(600h) 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 CO_2.

      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.