Deactivation of Nickel Catalysts by Sulfur and Carbon for the Pyrolysis

Feb 5, 2014 - Ibrahim F. Elbaba and Paul T. Williams*. Faculty of Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. Energy Fuels , 2014...
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Deactivation of Nickel Catalysts by Sulfur and Carbon for the Pyrolysis−Catalytic Gasification/Reforming of Waste Tires for Hydrogen Production Ibrahim F. Elbaba and Paul T. Williams* Faculty of Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom ABSTRACT: Hydrogen production from the two-stage pyrolysis−gasification/reforming of waste tires has been investigated using Ni/Al2O3 and Ni/dolomite catalysts in the relation to four cycles of use. The Ni/dolomite catalyst produced a higher hydrogen yield and the highest theoretical hydrogen potential to produce hydrogen gas compared to the Ni/Al2O3 catalyst. In addition, the used Ni/dolomite catalyst had the lower carbon deposition, as determined by temperature-programmed oxidation, being 18.2 wt % for the Ni/Al2O3 catalyst and 2.8 wt % for the Ni/dolomite catalyst. Detailed analysis of the reacted catalysts using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS) was used to characterize the presence of nickel, sulfur, and carbon on the used catalysts. The results showed that, for the Ni/Al2O3 catalyst, sulfur was mainly present on the surface of the nickel particles. However, for the used Ni/dolomite catalyst, no sulfur peaks were detected whether in the bulk metal or on the surface of the nickel particle. In addition, carbon deposition was closely associated with the nickel particles of the used Ni/Al2O3 catalyst but not for the used Ni/dolomite catalyst. The results suggest that nickel of the Ni/Al2O3 catalyst becomes deactivated because of sulfur reaction and carbon deposition, preventing reaction of the various reactant species generated from the pyrolysis of the tires. However, using dolomite as a support, the catalytic deactivation process was less pronounced and there was negligible deactivation of nickel by sulfur.

1. INTRODUCTION Approximately 1.5 billion new tires are produced worldwide each year, which will eventually become waste tires.1 For example, in the European Union (EU), more than 3 million tonnes of waste tires are generated annually, and in the U.S., more than 5 million tonnes of waste tires are generated annually.1,2 The majority of the waste tires are used for energy recovery, either directly or as tire-derived fuel in applications such as cement kilns, power stations, or co-combustion.3 Alternative options for the thermochemical treatment of waste tires is through the production of fuels such as oils or gases using pyrolysis or gasification. Pyrolysis of waste tires has been extensively researched and recently reviewed.3−5 Pyrolysis typically takes place at temperatures of about 500 °C in an inert atmosphere, where the organic components of the tires are thermally degraded to produce oil, gas, and char. The oil is a complex mixture of aliphatic and aromatic compounds, with fuel properties similar to a light fuel oil. The gases are typically composed of C1−C4 hydrocarbons and hydrogen, and the solid char consists of the carbon black filler used in the tire formulation as well as char produced during the pyrolysis of the rubber. Gasification of waste tires is normally carried out between 700 and 1400 °C, and several different partial oxidizing agents and a range of different reactors have been investigated, for example, using steam in a continuous screw kiln reactor,6 using air gasification of waste tires in a fluidizedbed reactor,7 using steam gasification of tires in a rotary kiln pilot plant gasifier,8 and gasification using air and carbon dioxide and also air and steam in a bubbling fluidized-bed reactor.9 The product gas from gasification is composed of mainly hydrogen, carbon monoxide, carbon dioxide, and methane; however, in the presence of steam, increased © 2014 American Chemical Society

hydrogen is produced, increasing the calorific value of the product gases. With the increasing interest in hydrogen as a future energy carrier and in processes that generate hydrogen from alternative sources, there has been several recent investigations into the production of hydrogen from waste tires.10−12 The enhanced production of hydrogen from waste tires involves catalysts, and nickel-based catalysts have been commonly used with tire gasification.10−12 Two-stage reactors have been used, where the first stage involves pyrolysis of the waste tire at 500 °C in an inert atmosphere, followed by catalytic gasification/reforming in a second stage using catalysts. Using two-stage reaction systems enable greater control of the catalytic process, because the temperature of each stage can be easily controlled and there is improved contact between pyrolysis products and the catalyst. Single-stage gasification tends to generate low-quality gas, which is commonly used for direct thermal use; however, two-stage reaction systems have been shown to increase the hydrogen/methane concentration of the product gases. Nickelbased catalysts are often used because there is extensive experience of their use in tar-reforming of syngas from biomass and other feedstocks.13,14 In addition, nickel catalysts have advantages of relatively low cost and commercial availability; however, their disadvantages include rapid deactivation because of carbonaceous coke formation on the catalyst, sintering of the support matrix, and poisoning of the nickel metal by sulfur.15,16 Of particular concern in the use of nickel catalysts for the processing of waste tires to produce hydrogen is the fact that Received: November 28, 2013 Revised: February 4, 2014 Published: February 5, 2014 2104

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tires contain significant concentrations of sulfur, typically about 1.4 wt %.17 Sulfur is used in the manufacture of the tire to cross-link the polymer chains within the rubber and also to harden and prevent excessive deformation at elevated temperatures. For a two-stage pyrolysis−gasification process, the pyrolysis of the tires will liberate some sulfur into the oil and gas phases; for example, aromatic sulfur compounds, e.g., thiophene and dibenzothiophene, and alkylated species are found in tire pyrolysis oils, and hydrogen sulfide is found in the off-gases.18,19 A significant proportion of sulfur is also retained in the char, typically between 1.9 and 2.7 wt %, representing ∼40% of the total sulfur, but the fraction of sulfur retained in the char is dependent upon the pyrolysis process and temperature.3 The high concentration of sulfur species evolved during the tire pyrolysis process passes to the catalyst and will deactivate the nickel metal of the catalyst, thereby reducing hydrogen production. There is, therefore, a need to understand the extent of such catalyst poisoning. Our previous work demonstrated the potential of two-stage pyrolysis−gasification/reforming of waste tires for hydrogen production using Ni/Al2O310 and Ni/dolomite11 catalysts. However, in that work, no consideration of the influence of deactivation by sulfur poisoning linked to carbon deposition was considered. In this paper, the two-stage pyrolysis− gasification/reforming of waste tires has been investigated with the aim of producing high yields of hydrogen from the waste tires. The first stage pyrolysis was undertaken at 500 °C, and the second stage gasification/reforming of the evolved pyrolysis gases took place at 800 °C in the presence of steam and catalyst. Two nickel-based catalysts were used, Ni/Al2O3 and Ni/dolomite, and their deactivation by carbon deposition and sulfur poisoning was investigated in relation to hydrogen production over four cycles of catalyst use.

Figure 1. Schematic diagram of the two-stage pyrolysis−gasification/ reforming experimental system. was carefully loaded onto a quartz wool support and inserted into the second reactor. The experimental procedure consisted of initial heating of the catalyst in the second-stage reactor to 800 °C. Once the catalyst had reached a stable temperature of 800 °C, the first-stage pyrolysis reactor containing the tire sample was heated to 500 °C at a heating rate of 40 °C min−1, where pyrolysis of the tire occurred. Water was introduced into the second stage at a flow rate of 4.74 mL h−1 to produce steam catalytic gasification/reforming conditions. Liquid products were collected in a condensation system, and noncondensed gases were collected in a Tedlar sample gas bag and analyzed off-line by packed column gas chromatography. The same catalyst was used over four cycles of experimentation to determine how the gas composition changed with repeated use of the catalyst. Therefore, for each catalyst, the catalyst was left in the second-stage catalytic reactor, without regeneration, whereas the first-stage reactor was cleaned and a fresh waste tire sample was added for each of the four experiments. It has been shown before that the rate of gas evolved and composition of the gases released during the pyrolysis of tires as they are heated to 500 °C will vary as the temperature is increased.20 Therefore, the interaction of the evolved pyrolysis gases with the steam and catalyst will not be uniform throughout the gasification/reforming stage. 2.3. Gas Analysis. The gases collected in the Tedlar gas sample bag were analyzed off-line with a system of gas chromatographs. Hydrocarbon gases were analyzed using a Varian CP-3380 gas chromatograph with a flame ionization detector (FID) and nitrogen as the carrier gas. The column was stainless-steel, 2 m long, and 2 mm diameter, packed with 80−100 mesh size Haysep. Hydrogen, nitrogen, carbon monoxide, and oxygen were analyzed with a separate Varian CP-3380 gas chromatograph with a thermal conductivity detector using a 2 m long and 2 mm diameter column packed with 60−80 mesh molecular sieve and argon carrier gas. Carbon dioxide was analyzed on the same gas chromatograph but with a separate injection, column, and detection system. The column was a 2 m long and 2 mm diameter column with Haysep 80−100 mesh molecular sieve and a thermal conductivity detector. The carrier gas was argon. 2.4. Characterization of Used Catalysts. Temperature-programmed oxidation (TPO) of the used catalyst was conducted in a thermogravimetric analyzer (TGA) to determine the amount and type of carbonaceous coke deposited on the catalyst. The TGA used was a

2. MATERIALS AND METHODS 2.1. Materials. The waste tires consisted of the rubber tread from automobiles. The rubber, with the steel removed, was shredded to a particle size of ∼6 mm. The tire rubber sample was analyzed by a Carlo Erba Flash EA 1112 elemental analyzer and was found to have carbon, hydrogen, nitrogen, and sulfur contents of 86.4, 8.0, 0.5, and 1.7 wt %, respectively. Two nickel-based catalysts were used in this research, Ni/Al2O3 and Ni/dolomite. The Ni/Al2O3 catalyst was prepared by an incipient wetness method using Al2O3 and an aqueous solution of Ni(NO3)2· 6H2O. After loading nickel onto Al2O3 (10 wt % Ni), the catalyst was dried at 105 °C overnight and calcined at 500 °C for 3 h under an air atmosphere. The Ni/dolomite catalysts were prepared by precipitating metallic nickel onto calcined dolomite. The dolomite was supplied by Warmsworth Quarry, Doncaster, U.K. Its composition was 21.3 wt % MgO, 30.7 wt % CaO, 0.3 wt % SiO2, 0.27 wt % Fe2O3, and 0.1 wt % Al2O3. Nickel nitrate hexahydrate was dissolved into distilled water before the addition of dolomite, which had been previously calcined at 1000 °C. The catalyst was dried at 105 °C overnight, followed by calcination at 500 °C for 3 h in air. A Ni loading of 10 wt % was prepared. Both catalysts were sieved to granules with a size less than 0.212 mm. 2.2. Experimental System. The pyrolysis−gasification/reforming of the waste tires was carried out in a two-stage fixed-bed reactor system (Figure 1). The first-stage pyrolysis of the tires at 500 °C produced pyrolysis gases, which were passed directly to the secondstage reactor, which contained the catalyst and was heated to 800 °C and where gasification/reforming of the evolved pyrolysis gases took place in the presence of steam. The reactors were constructed of stainless steel and were temperature-controlled using external ceramic furnaces. The carrier gas was N2 at a flow rate of 200 mL min−1. Approximately 1 g of tire and 0.5 g of catalyst were used. The catalyst 2105

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Stanton Redcroft TGA; the sample was heated at 15 °C min−1 in air; and the weight changes were recorded up to a sample temperature of 800 °C, with a final hold time of 10 min. The characteristics of the used catalysts and carbonaceous deposits were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS). SEM used was a Phillips XL30 environmental, and TEM was a Phillips CM FEGTEM coupled with EDXS. X-ray photoelectron spectroscopy (XPS) analysis of the used catalysts was carried out to determine the sulfur and type of carbon species formed on the catalyst using a VG Escalab 250 spectrometer, operated with an Al anode source at 10 mA and 12 kV. X-ray diffraction analysis of the used catalysts to determine the bulk crystal structure and chemical phase composition of the catalysts used a Phillips PW 1050 goniometer with a Cu Kα radiation X-ray tube operated at 40 kV and 40 mA.

3. RESULTS AND DISCUSSION 3.1. Product Yield and Gas Composition. Table 1 shows the product yield from the two-stage pyrolysis−catalytic

Figure 3. Product gas composition in relation to four cycle experiments of the Ni/dolomite catalyst.

Table 1. Product Yield for the Ni/Al2O3 and Ni/Dolomite Catalysts (Mass Balance in Relation to Tire) catalyst

Ni/Al2O3

Ni/dolomite

gas/tire (wt %) solid/tire (wt %) oil/tire (wt %) mass balance (wt %) reacted water (g)

39.8 41.7 31.9 113.5 0.14

50.1 38.0 32.7 120.8 0.26

Figure 2. Product gas composition in relation to four cycle experiments of the Ni/Al2O3 catalyst.

gasification/reforming of waste tires in relation to the Ni/ Al2O3 and Ni/dolomite catalysts for single use. The details of the influence of various process parameters on the yield and composition of products using the Ni/Al2O3 and Ni/dolomite catalysts has been reported previously.10,11 Table 1 shows that the gas yield for the Ni/dolomite (50.1 wt %) was much higher than that produced using the Ni/Al2O3 catalyst (39.8 wt %). The solid yield was slightly higher for the Ni/Al2O3 catalyst. In this work, the solid yield included not only the pyrolysis char in the first reactor but also carbon deposits on the quartz wool catalyst support and coke formation on the catalyst. The product yields (gas, oil, or solid) were calculated in relation to the mass of waste tires only and do not include the input water, therefore resulting in more than 100% mass balance, where the

Figure 4. TGA−TPO and DTG−TPO results of the reacted Ni/Al2O3 and Ni/dolomite catalysts.

added water will react to increase the gas production and, therefore, influence the mass balance. Figures 2 and 3 show the product gas composition and potential hydrogen production from the two-stage pyrolysis− catalytic gasification/reforming of waste tires in relation to the Ni/Al2O3 and Ni/dolomite catalysts over the four cycles of catalyst use. The potential hydrogen production is defined as the maximum theoretical weight of H2 produced from steam 2106

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Figure 5. TEM image of reacted catalysts: (a and b) Ni/Al2O3 catalyst and (c and d) Ni/dolomite catalyst.

53.6 vol % and the potential H2 production decreased from 11.4 to 10.1 wt % for the Ni/dolomite catalyst. Al2O3 is commonly used as a catalyst support for nickel because of its chemical and physical stability, high mechanical resistance, and production of high metal dispersion of nickel throughout the Al2O3 support. Ni/Al2O3 catalysts have been used extensively for tar reduction and enhanced hydrogen yield from biomass and wastes.14,23,24 Dolomite (CaCO3·MgCO3), with the addition of nickel, has also been used as a catalyst for tar reduction in the gasification of biomass and wastes.25,26 However, dolomite is less resilient, can be eroded, and produces quantities of fine particles during the gasification system, especially in the fluidized-bed process.27 A comparison of the catalytic activity of Ni/Al2O3 and Ni/dolomite catalysts has been made by Srinakruang et al.28 using feedstock composed of model tar compounds and with sulfur (as H2S) introduced into the feed stream. They reported that the Ni/ dolomite catalyst showed excellent catalytic activity, durability, and resistance to coke formation and sulfur poisoning

gasification of tire, which is estimated to be 38.7 g of hydrogen/ 100 g of tire based on the hydrogen elemental content of the tire and the hydrogen generated from the steam reaction with the tire, assuming conversion to CO2 and H2.21,22 The results show that the product gases consist of mainly hydrogen, with lower concentrations of methane, carbon monoxide, carbon dioxide, and C2−C4 hydrocarbons. The hydrogen gas yield decrease after the first cycle can be attributed to deactivation of the catalyst through carbon deposition, reaction of nickel with sulfur, and reducing the nickel active sites available for reaction. Although the gas composition appears to be stable for the second, third, and fourth cycles, in reality, catalyst stability tests require several hundred cycles to be completed to provide a proper assessment for commercial use. The Ni/Al2O3 catalyst appeared to be less effective in terms of hydrogen production and the extent of deactivation compared to the Ni/dolomite catalyst. For example, the H2 concentration decreased from 51.5 to 39.9 vol % and the potential H2 production decreased significantly from 7.2 to 4.1 wt % after four cycles of use of Ni/ Al2O3, whereas the H2 concentration decreased from 60.8 to 2107

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Figure 6. Carbon mapping analysis of reacted Ni/Al2O3 and Ni/dolomite catalysts: (a) TEM image of reacted Ni/Al2O3 catalyst, (b) carbon mapping of reacted Ni/Al2O3 catalyst, (c) TEM image of reacted Ni/dolomite catalyst, and (d) carbon mapping of reacted Ni/dolomite.

compared to the Ni/Al2O3 catalyst, which was less stable and more easily deactivated. 3.2. Carbon Deposition on the Used Catalyst. Deactivation of the catalysts is attributed to carbon deposition onto the catalyst surface, which blocks the pores and prevents the tire pyrolysis reaction products from interacting with the dolomite and nickel active sites. To understand the amount and nature of the carbons found on the catalyst surface, the used catalysts were characterized by thermogravimetric temperatureprogrammed oxidation (TGA−TPO) experiments. Figure 4 shows the TGA−TPO analysis of the used Ni/Al2O3 and Ni/ dolomite catalysts for the four cycles of use. From the TGA− TPO results, the amount of carbon deposited on the catalyst was higher for the Ni/Al2O3 catalyst compared to the Ni/ dolomite catalyst for the pyrolysis−gasification of waste tires. The carbon deposited on the surface of the Ni/Al2O3 catalyst was 18.2 wt %, while carbon deposited on the surface of the Ni/ dolomite catalyst was 2.8 wt % during pyrolysis−gasification after using the catalyst for the first cycle. Srinakruang et al.28

also showed that, during the steam reforming of model compounds representative of biomass tar, a Ni/dolomite catalyst showed lower carbon deposition compared to Ni/ Al2O3. They suggested that the basicity of dolomite reduced coke formation during the steam reforming process. Figure 4 also shows that, for the rate of weight loss thermograms for the temperature-programmed oxidation (DTG−TPO) results, one main oxidation peak was observed at a temperature of about 600 °C for both Ni/Al2O3 and Ni/ dolomite catalysts. The carbon type can be identified depending upon its oxidation temperature. Carbon oxidized at temperatures of ∼600 °C have been assigned as filamentous carbons, whereas amorphous-type carbons are oxidized at lower temperatures.29 Therefore, the carbon type deposited on the Ni/Al2O3 and Ni/dolomite catalysts were mainly of the filamentous type. This was also confirmed by SEM, which showed carbons to be a curled mass of carbon filaments several micrometers in length. The weight loss peak, which was around 710 °C, for the Ni/dolomite catalyst is suggested to be the 2108

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Figure 7. Elemental mapping analysis of the reacted Ni/Al2O3 catalyst: (a) TEM image, (b) Ni mapping, and (c) sulfur mapping.

TEM−EDXS was used to characterize the chemical composition of the used catalysts in terms of the surface carbon distribution. Figure 5 shows the TEM image of reacted Ni/Al2O3 and Ni/dolomite catalysts. Filamentous carbons are clearly seen on both catalysts, with filaments from 10 to 50 nm diameter and up to 5 μm in length. Nickel particles are seen at the tips of the carbon filaments, as reported by other researchers.33−35 It is suggested that hydrocarbons adsorb on the surface of the catalysts, followed by dissociation and diffusion of the hydrocarbons and carbon species over the catalyst surface or through the nickel crystallites to the nickel− support interface. Dependent upon the type of carbon structure formed, the nickel may then be detached from the catalyst surface and move away from the surface on the tip of the growing carbon filament. Filament growth ceases when nickel is encapsulated by carbon. Figure 5 also shows that the carbon layers encapsulate the nickel particles. Figure 6 shows the elemental mapping of carbon over the surface of the reacted

dissociation of magnesium carbonate and calcium carbonate present in the dolomite catalyst.30 As the number of cycles was increased, there was a slight shift in the weight loss of the carbon to higher temperatures of oxidation, suggesting that more of the less reactive filamentous-type carbon was being formed on the catalyst. Research on the two-stage pyrolysis gasification of polypropylene, a linear structured polymer, using a Ni/Al2O3 catalyst showed that the carbon was mainly amorphous, with less amounts of filamentous carbon.29 However, similar experiments with polystyrene, an aromatic structured plastic, using a Ni/Al2O3 catalyst produced mainly filamentous carbons.31 Similar aromatic styrene-based polymers, i.e., highimpact polystyrene and acrylonitrile−butadiene−styrene, have been shown to also produce mainly filamentous carbons.32 Consequently, the waste tire, which typically contains significant quantities of styrene−butadiene−rubber, would be expected to produce the filamentous-type carbons. 2109

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Figure 8. Elemental mapping analysis of the reacted Ni/dolomite catalyst: (a) TEM image, (b) Ni mapping, and (c) sulfur mapping.

a Ni−(PdRh)/γ-Al2O3 catalyst, nickel and sulfur align well during elemental mapping, suggesting that sulfur is predominantly deposited on the surface of nickel particles. They also reported that there was no bulk sulfur deposition on the catalyst; rather, sulfur was mostly associated with nickel. Lakhapatri and Abraham16,36 suggest that sulfur compounds present in the gasification/reforming environment decompose to hydrogen sulfide, which adsorbs dissociatively on the metal surface, forming a sulfur layer. The metal−sulfur surface layer inhibits chemisorption of small molecules, leading to catalyst deactivation for the important methanation, hydrogenolysis, hydrogenation, and water−gas shift reactions. Figure 9 shows the same TEM field of view as for Figure 6 but with sulfur and nickel identified using EDXS at specific points on the reacted Ni/Al2O3 and Ni/dolomite catalyst samples from the pyrolysis−gasification/reforming of waste tires. Figure 9a shows the TEM image of the reacted Ni/Al2O3 catalyst, and Figure 9b shows the EDXS analysis at different

Ni/Al2O3 and Ni/dolomite catalysts using TEM−EDXS. Carbon was observed extensively throughout the reacted catalysts; however, for the Ni/Al2O3 catalyst, carbon was closely associated with the nickel particles, but for the Ni/ dolomite catalyst, there was no such association. The filaments were shown to be both carbon fibers and also carbon nanotubes for both catalysts. 3.3. Sulfur Deposition on the Used Catalyst. Figures 7 and 8 show the same TEM field of view as for Figure 6 but with sulfur and nickel elemental mapping for the reacted Ni/Al2O3 and Ni/dolomite catalysts, respectively. For the Ni/Al2O3 catalyst, the high concentration of nickel for the metal particle is evident, but also associated with the nickel is sulfur, with only small amounts of sulfur associated with the alumina support. Figure 8, for the Ni/dolomite catalyst, shows that sulfur was found throughout the used catalyst and there was less direct association with the nickel metal. Lakhapatri and Abraham16 have also reported that, for the steam reforming of jet fuel using 2110

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Figure 9. TEM−EDXS analysis of reacted Ni/Al2O3 and Ni/dolomite catalysts: (a) TEM image of the reacted Ni/Al2O3 catalyst, (b) EDXS spectra at positions 1 and 2, (c) TEM image of the reacted Ni/dolomite catalyst, and (d) EDXS spectra at positions 1, 2, and 3.

was assigned to the presence of sulfur. Thus, sulfur was mainly present on the Ni surface, and no bulk sulfur was observed, as also suggested by Lakhapatri and Abraham.16,36 Panels c and d of Figure 9 show the TEM image and EDXS analysis with different chosen positions on the sample for the reacted Ni/ dolomite catalyst. From Figure 9d, no sulfur peaks were detected from EDXS analysis whether in the bulk or on the surface of the nickel particle in the case of the reacted Ni/ dolomite catalyst. It is suggested that the sulfur content on the Ni particles for the reacted Ni/dolomite catalyst was below the level that could be detected by TEM−EDXS analysis. XPS analysis was applied to the reacted nickel alumina and nickel dolomite catalysts to determine the type of sulfur and carbon species present (Figure 10). For the Ni/Al2O3 catalyst, the XPS peak of Ni 2p3/2 occurred at 852.9 eV, suggesting the formation of nickel sulfide. However, there was a much weaker XPS peak of nickel sulfide in the Ni/dolomite catalyst. Nickel sulfide has been proposed as the main poisoning species for Ni/ catalysts.16,37

Figure 10. Ni 2p XPS spectra for reacted Ni/Al2O3 and Ni/dolomite catalysts.

chosen positions on the sample. The results show that position 1, which represents the bulk composition of a Ni particle on the reacted Ni/Al2O3 catalyst, was predominantly Ni and showed no detectable peak for sulfur. Position 2, which represents the surface of the Ni particle, showed a clear peak at 230 eV that 2111

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Figures 2 and 3 showed that the hydrogen gas yield and potential hydrogen production were higher for the Ni/dolomite catalyst compared to the Ni/Al2O3 catalyst. Figures 7 and 8 suggest that nickel present in the Ni/Al2O3 catalyst becomes deactivated by sulfur but less so for the Ni/dolomite catalyst. Figure 6 also shows that there was increased carbon deposition associated with the nickel particles for the Ni/Al2O3 catalyst compared to the Ni/dolomite catalyst. The presence of sulfur at the nickel catalyst surface not only effects catalyst poisoning but also carbon deposition. The presence of sulfur in the catalytic process has been found to increase the carbon formation.38−40 For example, Delahay and Duprez38 studied the effect of sulfur on the coking of a Rh/ Al2O3 catalyst in the steam reforming of 1-methylnaphthalene containing sulfur. They found that the presence of sulfur compounds significantly increased the rate of catalyst coking. They suggested that increased carbon deposition was due to inhibition of the carbon−steam reaction by sulfur and via carbon deposition, resulting from the cracking of sulfurcontaining molecules. Ferrandon et al.39 investigated the autothermal reforming of gasoline doped with sulfur over the Rh/La−Al2O3 catalyst. They also reported that the presence of sulfur increases the rate of the carbon formation. Similar results were obtained by Xie et al.40 during the steam reforming of liquid hydrocarbons over the Rh−Ni/Al2O3 catalyst at a reaction temperature of 800 °C. They reported that the presence of sulfur can inhibit carbon gasification and enhance the formation of graphitic carbon on reforming catalysts as a result of the poisoning effect on metal sites. Overall, the work reported here suggests that Ni/dolomite catalysts show higher catalytic activity compared to Al2O3 catalysts in relation to hydrogen production from waste tires, in part because of the resistance to sulfur poisoning of nickel and consequent lower carbon deposition.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Libyan Ministry of Higher Education and Omar Al-Mukhtar University, Al Beida, Libya, for support for Ibrahim F. Elbaba.

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4. CONCLUSION Hydrogen production from waste tire by a catalytic steam pyrolysis−gasification/reforming process was investigated in a two-stage fixed-bed reactor over four cycles of use. The catalytic deactivation because of sulfur poisoning and carbon deposition on Ni/Al2O3 and Ni/dolomite catalysts was investigated. The results showed that the catalytic reactivity of the Ni/dolomite catalyst appeared to be more stable compared to that of the Ni/ Al2O3 catalyst after four cycles of use. For example, the theoretical potential H2 production decreased from 7.2 to 4.1 wt % for the Ni/Al2O3 catalyst but decreased from 11.4 to 10.1 wt % for the Ni/dolomite catalyst. Both the Ni/Al 2O 3 and Ni/dolomite catalysts were deactivated by sulfur poisoning. EDXS analysis of the used catalyst suggested that sulfur was mainly present on the Ni surface and no bulk sulfur was observed in the case of the reacted Ni/Al2O3 catalyst. However, no sulfur peaks were detected whether in the bulk metal or on the surface of the nickel particle in the case of the reacted Ni/dolomite catalyst. TPO of the used catalysts suggested that the carbon deposited on the surface of the Ni/Al2O3 catalyst was 18.2 wt %, while carbon deposited on the surface of the Ni/dolomite catalyst was 2.8 wt %. TEM−EDXS elemental mapping for carbon revealed that carbon was closely associated with the nickel particles of the used Ni/Al2O3 catalyst but less so for the used Ni/dolomite catalyst. 2112

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