Tuning the catalytic behavior of metal oxides
Excess oxygen on the surface of the metal oxide catalyst copper oxide promotes hydrogen oxidation but suppresses carbon monoxide oxidation.
image: Peroxide (OO) forms on CuO by annealing in O2, leading to a larger oxygen surface population. Greater oxygen coverage prevents organic species from reacting on the oxygen-rich CuO surface (bottom right) and promotes hydrogen gas decomposition (top right).
Credit: Image courtesy of University of Binghamton.
The Science
The behavior of catalysts that promote chemical reactions is not always straightforward. For example, adding oxygen enhances some catalytic oxidation reactions (reactions that introduce oxygen into chemical compounds) but suppresses others. Using a combination of experiments and computer simulations, scientists now understand how oxygen affects the way the catalyst copper oxide (CuO) reacts with hydrogen (H2) versus carbon monoxide (CO) gases. They found that peroxides (OO) form on the CuO surface when too much oxygen is present. These peroxides change how the copper and oxygen atoms are arranged on the surface of the CuO. This allows hydrogen gas to break apart on the surface but prevents CO adsorption.
The Impact
Catalysts—materials that make chemical reactions easier—are important to many industrial processes. Catalysts also play a vital part in the destruction of pollutants and the production of sustainable energy. The arrangement of oxygen on a catalyst surface determines its reactivity. Scientists now have a new understanding of how to control the arrangement of surface oxygen by controlling the availability of the atoms. These findings will allow scientists to control and enhance chemical reactions. This new understanding of catalysts could lead to increased energy efficiency and energy generation.
Summary
Oxidation reactions occurring on a catalyst rely on multiple steps. First, a reactant gas takes oxygen atoms from the catalyst surface. The resulting oxygen vacant sites are then filled by excess oxygen gas or from atoms surfacing from deeper within the catalyst. In this work, researchers demonstrated the tunability of these reaction steps generating surface peroxide species (OO), which are formed when excess adsorbed oxygen bonds to the catalyst surface. The presence of peroxide species allows the catalyst to lose surface oxygen groups and causes the exchange of copper and oxygen near the catalyst surface, thinning the active CuO layer. Additionally, the peroxides shield the catalyst surface from CO (and other organic groups) from absorbing, instead making the catalyst prone to H2 oxidation.
The researchers observed the competing absorption and reaction processes using surface sensitive methods such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and infrared reflection absorption spectroscopy (IRRAS). They also computationally modeled these reaction processes. These results are relevant to a wide range of catalytic oxidation reactions that can use excess adsorbed oxygen to activate lattice oxygen and tune the activity and selectivity of catalytic sites.
Funding
This work was supported by the Department of Energy (DOE) Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering. This research used the Proximal Probes Facility of the Center for Functional Nanomaterials and the 23-ID-2 (IOS) beamline at the National Synchrotron Light Source II, both of which are DOE Office of Science user facilities at Brookhaven National Laboratory, as well as the Scientific Data and Computing Center, a component of the Computational Science Initiative, also at Brookhaven National Laboratory.