University of Missouri-Columbia Columbia, MO 65211

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1 Surface chemistry studies of the reaction of CO 2 with MgO (100), TiO 2 (110), and TiO 2 (100) Juan Wang 1, Yuan Li 1, Thomas R. Marrero 2, and C. Michael Greenlief 1 1 Department of Chemistry 2 Depertment of Chemical Engineering University of Missouri-Columbia Columbia, MO Abstract There is significant interest in methods to aid in the removal of carbon dioxide from our environment. The methods range from sequestration to catalytic transformations. In sequestration, CO 2 is collected from emission sources, and may be stored in a variety of materials including geological formations. Catalytic research often involves the transformation of CO 2 to another compound such as methane. The majority of these latter studies focus on the use of photocatalysis for the conversion of CO 2. 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. The overall goal of these experiments is to elucidate the surface reaction mechanism for the reaction of carbon dioxide on metal oxide surfaces. We combine the use of X-ray photoelectron and Auger electron spectroscopies to examine the surface chemistry of CO 2. Carbon dioxide is exposed to the heated metal oxide surface and the surface atomic concentrations are measured after reaction. The substrate temperature and CO 2 exposure are varied in the experiment. Electron microscopy is also used to characterize the form of carbon deposited. Both MgO (100) and TiO 2 (100) and (110) single crystal substrates are used. Introduction Carbon dioxide (CO 2 ) is a major greenhouse gas that results from human activities and causes global warming and climate change [1-2]. There is significant interest in methods to aid in the removal of carbon dioxide from our environment. These methods range from sequestration to catalytic transformations. In sequestration, CO 2 is collected from emission sources, and may be stored in a variety of materials including geological formations [3,4]. Catalytic research often 1

2 involves the transformation of CO 2 to another hydrocarbon such as methane. The majority of the catalytic studies date focus on the use of photocatalysis for the conversion of CO 2 to CH 4 or CH 3 OH. Recent advances in this area are summarized in a review by Roy et al. published in 2010 [5]. Several metal oxide, or modified metal oxide, systems are covered in the review, including TiO 2. TiO 2 is one of the most studied photocatalysts. The direct band gap of TiO 2 is 3.75 ev (3.05 ev, indirect) and electrons can be excited from the valence band to the conduction band with UV light [6]. TiO 2 is often used as a model photocatalyst because it can be obtained in high purity and its behavior is well-known whether the substrate is a single crystal, powder, or nanoparticles [7]. Photo-activation of CO 2 on Ti-based heterogeneous catalysts was recently reviewed by Indrakanti et al. [8]. CO 2 adsorption through its conversion to methane or methanol was discussed. These same authors recently examined the photo-activation of CO 2 on TiO 2 crystals by computational methods [9]. Based on these calculations, the bonding configuration of CO 2 on titania was determined, as well as, its calculated behavior when illuminated. However, the experimental studies to verify all of the calculations have not been completed. While the conversion of CO 2 to methane and methanol has received attention, it is also highly desirable to directly reduce CO 2 to O 2 and C. Such a process could provide respirable oxygen useful in isolated habitats. The direct reduction of CO 2 was originally proposed by Sabatier [10]. In this reaction, CO 2 is reacted over Ni or Co metal catalysts at ºC with some carbon deposition, which indicated that the complete reduction of CO 2 to carbon and oxygen. However, the reaction has been shown to be incomplete over these metal catalysts, and requires the addition of hydrogen or steam [11-15]. The Sabatier reaction today involves the mixture of CO 2 and H 2 to produce CH 4 and H 2 O [16]. MgO is a wide band gap semiconductor (7.7 ev), and is not excited with UV light. This property offers the potential to separate and understand the role of photoexcitation in the surface reaction of CO 2. The adsorption of CO 2 on MgO(100) has been examined by several research groups. These studies examined the low temperature adsorption products and structures. CO 2 was determined to physisorb at low temperature through a precursor state with little mobility. Adsorption at room temperature led to a weakly bond carbonate-like intermediate [17-23]. In this paper, we describe a series of experiments to explore the interactions of CO 2 with the MgO surface. The goal is to understand the thermal decomposition of CO 2 so that the role of 2

3 light in the photocatalytic process can be elucidated. Our approach is to use ultra-high vacuum (UHV) based surface chemical studies with single crystal and high surface area powder substrates. In the UHV-based studies, X-ray photoelectron and Auger electron spectroscopies are used to examine the surface chemistry occurring upon adsorption and reaction of CO 2 as a function of surface temperature. Experimental All experiments were performed in an ultra-high vacuum (UHV) system. The base pressure for the system used in this study was torr. XPS was performed using a twin anode (Mg/Al) X-ray source. The Mg(K ) anode is operated at 15 kv and 20 ma (300 Watts). The Al(K ) anode is operated at 15 kv and 30 ma (450 Watts). A double-pass cylindrical mirror analyzer (CMA) operated is the fixed pass energy mode for XPS. A pass energy of 50 ev was used for high-resolution scans. A clean Au sample was used to calibrate the spectrometer. The following gold transitions were used for the calibration: Au 4f 7/2 (84.0 ev), Au 4d 5/2 (335.0 ev), and Au 4p 3/2 (546.0 ev). The Mg(2p 3/2 ) XPS transition for the clean surface was recorded and the spectrum was corrected to a binding energy of 51.7 ev, using the Au calibration. The XPS data are analyzed using the CASA XPS software package [24]. MgO(100) single crystal substrates (Sigma-Aldrich, mm, >99.9% purity, trace metal basis) were cleaned by cycles of exposing the crystal to oxygen (Linde, research grade, >9.998% purity) for minutes at a pressure of torr while maintaining a surface temperature of 800ºC. Surface cleanliness was measured by Auger electron spectroscopy (AES). A surface was considered clean when the measured carbon atomic concentration was less than one percent. Figure 1 is an example Auger spectrum of the clean MgO(100) surface. Carbon dioxide (Lindweld, research grade bone dry, >99.995% Figure 1. AES spectrum of the clean MgO(100) surface. 3

4 purity) was used as received and the purity of the gas is checked in-situ by mass spectrometry. The desired reagent gas is admitted to the chamber through an effusive doser directed onto the front face of the crystal for various periods of time. The actual pressure at the sample is higher than the chamber pressure as measured by the ion gauge and since these pressures should be proportional to one another, the chamber pressure is used as a measure of the overall sample exposure. The exposures (uncorrected for ion gauge C sensitivity to cyclopentene or cyclohexene) are reported in Langmuirs (1 L = 10-6 torr sec). Atomic % Carbon Temperature, C dn(e)/de (arb. units) 575ºC 600ºC Results and Discussion Figure 2 shows representative AES results for exposing MgO(100) to CO 2 (5000 L L) at several 650ºC different substrate temperatures. In this experiment, the MgO(100) substrate is held at a O Mg constant temperature Kinetic Energy (ev) while being exposed to CO 2. Three different Figure 2. AES spectra obtained after exposing MgO(100) to 5000 Langmuir of CO 2 at various surface temperatures. The surface temperatures shown are 575 C (red), 600 C (blue), and 650 C (black). surface temperatures are shown in Figure 2 (red C, blue C, and black 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 2 indicates the amount of carbon deposited on MgO at each 4

5 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 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 CO 2 at these elevated surface temperatures. Table 1 summarizes a series of experiments examining the influence of MgO surface temperature and CO 2 exposure. The sample is exposed to CO 2 at a variety of substrate temperatures. Three different CO 2 exposures and five different substrate temperatures are examined. After each exposure, the sample is cooled, and the Auger spectrum is taken. The amount of carbon at the surface is determined from the peak-to-peak intensities of a given elemental transition in the spectrum, along with the appropriate sensitivity factors. These results are shown in Table 1 and the percentages are averages of at least triplicate measurements on the same MgO(100) crystal. There is some scatter in the data and we are in the process of repeating the experiments on several MgO(100) crystals. In order to see the change of carbon percentage with substrate temperature more clearly, the data in Table 1 is presented as Figure 3. The general trend is for the carbon coverage to increase with exposure and temperature. Table 1. Average atomic percentage of carbon at different exposures by AES on different MgO(100) crystals CO 2 (L)/Temp. (ºC) 650ºC 625ºC 600ºC 575ºC 550ºC 500 L 1.7 (±0.3) 0.6 (±2.8) 1.9 (±0.5) 0.9 (±0.5) 1.1 (±0.5) 1000 L 1.2 (±0.2) 1.7 (±0.1) 1.4 (±0.3) 0.9 (±0.3) 1.1 (±0.6) 5000 L 2.8 (±1.3) 1.8 (±3.2) 3.2 (±0.1) 0.9 (±0.7) 1.5 (±0.7) We are also interested in comparing the amounts of C, O, and Mg at the surface as measured by XPS and AES at different temperatures with different exposures of CO 2. The probe depths of XPS and AES are different, even though both are surface-sensitive techniques, with AES measuring its signal over a greater depth. Table 2 summarizes these measurements. For each temperature/exposure pair, the atomic % as measured by AES and XPS is presented. 5

6 Atomic Percent of Carbon at Different Exposures by AES 3.5 Atomic percentage of Carbon L 1000L 5000L 550 C 575 C 600 C 625 C 650 C Figure 3. Atomic surface concentrations of carbon on MgO after reaction with CO 2 for three different exposures and a series of temperatures. Table 2. Surface Concentration of oxygen (atomic %) by XPS and AES after reaction with CO XPS 550 AES 575 XPS 575 AES 500L 47.8% 20.7% 48.2% 39.9% 1000L 49.7% 27.1% 48.4% 44.3% This general trend can be seen more clearly in Figure 3. Three different exposures are summarized in the figure. The surface temperature for each exposure was 550ºC. In all experiments, the oxygen surface concentration is significantly higher in the 600 XPS 48.9% 49.6% XPS measurements. XPS is a more surface 600 AES 37.8% 35.1% sensitive method than AES. The results 625 XPS 625 AES 650 XPS 650 AES 47.9% 52.1% 49.3% 39.3% 47.1% 33.7% 48.3% 39.2% indicate that the very near surface region is oxygen rich after the CO 2 exposure. Electron microscopy is also used to examine the surface after CO 2 exposure. Figure 4 is an electron micrograph of MgO(100) after CO 2 exposure. The black features in the micrograph are carbon deposits. The random distribution of the carbon deposits indicates the 6

7 reduction of CO 2 does not appear to be dominated by reactions at surface defect sites. We are planning to further characterize the carbon by electron microcopy. Complimentary studies on TiO 2 surfaces will also be presented. The band-gap of TiO 2 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 CO 2 at metal oxide surfaces. Future studies will explore the mass transfer aspects of this reaction. Figure 4.Electron micrograph of the MgO(100) crystal surface after exposure to CO 2 at elevated temperatures. The maker is 40 m. References 1. Houghton, J. Global warming. Rep. Prog. Phys. 2005, 68, Karl, T. R.; Trenberth, K. E. Modern global climate change. Science 2003, 302, R. Lal, Sequestration of Atmospheric CO 2 in Global Carbon Pool, Energy and Environmental Science 1 (2008) U.S. Department of Energy. Accessed September 24, S.C. Roy, O.V. Varghese, M. Paulose, and C.A. Grimes, Toward Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons, ACS Nano 4 (2010) Accessed February 6, M.A. Henderson, A Surface Science Perspective on TiO 2 Photocatalysis, Surface Science Reports 66 (2011) V.P. Indrakanti, J.D. Kubicki, and H.H. Schobert, Photoinduced Activation of CO 2 on Ti- Based Heterogeneous Catalysts: Current state, chemical physics-based insights and outlook, Energy & Environmental Science 2 (2009) V.P. Indrakanti, J.D. Kubicki, and H.H. Schobert, Photoinduced Activation of CO 2 on TiO 2 surfaces: Quantum chemical modeling of CO 2 adsorption on oxygen vacancies, Fuel Processing Technology 92 (2011) Sabatier, Hydrogen and Methane, French Patent (1905). 11. V.A. Naumov and L.I. Gavrilov, Determination of kinetic parameters of the Sabatier reaction 7

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