Page 227 - Advances in bioenergy (2016)

which can be further improved by addition of small amounts of alkali (Li, Na, K, Cs) or
alkaline earth promoters (Ca, Ba, Sr, Mg). 61-63 Optimum results have been obtained over Na-
and Ca-promoted samples, compared with samples containing Cs, Li, Sr, Ba, or Mg, 62,63 where

the specific activity (TOF) is about three times higher, compared with that of the unpromoted
catalyst. Qualitatively similar results were obtained for Ru and Pd catalysts. 61,62 The
beneficial effect of alkalis on WGS activity has been also observed over Pt/ZrO 2 64 and
Pt/CeO 65 catalysts doped with Na, K, Rb, or Cs. According to the authors, there is an
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optimum concentration of alkali where the CH bond of the surface formate intermediate is
weakened and enhances the WGS activity at low temperatures.

Mechanistic Aspects of the Ethanol Reforming Reaction

Investigation of the mechanism of ethanol SR has been the subject of several studies. 30,37,66-71 It
has been proposed that adsorbed ethanol or ethoxy species dehydrogenate toward

acetaldehyde, which can be either oxidized by hydroxyl groups over the alumina surface,
resulting in the formation of surface acetate species, or it can be decomposed to methane and
carbon monoxide. 30,37,66-71 Acetate species can be decomposed toward CH 3(ad) species, which

can further react with surface hydroxyl groups producing CO. 37,66,67,70,71 Although the overall
reaction network of ethanol SR is highly complicated, 24,37 it is generally agreed that ethanol
interacts strongly with the support, promoting mainly dehydration reactions. Platinum enhances
ethanol decomposition and dehydrogenation reactions at low temperatures, producing CH ,
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CO, and H , while reforming, WGS, and methanation reactions dominate at higher
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temperatures resulting in the formation of hydrogen and carbon oxides. 24,37

REFORMATION OF BIOGAS

Biogas refers to a mixture primarily composed of methane and carbon dioxide, produced via
anaerobic fermentation or digestion of organic matter due to microbiological action of
bacteria. The chemical composition and physical properties of biogas are strongly related to
the type of feedstock, the technical design of the digester–fermenter and operating conditions.
The two major components are always methane and carbon dioxide, but their ratio varies
significantly. Hydrogen sulfide is the impurity that can cause the most important nuisance in
biogas applications, whereas higher hydrocarbons, aromatics, chroro/fluorocarbons, organic
sulfur component, oxygenated compounds, chlorine, fluorine, as well as silicon compounds
(siloxanes) can also be present, especially in landfill-derived biogas. 72-74 Thus, purification is

necessary for almost any biogas use, especially if it is to be used as catalytic reaction
feedstock.

Various physicochemical methods, adopting chemical adsorption and absorption processes,
have been scientifically and technically explored for biogas purification and most of them are
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now commercially available. The most interesting H S and NH removal processes involve
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chemical absorption in aqueous solutions and can be divided into two categories, those