Oxidative-reforming of model biogas over NiO/Al₂O₃ catalysts: The influence of the variation of support synthesis conditions

Highlights

• Precipitation pH and ageing T °C of Al₂O₃ influenced the performance of Ni/Al₂O₃.

• Ni catalysts supported on Al₂O₃ obtained at pH 7 recorded high conversion values.

• Catalysts supported on Al₂O₃ obtained at pH 7 and 80 °C are promissory for reforming of biogas.

• Catalysts supported on Al₂O₃ obtained at pH 6 deactivated readily during reaction.

Abstract

In this study, nickel catalysts (20 wt%) supported on γ-Al₂O₃ were prepared by the impregnation method. The γ-Al₂O₃, was synthesized by precipitation of bayerite gel obtained from aluminum scrap. The synthetic conditions of the bayerite gel varied as follows: precipitation pH ranging from 6 to 7; ageing temperature ranging from 25 to 80 °C, the calcination temperature for all samples was 500 °C. The catalysts and the supports were analyzed by temperature programmed reduction (H₂-TPR), X-ray diffraction (XRD), physisorption of N₂ (BET), X-ray absorption near-edge structure (XANES) and scanning electron microscopy (SEM). Isopropanol decomposition reactions over the catalysts were carried out to evaluate their acidity. SEM images of the spent catalysts showed that the morphology of the carbon formed during the reaction is of the filamentous type. The TPR analysis of the catalysts showed the presence of NiO species weakly interacted with the support as well as stoichiometric and non-stoichiometric nickel aluminate, the reduction of these species was also observed by XANES analysis. XRD analysis of the fresh catalyst showed peaks assigned to NiO, NiAl₂O₄ and γ-Al₂O₃. The best catalysts (samples NiAl7-25 and NiAl7-80) synthesized in this report showed high stability and high conversion values (CH₄ (70%) and CO₂ (78%)). These catalysts showed better performance than the catalyst supported on commercial γ-Al₂O₃, which showed a high coke formation which affected the course of the reaction. The γ-Al₂O₃ synthesized from bayerite obtained at neutral pH conditions was the best support for nickel catalysts in the oxidative-reforming of model biogas.

Introduction

Biogas is produced by anaerobic fermentation of biomass and is composed primarily of methane and carbon dioxide (molar ratio CH₄:CO₂ = 1.5), and other gaseous by-products such as H₂S, NH₃, H₂. Its composition can vary with the origin of the biomass feedstock and the operational conditions during fermentation [1], [2]. Most of the hydrogen and syngas for industrial use is produced through steam reforming of methane. The biogas can be used as a substitute for natural gas in reforming processes, which is advantageous since biogas is considered a renewable source.

Thus, due to its principal components, biogas can be transformed into synthesis gas (syngas = H₂/CO) through the dry reforming of methane (DRM, reaction (1)) in a CH₄:CO₂ molar ratio of 1, and the excess methane can be partially oxidized by the POM reaction (reaction (2)). These coupled processes result in the oxidative-reforming of biogas [3], [4]:

DRM:     1.5CH₄ + 1CO₂ → 2CO + 2H₂ + 0.5 CH₄  ΔH° = 260.5 kJ mol⁻¹

POM:     0.5CH₄ + 0.25O₂ → 0.5CO + 1H₂    ΔH° = −22.6 kJ mol⁻¹

It is known that the POM reaction over nickel catalysts follows the combustion-reforming pathway [5], [6], where the total combustion of methane (TCM), dry reforming of methane (DRM) and steam reforming of methane (SRM) occur in parallel:

TCM:  CH₄ + 2O₂ → CO₂ + 2H₂O   ΔH° = −890 kJ mol⁻¹

DRM:  CH₄ + CO₂ → 2CO + 2H₂   ΔH° = 260.5 kJ mol⁻¹

SRM:  CH₄ + H₂O → CO + 3H₂   ΔH° = 225.4 kJ mol⁻¹

All these reactions (1), (2), (3), (4), (5), occur in parallel during the oxidative-reforming of biogas. The water–gas shift reaction (WGSR) is a reversible and exothermic reaction, and its reverse direction (RWGSR, reaction (6)) is favored at high temperatures where the reforming of methane takes place; so the occurrence of the RWGSR is very likely to occur during the reforming of biogas:

RWGSR:  CO₂ + H₂ ↔ CO + H₂O    ΔH° = 34.3 kJ mol⁻¹

The syngas (H₂/CO) produced by the reforming of the biogas can be used in the production of dimethyl ether (DME), a promising substituent for commercial diesel, through the syngas to DME process (known as STD process). Syngas can also be used as a feedstock in the Fischer-Tropsch process. The oxidative-reforming of biogas (reactions (1), (2), (3), (4), (5)) produces syngas with a H₂/CO ratio of 1.2, which allows its direct use in the STD process [7], [8].

Several metal catalysts have been proposed for methane decomposition: (i) the non-noble group VIII: Ni, Co, Fe; (ii) the noble group VIII: Ru, Rh, Pd, Pt, Ir; and (iii) transition metal carbide catalysts. Within non-noble group VIII, nickel based catalysts are the most promising as they are inexpensive, of high availability and show catalytic activity comparable to that of noble metals [9]. Nevertheless, nickel based catalysts are affected by carbon incrustation in the metallic nickel particles which leads to deactivation, additionally it was found that the carbon is more soluble in nickel than in noble metals (e.g. Rh, Ru, Ir, Pt) [10].

The methane reforming industry uses the commercial catalyst, nickel supported on alumina (Ni/Al₂O₃); however, it presents problems due to carbon deposits that lead to deactivation. This problem is related to coking (deposition of coke) caused by nickel incrustation, and can also be promoted by the acidity of the alumina that also favors the carbon deposition. Some pathways have been proposed to explain the carbon deposition during the reforming of methane, among them, are the cracking reaction of methane (reaction (7)) and the Boudouard reaction (8):

CH₄ → C + 2H₂      ΔH° = 74.8 kJ mol⁻¹

2CO → C + CO₂      ΔH° = −172.8 kJ mol⁻¹

Coke formation is inevitable under the reforming process operating conditions (<500 °C), hence various alternatives have been proposed to minimize the carbon deposits including: the use of solid solutions as catalytic supports [3], [6], [11], [12], the reduction of the metallic nickel crystallite size [13], [14] and the addition of promoters [4], [15], [16], among others. Another proposal to improve the catalytic performance, thereby reducing the coke deposits, is to modify the acidic–basic properties of the catalyst by the addition of alkaline-earth cations (e.g. Ca²⁺, Mg²⁺, Ba²⁺) or rare earth metal oxides (e.g. La₂O₃, Y₂O₃) [17].

The γ-alumina is a solid-acid which is extensively used as catalytic support due to its high surface area, acidic properties (due to Bronsted acid centers: Al–OH), thermal stability, low toxicity and low cost [18], [19], [20]. The properties of alumina depend on the synthetic method which can affect the catalytic properties of the resulting catalyst for the methane reforming reaction, and can lead to different conversion rates and different types of carbon deposits [21]. Currently, there are not many studies focused on the variation of the synthesis conditions of γ-alumina obtained by the sol–gel method, and its influence on the catalysis of the reforming of biogas.

Aluminum is an abundant raw material on earth and is mainly extracted from bauxite, much of this aluminum is recycled. In our previous work [18], various high-surface-area aluminas (the highest values recorded 371 m² g⁻¹) were obtained from aluminum scrap (aluminum cans of >99% purity) by the sol–gel method under moderate synthesis conditions using inexpensive starting materials. In that report, the influence of the variation of the synthesis conditions (precipitation pH and ageing temperature) in the physicochemical properties of the resulting γ-aluminas was studied. The present paper is a continuation of that previous report [18]; hence, the present study focuses on the influence of this variation, on the catalytic properties of the resulting nickel catalysts. The physicochemical properties of the resulting Ni/γ-Al₂O₃ catalysts were correlated with their catalytic behavior in the oxidative-reforming of model biogas and with their coke deposition rates; these results were compared with a nickel catalyst supported on a commercial γ-alumina.