Oxidative steam reforming of methane under atmospheric and pressurized conditions over Pd/NiO–MgO solid solution catalysts

doi:10.1016/j.apcata.2006.03.054

Applied Catalysis A: General

Volume 308, 10 July 2006, Pages 1-12

https://doi.org/10.1016/j.apcata.2006.03.054 Get rights and content

Abstract

The effect of the Pd loading in NiO–MgO solid solution was investigated in oxidative steam reforming of methane under atmospheric and pressurized conditions. The small amounts of Pd addition on Ni_0.2Mg_0.8O can inhibit the oxidation of Ni species and the mixture can maintain the reforming ability at low W/F conditions under atmospheric pressure, where Ni_0.2Mg_0.8O catalyst deactivated. In addition, Ni_0.2Mg_0.8O catalyst improved remarkably, gaining both high activity and excellent stability by the addition of small amounts of Pd under pressurized conditions. The deactivation rate was closely related to the amount of deposited carbon. Characterization results of TEM, TPR, EXAFS and FTIR suggest that Pd–Ni alloy particles are formed on Pd/Ni_0.2Mg_0.8O catalysts. The Pd–Ni alloy particles with a low ratio of Pd to Ni can play an important role in the high resistance to carbon deposition.

Introduction

Reforming of natural gas is a useful method for obtaining hydrogen for fuel cells as well as synthesis gas for producing chemicals such as methanol, dimethyl ether, Fischer–Tropsch oils and fuel-range hydrocarbons [1], [2], [3], [4], [5], [6]. It is well known that steam reforming of methane is an endothermic reaction (Eq. (1)). Therefore, in this conventional system, much external heat must been supplied from the outside of the reactor.CH₄ + H₂O → CO + 3H₂ ΔH₂₉₈ = 206 kJ/mol

On the other hand, oxidative steam reforming of methane can be an autothermal reaction when the partial pressure ratio is suitable. In such a reaction, oxygen is utilized together with steam, and the endothermic steam reforming is combined with exothermic combustion (Eq. (2)) and partial oxidation of methane (Eq. (3)) [7], [8], [9], [10].CH₄ + 2O₂ → CO₂ + 2H₂O ΔH₂₉₈ = −803 kJ/molCH₄ + 1/2O₂ → CO + 2H₂ ΔH₂₉₈ = −36 kJ/mol

Under the autothermal condition, the external heat supply becomes unnecessary. In the oxidative steam reforming of methane, the Ni catalysts easily deactivate due to the oxidation of Ni metal species [9], [10], [11], [12], [13]. On the other hand, it has been reported that additions of small amounts of noble metal on Ni catalysts enhanced the catalytic activity in the oxidative steam reforming of methane [9], [10], [11], [12], [13]. Another problem of the oxidative reforming of methane is hot spot formation on the catalyst surface, in particular, near the catalyst bed inlet. In the case of monometallic Ni catalysts, hot spot formation has been observed by the catalyst bed temperature profile [14], [15], [16]. According to the previous results, Pt and Rh are effective components to the suppression of hot spot formation in oxidative reforming of methane [17], [18], [19]. Furthermore, it has been reported that the addition of Pt to Ni catalysts is very effective to the suppression of hot spot formation [20], [21]. This function is explained by the overlap of the combustion and reforming reaction zones.

Another serious problem in oxidative steam reforming of methane is carbon deposition. This problem is common to the process for the synthesis gas production by reforming of methane and hydrocarbons [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Therefore, the catalysts should have high resistance to carbon deposition. Especially, since carbon deposition is more remarkable under more pressurized conditions, the tests under pressurized conditions are very important. In our previous reports, we reported that the additive effect of noble metals such as Rh and Pt over NiO–MgO solid solution can completely inhibit the carbon deposition and the catalyst deactivation in oxidative steam reforming of methane even under pressurized condition (1.0 MPa) [10], [12]. In this research, we investigated the role of the Pd addition on NiO–MgO solid solution in the oxidative steam reforming of methane under atmospheric and pressurized conditions. The resistance to carbon formation on Pd is not so high as that on Rh or on Pt [34], [35]. However, it is found that the addition of the optimum loading amount of Pd to Ni catalyst can give high resistance to carbon deposition and catalyst deactivation thanks to the synergistic effect of Pd and Ni. The role of Pd is discussed on the basis of the catalyst characterizations.

Section snippets

Catalyst preparation

Ni_0.2Mg_0.8O solid solution was prepared by the solid reaction method from NiO (Wako Pure Chemical Industries Ltd., Japan) and MgO (UBE Material Industries Ltd., Japan). The mixture of NiO with MgO was calcined in air at 1423 K for 12 h. The formation of Ni_0.2Mg_0.8O solid solution was identified by X-ray diffraction (XRD), as has been shown in our previous report [11]. After the calcination, the BET surface area of Ni_0.2Mg_0.8O was determined to be 2.6 m²/g. As a reference, MgO was calcined at the

Catalytic performance in oxidative steam reforming of methane under atmospheric pressure

Fig. 1 shows contact time (W/F) dependence of the methane conversion, H₂/CO ratio and CO selectivity in oxidative steam reforming of methane over Ni_0.2Mg_0.8O, Pd/Ni_0.2Mg_0.8O, MgO and Pd/MgO at 1073 K. As shown in Fig. 1(a)–(c), Ni_0.2Mg_0.8O gave high methane conversion at W/F = 0.4 g h/mol; however, methane conversion decreased with decreasing W/F drastically. In particular, methane conversion was about 25% at W/F = 0.13 g h/mol, where H₂/CO was almost zero, and CO selectivity was about 20%. This means

Conclusions

(1)
In oxidative steam reforming of methane under atmospheric pressure, the catalytic performance of Ni_0.2Mg_0.8O was improved drastically by the addition of small amounts of Pd metal, especially at low W/F values such as 0.13 g h/mol and high steam partial pressure conditions. In addition, Pd/Ni_0.2Mg_0.8O showed much higher catalytic performance than Pd/MgO in all the range of W/F conditions. The synergistic effect between Pd and Ni on catalytic performance in oxidative reforming of methane, in

Acknowledgements

A part of this study is supported by a Industrial Technology Research grant (05A43002C) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. EXAFS studies has been performed under the approval of the Photon Factor Advisory Committee (proposal no. 2006G095).

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