Effect of Cu–Mo Activities on the Ni–Cu–Mo/Al₂O₃ Catalyst for CO₂ Reforming of Methane

Abstract

A series of Ni–Cu–Mo/Al₂O₃ catalysts with different metal contents were prepared by sequential impregnation method. The performance of the catalysts for carbon dioxide reforming of methane was investigated in a quartz tubular reactor at CH₄/CO₂ ratio of 4/6, feed gas flux of 100 mL/min, catalysts 460 mg, temperature range of 600–900 °C and atmospheric pressure. The catalysts were characterized by XRD, XPS, BET, CO₂-TPD, TG, EDS and SEM. The performance and characterization results showed that the addition of Cu and Mo was beneficial for the reaction. The formation of spinel (NiMoO₄, NiAl₂O₄, Cu₆Mo₄O₁₅) and alloy (Ni₃Mo, Cu_3.8Ni) structures could significantly improve the catalytic activity and prevent the generation of carbon deposition. XRD patterns of the catalyst with the mass ratio of Ni:Mo = 0.75 after reaction suggested Mo₂C formation which could eliminate the coke and extend the stability. The catalyst with the mass ratio of Ni:Mo = 0.75 showed better activity and still remained highly active under the condition of 800 °C for 60 h. Therefore, the highly effective Ni–Cu–Mo/Al₂O₃ catalyst could be a potential catalyst for carbon dioxide reforming of methane.

Graphical Abstract

1 Introduction

Carbon dioxide reforming of methane has great potential and attracts many researchers’ attention during recent years. It is of great significance because of the greenhouse effect, as well as energy and resource challenges. Nickel based catalysts have been extensively used in carbon dioxide reforming of methane due to relatively high activity and low cost, but sintering and carbon deposition over them prevent their application [1]. Improvement of nickel based catalysts is favorable.

To improve the performance of nickel based catalysts, various metallic active components were added into nickel based catalysts. Xinfa Dong et al. [2] investigated the effect of transition metal (Cu, Co and Fe) on the modification of nickel based catalysts for the autothermal reforming of methane. The results showed that the addition of transition metal Cu improved the dispersion of NiO, inhibited the formation of NiAl₂O₄, and thus significantly increased the activity of catalyst. Jae-Sung Choi et al. [3] studied CO₂ reforming of methane over modified Ni/Al₂O₃ catalysts. Catalysts modified with Cu showed slightly improved activity.

Moreover, investigations before had shown that Mo, as well as the oxides and carbides of Mo were excellent promoters to improve the performance of nickel based catalysts. The addition of Mo into nickel catalysts could effectively improve the metal-support interaction and lead to a decrease in carbon deposition [4–6]. The addition of MoO₃ enhanced the electron density of Ni and improved the stability of the catalysts [7]. Chuan Shi et al. [8] studied Ni-modified Mo₂C catalysts for methane dry reforming. Ni-Mo₂C was a typical bi-functional catalyst for methane dry reforming, and a catalytic redox cycle could be established by regulating the mass ratio of Ni and Mo₂C. The formation of the alloy between the active ingredients could basically eliminate carbon deposition on the catalyst and improve the catalytic performance of catalysts [9]. A. Djaidja [10] studied the Ni–M–Mg/Al (M = Fe or Cu) catalysts for CH₄–CO₂ reforming, the formation of Ni–Fe or Ni–Cu alloys led to a very good dispersion of the metal dispersion of the metal particles which explained the good catalytic stability observed in the CH₄−CO₂ reforming.

In the present work, Ni–Cu–Mo/Al₂O₃ catalyst was proposed for carbon dioxide reforming of methane. Comparing the properties and catalytic performance of Ni/Al₂O₃, Ni–Cu/Al₂O₃, Ni–Mo/Al₂O₃, Ni–Cu–Mo/Al₂O₃ catalysts, Ni–Cu–Mo/Al₂O₃ catalyst showed higher activity for the carbon dioxide reforming of methane reaction, affording high selectivity of CO and H₂.

2 Experimental

2.1 Catalyst Preparation

The 12 wt% Ni/Al₂O₃, Ni–Cu/Al₂O₃, Ni–Mo/Al₂O₃ and Ni–Cu–Mo/Al₂O₃ (Cu proportional content: 10 wt%) catalysts were prepared by impregnation method as shown in Fig. 1. γ-Al₂O₃ was first impregnated with aqueous solution of (NH₄)₆Mo₇O₂₄·H₂O, and dried at 110 °C for 5 h, followed by calcination in a muffle furnace at 500 °C for 4 h to form MoO₃/Al₂O₃ compound. Trimetallic catalysts Ni–Cu–Mo/Al₂O₃ were prepared by impregnation of MoO₃/Al₂O₃ with mixtures of aqueous solutions of Ni(NO₃)₂·6H₂O and Cu(NO₃)₂·3H₂O in corresponding amount. Then, the catalysts were dried at 110 °C for 5 h, followed by calcination in a muffle furnace at 500 °C for 4 h and reduction at 700 °C for 2 h.

Fig. 1

Schematic flow chart for the preparation steps of the four kinds of the catalysts

2.2 Catalytic Performance Tests

The catalytic performance tests were carried out in a quartz fixed-bed reactor at 600–900 °C. The feed mixture of 100 mL·min⁻¹ with CH₄:CO₂ mol ratio of 4:6 was introduced into the reactor, and 460 mg of catalyst was charged in the reactor. The product composition was analyzed by an on-line gas chromatography (GC-1100) equipped with a TDX-01column (2 m × 3 mm) and with TCD detector (Fig. 2).

Fig. 2

Schematic flow chart for carbon dioxide reforming of methane

The conversions (X) of CH₄ and CO₂, the selectivity (S) of CO and H₂ were shown in form (1)–(4).

$$X(CH_{4} )\% = \frac{{(F_{{CH_{4,IN} }} - F_{{CH_{4,OUT} }} )}}{{F_{{CH_{4,IN} }} }} \times 100\,\%$$

(1)

$$X(CO_{2} )\% = \frac{{(F_{{CO_{2,IN} }} - F_{{CO_{2,OUT} }} )}}{{F_{{CO_{2,IN} }} }} \times 100\,\%$$

(2)

$$S(H_{2} )\% = \frac{{F_{{H_{2,OUT} }} }}{{2 \times (F_{{CH_{4,OUT} }} - F_{{CH_{4,IN} }} )}} \times 100\,\%$$

(3)

$$S(CO)\% = \frac{{F_{CO,OUT} }}{{[(F_{{CH_{4} ,IN}} - F_{{CH_{4} ,OUT}} ) + (F_{{CO_{2} ,IN}} - F_{{CO_{2} ,OUT}} )]}} \times 100\;\%$$

(4)

2.3 Catalyst Stability Test

The reforming reaction was carried out at 800 °C for 80 h in order to study the long term stability of the catalyst with Ni:Mo mass ratio of 0.75. The feed mixture of 100 mL min⁻¹ with CH₄:CO₂ mol ratio of 4:6 was introduced into the reactor, and 460 mg of catalyst was charged in the reactor.

2.4 Catalyst Characterization

X-ray diffraction (XRD) of the catalysts was recorded with an X-ray diffractometer (RINT2000, Japan) using monochromatic Cu Kα radition at 40 kV and 150 mA. The diffraction patterns were recorded in the 2θ range of 10–80° with a scanning speed of 10°min⁻¹ and a step size of 0.02°s⁻¹. X-ray photoelectron spectra (XPS) was measured on an Thermo ESCALAB 250Xi (US) spectrometer using monochromatized Al Kα radiation at electron gun power of 150 W, and the Al Kα monochromatized line (1486.6 eV) was adopted during the analysis. The BET surface area measurements were performed with N₂ adsorption at −196 °C using an adsorption unit ASAP2020. CO₂ temperature-programmed desorption (CO₂-TPD) was performed on an AutoChemII 2920 Analyzer. 400 mg of catalyst was heated to 350 °C and kept for 60 min in Ar stream. Then, the catalyst was cooled down to ambient temperature and followed by adsorption with pure CO₂. After adsorption, the catalyst was purged with He stream. The desorption of CO₂ was carried out with a heating rate of 6 °C·min⁻¹ to 500 °C and measured by a mass spectrometer. Scanning electron microscope (SEM) images were carried out with a JSM-6700F instrument operated at 8 kV. The thermal analyses were carried out on a NETZSCH TG/209/F1 thermoanalyzer. The measurements were conducted in dynamic air atmosphere (50 mL min⁻¹), using the alumina plates crucibles of 150 mL. The heating rate was of 20 K min⁻¹ in the range of temperature 30–900 °C and the mass samples were about 4 mg. Energy Dispersive Spectroscopy (EDS) was also used for qualitative detection of Ni, Cu, Mo elements in the catalyst matrix.

3 Results and Discussion

3.1 Catalytic Performance

The Ni content 12 wt% with the Ni/Al₂O₃, Ni-Cu/Al₂O₃, Ni-Mo/Al₂O₃, Ni–Cu–Mo/Al₂O₃ catalytic performances for carbon dioxide reforming of methane were shown in Fig. 3, which showed the reactants conversion as a function of temperature. From the results, it was apparent that the catalytic activities of four kinds of catalysts were presented the same trend, namely the reactants conversion increased significantly with increasing temperature. In fact, the increase in catalytic performance with temperature conformed to the thermodynamics of the process since it was an endothermic reaction. The Ni/Al₂O₃ catalyst conversion rate was the lowest. The conversion rates of the Ni–Cu/Al₂O₃ and Ni-Mo/Al₂O₃ were similar and higher and the conversion rate of the Ni–Cu–Mo/Al₂O₃ was the highest. This showed that on the basis of Ni catalysts added Cu or Mo fertilizer could enhance the catalytic activity of catalysts, adding the two Cu and Mo fertilizers could further increase the catalytic activity of the catalysts, especially at low temperatures (650–750 °C). The reason may be that between Ni and Cu or Mo formed some kinds of new structure and this could be achieved by characterization of catalysts.

Fig. 3

Four catalytic performances for carbon dioxide reforming of methane

Figure 4 illustrated the catalytic performance of the Ni–Cu–Mo/Al₂O₃ catalyst with the different Ni:Mo mass ratio for carbon dioxide reforming of methane, showing the reactants conversion and products selectivity as a function of temperature. From the results, it was apparent that the reactants conversion increased with increasing temperature, while there was negligible difference in products selectivity. In 600–800 °C temperature range, the conversions increased rapidly with the increase of reaction temperature. But as the temperature increased, the conversion rate increased slower. When temperature was higher than 750 °C, as the temperature increased, the conversion rates basically remained unchanged. Therefore, the proper temperature for the Ni–Cu–Mo/Al₂O₃ catalysts was 750 °C. The reactants conversion and products selectivity also varied with the Ni:Mo mass ratio. The catalyst with the mass ratio of Ni:Mo = 0.75 showed higher activities. The conversions of CH₄ and CO₂ reached 97.68 and 99.12 %, and the selectivity of CO and H₂ reached 92.96 and 90.01 % at 800 °C, respectively.

Fig. 4

Catalysis activities of the Ni–Cu–Mo/Al₂O₃ with different Ni/Mo mass ratio. 1 Ni:Mo = 0.15; 2 Ni:Mo = 0.30; 3 Ni:Mo = 0.45; 4 Ni:Mo = 0.60; 5 Ni:Mo = 0.75

3.2 Catalyst Stability

Figure 5 illustrated the stability of the catalyst with the mass ratio of Ni:Mo = 0.75. The stability of the catalyst was tested in the quartz fixed-bed reactor at 800 °C. As to the catalyst with Ni:Mo mass ratio of 0.75, the conversions of methane and carbon dioxide were over 90 %, and the conversions still remained at 90 % till 60 h. The Reaction conversion rate after 60 h began to gradually reduce. This showed that the Ni–Cu–Mo/Al₂O₃ catalyst was highly active.

Fig. 5

The stability of the catalyst with the mass ratio of Ni:Mo = 0.75

3.3 Catalyst Characterization

Figure 6 showed the XRD patterns in the range of 10–80° of the catalysts with different mass ratio of Ni:Mo. It appeared that nickel was mainly in the form of elemental distribution in the catalyst, a few in the form of spinel (NiAl₂O₄) or alloy (Ni₃Mo, Cu_3.8Ni) and the Cu or Mo additives formed the Cu₆Mo₄O₁₅ metal oxides. The structures of NiMoO₄ and NiAl₂O₄ could effectively prevent the sintering of nickel and improve the activity of catalysts. The broadness of Ni₃Mo and Cu_3.8Ni characteristic peaks increased with the increasing mass ratio of Ni:Mo. The NiMoO₄ characteristic peak of the catalyst with Ni:Mo mass ratio of 0.75 was high and sharp, which explained that the crystal shape was perfect. The characteristic peaks of the alloy Ni₃Mo and Cu_3.8Ni were wide and low, suggesting that the formation of the grain size was small and the dispersion was high. It suggested that the alloy structure formed between Ni and Cu or Mo could significantly improve the catalytic activity of catalysts, which was consistent with the results of the catalytic performance tests. The Ni surface area and average Ni particle size as shown in Table 1 were calculated according to the method described in Ref. [11].

Fig. 6

XRD patterns of Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo. 1 Ni:Mo = 0.15; 2 Ni:Mo = 0.30; 3 Ni:Mo = 0.45; 4 Ni:Mo = 0.60; 5 Ni:Mo = 0.75

Table 1 Ni dispersion on Ni–Cu–Mo/Al₂O₃ catalyst

Figure 7 Showed the XRD patterns of Ni–Cu–Mo/Al₂O₃ catalyst with the mass ratio of Ni/Mo = 0.75 after reaction. The spinel and alloy structures disappeared after reaction, while MoO₂, Mo₂C, Al_0.9Ni_4.22 and Cu_0.8Ni_0.19 species formed. The results suggested that the addition of Cu and Mo induced strong metal-support interaction, which was reported as one of the contributions to good catalytic performance [12]. Mo₂C, which was also catalytically active for carbon dioxide reforming of methane, contributed to the prevention of carbon deposition and extended the stability of the catalyst.

Fig. 7

XRD patterns of Ni–Cu–Mo/Al₂O₃ catalyst with the mass ratio of Ni/Mo = 0.75 after reaction

X-ray photoelectron measurements were performed over the Ni–Cu–Mo/Al₂O₃catalyst with Ni:Mo mass ratio of 0.75 to investigate the surface properties and elemental oxidation states. Figure 8 showed the XPS patterns of the catalyst with Ni:Mo mass ratio of 0.75, and the Al Kα monochromatized line (1486.6 eV) was adopted during the analysis. The samples were pressed into a thin pellet, and the binding energys (BEs) were corrected with the reference BE of C1 s (284.6 eV).

Fig. 8

XPS patterns of Ni–Cu–Mo/Al₂O₃ catalyst with Ni:Mo mass ratio of 0.75

The spectra of Ni2p, Cu2p, Mo3d, Al2p, O1 s were taken at the constant analyzer energy mode (30 eV pass energy) and C1 s at the 50 eV pass energy. XPS analysis was performed to examine the elemental composition and to determine the valence state of elements of the catalysts. For the O1 s spectra, the O1 s peak is generally complicated because of the O ions from different oxides. As per the literature, the O²⁻ was existed in CeO₂ at the peak of 529.6 eV [13]. And the O1 s binding energy value reported for Rh₂O₃ was 530.0 eV [14]. On this basis, the peak at 529.87 eV may be attributed to O²⁻ of the NiAl₂O₄, NiMoO₄ and Cu₆Mo₄O₁₅. The BEs of Al2p on the Ni/Al₂O₃ catalysts were found to be in the range of 73.7–74.1 eV without significant variations [15], which suggested the Al³⁺ of the formation of spinel NiAl₂O₄ species. For the Ni2p spectra, Ni mainly existed as Ni⁰ (852.48 ± 0.5 eV) in Zr-based catalyst [16] and Ni⁰ (852.7 eV) in Ni–Co catalyst [17]. Ni mainly existed as Ni²⁺ (853.5–855.6 eV) [18] or (856 or 874 eV) [19]. As it is shown in Fig. 8, the low-intensive peak of 852.06 eV could be assigned to Ni elemental. The peak of 852.06 and 872.74 eV were attributed to Ni²⁺, which may be the spinel NiAl₂O₄ or the NiMoO₄. In addition, the higher energy peak (860.85 eV) may be associated with the Ni₃Mo or Cu_3.8Ni alloy, which combined with the XRD analysis. By the peak intensity, it could also find that the more the alloy component content, the better the CH₄ and CO₂ conversions. The observed binding energy values for various Mo phases differ from the one reported in the Ref. [19] and the BEs were performed Mo⁰ (227.9–228.2 eV), Mo⁴⁺ (229.4–229.6 eV) and Mo⁶⁺ (232.5–232.6 eV) [19]. We could speculate that the peak of 227.52 eV was the metal state Mo⁰ (which was consistent with the results of XRD). The 234.41 eV may be Mo⁶⁺ existed in the NiMoO₄ and the 231.2 eV may be the Ni₃Mo alloy structure. Due to the different chemical environment of the Cu element, the observed binding energy values for various Cu phases were very complex. The satellite peak at 940–950 eV was the copper oxide [20]. The peak at 932.4 and 934.3 eV could be attributed to presence of Cu¹⁺ and Cu²⁺, respectively [21]. The binding energies at 952.2 and 932.3 eV can be attributed to Cu⁰ and Cu¹⁺, respectively [22]. From the XRD analysis, we could know that the Cu element existed in the Cu₆Mo₄O₁₅ metal oxides and the Cu_3.8Ni alloy. Due to the Ni²⁺, Mo⁶⁺, Al³⁺, O²⁻, the valence state of the Cu element should be Cu¹⁺. On this basis, the peak at 944.74 and 954.48 eV may be the Cu₆Mo₄O₁₅ metal oxides. The higher-intensive BEs (934.64 and 936.65 eV) may be attributed to Cu_3.8Ni alloy.

On the one hand, the NiAl₂O₄, Cu₆Mo₄O₁₅ and NiMoO₄ played a fixed role for the active components; on the other hand, they also provided the oxygen in the process of the reaction to prevent the generation of carbon deposition. Since the presence of the metal alloys could increase the dispersion of the active components, the existence of Ni₃Mo and Cu_3.8Ni alloy could also improve the catalytic activity of the Ni–Cu–Mo/Al₂O₃ catalyst.

Figure 9 presented the SEM images of Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo. The images showed that the metals dispersed uniformly on the support.

Fig. 9

SEM images of Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo a Ni:Mo = 0.15; b Ni:Mo = 0.30; c Ni:Mo = 0.45; d Ni:Mo = 0.60; e Ni:Mo = 0.75

Table 2 showed the surface and pore properties of Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo. The BET surface area, average pore diameter and pore volume did not vary much between Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo. The catalyst with Ni:Mo mass ratio of 0.75 exhibited the biggest BET surface area, which might be favorable for the catalytic reaction.

Table 2 The surface and pore structure of Ni–Cu–Mo/Al₂O₃ catalysts

The CO₂-TPD curves of Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo were presented in Fig. 10. It was apparent from TPD profiles that all catalysts showed three CO₂ desorption peaks at about 100, 200 and 450 °C, respectively. The research estimated that the CO₂ adsorbed on weaker sites was desorbed at low temperature while that adsorbed on strong sites was desorbed at high temperature [23]. The increase in Ni:Mo mass ratio led to the increase in peak areas toward high temperature, which were assigned to medium and strong sites.

Fig. 10

CO₂-TPD patterns of Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo 1 Ni:Mo = 0.15; 2 Ni:Mo = 0.30; 3 Ni:Mo = 0.45; 4 Ni:Mo = 0.60; 5 Ni:Mo = 0.75

In order to further verify the Ni–Cu–Mo/Al₂O₃ catalyst with the Ni/Mo ratio of 0.75 could simultaneously lead to both high stability and high activity, the catalyst was tested in the same reaction for 80 h at the 800 °C.

Zhang et al. [24] found that on the surface of Ni/Al₂O₃ catalyst there were three forms of carbon deposition: the surface of the catalyst was mainly C_α at 150–220 °C, which was a kind of active carbon mainly responsible for generating the syngas; At 500–600 °C, the catalyst surface were mainly C_β, which may be involved in generating CO at more than 600 °C; When temperature greater than 650 °C, the catalyst surface mainly existed in the form of C_γ, which was the main factor that led to the catalyst deactivation. Figure 11 showed the TG analysis of the Ni–Cu–Mo/Al₂O₃ catalyst which reacted in CH₄–CO₂ reforming reaction at 800 °C for 80 h. Figure 11 suggested that the amount of carbon deposition C_α and C_β were 1.17 and 51.57 %, respectively. And the C_γ was not found in the catalyst after reaction. Since the C_γ was the main factor that led to the catalyst deactivation, the absent of C_γ may be the reason for high catalytic stability of the Ni–Cu–Mo/Al₂O₃ catalyst for CH₄–CO₂ reforming reaction at 800 °C after 80 h. TG analysis further verified the existence of the alloy structure may inhibit carbon deposition.

Fig. 11

TG analysis of Ni–Cu–Mo/Al₂O₃ catalyst after reaction with the mass ratio of Ni:Mo = 0.75

The EDS quantitative analysis was carried out on the catalyst surface element and the relative contents of surface Ni, Cu and Mo were obtained for Ni–Cu–Mo/Al₂O₃ catalyst with the Ni/Mo ratio of 0.75. Data results as shown in Table 3 indicated that Ni–Cu–Mo/Al₂O₃ catalyst had good dispersion which could promote the reaction.

Table 3 Final composition of Ni–Cu–Mo/Al₂O₃ catalyst with Ni:Mo mass ratio of 0.75

4 Conclusions

With the addition of Cu and Mo, a series of Ni–Cu–Mo/Al₂O₃ catalysts with different mass ratio of Ni:Mo were prepared and applied for CH₄–CO₂ reforming. The spinel (NiMoO₄, NiAl₂O₄, Cu₆Mo₄O₁₅) and alloy (Ni₃Mo, Cu_3.8Ni) structures were formed. The chemical composition of the Ni–Cu and Ni–Mo alloys were found to have a higher dispersed state in the Ni–Cu–Mo/Al₂O₃ catalyst. The Ni–Cu–Mo/Al₂O₃ catalysts had high BET surface area and uniform distribution, and showed potential for carbon dioxide reforming of methane. The formation of Mo2C after reaction contributed to the prevention of carbon deposition and high stability of the catalyst.