267673 Surface Chemistry Studies of the Reaction of CO2 with MgO(100) and TiO2(100)
Surface chemistry studies of the reaction of CO2 with MgO(100) and TiO2(100)
Juan Wang1, C. Michael Greenlief1, and Thomas R. Marrero2
1Department of Chemistry
2Department of Chemical Engineering
University of Missouri
Columbia, MO 65211
There is significant interest in methods to aid in the removal of metabolic carbon dioxide from confined spaces. The methods range from sequestration to catalytic transformations. In sequestration, CO2 is collected from emission sources, and may be stored in a variety of materials including metal-organic frameworks. Catalytic research often involves the transformation of CO2 to a hydrocarbon such as methane. The majority of these latter studies focus on the use of photocatalysis for the conversion of CO2. An alternative method, discussed in this presentation, is to examine the surface chemistry for the adsorption and thermal reduction of carbon dioxide with magnesium oxide and titanium oxide surfaces. We are also interested in gaining a better understanding of the refractory nature of CO2.
One of the main goals of these studies is to elucidate the surface mechanisms for the reduction of carbon dioxide on metal oxide surfaces. We are using mass spectrometry, coupled with surface spectroscopies, to characterize the surfaces before and after reaction with CO2 and the composition of gaseous products. Electron microscopy is also used to characterize the form of carbon deposited. Both single crystal substrates and high surface area powder samples are used.
Figure 1 shows representative Auger electron spectroscopy (AES) results for exposing MgO(100) to CO2 (5000 Langmuir (L), 1 L= 10-6 torr·s) at several different substrate temperatures. In this experiment, the MgO(100) substrate is held at a constant temperature while being exposed to CO2. Three different surface temperatures are shown in Figure 1 (red - 575°C, blue - 600°C, and black - 650°C). At each of these temperatures, carbon is deposited on the surface and is detected by AES after the exposure. The inset graph to Figure 1 indicates the amount of carbon deposited on MgO at each temperature for the 5000 Langmuir exposure. The amount of carbon deposited changes from 1.5 atomic % to 2.4 % as the temperature is changed from 550°C to 650°C. It should be noted that there is no carbon deposition when the same exposure is made at room temperature. The presence of carbon indicates there is some dissociation of CO2 at these elevated surface temperatures.
Figure 2 is an electron micrograph of a MgO(100) crystal after exposure to CO2 at elevated temperatures. The black spots are due to the presence of carbon deposited on the surface. The random distribution of the carbon deposits indicates the reduction of CO2 does not appear to be dominated by reactions at surface defect sites.
X-ray photoelectron spectroscopy (XPS) is also used to follow the surface conditions after CO2 exposures. The outermost surface layers of the MgO surface are nearly stoichiometric after CO2 exposure as measured by XPS, while the deeper layers are oxygen deficient.
Complimentary studies on TiO2 surfaces will also be presented. The band-gap of TiO2 is smaller compared to MgO and UV light has sufficient energy to overcome the band-gap. Studies with and without the influence of UV light will be discussed with an emphasis on the role of light for the reduction of CO2 at metal oxide surfaces.
Future studies will explore the mass transfer aspects of this reaction.
Figure 1. AES spectra obtained after exposing MgO(100) to 5000 Langmuir of CO2 at various surface temperatures. The surface temperatures shown are 575°C (red), 600°C (blue), and 650°C (black).
Figure 2. Electron micrograph of the MgO(100) crystal surface after exposure to CO2 at elevated temperatures. The maker is 40mm.
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