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Water for Energy and Fuel Production
at 400°C. For both compounds, ZrO 2 catalyst gave hydrogen yield twice higher than
those obtained without the catalyst. With NaOH, the yield increase was four times.
Both catalysts enhanced the decomposition of aldehyde and ketone intermediates into
CO. For lignin, both catalysts enhanced the decomposition of carbonyl compounds,
which in turn inhibited the char formation and promoted the formations of CO and H 2 .
10.4.1 CATAlySTS For SCWo
Ding et al. [15] showed that it is possible to develop effective catalysts for SCWO
applications. These catalysts can be used either to enhance oxidation rates of organic
compounds or to increase destruction of refractory products. The catalyst can be
designed to increase the selectivity of certain products. Because of a wide variation in
the nature of aqueous wastes, understanding the unique characteristics of SCW and its
effect on the catalyst surface, reaction activity, and mechanisms, and the knowledge
of preferred crystalline phases of metal oxides is essential for the development and
design of an effective SCWO catalytic system. Oxides of Ce, Co, Fe, Mn, Ti, and Zn
may be used as catalysts, and their supports can be selected from the oxides of Al, Hf,
Zr, and Ti. These supports have been found to be stable in SCWO environments. The
additives that can increase the physical strength or stabilize the activity of a catalyst
may be an oxide of Bi, Cd, Ga, Ir, K, Mo, Ta, or W. An effective SCWO catalyst must
have large surface area and be able to withstand larger surface area changes.
Catalyst activity and stability is affected by the preparation methods. Traditionally,
catalysts are produced by coprecipitation, impregnation (coating), fused alloy, fused
metal oxide, and crystal growth processes [62–72]. Coprecipitation and impregnation
are two of the most popular methods for the preparation of metal and metal oxide
catalysts [17,20,62–72]. Many commercial oxidation catalysts are prepared by coating
noble metals on metal oxide supports to modify catalyst surface structure and active
sites that can result in the increase in catalyst activity and stability. While the physical
conditions of these catalysts are adequate for the gas-phase oxidation, they may not
be completely suitable for the SCW conditions. Since transition metal oxide catalysts
are major components of ceramics, the common methods of ceramic preparation such
as sol-gel, coprecipitation, polymeric sponge, and high-temperature aerosol decomposition methods have been adapted for the preparation of metal oxide catalysts. The
structure and properties of ceramic catalysts depend on the process parameters such
as solvents, pH, temperature, and aging time. Numerous reported studies have evaluated these effects [62–72]. In the final analysis, preparation method must be chosen that gives the desired activity, selectivity, stability, and prepares catalyst that can
handle refractory materials and possible poisons in the waste feed.
10.5 deCOmPOsitiOn and eXtraCtiOn
OF materials By sCW
SCW is a good extracting and decomposition agent for many complex organic materials [73–118] (Kim and Mitchell, 2012, pers. comm.; Swanson et al., 2012, pers.
comm.). This application generally produces useful liquids that can be either a fuel
or raw materials for various downstream chemicals. Feedstock normally used for