Low-Temperature Catalytic Performance of Ni-Cu/Al₂O₃ Catalysts for Gasoline Reforming to Produce Hydrogen Applied in Spark Ignition Engines

Abstract

The performance of Ni-Cu/Al₂O₃ catalysts for steam reforming (SR) of gasoline to produce a hydrogen-rich gas mixture applied in a spark ignition (SI) engine was investigated at relatively low temperature. The structural and morphological features and catalysis activity were observed by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and temperature programmed reduction (TPR). The results showed that the addition of copper improved the dispersion of nickel and therefore facilitated the reduction of Ni at low temperature. The highest hydrogen selectivity of 70.6% is observed over the Ni-Cu/Al₂O₃ catalysts at a steam/carbon ratio of 0.9. With Cu promotion, a gasoline conversion of 42.6% can be achieved at 550 °C, while with both Mo and Ce promotion, the gasoline conversions were 31.7% and 28.3%, respectively, higher than with the conventional Ni catalyst. On the other hand, initial durability testing showed that the conversion of gasoline over Ni-Cu/Al₂O₃ catalysts slightly decreased after 30 h reaction time.

1. Introduction

Hydrogen is an emerging alternative used increasingly to replace the depletion of conventional fossil fuels—it has long been recognized as a fuel having some highly desirable properties for application in engines [1]. These features make hydrogen an excellent fuel to potentially meet the increasingly stringent targets for exhaust emissions from combustion devices, including the reduction of greenhouse gas emissions. Hydrogen can be used in internal combustion engines (ICE) on its own or as a partial substitute for traditional ICE fuels to improve performance and emissions, as has been reported by many researchers [2,3,4,5,6,7,8,9,10,11].

The combustion speed of the mixture of hydrogen and air is much higher than other petroleum mixtures. Burning its mixture with air requires ignition by a spark. However, the spark needs only a minuscule amount of energy to ignite this mixture [12]. A hydrogen engine is easy to start in a cold winter because hydrogen remains in a gaseous state until it reaches the low temperature of −253 °C [13]. Its combustion products are clean, consisting of water and a tiny amount of NO_x [14].

Hydrogen-rich gaseous fuel was produced by exhaust gas assisted reforming of gasoline or diesel fuel with steam (on-board production) [8,15]. This technique can potentially provide a feasible and practical engine system. It does not require a secondary fueling system, which is undesirable for the driver/operator.

Hydrogen-rich gaseous fuel involves hydrogen generation by direct catalytic interaction of liquid fuels with steam (steam reforming). Krumplet et al. reported the activity of different transition metals (Fe, Cu, Co, Ag, Ru, Ni, Pt, Pd, Rh) for converting liquid fuels into hydrogen-rich products; all metals exhibited 100% conversion above 700 °C [16,17,18,19]. They also reported that below 600 °C, conversion drops more quickly for first-row transition metals (particularly Ni and Co) than for second-row (Ru) and third-row (Pt, Pd) ones. The second- and third-row transition metals exhibit higher H₂ selectivity (>60%) than the first-row transition metals at temperatures above 650 °C [16]. Nickel has been widely used as a catalyst for steam reforming of gasoline fuels because of its high electronic conductivity, thermal stability, activity, and low cost [20]. However, it showed low conversion of gasoline fuels at low temperatures and was easily deactivated at high temperatures [21,22,23]. Addition of a second metal (Co, Mo, W, Re, Pd) to the Ni-alumina catalyst also resulted in better activity at much lower operating temperatures [21,22,23,24,25,26]. The activity, durability, and hydrogen selectivity of nickel catalysts doped with a thermal stabilizer and activity promoter, such as lanthanum and cerium oxide, respectively, were reported [27]; a lower carbon deposition and a higher thermal stability of metallic Ni particles under the reaction conditions were also observed [27]. Ni catalysts containing Cu have already been found to have significantly different catalytic activity and yield distribution compared to monometallic Ni catalysts in many reactions [28,29,30,31,32]. For example, the addition of Cu to Ni enhanced the catalytic effect of steam reforming of dimethyl at low temperatures [28]. It is also reported that adding Cu to Ni in the ethanol steam reforming reaction not only reduced CO generation, but also improved resistance to coke formation on the Ni catalysts [29]. It has been suggested that changes in the catalytic properties induced by Cu addition to Ni can be caused by changes in the electronic and/or geometric properties of the homogeneous bimetallic particles [15]. Huang et al. reported the WGS (water gas shift) reaction during steam reforming of methane over supported Cu-Ni catalysts and showed that the addition of Cu to Ni catalysts enhanced the WGS activity [16]. The presence of Cu in Ni catalysts may suppress methane steam reforming activity and thus enhance the extent of the WGS reaction in methane steam reforming [16]. In addition, Li et al. also reported that the Cu- and Ni-loaded ceria catalyst retained high activity in the WGS reaction [20]. However, the Ni-Cu catalysts for steam reforming of gasoline fuels to produce hydrogen have not been investigated. Therefore, the aim of this study was to investigate the catalytic performance of Ni-Cu/γ-Al₂O₃ for steam reforming of gasoline to produce hydrogen at relatively lower temperatures, which is suitable for the exhaust gases of spark ignition (SI) engines.

2. Results and Discussion

2.1. Textural, Structural, and Morphological Characterizations

2.1.1. Textural Characterizations

Table 1. Surface area and pore structure of different catalysts.

Sample	SBET (m2/g)	Total Pore Volume (cm3/g)	Average Pore Diameter (nm)
18 wt. % Ni/γ-Al2O3	152.8	0.22	2.2
18wt. %Ni0.9-Cu0.1/γ-Al2O3	153.1	0.22	2.3
18wt. %Ni0.7-Cu0.3/γ-Al2O3	154.4	0.23	3.1
18wt. %Ni0.5-Cu0.5/γ-Al2O3	156.8	0.24	4.6
18wt. %Ni0.3-Cu0.7/γ-Al2O3	160.4	0.28	5.1
18 wt. % Cu/γ-Al2O3	165.9	0.29	5.8
18 wt. % Ni0.5-Mo0.5/γ-Al2O3	154.7	0.23	3.1
18 wt. % Ni0.5-Ce0.5/γ-Al2O3	171.4	0.31	6.6
6wt. %Ni0.5-Cu0.5/γ-Al2O3	197.2	0.35	7.4
36 wt. % Ni0.5-Cu0.5/γ-Al2O3	105.4	0.17	1.6

shows the surface and pore properties of different catalysts. The specific surface area, pore volume, and average pore diameter of (Ni_0.5-Cu_0.5)_x/(Al₂O₃)_1−x catalysts (x = 6 wt. %, 18 wt. %, and 36 wt. %) decreased with the increase of Ni-Cu loading; the reason could be the depositions of the active components in the pores and the micropores of γ-Al₂O₃. The surface area (152.8 m²/g) and total pore volume (0.22 cm³/g) of 18 wt. % Ni/γ-Al₂O₃ catalyst demonstrated progressive substitution of Ni by Cu of 18 wt. % Ni_1−x-Cu_x /Al₂O₃ catalysts; this results in a slight increase of the BET area and total pore volume, which may be caused by the different CuO and NiO crystallite sizes and formation of mixed oxides. In addition, the surface area and pore structure of different catalysts are also shown in Table 1.

2.1.2. Structural Characterizations

Figure 1 shows the X-ray diffractometry (XRD patterns of Ni_0.5-Cu_0.5/Al₂O₃ catalysts (with varying Ni_0.5-Cu_0.5/Al₂O₃ ratios). The γ-Al₂O₃ peaks were observed with other ratios; the intensity of the γ-Al₂O₃ peaks slightly decreases, indicating that CuO and NiO particles could be highly dispersed in γ-Al₂O₃ [29,33]. The reflection peak intensities of the CuO and NiO phase occurred and became sharper with the increase of Ni-Cu loading, while no appearance of other phases was detected (Figure 1c,d).

Figure 2 showed XRD patterns of 18 wt. % Ni-Cu/Al₂O₃ catalysts with various Cu ratios. For pure CuO/Al₂O₃ (Figure 2a), only the phases of CuO were observed. With the decrease of CuO loading, the reflection peak intensities of the CuO phase were largely reduced with peak broadening (Figure 2a–c). At 30 mol % CuO ratio, only the cubic NiO phase was observed (Figure 2d). On further reducing the CuO contents (Figure 2e,f), the emerged intensity of the cubic NiO phase was gradually increased, while no reflection peaks of CuO were observed.

XRD patterns of 18 wt. % different catalysts are shown in Figure 3. The reflection of CuO and cubic CeO₂ are evidently observed (Figure 3a,b), while no separate reflections from Ni are detected for all samples. This provided further evidence that nickel promoted the dispersion of Cu and Ce [28]. It is surprising to note that the reflections of NiMo₄ were observed over 18 wt. % Ni_0.5-Mo_0.5/Al₂O₃ catalysts. The presence of the NiMo₄ phase was possibly due to the preparations of Ni-Mo catalysts [34,35].

2.1.3. Morphology Characterizations

Figure 4 shows a scanning electron microscopy (SEM) micrograph of Ni_0.5-Cu_0.5/Al₂O₃ catalysts with various Ni-Cu content ratios; high distributions of CuO-NiO nanoparticles on large spongy clusters of γ-Al₂O₃ were observed for all the samples. Ni-Cu nanoparticles of 10 nm were observed on the 6 wt. % Ni_0.5-Cu_0.5/Al₂O₃ catalysts (Figure 4a). With the increase of Ni-Cu loading, an increase in the particle size of Ni-Cu was clearly observed (Figure 4b,c) where aggregations of packed particles from a few nanometers to few hundred nanometers in size exist, which may cause agglomeration of powders, indicating sintering of the catalysts.

Figure 5 shows the corresponding SEM micrograph and energy dispersive X-ray (EDX) mapping of Ni_0.5-Cu_0.5/Al₂O₃ catalysts; it clearly shows the presence of Ni, Cu, and Al elements. In addition, the element mapping revealed a uniform distribution for Ni, Cu, and Al.

2.2. Temperature Program Reduction (TPR)

Figure 6 shows the TPR profiles of 18 wt. % Ni-Cu/γ-Al₂O₃ catalysts with various Cu ratios. For the Ni/γ-Al₂O₃ catalysts (Figure 6a), the TPR pattern shows that the broad reduction signal appears from 400 °C, which can be attributed to the reduction of NiO particles [36,37]; it recognizes that with the Ni/γ-Al₂O₃ catalysts reduced at 400 °C, there might be a reduction of dispersed Ni²⁺ and a more intense broad reduction corresponding to bulk NiO species [38]. At lower Cu ratios, only two apparent peaks were observed (Figure 6b,c): A low-temperature peak between 200 and 270 °C, and a high-temperature peak between 580 and 620 °C. With the increase of Cu ratios, the H₂-TPR profile patterns showed three reduction peaks (Figure 6d,e); one more low temperature peak between 260 and 380 °C was detected. The intensity of the low-temperature peaks increased with increasing Cu content; while the high-temperature peak shifted to a lower temperature, the range of reduction temperatures was wider. For pure Cu/γ-Al₂O₃ catalysts, the reduction is characterized by rather-combined two peaks in the range of 200–400 °C (Figure 6f).

Fierro et al. [39] reported that the TPR characteristics could be affected by mass transfer limitations and experimental operating variables such as the initial amount of reducible species, the initial H₂ concentration, the total gas flow rate, the heating rate, and the activation energy of the reaction. They claimed that desorption of H₂ attached on the reduced Cu metal surface could exhibit the apparent double-peak behavior, which could be affected by water vapor produced by the reduction process, and that the H₂-TPR profile of CuO depends on the particle size and surface area, where the peak top is higher by 288 °C for particle sizes of 425–850 microns compared with <100 microns [39]. A similar tendency but with a much larger peak was also reported by Luo et al. [40], who claimed that the hydrogen spillover effect was the reason for the difference between CO-TPR and H₂-TPR.

Kim et al. [41] reported that there is an incubation period prior to reduction, which is longer at lower temperatures. The tendency was in agreement with the general theory of nucleation and growth, where the number of newly-formed nuclei was copious at lower temperatures but since the growth was very much limited at lower temperatures, the phase existence was not easily detected by X-ray diffractometry. They claimed that CuO reduction was generally easier than Cu₂O reduction with H₂-TPR and that the apparent activation energy for Cu₂O was close to twice that of CuO, but when the H₂ flow rate was not high enough to avoid the rate-limiting step of the reduction process, a sequential reduction process such as CuO → (Cu₄O₃→) Cu₂O → Cu could occur.

Hierl et al. [42] reported that the lowering of the reduction temperature was partially caused by the effect of Ni on the segregation of Cu from the surface in preference of Ni to occupy subsurface or bulk coordination sites. The enrichment of Cu may be enhanced by an increase in the nickel content. On the other hand, the reduction of CuO species could accelerate the reduction of NiO species due to the formation of atomic hydrogen in the environment [43]. Xiaolei Wang et al. [28] reported the interaction between CuO and NiO as well as their occupancy on surface acidic sites of γ-Al₂O₃ reducing the interaction between metals and support, leading to lowering of the reduction temperature of CuO and NiO.

Consequently, the peak for pure CuO reduction in Figure 6f can be mainly attributable to direct reduction of CuO particles to Cu, contributing a low-temperature peak. The extended TPR peak profile indicates the difficulty of the reduction due to a longer diffusion path for H atoms or H₂ molecules as well as the escape of the produced H₂O molecules. TPR profile analysis of 18 wt. % Ni-Cu/γ-Al₂O₃ catalysts at small Cu ratios shows a low-temperature peak (Figure 6b,c) that corresponds to the reduction of CuO particles to Cu. The high-temperature peak corresponding to the reduction of NiO interacted with alumina is also observed in the Ni-Al profile. With increase of Cu content, the first peak at low temperature between 200 and 270 °C could be due to the reduction of CuO particles while the second peak between 260 and 380 °C could correspond to the reduction of CuO bulk [44]. The range of reduction temperatures is wider, which could possibly be due to agglomeration of the CuO bulk to form larger crystallites [45]. Furthermore, the high-temperature peak shifted to a lower temperature with an increase in the Cu content, which is due to the added Cu that produces spillover hydrogen, which considerably accelerates the nucleation of the Ni metal under these reduction conditions, and enhances the reducibility of Ni [46]. This suggests that Cu could enhance the reducibility of the dispersed NiO species, resulting in the shift to lower temperatures.

Figure 7 shows the TPR profiles of different catalysts. In the case of the 18 wt. % Ni_0.5-Mo_0.5/γ-Al₂O₃ catalysts (Figure 7a), a reduction peak at about 532 °C was observed and another peak could be assigned at 700 °C. It was reported that the presence of a Mo species impeded the incorporation of the Ni species into the lattice of alumina, thus preventing the growth of Ni particles, leading to less lattice expansion of alumina and highly dispersed Ni-Mo catalysts. This resulted in a weak metal-support interaction and, consequently, improved the reducibility of the catalysts [34,35,47]. Thus we assigned the peak at 532 °C to the reduction of NiMoO₄ (the XRD data showed a NiMoO₄ peak); the latter peak could be assigned to the reduction of NiO interacting with MoO₃. This result is consistent with a previous report [34]. Figure 7b shows the TPR of 18 wt. % Ni_0.5-Ce_0.5/γ-Al₂O₃ catalysts. It shows that a peak at about 535 °C is observed and it has been reported that the presence of Ce improves Ni dispersion, thus promoting the reduction of nickel species [48,49,50]. Incidentally, the reduction peak could be assigned to the strong interaction between NiO and CeO₂. For Ni_0.5-Cu_0.5/Al₂O₃ catalysts with varying Ni_0.5-Cu_0.5/Al₂O₃ ratios (Figure 7c–f), the three reduction peaks were clearly observed at 36 wt. % Ni_0.5-Cu_0.5/Al₂O₃ catalysts; the intensity of the reduction peaks decreased with no change to the position of peaks due to decreasing the Ni-Cu content. A summary of the TPR results including the temperature peak at maximum H₂ consumption and estimated quantity H₂ consumption during the catalysts reduction is also presented in

Table 2. Summary of H₂-Temperature program reduction (TPR) of different catalysts.

Sample	Temperature (°C)				H2 Consumption (mmol·g−1)
	T°C (Initial)	Peak I	Peak II	Peak III
18 wt. % Ni/γ-Al2O3	400	-	-	600	1.67
18 wt. % Ni0.9-Cu0.1/γ-Al2O3	208	260	-	586	2.23
18 wt. % Ni0.7-Cu0.3/γ-Al2O3	220	262	-	568	3.31
18 wt. % Ni0.5-Cu0.5/γ-Al2O3	185	258	295	514	4.75
18 wt. % Ni0.3-Cu0.7/γ-Al2O3	180	258	300	492	4.83
18 wt. % Cu/γ-Al2O3	185	295	-	474	4.39
18 wt. % Ni0.5-Mo0.5/γ-Al2O3	425	-	532	-	3.68
18 wt. % Ni0.5-Ce0.5/γ-Al2O3	350	-	535	-	3.73
6 wt. % Ni0.5-Cu0.5/γ-Al2O3	210	262	316	-	2.84
36 wt. % Ni0.5-Cu0.5/γ-Al2O3	205	258	312	514	3.81

. The amount of H₂ consumption during the reduction of Ni-Cu/Al₂O₃ catalysis is higher than that for Ni-Mo/Al₂O₃ and Ni-Ce/Al₂O₃ catalysis and with a lower temperature peak.

2.3. Catalytic Activity

2.3.1. Effect of Steam/Carbon Molar Ratios

Figure 8 shows the effect of steam/carbon (S/C) molar ratios on product selectivity over 18 wt. % Ni_0.5-Cu_0.5/γ-Al₂O₃ catalysts at 550 °C. At a small S/C molar ratio of 0.3, less H₂ was formed, with selectivity in favor of CH₄ and low formation of CO₂ and CO was observed. When the ratio of S/C increased from 0.3 to 0.9, H₂ selectivity gradually increased and the best H₂ selectivity was obtained with a 0.9 ratio of S/C, the formation of CH₄ decreased while the formation of CO and CO₂ did not change much. When the S/C ratio increased over 0.9, H₂ production decreased and the formation of CO₂ increased slightly. This suggests that the formation of products derived from thermal decomposition, WGS, and catalytic cracking reactions is favored at stoichiometric reaction conditions due to the limited amount of steam for reforming reactions [44,45,46]. The 0.9 ratio of S/C is used for the rest of the Ni-Cu/γ-Al₂O₃ catalysts.

2.3.2. Effect of Temperature on Reformation Composition

Steam reforming of iso-octane is a complex process including several gas phase and chemical reactions, such as pyrolysis, oxidation, reforming, methanation, and WGS reactions [46]. Savage et al. [51] reported that hydrogen selectivity occurs through multiple pyrolysis reactions, as hemolytic dissociation of the C–C bond, radical recombination, beta scission, isomerization, and hydrogen abstraction. The product selectivity is determined by the strength of the C–H and C–C bonds in the reactant molecules; an alternative approach for H₂ selectivity through steam reforming (SR) of iso-octane was proposed by Kopasz et al. [16]. iso-Octane is initially broken down by thermolysis into lighter hydrocarbons, which are reformed by steam on the catalytic surface. The following simplified reaction scheme has been proposed and includes reforming/decomposition reactions and secondary chemical steps involving the different intermediate products [45].

C₈H₁₈ + 8H₂O → 8CO + 17H₂   ∆H⁰ = 1310 kJ/mol

(1)

C₈H₁₈ + 16H₂O → 8CO₂ + 25H₂  ∆H⁰ = 1684 kJ/mol

(2)

C₈H₁₈ → 8C + 9H₂     ∆H⁰ = 391 kJ/mol

(3)

C₈H₁₈ + 8CO₂ → 16CO + 9H₂   ∆H⁰ = 1639 kJ/mol

(4)

CO + H₂O → CO₂ + H₂    ∆H⁰ = –41 kJ/mol

(5)

CO + 3H₂ → CH₄ + H₂O    ∆H⁰ = –206 kJ/mol

(6)

CO₂ + 4H₂ → CH₄ + 2H₂O   ∆H⁰ = –165 kJ/mol

(7)

C + 2H₂ → CH₄      ∆H⁰ = –75 kJ/mol

(8)

CO + C → CO₂      ∆H⁰ = –172 kJ/mol

(9)

C + H₂O → CO + H₂     ∆H⁰ = 131 kJ/mol

(10)

Figure 9 shows the effects of temperature on conversion and product selectivity over 18 wt. % Ni_0.5-Cu_0.5/γ-Al₂O₃ catalyst at a 0.9 ratio of S/C. Steam reforming of iso-octane results mainly in H₂, CO, CO₂, and CH₄. As the reforming temperature increases from 500 to 750 °C, the CO₂ concentration decreases slightly from 15.1% to 9.4%; the CO concentration remained almost constant (7%), while a strong increase in the CH₄ concentration from 6.9% to 21.2% was obtained. The concentration of H₂ dropped from the highest concentration of 71% to 61.8% due to the reforming temperature increasing from 500 to 750 °C. This indicates that the methanation reactions between the carbonaceous compounds (C, CO, CO₂) and H₂ are favored at high temperatures (reaction 10, 11, and 12). Therefore, a decrease in H₂ was observed at high temperatures, accompanied by an increase in CH₄ concentration. It should also be noted that the CO concentration was almost stable at high temperatures, probably implying a balance between the CO production and CO consumption (reactions 5, 8–10, and 13–14). A similar result on product selectivity was also observed by Ming [52]. The conversion of iso-octane is also shown in Figure 9. At 500 °C, only 38.1% of iso-octane was converted to gas compositions, while nearly 100% conversion was observed at 750 °C. As the reaction temperature increased, the energies for steam reforming (reactions 5 and 6), cracking (reaction 7), and dry reforming reactions (reaction 8) were promoted, therefore conversion performance was improved.

2.3.3. Effect of Ni-Cu Loading on γ-Al₂O₃ Support

Figure 10 shows the effects of conversion and product selectivity when Ni_0.5-Cu_0.5 increased from 6 wt. % to 36 wt. %. The results show that the iso-octane conversion increased with increasing Ni-Cu content. It is a fact that with the increase of Ni-Cu loading, a higher interaction between CuO and NiO as well as their occupancy of surface sites of γ-Al₂O₃ were seen, therefore enhancing the iso-octane conversion [28]. The product selectivity were not dependent upon the metal loading; the results showed that product selectivity was almost identical for 6 wt. %, 18 wt. %, and 36 wt. % Ni-Cu loading (Figure 10).

Figure 11 shows the effect of the Cu molar ratios on conversion and product selectivity over 18 wt. % Ni_x-Cu_1−x/Al₂O₃ (x = 1, 0.7, 0.5, 0.3, 0) catalysts at the low temperature reaction of 550 °C. For the 18 wt. % Ni/γ-Al₂O₃ catalysts, it exhibited poor catalytic activity with a maximum iso-octane conversion of 20.6%. It was also observed that iso-octane conversion increased from 20.6% to 42.6% with increasing Cu loading up to 0.5 molar, while H₂ and CH₄ selectivity reached values of 70.5% and 6.7%, respectively; CO and CO₂ selectivity decreased slightly. This fact was probably caused by Cu, which enhanced the reducibility of the dispersed Ni species, and/or by Ni, which favored the segregation of Cu²⁺ ions to the surface [28,53]. This is in agreement with the analysis of XRD and H₂-TPR; in this way the iso-octane conversion was enhanced. It seems that a catalyst containing about 0.5 molar of Cu is suitable for steam reforming reactions (reactions 5–8); as a result, high hydrogen concentration is observed. At higher Cu loadings of 0.5 molar, iso-octane conversion and H₂ selectivity decreased while higher CH₄ formation was favored. The phenomenon could be caused by agglomeration of bulk Cu particles that were dispersed on the Ni surface, therefore inhibiting the reducibility of Ni. This indicated that the methanation reaction (reactions 10–12) should take preference at higher Cu loadings of 0.5 molar [54]. For 18 wt. % Cu/γ-Al₂O₃ catalysts, it also showed poor catalytic activity, with a maximum iso-octane conversion of 32.4%; hydrogen selectivity reached a poor maximum value of 55.4% which is much lower than that obtained for the bimetallic Ni-Cu catalyst, and the highest CH₄ selectivity of 21.5% was observed.

2.3.4. Effect on Catalytic Performance

Conversion and product selectivity for steam reforming of iso-octane on Al₂O₃-supported catalysts at the low temperature of 550 °C are summarized in

Table 3. Conversion and product selectivity for steam reforming of iso-octane on Al₂O₃ supported catalysts, T = 550 °C, N₂ = 15 cm³/min; iso-octane feed rate was 0.03 g/min; S/C molar ratio: 0.9.

Catalysts	Conversion (%)		Product Selectivity (%)
	iso-Octane	Water	H2	CO	CO2	CH4
18 wt. % Ni0.5-Cu0.5/γ-Al2O3	42.6	12.2	70.6	7.4	15.3	6.7
18 wt. % Ni0.5-Mo0.5/γ-Al2O3	31.7	9.7	71.9	7.1	15.6	5.4
18 wt. % Ni0.5-Ce0.5/γ-Al2O3	28.3	8.2	72.0	7.6	16.1	4.4

. Here it is presented that iso-octane conversion over 18 wt. % Ni_0.5-Cu_0.5/γ-Al₂O₃ catalyst was much higher than that of 18 wt. % Ni_0.5-Mo_0.5/γ-Al₂O₃ and of 18 wt. % Ni_0.5-Ce_0.5/γ-Al₂O₃ catalysts; similar product selectivity was observed for all catalysts. Wang et al. [55] reported steam reforming of gasoline over the Ni-Ce/γ-Al₂O₃ catalyst and found that the Ce addition improved oxygen mobility and therefore enhanced the activity maintenance of the catalyst. A better activity maintenance of Ni-Ce/γ-Al₂O₃ than Ni/γ-Al₂O₃ was exhibited at a temperature of 690 °C. It was also reported that the largest and most desirable changes of nickel catalytic properties in steam reforming of hydrocarbon were observed with small amounts of Mo at high temperatures [47]. The best catalytic performance of Ni_0.5-Cu_0.5/γ-Al₂O₃ catalyst demonstrated in Table 3 is consistent with TPR results (Figure 6), of which the superior reducibility of the Ni-Cu/γ-Al₂O₃ catalyst was reached at low temperatures.

2.3.5. Stability Testing

Figure 12 shows hydrogen selectivity and iso-octane conversion over different catalysts with the time on steam of 30 h. For Ni_0.5-Cu_0.5/γ-Al₂O₃ catalyst, the conversion of iso-octane and H₂ selectivity were stabilized for the first hours and thereafter slightly decreased when the time on steam was extended to 30 h. The same tendency of hydrogen selectivity and iso-octane conversion was observed over the Ni_0.5-Ce_0.5/γ-Al₂O₃ and Ni_0.5-Mo_0.5/γ-Al₂O₃ catalysts, this result is consistent with previous studies [47,48]. It is widely accepted that the deactivation of the Ni catalyst in the steam reforming of fossil fuels is mainly due to carbon deposition on the catalyst surface [20]. A large amount of coke was deposited on the surface of the Ni catalyst, resulting in a loss of catalytic activity for C–C bond cleavage. It is believed that Mo species incorporated with the Ni catalyst suppress coke formation. Mo species not only decrease the coking rate but also prolong the induction period of coke formation [56]. It was also reported that the highly dispersed Mo species served as a barrier for preventing the growth of Ni particles [56]. This is the reason for the low rate of coke deposition on the Ni-Mo catalyst. It was also reported that the Ce content in Ni_0.5-Ce_0.5/γ-Al₂O₃ catalyst can make highly dispersed Ni particles with a strong metal-support interaction, resulting in high coke resistance [48]. The results in Figure 12 also show a higher decrease in the conversion of iso-octane over Ni_0.5-Cu_0.5/γ-Al₂O₃ catalyst. This phenomenon could be caused by agglomeration of copper particles. It was reported that Cu-based catalysts could easily be deactivated, caused by agglomeration of copper particles when reaction temperatures are over 300 °C [57]. Figure 13 shows SEM micrograph and EDX mapping of Ni_0.5-Cu_0.5/Al₂O₃ catalysts after 30 h on steam. The results show that the Cu particles become larger and the carbon particles deposited on the surface can also be clearly observed.

3. Experimental Section

3.1. Catalyst Preparation

γ-Al₂O₃ powder (Wako Pure Chemical, Osaka, Japan, pure 99.98%, 216.4 m²/g), Cu(NO₃)₂·3H₂O, Ni(NO₃)₂·6H₂O, Ce(NO₃)₃·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O (Merck Chemicals, Darmstadt, Germany, pure 99,99%) were used as starting materials. Ni-Cu/γ-Al₂O₃ (with various Ni-Cu ratio and loading), Ni-Mo/γ-Al₂O₃ and Ni-Ce/γ-Al₂O₃ were prepared by co-impregnating γ-Al₂O₃ support with Ni(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, Ce(NO₃)₃·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O aqueous solutions, respectively. After impregnation, all samples were dried at 110 °C for 6 h, followed by calcination at 500 °C for 6 h [18]. The molar ratios of Ni:Mo and Ni:Ce on Ni-Mo/γ-Al₂O₃ and Ni-Ce/γ-Al₂O₃ catalysts were kept at 1:1. The loading of Ni-Mo and Ni-Ce was maintained at 18 wt. %. The molar ratios of Cu in Ni-Cu/γ-Al₂O₃ catalysts were varied by 0, 10, 30, 50, 70, and 100%. Loading of Cu and Ni on Ni_0.5-Cu_0.5/γ-Al₂O₃ catalysts was changed from 6 to 36 wt. %.

3.2. Catalyst Characterization

The structures, the morphological aspects and the compositions of catalysts were analyzed by X-ray diffractometry (XRD) using Cu Kα radiation (RIGAKU RINT-2100CMT, Tokyo, Japan), scanning electron microscopy (SEM, Hitachi, SU6600 EVACSEQ, Tokyo, Japan and JOEL JSM7600F, Tokyo, Japan) and EDX (energy dispersive X-ray, X-Max50, Tokyo, Japan). The surface area was estimated by the N₂ adsorption at −196 °C, using the multipoint BET analysis method (ASAP 2010, Norcross, GA, USA). The total pore volume was calculated based on the adsorbed nitrogen at the highest relative pressure, while the average pore size diameter was determined by the Barrett, Joyner, and Halenda (BJH) method [25].

Hydrogen temperature programmed reduction (H₂-TPR) was conducted to determine the reductive properties of catalysts; 500 mg of catalyst was put in a quartz flow micro reactor and heated to 700 °C at a heating rate of 3 °C/min under 10% H₂/He with a total flow of 25 mL/min. Prior to TPR profile measurement, all samples were pretreated at 600 °C for 1 hour under 20% O₂/N₂ and cooled to room temperature. The consumption of H₂ was detected by a gas chromatography (Thermal Trace GC, San Jose, CA, USA).

3.3. Catalytic Activity

iso-Octane (Merck Chemicals, Darmstadt, Germany, pure 99.999%) was used as a gasoline surrogate for the simplicity of the experimental procedure. Catalytic activities were carried out with about 500 mg of catalyst placed in a fixed bed continuous-flow quartz reactor. Before the steam reforming reaction, all catalysts were preheated by flowing air at 600 °C for 1 hour then reduced under 10% H₂/He at 400 °C for 2 h. Flows of liquid hydrocarbon (iso-octane) and water were controlled by liquid pumps and preheated in an evaporator before being fed into the reactor. Pure 99.99% N₂ was used as the carrier gas and internal standard; the flow rate of N₂ was controlled by a mass flow rate-controller at a value of 15 mL/min. The products were withdrawn periodically from the outlet of a condenser installed at the reactor exit to collect water. The gas mixture was analyzed by a gas chromatograp, (Thermal Trace GC RGA) equipped with a thermal conductivity detector. Conversion, selectivity, and formation rates of products were calculated by an internal standard analyzing method as reported earlier [10,18],whereby, the flow rate of nonreactive internal standard gas (F_st) was kept constant.

$$F_{st}=F_{total}^{outlet}{X}_{st}^{outlet}=F_{total}^{inlet}{X}_{st}^{inlet}$$

(11)

Here, $F_{total}^{inlet}$ and $F_{total}^{outlet}$ were the total flow rates at inlet and outlet of the reactor, respectively. X_st refers to the concentration of the internal standard gas.

The formation rate of i product (F_i) can be calculated as:

$$F_i=F_{total}^{outlet}{X}_i^{outlet}=F_{st}\frac{X_i^{outlet}}{X_{st}^{outlet}}$$

(12)

Hydrocarbon conversion (C_HC) can be calculated on the basis of the carbon balance as

$$C_{HC}=\frac{F_{CO}^{outlet}+F_{C{O}_2}^{outlet}+F_{C{H}_4}^{outlet}}{F_{gasoline}^{inlet}}$$

(13)

In the same way, H₂O conversion ($C_{H_{2}O}$) is calculated as

$$C_{H_{2}O}=\frac{F_{CO}^{outlet}+2F_{C{O}_2}^{outlet}}{F_{H_{2}O}^{inlet}}$$

(14)

$F_{gasoline}^{inlet}$ and $F_{H_{2}O}^{inlet}$ are the flow rates of the gasoline and water feed, respectively, which were controlled by liquid pumps.

4. Conclusions

Steam reforming of gasoline with hydrogen selectivity over Ni-Cu/γ-Al₂O₃ catalysts was investigated at low temperatures. TPR analysis of Ni-Cu/Al₂O₃ catalysts indicated a shift in T_max towards a lower temperature; the addition of copper improved the dispersion of nickel and therefore facilitated the reduction of the Ni at lower temperatures. The catalytic performance of Ni-Cu/γ-Al₂O₃ catalysts was improved by the addition of Cu, which enhanced the reducibility of the dispersed Ni species. The conversion of gasoline with Ni-Cu/Al₂O₃ catalysts was significantly higher than that with nickel catalysts at a low temperature of 550 °C close to the normal SI engine exhaust gas temperature. On the other hand, initial durability testing showed that the conversion of gasoline slightly decreased after 30 h with steam, which was ascribed mainly to carbon deposition and agglomeration of copper particles. Therefore, Ni-Cu/Al₂O₃ could be effective in hydrogen selectivity when applied in SI engines.