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Photocatalytic Oxidation (PCO) Air Cleaners: Reducing Energy Use While Clearing the Air

The concentrations of volatile organic compounds (VOCs) found in offices, classrooms, and homes could be greatly reduced in the coming years as the result of indoor air cleaner research being conducted at Lawrence Berkeley National Laboratory's Indoor Environment Department. Begun by Berkeley Lab's William Fisk and Alfred Hodgson, the department's indoor air cleaner research is now led by Hugo Destaillats and is focused on photocatalytic oxidation (PCO)—a promising technology for reducing VOCs and energy use simultaneously.

The need for VOC reduction is great. "Indoor air measurements typically find twenty high-concentration VOCs, fifty low-concentration VOCs, and many more very low concentration VOCs," says Destaillats. Depending on exposure levels, several of these compounds may have health consequences individually, and some of them the potential to react with reactive atmospheric species (such as ozone) to form a secondary pollutant.

The compounds come from a variety of sources—some avoidable and some less so. One VOC, cancer-causing benzene, for example, is found in tobacco smoke, and it can be reduced by maintaining a smoke-free environment. Because it is also a product of motor vehicle exhaust, however, it can enter through open windows or through air vents. Formaldehyde, a carcinogen, irritant, and possible source of asthma exacerbation in indoor air often comes from pressed wood products as they off-gas, so avoiding the excessive use of pressed wood furniture can reduce indoor formaldehyde concentrations. However, formaldehyde can also be produced as a secondary pollutant, when ozone and other substances from outdoor air react with those indoors.

Indoor air cleaners eliminate some pollutants, but most concentrate primarily on filtration of particles, rather than on gaseous VOC removal. Overall, particulate filtering has become efficient and cost-effective, but as VOC measurements show, most air cleaners' ability to reduce indoor VOCs is much less advanced. Destaillats, along with Berkeley Lab's Michael Apte, William Fisk, and Mohamad Sleiman are looking at more-effective air cleaning strategies for commercial and residential applications. Photocatalytic oxidation is offering some promising results.

The Indoor Environment Department's research addresses the dual challenge of reducing both VOCs and energy use. If energy efficient photocatalytic oxidation reductions of VOC are effective, then energy use can be reduced, because less outdoor air, which must be heated or cooled, will be needed to "dilute" the indoor air to healthy levels.

Two Means to the Same End

Broadly, photocatalytic oxidation systems can be divided into two categories: active PCO and passive PCO. Each system takes a different approach to eliminating indoor air pollutants, but they differ in their complexity and cost.

Active PCO for Indoor Air Systems

In active PCO systems, a high-surface area support irradiated with UV light captures the VOCs as they are entrained in the intake air of a heating, ventilating, and air conditioning (HVAC) system. A photocatalytic coating applied to the support then interacts with the compound under UV irradiation, converting it to end products such as carbon dioxide and water, or to other reaction intermediates.

One configuration being evaluated by Berkeley Lab is an in-duct air cleaner that circulates air through a series of honeycomb monolith filters coated with titanium dioxide (TiO2) as a catalyst. Ultraviolet lamps (either UVA or UVC) irradiate the coated filters to activate the TiO2, and as air passes through the system, VOCs are adsorbed on the catalyst and react.

Outdoor or recirculated indoor air with VOCs goes through UVPCO resulting in 'Clean' air withlower VOCs. Images of in-duct systems and standalone home air cleaner

UV photocatalytic oxidation air cleaners.

To better understand the potential for PCO technology, Berkeley Lab measured conversion efficiencies and clean air delivery rates for individual VOCs in several indoor mixtures likely to occur in commercial buildings. The research team also investigated the formation of gas-phase products of incomplete conversion.

Researchers estimated that conversion efficiencies of the PCO installed in an office building HVAC system would have to surpass 17 percent to enable a 50 percent reduction in building ventilation. The Berkeley Lab experiment showed conversion of most VOCs surpassing 19 percent, confirming its effectiveness. In several cases, conversion was as high as 75 percent.

However, the test also showed generation of formaldehyde and acetaldehyde from partial oxidation of VOCs. In follow-up research, the Indoor Environment Department conducted an experiment to determine if generation of those compounds could be reduced by using a chemisorbent oxidizer—sodium permanganate—downstream of the PCO device. They found that a four-panel, folded, media bed installed downstream of the reactor removed formaldehyde with greater than 90 percent efficiency and removed acetaldehyde at about 70 percent efficiency—resulting in net formaldehyde and acetaldehyde reductions of 50 to 70 percent. As a result, the combined PCO air cleaner and chemisorbent system appears to have sufficient VOC removal efficiency to reduce ventilation rates by 50 percent without increasing indoor aldehyde concentrations.

"It worked beautifully," says Destaillats, "but there is a cost problem because it's expensive. Not everyone would be able to afford it." However, cost is not the only issue—in indoor air applications, the PCO catalysts can become poisoned and deactivated.

Passive PCO: Invisible but Potentially Effective

Passive PCO air cleaning approaches could also use catalysts to reduce VOC concentrations, but rather than using them in the HVAC system, they would be incorporated into the building surfaces themselves and use catalysts that are activated by ambient light rather than UV. For example, they might be incorporated into a building's painted surfaces. Although passive PCO systems are still in the experimental stage right now, some early results have shown promise and have led to a supporting material that may be effective in both passive and active systems: clays.

The Quest for More Effective Photocatalyst Coatings

The performance of PCO systems may be improved through supporting the TiO2 nanoparticles in matrices that increase the effective dwelling time of VOCs in the proximity of the photocatalyst. This knowledge prompted Berkeley Lab to examine different clays as a support of a TiO2 catalyst. Clays often present a large surface area for adsorption of organic pollutants, and can also interact directly with VOCs through redox and acid-base interactions.

"You need to find a material where the compound can penetrate a porous structure," says Destaillats. The porous material ensures that the VOC resides there long enough for the catalyst to work. "Most VOCs can be absorbed very efficiently with clay."

Three graphs illustrating the porosity of hectorite, hecto-TiO2, kaolinite, kao-TiO2 and P25. Top: hectorite viewed at 10Ám; Bottom: hectorite-TiO2 viewed at 5Ám.

Porosity of clay-TiO2 nanocomposite photocatalysts.
Source: Kibanova et al. 2009 — Applied Clay Sci.

Collaborating with clay experts Daria Kibanova, Martin Trejo, and Javiera Cervini-Silva of the National Autonomous University of Mexico, Destaillats conducted a bench-scale test looking at the effectiveness of using hectorite and kaolinite clays combined with TiO2 to remove toluene. The research revealed that reaction rates were comparable to a reference TiO2 material (P25) in dry air for the best performing clay-TiO2 composite, but showed lower removal of a model hydrophobic pollutant under high relative humidity conditions. Destaillats sums up the findings: "Competition of hydrophobic VOCs with an excess of water on the surface of the catalyst is an important factor affecting overall conversion."

Air Cleaner Readiness

Small, stand-alone active PCO units are available commercially, but Destaillats warns buyers that the performance of current products is uncertain. It is possible that some units produce harmful aldehydes and that catalysts become deactivated.

Berkeley Lab's current research addresses only the large systems that can be incorporated into a building's HVAC system. These systems are still in the experimental phase. Ideally, in-duct air cleaners should include several stages (such as the use of a chemisorbent after the PCO), but the cost of a multi-stage system may be cost-prohibitive for some.

With a basic PCO system, maintenance is fairly easy and inexpensive. Users need to replace the UV light bulbs once every few years, and the catalyst-coated surfaces may need to be replaced periodically. However, if the system is using a chemisorbent, which is expensive, maintenance will be more costly. It is not clear yet how often the chemisorbent would need to be changed, but it could require replacement two or three times per year.

Next Steps on the Road to Cleaner Indoor Air

Because of Berkeley Lab's successful tests on both types of air cleaner system, Hugo Destaillats would like to keep the department's air cleaner research alive on all fronts.

"I would like to explore further active PCO cleaners and passive methods such as photoactive paints," he says. "I would also like to explore other support materials, such as carbon nanofibers, and to experiment with other catalysts. TiO2 is not the only one."

But PCO is only one avenue of research in what he sees as a much larger field, and he's eager to explore other possibilities as well. "There are other options to clean indoor air that don't involve photochemistry at all."

— Mark Wilson

For more information, contact:

  • Hugo Destaillats
  • (510) 486-5897

Mark Wilson is Contributing Editor of EETD News and a principal in Drewmark Communications.

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