Article pubs.acs.org/IECR
Metal-Free and Pt-Decorated Graphene-Based Catalysts for Hydrogen Production in a Sulfur−Iodine Thermochemical Cycle Zheng Bo,*,† Xiuyan Zhang,† Kehan Yu,‡ Zhihua Wang,† Xiangdong Lin,† Jianhua Yan,† and Kefa Cen† †
State Key Laboratory of Clean Energy Utilization, Department of Energy Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China ‡ Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States ABSTRACT: Graphene oxide (GO), GO reduced by hydrazine hydrate (rGO-HH), GO reduced by ethylene glycol (rGO-EG), Pt-decorated rGO-HH composite (Pt/rGO-HH), and Pt-decorated rGO-EG composite (Pt/rGO-EG), are fabricated for the heterogeneous catalytic decomposition of HI in a sulfur−iodine thermochemical cycle. Corresponding material characterization on various catalysts are performed to gain insight into the catalytic mechanism. rGO-HH presents better catalytic activity than the GO and rGO-EG counterparts, due to the increase of active sites (unsaturated carbon atoms) with the reduction of oxygencontaining groups and the formation of edge planes. Homogeneously dispersed fine Pt nanoparticles are obtained with employing rGO-EG as the support. The corrugation morphology and graphene edges benefit the nucleation, dispersion, and immobilization of Pt nanoparticles. As a consequence, Pt/rGO-EG presents better catalytic activity and stability than the convenient Pt/activated-carbon (Pt/AC) counterpart. Results indicate that the graphene-based catalysts hold a great promise for the catalytic decomposition of HI.
1. INTRODUCTION The sulfur−iodine (SI) thermochemical cycle has been recognized as a cost-effective and environmentally attractive route for the large-scale production of hydrogen.1,2 In a typical SI cycle, solutions of HI and H2SO4 from the Bunsen reaction (I2 + SO2 + 2H2O = 2HI + H2SO4) are first separated by a liquid−liquid phase separation,3 followed by an HI decomposition reaction (2HI = I2 + H2) leading to the production of hydrogen. In the absence of catalyst, an elevated temperature (commonly >500 °C) is required to realize the endothermic HI decomposition process. Consequently, continuous endeavors have been directed toward the development and optimization of advanced catalysts for the high-efficiency heterogeneous catalytic decomposition of HI.4−19 The great interest of employing carbonaceous materials for catalytic decomposition of HI stems from their excellent corrosion resistance, high temperature stability, controlled pore structure, and usually high specific surface area. To date, many carbon-based catalysts have been proposed for HI decomposition, including various allotropes of carbon such as carbon molecular sieve (CMS),13 graphite (GR),13 carbon nanotubes (CNT),13 and activated carbon (AC),7,8,13−15 as well as their metal-decorated derivatives such as Pt/CNT, 12−14 Pt/ AC,4,10,12−14 Pt/CMS,12,13 Pt/GR,12,13 Ni−Pd/AC,16 Pt−Ir/ AC,17 Pd−Pt/AC,18 and Pt−Ni/AC.19 In the absence of metal, the catalytic activity of carbon materials is not only affected by the specific surface area but also the amount of active sites. For example, although the specific surface areas were ranked as AC > CMS > CNT > GR, the catalytic activities were found in the order of CMS > AC > GR > CNT.13 It was further supported by our previous work on the catalytic performance of AC catalysts with various specific surface areas and different levels of lattice defects.15 As for the metal-decorated counterparts, the unique properties of carbon nanostructures, e.g., high specific © 2014 American Chemical Society
area and well-defined structure, make them competitive with conventional catalyst supports.20 Typically, Pt/CNT composite was demonstrated to be capable of providing higher catalytic activity and stability than that of Pt/AC.14 Graphene, a single-atom-thick planar sheet of hexagonally arrayed sp2 boned carbon atoms,21 is an ultimate twodimensional (2D) nanostructure exhibiting a series of unique, outstanding physical, chemical, and mechanical properties.21,22 The feasibility and great potential of metal-decorated graphene catalysts have been readily demonstrated in a wide range of fields, such as C−C cross-coupling, electrochemical, and photocatalytic reactions.23−29 Compared with traditional metal/activated carbon and metal/carbon black counterparts, the metal/graphene catalysts can provide a higher degree of the dispersion of metal nanoparticles for enhanced catalytic activity, as well as a better immobilization/intercalation of metal nanoparticles for improved catalytic stability and lower metal leaching.23,30−32 On the other hand, metal-free graphene-based catalysts also received a great interest for both scientific investigation and practical applications. Some chemically converted derivatives of graphene, typically graphene oxide (GO) and reduced graphene oxide (rGO), are quite promising for particular applications, such as reduction of nitrobenzene for hydrogenation,33 selective oxidative dehydrogenation of hydrocarbons,34 and generation of aldehydes or ketones from alcohols, alkenes, and alkynes.35 In these cases, the presence of functional groups (typically within GO) and unsaturated carbon atoms (typically within rGO) can provide active sites for catalysis. However, to the best of our knowledge, there was Received: Revised: Accepted: Published: 11920
May 5, 2014 July 7, 2014 July 8, 2014 July 8, 2014 dx.doi.org/10.1021/ie5018332 | Ind. Eng. Chem. Res. 2014, 53, 11920−11928
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Figure 1. TEM images of (a) GO, (b) rGO-EG, and (c) rGO-HH. Insets: digital photographs of GO, rGO-EG, and rGO-HH suspension. Scale bars: (a) 200 nm; (b) 200 nm; and (c) 100 nm.
a 1000 mL round-bottomed flask for a 24 h reflux at 120 °C under magnetic stirring. Finally, the as-obtained rGO-EG was filtered, washed, and dried in vacuum. 2.2. Preparation of Metal-Decorated Catalysts. Pt/AC, Pt/rGO-HH, and Pt/rGO-EG were used as the metaldecorated catalysts for HI decomposition tests. As for Pt/AC and Pt/rGO-EG, 13.98 mL solution of H2PtCl6 (concentration: 0.019 M), 97.13 mL of deionized water, and 1 g of AC (woodbased, Xilong Chemical Company) or GO were added into 450 mL of EG. The GO mixture was ultrasonically treated for 3 h − a 24 h reflux at 120 °C under magnetic stirring. The as-obtained Pt/AC or Pt/rGO-EG composite was filtered and washed with ethanol and deionized water with a subsequent drying in vacuum. The preparation of Pt/rGO-HH was conducted with the following steps. First, 1 g of the as-prepared GO powder and 13.98 mL solution of H2PtCl6 (concentration: 0.019 M) were dispersed in 1 L of deionized water, followed by a 2 h ultrasonication (FB15150, 300 W, Fisher Scientific). The resulting dispersion was mixed with 840 μL of HH (85 wt % in water, Sinopharm Chemical Reagent Co. Ltd.). The mixture was then kept in a 95 °C oil bath and stirred for 2 h. The asobtained Pt/rGO-HH was filtered and washed with ethanol and deionized water and finally dried in vacuum. According to the energy dispersive X-ray (EDX) analysis, the weight percentages of Pt nanoparticles in Pt/rGO-EG, Pt/rGO-HH, and Pt/AC were measured as 4.61%, 4.07%, and 4.92%, respectively. 2.3. Material Characterization. The material morphology and crystal structure were investigated by a Tchnai G2 F30 STwin transmission electron microscopy (TEM, Philips-FEI). To perform TEM and high-resolution TEM (HRTEM) characterizations, samples was wetted with ethanol and contact-transferred to a 230 mesh holey carbon grid. The energy dispersive X-ray (EDX) analysis was also performed. A DXR 532 Raman spectrometer (Thermo Fisher Scientific) was used to obtain the Raman spectra of the samples with an excitation wavelength of 532 nm. X-ray diffraction (XRD) patterns were collected with a XRD-6000 Diffractometer using Cu Ka radiation (λ = 0.15425 nm, Shimadzu). X-ray photoelectron spectroscopy (XPS) investigation was carried out in a VG Escalab Mark II system employing a monochromatic Mg Ka X-ray source (hm =1253.6 eV, West Sussex). Measurements of the Brunauer−Emmett−Teller (BET) surface area, pore volume, and average pore diameter
few literature on the performance of graphene-based catalysts for HI decomposition. Inspired by the above facts, we herein attempt to extend the utilization of graphene-based catalysts into the field of thermochemistry reactions. This paper reports the catalytic performances of a series of metal-free and metal-decorated graphene-based catalysts, including (i) GO, (ii) GO reduced by hydrazine hydrate (rGO-HH), (iii) GO reduced by ethylene glycol (rGO-EG), (iv) Pt-decorated rGO-HH composite (Pt/ rGO-HH), and (v) Pt-decorated rGO-EG composite (Pt/rGOEG), for HI decomposition reaction. Corresponding material characterization of various catalysts were performed, attempting to gain insight into the relationship between material morphology/structure and catalytic behaviors. Finally, the catalytic activity and stability of Pt/rGO-EG were compared with those of Pt/AC, a conventional catalyst with high activity for HI decomposition.4,10,12−14
2. EXPERIMENTAL SECTION 2.1. Preparation of Metal-Free Catalysts. GO was synthesized from graphite powder (XF010, XF NANO) using a modified Hummer’s method. 1 g of graphite powder was mixed with 25 mL of concentrated H2SO4 in a flask at room temperature. The flask was cooled to 0 °C in an ice bath with the slow addition of 3.5 g of KMnO4. The mixture was stirred for 2 h in a 35 °C water bath, followed by an ice bath with adding 100 mL of deionized water into the mixture. H2O2 (30 wt %, 8 mL) was added until the gas evolution ceased. The solid obtained from centrifugation (8000 r.m.p., 10 min) was washed with 10 vol % HCl and deionized water, with repeating the washing process for four times. GO powder was finally obtained with a drying process under vacuum at 35 °C. The preparation of rGO-HH was conducted with the following steps. One g of the as-prepared GO powder was dispersed in 1 L of deionized water, followed by a 2 h ultrasonication (FB15150, 300 W, Fisher Scientific). The resulting dispersion was mixed with a 840 μL of hydrazine hydrate (85 wt % in water, Sinopharm Chemical Reagent Co. Ltd.). The mixture was then kept in a 95 °C oil bath and stirred for 2 h. The as-obtained rGO-HH was filtered and washed with ethanol and deionized water and finally dried in vacuum. To obtain rGO-EG, 97.13 mL of deionized water and 1 g of GO were added into 450 mL of ethylene glycol. The mixture was first ultrasonically treated for 3 h and then transferred into 11921
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Figure 2. (a) Raman spectra of GO, rGO-HH, and rGO-EG. Gaussian line fitted C 1s XPS spectra of (b) GO, (c) rGO-HH, and (d) rGO-EG. Insets: wide energy survey scan XPS spectra of samples.
Further measurements on the crystal structures and elemental composition of GO, rGO-EG, and rGO-HH were conducted with Raman and XPS analysis. Figure 2a compares the Raman spectra of GO, rGO-HH, and rGO-EG. The two most intense features of the Raman spectra are the disorder-induced D peak at ∼1335−1350 cm−1 and the G peak at ∼1584−1590 cm−1 corresponding to the formation of graphitized structure.37 With the reduction of GO, the structural changes occurred due to the loss and rearrangement of oxygen and carbon atoms, which was clearly reflected by the Raman spectra. One of the most significant evidence is the change of intensity ratio of the D peak to the G peak increased with the chemical reduction process (0.99, 1.07, and 1.37 for GO, rGO-EG, and rGO-HH, respectively), suggesting the increasing material lattice defects and graphene edges. XPS measurements provided further information on the elemental composition within the GO and rGO samples. Figures 2b-d present the XPS spectra of GO, rGO-EG, and rGO-HH. Each C 1s XPS spectrum was decomposed with a Gaussian fit, and five fractions were obtained, i.e., the sp2 and sp3 hybridized carbon (CC−C) at ∼284.6 eV, the hydroxyl carbon (C− OH) at ∼286.4 eV, the carbonyl carbon (CO) at ∼287.4 eV, and the carboxylate carbon (OC−O) at ∼298.9 eV, respectively. Table 1 lists the atomic ratio of carbon to oxygen (area of the C 1s peak divided by area of the O 1s peak, multiplied by the ratio of the photoionization cross sections), and the fractions of C atoms in CC, C−C, C−OH, CO, and OC−O, for GO and rGO samples. In brief, the rGOHH samples presented both the highest C/O ratio (up to 9.50) and C fraction (61.7%) in the nonoxygenated functional group (CC−C).
were conducted with a Micromeritics ASAP 2010 BET analyzer using nitrogen as the adsorbent. 2.4. Catalytic Activity Tests. The catalytic activities of different catalysts for HI decomposition reaction were evaluated via a lab-scale fixed-bed reactor. Details on the reactor can be found in our previous work.36 Catalysts (1 g) mixed with coarse quarts particles were placed into a quartz tubular reactor (inner diameter 18 mm) housed by a furnace. The temperature was well adjusted in the range of 300−550 °C. A peristaltic pump (BT100-2J) was used to pump the 55 wt % hydriodic acid into an evaporator with a rate of 0.7 mL/min. In the evaporator (160 °C to ensure the flash vaporization of HI solution), the solution was vaporized in a 60 mL/min nitrogen (carrier gas). The as-produced gases were trapped into a spiral condenser, with any residual HI, I2, and H2O being absorbed by the following two downstream scrubbers. The hydrogen concentration was analyzed by a thermal conductivity gas sensor (K522, Hitech Instruments Ltd.), and the converted HI as well as the HI conversion rate were calculated accordingly. All the experiments were conducted at atmospheric pressure.
3. RESULTS AND DISCUSSION 3.1. Material Characterization of GO, rGO-EG, and rGO-HH. Figure 1 shows the digital photographs of GO, rGOEG, and rGO-HH aqueous dispersions and the correspondingly TEM images. The yellow brown GO suspension changed its color to black (for both rGO-EG and rGO-HH suspensions), indicating the reduction of GO and the restoration of aromatic graphene structure. All the TEM images exhibited a similar wrinkled structure of transparent thin nanosheets, intrinsic to the graphene-based materials obtained from chemical routes. 11922
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and rGO-HH with an increasing reaction temperature. The thermodynamic yield A was obtained from O’Keefe’s work,4 and the thermodynamic yield B was calculated by FactSage based on complex thermochemical databases. The blank plot was obtained from the tests in the absence of catalyst. As shown in Figure 3, for all the applied catalysts, the HI conversion rates increased with an elevated operation temperature due to the kinetics of this endothermic reaction. For a certain operation temperature, the HI conversion rates of different catalysts were found in the order of rGO-HH > rGO-EG > GO. In the absence of defects or metal-loading, the graphene basal planes are commonly considered as not very reactive, where unsaturated carbon atoms are only from the edges of graphene layers.22 With the reduction of GO, more unsaturated carbon atoms will be formed due to the reduction of oxygen-containing functional groups as well as the newly formed graphene edges, which favors the catalytic performance. The XPS results, as presented in section 3.1, indicate that rGO-HH owns higher level reduction of GO than the rGO-EG counterparts with more active sites from the unsaturated carbon atoms. Furthermore, the presence of abundant oxygen-containing groups with acidic characters could reduce the electron transfer effects associated with the graphene basal planes.38 Meanwhile, the higher intensity ratio of the D peak to the G peak of rGOHH samples with respect to GO and rGO-HH suggests the increase of newly formed, smaller graphitic domains and the increased fraction of graphene edges,39 both of which could favor the catalytic performance due to the increase of active sites. As a consequence, it is reasonable that rGO-HH with the most active sites presented the best catalytic ability among the three types of metal-free graphene-based catalysts.
Table 1. Atom Ratio of Carbon to Oxygen and Fractions of Four Components Corresponding to Carbon Atoms in Different Functional Groups for GO, rGO-HH, and rGO-EG relative intensity of C species in C 1s peak samples
C/O
CC−C
C−OH
CO
OC−O
GO rGO-HH rGO-EG
2.05 9.50 3.31
49.40 61.70 54.26
20.16 5.83 15.30
18.19 15.29 17.39
12.25 17.17 13.05
3.2. Catalytic Performance of GO, rGO-EG, and rGOHH. Figure 3 shows the HI conversion rates over GO, rGO-EG,
Figure 3. Dependence of HI conversion on operation temperature over GO, rGO-EG, and rGO-HH.
Figure 4. (a) HI conversion over rGO-HH during the continuous HI decomposition experiment (90 min, 486 °C). (b) Nitrogen physisorption isotherms at 77.4 K of rGO-HH before and after 90 min HI decomposition. (c-d) XPS spectrum of rGO-HH after 90 min HI decomposition. 11923
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Figure 5. HRTEM images of (a) Pt/rGO-HH and (b) Pt/rGO-EG. (c) TEM image of Pt/rGO-EG. (d) XRD patterns of rGO-EG and Pt/rGO-EG. Scale bars: (a) 20 nm; (b) 5 nm; and (c) 200 nm.
volume (from 5.62 to 9.00 nm and from 0.84 to 1.12 cm3/g, respectively). Moreover, according to the XPS spectrum shown in Figures 4c and d, the C/O ratio of rGO-HH after 90 min HI decomposition was calculated as high as 10.2, confirming the further reduction of rGO-HH at a temperature of 486 °C. Taking the advantages of enlarged pore size and increased pore volume as well as the newly formed unsaturated carbon atoms, the rGO-HH catalyst presented a slightly increased HI conversion during the 10 min operation and finally a stable catalytic performance for HI decomposition. 3.3. Material Characterization of Pt/rGO-EG and Pt/ rGO-HH. Figure 5 exhibits the TEM and HRTEM images of Pt/rGO-EG and Pt/rGO-HH. The Pt (111) facets with a dspacing of ∼0.21−0.23 nm indicates the existance of Pt metallic phase on both rGO-EG and rGO-HH supports. However, obvious differences in terms of Pt size and Pt dispersion on rGO-EG and rGO-HH were identified. As shown in Figure 5a, strong aggregation of Pt nanoparticles was observed when rGO-HH was used as the support, resulting in a metal particle size of ∼10−30 nm. In contrast, as shown in Figures 5b and c, the dimension of Pt nanoparticles on rGO-EG was much smaller, and no obvious aggregation was found. The above results are consistent with previously reported work where Pt/ rGO-EG and Pt/rGO-HH were used as the catalysts for hydrogenation of nitroarenes,41 suggesting that EG is more
The catalytic stability of rGO-HH was further evaluated via a continuous HI decomposition process. Figure 4a shows the HI conversion over rGO-HH during a 90 min experiment with a fixed operation temperature of 486 °C. During the first 10 min, the HI conversion rate increased from 18.41% to 18.94%, which slightly decreased to 17.01% with a further increase of the operation time up to 90 min. Finally, the rGO-HH catalyst presented a stable catalytic performance for the 90 min HI decomposition with an overall HI conversion drop of 1.40%. The above observation could be related to the structural changes of rGO-HH during the continuous HI decomposition process. Figure 4b shows the nitrogen physisorption isotherms (77.4 K) of rGO-HH before and after 90 min HI decomposition. The BET specific surface area of rGO-HH decreased from 601 to 500 m2/g, possibly due to the trapping of I2 in the graphene interlayer/intersheet spacings during the HI decomposition process. However, enlarged pore volume/ pore size and increased unsaturated carbon atoms were observed, which could be attributed to the thermal reduction of rGO-HH. At a temperature of 486 °C, gaseous products such as CO2, H2O, and CO were formed through the reduction of residual oxygen-containing functional groups in rGO-HH, resulting in a rapid buildup of sufficient pressure within the graphene interlayers and the expansion of graphene layers.40 It was evidenced by the increasing average pore size and pore 11924
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suitable than HH for the formation and dispersion of fine Pt nanoparticles on rGO surface. In this case, ethylene glycol could take a dual role as the reductive agent for the reduction of GO and also the dispersing agent for the deposition of Pt nanoparticles on rGO sheets.26 Estimated from the HRTEM image shown in Figure 5b, the diameter of Pt nanoparticles on rGO-EG was ∼2−4 nm, similar to those on CNTs (2 nm) and carbon molecular sieve (5 nm) but obviously smaller than that on graphite (25 nm).12 The XRD patterns of rGO-EG and Pt/rGO-EG are presented in Figure 5d. For Pt/rGO-EG, the peaks at around 39.7°, 46.6°, 67.7°, and 81.6° were assigned to the diffraction peaks of Pt (111), Pt (200), Pt (220), and Pt (311), respectively, confirming the formation of highly crystalline face-center cubic (fcc) structure of Pt nanoparticles on rGOEG. The diffraction peak for Pt (111) was used to calculate the average Pt nanoparticle size by the Scherrer’s formula. The ascalculated mean size of Pt on rGO-EG is ∼3.51 nm, close to the value estimated from the TEM/HRTEM images (Figures 5b and c). 3.4. Catalytic Performance of Pt/rGO-EG and Pt/rGOHH. Figure 6 shows the HI conversion rates over Pt/rGO-EG
3.5. Comparison between Pt/rGO-EG and Pt/AC. Pt/ AC has been extensively demonstrated as an excellent catalyst for HI decomposition.4,10,12−14 Figure 7a compares the HI
Figure 7. (a) Comparison of HI conversion over Pt/AC and Pt/rGOEG. (b) HI conversion over Pt/AC and Pt/rGO-EG during the continuous HI decomposition experiment (90 min, 486 °C).
conversion rates over Pt/rGO-EG and Pt/AC with an increasing reaction temperature. For each certain operation temperature, Pt/rGO-EG presented a better catalytic activity than that of Pt/AC, especially for the relatively low temperature range of 329−438 °C. As for the catalytic stability, Figure 7b presents the catalytic performances of Pt/AC and Pt/rGO-EG for the continuous HI decomposition experiment with a fixed operation temperature of 486 °C. After 90 min HI decomposition, the HI conversion rate over Pt/AC decreased by 4.25%, while that over Pt/rGO-EG was almost kept as a constant (only dropped by 0.12%). This result indicates that Pt/rGO-EG owns a much better catalytic stability than that of Pt/AC. rGO supports have been extensively demonstrated as useful substrates for immobilizing metal nanoparticles due to its high conductivity, huge specific area, and excellent thermal stability (see ref 42 and the references therein). Meanwhile, the residual oxygen-containing groups can enhance the interactions between the adsorbing nanoparticles and the graphene surfaces.26,42 Compared with the previously reported data, the catalytic stability of the Pt/rGO-EG catalyst is also better than those of Pt/AC, Pt/CNT, Pt/CMS, Pt/GR, and Pt/Al2O3 catalysts.12−14 With comparing the HRTEM images of Pt/AC (Figure 8a) and Pt/rGO-EG (Figure 5b), it was clearly observed that the Pt
Figure 6. Dependence of HI conversion on operation temperature over Pt/rGO-EG and Pt/rGO-HH.
and Pt/rGO-HH with an increasing reaction temperature. For the same operation temperature, Pt/rGO-EG presented a much higher HI conversion rate than that of Pt/rGO-HH. Typically, the HI conversion rate of the Pt/rGO-EG catalyst could reach 11.6% at a temperature as low as 329 °C, which further approached the equilibrium value with an increasing temperature up to 438 °C. As is well-known, the size and dispersion of metal nanoparticles on supports play important roles on the catalytic activity of metal-decorated catalysts.23,30−32 Highly distributed catalyst particles with relatively smaller size could lead to a higher active surface, benefiting the mass specific activity.22 Furthermore, the decrease of Pt size could increase the adhesion energy, which favors the catalytic stability. The homogeneously dispersed fine Pt nanoparticles on rGO-EG, evidenced by both the TEM/HRTEM images and XRD patterns, resulted in an effective utilization of Pt nanoparticles with excellent catalytic activities. The above results suggest that different preparation methods/reagents may play a significant role in the development of physicochemical properties of Pt/ graphene that eventually affects the catalyst activity. 11925
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Figure 8. (a) HRTEM images of Pt/AC before use. (b) TEM and (c) HRTEM images of Pt/AC after 90 min HI decomposition. (d) HRTEM image of Pt/rGO-EG after 90 min HI decomposition. Scale bars: (a) 5 nm; (b) 100 nm; (c) 5 nm; and (d) 5 nm.
catalysts, the catalytic activities were on the order of rGOHH > rGO-EG > GO. The reduction of GO led to the increases of unsaturated carbon atoms (from both the reduction of oxygen-containing groups and the formation of graphene edge planes), providing more active sites for improved catalytic activity. Within the 90 min continuous HI decomposition process, thermal reduction of rGO-HH was observed. It resulted in the enlarged pore size/volume and increased unsaturated carbon atoms, benefiting the catalytic stability. For metal-decorated graphene-based catalysts, well dispersed Pt nanoparticles with a uniform size of ∼5 nm were obtained with employing rGO-EG as the support. Taking the advantages of corrugation morphology and exposed edge planes, Pt/rGO-EG exhibited higher activity and better catalytic stability than the convenient Pt/AC counterpart, showing a great promise for the effective heterogeneous catalytic decomposition of HI in a SI cycle. Further investigation on the stability of graphene-based catalysts in the long term operation of the HI reaction would be beneficial for their practical applications.
nanoparticles on rGO-EG were obviously less aggregated than those on AC. The corrugation morphology as well as the graphene edges of rGO-EG could benefit the Pt precursor nucleation and immobilization.22 For Pt/AC, the obvious HI conversion drop after the continuous HI decomposition process could be attributed to the enlarged size of Pt nanoparticles. As evidenced by the TEM/HRTEM images, the dimension of Pt nanoparticles on AC significantly increased from ∼5 to ∼10−40 nm (Figures 8a-c). On the other hand, the size of Pt nanoparticles on rGO-EG were kept as
