Promoting effect of Sn on supported Ni catalyst during steam reforming of glycerol

Highlights

• The characterization results revealed the formation of NiSn alloy.

• Sn-doped catalyst exhibited a high activity.

• Sn addition increase the durability by decreasing the coke deposition.

Abstract

The promoting effect of Sn on the catalytic performance of supported Ni catalyst in the reaction of glycerol steam reforming was studied. The physico-chemical properties of the prepared samples were investigated by X-ray fluorescence (XRF), BET surface area, in situ X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO) techniques. The characterization results of the samples after reduction treatment (in the same conditions than the activation before catalytic activity measurements) revealed the formation of NiSn alloy. The Sn-doped catalyst exhibited a high activity and it was demonstrated that the Sn addition increase the catalyst stability and durability by decreasing the coke deposition.

Introduction

During the last decades important efforts are ongoing to reduce fossil fuel dependency and greenhouse gas emissions. Among the different possibilities to find a viable option, the use of hydrogen as an energy carrier is considered to be an interesting alternative for the future and could have a crucial role in reducing atmospheric pollution. Biomass is considered as one of the most attractive renewable source for hydrogen production and numerous studies are being directed toward the development of novel technologies to produce hydrogen from biomass [1], [2], [3], [4]. Steam reforming of glycerol is an important approach for hydrogen production from biomass. Glycerol is the main by-product generated in the biodiesel production. Biodiesel is one renewable biofuel obtained by catalytic trans-esterification of triglycerides with methanol [5]. About 10 wt.% of glycerol can be produced during the conversion of vegetable oils into biodiesel. The use of glycerol for hydrogen generation is a very advantageous option since its use would decrease the price of biodiesel making it more competitive [6].

The overall reaction of glycerol steam reforming is given by the following equation:

C₃H₈O₃ + 3H₂O → 3CO₂ + 7H₂

which can be expressed as a combination of glycerol decomposition (2) and the water-gas shift reaction (3):

C₃H₈O₃ → 3CO + 4H₂

CO + H₂O ↔ CO₂ + H₂

Theoretically, a maximum of 7 mol of H₂ per mol of glycerol can be produced, although this ratio depends on the reaction conditions such as temperature, pressure and steam-to-glycerol steam ratio.

Although the glycerol steam reforming process is very attractive and it could be developed on an industrial-scale, it has some challenges that must to be overcome in order to accomplish its effective commercialization. For example, the process is an endothermic reaction and requires high temperatures increasing the operation costs. Besides, the catalyst deactivation by coke deposition is also an issue since it affects hydrogen yield and long term operation. With the aim of overcoming these challenges, the development of active, stable and inexpensive catalytic materials is mandatory. Catalysts containing group 8–10 metals such as Ni [7], [8], [9], [10], [11], [12], Co [13], [14], [15], [16], Pt [17], [18], [19], [20], [21], Ru [22], [23], [24], Rh [25], Pd [26] or Ir [27] on different oxides have been largely investigated as active catalysts for glycerol steam reforming. Ni-supported is one of the most promising active metals for such an application because of its high activity, low cost and wide availability. However, Ni-based catalysts suffer deactivation by coke deposition on the catalyst surface that block active sites and favour side reactions. Deactivation of nickel-based catalysts by sintering of nickel crystallites is another important drawback. Promoting nickel catalysts with a second metal has been proven to be one of the most promising approach to obtain more stable and optimal catalysts [28]. Bimetallic catalysts comprising Sn as a Ni promoter have been proved to outperform Ni monometallic catalyst in steam reforming processes [29]. The coke deposition can be markedly reduced by using Sn-doped catalysts in the steam reforming reaction to generate hydrogen from hydrocarbons. Sn alloyed with nickel prevents the formation of nickel atom ensembles, which are the responsible of the coke formation, and avoids the diffusion of carbon to form larger coke agglomerates [30]. Therefore, NiSn-based catalysts have the potential to decrease the catalyst deactivation caused by coking maintaining its high specific activity. Sn-doped Ni catalysts have been reported for aqueous phase reforming [31], [32] and methane steam reforming [33] where the formation Ni_xSn_y alloys play a key role to inhibit coke deposition. Pengpanich et al. [34] reported a clear example of this positive effect in the partial oxidation of iso-octane. These authors found that the addition of small amounts of tin decreased by more than 50% the formation of carbon deposits without changes in the conversion. This enhancement was ascribed to the ability of Sn to reduce the growth of carbon filament by retarding carbon solubility in the Ni particles [28]. Moreover, Saadi et al. [35] have demonstrated, using density functional theory (DFT) calculations, the ability of NiSn to inhibit graphite formation during steam reforming reactions. They demonstrated that the presence of Sn increases the CC bond formation barrier.

We have investigated previously the effect of the nature of the support (acidity, basicity and redox properties) in the catalytic performance of Sn-doped Ni catalysts during the steam reforming of alcohols [36], [37], [38]. The objective of the present work is to investigate the effect of tin on the catalytic performance of Ni-supported catalysts in terms of activity, selectivity and durability in the steam reforming of glycerol.