Carbon intermediates during CO2 reforming of methane over Nisingle bondCaOsingle bondZrO2 catalysts: A temperature-programmed surface reaction study

https://doi.org/10.1016/j.ijhydene.2016.08.128Get rights and content

Highlights

  • The carbon elimination sequence of TPH over Nisingle bondCaOsingle bondZrO2 was different from TPO.

  • There existed an interaction between the carbon intermediate and catalyst support.

  • 2 was difficult to be removed by hydrogen or CO2 but easily to oxygen.

  • C-NCZ shows more Cβ2 species due to its strong redox property.

Abstract

The carbon intermediates for dry reforming of methane over Nisingle bondCaOsingle bondZrO2 catalysts during the initial reaction period were studied by temperature-programmed surface reaction (TPSR) techniques, including temperature-programmed hydrogenation (TPH), temperature-programmed CO2 reaction (TPRn CO2) and temperature-programmed oxidation (TPO). Three carbon species were detected on the catalysts surface during dry reforming, namely Cα, Cβ and Cγ. However, due to the different redox properties of Nisingle bondCaOsingle bondZrO2 catalysts, the individual carbon intermediate amount and interaction with the catalyst surface was distinct, which in turn led to the different reaction performance of both catalysts. The TPH-TPO and TPRn CO2-TPO tests showed that H2 and CO2 had weaker elimination ability than O2 and their elimination ability was lied on the carbon intermediate properties and the interaction with the support. According to the TPSR results, a possible mechanism of coking for dry reforming over Nisingle bondCaOsingle bondZrO2 catalysts was proposed, based on which the carbon intermediates might convert to the others or dissolve on the metal sites forming carbon deposits.

Introduction

During the past 30 years, carbon dioxide reforming of methane to produce synthesis gas is becoming more attractive due to the increasing interest in reduction of carbon dioxide emissions and efficient utilization of natural gas [1], [2], [3]. As shown in Eq. (1), dry reforming of methane (DRM) reaction can utilize two major greenhouse gases, CO2 and CH4, to produce hydrogen and carbon monoxide with a H2/CO molar ratio close to unity [4], [5], which could be further employed to synthesize high-value added products.CH4+CO2=2CO+2H2(DRM),ΔH298=247.3kJ/mol

Noble metals like Pt, Rh and Ru are reported to be highly active towards DRM and more resistant to carbon formation than other transition metals but the drawback is their high cost and limited availability [6], [7], [8], [9]. Ni-based catalysts were widely investigated in carbon dioxide reforming of methane due to their low cost and high activity comparable with supported noble metals [10], [11], [12], [13]. Unfortunately, Ni-based catalysts for dry reforming reaction have limited life time due to rapid and severe carbon deposition problem and thus the catalyst deactivation [1]. For example, Barama et al. [14] compared the performance of supported Rh, Ni, and Pd in DRM, equally high conversion was observed on Rh and Ni-based catalysts, but significant higher amount of carbon formation was detected on Ni, which may be a potential risk for deactivation. Therefore, many efforts have been focused on the development of Ni-based catalysts bearing both high activity towards synthesis gas production and high resistance towards carbon formation.

Coke originates mainly from two reasons, i.e. CH4 decomposition (MD, Eq. (2)) and CO disproportionation or Boudouard reaction (BR, Eq. (3)).CH42H2+C(MD),ΔH298=74.9kJ/mol2COCO2+C(BR),ΔH298=172.5kJ/mol

These carbon deposition causes catalyst deactivation or plugging of the reactor [15], [16]. From the industrial viewpoint, it is the chief target to develop cheap and economical catalysts which are resistant to coke.

In recent years, some workers began to find the relationship between catalytic deactivation and carbon deposition [17], [18], [19]. However, most of them concerned on the deposited carbon covered on the catalysts used for a long time, very less works considered the carbon intermediates during the initial period of reaction. Generally, the coke deposits came from various forms of carbon intermediates which had different reactivity toward hydrogenation and oxygenation. Zhang et al. [20] have found that three types of carbonaceous intermediates, i.e., Cα, Cβ and Cγ was formed during DRM reaction via temperature-programmed oxidation test. Chen [21] studied the temperature-programmed hydrogenation profiles of carbon intermediates on magnesia supported nickel catalyst, their result led us to conclude that two forms of carbonaceous species Cα and Cβ should play different roles in the reforming reaction. Guo [22] studied the activity of different coke intermediates from CH4 decomposition over a Ni/MgAl2O4 catalyst by temperature-programmed reactions with O2, CO2 and H2, respectively. The comparison of temperature-programmed oxidation profiles of these catalysts showed that the activities of different carbon species toward O2 were in the order: Cα > Cβ > Cγ while they had a more complicated activities towards H2 and CO2. Furthermore, it is more important to correlate these properties of carbonaceous species with the catalytic activity. By such a work, it was expected that more informative results would be provided not only for understanding the reaction mechanism, but also for finding a way to control the anti-coking performance of a catalyst.

Recently, due to thermal stability and redox property, the synthesis as well as application of ZrO2-based solid bases were carried out by several authors [23], [24]. In the previous work of our group [25], [26], [27], Nisingle bondCaOsingle bondZrO2 catalysts were prepared by different procedures and tested in CO2 reforming of CH4, and we found the co-precipitated nano-crystalline Nisingle bondCaOsingle bondZrO2 catalyst with strong metal-support interaction (SMSI) effect showed an excellent activity and stability during dry reforming. In this catalyst system, CaO played a significant role as strong solid base to prompt the adsorption of slightly acidic CO2 in the gaseous phase and facilitated the carbon elimination process through the reverse BR reaction (Eq. (3)) [28]. At the same time, ZrO2 was reported to have redox properties which was able to release lattice oxygen and be quickly re-oxidized in the presence of oxidative agents during reforming reaction [29]. And the cooperation between CaO and ZrOx was benefit for the fast dissociative adsorption or conversion of carbon intermediates. In order to have a further investigation on the coke behavior as well as the carbon formation mechanism of Nisingle bondCaOsingle bondZrO2 system at the initial reaction period, the catalysts were performed for a short time on stream of less than 1 h and temperature-programmed surface reactions with H2, CO2 and O2 were used to distinguish the coke intermediates according to their activity to reactants and interaction with catalyst surface.

Section snippets

Preparation of the Nisingle bondCaOsingle bondZrO2 catalysts

Nisingle bondCaOsingle bondZrO2 catalysts were prepared by two different co-precipitation methods. Ni(NO3)2·6H2O, Ca(NO3)2·4H2O and Zr(NO3)4·5H2O were used as precursors. For the co-precipitated samples, 1.00 g Pluronic P123 block copolymer (EO20PO20EO20), 0.04 mol Ni(NO3)2·6H2O, 0.02 mol Ca(NO3)2·4H2O and 0.10 mol Zr(NO3)4·5H2O (amounts of the used precursors were previously optimized) were dissolved in de-ionized water, the green solution was co-precipitated by NaOH aqueous solution at pH = 11–12, the obtained

Catalytic activity and catalyst deactivation

The catalytic activity and deactivation of C-NCZ and CN-NCZ catalysts in CO2 reforming of methane expressed as CH4 and CO2 conversion were shown in Fig. 1. C-NCZ showed a higher initial CO2 conversion in 1 h at 84.3% and deactivated slightly to 83.1% in 21 h. On the other hand, CN-NCZ showed a much lower initial CO2 conversion of 78.5% and fastly deactivated to 67.5% after reaction. Regardless of the catalysts used, the CO2 conversion was slightly higher than the corresponding CH4 conversion

Conclusion

Temperature-programmed surface reactions were carried out over Nisingle bondCaOsingle bondZrO2 catalysts used in 1 h of methane dry reforming to characterize the coke intermediates. TPH, TPRn CO2 and TPO reaction profiles showed that there were three carbon intermediates (i.e. Cα, Cβ and Cγ) formed on the catalyst surface. TPO plots showed that the activities of different carbon species toward O2 were in the order: Cα > Cβ > Cγ. However, due to the redox property of Nisingle bondCaOsingle bondZrO2 catalysts, they had more complicated

Acknowledgments

We gratefully appreciate the financial support of the International S&T Cooperation Program of China (2013DFA40460), the National Natural Science Foundation of China (21203230), the Coal Based Key Scientific and Technological Project of Shanxi Province (MH2014-06) and the Talent Development Funds of Shanxi University. N. Sun also wish to acknowledge the financialsupport from the “Youth Innovation Promotion Association, CAS”.

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