From the Surface Reaction Control to Gas-Diffusion Control: The

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From the Surface Reaction Control to Gas-diffusion Control: the Synthesis of Hierarchical Porous SnO Microspheres and Their Gas-sensing Mechanism 2

Xiaobing Wang, Yuanyuan Wang, Fei Tian, Huijun Liang, Kui Wang, Xiaohua Zhao, Zhansheng Lu, Kai Jiang, Lin Yang, and Xiangdong Lou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01397 • Publication Date (Web): 10 Jun 2015 Downloaded from http://pubs.acs.org on June 19, 2015

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The Journal of Physical Chemistry

From the Surface Reaction Control to Gas-diffusion Control: the Synthesis of Hierarchical Porous SnO2 Microspheres and Their Gas-sensing Mechanism Xiaobing Wang,†,‡ Yuanyuan Wang,†,‡ Fei Tian,†,‡ Huijun Liang,† ,§,‡ Kui Wang,† Xiaohua Zhao,† ¶

Zhansheng Lu, Kai Jiang,† Lin Yang,*,† and Xiangdong Lou*,† † Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China; § College of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang 453003, P. R. China; ¶College of Physics and Information Engineering, Henan Normal University, Xinxiang 453007, P. R. China. KEYWORDS: Gas Sensors, Mechanisms, Control Factors, Tin Dioxide, Hierarchical Microspheres

ABSTRACT: A series of hierarchical porous SnO2 microspheres (SnO2-Ms) with same sizes of nanoparticles were fabricated through increasing the reaction time of the one-step hydrothermal method. Especially, these SnO2-Ms also have the different specific surface areas and pores sizes. When they are applied in sintering type thick film gas sensors, through comparing the gas-sensing property of the as-prepared SnO2-Ms, it can clearly demonstrate that the surface chemical reaction (SCR) control of the sensing properties of sensors is gradually replaced by the gas diffusion control with the operation temperature (To) increasing. For the first time, this dually control is discovered through contrast experiment. According to the testing results, the sensing mechanism of sensors can be explained by many factors, such as the reaction rate constant of the SCR, the Knudsen diffusion coefficient of the target gas, the To, the specific surface area, the pore size and the change of the H2O, etc. A pore canal model and a hollow sphere model are introduced, which can effectively explain the sensing mechanism of gas sensors. This discovery can make up for the inadequacy of the surface-control and the diffusion-control theory, and expound their interrelation. This discovery also provides a novel strategy for studying the sensing mechanism of sensors, which is expected to open up exciting opportunities for improving the sensing properties of the gas-sensing materials and studying some gas-solid catalytic phenomena.

1. Introduction Now gas sensor has been considered as one of the most important electronic device which can be widely applied in a large variety of fields, such as industrial manufacture, scientific exploration, daily life, environmental monitoring, biomedical and agricultural domains, and so on.1-5 In practical applications, extensive studies have revealed that many complex factors affect the sensing properties,6-8 such as natural properties of base materials, surface areas and microstructure of sensing layers, surface additives, temperature and humidity, etc.6, 9, 10 Meanwhile, the sensing mechanisms of metal oxide semiconducting (MOS) gas sensors have also been investigated extensively, and form the surface-controlled mechanism and bulk-controlled mechanism.11 For the surface-controlled mechanism, it is suitable for mostly MOS gas sensors, and that the surface chemical reaction (SCR) is a key factor for triggering the response of sensors. However, the SCR is always affected by the gas diffusion, the analyte gas (or target gas, GT) species and the gas-sensing

materials, etc. In this context, the surface-controlled mechanism is further developed and forms the SCR control theory and the gas diffusion control theory. For the SCR control theory, the basic understandings have been deepened through elucidating the role of adsorbed oxygen (surface oxygen, Oad),12-14 Schottky barrier mechanism,11 grain size effects,15 chemical and electronic sensitization mechanisms, and so on.1, 8, 16, 17 For the gas diffusion control theory, there are also some base understands in the developing process of surfacecontrolled mechanism. For example, in 1989 and 1990, Gardner proposed a linear diffusion-reaction model 18 and a nonlinear diffusion-reaction model,19 respectively. Thereafter, Vilanova et al.20 proposed that the conductance transients have been modelled taking into account a non-linear diffusionreaction model in 1996. Here, the basic assumption is that the reaction rate is faster compared with the diffusion rate. Then Lu et al. proposed that the gas sensing mechanism also included the function of diffusion-reaction in 2000.21 Before long, Sakai and Matsunaga et al.1, 15, 22-25 proposed that the theory of gas-diffusion controlled sensitivity for thin film semiconductor

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gas sensor. The reaction rate constant (k) and Knudsen diffusion coefficient (DK) were introduced, which replace the assumption that the chemical kinetics rate is faster than the gas diffusion rate. All these theoretical researches should be a new trial to the study of sensitization mechanism of the thick film sensors and the thin film sensors. However, compared with the SCR theory, the development of gas diffusion control theory is very slowly. On the one hand, the relevance of gas diffusion to the gas sensing characteristics is too hard to explain.1, 26-28 This relation involves various physical and chemical processes, including dissociation of the analyte gas (target gas, GT) molecules, migration of the species split from the molecules, and ensuing redox reactions on the surface of the MOS grains.29-31 On the other hand, the operation temperature (To) of the experimental data is usually more than 300℃,21-23 even some studies have not gave out the To range,18-20, 24, 25 because To has a strong effect on the SCR rate, gas diffusion, gas sensing properties and so on, the result is that the further application of diffusion control theory is limited. In addition, to facilitate the reasoning of mathematical formulae, the effect of GT species and reaction products are neglected in more studies. Even some assumptions are too simplistic 32 to conform their conclusions by the test results.33 In the last decade, except for these theoretical studies, some experimental phenomena could be explained by diffusion control in the many literature.33 Unfortunately, in these literatures, they pay more attention to how to use gas-diffusion to explain the experimental phenomena, and that neglected the effect of GT species, To, SCR, gas-sensing materials, etc. To solve the above issues, we try to design an experiment for evaluating the relationship among the gas sensing property, the SCR, the gas diffusion, the To, the GT species, reaction products and the gas-sensing materials, etc. Especially the To less than 300 ℃ and the thick film sensors. Fortunately, we unexpectedly discovered that the gas-sensing mechanism of gas-sensors was controlled by two factors at the To less than 300 ℃. One is the SCR rate, the other is the gas diffusion rate. Especially, the SCR control of the sensing properties is gradually replaced by the diffusion control with the To increasing. It suggests that the control factors (or theory) of the sensing properties are able to change when the To changes. For this, we build a pore canal model and hollow sphere model to explain the alternate change between the SCR control and the gas diffusion control. All these results are performed by three the hierarchical porous SnO2 microspheres which were fabricated through increasing the reaction time of one-step hydrothermal method. Except for the excellent gas-sensing performances, these SnO2 microspheres have the same nanoparticles size, different specific surface areas and pores size. Especially, this discovery would be more helpful to understand the sensing mechanism of sensors, improve the sensing properties of sintering type thick film sensor. It is also helpful to study some gas-solid catalytic phenomena.

2. Experimental Section All the chemical reagents used in this work were of analytical grade without any modification. The solvent medium used for the reaction system was distilled water. Synthesis of SnO2 hollow microspheres. In a typical experimental procedure, 3.70 g SnCl2·2H2O and 2.46 g C4H6O6

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were dissolved in 30 mL distilled water and stirred for 10 min, respectively. Then the two solutions were mixed together under constant stirring with a magnetic stirrer. 0.5 g polyvinylpyrrolidone (PVP) was put into the suspension obtained. The mixed solution was stirred for 30min, and then transferred to 100 mL Teflon-lined stainless steel autoclave filled with distilled water up to 80% of its total volume. The autoclave was sealed and heated at 180 ℃ for 20 h in an electrical oven. After heating treatment, the autoclave was cooled to room temperature. Finally, the light yellow precipitates were centrifuged and washed with deionized water and absolute ethanol several times, then dried at 80 ℃ for 12 h. As a result, the sample was obtained (denoted as B25). When the hydrothermal time prolonged to 30 h and 40 h, the product was named B58 and B93, respectively. As a control, the precursor is synthesized using the same method except that without the hydrothermal progress. Characterizations. The crystal structure was determined using X-ray diffraction (XRD) (Bruker advance-D8 XRD with Cu Kα radiation, λ=0.154178 nm, the accelerating voltage was set at 40 kV with a 100 mA flux). Microstructures and morphologies were investigated using scanning electronic microscopy (SEM, JEOL JSM-6390LV), field emission scanning electron microscopy (FESEM, ZEISS SUPRA-40 VP) and transmission electron microscope (TEM, JEM-2100). The chemical state of the sample was examined by X-ray photoelectron spectroscopy (XPS) on a VG Scientific ESCALABMKLL spectrometer using Al Kα X-ray source (10 mA, 15 kV). Nitrogen adsorption/desorption isotherms were determined with an ASAP 2020 (Micromeritics Instruments). Surface-area determination and pore analysis were performed by using the Brunauer-Emmet-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method. Fourier transform infrared spectrographs (FT-IR) were recorded on a Bio-Rad FTS-40 Fourier transform infrared spectrophotometer in the wavelength range of 4000-400 cm-1. The spectra were collected at 2 cm-1resolution with 128 scans. For all FT-IR measurements, the dried samples were mixed with KBr. Sensor fabrication and gas sensing test. The gas-sensing properties were measured statically using the gas-sensing measurement system of WS-30A (Weisheng Instruments Co., Zhengzhou, China). In a typical fabrication procedure, a proper account of sample powder was slightly grinded with several drops of terpineol in an agate mortar to form slurry. The paste was coated onto an alumina tube (the coating thicknesses are all about 0.25 mm and thus the sample density on the ceramic tube of each sensor is similar) on which a pair of Au electrodes was previously printed, and dried at 80 ℃ for 2 h in air. After annealed at 300℃ for 2 h, A small spring-like Ni–Cr alloy (34~35Ω) was inserted into the ceramic tube was employed as a heater by tuning the heating voltage which is equivalent to the To (The operation temperature estimate, See Supporing Information). The gas sensors were aged at 300 ℃ for 10 days to improve the stability before the first measurement. In a typical test, the desired concentration of the volatile organic compounds gas was obtained by the static liquid gas distribution method (The concentration of target gas, Supporting Information). In this paper, we described the sensitivity as


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Figure 1. SEM, FESEM (insert image), TEM, HRTEM and SAED images of B25 (a1, a2, a3 and a4), B58 (b1, b2, b3 and b4) and B93 (c1, c2, c3 and c4)，respectively.

gas response (S), which was defined as S= Ra/Rg, where Ra and Rg were resistances of the sensors in air and reducing gases vapor, respectively.

3. Results and discussion 3.1. Synthesis and Morphological Characterization A possible mechanism for the formation of the SnO2 microspheres with different pore size and the specific surface area is illustrated in Scheme 1. The Sn2+ of SnCl2 first reacts with tartaric acid to produce SnC4H4O6. In the process of hydrothermal reaction, the PVP molecules would react with SnC4H4O6 to form microspheres.34,

35 With the hydrothermal reaction time increasing, the organic residues in the microspheres are gradually decomposed.36 When the reaction time is prolonged to 20h, a solid microsphere (B25) is obtained. When the reaction time is prolonged to 30h and 40h, a thick wall hollow microsphere (B58) and a thin wall hollow microsphere (B93) are obtained, respectively. For these SnO2 microspheres, the sizes of nanoparticles are same or similar, but the specific surface areas and pore sizes are difference. All of these can be verified by the further characterization.



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cm−1. It means that PVP did not take part in the reaction before the hydrothermal process.

Scheme 1. Schematic illustration of the possible process for the formation of SnO2 hollow microspheres.

Figure 1 shows the SEM, FESEM, TEM, HRTEM and the selected area electron diffraction (SAED) images of the B25, B58 and B93, respectively. The TEM images of B25 (Figure 1a2), B58 (Figure 1b2) and B93 (Figure 1c2) can prove that they are gradually changed from the solid microspheres to the hollow microspheres. Moreover, when the hydrothermal reaction time increased to 30 h, the microspheres are hollow clearly and the shell is a thickness of about 0.46µm (Figure 1b1). When the reaction time increased to 40 h, the thickness of shell is decreased to about 0.30 µm (Figure 1c1). TEM (Figure 1b2, c2) can further proved that B58 and B93 have a wall thickness of about 0.46 µm (Figure 1b2) and 0.35 µm (Figure 1c2), respectively. FESEM and HRTEM images indicate that the shells show a rough surface. HRTEM images further show that they are constituted with nanoparticles of about 10 nm (Figure 1a3, b3 and c3), and the typical lattice fringe spacing was measured to be 0.334 nm, corresponding to the (110) crystal plane of SnO2. The ring pattern of SAED (Figure 1a4, b4 and c4) demonstrates the polycrystalline of the SnO2. It means that all specimens are the hierarchical structure SnO2 microspheres, and they can be fabricated by one-step hydrothermal method. However, the size of the as-prepared SnO2 microspheres (include B25, B58 and B93) is not uniform. The average diameter of B25, B58 and B93 are 2.03µm, 3.23µm and 3.53µm according to the diameter distribution of microspheres, respectively (Figure S1, Supporting Information). In addition, no microspheres were observed in the precursor (Figure S2, Supporting Information). It means that PVP did not take part in the reaction before the hydrothermal process, which can be verified by FT-IR measure. The FT-IR spectra are shown in Figure 2. As for pure tartaric acid (Figure 2a), the absorption band at 1743 cm−1 is characteristic of the C=O stretching vibration.37 and the weak absorption bands can be observed at the precursor (Figure 2b). However, when the reaction time was prolonged to 20 h, the C=O absorption bands disappear completely, it should due to aldol condensation in the self-assembly process. The peaks around 2359 cm-1 belong to residual CO2 traces in the spectrometer’s atmosphere.37 The absorption bands at 3200–3600 cm−1 can be assigned to the O—H stretch and hydrogen bonded.38 When the precursor was obtained, the strong absorption bands at ~ 3415 cm-1 and ~ 1628 cm-1 can be observed (Figure 2b), which can be attribute to the stretching and bending vibration of the OH group. The bands in the range of 536-669 cm-1 can be assigned to the antisymmetric and symmetric vibrations of SnO-Sn.36, 39-41 However, the characteristic bands of PVP were not observed in the precursor, such as 1423 cm−1 and 1290

Figure 2. FT-IR spectra of (a) tartaric acid, (b) the precursor, (c) PVP, (d) B25, (e) B58 and (f) B93.

When the reaction times were prolonged to 20h (Figure 2d), 30h (Figure 2e) and 40h (Figure 2f), the characteristic bands of PVP (1463 cm−1, 1437 cm−1 and 1423 cm−1, especially 1290 cm−1, Figure c) were observed.42 It could be speculated that the concentration of tartaric acid inside the as-prepared SnO2 microspheres was gradually reduced with prolonging reaction time, and the long polymeric chain of PVP would be selectively absorbed on the surfaces of SnO2 nanoparticles. This selectively absorption is evidenced by the weakening of —CH— stretching vibrations at 2953 cm−1 from Figure 2d-f.43 It also confirms that a few of organics include in the single SnO2 nanocrystals which can not be detected by XRD. In the crystallize process, these organics will prevent the SnO2 nanocrystals together and stop the nanoparticle size increase. Therefore, according to our obtained experimental data, the growth mechanism for the formation of SnO2 hollow microspheres should be appropriate.

Figure 3. XRD patterns of the precursor, B25, B58 and B93.


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Figure 4. Survey scan XPS spectrum of the as-prepared SnO2 (a) and high resolution XPS spectra in the vicinity of the Sn 3d (b), O 1s (c), and C 1s peaks (d). The phase analysis of the precursor and the as-prepared SnO2 microspheres were investigated by XRD. As shown in Figure 3, all the strong diffraction peaks of the samples except the precursor can be indexed to the tetragonal rutile structure of SnO2 (cassiterite, JCPDS No. 41-1445, a=b=4.738 Å and c=3.188 Å.) and belong to space group P42/mnm (number 136). The broader and lower intensity of the diffraction peaks of the precursor indicate the poor SnO2 crystal nature of the compound. The average grain size of the nanocrystals calculated along the (110) plane using the Scherrer formula is about 11nm for the as-prepared products, which agrees well with the crystallites size measured of the HRTEM. It is important that the average sizes of nanoparticles have not significantly change, which can keep the catalytic activity unchanged when they are used as the sensing material. So we consider that these samples should consist of a type SnO2 material, the change is only specific surface area and the pore size. The chemical composition and surface state of the asprepared products were further investigated by X-ray photoelectron spectroscopy (XPS) (Figure 4). Figure 4a shows the survey scan XPS spectrum of the B25, B58 and B93, respectively. Peaks of Sn, O and C can be clearly observed in survey spectrum (Fig. 4a), and no peaks of other elements are observed. The high-resolution XPS spectra of Sn, O and C elements are shown in Fig. 4b, c and d, respectively. In Fig. 4b,

there are two peaks locate at 495.0 and 486.6 eV, respectively, with a peak separation of 8.4 eV, indicating the Sn4+ oxidation state.44-46 The asymmetric O 1s spectrum (Figure 4c) can be resolved into several peaks that correspond to O2(ad)−, O(ad)− and O2− with binding energies of 530.2, 532.1 (B93 is 531.8 eV), and 532.9 eV, respectively.46 These binding energies are characteristic of ionized oxygen species at the SnO2 surface.47 With the specific surface area of the as-prepared samples increasing, the relative peak area percentages of O2(ad)−, O(ad)− and O2− in their peak area of O 1s are gradually increase, suggesting that the amount of O2(ad)−, O(ad)− and O2− gradually increase. Additional peaks at 533.4 and 531.6 (B93 is 531.5) eV (Figure 4c) represent C=O groups and C-OH groups belonging to the tartaric acid and PVP.48 Their relative peak area percentages decrease gradually with the reaction time increasing, suggesting that the organic residues in the microspheres are gradually decomposed. The C 1s high resolution spectrum (Figure 4d) has a main peak in the range of 284−285 eV that corresponds to C−C and C−H bonding, two minor peaks at 286.1 and 288.4 eV that correspond to C−OH and O=C groups, respectively.46 All these results further show that the organic residues in the microspheres are gradually decomposed with the hydrothermal reaction time increasing, so that different specific surface area and pore size can be obtained.



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are highly porous. It can be further verified by the FESEM images (Figure 1). The more parameters are listed in table 1 according to the nitrogen adsorption/desorption experiment. 3.2. Gas-sensing property and the gas-sensing mechanism discussion The gas sensing experiment was performed using the static test method to examine the gas-sensing property of the asprepared SnO2 microsphere with acetone, ethanol and H2S, respectively. The concentrations of GT (diluted in air) were varied from 5 to 100 ppm (The concentration of target gas, See Supporting Information). The operation temperature is estimated by the heater powers (The estimate of operation temperature, See Supporting Information). In addition, we described the sensitivity as the response (S). As shown in Figure 6, all the as-prepared SnO2 microspheres exhibited excellent sensing properties to aceton, ethanol and H2S. The black, red and blue line in Figure 6 express concentration-responses curve (CRC) of the as-prepared B25, B58 and B93, respectively. The change of To is from 180℃ to 300℃.

Figure 5. (a) N2 adsorption-desorption isotherm of the products. (b) The pore size distribution calculated from the desorption branch.

Table 1. The specific surface area, the main distribution rang of pore size, pore volume and average pore size of the products.

The products

Specific surface area [m2 g-1]

The main distribution range of Pore size [nm]

Total pore volume [cm3 g-1]

Average pore size [nm]

HS-20h

25.78

3-50

0.071

11.0

HS-30h

58.54

2-30

0.12

8.2

HS-40h

93.75

1.7-15

0.14

5.4

The specific surface areas and porosity of the as-prepared microspheres were investigated by nitrogen adsorption/desorption experiment. The N2 adsorption/desorption isotherms of the as-prepared SnO2 microspheres show characteristics of mesoporous materials (Figure 5a). The BarrettJoyner-Halenda (BJH) pore size distribution (Figure 5b) is obtained from the isotherm. As can be seen, the specific surface areas of B25, B58 and B93 are 25.78 m2 g-1, 58.54 m2 g-1 and 93.75 m2 g-1, the average pore sizes are 11.0 nm, 8.2 nm and 5.4 nm, the pore diameter centers are 31 nm, 9 nm and 1.8 nm, respectively. It means that the as-prepared microspheres

As shown in Figure 6, with the To increasing (from a to d), the CRC position of B25 (black line) rise gradually highly relative to the B58 (red line) and B93 (blue line). However, the CRC position of B93 shows a reverse pattern. The CRC position of B58 is the highest at 220℃ (Figure 6b), then it is located in the middle with the To further increasing (Figure 6c and d). This phenomenon also occurs on the responses to ethanol (Figure S5 a-e, Supporting Information). However, only at higher temperature (260℃), the CRC position of H2S (Figure S5i, Supporting Information) is similar to acetone (Figure 5c and d) and ethanol (Figure S5d and e). It means that the CRC position of the different sample can change alternately with the To increasing. It is well known that two processes can take place on the surface of the MOS film: gas adsorption followed by diffusion into the bulk of MOS film and catalytic oxidation of hydrocarbon with oxygen chemisorbed on MOS.16, 44 It means that the total sensor sensitivity is not only dependent on the SCR rate but also the rate of gas diffusion.1 However, when the sensors are made of the same material, the SCR rates are dependent on the catalytic activity of a sensing layer and the To,1, 49 the gas diffusion rate is limited by the microstructure of the sensing layer and the size of the GT molecules.49 The surface-control theory believes that if the gas-sensing property is controlled by the SCR rates, the larger the specific surface area, the more the active site, the sensitivity of the largest specific surface area should be the biggest at this time. Otherwise, it should be the diffusion-control. The gas-diffusion rate will become the ratelimiting step. The larger the pore size, the faster the diffusion rate, the sensitivity of the largest pore size should be the biggest. However, as shown in Figure 6, the surface-control theory can not explain why the sensitivity of B25 is the largest at the high To, (Its specific surface area is the smallest, but its sensitivity is the biggest.) just as the diffusion-control theory can not explain why the sensitivity of B93 is the largest at the low To (Its pore size is the smallest, but its sensitivity is the biggest.). It means that this very interesting phenomenon is hardly explained by a single surface-control theory or a single diffusion-controlled theory. If the gas-sensing properties of the


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Figure 6. The concentration-responses curve of acetone at the different operating temperature. (a) 180℃, (b) 220℃, (c) 260℃, (d) 300℃, respectively.

as-prepared SnO2 microspheres can be understood as dually controlled between the SCR and the gas-diffusion, the change of the CRC position is reflected that the SCR control is gradually replaced by the gas-diffusion controlled with the To increasing, this phenomenon can be explained more easily. 3.2.1. Surface Reaction and gas-diffusion dually control the sensitivity of the SnO2 sensors As shown in Figure 6, according to the control factors and the test data, the response of the sensors can be divided into three stages. i) The SCR control stage. At the lower To (e.g., To200℃), more Oad convert to O(ad)− and O(ad) 2− through accepting electrons from tin oxide. This result was used the electron paramagnetic resonance (EPR) measurements. In the recently, the Oad on the SnO2 was also investigated in more detail by using the temperature programmed desorption (TPD),3, 59, 60 their temperature ranges are difference to the S.C. Chang’s result. In this paper, the To is estimated by the heater powers, it should have a certain degree of error. But the existence forms of Oad are agrees well with the S.C. Chang, so we adopt our To ranges to discuss the influence of Oad, GT species and the To. Actually, the influence of the Oad, the GT species and the To, especially their gas-sensing mechanisms have been investigated extensively through experiments and theories in many literature.6, 60, 61 For example the gas-sensing mechanisms of CO, we used the spin-polarized density functional theory (DFT) to investigate the most likely reaction steps involved in CO oxidation and the refresh of the SnO2 surface. The proposed process for the adsorption and conversion of O2 are that O2 (gas)↔ O2(ad) ↔ O2(ad)− ↔ O2(ad)2− ↔ O(lattice)2−+O(ad)−, which can happen only on the SnO2−x support but not on the perfect SnO2 (110) support.62 Therefore, the To not only directly affects the SCR, the gas diffusion and the Oio species on the MOS surface, but also further affects the sensitivity of sensors. Because Oio species will change with the change of To, the influence of Oio species at the different To can be seen that the reactivity of Oio is not same, even they are same or opposite. So we assume that the Oio have different reactivity [O2(ad)−

From the Surface Reaction Control to Gas-Diffusion Control: The

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