Coking and Regeneration of Nickel Catalyst for the Cracking of

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Coking and regeneration of Ni catalyst for the cracking of toluene as a tar model compound Peng Lu, Qunxing Huang, Yong Chi, and Jianhua Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01218 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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Coking and regeneration of Ni catalyst for the cracking of toluene as a tar model compound Peng Lu, Qunxing Huang*, Yong Chi, Jianhua Yan State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, People’s Republic of China *Corresponding author, E-mail: [email protected] KEYWORDS: Tar; Ni catalyst; coking; regeneration; filamentous coke ABSTRACT: The coking and regeneration of Ni/γ-Al2O3 for the cracking of toluene as a tar model compound were investigated. Special attention has been paid on the effect of coke nature, especially filamentous coke, on the regeneration performance. The catalyst was deactivated by cracking toluene at 700 °C for 0.5 hour. The deactivated catalyst was then regenerated by calcination in air at 600 °C for 3 hours. The cracking-regeneration cycle usage was carried out up to four times. The results showed that the toluene conversion decreased slowly from 28.2% by fresh catalyst to 22.6% by the third regenerated catalyst, then decreased quickly to 16.3% by the fourth regenerated catalyst. The increase of NiO crystal size from 12.8 to 18.0 nm and the decrease of Brunauer-Emmett-Teller (BET) surface area from 97.7 to 86.9 m2/g caused by sintering and remaining coke were the major reasons for the loss of catalytic activity of Ni catalyst after cycle usage. The cycle usage made the deposited coke more graphitized by slightly enhancing the formation of filamentous coke, but reduced the total quantity of coke. The existence of filamentous coke required a higher regeneration temperature (600 °C) due to a more graphitized nature than 1 ACS Paragon Plus Environment

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amorphous coke. The formation and the growth of filamentous coke decreased the Ni particle size from 13.0 nm at 10 min to 11.4 nm at 30 min in the fresh cycle and weakened the interaction between metal and support.

1. INTRODUCTION Gasification is an advanced thermal treatment method, which converts the waste materials (such as biomass, municipal solid waste and sewage sludge) into useful syngas. The major barrier to the utilization of syngas is the byproduct tar, which is a complex mixture of condensable hydrocarbons. Tar must be removed from the syngas because it will deposit on the surface of downstream pipes and equipment and cause serious secondary pollution. There are two types of methods for the removal of tar, namely cold gas cleaning methods and hot gas cleaning methods. Water scrubbing is the most commonly used cold gas cleaning method. However, some of the tars are non-polar and do not dissolve in water resulting in the insufficient separation of tar.[1] Besides, water scrubbing produces a large amount of waste water. Compared with cold gas cleaning methods, hot gas cleaning methods, especially catalytic conversion, can utilize the energy of tar and improve the H2 to CO ratio in the syngas without cooling and secondary waste water pollution. Ni based catalysts are widely used to eliminate tar produced from gasification due to higher activity than Fe and Cu, and relatively lower cost than other transition metals, like Ru and Pt.[2-4] However, previous research has reported the strong coke deposition tendency of Ni catalysts during tar cracking.[5] Coke deactivation occurs 2 ACS Paragon Plus Environment

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through complex mechanisms including polymerization, dehydrogenation, Boudouard or polycondensation.[6] Both amorphous and filamentous coke were found on the surface of Ni catalysts during cracking or reforming of hydrocarbons.[7-9] Barbarias et al.[10] reformed n-hexane, 1-hexene, tetradecane and toluene over a Ni commercial catalyst at 700 °C. The following order of catalyst deactivation rate was observed: toluene > 1-hexene > tetradecane > n-hexane. The deactivation rate depended on the nature of coke, with olefins (1-hexene) and aromatics (toluene) being encapsulating coke precursors (amorphous and structured, respectively) and paraffins (tetradecane and n-hexane) being filamentous and inert coke precursors. They concluded that the filamentous coke had a weaker effect on deactivation than encapsulating coke. The formation of filamentous coke lifted the Ni particles from the support, which allowed the continuous contact between the active sites and reactants. Nevertheless, coke deactivation is reversible and the deposited coke or coke precursors can be removed by calcination or gasifying to recover the catalytic efficiency partially or completely. Calcination in air is an easy, cheap and effective way. Alenazey et al.[11] studied coke removing with O2, air, CO2 and H2. They found that the reactivity coefficient of the gasifying agent decreased in the order, O2 > air > CO2 > H2. The cost of pure oxygen is relative high and the gasification rate of the coke with CO2 and H2 is relatively low. The carbonaceous deposits can also be removed by washing with organic solvents, supercritical fluids or plasma, abrasion or ultrasonication.[12] But these methods are more complicated and expensive than calcination. 3 ACS Paragon Plus Environment

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Calcination is a highly exothermal reaction that may damage the catalyst by collapse of porous structure, rearrangement of metals, alloys formation or phase segregation.[13] Thus, the calcination conditions for the removing of coke are important, especially temperature. Wang et al.[14] studied the effects of regeneration temperature (450 ~ 600 °C) on catalytic performance of supported skeletal Ni catalyst for the hydrogenation of indene and styrene. They suggested that the deactivation of catalyst was probably caused by the formation of coke precursors and aggregation of skeletal Ni. Calcination at 550 °C can obtain the best activity recover. Calcination at lower temperature cannot oxidize the deposited coke completely and a higher temperature will cause sintering problem. Takeshi et al.[15] conducted the cycle usage of Ni/MgO catalyst for the steam reforming of naphthalene/benzene as model tar compounds. The reforming was carried out at 800 °C for 3 h, followed by calcination regeneration at 600 °C for 40 min. They reported that the catalyst activity (represented by the carbon conversion to gas) was stable at around 55% from the fresh to the third cycle, then suddenly decreased to 30% in fourth cycle. The calcination temperature and catalyst properties, such as crystal size and interaction between active sites and support, were affected by the coke nature. However, previous researches concerning Ni catalyst regeneration have not taken coke nature into consideration. The purpose of this research is to study the cracking-regeneration cycle usage performance of Ni catalyst for the cracking of tar and the effect of coke properties (especially filamentous coke) on the cycle usage. Similar to our previous research,[16] toluene was chosen to represent dominating tar 4 ACS Paragon Plus Environment

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component derived from solid waste gasification. The activity of catalyst was represented by the toluene conversion and hydrogen production. Special attention has been paid on the properties of deposited coke and regenerated catalysts. The coke properties were characterized by SEM, TEM, TPO and Raman spectrum. The regenerated catalysts were characterized by XRD, XPS, H2-TPR and N2 adsorption-desorption tests.

2. MATERIALS AND METHODS 2.1 Catalyst preparation The Ni/γ-Al2O3 with a nickel loading of 20% was prepared by impregnation method. The Ni(NO3)2·6H2O was mixed in 60 ml ethanol, stirring until fully dissolved. Then, γ-Al2O3 was added into the mixture, followed by stirring at 80 °C until the ethanol was fully evaporated. The prepared sample was dried at 105 °C overnight and calcined at 800 °C in air for 3 hours. The catalyst was then crushed, sieved to particle sizes less than 0.075 mm, dried and stored for later tests.

2.2 Experimental setup Figure 1(a) shows the experimental apparatus. The catalytic cracking of toluene and regeneration of catalyst were carried out in a laboratory fixed bed reactor which was made up of a quartz tube with a length of 50 cm and an internal diameter of 3.5 cm. The reactor was supported and heated by a tubular electrical furnace and the temperature was controlled by a K-type thermocouple placed near the center of the 5 ACS Paragon Plus Environment

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bed. The variability of the temperature controlling system was ±1 °C. In order to achieve a quick and uniform coke deposition, a relatively high initial toluene concentration (15 g/Nm3) was adopted and 0.4 g fresh or regenerated catalyst was placed uniformly in a quartz boat (150×30×8 mm) with a thickness of 1 ~ 2 mm placed into the center of a quartz tube, which leaded to a low toluene conversion. N2 was used as the carrier gas in the cracking test and the residence time was 1.5 s, which was controlled by a mass flow controller. Before each cracking test, the catalyst was stabilized at 700 °C for 5 min. Then toluene was injected through a syringe pump and preheated to 250 °C before entering the reactor. Gas product was sampled by gas bags every 5 min and analyzed through a gas chromatograph (Agilent Micro GC 490). The unreacted toluene was collected by n-Hexane and determined by GC-MS (Thermo Scientific ISQ). Each cracking test lasted for 0.5 h. The toluene conversion ratio and yield ratio of H2 were calculated by Eq. (1) and Eq. (2), respectively.

Toluene conversion (η ) =

Moles of toluenein − Moles of tolueneout ×100% (1) Moles of toluenein

( )

Yield ratio of H 2 YH 2 =

Moles of H 2 produced × 100% (2) 4 × Moles of toluenein

After cracking test, the used catalyst was cooled down to 600 °C and was oxidized in air with a flow rate of 0.4 L/min for 3 hours by the same experimental apparatus. Dilute nitric acid was used to absorb the Ni particles which may be carried by the effluent stream. Then the regenerated catalyst was heated up to 700 °C and undertook the cracking test again. The cracking test using fresh catalyst was defined as fresh cycle, as shown in Figure 1(b). The regeneration and cracking tests were carried out 6 ACS Paragon Plus Environment

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up to four times, which were respectively named as cycle 1 ~ 4.

2.3 Characterization methods The morphology of coke was visualized by the S-4800 scanning electron microscope (SEM) and transmission electron microscope (TEM, JEM-2100F). Raman spectroscopy (LabRam HRUV) was used to characterize the carbon structure of the coke. The quantity and type of coke were identified by temperature programmed oxidation (TPO), which was carried out on 5 ~ 6 mg used catalysts using a ramp rate of 15 °C/min, to the final temperature of 800 °C in air with a gas flow rate of 80 ml/min. The chemical species and surface composition of regenerated catalyst were determined by X-Ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) accordingly. XPS is a useful tool to determine the surface chemical composition with a working depth of several nm and a diameter of 250 µm. N2 adsorption - desorption tests were applied for the determination of surface area, pore volume and pore size distributions. Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) was used to confirm the existence of Ni particles in the effluent stream. The reducing properties of catalysts have been determined by hydrogen temperature programmed reduction (H2-TPR) in a Micromeritics AutoChem II 2920 system. Initially, the samples were outgassed under He flow at 300 °C for 1 h. The analysis was stabilized under a flow (35 ml/min) of 10% H2 in Ar at 50 °C for 1 h and followed by heating up to 900 °C with a ramp of 10 °C /min. A thermal conductivity 7 ACS Paragon Plus Environment

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detector (TCD) was used to monitor hydrogen consumption. Prior passing through the detector, the effluent gases were cooled in liquid nitrogen-cooled isopropanol trap at 80 °C.

3. RESULTS AND DISCUSSION 3.1 Cycle usage performance As illustrated in Figure 1(a), not all the reactants contacted with the catalyst resulting in a relatively low average toluene conversion of 28.2% and H2 yield ratio of 22.5% with fresh catalyst for 30 min. But the coke deposited uniformly, which guaranteed a representative sampling of the deactivated and regenerated catalysts. The catalyst turned black within one minute due to the quick coverage of coke, as shown in Figure 1(b). The average toluene conversion ratio, as shown in Figure 2(a), decreased slowly from 28.2% by fresh catalyst to 22.6% by the catalyst after third regeneration, then decreased quickly to 16.3% in the cycle 4. The catalytic activity of Ni catalyst (represented by toluene conversion ratio) decreased by 42.2% from the fresh cycle to the cycle 4. Coke and H2 were the major products of toluene catalytic cracking using Ni/Al2O3 by Eq. (3).[2] The average H2 yield ratio (22.5 ~ 13.0%) showed the same tendency as the average toluene conversion ratio (Figure 2(b)). Ni/ γ − Al2O3 C7 H 8 ( g ) → 7C ( s ) + 4 H 2 ( g )

− 50 kJ / mol (3)

150 min test by fresh catalyst without regeneration was also carried out for comparison purpose. The yield of H2 increased slightly at the first 20 min 8 ACS Paragon Plus Environment

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(presumably the effect of filamentous coke), then decreased dramatically in the next 70 min. Finally, the yield ratio of H2 decreased to 4.86% after 90 min reaction time, indicating that the catalyst lost its catalytic activity completely. The regeneration of catalyst significantly improved the H2 production when compared with the 150 min test. After the first regeneration, the yield ratio of H2 (22.4%) returned to the same level of fresh catalyst (22.5%). After the second regeneration, the H2 yield ratio (19.6%) was a little smaller compared to that of cycle 1. In the cycle 3, the H2 yield ratio (18.8%) was nearly the same as cycle 2. In the cycle 4, the H2 production (13.0%) decreased rapidly compared with other cycles. The H2 production showed slight decreasing trend along the 30 min reaction time in all cycles. The performance of regenerated catalyst indicated that the calcination at 600 °C maintained the activity of Ni/Al2O3 within three regeneration cycles. Further regeneration showed a significant loss of activity. These results will be explained by the following analysis on the properties of regenerated catalysts and deposited coke.

3.2 Characterization of regenerated catalysts Figure 1(b) presents the appearances of fresh, deactivated and regenerated catalysts. The color of fresh catalyst was blue, changed to dark blue after the first, second and third regenerations. The color finally turned to green after the fourth regeneration. The change of color was mainly caused by the evolution of crystal phase of Ni. Figure 3 presents the XRD patterns of Ni catalyst at different reaction time 9 ACS Paragon Plus Environment

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(cracking process, 10, 20 and 30 min) in the fresh cycle. NiAl2O4 gradually changed to Ni0 at 2θ of 44.5°, 51.8° and 76.4°, corresponding with planes (111), (200) and (220) respectively. The peak at 2θ of 51.8° (200) was used to calculate the average crystal size of Ni0 based on the equation by Scherrer.[17,18] The mean crystal size of Ni0 decreased from 13.0 nm at 10 min to 11.4 nm at 30 min. Besides, a carbon peak at 2θ of 26° (002) was observed due to the deposition of coke. Figure 4 shows the XRD patterns of fresh and regenerated Ni catalysts. In the fresh catalyst, there were only NiAl2O4 and Al2O3 due to a high calcination temperature of 800 °C, which was in agreement with the results of Srinivas et al.[19] After the first regeneration, a part of NiAl2O4 was converted into NiO. NiAl2O4 and NiO were firstly reduced to metallic Ni0 by H2 and C(s) in the cracking test (Eqs. (4) ~ (7)). Then, metallic Ni0 was oxidized to NiO by air in the regeneration test at 600 °C (Eq. (8)).

NiAl2O4 ( s ) + C ( s ) → Ni ( s ) + Al2O3 ( s ) + CO ( g ) (4) NiAl2O4 ( s ) + H 2 ( g ) → Ni ( s ) + Al2O3 ( s ) + H 2O ( g ) (5)

NiO ( s ) + C ( s ) → Ni ( s ) + CO ( g ) (6) NiO ( s ) + H 2 ( g ) → Ni ( s ) + H 2O ( g ) (7) Ni ( s ) + 1 2 O2 ( g ) → NiO ( s ) (8) There were five diffraction peaks of NiO of regenerated catalysts in the cycles 1 ~ 4 at 2θ of 37.0°, 43.2°, 62.8°, 75.4° and 79.2°, corresponding with planes (101), (012), (110), (021) and (222) respectively.[14] The peak at 43.2° was used to estimate the average NiO crystal size in the (012) plane. The mean crystal size of NiO increased from 12.8 nm in cycle 1 to 18.0 nm in cycle 4. Takeshi et al.[15] also found the 10 ACS Paragon Plus Environment

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increase of Ni metal particle size after regeneration, but they didn’t regard it as the major reason for remarkable decrease of catalytic activity for the steam reforming of naphthalene and benzene in fourth cycle. The oxidation of coke in the regeneration test refreshed the catalyst. However, as shown in Figure 1(b), some coke still remained on the surface of the catalyst, which changed the textural structure of regenerated catalyst. The effects of cycle usage on the textural properties of fresh and regenerated catalysts are presented in Table 1 and Figure 5. The BET surface area and pore volume decreased slightly from 97.7 to 86.9 m2/g and 0.552 to 0.478 cm3/g, respectively. But the micropore area and volume increased after the first and second regenerations, then decreased. Also, the average pore width firstly decreased from 22.6 in the fresh cycle to 20.4 nm in cycle 2, then increased to 23.5 nm in cycle 4. The pore size distributions after regeneration showed different trends in different ranges of pore size. The volume of pores smaller than 4 nm increased before cycle 2, then decreased in cycle 3 and 4. While, the volume of pores between 4 and 20 nm monotonous decreased after regeneration. There were no significant differences when the pore size was larger than 20 nm between different cycles. The changes of textural properties suggested the presence of remaining coke on the regenerated catalyst both inside the micropores and mesopores, causing the decrease of area and volume. Besides, the regeneration-cracking cycles resulted in the Ni sintering or Ni particles movement from the micropores towards the external surface and mesopores, which is expected to decrease the mesopore volume and to increase 11 ACS Paragon Plus Environment

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the micropore volume.[13] The movement of Ni particles can explain the area and volume increase of smaller pores than 4 nm in cycles 1 and 2. The remaining coke explained the decrease of total surface area and pore volume. After two regenerations, the effect of remaining coke overlapped the effect of Ni movement, causing the area and volume decrease of both micropore and mesopore. TPR measurements, as shown in Figure 6, were carried out to investigate the metal-support interaction and the reducibility of Ni species. The reduction temperature of Ni species depends on the nature of the interaction between Ni and support. There were four significant peaks in the temperature ranges of below 400, 400 ~ 500, 500 ~ 600 and above 700 °C. According to the summary of previous researches[20-24]: i) the peak below 400 °C corresponded to the reduction of bulk NiO clusters with low interaction with the support; ii) the two peaks in the 400 ~ 500 and 500 ~ 600 °C ranges corresponded to two types of surface spinels formed by accommodating Ni ions or NiOx species in the octahedral and tetrahedral sites of Al2O3, respectively; iii) the peak above 700 °C was associated with the NiAl2O4 spinel, which formed a very strong interaction between Ni and support. NiAl2O4 was the major Ni species in the fresh catalyst, resulting in a higher reduction temperature than 800 °C. The regeneration cycles decreased the reduction temperature of peak 4 from 801 to 752 °C, revealing a weaker interaction between NiAl2O4 spinel and the support. The reduction temperatures of peaks 1 ~ 3 shifted to higher temperatures after more times of regeneration process, indicating that the cycle of cracking and regeneration slightly increased the interactions between NiOx species 12 ACS Paragon Plus Environment

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and Al2O3 support.

3.3 Characterization of deposited coke The coke characterization was affected by the properties of catalysts. Both amorphous and filamentous cokes were observed on the surface of catalyst, as illustrated in Figure 7 by SEM. The coke structure and distribution on the fresh and regenerated catalysts didn’t show much differences. The amorphous coke showed sparse distribution and agglomerating tendency. While the filamentous coke with a length of several micrometers and a diameter of less than 50 nm, twisted with each other, which was confirmed as DWCNTs (double wall carbon nanotubes) by TEM in Figure 7 (f). Ni particles were found on the top or in the middle of the filamentous coke.

The

formation

and

growth

of

filamentous

coke

followed

the

dissociation-diffusing-precipitation mechanism.[25] The carbon source was firstly dissociated by nanoparticles, which subsequently dissolved the carbon produced by the dissociation reaction, and once supersaturated, carbon precipitated from the nanoparticles to form filamentous coke (CNTs). This coke dragged Ni particles during its growth and separated from Ni crystallites, thus causing the decrease of Ni particle size.[26] From the TPO results in Figure 8 (a) and Table 2, the coke yield with the fresh catalyst was 0.35 g/g-catalyst. The amount of deposited coke decreased with the regeneration cycles. The coke production decreased from 0.35 to 0.22 g/g-catalyst after the first regeneration. Then, the yield of coke decreased to 0.12 g/g-catalyst in 13 ACS Paragon Plus Environment

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the cycle 4 due to the loss of catalytic activity, which was in agreement with the results of toluene conversion. There were two peaks by deconvolution of DTG (derivative thermogravimetric analysis) curves, one at around 450 °C, and another at about 550 °C, corresponding with amorphous and filamentous coke respectively, as presented in Figure 8 (b) (using fresh cycle as an example). Filamentous coke was reported to show a peak at higher temperatures than amorphous coke due to being more graphitized.[27] Thus, the regeneration temperature of this research was chosen to be 600 °C in order to remove the deposited coke effectively. The If/Ia (the mass ratio of filamentous coke to amorphous coke) was calculated by the area ratio of the second peak (filamentous coke) to the first peak (amorphous coke). The If/Ia of deactivated catalyst in fresh cycle was 0.85, indicating that the production of filamentous coke was close to that of amorphous coke. The regeneration process seemed to have positive effect on the formation of filamentous coke due to the larger If/Ia values (0.93, 0.84, 0.87 and 0.89) of deactivated catalysts in the cycles 1 ~ 4 than that in the fresh cycle. There are two typical peaks appeared in the first region of Raman spectra (Figure 9). D peak (1360 cm-1) represented the amorphous carbon and G peak (1580 cm-1) was associated with the graphitic carbon. The intensity of Raman peaks in cycle 4 was significantly smaller compared with other cycles, indicating the amount of coke was smaller resulting from the less active nature of catalyst after the fourth regeneration process. The intensity ratio ID/IG was usually used to evaluate the graphitization of carbon materials.[28] The ID/IG values of deactivated catalysts in cycles 1 ~ 4 (1.21, 14 ACS Paragon Plus Environment

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1.14, 1.09 and 1.17) were smaller than that in fresh cycle (1.29), indicating a more ordered carbon structure of the deposited coke on the surface of regenerated catalysts, which was in agreement with the results of TPO.

3.4 Effect of filamentous coke on the cycle usage The coke morphology also affected the properties of regenerated catalyst and subsequently the cycle usage performance. There were three major effects of filamentous coke on the cycle usage of Ni catalyst during cracking of tar based on the aforementioned results. Firstly, the formation and growth of filamentous coke reduced the Ni particle size in the cracking test, which subsequently decreased the extent of aggregation during regeneration. The crystal size of NiO after regeneration still increased from 12.8 nm in cycle 1 to 18.0 nm in cycle 4 due to the thermal effect of coke oxidation in regeneration test. A larger Ni crystal size enhanced the formation and growth of filamentous coke,[29] which explained the increase of If/Ia in DTG curves and the decrease of ID/IG in Raman spectra after regeneration. Secondly, filamentous coke was more graphitized than amorphous coke resulting in a higher calcination temperature in the regeneration process. The second peak corresponding with filamentous coke in the DTG curves was about 550 °C. Thus, the calcination temperature must be higher than 550 °C to ensure an effective coke removal efficiency. From the results of toluene conversion and regenerated catalysts properties, the calcination temperature of 600 °C achieved acceptable coke removal 15 ACS Paragon Plus Environment

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efficiency without causing serious sintering problem. Finally, the filamentous coke lifted the Ni particles from the support, causing a weaker interaction of metal and support. On the one hand, this effect avoided the cover of active sites and subsequently decreased the deactivation rate. On the other hand, the weaker interaction between metal and support was bad for the activity of catalyst and was easy to lose the active sites. In the regeneration process, the remove of coke released the Ni particles, which may be blown away by the carrier gas. Dilute nitric acid (200 ml) was used to absorb the Ni particles in the effluent stream of regeneration process and analyzed by ICP-MS. The amounts of taking-away Ni were 0.564, 0.208, 0.198 and 0.248 µg from the first to the fourth regeneration, which verified the loss of Ni particles during regeneration, but was negligible compared with the Ni content in the raw catalyst (80 mg). The XPS results (Figure 10) showed insignificant differences, which demonstrated that the Ni content on the surface of support was stable during the cracking-regeneration cycle usage.

4. CONCLUSIONS The cracking-regeneration cycle usage of Ni/γ-Al2O3 for toluene cracking and the effect of coke properties (especially filamentous coke) on the cycle usage performance were investigated. The average toluene conversion ratio decreased slowly from 28.2% by fresh catalyst to 22.6% by the catalyst after third regeneration, then decreased quickly to 16.3% after cycle 4. The catalytic activity of fresh Ni catalyst (represented by toluene conversion ratio) decreased by 42.2% after fourth 16 ACS Paragon Plus Environment

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regeneration. The average H2 production showed similar tendency. The NiO crystal size of the regenerated catalysts increased from 12.8 nm in cycle 1 to 18.0 nm in cycle 4 due to the effect of gradual sintering. The movement of Ni particles to the external surface and mesopores caused the increase of micropore area and volume in cycles 1 and 2. The remaining coke caused the decrease of BET surface area and pore volume. The increase of Ni crystal size and the decrease of BET surface area probably were the major reasons of partial deactivation of Ni catalyst after cracking-regeneration cycle usage. Both amorphous and filamentous deposition coke were observed on the surface of catalysts. The coke yield was between 0.35 ~ 0.12 g/g-catalyst with a decreasing trend after regeneration. The mass ratio of filamentous coke to amorphous coke was between 0.84 ~ 0.93 with a slight increasing trend, indicating the cycle usage promoted a more ordered carbon structure of deposited coke. The results of Raman spectra also demonstrated this viewpoint. Filamentous coke reduced the Ni particle size and weakened the interaction of metal and support. Besides, the formation of filamentous coke required a higher calcination temperature than 550 °C during regeneration process.

AUTHOR INFORMATION Corresponding Author *Telephone: +86-571-87952834; Fax: +86-571-87952438. E-mail: [email protected]

ACKNOWLEDGEMENT The authors would like to greatly acknowledge the Environmental Protection Special Funds for Public Welfare (201509013). 17 ACS Paragon Plus Environment

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Table 1. Textural properties of the fresh and regenerated catalysts.

Fresh Cycle 1 Cycle 2 Cycle 3 Cycle 4

BET surface area (m2/g)

Micropore area (m2/g)

Pore volume (cm3/g)

97.7 95.6 96.1 89.0 86.9

8.8 11.4 9.2 8.7 7.8

0.552 0.534 0.491 0.490 0.478

Micropore volume (cm3/g) 0.0040 0.0054 0.0043 0.0039 0.0035


Average pore width (nm) 22.6 22.3 20.4 22.9 23.5

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Table 2. Coke yield and the mass ratio of filamentous coke to amorphous coke (If/Ia). Coke (g/g-catalyst) If/Ia

Fresh cycle 0.35 0.85

Cycle 1 0.22 0.93

Cycle 2 0.18 0.84


Cycle 3 0.17 0.87

Cycle 4 0.12 0.89

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(a)

Gas samples

Tubular furnace

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Quartz tubular reactor Syringe pump for toluene

Gas outlet Cold water Quartz boat

n-Hexane (cracking test)

Catalysts

or

N2

Air

Dilute nitric acid (regeneration test)

(b) Fresh cycle Fresh catalyst

Cycle 1

Cracking 0.5 h, 700 °C

Regeneration 3 h, 600 °C



Cycle 2






Cycle 3

Cycle 4

Figure 1. (a) Schematic drawing of experimental apparatus and (b) Cracking and regeneration cycles.


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Average toluene conversion (%)

30

(a) 4.3%

28.2%

14.5%

27.0%

25

19.9%

42.2%

24.1% 22.6%

20 15

16.3%

10 5 0

Fresh cycle Cycle 1

25 Fresh cycle

Cycle 2

Cycle 3

Cycle 4

(b) Cycle 1 Cycle 2

22.5%

Cycle 3

22.4%

20

Yield ratio of H2 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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19.6% 18.8%

Cycle 4

15 16.2% 13.0%

10 7.84%

Without regeneration Cycles after regeneration

5 0

20

40

5.37%

60

80

100

4.86%

120

140

Reaction time (min)

Figure 2. (a) The average toluene conversion ratio and (b) the evolution of H2 yield ratio with time in different cycles.


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13 2

32 30 min

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

4

3

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1: Ni

2: Al2O3

3: NiAl2O4

4: Carbon

32

2 1

32

13

20 min

10 min

Fresh

10

20

30

40

50

60

70

80

2θ θ (°°)

Figure 3. XRD patterns of Ni catalyst at different reaction time (cracking process, 10, 20 and 30 min) in the fresh cycle.


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132 1 Cycle 4

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

1: NiO 2: Al2O3 3: NiAl2O4

2 3

2 3

3

2 21 3 3

3 1 1

Cycle 3

Cycle 2

Cycle 1

Fresh

10

20

30

40

50

60

70

80

2θ θ (°°)

Figure 4. XRD patterns of fresh and regenerated catalysts.


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0.010

0.008

Pore volume (cm3/g⋅⋅nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fresh

Cycle 2 0.006

Cycle 4

0.004

Cycle 1

Cycle 3 0.002

< 4 nm

> 20 nm

4 ~ 20 nm

0.000 2

4

8

16

32

64

Pore diameter (nm)

Figure 5. Pore size distributions of fresh and regenerated catalysts.


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Peak 4 Fresh Cycle 2 Cycle 4

Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cycle 1 Cycle 3

Peak 3 Peak 2 Peak 1

NiO clusters

200

300

400

Spinels formed by Ni or NiOx

500

600

NiAl2O4 spinel

700

800

900

Temperature (°°C)

Figure 6. H2-TPR results of fresh and regenerated catalysts.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 7. SEM of coke deposited on the surface of (a) fresh catalyst, (b) first regeneration catalyst, (c) second regeneration catalyst, (d) third regeneration catalyst, (e) fourth regeneration catalyst and (f) TEM of filamentous coke.


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0.0

(a) DTG (%/min)

-0.5

-1.0

Filamentous coke ~ 550 °C

-1.5 Fresh cycle Cycle 1 Cycle 2 Cycle 3 Cycle 4

-2.0 Amorphous coke ~ 450 °C -2.5 200

300

400

500

600

700

800

Temperature (°°C)

Fresh cycle

0.0

(b)

DTG (%/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.5 Peak 2

Raw line

-1.0 Peak 1

-1.5 Fitted line

-2.0

-2.5 200

300

400

500

600

700

800

Temperature (°°C)

Figure 8. (a) TPO results of coke deactivated catalysts after cracking tests and (b) curve-fitting method for the DTG curve of the fresh cycle as an example.


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G peak

D peak Cycle 4

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ID/IG = 1.17

Cycle 3

ID/IG = 1.09

Cycle 2

ID/IG = 1.14

Cycle 1

ID/IG = 1.21

Fresh cycle ID/IG = 1.29

800

1000

1200

1400

1600

1800

2000

-1

Raman shift (cm ) Figure 9. Raman spectra of coke deactivated catalyst after cracking test.


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Fresh Cycle 1 Cycle 2 Cycle 3 Cycle 4

Ni 2p3/2&1/2 Ni LMM

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

200

400

600

800

1000

Binding Energy (ev)

Figure 10. XPS results of fresh and regenerated catalysts.


Coking and Regeneration of Nickel Catalyst for the Cracking of

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