Spectroscopy Techniques Investigate X-Ray Radiolysis

Daniel S. Burgess

Using UV-VIS and x-ray absorption near-edge spectroscopy, scientists at the department of chemistry at Universiteit Utrecht in the Netherlands have investigated the effects of synchrotron x-ray radiation on catalytic processes involving copper solutions. The results offer insights into the utility of combined analytical techniques, suggesting that it is crucial to understand whether and how such approaches to measurement influence the reaction under study so that these effects can be minimized.

When illuminated with high-energy synchrotron radiation for x-ray absorption spectroscopy measurements, gas bubbles form in aqueous solutions by radiolysis. The products of this phenomenon can react with solutes, affecting the chemical reaction under study. At high concentrations, radiolysis of the solute also can occur. Courtesy of Bert M. Weckhuysen.

Bert M. Weckhuysen, a professor of inorganic chemistry and catalysis at the university, said that the research team is interested in using combined spectroscopic methods to probe chemical reactions — including by x-ray absorption near-edge spectroscopy, in which the response of a sample to monochromatic x-ray photons with energies near its absorption edges is collected and compared with standards to determine the presence and relative abundance of chemical species.

“The chemical reactions we study are complex, and as a result, one technique is often unable to provide all of the information necessary to understand them,” Weckhuysen explained. “Therefore, collecting multiple data in the same experiment enables a potentially deeper insight into understanding a catalytic process.”

The problem, he said, is that the energies involved in x-ray absorption near-edge spectroscopy can influence the reaction. For example, photons with energies greater than 100 eV can break the molecular bonds of water to produce hydrogen peroxide, hydrogen, ozone and oxygen in a process known as radiolysis. They also generate free hydrated electrons and radicals that interact with other species in solution, and they may yield new products by the radiolysis of the solutes.

To better understand this phenomenon, the scientists simultaneously collected x-ray absorption data and either 200- to 735-nm UV-VIS spectra or 25-fps video images of various copper-containing metal salts and ligands in solutions of water and of 1-methyl-2-pyrrolidinone that were exposed to x-rays with photon energies from 8910 to 9250 eV at the European Synchrotron Radiation Facility in Grenoble, France. The experimental setup incorporated a Princeton Instruments phosphorus-masked cooled CCD camera for the x-ray absorption measurements, a Newport multimodal multiplex spectrometer for the UV-VIS data and a Sony hole-accumulation-diode CCD camera for video monitoring.

The results confirm that gas bubbles form in aqueous solutions by radiolysis when illuminated with high-energy x-rays. The products in these bubbles can react with the solutes, including forming colloidal copper nanoparticles. Depending on the type of ionic salt used, the ligands and the reduction potential of the copper complexes, the copper solutions undergo various degrees of reduction — either no reduction, or to Cu¹⁺ or Cu⁰.

Weckhuysen said that there are potential solutions to minimize the effects of the irradiation. He also noted that the work has implications for any research in which high-energy photons are involved. Moreover, he said, the scientists are investigating whether certain features in the UV-VIS spectra are effects of the near-UV illumination.

Journal of Physical Chemistry B, Sept. 7, 2006, pp. 17671-17677.