Page 226 - Advances in bioenergy (2016)

Investigations regarding the contribution of the metal phase and the support material in ethanol
conversion as well as in products selectivity showed that neither Pt alone nor Al O alone are
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capable of effectively catalyzing the reformation of ethanol. Product distribution results
demonstrated that the carrier catalyzes mostly dehydration reactions. Its specific activity is low
but it provides a large surface area and its overall contribution may be significant. Platinum
catalyzes the dehydrogenation reaction, and, mostly, the decomposition/reformation of
dehydration products, as well as the WGS reaction. At the metal/support interface, Al O
2 3
functions to adsorb ethylene and diethyl ether, whereas Pt functions to promote their
fragmentation and decomposition toward smaller molecules. The WGS reaction also takes
place at the metal/support interface.

The WGS reaction is a critical step in fuel processors, which is used to improve the H yield
2
and to reduce CO levels in the reformate gas, before a final purification step, which may
involve preferential oxidation or methanation of residual CO. 38,39 WGS catalysts for fuel cell
applications should be sufficiently active and selective in the temperature range of 200–280°C,
thermally stable, and resistant to poisoning under the reformer's conditions. Noble metal
catalysts may offer significant advantages, compared with conventional WGS catalysts and
therefore have been widely investigated in the last few years.

Among the various metals examined so far, including Pt, 40-46 Rh, 40-43,45,47 Ru, 40,42,43,45,48
40
40
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Pd , 40,42-45,49 Au, 40,46,50-53 Re, Os, Ir, 40,44,54 Ag, Cu, 40,52,55 Ni, 40,49,55 Fe, 40,49 and Co,40,49
Pt catalysts seem to be promising candidates, characterized by high activity at low-to-medium
reaction temperatures. In particular, investigations over Pt, Rh, Ru, or Pd catalysts supported
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43
on TiO , CeO , or Al O 43 showed that the turnover frequency (TOF) of CO conversion at
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250°C varies in the order of Pt > Rh ≈ Ru > Pd, with Pt being 20–50 times more active than
Pd, depending on the metal oxide support, whereas Rh and Ru exhibited an intermediate
44
performance. 42,43 Similarly, Sato et al. reported that Pt/TiO catalyst exhibits higher activity
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compared to Pd and Ir, whereas Radhakrishnan et al. found that the order of activity for the
WGS reaction is Pt > Rh > Ru > Pd over catalysts supported on ceria–zirconia oxides.
Results of kinetic measurements obtained with the use of Pt or Ru catalysts of variable metal
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content (0.1–5 wt%) supported on TiO , CeO , or Al O 43 showed that the specific
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reaction rate (TOF) does not depend on metal loading (0.1–5.0 wt%) or Pt crystallite size
(1.2–16.2 nm). Thus, it can be concluded that the WGS reaction over supported noble metal
catalysts is structure insensitive, as far as the metallic phase is concerned, in agreement with
other studies. 40,56-58

Detailed investigations of a variety of metal–support combinations showed that the key
parameters that determine the WGS activity of dispersed noble metal catalysts are related
mainly to the nature and the physicochemical characteristics of the metal oxide support. 43,59
Generally, it has been shown that noble metals exhibit significantly higher activities when
supported on ‘reducible’ (e.g., TiO , CeO , La O ) compared with ‘irreducible’ oxides (e.g.,
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Al O , MgO, SiO ). 43,59,60 Titania-supported Pt catalysts exhibit sufficiently high activity,
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2