Page 222 - Advances in bioenergy (2016)

activity and selectivity toward H . Dispersion of Rh on MgAl-based spinel oxide supports
2
results in a significant decrease of surface acidity compared with Rh/Al O catalysts,
2 3
21
improving catalyst stability. A PdZn alloy is formed by codeposition of Pd and Zn on ZnO
support, which enhances dehydrogenation reaction and, consequently, hydrogen production. 22

One of the major problems in ethanol SR reaction is related to coke formation and deposition
onto the catalyst surface, which may lead to catalyst deactivation. Carbon formation may take
18
place via several processes, such as the ‘Boudouard’ reaction [Eq. (10)], methane
23
decomposition [Eq. (11)], and cracking of ethylene produced by dehydration of ethanol [Eq.
(12)]. 18,24

Carbon deposition can be avoided at high temperatures, where high hydrogen yield can be also
achieved. 23,25 However, high reaction temperatures enhance the formation of byproduct CO,
due to the thermodynamically favored reverse water–gas shift (WGS) [Eq. (6)] and dry
methane reforming [Eq. (7)]. For PEM fuel cell applications, the concentration of CO should
26
be reduced to less than 50 ppm, as dictated by the poisoning limit of Pt electrodes. This is
usually accomplished by treatment of the reformate gas in a WGS unit, followed by a final
purification step, which may involve preferential oxidation or methanation of residual CO, 27,28
resulting in higher hydrogen cost and larger size and weight of the fuel processor.

Low Temperature SR of Ethanol

As ethanol can be decomposed at low temperatures (300–400°C), it is of interest to develop
alternative processes and catalytic materials for hydrogen and/or power production. For
example, ethanol could be reformed at low temperatures (300–400°C) toward a gaseous
mixture containing H , CH , and carbon oxides. Depending on operating conditions and
4
2
catalyst, the amount of byproduct CO may be significant and should be reduced. This could be
accomplished by the development of novel catalytic materials capable to simultaneously
enhance the WGS reaction. The elimination of CO from the reformate gas can also result in a
decrease of CH selectivity by retarding CO methanation, which may run in parallel in the
4
temperature range of 300–400°C, consuming significant quantities of hydrogen. The resulting
gas mixture can then be used to feed fuel cells for the production of electricity and heat.

Advantages of the low temperature SR (LTSR) of ethanol include: (1) the reformer requires
lower quality heat to operate at low temperatures, making the thermal independence of the
process easier, (2) heat can be obtained from other processes, thus increasing system
efficiency, and (3) CO levels can be very low, making the final CO clean-up step easier.

The LTSR of ethanol has been investigated over a variety of metal catalysts, including Ni, Co,
Cu, Ir, Pt, Rh, Pd, and Ru dispersed on different supports (e.g., Al O , CeO , La O , ZrO ,
2
2
2 3
2 3
Y O , ZnO, Sm O , TiO , V O , and MgO). 24,29-32 In particular, it has been found that activity
2 3 2 3 2 2 5
and selectivity varies in the order of Ir > Ni > Co over CeO -supported catalysts, with Ir
2
exhibiting complete conversion of ethanol to H , CO , and CH and zero concentration of CO
2
2
4
32
at 350°C. Cobalt catalysts supported on ZnO exhibit high activity and selectivity toward H ,
2