To avoid the use of rare metal catalysts (e.g. Pt, Ru, Pd), cheaper alternatives are
being considered. Simple metal oxides may prove interesting catalyst materials
for HTL due to their low cost and known catalytic activity. Table 2.2 list a couple
of such catalysts which effect has already been investigated. Although all of these
metal oxide catalysts did appear to have a catalytic effect, few of them significantly
benefited the HTL process. Zirconium dioxide (ZrO) has also shown promising
catalytic properties in biomass gasification. This catalyst reported to reduce char
formation, enhance the hydrogen production, and induces lignin decomposition
through targeted decarbonylation at supercritical conditions [112, 113]. Moreover,
the base properties of ZrO2 has been suggested to enhance the water-gas shift reaction.