethanol
sample exhibited a rapid weight loss at about 440 C, corresponding to the formation of NiO or Ni [42]. With increasing temperature, a broad peak centered at 600 C appeared, ascribed to the reduction of residual NiO by carbon. Therefore, the Ni/GC was held at 330 C, 440 C, 600 C for 10 min, respectively, during the thermal treatment. The complete release of volatile components improved the dispersion of nickel nanoparticles. Graphene growth was promoted on Ni after being held at 950 C for 90 min. Low temperatures tend to give amorphous or poorly crystalline carbon [38,43]. However, longer retention time, in the carbonization stage, could cause the excessive thickness of graphene shell. Thicker graphene shell would limit electron penetration, which affects the catalytic activity. The XRD results of all the catalysts are presented in Fig. 5. There is only slight difference in 2q values among Ni@G2, Ni@G4, Ni@G8 and Ni@G12 chainmail catalysts. All samples exhibited three peaks at 44.5 , 51.8 , and 76.4 , corresponding to (111), (200) and (220) of metal phase (JCPDS #04-0850), respectively. The diffraction peaks of were sharp and intense, indicating the highly crystalline nature of Ni. No peaks of NiO were observed, which confirmed that NiO was completely reduced to Ni in the calcination stage. With decreasing Ni content, the peaks of Ni were severely broadened and reduced in intensity, illustrating that crystallite size of Ni core was reduced [44]. According the Scherrer formula, the Ni core size
(12)
Result and discussion Catalysts characterization The results in Fig. 4 show TGA of the nickel-impregnated glucose char (Ni/GC) under nitrogen atmosphere. Weight loss of Ni/GC was observed as the temperature increased. Specifically, there was a broad and weak peak at 130 C registered in the DTG curve, assigned with the dehydration of crystallized water in nickel acetate tetrahydrate and acetic acid generation concurrently. A sharp peak was clearly observed at 330 C, which was attributed to the further decomposition of dehydrated intermediate. Subsequently, the
Fig. 4 e TGA of Ni/GC in N2 at a heating rate of 5 C·min¡1.
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Fig. 5 e XRD patterns of Ni@G2, Ni@G4, Ni@G8 and Ni@G12 from the peak at 2q ¼ 44.5 for Ni@G2 is 56 nm, for Ni@G4 is 38 nm, for Ni@G8 is 30 nm and for Ni@G12 is 26 nm. In addition, the broad and weak (002) peaks at 2q angle of 26.4 were seen for all samples, indicating the distance between stacked graphene layers [45].average stacking number of a graphene layer was estimated via the d-spacing and size of the crystallite [46]. The detailed X-ray crystalline parameters are given in Table 1. Graphene interlayer spacing of all samples were larger than that of graphite (3.35 A) [47]. And there were no obvious changes in the number of graphene layers among catalysts, which was attributed to the consistency of retention time during catalyst preparation. The existence of graphene was further confirmed by FTIR spectra. In Fig. 6, both the peak at 1397 cm1 and broad peak centered at 3500 cm1 are related to vibrations of adsorbed moisture. The characteristic at 1627 cm1 could be ascribed to C¼C bending vibrations [48]. In addition, Raman analysis (Fig. 7) was carried out to identify the carbon structure of fresh catalysts. The three characteristic peaks at around 1345, 1580, and 2690 cm1 were in good agreement with the typical Raman modes of the D, G, and 2D bands of graphene, respectively, confirming the existence of graphene in samples. The D peak originated from the presence of disordered amorphous carbon while the G peak referred to the tangential vibrations of the graphitic carbon sp2 bonds [49]. The larger ID/ IG apparent in the Raman spectra indicated that more amorphous carbon was present in Ni@G12. Transmission electron microscopy (TEM) micrograph in Fig. 8a shows thatNi@G4 exhibits a coreeshell structure in the range of 35e43 nm. The high-resolution TEM (HRTEM) image
Fig. 6 e FTIR spectras of Ni@G2, Ni@G4, Ni@G8 and Ni@G12. shown in Fig. 8b reveals the graphene shell with an interlayer of about 3.452 A, corresponding to the graphene (002) plane. As depicted in Fig. 8c, the thickness of graphene shell is about 8 nm. These multilayered graphene could protect Ni core from oxidation and sintering. In the core, lattice distances of 1.792 A and 2.065 A in Fig. 8d are indexed to the (200) and (111) crystal planes of Ni. According to the above results of XRD, FTIR, Raman and TEM analyses, the chainmail catalysts are heterogeneous with metallic nickel for and graphene for shell. The graphene shell can protect the inner nickel well enough from oxidation in air at high temperatures. The TGA data of all catalysts in Fig. 9 show an interesting trend of weight change during the thermal treatment procedure in air. Different from common metal nanoparticles, the encapsulated particles started to gain weight after a slight weight loss below 400 C. This indicated that the metallic Ni was completely coated by graphene because of no oxidation of Ni. A dramatic weight loss was noticed in the range of 600e750 C, corresponding to the combustion of the carbon skeleton of graphene [50]. Above 750 C, the rate of weight loss slowed
Table 1 e X-ray structural parameters of catalysts from (002) band. Sample Ni@G2 Ni@G4 Ni@G8 Ni@G12
2q, Plane (002)
Value of d ( A)
Value of L002 ( A)
No. of layers
26.2 26.4 26.3 26.2
3.432 3.445 3.426 3.438
87 76 81 74
26 23 24 22
Fig. 7 e Raman spectras of fresh Ni@G4 and Ni@G12.
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Fig. 8 e HRTEM images of the Ni@G4 chainmail catalyst at different magnification. down due to the oxidation of the exposed Ni core while the carbon skeleton of graphene was burnt. Ni@G8 and Ni@G12 began to lose weight obviously previous to Ni@G2 and Ni@G4, attributed to more amorphous carbon existing on the surface
of catalysts with lower nickel content, which coincided with Raman analysis of fresh Ni@G4 and Ni@G12.
Catalytic activity Effect of steam to ethanol molar ratio
Fig. 9 e Thermal stability curves of Ni@G2, Ni@G4, Ni@G8 and Ni@G12 catalysts. The content of Ni in all samples estimated from the thermal analysis were 38.52 wt%, 22.80 wt%, 13.29 wt% and 8.61 wt% respectively.
S/E molar ratio plays an important role in ESR, influencing the yield, selectivity and deactivation of catalysts. Overstoichiometric mixtures are commonly used in ESR reaction to facilitate the WGS reaction via shifting the equilibrium conversion. In addition, it intensifies coke gasification by steam and consequently, renews catalysts to obtain activity and stability [51]. Recently, diluted ethanol, characterized by lower purification cost, proved effective in improving efficiency of steam reforming [52]. However, higher S/E molar ratio associated with steam reforming requires more energy consumption to vaporize the feedings. In this work, the S/E molar ratio began with the stoichiometric value. All the experiments were conducted in argon (50 mL min1) at 550 C and atmospheric pressure. Typical experimental results are presented in Fig. 10 in terms of H2 yield, selectivity to products, and weight changes of catalysts. Data reports for each catalyst were collected under steady-state conditions. Fig. 10a shows the H2 yields of Ni@G4 catalyst at various S/E molar ratios (varying from 3 to 7). Fig. 10b shows products selectivity for the first 4 h of operation.
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5, because high concentration of ethanol had beneficial influence on the adsorption of ethanol on the external graphene shell. In the catalytic reaction, ethanol was preferentially adsorbed on the surface of the Ni@G4 catalyst, which can be verified by another reference experiment as follows. The Ni@G4 catalyst was tested under similar conditions but feeding deionized water (13 mL min1). In Fig. 11, we plot the H2 yield at 550 C as a function of the time on stream. The Ni@G4 catalyst suffered a strong gasification. A weight loss of 45.32% was estimated at the end of the test, while weight loss of the same catalyst with the corresponding S/E ratio of 6 was 6.28%. The results indicated that ethanol was preferentially adsorbed on the external surface of the chainmail catalyst. As to S/E molar ratio of 7, it showed higher H2 yield than that with S/E ratio of 6. Combined with weight loss after 4 h of reaction in Fig. 11c and products selectivity in Fig. 11b, the selectivities to CO and CH4 accounted more, and the spent catalyst weight lost more. As listed in Table 2, the carbon conversion at S/E molar ratio of 7 was 128.9%. Higher S/E molar ratio induced excessive carbon gasification, and increased H2 yield to some extent. Therefore, the S/E molar ratio for ESR was fixed at 6 in the following tests.
Effect of nickel content The catalytic performance of all samples were tested for ESR at 550 C and S/E molar ratio of 6, aiming to study the effect of Ni content on each catalyst. As shown in Fig. 12a, the H2 yield was high up to 3.4 mol H2/mol ethanol for the first 150 min with the Ni@G2 catalyst, and then decreased quickly. This was because multilayered graphene could not protect the large Ni core (56 nm) sufficiently enough with time on stream. Jens et al. [54] reported that smaller Ni particles conduced to an increase in catalytic activity of steam reforming. Ni@G4 catalyst showed the best activity and stability in the ESR reaction, with the H2 yield ranging from 3.33 to 3.47 mol H2/mol ethanol. As Fig. 12b shows, the selectivity of Ni@G4 to H2 (65.34%) and CO2 (24.56%) were significantly higher, and the selectivity to CH4 was relatively low (0.66%). It indicated that the methanation reaction was not dominant in ESR on Ni@G4. For the Ni@G8 and Ni@G12 catalysts, the initial H2 yields were
Fig. 10 e Variation of H2 yield (a), products selectivity (b) and weight changes of spent catalysts (c) as a function of S/E molar ratio on Ni@G4 catalyst at 550 C.
In general, increasing S/E molar ratio from 4 to 6 increased H2 yield and reduced CO selectivity, due to the promotion of the WGS reaction according to the thermodynamic equilibrium [53]. Most surprisingly, higher H2 yield was obtained at the S/E molar ratio of 3, compared to S/E molar ratios of 4 and
Fig. 11 e Comparison between S/E ¼ 6 and pure water on Ni@G4 catalyst at 550 C.
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Table 2 e Carbon conversion (%) at various S/E molar ratios. 3:1
4:1
5:1
6:1
7:1
86.7
70.5
80.9
97.2
128.9
at low level, and then increased continuously. The existence of amorphous carbon (based on previous Raman analysis of fresh catalyst in Fig. 7) inhibited electron transmission from Ni core to the catalyst surface reducing the catalytic activity. With the gasification of amorphous carbon, graphene was exposed to facilitate catalysis, coinciding with the more weight loss of spent catalyst (see Fig. 12c). Table 3 lists the carbon conversion of chainmail catalysts with different nickel content. Carbon balance occurred on Ni@G4 catalyst, indicating that the amount of condensed products could be very small. The carbon imbalance occurred on Ni@G8 and Ni@G12 catalysts, attributed to the gasification of more amorphous carbon on the surface of chainmail catalysts. In general, Ni@G4 was more suitable for ESR.
Effect of reaction temperature The effect of reaction temperature on ESR was investigated from 400 to 600 C on Ni@G4 catalyst at S/E molar ratio of 6. As can be seen from Fig. 13, the reaction temperature significantly affected the catalytic activity. Ni@G4 showed the lowest catalytic activity at 400 C, and the H2 yield gradually decreased. Combined with the weight gain of spent catalyst in Fig. 13c, carbon deposition could be the primary reason for catalyst deactivation. Compared to fresh Ni@G4, the ID/IG value of the Ni@G4 after reaction at 400 C in the Raman spectroscopy (Fig. 14) increased to 0.99, indicating that there were a variety of amorphous carbon in the spent catalyst caused by carbon deposition. Moreover, Ni could not efficiently split CeC bond at low temperatures [55]. With the temperature up to 550 C, the H2 yield achieved 3.46 mol H2/mol ethanol and remained stable in the test. As the temperature further increased to 600 C, the initial H2 yield was over 4.77 mol H2/mol ethanol, which was quite higher than that at 550 C. It was attributed to extensive steam gasification of the carbon on the surface of the catalyst, coinciding with the more weight loss of spent catalyst (see Fig. 13c). Nevertheless, the H2 yield dramatically decreased after 75 min and fluctuated later. As shown in Fig. 13b, from 400 to 550 C, a certain quantity of CH4 and CO could be presented in the gas mixture and their variation followed an identical pattern, the selectivity to which decreased with increasing temperature. On the contrary, CO2 showed the opposite tendency for selectivity. At 400 C, the H2 selectivity was about 49.75%, along with a certain amount of CO (18.04%), CO2(15.83%) and CH4 (16.38%) as products. Combined with the weight gain of spent catalyst, the phenomena were probably related to MSR [Eqs. (3) and (4)], and Boudouard reaction [Eq. (6)]. It also implied that ethanol decomposition [Eq. (1)] was the first step in the ESR reaction. At higher steam-reforming temperatures, the CH4 selectivity decreased while the H2 selectivity increased concurrently,
Table 3 e Carbon conversion (%) of chainmail catalysts with different nickel content. Fig. 12 e Variation of H2 yield (a), products selectivity (b) and weight changes of spent catalysts (c) as a function of nickel content on different catalysts.
Ni@G2 95.5
Ni@G4
Ni@G8
Ni@G12
97.2
102.9
108.2
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Fig. 14 e Raman spectras of fresh Ni@G4 and spent Ni@G4. Spent catalyst was collected after reaction at 400 C (S/E molar ratio: 6).
surface gasified by steam and the thermodynamics of the WGS, a moderately exothermic reaction. It was coinciding with the results of Gerd et al. [53]. The carbon conversion listed in Table 4 also indicated that the more favorable reaction temperature for ESR on Ni@G4 catalyst is 550 C.
Special function of chainmail catalyst for ESR
Fig. 13 e Variation of H2 yield (a), products selectivity (b) and weight changes of spent catalyst (c) as a function of reaction temperature on Ni@G4 catalyst at S/E ratio of 6.
showing that the MSR favored high temperature. Reasonable H2 selectivity (64.9%) and negligible CH4 selectivity (0.62%) were achieved at 550 C. Additionally, the H2 selectivity strongly decreased and the CO selectivity strongly increased at 600 C. This was because of the carbon on the catalyst
Several types of catalysts such as calcined rocks, iron ores, alkali metals, transition metals, and noble metals have been employed for ESR. In consideration of catalytic activity and economic reasons, the nickel catalysts have been widely used in ESR. Nickel catalysts are usually supported by metal oxides, zeolites or minerals [56e58]. These supports are rather expensive, and the carbon deposition and metal sintering are relatively obvious. As an alternative, char has been employed as a low-cost catalyst carrier in ESR due to its inert chemical property and high hydrothermal stability [59]. Hence, in this work, glucose was chosen as the carbon precursor due to the absence of metal elements. The unique chainmail catalyst was fabricated via dissolved carbon segregating from nickel. Monolayered graphene is a zero-overlap semimetal with extremely low density of states (DOS) close to the Fermi level, which is actually very inert in catalysis [32,60]. In this sense, it is necessary to increase the DOS near the Fermi level to improve graphene catalytic performance. Metal-anchored graphene exhibits highly catalytic activity due to the interaction of the metal species with graphene [61]. The nickel core donates electrons to the p-band of
Table 4 e Carbon conversion (%) on Ni@G4 at different temperatures. 400 C 45.4
450 C
500 C
550 C
600 C
51.7
60.2
98.9
144.3
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graphene to tune the electronic state of graphene surface. Kozlov et al. [34] proved that graphene also back-donated electrons from its s-band. Therefore, a chemical bond was formed via a donation/back-donation mechanism. The graphene shell of the chainmail catalyst can completely prevent reaction molecules and medium contacting the Ni core, protecting the Ni core from sintering, oxidation, or corrosion. To demonstrate the role of graphene shell, Ni/Al2O3 was used as the reference catalyst under the same condition. As shown in Fig. 15a, the reference Ni/Al2O3 catalyst significantly deactivated in terms of H2 yield with time on stream. XRD profiles of fresh and spent Ni/Al2O3 (see Fig. 15b) show that NiO and graphitic carbon were generated after reaction. Structural analysis also revealed that Ni0 sites were much more active for CeC bond cleavage and for the WGS reaction than Ni2þ species [62,63]. According the Scherrer formula, the Ni particle size from 2q ¼ 44.5 for fresh Ni/ Al2O3 is 29 nm, for spent Ni/Al2O3 is 48 nm. Carbon deposition, oxidation of Ni0 and metal sintering were the primary reasons for catalyst deactivation. Whereas the XRD profiles of Ni@G4 chainmail catalyst in Fig. 15c revealed no NiO peak after reaction, even after reaction at 600 C. And the Ni particle sizes were in the range of 33e38 nm without apparent metal sintering. TEM micrograph of Ni@G4 after reaction at 550 C (see Fig. 15d) shows that there were no obvious changes in nanoparticle's size and morphology, and the graphene shell still
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survived after ESR. The core-shell structure of the chainmail catalyst not only prevented the sintering and oxidation of Ni, but also improved the stability of chainmail catalysts. In addition, the spent catalyst could be separated easily by an external magnetic field due to the ferromagnetism of nickel core. Ethanol undergoes a dissociative adsorption on the catalyst surface [64]. The strong electron donating ability of the hydroxyl group makes ethanol a p-electron rich system [65]. Thus, it induces an interaction between the external graphene shell and hydroxyl group via the formation of pep conjugated system, which is the primary driving force in adsorption of graphene shell. Furthermore, electrons donated by the hydroxyl group also increase the DOS of graphene at the Fermi level, thus improve catalytic performance. In conclusion, ethanol is preferentially adsorbed on the external surface of the Ni@G4 catalyst, as the results of the reference test depicted in Section Effect of stream to ethanol molar ratio. Thermodynamic equilibrium calculation for ESR reveals that H2, CO, CO2, and CH4 are the main products [53]. At higher temperatures, the formations of H2 and CO are thermodynamically favored [51]. Therefore, in this work, taking into account the products selectivity, the first step of ESR on chainmail catalyst could be regarded as ethanol decomposition, and then the reactive CO and CH4 were reformed by steam, as illustrated in Fig. 16.
Fig. 15 e (a) H2 yields of ESR on Ni/Al2O3 and Ni@G4 chainmail catalyst (Temperature: 550 C, S/E molar ratio: 6); (b) XRD patterns of fresh and spent Ni/Al2O3 catalysts; (c) XRD patterns of Ni@G4 catalysts after reaction at different temperatures; (d) TEM images of the Ni@G4 chainmail catalyst after reaction at 550 C (S/E molar ratio: 6). The inset in (d) shows the HRTEM image of graphene shell.
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Fig. 16 e Schematic diagrams of the ESR reaction on chainmail catalyst.
Conclusion [3]
A novel nickel-based chainmail catalyst was successfully fabricated for H2 production from ESR. Compared to Ni@G2, Ni@G8 and Ni@G12, the Ni@G4 as a promising catalyst displayed superb catalytic activity with a high H2 selectivity (65.34%) as well as a high stability at 550 C. At the S/E molar ratio of 6, the H2 yield was high up to 3.47 mol H2/mol ethanol, and the selectivity to CH4 was rather low (0.66%). According to the product distribution, a relatively reasonable ESR reaction pathway on the catalyst was proposed. Ethanol was firstly decomposed to H2, CO and CH4. Then the CO and CH4 were converted via WGS and MSR, respectively. From the results of XRD, Raman and TEM of fresh and spent catalysts, the coreshell structure of the chainmail catalyst not only prevented the sintering and oxidation of Ni, but also effectively improved the H2 production and reduced the undesired products. In ESR, ethanol was preferentially adsorbed on the surface of the Ni@G4 catalyst. Furthermore, the spent catalyst could be separated easily by an external magnetic field owing to the ferromagnetism of nickel core.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Acknowledgments This work is financially supported by National Natural Science Foundation of China with Grant No. 21676148 and Qing Lan Project.
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