Catalysis for Clean Water and Clean Air

There are two articles on water purification, each using a different catalytic strategy to target the removal of organic contaminants from industrial wastewater. Saunders et al. (2) describe the use of catalytic wet air oxidation, in which the challenge is to develop catalysts that will operate at temperatures below 100ºC, avoiding the need for elevated pressures to keep the water in the liquid phase. Perhaps surprisingly, some of the best new catalysts appear to repel water, but it is their relative wettability on the microscopic scale that is critical, helping direct the oxygen (from the air feed) and the organic molecules (in the contaminated water) to the catalytically active sites. This is all a bit like the hydrophilic and hydrophobic patterning on the wings of a Namib beetle (1), which enables moisture from the air to be directed to the mouth of the insect!

Underhill et al. (3) take a different approach to the decontamination of wastewater, though their aim is still to achieve full oxidation of dissolved organic molecules. In their approach, they use catalysis to generate peroxide species as the oxidising agent at the point of use. Significantly, they show that when the peroxide species are formed in situ, they are more effective than when added as hydrogen peroxide solution. As Winkless has pointed out (1), this approach could be adapted for the treatment of household greywater, which is the lightly contaminated wastewater from washing and bathing. The water could be re-used following treatment in a domestic appliance that combines filtration with catalytic disinfection based on the type of catalysis described by Underhill et al.

The catalytic theme persists throughout this issue. Whereas Winkless has suggested that “catalytic converters are due a reboot” (1), Leung et al. (4) describe here how a catalytic reformer – similar in composition and design to that of a three-way catalytic converter – can be used to recover waste heat from vehicle exhaust. This type of catalytic heat recovery could allow gasoline fuelled vehicles to match the low fuel consumption and CO₂ emissions of diesel vehicles, without needing to ‘reboot’ the three-way catalytic converter that does so well in controlling the release of NOx, CO and hydrocarbons. The technology could prove vital in stabilising urban air quality over the next two decades, while road transport transitions from the internal combustion engine to the electric powertrain.

Lowering Carbon Dioxide Emissions

On a different scale of application, but with the same aims of reducing CO₂ emissions and increasing energy efficiency, van Dijk et al. (5) provide an overview of the multi-partner STEPWISE project. The technology, which is custom designed for the iron and steel manufacturing industries, is at a much more advanced stage of development than any of the others described in this issue. During the lifetime of the project, pilot plant testing will be carried out on a carbon-storage system for processing up to 14 tonnes of CO₂ per day. As van Dijk et al. state in their article: “To put that in context, on average a car emits 4.75 tonnes in a year”, and yet the pilot plant will be processing only 1% of the CO₂ released by the blast furnace of a typical steel manufacturing plant.