The Interaction of CO

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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The Interaction of CO with CeO Powder Explored by Correlating Adsorption and Thermal Desorption Analyses Danielle Schweke, Shimon Zalkind, and Smadar Attia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01299 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

The Interaction of CO2 with CeO2 Powder Explored by Correlating Adsorption and Thermal Desorption Analyses Danielle Schweke1*, Shimon Zalkind1, Smadar Attia2 and Joseph Bloch1# 1. Nuclear Research Centre- Negev, Beer-Sheva, Israel 2. Israel Atomic Energy Commission, Tel-Aviv, Israel #

Deceased

*Corresponding author email: [email protected]

Abstract

Understanding the interplay between thermodynamics and kinetics is of high importance for the optimization of catalytic reactions involving adsorption of CO2 on CeO2 (ceria). The present study explores the interaction of CO2 with ceria powder in near-realistic conditions by correlating adsorption and thermal desorption analyses. Activation energies for desorption Ea and kinetic parameters (adsorption time constants τ and sticking coefficients s0) are determined using a new methodology, based on surface science models. The sticking coefficients obtained for CO2

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on ceria powder are significantly lower than for CO2 on flat surfaces. CO2 is found to adsorb most rapidly on sites attributed to surface defects. CO2 adsorption is slower on non-defective active sites, leading to the formation of various carbonate species. The desorption analysis indicates that each peak in the CO2-TPD profiles is composed of several sub-peaks, resulting from various binding sites for CO2 on the polycrystalline powder. The distribution of the chemisorbed CO2 species between the different sites, the corresponding adsorption energies and the influence of coverage on those energies are thus determined. In addition, the correlation between adsorption and desorption analyses indicates the influence of heating on the distribution of the chemisorbed species.

INTRODUCTION Ceria is widely used as catalyst, often as a mixture with other oxides1 or as a support to metallic catalysts.2-4 Its main applications include the oxidation of CO and hydrocarbons to CO2 and H2O and the simultaneous reduction of NOx to N2 (the so-called three-way catalysts TWC in automotive exhaust convertors)5,6 and the conversion of CO2 to methanol and other valuable fuels.7-9 The basic reaction in those processes is the well-known reverse water gas shift (RWGS) reaction.7,10 The ability of ceria to liberate oxygen easily (due to the Ce4+/Ce3+ redox couple) is at the origin of its unique efficiency as a catalyst in oxidation-reduction reactions. Indeed, several studies point to the dominant role of oxygen vacancies in surface catalytic reactions.10,11,12 The oxygen vacancies also create pathways for ionic diffusion, as shown by quantum-mechanical calculations.13-16 This property is at the origin of the application of ceria-based materials also as electrolytes in solid oxide fuel cells.17-20


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Thermal programmed desorption (TPD) is a particularly well-adapted method for the experimental study of surface catalytic processes. When applied to the study of flat samples of low surface-area, such as single crystals or thin films, high heating and desorption rates (of the order of 1K/s) are usually applied, leading to relatively narrow peaks. In that case, analysis of the thermal desorption spectrum (TDS) is well established and supported by theory.

21-23

The

procedure mostly used is the "full analysis", based on the rigorous application of the Polanyi−Wigner equation.

21,24

This approach has been applied recently for the study of

adsorption and desorption of H2O and CO2 on TiO2(110)25 and for the study of CO2 adsorption on Fe3O4. 26 Both studies showed that adsorption occurs initially and most strongly at defects. Nevertheless, studies on clean and well-ordered surfaces lack important ingredients to successfully represent processes occurring in "real life" catalytic process.27 TPD can also be used for the study of powders. Then, samples are heated (typically at 1-10 K/min) using an external oven and the desorbing gas is generally transferred to a quadrupole mass spectrometer (QMS) detector through an inert gas. The peaks are generally broad and the application of the theory for thermal desorption analysis is more restricted.28,29 The results obtained by this technique are therefore generally considered as qualitative and used to determine the activity of catalysts as a function of various parameters. For instance, in a TPD study on the catalytic performance of CuO-CeO2 catalysts for CO oxidation, it was found that the optimal composition is the one leading to the highest concentration of easily reduced sites.30 In another study on the influence of CO2 on the performance of CeO2/Al2O3 catalysts for propane oxidation, the catalytic activity was found to decrease in the presence of CO2, as a result of the formation of surface carbonates.31 Despite their importance for optimizing such catalytic processes, basic studies on CO2 interaction with ceria powder under realistic pressures are lacking. In the present work, we


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combine volumetric adsorption and TPD in vacuum conditions for studying kinetic and thermodynamic aspects of this interaction. We develop a new methodology for the analysis of TPD profiles from powder samples and apply it to determine the activation energies for desorption Ea and the distribution of the chemisorbed species between the various adsorption sites. The adsorption rate constants for CO2 on the different types of sites are derived from the kinetic analysis of the adsorption. The initial sticking coefficients for CO2 on ceria powder are also deduced by combining the kinetic and TPD data. Furthermore, by correlating the adsorption and desorption data, the effect of heating on the distribution of the chemisorbed molecules could be elucidated. The presented results points to the determinant role of surface defects on the kinetics and thermodynamics of CO2 adsorption on ceria.

EXPERIMENTAL SECTION The system: The experimental setup (combining Sievert’s and QMS apparatus) is shown in Figure 1. The powder sample was placed in a tubular quartz reactor heated by an external resistivity oven. Two thermocouples were used to monitor the sample's temperature: an internal one (TC1) used to record the sample's temperature and an external one (TC2) used to control the oven's temperature for the TPD experiments. The base pressure in the system, achieved by a diffusion pump, was ~10-8 Torr. Experimental procedure: In each experiment, a predetermined initial pressure of CO2 was introduced into the pre-determined volume enclosed by valves 1, 2 and 3 and the sample was exposed to the gas at room temperature by opening valve 1. Several exposure pressures, ranging from a few mTorrs to ~0.5 Torr, were used. The pressure during the adsorption stage was recorded using a diaphragm capacitance manometer until the system reached equilibrium (in the absence of any filter between the powder and the pressure gauge). At low initial pressures, the


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amount of CO2 available was significantly lower than the sample’s adsorption capacity and all the CO2 available adsorbed, leading to a negligible equilibrium pressure. In experiments performed at higher initial pressures, a finite equilibrium pressure was recorded at the end of the adsorption stage. The remaining CO2 above the sample at equilibrium was pumped. Subsequently, the weakly adsorbed molecules were pumped while monitoring the residual gases by QMS, using valve 3 above the QMS as a leak valve. When the CO2 signal intensity became minor (below ~5·10-8 Torr), we started heating the sample at a linear heating rate of 9K/min with valve 3 fully opened to achieve the fastest pumping rate. Both QMS and temperature signals were measured as a function of time and the thermal desorption spectrum (TDS) was extracted. Following thermal desorption, the sample was cooled down to room temperature.

Figure 1: Schematic drawing of the experimental set-up used for CO2-ceria experiments. The system combines volumetric gas adsorption and Thermal Programmed Desorption (TPD) apparatus for studying adsorption on polycrystalline powder under "realistic" conditions.


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The sample: Commercial 99.9% CeO2 powder (~1g) was used. An X-ray diffraction (XRD) pattern of the sample indicated only the fluorite oxide diffraction pattern of CeO2. The BET surface area of the sample was measured in a separate system, after pre-treating the sample. A surface area of 2.099±0.004 m2/g was obtained. Assuming spherical particles of uniform size, this value corresponds to an average particle diameter of ~0.2 µm. The surface area was used to determine the surface coverage corresponding to any amount of CO2 adsorbed. Scanning Electron Microscope (SEM) micrographs of the sample confirmed this particle size and showed that the particles tend to agglomerate to form apparently larger particles. The results of the powder characterization are provided as Supplementary material. Before running the experiments, the sample was thoroughly pumped under heating up to ~950K until water and hydrogen levels dropped down to the background level. For the experiments performed on the oxidized ceria powder, the sample was exposed to about 0.5 Torr O2 at ~570K for a few hours before cooling, in order to refill the oxygen vacancies which are known to be created by outgassing.

RESULTS AND DISCUSSION 1. Thermal desorption profiles of chemisorbed CO2 Effect of initial pressure Figure 2 represents the total amount of adsorbed CO2 on the oxidized ceria powder as a function of initial pressure (P0). The adsorbed CO2 includes weakly bound molecules, which desorb from the surface during pumping at room temperature (that we refer to as "physisorbed") and strongly bound molecules, which desorb from the surface only upon surface heating during the TPD stage (that we refer to as "chemisorbed"). The total amount of adsorbed CO2 (black


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dots) is deduced from the volumetric measurement. The chemisorbed fraction (red dots) is obtained by integrating the TDS signal of CO2. Finally, the physisorbed fraction (blue dots) is obtained by subtracting the chemisorbed amount from the total amount of adsorbed CO2. It can be seen that at initial pressures up to P0~50 mTorr, the CO2 molecules adsorb chemically. For higher exposure pressures, the physisorbed amount becomes noticeable and continues to increase with increasing exposure pressure. In contrast, the chemisorbed amount apparently reaches saturation at CO2 pressures of a few Torrs, since TPD profiles taken at such pressures look similar in shape and intensity to those recorded at P0~0.5 Torr (see Figure 4).

2.5

Total amount of adsorbed CO2 Chemisorbed CO2

Adsorbed CO2(µmole/gr)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Physisorbed CO2

2.0 1.5 1.0 0.5 0.0 0

100

200

300

400

500

P0 (mTorr)

Figure 2: Chemisorbed and physisorbed amounts of CO2 as a function of exposure pressure of CO2 (P0). The total amount of adsorbed CO2 was determined volumetrically while the chemisorbed amount was obtained by integrating the Thermal Desorption Spectrum (TDS) curve of CO2 and using an appropriate calibration. The physisorbed amount was obtained by subtracting the chemisorbed amount from the total amount of adsorbed CO2. (The calibration is


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the main source of error in the chemisorbed and physisorbed amounts and may reach 0.25 µmole/gr at high exposure pressures). Figure 3 shows a series of TDS (Thermal Desorption Spectroscopy) spectra obtained following the adsorption of CO2 at different initial pressures on the oxidized sample (i.e. sample exposed to O2 at ~570K after outgassing). Clearly the spectra are composed of several peaks, whose relative intensity depends on the initial pressure. The CO2 desorption signal extends up to the temperature of ~750K. Based on previous studies discussed below, the strongly chemisorbed species forming upon CO2 adsorption on ceria are attributed essentially to surface carbonates.32,33 The intensity of the CO signal resulting from CO2 dissociation, obtained upon subtracting the CO signal resulting from CO2 fragmentation (~8% of CO2-TPD signal) from the CO-TPD one, was found to be negligible in those experiments. The CO-TPD profiles exhibited weak peaks at ~570K and above.

P0

496

200 CO2intensity (arbitrary units)

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4 mTorr 7 mTorr 15 mTorr 26 mTorr 39 mTorr 77 mTorr 98 mTorr 208 mTorr 496 mTorr

150 208

100 98 77

50

400

39 26 15

500

600

700

800

Temperature (K)

Figure 3: TPD profiles of CO2 obtained for various (denoted) initial pressures of CO2.


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Effect of partial reduction CO2 and CO TPD profiles recorded upon CO2 adsorption on partially reduced ceria samples (reduced by outgassing up to ~950K) are shown in Figure 4a. For comparison, similar profiles, recorded following prior exposure of the sample to O2 at room temperature or under heating to ~550K, are shown in Figure 4b. It can be seen that the CO2-TPD profile obtained upon CO2 adsorption on the reduced sample clearly differs from that obtained upon CO2 adsorption on the pre-oxidized samples. In addition, a high intensity CO signal is obtained upon CO2 adsorption on the reduced sample, indicating surface dissociation of CO2. The CO-TPD profiles are composed of two distinctive peaks, at ~550K and ~750K. In contrast (as previously mentioned), the CO signal is negligible following sample pre-oxidation.

Figure 4: CO2-TPD and CO-TPD profiles obtained on partially reduced ceria powder. a/ Powder exposed to CO2 at initial pressure of ~ 2 Torr. b/ Powder exposed to CO2 at initial pressure of ~ 11 Torr following pre-exposure to O2 at room temperature or under heating. The CO signal was


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obtained by subtracting the signal resulting from CO2 fragmentation (~8% of CO2-TPD signal) from the measured CO-TPD curve. 2. Characterization of the chemisorption sites by TPD Kinetic modeling of the desorption The TPD spectra were analyzed using the Polanyi-Wigner equation21,22. The desorption rate for a first order kinetics process is given by: (1) rdes = − d [c ] dt = ν ⋅ [c ] ⋅ exp( − E a k B T ) where [c] is the surface concentration of the adsorbate, T is the temperature, Ea is the activation energy for desorption, kB is the Boltzmann constant and ν is the pre-exponential factor. Examination of the series of spectra shown in Figure 3 indicates that each CO2-TPD spectrum is composed of four to five peaks, resulting from different desorbing entities. Adding the contribution from each desorbing entity i (of characteristic surface concentration c0i and activation energies Eai) leads to:

(2) c = ∑ ∙ exp (− ∙ , ⁄() ∙ ) where νi is the pre-factor corresponding to each entity. The pre-factor values νi were determined based on statistical thermodynamics considerations. TS According to statistical thermodynamics24,34, the pre-factor is given by ν TST = (kT h ) ⋅ ( q q ads )

(3) where qTS is the partition function of the molecule in the transition state (just before desorbing) and qads is the partition function of the adsorbed molecule (initial state of the desorption process). Calculation of the exact partition functions of the adsorbed molecule, qads, requires a detailed knowledge of the adsorbate-substrate interaction potential. However, the upper and lower limits of qads can be calculated. The lower bound value of qads is taken to be one,


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as we assume that the adsorbate is fully hindered in all translational, rotational and vibrational modes. The upper bound value is obtained by considering the highest possible number of degrees of freedom for the adsorbate (as in the transition state), assuming the adsorbate’s rotational and vibrational degrees of freedom are allowed whereas the translational motion is hindered since the adsorbates considered are strongly chemisorbed on the surface. (In contrast, the maximal partition function for physisorbed adsorbates is usually calculated assuming a "2D gas" on the surface).24,34 The upper and lower limits of νi were thus calculated for each peak, at the temperature of the peak maximum. Figure 5 illustrates the fitting process to the above model, using non-linear regression, on the experiment performed at P0=4 mTorr. Using a single activation energy for desorption for each TPD peak (denoted A to E in increasing order of stability) do not lead to an adequate fitting, as can be seen in Figure 5a. A significantly better fitting to the experimental spectrum is obtained assuming that each TPD peak is composed of several sub-peaks of different energies, distributed normally around the central Ea,i value (Figure 5b). This indicates that, unlike the case of single crystal experiments under UHV conditions, a variety of binding sites for CO2 on the polycrystalline powder (sub-peaks) are possible for each adsorbate entity. The upper and lower limits values of νi used in the calculation (equation 2) are listed in Table 1.


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Figure 5: (a) Experimental TPD spectrum (obtained upon exposure to 4 mTorr CO2) and best fitted spectrum obtained using a single Ea value for each peak. The energies (Eai) and relative intensities (c0i) obtained for the different peaks are represented graphically on the right. (b) Experimental TPD spectrum and best fitted spectrum assuming each peak (A to E) is composed of sub-peaks of activation energies normally distributed around the central value, as represented on the right. The energy step in Ea scale (0.08eV) was chosen as the maximal value leading to a good fitting to all the TPD spectra. Table 1: List of the pre-exponential factor values νi used for modeling desorption (using equation 2). The upper (νhigh) and lower (νlow) limits of ν for each peak were determined at the temperature of the peak maximum, using the method described in the text.

νhigh (s-1)

Peak A

Peak B

Peak C

Peak D

Peak E

1.4 ×1014

2.1 ×1014

5.1 1014

8.0 1014

1.2 1015


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when qads=1

νlow (s-1)

2.1 1012

2.9 1012

6.0 1012

8.5 1012

1.1 1013

when qads=qTS=qrotqvib

The fitting was performed from the TPD spectrum obtained at the lowest pressure (in which the different peaks are discerned most clearly) to that obtained at the highest pressure. To get fitting results consistent with the whole set of TPD spectra, the activation energy for each peak was allowed to vary between lower and upper bounds defined based on the results obtained from the preceding fitting. The results obtained by fitting the series of CO2-TPD profiles are shown in Figure 6 for five representative spectra. It can be seen that the width of the peaks, especially of peak C, increases significantly with increasing coverage. This may indicate that the number of adsorption sites for that species increases with coverage. Note that peak A can no longer be discerned from peak B in the fitting of the TPD spectra for P0 ≥ 77 mTorr. Site A may be included in the low energy side of site B or may have moved to lower energy sites, which are pumped before the TPD measurement.


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Figure 6: Series of representative TPD spectra obtained upon exposure the sample to CO2 at different initial pressures (P0), along with fitting results. The specific exposure pressure in each measurement is written on the right. Note the different y-scale in each graph. Site energies vs coverage The activation energies Ea obtained by fitting the TPD spectra according to the above procedure are listed in Table 2 for three representative initial CO2 pressures. Ea averaged values between the minimal (Ea,min) and maximal (Ea,max) values, obtained using the upper and lower limits of ν respectively, are mentioned for peaks A to D. Peak E is not listed since its position could not be determined accurately (peak maximum out of measured temperature range).


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Table 2: Activation energies for desorption (Ea) obtained by fitting the TPD spectra according to the procedure described in the text. For each peak, the upper and lower bounds (obtained by using the pre-factor limits listed in Table 1) as well as the averaged values are given for three representative experiments (performed at exposure pressures 4 mTorr, 77 mTorr and 208 mTorr). The assignment proposed for each peak is shown in Table 3 and illustrated schematically in Table 4. P0 = 4 mTorr

P0 = 77 mTorr

P0 = 208 mTorr

Peak

Ea,min Ea,max (eV)

Ea,min - Ea average Ea,max (eV) (eV)

Ea,min - Ea average Ea,max (eV) (eV)

A

1.05 -1.18

1.11

-

-

-

-

B

1.30 -1.50

1.40

1.22 - 1.36

1.29

1.13 - 1.27

1.20

C

1.75 -1.97

1.86

1.50 - 1.69

1.60

1.45 - 1.60

1.52

D

2.20 -2.50

2.35

2.00 - 2.24

2.12

1.94 -2.20

2.07

- Ea average (eV)

As shown in Table 2, the activation energies for desorption significantly decrease with increasing coverage. The effect of coverage obtained in the current study is consistent with DFT calculations of CO2 adsorption on CeO2(111).35 Those calculations report the formation of bent CO2 species at low coverage (monodentate and bidentate carbonate, with adsorption energies of 0.31eV and -0.12eV respectively). Increasing the coverage led to a destabilization of those adsorbates attributed to the repulsive interactions between adjacent adsorbates or to the limited capacity of the ceria surface to donate electrons to the adsorbates (though the combined adsorption of specific species was found to reduce this effect).35


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The activation energies for desorption obtained in the present analysis are particularly high. However, the desorption temperatures are in relatively good agreement with those of previous works, performed under similar conditions.30-33 To our knowledge, the most stable CO2 adsorbate detected experimentally on ceria is a tridentate carbonate on CeO2(100).36 It led to TPD desorption peaks up to 700K onto the oxidized surface, which persisted above 700K and even led to a new, intense, desorption peak at 765K on the reduced CeO1.7(100) surface. Angle dependent C k-edge NEXAFS showed that the carbonate species were lying parallel to the surface, suggesting a tridentate configuration. The carbonates were slightly and uniformly inclined following adsorption and got closer to an in-plane orientation following heating (from the deposition temperature of 180K to 400K). The assignment proposed was supported by DFT PBE+U calculations which led to a flat-lying, tridentate carbonate configuration with adsorption energy of −1.93 eV. Previous DFT calculations for CO2 adsorbed on CeO2(111)35 and CeO2(110)37 may not have led to the most stable carbonate species on the considered surfaces. Alternatively, the lower binding energies obtained in those studies may be due to the ideal surfaces considered in the calculations and to the fact that CO2 bounds less strongly on CeO2(111) and CeO2(110) surfaces than CeO2(100).36 Site occupation vs coverage Figure 7 shows the distribution of the chemisorbed CO2 as a function of exposure pressure (P0). The site occupations correspond to the c0i coefficients obtained from fitting the TPD spectra to equation (2). This distribution may differ from that formed directly after adsorption, due to surface processes occurring during heating as discussed below. It can be seen that, in the experiments performed at very low initial pressures, desorption occurs essentially from the most stable sites (E). Those sites are associated with structural defects (such as grain boundaries,


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dislocations, kinks etc as will be shown in section 3.4). An additional peak, resulting from desorption from D sites, can already be discerned at very low pressures. Sites E and D saturate at coverage of ~2·1012 and ~5·1012 molecules/cm2 respectively. Those coverages are considerably lower than a monolayer of CO2 on ceria which is in the order of 1014 molecules/cm2.38 As initial pressure increases, the amount of desorbing C and A/B species becomes noticeable and continuously increases. In relative amounts (normalized to the total amount of chemisorbed CO2), the fraction of molecules adsorbed on the highest energy sites (E) decreases very rapidly with increasing exposure pressure (finally reaching ~10% of the chemisorbed molecules). In contrast, the fraction of chemisorbed CO2 forming C and A/B species increases consistently with increasing pressure (reaching ~80% of the chemisorbed species at ~0.5 Torr). This is in agreement with studies showing that adsorption occurs initially and most strongly on defects.25,26

Figure 7: Distribution of chemisorbed CO2 between the different adsorbates as a function of exposure pressure (P0). The site occupations correspond to the c0i coefficients obtained from fitting the TPD spectra to equation (2). 3. Kinetics of CO2 adsorption on stoichiometric ceria


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Kinetic modeling of the adsorption The analysis presented in this section concerns the experiments for which P0 < 0.1Torr, in which desorption can be neglected and the overall coverage remains very low at the end of the adsorption stage (1.5·1013 molecules/cm2 which is approximately one order of magnitude lower than a monolayer of CO2 on ceria). Assuming there is no energy barrier to CO2 adsorption39, the rate of adsorption of CO2 on the ceria surface at temperature T, obeys the equation: (4)

= #

!"

!" ()⁄

=$

!"

∙ %() ∙ & ∙ '

where nads is the number of moles of adsorbed CO2 per cm2, P is the pressure, s is the initial sticking coefficient (0

The Interaction of CO

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