Catalytic Performance and Characterization of Ni-Co Bi-Metallic Catalysts in n-Decane Steam Reforming: Effects of Co Addition

Abstract

Co-Ni bi-metallic catalysts supported on Ce-Al₂O₃ (CA) were prepared with different Co ratios, and the catalytic behaviors were assessed in the n-decane steam reforming reaction with the purpose of obtaining high H₂ yield with lower inactivation by carbon deposition. Physicochemical characteristics studies, involving N₂ adsorption-desorption, X-ray diffraction (XRD), H₂-temperature-programmed reduction (H₂-TPR), NH₃-temperature-programmed desorption (NH₃-TPD), SEM-energy dispersive spectrometer (EDS), and transmission electron microscope (TEM)/HRTEM, were performed to reveal the textural, structural and morphological properties of the catalysts. Activity test indicated that the addition of moderate Co can improve the hydrogen selectivity and anti-coking ability compared with the mono-Ni/Ce-Al₂O₃ contrast catalyst. In addition, 12% Co showed the best catalytic activity in the series Co-Ni/Ce-Al₂O₃ catalysts. The results of catalysts characterizations of XRD and N₂ adsorption-desorption manifesting the metal-support interactions were significantly enhanced, and there was obvious synergistic effect between Ni and Co. Moreover, the introduction of 12% Co and 6% Ni did not exceed the monolayer saturation capacity of the Ce-Al₂O₃ support. Finally, 6 h stability test for the optimal catalyst 12%Co-Ni/Ce-Al₂O₃ demonstrated that the catalyst has good long-term stability to provide high hydrogen selectivity, as well as good resistance to coke deposition.

1. Introduction

Nowadays, hydrogen is recognized as a clean fuel and energy carrier since its combustion produces only water as product [1,2]. However, how to produce hydrogen from primary energy sources (such as hydrocarbons) in an efficient and economic way should be further researched and developed [3,4,5,6]. In the past few decades, the most effective approach was catalytic reforming of hydrocarbons. Currently, over 50% of the world’s hydrogen supply is from steam reforming of hydrocarbons [7].

Nowadays, H₂ is mainly produced by steam reforming of CH₄ and other high-energy density liquid fuels, including ethanol, gasoline, diesel, or jet fuel [8,9,10]. An interesting option is hydrogen production from diesel steam reforming. n-decane, one of the main components of diesel, is considered as an ideal source of hydrogen since its availability, easy handling and storage and, relatively high H/C ratio (produce 31 mol of H₂ per mole of reacted n-decane) [11,12]. However, the n-decane steam reforming reaction is different with CH₄, CH₃OH, C₂H₅OH etc., and always accompanied by other side effects (cracking, isomerization, hydrogen transfer reaction). More so, the catalysts used in n-decane steam reforming reaction are easily to lose activity caused by carbon deposition, especially at higher temperatures [13,14,15]. Therefore, the catalysts use in n-decane steam reforming reaction are put forward higher requirements.

C₁₀H₂₂ + 20H₂O → 31H₂ + 10CO₂

Hydrocarbons steam reforming reactions have been extensively investigated over noble and transition metals (Pt, Pd, Rh, Ni, Co, etc.) and several oxide supports (Al₂O₃, CeO₂, MgO, ZrO₂, zeolite, etc.) [16,17,18,19,20,21,22,23], so as to develop excellent catalysts to obtain hydrogen as high yield as possible together with high resistance of coke deposition. Transition metals (especially Ni-based) catalysts, which have the high C–C and C–H bonds breaking activity, have been proved to be very effective for hydrocarbons steam reforming reactions as noble metal catalysts [16,17,18]. Moreover, the lower cost improved its applicability. Therefore, more and more researchers focused on studying hydrocarbon steam reforming over Ni-based catalysts [16,17,18,19,20,21]. However, coking is easily deposited on the surface of the active phase Ni, which can lower the catalytic activity [24,25,26,27]. Therefore, various promoters were introduced into Ni-based catalysts to improve catalytic activity and coking resistance. Lanthanide metals (La, Ce), alkali metals (Na, K), and alkali earth metals (Mg, Ca, Sr, Ba) promoters [28,29,30,31,32,33], have been found to be effective for improving coking-resistant capacity. However, the addition of these additives influenced Ni dispersion, due to a part of the promoter is in an intimate contact with nickel [34,35,36].

In order to improve the anti-coking ability of Ni-based catalyst, and have a slight influence on catalytic activity, many scholars introduced another active metal into Ni-based catalyst to form bi-metallic catalysts [37,38,39,40,41,42]. Wang et al. [37] introduced Pd into Ni-alumina catalysts, the catalytic activity and stability was obviously improved. Vizcaino et al. [43] found that Cu modified Ni-based catalyst showed better anti-coking ability. The addition of Cu is helpful for the process of eliminating the deposited carbon. In our previous work [44], we have added M (Fe, Co, Cu, Zn) as a promoter into the Ni/Ce-Al₂O₃ catalyst in order to improve the anti-coking ability. Clearly, Co doped Ni/Ce-Al₂O₃ showed an excellent coking-resistant effect. But, the catalytic activity have a slightly reduction at high temperature (650~800 °C). In another study [45], we added Co as another active species into Ni/Ce-Al₂O₃ to form Ni-Co bi-metallic catalyst and investigated the catalytic activity, stability and coking inhibition effect during n-decane reforming. The results showed that the introduction of Ni and Co synchronously can effectively suppress carbon deposition and obviously improve catalytic activity. There was obvious synergistic effect between Ni and Co. However, the difference among different Co content on the Co-Ni/Ce-Al₂O₃ bi-metallic catalyst has not been discussed. Consequently, it would be valuable to investigate the influence of the content of Ni on n-decane steam reforming.

In this paper, the steam reforming experiments of n-decane over x%Co-Ni/Ce-Al₂O₃ catalysts with different Co loading were carried out. The effect of different Co loading on catalytic activity and the amount of deposited carbon were discussed. The purpose of this work is screening the suitable catalysts for steam reforming process in order to maximize n-decane conversion and H₂ yield, and minimize the formation of byproducts and carbon deposition. This work provided some positive suggestions for catalysts preparation and optimization by studying the structure-activity correlations.

2. Results and Discussion

2.1. Catalytic Performance

2.1.1. n-Decane Conversion and H₂ Selectivity

n-decane steam reforming is used as the probe reaction. The initial activity tests over the series catalysts were performed from 650 to 800 °C in order to examine the influence of temperature and different promoters on catalytic performance. n-decane conversion and H₂ selectivity are considered the main parameters to check the advantages and disadvantages of the catalysts, and the results are shown in Figure 1a–d. The catalytic activities over these catalysts gradually increase with the temperature. Obviously, the presence of Ni or/and Co can effectively promote the rate of the steam reforming reaction, as well as the selectivity of H₂ and n-decane conversion. Moreover, the synchronous introduction of Co and Ni further enhanced the catalytic activity compared with the 6%Ni/Ce-Al₂O₃(NCA) catalyst. This demonstrates that the addition of Co could provide sufficient Ni active sites for the reactants. In addition, the catalytic activity of x%Co-Ni/Ce-Al₂O₃ (CNCA) bi-metallic catalysts with different Co content increases firstly and then decreases with Co addition. The catalytic activity reaches the best when the Co content is 12%. This indicated that moderate Co is favor for promoting the activity. There is a synergistic effect between Co and Ni.

2.1.2. Thermal Stability and Regeneration of C12-NCA

To better understand the effect of the ordered co-modification in n-decane steam reforming, the 12%Co-Ni/Ce-Al₂O₃ (C12-NCA) catalyst was screened out with a 6 h stability test at 750 °C and 800 °C, and the results are displayed in Figure 2a. It can be seen in Figure 2a that the H₂ selectivity and n-decane conversion have a slightly change within 6 h. The C12-NCA catalyst has a good thermal stability. The C12-NCA catalyst also screened out for regeneration experiment. Carbon deposition on used C12-NCA catalyst was removed by oxygen enriched calcinations at 650 °C. The regenerative C12-NCA catalyst was carried out again in the same reactor as the fresh ones, and the contrast results are presented in Figure 2b. It is found that the n-decane conversions and H₂ selectivity over the reused-1(-2) C12-NCA catalyst are approximately equal to the results of fresh one. Therefore, the C12-NCA catalyst is renewable.

2.2. Fresh Catalyst Characterization

2.2.1. N₂ Adsorption-Desorption Measurements

Table 1. The textural properties of the fresh and used catalysts.

Catalysts	Textural Properties
	Surface Area *(m2/g)	Pore Volume (mL/g)	Mean Pore Diameter (nm)
CA	155.9 (69.3) *	0.49	5.42
NCA	150.2 (83.6)	0.47	5.35
C3-NCA	149.3 (89.1)	0.47	5.32
C6-NCA	147.6 (88.6)	0.47	5.34
C9-NCA	143.9 (92.3)	0.45	5.28
C12-NCA	140.5 (96.7)	0.44	5.26
C15-NCA	136.9 (92.4)	0.44	5.26

* The numbers in the parentheses represent the surface area of used catalysts.

shows the results of N₂ adsorption-desorption results of the fresh and used catalysts. The surface areas of different samples in this work are in the range of similar CA support, even if the introduction of Ni and Co species by impregnation method. The value has a slightly decrease with the addition of Co and Ni, and gradually drops with the increase of Co. The loss can be attributed to the fact that the internal surface area of the CA pore system is progressively covered by Ni, Co species forming a layer [45,46,47].

On the other hand, the surface areas of all catalysts used decrease with different levels after n-decane reforming reactions. CA and NCA catalysts decreased by 56% and 44% respectively compared with the fresh ones. Fortunately, the falling range gradually reduced with the addition of Co. Co as the active species showed a better carbon-resistant ability. The results are consistent with the results of the catalyst characterization.

2.2.2. X-ray Diffraction (XRD) Analysis

Figure 3 depicts the X-ray diffraction analysis of the fresh CA, NCA and bi-metallic CNCA catalysts. All the samples present similar characteristic features of γ-Al₂O₃ at 2θ = 45.7°, 66.8°; cubic fluorite structural CeO₂ at 2θ value of 28.5°, 56.3°; and the Ce crystallite at 2θ = 34.7°, 49.8° and 59.2° by Bragg’s refections [44,48]. For all the catalysts, there was no CoO_x, CoAl₂O₄, NiO, or NiAl₂O₄ diffraction peaks detected. This was probably due to the highly dispersion of CoO_x and NiO particles are not easy to be detected by XRD [49]. Moreover, the synchronous addition of Co and Ni did not form Ni-Co alloy phase. This indicated that active species were strongly interacted with CA support, and all of the as-prepared have a good thermal stability. It also can be seen that the degree of crystallization of all the fresh catalysts are smaller, suggesting that these catalysts are stable at high temperatures, which is coincides with the surface area analysis [50,51]. This is in agreement with the XRD analysis previously shown and with the observations made in other studies.

2.2.3. H₂-Temperature-Programmed Reduction (H₂-TPR) and NH₃-Temperature-Programmed Desorption (NH₃-TPD) Analysis

Figure 4 shows the reduction profiles of the CA, and Co, Ni modified CA. It can be seen that there is one or two H₂ consumption peaks for CA support at the region of 250~350 °C, which could be assigned to the reduction of a small amount of CeO₂ to CeO_x [28,29]. For NCA catalyst, reduction peaks of ~460 °C and ~823 °C which are attributed to the reduction of NiO and NiAl₂O₄ [40]. The highest reduction temperature is between 780 and 900 °C indicate the existence of species of NiO with strong interaction with Al₂O₃, resulting from the formation of the NiAl₂O₄ [42]. For CNCA catalysts with different Co loading, two reduction peaks around ~300 °C and ~610 °C are ascribed to the reduction of Co₂O₃ and CoO, respectively [41]. Significantly, the reduction temperatures differences of NiO and Co₂O₃ between these CNCA catalysts can be attributed to the existence of different interaction between Co and Ni. It is noteworthy that Co addition can obviously promote the reduction of NiO, and reach the optimal effect at 12% Co loading. There was obvious synergistic effect between Ni and Co, which is consistent with the results of the work reported by Jiao et al. [48,52,53,54,55,56].

NH₃-TPD technology was used to investigate the acidities of the series catalysts, such as total amount, nature, and strength distribution, in order to look for the possible interpretation for the above experimental results, and the profiles are shown in Figure 5. The area of desorption peak goes hand in hand with the total amount of surface acid sites, while the peak temperature is closely related to the strength of individual acid site. The peak temperature in the range of 80 to 200 °C is regarded as weak acid sites, and the desorption temperature between 200 to 400 °C is considered the medium acid sites, while the peak temperature locates at 400 to 700 °C corresponds to strong acid sites. Figure 5 shows that CA and NCA catalysts have three desorption peaks at the region of 100~500 °C, which is regarded as the desorption peak of the weak and medium acid. Moreover, all the Co doped NCA catalysts have one strong desorption peak at 100~400 °C. Obviously, the desorption peak temperature moves to a low temperature area by adding Co, suggesting that the amount of acid sites decrease with the introduction of Co. In our previous studies [12,18], we found that the larger acidity and active strong acid centers are easy to give rise to rapid deactivation of the catalyst due to carbon deposition. It is noteworthy that the addition of Co modifier increases the basicity of the NCA catalyst. This process will result in preventing alkenes further reacting into aromatic or heavier products, which is beneficial to reduce the carbon deposition over catalysts and prolong the work life of catalysts.

2.2.4. Transmission Electron Microscope (TEM) Analysis

In this work, the catalytic activity over the C12-NCA catalyst is better than C15-NCA. However, there is somewhat different texture and structural properties between the two. In order to further study the difference between C12-NCA and C15-NCA, the TEM analysis was used to find the influence of microscopic appearance and dispersion on catalytic activity. Figure 6 shows TEM images and Ni or/and Co particle size distributions of C12-NCA and C15-NCA catalysts. It is found that the Ni or/and Co particle size over C12-NCA is mainly focused on 11–20 nm, while the value for C15-NCA is about 16–25 nm. The average particle size of Ni or/and Co of C15-NCA is significantly larger than C12-NCA. Obviously, poor Ni or/and Co distribution over C15-NCA are observed. It may be the reason of the weaker catalytic activity of C15-NCA catalyst. The results indicate that 6% Ni and 15% Co loading is easy to aggregate after 800 °C calcination, which may be due to redundant Co enriching on the catalyst surface and exceed the monolayer saturation capacity of the CA support.

In order to further understand the Ni dispersion, the energy dispersive spectrometer (EDS) mappings of C12-NCA and C15-NCA catalysts were examined. As shown in Figure 7, the red sections correspond to Co, the blue sections correspond to Ce, the green sections correspond to Ni, while the yellow sections correspond to Al. Obviously, Ni and Co particles evenly dispersed on the surface of the CA support over C12-NCA. However, some regions overlap each other for C15-NCA. This may be caused by the partial sintering of 6% Ni and 15% Co loading, and form bigger crystallite and lower metal dispersion. Poor nickel dispersion not only notably impact the physiochemical properties of the catalyst, but also impact the catalytic performance in steam reforming of reactions. Therefore, the dispersion state of active species is one of the main reasons of the activity difference among these catalysts.

2.3. Carbon Deposition

Table 2. The amount of carbon deposition over used catalysts.

Catalysts	Carbon Deposition
	[mg/(gCat·h)]	[mg/gCFeed]
CA	258.9	11.6
NCA	194.3	8.55
C3-NCA	178.8	7.87
C6-NCA	143.2	6.30
C9-NCA	114.1	5.02
C12-NCA	88.6	3.90
C15-NCA	75.0	3.30

presented the amount of carbon deposition over the series catalysts determined by the temperature programmed oxidation (TPO)-infrared spectrum (IR) analysis. It can be seen that the amount of carbon deposition is varied with the introduction of Ni and Co. Clearly, the amount of carbon deposition over CA catalyst is larger than others, and the values drop off with the Co content increased. For the bi-metallic C12(C15)-NCA catalyst, the amount of carbon deposition (<4.0 mg/g-fuel) is much less than conventional Ni-alumina catalysts. The possible reasons are that the addition of Co reduces the acidity of the CA catalyst and carbon dioxide adsorption, and decreases carbon deposition generation during n-decane steam reforming process [48]. According to the literature [57], Co addition can increase the dispersion state of Ni, prevent metal sintering, and inhibit carbon deposition, which is consistent with our results in this work.

2.4. Used Catalyst Characterization

The formation of carbon deposition on the used catalyst was also evidenced by SEM observations. In Figure 8a–g, the micrographs of used catalysts are presented. Carbon deposition grows on the surface of catalysts in different range. It is observed from Figure 8h that the Ni species is occupied at the tip of the filamentous carbon and the Ni metal particle size is found to be ca. 30 nm. This can demonstrate that active metal has removed from the surface of catalyst with the generation of filamentous carbon. The results are consistent with the mechanisms of filamentous carbon growth in literature [58]. In addition, the C content over the used CA, NCA, C3-NCA, C6-NCA, C9-NCA, C12-NCA, C15-NCA catalysts are 91.14%, 63.90%, 56.46%, 49.89%, 34.79%, 14.94% and 8.31%, respectively. It clearly indicates that the C content over the used catalysts has a close relationship with the surface area. Obviously, the Brunauer–Emmett–Teller (BET) specific surface area decrease caused by the formation of the carbon deposition is another main reason of the activity difference among these catalysts.

3. Experimental Section

3.1. Catalysts Preparation

Ce-Al₂O₃ composite oxides (5 wt.% Ce) support was prepared by coprecipitation method, and the complete experimental procedure is detailed in references [8,44]. The catalysts were prepared by incipient wetness impregnation using Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O as the precursors of Ni and Co, respectively. After impregnation and drying (120 °C, 3 h), the samples were calcined in air at 800 °C for 4 h. Finally, the mono-metallic Ni catalyst (Ni/Ce-Al₂O₃, 6 wt.% Ni), and Co-Ni bi-metallic catalysts {Co-Ni/Ce-Al₂O₃, 6 wt.% Ni, x wt.% Co (x = 3, 6, 9, 12, 15)} were made into columnar particles (length of 5 mm, diameter of 5 mm), and marked as NCA, C3-NCA, C6-NCA, C9-NCA, C12-NCA and C15-NCA, respectively.

3.2. Catalysts Characterization

Surface area and pore volume were obtained from N₂ adsorption-desorption isotherms at 77 K performed on a Quanta chrome autosorb-1 analyzer. Before analysis, the samples were dried at room temperature and then vacuum degassed at 300 °C for 5 h. X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance diffractometer with a copper Kα radiation source operated at 40 kV and 40 mA. During the analysis, the catalysts were scanned from 20° to 80° at a speed of 5 °/min. Furthermore, temperature-programmed reduction (TPR) profiles of fresh catalysts under H₂-blanket were carried out using a TP-5076 TPR instrument. The samples (100 mg) were heated to 400 °C at a heating rate of 8 °C/min in a flow of Ar, kept for 45 min, and then cooled to 30 °C. The flow gas was switched to H₂ (5%) in Ar (25 mL/min). The reduction was carried out from 30 °C to 920 °C, at a heating rate of 8 °C/min. NH₃-temperature-programmed desorption (NH₃-TPD) experiments were performed in a TP-5076 TPD instrument (Tian Jin Xian Quan Instrument Co., Ltd., Tianjin, China) equipped with a thermal conductivity detector (TCD) to investigate the surface acidity of catalysts. Prior to analysis, catalysts were pretreated at 400 °C for 1 h in a flow of N₂ to clean the catalyst surface and then cooled to room temperature. After the pretreatment, a 2% NH₃/N₂ gas mixture was passed through the sample at 20 mL/min and the temperature was raised from 50 to 750 °C at a heating rate of 8 °C/min. In addition, morphological characterization was examined by field emission scanning electron microscopy (FESEM) using a Quanta 600FEG Field emission scanning electron microscope (FEI, Hillsboro, OR, USA) and coupled with Oxford-IE-250 energy dispersive spectrometer (EDS) for local elemental composition determination.

Transmission electron microscope (TEM) analysis was obtained using a FEI titan themis 200 transmission electron microscope (FEI, Hillsboro, OR, USA) equipped with Bruker super-X energy-dispersive spectrometer (EDS). Prior to measurement, the samples were dispersed in ethanol at first and then collected on a Cu grid which was covered with carbon film.

3.3. Catalytic Performance Test

n-decane steam reforming reaction was carried out in a fixed-bed stainless steel tubular reactor (i.d. = 12 mm) at atmospheric pressure. 3.0 g catalyst diluted with equi-volume quartz particles was charged for each catalytic assessment. The schematic diagram was shown in Figure 9. In each run, water and n-decane were pre-mixed quantitatively at 350 °C and the mixture vapor was fed into the reactor. The volumetric feed flow of water and n-decane are both 2.5 mL/min. The reactor was set at 650 °C, 700 °C, 750 °C or 800 °C respectively for evaluating the catalysts’ activity.

The reaction effluents were on-line analyzed by Gas Chromatography with FID detector (GC-2010, SHIMADZU Co., Ltd., Kyoto, Japan) equipped with HP-Al/S separation capillary column (CH₄, C₂H₄, C₂H₆, C₃H₈, C₃H₆, C₄); and TCD detector with TDX-01 packed column (H₂, CO and CO₂). Solid products deposited covering the catalysts were investigated by temperature programmed oxidation (TPO), and the characteristics signals of CO and CO₂ were tested by IR analyzer.

Activity data were reported as n-decane conversion X_A (%), and H₂, CO and CO₂ selectivity (S_i), which are defined as follows:

$$X_A(\%)=\frac{{\mathsf{\Delta }\mathrm{m}}_A}{{\mathrm{m}}_{A,in}}=\frac{{\mathrm{m}}_{A,in}-{\mathrm{m}}_{A,out}}{{\mathrm{m}}_{A,in}}\times 100$$

(1)

$$S_i=\frac{\mathrm{moles}{P}_i}{{\displaystyle {\sum }_{i=1}^n\mathrm{moles}{P}_i}}\times 100\%$$

(2)

where i represents the different gas products detected.

4. Conclusions

Series Co-Ni bi-metallic catalysts supported on Ce-Al₂O₃ (CA) with different Co ratios are prepared for n-decane steam reforming towards H₂ preparation, so as to screen appropriate Co content. N₂ adsorption-desorption, XRD, NH₃-TPD, SEM-EDS, and TEM were used to explain the relationship between catalytic activity and textural-structure properties, microscopic appearance and dispersion state of the catalysts. The activity test indicated that the addition of Co resulted in higher selectivity of hydrogen compared with NCA contrast catalyst. The results indicated that 12% Co showed better hydrogen selectivity (74%), n-decane conversion (100%), and anti-carbon ability (3.90 mg/g-fuel). Moreover, thermal stability and regeneration performance were also tested, and the results showed that reforming activity over C12-NCA catalyst was stable in 6 h, and the hydrogen selectivity and n-decane conversion over regenerated catalysts were almost close to the fresh ones.

The results were mainly interpreted on the basis of characterization studies, which revealed the significant role of textural and structural properties, microscopic appearance and dispersion state on reforming performance. Obviously, the dispersion state and the BET-specific surface area decrease caused by the formation of the carbon deposition are the main reasons for the activity difference among these catalysts.