The ambient pressure X-ray photoelectron spectroscopy instrument at CFN. Photo: Brookhaven National Laboratory.
The ambient pressure X-ray photoelectron spectroscopy instrument at CFN. Photo: Brookhaven National Laboratory.

Researchers at Binghamton University have partnered with colleagues at Brookhaven National Laboratory’s Center for Functional Nanomaterials (CFN) to investigate how peroxides on the surface of copper oxide can promote the oxidation of hydrogen but inhibit the oxidation of carbon monoxide, potentially offering a way to steer redox reactions. They were able to observe these quick changes with two complimentary spectroscopy methods that have not been used in this way before. The researchers report their findings in a paper in the Proceedings of the National Academy of Sciences.

“Copper is one of the most studied and relevant surfaces, both in catalysis and in corrosion science,” explained Anibal Boscoboinik, materials scientist at CFN. “So many mechanical parts that are used in industry are made of copper, so trying to understand this element of the corrosion processes is very important.”

“I’ve always liked looking at copper systems,” said Ashley Head, also a materials scientist at CFN. “They have such interesting properties and reactions, some of which are really striking.”

Gaining a better understanding of oxide catalysts could give scientists more control over the chemical reactions they catalyze, including solutions for clean energy. Copper oxide, for example, can catalytically convert methanol into valuable fuels, so being able to control the amount of oxygen and the number of electrons on copper surfaces is a key step to efficient chemical reactions.

Peroxides are chemical compounds that contain two oxygen atoms linked by shared electrons. The bond in peroxides is fairly weak, allowing other chemicals to alter its structure, which makes peroxides very reactive. In this study, the researchers were able to alter the redox steps of catalytic oxidation reactions on an oxidized copper surface (CuO) by identifying the peroxide species formed with different gases – oxygen (O2), hydrogen (H2) and carbon monoxide (CO).

Redox is a combination of reduction and oxidation reactions. In this process, the oxidizing agent gains an electron and the reducing agent loses an electron. When comparing the different peroxide species and how these steps played out, the researchers discovered that a surface layer of peroxide significantly enhanced CuO reducibility in favor of H2 oxidation. On the other hand, they also discovered that the surface acted as an inhibitor to suppress CuO reduction against CO oxidation. This opposite effect of the peroxide on the two oxidation reactions stems from the modification of the surface sites where the reaction takes place.

By identifying these bonding sites and learning how they promote or inhibit oxidation, scientists could use these gases to gain more control over how the reactions play out. In order to tune the reactions, however, the researchers had to get a clear look at what was happening.

Studying this reaction in situ was important to the team, since peroxides are very reactive and these changes happen quickly. Without the right tools or environment, it’s hard to capture such a brief moment on the surface.

Peroxide species on copper surfaces had never been observed before with in-situ infrared (IR) spectroscopy. This technique uses infrared radiation to gain a better understanding of a material’s chemical properties by looking at the way the radiation is absorbed or reflected under reaction conditions. In this study, the researchers used IR spectroscopy to differentiate ‘species’ of peroxide, based on very slight variations in the oxygen they were carrying, which would otherwise have been very hard to identify on a metal oxide surface.

“I got really excited when I was looking up the infrared spectra of these peroxide species on a surface and seeing that there weren’t many publications,” said Head. “It was exciting that we could see these differences using a technique that’s not widely applied to these kinds of species.”

The IR spectroscopy results weren’t conclusive on their own, however, which is why the team also used another spectroscopy technique called ambient pressure X-ray photoelectron spectroscopy (XPS). XPS uses lower energy X-rays to kick electrons out of a sample, with the energy of those electrons revealing information about the chemical properties of the atoms in the sample. Having both techniques available through the CFN User Program was key to making this research possible.

“One of the things that we pride ourselves in is the instruments that we have and modified here,” said Boscoboinik. “Our instruments are connected, so users can move the sample in a controlled environment between these two techniques and study them in situ to get complementary information. In most other circumstances, a user would have to take the sample out to go to a different instrument, and that change of environment could alter its surface.”

“A nice feature of CFN lies not only in its state-of-the-art facilities for science, but also the opportunities it provides to train young researchers,” said Guangwen Zhou, professor in the Department of Mechanical Engineering and the Materials Science program at Binghamton University. “Each of the students involved have benefited from extensive, hands-on experience in the microscopy and spectroscopy tools available at CFN.”

The results of this study could also apply to other types of reactions and other catalysts besides copper. Metal oxides are widely used as catalysts themselves or as components in other catalysts. Tuning peroxide formation on other oxides could provide a way to block or enhance surface reactions during other catalytic processes.

“I’m involved in some other projects related to copper and copper oxides, including transforming carbon dioxide to methanol to use as a fuel for clean energy,” said Head. “Looking at these peroxides on the same surface that I use has the potential to make an impact on other projects using copper and other metal oxides.”

This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.