Elsevier

Chemosphere

Volume 48, Issue 10, September 2002, Pages 1047-1060
Chemosphere

Heterogeneous water phase catalysis as an environmental application: a review

https://doi.org/10.1016/S0045-6535(02)00168-6Get rights and content

Abstract

Catalytic water phase processes as an environmental application is a relatively novel subject with tremendous potential in the near future. This review of 120 references presents the wide scale of heterogeneous water phase applications studied mainly within past five years. Both oxidation and hydrogenation processes are included as well as TiO2 assisted photocatalysis. According to the references, heterogeneous catalysis is developing rapidly. New bimetallic catalysts and supports with higher surface area have improved catalytic efficiency in both oxidation and hydrogenation processes. It also seems that study on use of some waste materials such as red mud as catalyst is a very progressive field. On the whole, the chemical aspects are pretty well known, but the catalyst durability, and in many cases activity as well, has to be improved.

Introduction

Scientific interest of catalytic water phase processes has awakened within past 10 years. For the time being catalytic water treatment processes are for very limited use only. However, ever tightening regulations in treatment of aqueous waste force to search novel methods in the near future. Many industrial waste streams are not suitable for biological processes due to their inherent toxicity, but their treatment by traditional non-catalytic chemical processes or by incineration may be too energy intensive. With adaptable catalyst, energy consumption of various well-known oxidation procedures e.g. supercritical water oxidation, wet peroxide oxidation, wet oxidation, and wet air oxidation of stable organics may be decreased (Luck, 1999; Larachi et al., 2001). Supposedly also novel procedures for degradation of stable compounds yet not efficiently degraded will arise. One interesting observation is advanced removal of nitrogen containing compounds in wet air oxidation when metal oxide catalyst is present (Deiber et al., 1997).

Quality problems of groundwater and freshwater along with better understanding of importance of drinking water quality have already generated several studies dealing with heterogeneous catalytic raw water treatment processes including oxidation and hydrogenating processes.

If catalyst in water phase process is dissolved a.k.a. homogeneous, separation processes possibly have to be adapted. In many cases, separation would be technically and/or economically unachievable. In addition, many active homogeneous catalysts, such as some metal salts are a potential environmental problem. Under these circumstances, there is a need for heterogeneous catalytic procedures where the catalyst is in different phase and therefore more likely easy to separate. However, heterogeneous processes are usually more complicated to control. According to Turner (1981), the following five steps are needed to enable heterogeneous reaction: (i) diffusion of reactants to the surface, (ii) adsorption of reactants onto the surface, (iii) reaction on the surface, (iv) desorption of products off the surface, and (v) diffusion of products from the surface.

Since surface plays an important role in adsorption and desorption, appropriate support selection may therefore have a remarkable effect on reaction rate. This is possibly a field of the most rapid development in heterogeneous catalysis during past 10 years. This review consists of publication concerning environmental heterogeneous catalytic water phase applications. All the most important processes, catalysts, and supports are listed. The reference material included has mostly been published during past five years. However, some older papers were included when found fruitful.

Section snippets

Catalysts

Catalytic gas phase processes have established their strong position among all exhaust gas treatment processes. In water phase, the development has not been as rapid partly due to the difficulties in finding active and stable catalysts. In water phase, the economical facts typically force to use lower reaction temperatures than in gas phase. This alone often limits the use of similar catalysts as in gas phase.

Various noble metals (Ru, Pt, Rh, Ir, and Pd) and some metal oxides (Cu, Mn, Co, Cr,

Catalyst support

Usually supports are classified by their chemical nature to organic and inorganic supports. Whatever the support is, it plays an important role in immobilizing active catalyst. Principally, the support has three main functions: (i) to increase the surface area of catalytic material, (ii) to decrease sintering and to improve hydrophobicity and thermal, hydrolytic, and chemical stability of the catalytic material, and (iii) to govern the useful lifetime of the catalyst (Matatov-Meytal and

Oxidation processes

Traditional non-catalytic water phase oxidation processes need long reaction time, relatively high temperature (⩾200 °C) and pressure (70–250 atm) (Matatov-Meytal and Sheintuch, 1998). When heterogeneous catalysts utilized, reaction conditions in many cases can be turned milder. In environmental catalytic water phase processes an oxidant used is typically air or dioxygen. However, hydrogen peroxide and ozone are used as well. The low solubility of oxygen in water and the slow rate of

Hydrodechlorination

Hydrodechlorination is commonly more energy efficient than oxidation in treatment of polychlorinated organic compounds. In addition, contrary to the oxidative treatment methods there is no risk of formation of polychlorinated dibenzo-p-dioxines (PCDD) or polychlorinated dibenzofurans (Frimmel and Zdražil, 1997). In the presence of noble metal catalyst a conversion of 50–90% might be achieved with various polychlorinated hydrocarbons at 25–50 °C temperature and near atmospheric pressure (

Conclusion

Solid catalysts offer a novel approach in liquid phase environmental processes. Many of above explained applications are and presumably will be limited to narrow field of waste streams. However, if these processes are capable to replace other application with higher energy consumption there is a niche for them. Especially in photocatalytic processes the most rapid development and also the greatest demand of development is in engineering related aspects, such as specific reactor design and novel

Acknowledgements

Financial support from the Maj and Tor Nessling Foundation, the Academy of Finland, and the Kemira Foundation is greatly acknowledged.

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