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Introduction

Adsorption Enhanced Reforming (AER) is a novel steam reforming technique that enables the production of fuel-cell grade hydrogen in a single step. When CO₂ is removed from the reforming process, the water gas shift (WGS) reaction (CO + H₂O = CO₂ + H₂) shifts to the right which results in a relatively lower reforming temperature and also an enhancement of the yield and purity of H₂. The major CO₂ adsorbents reported in AER are hydrotalcites (Mg₆Al₂(CO₃)(OH)₁₆4H₂O) and CaO-based materials. This research project is to investigate the potential application of MgO aerogels as a new adsorbent at low temperatures. The research aspects include materials synthesis and characterization as well as thermodynamic modeling.

Synthesis and Characterization of MgO Aerogels

In order to study the effects of surface defects, dopant modification, and composition on CO₂ absorption of sol-gel derived MgO and MgO composites, the following materials were synthesized and characterized.

Amorphous MgO aerogels (with surface areas ~ 500 m²/g) and xerogels (with surface areas ~ 100 m²/g) were synthesized through the sol-gel process using a modified Pechini process.  The MgO aerogels and xerogels are amorphous and crystallize to the periclase phase at ~ 450°C in air, without passing through the brucite phase (Figure 1).

Figure 1: XRD shows that sol-gel derived MgO are amorphous and crystallize to the periclase phase at ~ 400°C.  The commercial MgO contains both brucite and periclase forms.

This is important to note since the crystalline MgO synthesized from sol-gel derived aerogels and xerogels are purer forms of MgO, whereas commercially available MgO contain traces of the brucite (Mg(OH)₂) phase.  The periclase phase has XRD peaks at 2¦È = 38^o, 43^o, and 62° whereas the brucite phase has primary peaks at 2¦È = 19^o and 39°.

When sol-gel derived MgO were heat treated in CO₂ instead of air, the transition to the periclase phase was delayed until 600°C (Figure 2). Brucite formation was also not observed for this treatment process.

                           Figure 2: XRD of sol-gel derived MgO heat-treated under CO2.

MgO doped with 3 mol% of Ni and 2 mol% of Co were also synthesized.  Neither dopants changed the crystallinity of the MgO matrix and were too low in concentration to be detected with XRD.  For CO₂ adsorption between ambient to 800°C a thermogravimetric analyzer (TGA, TA Instruments, SDT 2960) will be used.  Preliminary studies with TGA under air were conducted to determine material evolution with heat treatment.  Figure 3 shows that all samples lost approximately 60% of their initial mass due to loss of surface adsorbed water, organic solvents (80 - 180°C, physisorbed water and organic solvents (300 - 400°C).  Doping with Ni did not change the mass loss profile of the MgO.  Doping with Co appeared to slow the desorption process.  A similar study under wet-CO₂ and dry-CO₂ will be conducted to determine the adsorption behavior of these materials.

              Figure 3:  Mass loss profiles of MgO, Ni-doped MgO, and Co-doped MgO.

Current work includes synthesizing Rh-doped MgO as well as a sol-gel derived MgO-Al₂O₃ composite that is similar to hydrotalcite but with higher surface areas and exhibit metastable phases not available through the traditional synthesis process. 

Thermodynamic analysis of CO₂ adsorption on steam reforming

A thermodynamic model has been developed to study the effect of CO₂ adsorption on steam reforming. The model is based on the minimization of Gibbs free energy of the reactive system with the assumption of Langmuir isotherm:

(1)

The above model was applied to the steam reforming of ethanol at T = 500°C, P = 5 bar and molar ratio H₂O/EtOH = 3. The model was solved based on Langrage multiplier method for and some of the results are summarized below.

The gas composition in the product mixture under different CO₂^(ads)/CO₂ (a parameter approximately proportional to the capacity of the adsorbent) is shown in Figure 4. As CO₂^(ads)/CO₂ increases, the fraction of CO₂(g) rapidly decreases to a minimal level (at CO₂^(ads)/CO₂=10^1~2), beyond which a significant reduction in the CO or CO₂ concentration is not obvious. The low partial pressure of CO₂ in the gas phase makes it difficult for a further adsorption. When CO₂ is gradually adsorbed on the surface, the forward reaction in the reversible water gas shift reaction CO + H₂O = CO₂ + H₂ is favored. As a result, part of the CH₄ is converted to CO through the methane reforming reaction CH₄ + H₂O = CO + 3H₂. In this sense, the entire system behaves like a buffer solution. Due to the coupling of multiple equilibrium phenomena, a complete removal of CO₂ is very difficult. However, the fact that the primary carrier of carbon in the gaseous phase is CH₄ (instead of CO or CO₂) at CO₂^(ads)/CO₂ > 10¹ implies that a complete removal of CO₂ is not necessary.

Figure 4: Gas compositions of ethanol reforming at equilibrium as a function of CO₂^(ads)/CO₂ (H₂O/EtOH = 1/3, P = 5 bar and T = 500°C).

A parametric analysis based on Eq. (1) was made in a wide range of operating conditions such as temperature, pressure, steam-to-carbon ratio and with/without the presence of oxygen. It was shown in all cases that CO₂ adsorption not only enhances H₂ yield and purity, but also suppresses the formation of coke, an undesired material that could deactivate the reforming catalyst. For example, Figure 5 shows clearly that the coke formation is suppressed by increased CO₂ adsorption at a fixed pressure.

Figure 5: Contour of coke formation as functions of system pressure and CO₂ adsorption.