Hydrogen peroxide is among the most common and versatile of household products: disinfecting wounds, bleaching hair, whitening teeth and removing stains, cleansing contact lenses and killing mold.
Zirconium, one of the most abundant elements in the world, is stable and just as chemically innocuous. A corrosion resistant metal used in nuclear reactors and high performance pumps and valves, in oxide form it even serves as jewelry and in treating poison ivy.
In two separate nano-inspired projects, Lehigh researchers are working to add to the credit reels of these workaday chemicals a shared, and perhaps loftier usage than ever before -- providing healthy water to parts of the world that need it most.
Quenching global thirst
Nanoparticles of zirconium oxide, says Arup SenGupta, P.C. Rossin Senior Professor of civil, environmental, and chemical engineering, possess adsorption properties that make them uniquely beneficial to human beings. They can remove four major toxins -- arsenic, fluoride, phosphate and lead -- from water.
SenGupta, a highly-awarded, three-decade pioneer in engineering for clean water around the world, and his students have extended their lab’s impact on global health, using zirconium oxide nanoparticles to invent the world’s first filter capable of removing both fluoride and arsenic from groundwater.
Nearly 400 million people in Asia and Africa drink groundwater that contains toxic levels of these contaminants. Exposure to excessive amounts of fluoride can cause skeletal fluorosis; this decreases the elasticity of bones, making them more prone to fracture and causing bone and joint damage. Elevated levels of arsenic can cause skin lesions, a variety of cancers and blood vessel disorders.
The materials used in the filter are polymeric ion exchangers doped with zirconium oxide nanoparticles. The doping procedure, says SenGupta, plays a critical role in producing optimum hybrid particles that are robust and accessible to water while maintaining their integrity. In late 2015, SenGupta and Surapol Padungthon ‘13PhD earned a U.S. patent for the invention.
SenGupta’s team was initially attracted to zirconium because of its filtering potential. Zirconium oxide nanoparticles pose no health hazard, says SenGupta, and they offer unique properties that help remove harmful contaminants from water.
“This is the only material currently available that can remove both arsenic and fluoride, and it can be reused for years without being wasted,” he says.
SenGupta has also developed a business model that enables people who lack clean groundwater to install and operate purification systems in an economically sustainable way. He and PhD candidate Mike German recently received the 2016 VentureWell-Lemelson Sustainability Award for co-founding DrinkWell, an organization that provides purification technologies and business opportunities to people who lack access to clean water.
Greener greywater
Around the world, many people still rely on decentralized community- or household-based systems for a safe source of water. For many in these communities, the reuse of so-called “greywater“ — wastewater streams from showers, baths, basins and washing machines — can help ensure that water needs are met. Greywater can be contaminated with a range of substances such as soaps and detergents, skin, and dirt, yet it can be recycled for toilet flushing, irrigation, and other non-potable uses.
Hydrogen peroxide can be used to prepare greywater to be reused safely; however, it is typically made in a multi-step, energy-intensive process. Large quantities are produced, shipped and stored in a highly concentrated form, and eventually diluted for personal or commercial use. The sheer logistics of this have stymied the use of hydrogen peroxide for water treatment in underdeveloped or disaster-stricken regions -- until now.
A group of researchers from Lehigh, Cardiff University in Wales, and Oak Ridge National Laboratory has developed a way to produce hydrogen peroxide through a simple, economic, one-step process. In an article published earlier this year in the journal Science, the group reported that bimetallic compounds consisting of palladium and any one of six other metallic elements can effectively catalyze the hydrogenation of oxygen to form hydrogen peroxide.
The new catalysts, says Christopher J. Kiely, professor of materials science and engineeringand director of Lehigh’s Electron Microscopy and Nanofabrication Facility, can be made by combining palladium with tin, cobalt, nickel, gallium, indium or zinc.
“Using our new catalyst, we’ve created a method of efficiently producing hydrogen peroxide on demand in a quick, one-step process,” said Simon J. Freakley of the Cardiff Catalysis Institute, the current article’s lead author. “Being able to produce hydrogen peroxide directly opens up a whole host of possibilities, most notably in the field of water treatment.”
The latest Science article is the fourth on the topic from Lehigh and Cardiff researchers over the past decade. Their previous three Science papers dealt with advances in creating and using an alloy catalyst of gold and palladium; one, published in 2009, discussed the catalyst’s potential to produce hydrogen peroxide quickly and efficiently while preventing its decomposition.
“Scientists have known for more than a century that palladium can catalyze the direct reaction of hydrogen and oxygen into hydrogen peroxide,” Kiely says. “Unfortunately, palladium also rapidly decomposes the peroxide. Previously, we discovered that gold-palladium nanoparticles prevented this undesired reaction. But gold is expensive. For widescale adoption, especially in the developing world, we needed cheaper metals.”
According to Kiely, all of the six new alloying elements are cheaper than gold. Furthermore, the catalysts can be made without nitric-acid pre-treatment, a requirement of the gold-palladium alloy.
In the current research, the team found that palladium, combined, for instance, with tin, promoted the reaction as effectively as gold-palladium catalysts -- and electron microscopy helped explain why.
Variation in particle size and composition -- a given in catalyst production -- turned out to be the culprit. Researchers detected that larger alloy particles, roughly 5-10nm in diameter, were slightly palladium-deficient, and were responsible for producing the desired hydrogen peroxide. The source of the subsequent undesired peroxide decomposition was traced back to the presence of ultra-small, palladium-rich particles with diameters of a single nanometer.
To stop these smaller particles in their tracks, the team deposited a palladium-tin catalyst onto a titanium dioxide support. Some tin formed into an oxidized layer across the support, while the remainder made palladium-tin alloy particles. A simple, three-step heat treatment process caused the tin-oxide films to encapsulate the smaller, palladium-rich particles, thus preventing them from decomposing the peroxide, while leaving the larger productive alloy particles uncovered.
“The heat treatment buried the small palladium-rich particles in the tin-oxide and averted the undesired reaction,” says Kiely. “This so-called ‘strong-metal-support-interaction’ phenomenon is normally unwelcome, yet in this case we have used it to our advantage.”
The group spent five years developing the palladium-tin catalyst and optimizing the heat treatment regimen; according the Science article, the team has achieved overall selectivity to hydrogen peroxide of more than 95 percent.
