Dry Reforming of Methane over Zr,Y-Modified Ni/Mg/Al Hydrotalcite Catalysts for Hydrogen and Carbon Monoxide Production

Dry reforming of methane over Zr,Y-modified Ni/Mg/Al hydrotalcite catalysts for hydrogen and carbon monoxide production

Katarzyna Świrk^a,b, Maria Elena Gálvez^b, Monika Motak^a, Teresa Grzybek^a, Patrick Da Costa^b

 

^a AGH University of Science and Technology, Faculty of Energy and Fuels, Al. A. Mickiewicza 30, 30-059, Krakow, Poland

^b Sorbonne Universités, UPMC Paris 6, CNRS, UMR 7190, Institut Jean Le Rond d'Alembert, 2 Place de la Gare de Ceinture, 78210, Saint-Cyr-L'Ecole, France

1. Introduction

Dry reforming of methane (DRM) is a method of producing hydrogen and carbon monoxide while utilizing CO₂. The process has not been launched yet on an industrial scale due to lack of stable, active and selective catalyst [1]. Ni-based catalysts, used e.g. in steam reforming, seem to be a great alternative for expensive noble metal materials. However, the main drawbacks of nickel catalysts is their fast deactivation caused by carbon deposition and/or sintering [2].

The hydrotalcites (HTs, or layered double hydrotalcites LDHs, which exhibit brucite-like structure) appear to be promising support for catalysts due to their strong interrelated nanostructure which hinders Ni from sintering at high temperatures needed for DRM [3]. On the other hand, an addition of metallic promoters may positively influence catalytic activity and stability in DRM process [4].

Zr and Y species seems to be good components for the DRM catalyst, because only a slight addition of yttrium may stabilize the zirconia phase resulting in high thermal stability of catalysts [5]. Moreover, the yttrium as a promoter, enhances activity and stability of the used catalyst in DRM process, due to ability of oxygen vacancies creation [6].

Thus, the goal of this work was to examine the effect of the promotion of Ni/Mg/Al hydrotalcites with Zr and Y on their structure and performance in dry methane reforming.

2. Experimental

The HT sample was prepared by the co-precipitation method at pH of 10 ± 2, from the aqueous solution of nickel, magnesium and aluminum nitrates (Ni/Mg=0.33, M²⁺/M³⁺=3.0). Subsequently, the fresh HT material was modified with zirconium (5 wt.%) and yttrium (0.2 wt.%, 0.4 wt.%, or 0.6 wt.%), resulting in HT/Zr5 Y0.2, HT/Zr5 Y0.4, HT/Zr5 Y0.6 catalysts. The introduction of these two metals occurred through wetness impregnation method (co-impregnation). So synthesized materials were dried at 80^oC overnight, and calcined in air at 550^oC for 6 hours. The obtained catalysts were characterized by XRD, TPR-H₂, TPD-CO₂, and FTIR.

The dry reforming tests were carried out in a fixed-bed quartz reactor at temperature range of 850-600°C. Before each catalytic test, the samples were reduced in situ at 900^oC for 1 hour with hydrogen (5% H₂ in Ar, flow 10^oC/min). The gas composition was adjusted to 1/1/8=CH₄/CO₂/Ar, with total flow of feed gases of 100 cm³/min (GHSV=20,000h^-1). The final products were analyzed by Varian GC-490 micro-GC.

3. Results and discussion

The X-ray analysis for the fresh samples confirmed successful synthesis of hydrotalcite-like materials (reflections at 2θ equal to ca. 11, 22 and 35^o). Also, the periclase-like structure was registered after thermal treatment in air. On top of this, separate phase of Zr compound (ca. 31^o and 51^o) was observed. However, no separate phase was registered for yttrium in the XRD patterns of calcined and reduced samples.

The TPR-H₂ profiles show wide peaks with maximum value at ca. 830-860^oC, which arise from reduction of Ni particles derived from mixed oxides in hydrotalcite structure. The reduction temperature is dependent on the presence of promoters. There was observed a shift of the wide peak towards higher temperatures in Zr, and Zr,Y-modified samples (847^oC), with the highest temperature reached at 860^oC for HT/Zr5 Y0.4 sample. This shift indicates decrease in reducibility, and stronger interaction of Ni with the support. The observed peaks at lower temperature, ca. 400^oC, arise from NiO weakly bonded with the support.

The Zr,Y-modification influenced basicity of tested samples by expanding the number of medium sites while increasing Y content. Although, the total basicity decreased with promoters addition.

The FTIR measurements showed typical spectra for hydrotalcite-derived material.

The DRM tests (CH₄/CO₂/Ar=1/1/8) showed increase in CH₄ and CO₂ conversions while increasing temperature (Fig. 1 and Fig.2). The best results in the low temperature region (600-700^oC) were observed for Zr,Y-modified samples. At 700°C the conversions obtained following sequence: HT/Zr5 Y0.6 > HT/Zr5 Y0.4 > HT/Zr5 > HT > HT/Zr5 Y0.2. Above this temperature the best performance was found for the catalyst impregnated only with Zr. The presence of side reactions, such as RWGS was deduced through CO₂ conversion greater than CH₄ conversion, and H₂/CO lower than 1.

Fig. 1 CH₄ conversion as a function of reaction temperature (DRM process CH₄/CO₂/Ar=1/1/8, GHSV=20,000h^-1) of hydrotalcite catalysts modified with Zr and Y

Fig. 2 CO₂ conversion as a function of reaction temperature (DRM process CH₄/CO₂/Ar=1/1/8, GHSV=20,000h^-1) of hydrotalcite catalysts modified with Zr and Y

4. Conclusions

All prepared catalysts were catalytically active in the DRM process. The presence of Zr and Y in Ni-containing hydrotalcite-derived catalysts resulted in a higher number of medium basic sites, which contributes to enhanced catalytic activities in DRM at low temperatures. The DRM tests show that the HT/Zr5 Y0.6 gives the highest CH₄ and CO₂ conversion, and the highest H₂/CO ratio in the temperature range of 600^oC to 700^oC. At temperatures higher than 700^oC the presence of yttrium did not increase the catalytic activity of the Zr-modified sample. The effect of the reverse water gas shift reaction, on the hydrogen production and the molar ratio H₂/CO has been observed.

Acknowledgments

K. Świrk would like to acknowledge for financial support the French Embassy in Poland and InnoEnergy PhD School.

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