Enhanced Ni-Al-Based Catalysts and Influence of Aromatic Hydrocarbon for Autothermal Reforming of Diesel Surrogate Fuel

Abstract

Aromatic hydrocarbons along with sulfur compounds in diesel fuel pose a significant threat to catalytic performances, due mainly to carbon deposition on the catalytic surface. In order to investigate the influence of an aromatic hydrocarbon on the autothermal reforming of diesel fuel, 1-methylnaphthalene (C₁₁H₁₀) was selected as an aromatic hydrocarbon. Two types of diesel surrogate fuel, i.e., DH (dodecane (C₁₂H₂₆) and hexadecane (C₁₆H₃₄) mixture) as well as DHM (DH fuel and C₁₁H₁₀ mixture) fuel, were prepared. A Rh-Al-based catalyst (R5A-I) was prepared using a conventional impregnation method. Various Ni-Al-based catalysts with Fe and Rh promoters were prepared via a polymer modified incipient method to improve the carbon coking resistance. These catalysts were tested under conditions of S/C = 1.17, O₂/C = 0.24, 750 °C, and GHSV = 12,000 h^-1 at DH or DHM fuel. R5A-I exhibited excellent catalytic performance in both DH and DHM fuels. However, carbon coking and sulfur poisoning resistance were observed in our previous study for the Ni-Al-based catalyst with the Fe promoter, which became deactivated with increasing reaction time at the DHM fuel. In the case of the Rh promoter addition to the Ni-Al-based catalysts, the catalytic performances decreased relatively slowly with increasing (from 1 wt.% (R1N50A) to 2 wt.% (R2N50A)) content of Rh₂O₃ at DHM fuel. The catalysts were analyzed via scanning electron microscopy combined with energy dispersive X-ray, X-ray diffraction, and X-ray photoelectron spectroscopy. Gas chromatography-mass spectrometry detected various types of hydrocarbons, e.g., ethylene (C₂H₄), with catalyst deactivation. The results revealed that, among the produced hydrocarbons, C₂H₄ played a major role in accelerating carbon deposition that blocks the reforming reaction. Therefore, Rh metal deserves consideration as a carbon coking inhibitor that prevents the negative effects of the C₂H₄ for autothermal reforming of diesel fuel in the presence of aromatic hydrocarbons.

1. Introduction

Compared with other energy sources, hydrogen energy represents a cleaner and more sustainable energy, and has become one of the clean fuel options for reducing motor vehicle emissions. Hydrogen can be produced from a mixture of clean coal, fossil fuels, and nuclear power [1]. A hydrogen energy system is a complex system that includes production, utilization, and storage parts, and hydrogen utilization is difficult, due to the lack of hydrogen-supply infrastructure. In particular, hydrogen storage in safe, inexpensive, and lightweight storage vessels has been accomplished [2,3]. Furthermore, auxiliary power unit (APU) systems have been considered as on-board fuel processors. Hydrocarbon fuels, such as gasoline and diesel fuel, are used to operate the engines of most vehicles. Hydrogen, which is attainable through a catalytic reforming process performed on hydrocarbon fuels, can be used to generate electrical energy for fuel cells [4,5,6]. Notably, solid oxide fuel cell (SOFC)-based APU systems that use diesel fuel are unique alternatives because this fuel takes advantage of the high hydrogen density and existing fuel infrastructure [7,8,9]. Various reforming methods, including steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR), may be used to generate hydrogen-rich gases for APU systems. A water gas shift reaction (WGSR) may occur in the presence of CO and excessive steam. The corresponding reforming reactions are given as follows:

(1)

SR: C_nH_m + nH₂O → (m/2+n)H₂ + nCO

(2)

POX: C_nH_m + (n/2)O₂ → (m/2)H₂ + nCO

(3)

ATR: C_nH_m + (n/2)H₂O + (n/4)O₂ → (m+n)/2H₂ + nCO

(4)

WGSR: CO + H₂O ↔ H₂ + CO₂

ATR combines endothermic SR with exothermic POX. In other words, hydrocarbon fuels react with air and steam, which generates hydrogen-rich gas. Compared with the conventional reforming methods, ATR as a potential reforming method exhibits higher energy efficiency and thermal stability [10,11,12,13,14,15,16,17]. The production technologies of green diesel have studied for combustion engines with remarkable properties and significantly lower emissions [18]. The complexity of a typical commercial diesel fuel composition prevents exact modeling of such compositions. Due to sulfur poisoning and carbon deposition, low aromatic and sulfur compounds in diesel fuels lead to significant deterioration of the catalytic performance [19,20,21]. Many catalysts have been prepared and tested for reforming hydrocarbon fuel. Methane, biogas, and glycerol were employed as feedstock for the production of syngas (CO + H₂) as well as gasoline and diesel fuel [22,23,24,25]. Additionally, the role of the oxide support in the catalytic performance of ATR performed on a gasoline surrogate fuel was determined by testing Rh catalysts with various supports (Gd-CeO₂, Y-ZrO₂, γ-Al₂O₃, La-Al₂O₃, and CaAl₁₂O₁₉) [26]. Ni-catalysts supported on modified alumina (-La, -Ba, and -Ce) were studied with the aim to improve the catalytic performance associated with steam reforming of diesel fuel [27]. Pt/CeO₂-Al₂O₃ was investigated for numerical simulation of diesel reforming [28]. Rh-Ni-based catalysts showed high activity and stability for methane reforming by the addition of Rh [29,30]. Jet fuel in the absence and presence of sulfur was conducted by Rh and Rh–Ni loaded on CeO₂-modified Al₂O₃ support for steam reforming [31]. Furthermore, novel-metal-based (RhPt/ZrO_2, Al₂O₃, CeO₂-ZrO₂, SiO₂, and TiO₂, Rh-Ni/Al₂O₃) catalysts were evaluated for reforming diesel fuel in the presence of sulfur compounds [32,33,34,35,36]. Rh-based catalysts were prepared via microemulsion and a zone-coating preparation method for diesel reforming [37,38] and also sorption-hydrolytic deposition conducted for highly dispersed Rh-, Pt-, and Ru/Ce_0.75Zr_0.25O₂–δ catalysts [39]. Rh/Ce_0.75Zr_0.25O_2-δ-ƞ-Al₂O₃/FeCrAl were tested in pilot scale reactors with autothermal reforming of n-hexadecane. The produced syngas can be supplied as a fuel for power generation units based on SOFC [40]. In our previous studies, a polymer modified incipient method (PM) was introduced as a timesaving and promising alternative preparation method to conventional preparation methods, such as impregnation or co-precipitation methods [41]. Moreover, sulfur-resistant Ni-Al-based catalysts with added Fe promoters were prepared by means of the PM method for autothermal reforming of dodecane [42]. The role of these catalysts in diesel surrogate fuel containing aromatic hydrocarbons has, however, remained unexplored. Therefore, in this study, Ni-Al-based catalysts were tested in the presence of aromatic hydrocarbon such as 1-methylnaphthalene (C₁₁H₁₀) and, ultimately, attempt to enhance catalytic performances. The ethylene (C₂H₄) was considered a major contributor to catalyst deactivation that results in rapid coke formation during the reforming process [43,44]. The produced hydrocarbons (e.g., C₂H₄) were detected via GC-MS to identify the hydrocarbon that plays the major role in this deactivation in the presence of aromatic hydrocarbon. Furthermore, the catalysts were characterized by means of field emission scanning electron microscopy (FE-SEM) in conjunction with energy dispersive X-ray (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).

2. Results and Discussion

2.1. Catalytic Performances in the Absence and Presence of Aromatic Hydrocarbon

The experiments were performed at S/C = 1.17, O₂/C = 0.24, 750 °C, and GHSV = 12,000 h^-1, for comparison with the catalytic performances realized in the presence of aromatic hydrocarbons such as 1-methylnaphthalene (C₁₁H₁₀). Figure 1 shows the conversions obtained for the blank test, R5A-I, N10A, and F10N10A at DH (dodecane (C₁₂H₂₆) and hexadecane (C₁₆H₃₄) mixture) and DHM (DH fuel and C₁₁H₁₀ mixture) fuels. A conversion of ~33% was obtained for the blank test, with/without C₁₁H₁₀ in the diesel surrogate fuel. As shown in Figure 1a, conversions of ~80% were obtained for 5-hour periods of each catalyst at the DH fuel. However, the catalytic performances varied with the addition of C₁₁H₁₀ to the diesel surrogate fuel (see Figure 1b). The catalytic performances of N10A and F10N10A deteriorated significantly within 1 hour and, thereafter, the conversions remained the same as that of the blank test, which suggests that the catalysts were completely deactivated. The addition of Fe as a promoter to Ni-Al-based catalysts enhances carbon coking and the sulfur poisoning resistance, as reported in our previous study [42]. However, this addition had a negative effect in the presence of the aromatic hydrocarbon. Although the conversion of R5A-I at DHM fuel was 10% lower than that at the DH fuel, the catalytic performance of R5A-I was maintained for 5 hours. This indicated that R5A-I including Rh metal acted as an excellent catalyst at the DHM fuel.

The selectivity of product gas (%) of the blank test, R5A-I, N10A, and F10N10A at DH and DHM fuel is shown in Figure 2. For the blank test and R5A-I, the percentage of H₂, CO, CH₄, and CO₂ produced in the absence and presence of C₁₁H₁₀ was almost the same. Additionally, at the DH fuel, selective percentages of H₂, CO, CH₄, and CO₂ produced by R5A-I was similar to those generated by N10A and F10N10A. Approximately 59%, 23%, 1%, and 17% of H₂, CO, CH₄, and CO₂ selectivity, respectively, were initially generated by deactivated catalysts such as N10A and F10N10A. The subsequent selectivity were nearly the same as those obtained for the blank test. In other words, selectivity of H₂ and CO₂ produced by N10A and F10N10A decreased with increasing time, while that of CO and CH₄ increased until these catalysts were completely deactivated. Therefore, compared with the presence of C₁₁H₁₀, reaction conditions, such as S/C, O₂/C, and temperature, played a greater role in selectivity of product gas (%), which is consistent with the findings of our previous study [45].

The hydrocarbons produced after reforming were separately analyzed via GC-MS (see

Table 1. Data obtained from the GC-MS of the blank test, R5A-I, N10A, and F10N10A after 30-minute and 60-minute reactions at the DHM fuel.

	Blank Test		R5A-I		N10A		F10N10A
Reaction Time (min)	30	60	30	60	30	60	30	60
Carbon dioxide (CO2)	8.2	8.2	98.3	98.2	39.5	21.2	55.7	25.8
Ethane (C2H6)	2.3	2.3	-	-	3.2	2.8	2.9	2.7
Ethylene (C2H4)	32.2	32.2	-	-	28.6	30.9	20.7	29.9
Propylene (C3H6)	16.4	16.4	-	-	13.1	15.9	7.8	14.7
1,3-Butadiene (C4H6)	9.4	9.4	-	-	4.9	8.1	3.0	7.7
1,3-Cyclopentadiene (C5H6)	4.5	4.5	-	-	0.9	2.9	0.4	2.6
Benzene (C6H6)	16.2	16.2	0.2	0.3	9.7	11.1	7.9	11.7
Toluene (C7H8)	4.2	4.2	-	-	-	2.4	1.1	2.1

for the blank test, R5A-I, N10A, and F10N10A data corresponding to 30-minute and 60-minute reactions at the DHM fuel). Small amounts of benzene (C₆H₆) were produced in the case of R5A-I, but various types of hydrocarbons, which resulted in catalyst deactivation, were detected in N10A and F10N10A. High fractions of ethylene (C₂H₄) formed in N10A and F10N10A after 60 minutes of reaction. The previously mentioned results suggested that the hydrocarbons produced after reforming had an adverse effect on the catalytic performance.

2.2. Catalyst Analysis before and after Reaction in the Absence and Presence of Aromatic Hydrocarbon

Catalyst deactivation by hydrocarbons production was verified via SEM-EDX, XRD, and XPS analysis. The morphology and amounts of carbon on the catalytic surface were determined by SEM-EDX analysis. High-magnification (10,000x) SEM images reveal the wire shape of carbon deposited on the surface of N10A and F10N10A after a 5-hour reaction at DH and DHM fuel (see Figure 3). Moreover, the amount of carbon coking at DH fuel was expected to be lower than that at the DHM fuel because the previously mentioned wire shape occurred more frequently at DHM fuel. The amounts of carbon on the catalytic surface of R5A-I, N10A, and F10N10A after 5-hour reactions at DH and DHM fuel are listed in

Table 2. Amounts of carbon on the catalytic surface of R5A-I, N10A, and F10N10A after 5-hour reactions at DH and DHM fuel.

	R5A-I		N10A		F10N10A
Fuel	DH	DHM	DH	DHM	DH	DHM
Carbon (wt.%)	21.59	20.89	19.96	47.21	20.78	45.08

. In the case of R5A-I, ~21 wt.% of carbon occurred on the surface irrespective of the diesel surrogate fuel considered. At the DHM fuel, 47.21 wt.% and 45.08 wt.% of carbon were deposited on the catalytic surface of N10A and F10N10A, respectively, and these amounts were more than twice those occurring at DH fuel. In our previous study, Ni-Al-based catalyst and Fe-Ni-Al-based catalyst at D fuel (C₁₂H₂₆) in the presence of DBT were 33.4 wt.% and 8.58 wt.%, respectively [42]. In particular, the amount of carbon increased more in the case of Fe-Ni-Al-based catalyst at DHM fuel, compared to those at D fuel in the presence of DBT and DH fuel. Thus, it was concluded that the addition of Fe to Ni-Al-based catalyst had no effect to prevent carbon deposition on the catalytic surface. Moreover, SEM-EDX analysis revealed that hydrocarbon production promotes carbon deposition on the catalytic surface.

The crystal structure of R5A-I, N10A, and F10N10A were characterized via X-ray diffraction (XRD) analysis (see Figure 4). Rh₂O₃, NiO, and Fe₂O₃ were undetected in the fresh state. The incorporation of Ni²⁺ ions into –Al₂O₃ (NiAl₂O₄) resulted in the lattice expansion of –Al₂O₃ and, hence, the diffraction peak of NiAl₂O₄ is shifted to a lower diffraction angle, i.e., 66.84°, compared with that of –Al₂O₃. The NiAl₂O₄ phase is inactive in the reforming reaction of hydrocarbons [45,46,47]. As shown in Figure 4b,c, the Ni and FeNi phase were observed after the H₂ pre-reduction treatment and 5-hour reaction at DH fuel. However, these phases were absent from the XRD pattern associated with the 5-hour reaction at DHM fuel. The XRD pattern of each catalyst was almost identical to those of the fresh state, which suggests that detection of active metals (e.g., Ni or FeNi) was difficult, due to carbon deposition on the catalytic surface.

The phases comprising R5A-I, N10A, and F10N10A were investigated further via XPS analysis (see Figure 5). Rh₂O₃, NiO, and Fe₂O₃ phases, which were undected by XRD, were observed in the fresh state. The XPS peaks corresponding to Rh3d5/2, Rh₂O₃, and Rh3d3/2 occurred at binding energies of 307.6 eV, 308.3 eV, and 312 eV (see Figure 5a), respectively [48]. The peaks of Rh3d5/2 and Rh3d3/2 appeared after the H₂ pre-reduction treatment and the 5-hour reaction at DH and DHM fuel. The Rh3d5/2 and Rh3d3/2 peaks appearing after this reaction were relatively sharper than those observed after the H₂ pre-reduction treatment. The peaks of Ni, NiO, and NiAl₂O₄ are associated with binding energies of 853 eV, 854 eV, and 856 eV, respectively [49], as shown in Figure 5b. However, the 5-hour reaction at DH and DHM fuels yielded indistinguishable Ni peaks. The peaks of FeNi and Fe₂O₃ occurred at binding energies of 710 eV and 711 eV (see Figure 5c), respectively [50]. The FeNi peak after the 5-hour reaction at DHM fuel was slightly smaller than that associated with the 5-hour reaction at DH fuel. The results indicated that the changes in the Ni and FeNi peaks result from carbon deposition on the catalytic surface during the reaction. Therefore, the produced hydrocarons such as C₂H₄ accelerated carbon deposition and inhibited the reforming reaction required for autothermal reforming of the diesel surrogate fuel.

2.3. Catalytic Performances of Ni-Al-Based Catalysts with Fe and Rh Promoters in the Presence of Aromatic Hydrocarbon

The resistance properties of aromatic hydrocarbon were investigated under the previously mentioned conditions. The catalytic performance of Ni-Al-based catalysts improved with increasing content of Ni metal (Supplementary Materials Figure S1). Research regarding the content of Ni metal for autothermal reforming of a diesel surrogate fuel will be presented in another study. Ni-Al-based catalysts with a NiO content of 50 wt.% were prepared and various amounts of Fe and Rh promoters were added to these catalysts to improve and prolong the catalytic performances of DHM fuel. The Ni-Al-based catalysts with different amounts (Fe₂O₃ 10 wt.%, 20 wt.%, and 30 wt.%) of Fe promoter were tested using DHM fuel. The Fe promoter improves carbon coking and the sulfur poisoning resistance for autothermal reforming of dodecane [42] and was, hence, expected to prevent carbon deposition in the presence of aromatic hydrocarbons. Figure 6a reveals initial conversions of 72%, 68%, 68%, and 55% for the N50A, F10N50A, F20N50A, and F30N50A, respectively, using DHM fuel. Contrary to expectations, these catalysts were deactivated with increasing reaction time. A conversion of ~33% was obtained for the blank test in Figure 1. This conversion was a similar value with those for the N50A, F10N50A, F20N50A, and F30N50A at higher times. This conversion was obtained from DHM fuel thermally decomposed to CO, CO₂, and CH₄ after catalyst deactivation. Furthermore, catalytic performances of DHM fuel deteriorated with the increasing amount of Fe promoter, i.e., this promoter yielded no improvement in the catalytic performance in the presence of aromatic hydrocarbons, such as C₁₁H₁₀. Small amounts (Rh₂O₃ 1 wt.% and 2 wt.%) of Rh promoter as alternatives to the Fe promoter were added to the Ni-Al-based catalysts to resolve the problem of catalyst deactivation. Figure 6b reveals an initial conversion of ~70% for the R5A-I, N50A, R1N50A, and R2N50A using DHM fuel. The conversion of R5A-I was maintained for 5 hours, but the catalytic performances of N50A, R1N50A, and R2N50A deteriorated with increasing reaction time. R1N50A and R2N50A were more slowly deactivated than N50A. The deactivation rate of R2N50A was moderately reduced by increasing the content of Rh promoter. The addition of Rh to Ni-Al-based catalysts improved the dispersion of Ni and retarded the sintering of Ni by the strong interaction between Rh and Ni [29,30,33]. Although metallic Rh is a costly metal, the Rh promoter deserves consideration as a promoter for Ni-Al-based catalysts. For improved catalytic performance, further studies are required to identify a suitable promoter that yields autothermal reforming of diesel fuel.

The basis for differences in the catalytic performance was investigated via GC-MS analysis of hydrocarbons such as C₂H₄.

Table 3. Data obtained from GC-MS of R5A-I, N50A, R1N50A, and R2N50A after 30-minute and 300-minute reactions of DHM fuel.

	R5A-I		N50A		R1N50A		R2N50A
Reaction Time (min)	30	300	30	300	30	300	30	300
Carbon dioxide (CO2)	98.3	75.5	94.1	23.8	98.7	38.7	98.7	45.6
Ethane (C2H6)	-	0.5	-	2.8	-	2.7	-	2.4
Ethylene (C2H4)	-	0.3	-	23.8	-	18.2	-	16.5
Propylene (C3H6)	-	-	-	12.2	-	8.8	-	7.9
1,3-Butadiene (C4H6)	-	-	-	5.9	-	4.3	-	4.1
1,3-Cyclopentadiene (C5H6)	-	-	-	3.5	-	2.7	-	2.7
Benzene (C6H6)	0.2	23.7	-	17.9	-	16.6	-	14.1
Toluene (C7H8)	-	-	-	4.8	-	4.1	-	3.2

shows the data corresponding to R5A-I, N50A, R1N50A, and R2N50A after the reaction for 30-minute and 300-minute using DHM fuel. The amount of hydrocarbons increased, in general, with increasing reaction time, whereas the amount of CO₂ decreased. At fractions of 23.8%, 18.2%, and 16.5%, C₂H₄ accounted for the highest proportion of produced hydrocarbons in the case of N50A, R1N50A, and R2N50A, respectively, which reacted for 300 minutes. C₆H₆ accounted for the second-highest fraction of produced hydrocarbons. Neverthless, C₆H₆ had a negligible effect on catalyst deactivation, since R5A-I exhibited excellent catalytic performance, despite the high fraction of this hydrocarbon. As previously mentioned, we assumed that the produced hydrocarbons were associated with carbon deposition on the catalytic surface. The results indicated that, compared with the other hydrocarbons, the produced C₂H₄ plays a crucial role in catalyst deactivation, which is consistent with the results of previous studies [43,44]. More importantly, Rh metal seemed to play an important role in reducing C₂H₄ production, which accelerates carbon deposition.

2.4. Analysis of Rh-Ni-Al Catalyst Before and After Reaction in the Presence of Aromatic Hydrocarbon

The morphology and amounts of carbon on the catalytic surface were observed via SEM-EDX analysis. High-magnification (10,000x) SEM images are shown in Figure 7. Wire-shaped carbon occurred in N50A, but classification of the carbon morphology occurring in R5A-I, R1N50A, and R2N50A was difficult.

Table 4. Amount of carbon on the catalytic surface of R5A-I, N50A, R1N50A, and R2N50A after 5-hour reactions of DHM fuel.

	R5A-I	N50A	R1N50A	R2N50A
Carbon (wt.%)	20.89	40.59	22.05	21.21

lists the amount of carbon formed on each catalytic surface after 5-hour reactions as DHM fuel. As shown, 20.89 wt.%, 40.59 wt.%, 22.05 wt.%, and 21.21 wt.% of carbon were formed on R5A-I, N50A, R1N50A, and R2N50A, respectively. The amount of carbon in N50A was about two times higher than that of the other catalysts containing Rh metal. Rh metal promoted the gasification or reforming of adsorbed carbon species on Rh-Ni-Al-based catalysts during steam reforming of sulfur-doped n-hexadecane [33]. Therefore, this metal was helpful in resisting deposition of carbon from the C₂H₄ product onto the catalytic surface.

3. Materials and Methods

3.1. Catalyst Preparation

In our previous studies, various Ni-Al-based catalysts with Fe and Rh promoters were prepared by means of a polymer modified incipient method, to demonstrate the potential of this catalyst preparation method for hydrocarbon fuel reforming [41,42]. The following nomenclature was used for the catalysts: The letters ‘N’, ‘R’, ‘F’, and ‘A’ represent Ni, Rh, Fe, and Al metal, respectively. In addition, the figures revealed the content (wt.%) of metal oxide. During the catalyst preparation procedure, known amounts of Ni(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O were dissolved in ethylene glycol (15 mL) and methyl alcohol (15 mL). Fe(NO₃)₃·9H₂O and Rh(NO₃)₃·xH₂O were added as the precursors for Fe and Rh metal. The solid products were obtained by heating the solutions with PMMA colloidal crystals (1.5 g) on a hot plate at 100 °C and drying at 300 °C for 4 hours. Afterward, the products were calcined at 800 °C for 4 hours. An Rh-Al-based catalyst (R5A-I) was prepared using a conventional impregnation method, and the catalytic performance of this catalyst was compared with that of the other catalysts. Rh(NO₃)₃·xH₂O and γ-Al₂O₃ were dissolved in water. The resulting solution was stirred at room temperature and the suspension was then evaporated via vacuum evaporation. Afterward, the powder was dried at 300 °C for 4 hours and then calcined at 800 °C for 4 hours as per the above conditions.

3.2. Catalytic Reforming Tests

The catalysts (mass: 0.5 g, size: 150 to 250 μm) were supported by quartz wool on either side and packed in the middle of a fixed-bed quartz reactor. The quartz reactor was generally used to minimize coke formation on the inner walls. Thermocouple was placed at the center of the catalytic bed and controlled the temperature of the electric furnace. In addition, the temperature was monitored in the inlet and outlet of the reactor. The temperature gradient between the inlet and outlet of the reactor and the catalytic bed was negligible. The mass flow controllers were used to adjust the flow rate of gases such as H₂, N₂, and O₂. Enough gas was introduced and the line was heated to prevent condensation. The diesel surrogate fuel was prepared from dodecane (C₁₂H₂₆), hexadecane (C₁₆H₃₄), and 1-methylnaphthalene (C₁₁H₁₀). This fuel varied depending on whether aromatic hydrocarbons (e.g., C₁₁H₁₀) were contained in diesel surrogate fuels such as DH (C₁₂H₂₆ and C₁₆H₃₄ mixture) and DHM (DH fuel and C₁₁H₁₀ mixture) fuel. The selected diesel surrogate fuel and H₂O were injected by syringe pumps and completely vaporized with N₂ at 400 °C in the vaporizer. Afterward, the condensable liquids were completely removed before injection into gas chromatograph (GC, Agilent 6890, Santa Clara, CA, USA). Product gases such as H₂, N₂, CO, CH₄, and CO₂ were then analyzed via GC-TCD (thermal conductivity detector). Supplementary Materials Figure S2 shows the schematic diagram of experimental apparatus. In addition, CO₂ and hydrocarbons, such as ethane (C₂H₆), ethylene (C₂H₄), propylene (C₃H₆), 1,3-butadiene (C₄H₆), 1,3-cyclopentadiene (C₅H₆), benzene (C₆H₆), and toluene (C₇H₈), were detected via GC-MS (Agilent (7890B-5977B GC/MSD), Santa Clara, CA, USA). The H₂ pre-reduction treatments were conducted for 2 hours at 750 °C prior to the tests. The catalytic performance of each catalyst was investigated under the following conditions: S/C = 1.17, O₂/C = 0.24, 750 °C, and GHSV = 12,000 h^-1 at DH or DHM fuel. The diesel surrogate fuel conversions and the selectivity of product gases were calculated as follows.

$$\mathrm{Conversion} (\%)=\left(\frac{{\mathrm{F}}_{CO·Out}+{\mathrm{F}}_{C{O}_2·Out}+{\mathrm{F}}_{C{H}_4· Out}}{{\mathrm{F}}_{C_{12}{H}_{26}·In}\times 12+{\mathrm{F}}_{C_{16}{H}_{34}·In}\times 16+{\mathrm{F}}_{C_{11}{H}_{10}·In}\times 11}\right)\times 100$$

(1)

$$\mathrm{Selectivity} \mathrm{of} \mathrm{product} \mathrm{gas} (\%)=\left(\frac{{\mathrm{F}}_{product·Out}}{{\mathrm{F}}_{H_2·Out}+{\mathrm{F}}_{CO·Out}+{\mathrm{F}}_{C{H}_4·Out}+{\mathrm{F}}_{C{O}_2·Out}}\right)\times 100$$

(2)

3.3. Characterization

The morphology and the amounts of carbon on the catalytic surface were determined via FE-SEM (S-4300, Hitachi, Tokyo, Japan) and EDX (EDX-350, Hitachi, Tokyo, Japan), respectively. The crystal structure of each catalyst was determined via XRD using a CuKα radiation source (Phillips XPERT instrument, Korea Basic Science Institute, Daegu). Furthermore, the phases comprising the mixed metal oxides were identified via XPS (Quantera SXM, ULVAC-PHI, Chigasaki, Japan).

4. Conclusions

To investigate the effect of aromatic hydrocarbon on the autothermal reforming of diesel, 1-methylnaphthalene (C₁₁H₁₀) was selected and blended as an aromatic hydrocarbon. Two types of diesel surrogate fuel, i.e., DH (C₁₂H₂₆ and C₁₆H₃₄ mixture) and DHM (DH fuel and C₁₁H₁₀ mixture) fuels, were prepared. The R5A-I, Ni-Al-based catalyst, and Fe-Ni-Al-based catalyst maintained catalytic performances for 5 hours using DH fuel, whereas the catalysts except for R5A-I became deactivated at DHM fuel. Unsurprisingly, the Fe promoter of the Ni-Al-based catalysts yielded no improvement in the catalytic performance at DHM fuel. When an Rh metal was added to these catalysts, the resulting catalytic performance was superior to that achieved for the other catalysts. The extent of catalyst deactivation decreased with greater content of this metal as a promoter. Although expensive, the Rh metal acted as a remarkable promoter that improved the resistance property of aromatic carbon for autothermal reforming of diesel fuel. The GC-MS data revealed that, among the produced hydrocarbons, C₂H₄ played a major role in deactivating the catalysts, since this hydrocarbon resulted in carbon deposition that covered the Ni metal active sites. In conclusion, the catalytic performance of Ni-Al-based catalysts, which are susceptible to accelerated carbon deposition from the C₂H₄ product, can be improved through the addition of small amounts of Rh metal.