Resonant-gravimetric Identification of Competitive Adsorption of

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Resonant-gravimetric Identification of Competitive Adsorption of Environmental Molecules Pengcheng Xu, Tao Xu, Haitao Yu, and Xinxin Li Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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Analytical Chemistry

Resonant-gravimetric Identification of Competitive Adsorption of Environmental Molecules Pengcheng Xu, Tao Xu, Haitao Yu, and Xinxin Li* State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, China. *Corresponding Author. Xinxin Li, E-mail: [email protected]; Fax: +86-21-62131744; Tel: +86-21-62131794. ABSTRACT: Understanding competitive adsorption relationship among various ambient gases is important in adsorbing-material development for capturing environmental harmful gas. For example, environmental interfering factors (e.g., moisture) can affect the competitive gas-molecule adsorption that needs to be clarified. Due to lack of method to quantitatively study dynamic adsorbing process (e.g., real-time counting adsorbed molecule number), it is difficult to reveal the competitive adsorption mechanism. Still using conventional “trial-and-error” method hinders the development of high-performance adsorbing materials, thereby new technology is highly demanded to address the issue. This study opens up a three-step resonant-gravimetric analysis method by using ultra-sensitive resonant-cantilevers. The three experimental steps are sequentially for qualitative analysis, quantitative determination and thermodynamic-level identification about competitive adsorption relationship among the environmental gas-molecules. Previous studies indicate that zeolitic-imidazolate-framework (ZIF) of ZIF-8 nanocrystals have low affinity to environmental CO2. This conclusion is confirmed in this study by evaluating ZIF-8 with the three experimental steps, sequentially for qualitative judgment of adsorbability, quantitative determination of hydrous molecule-structure in real air and quantitative extraction of thermodynamic enthalpy ∆H°. By figuring out competitive interface-adsorption relationship, we verified that ZIF-8 cannot adsorb CO2 in real air. However, for the first time, ZIF-8 is identified as an excellent adsorbent to environmental NO2.

INTRODUCTION Development of advanced materials for capture and sequestration of harmful/greenhouse gas molecules from ambient atmosphere is becoming an intensive research topic. Such advanced materials are highly demanded in various fields like environmental protection.1-3 To date, design and optimization of new adsorbing materials have been mainly based on the socalled “trial-and-error” method.4-6 Besides labourintensive and time-consuming, using the “trial-anderror” method is difficult to improve the material to reach the best performance. Let us take CO2 capture material as example. Conventional efforts have been made mainly on increasing specific surface area to enhance CO2 adsorbing capacity. Some recently developed nano-materials, such as metal-organic frameworks (MOFs), nanoporous graphene and surface-modified mesoporous-silica, are considered as promising candidates for capture and fixation of CO2, due to their huge specific surface areas.7-16 For practical application however, more aspects of adsorbing performance should be taken into account, including adsorbing speed and regeneration capacity, etc.17 By simply using the “trial-and-error” method, it is really difficult to comprehensively understand the correlated and interacted relationship among different character-

istics of the adsorbing material, thereby, collaborative optimization among the multiple properties is hard to be achieved.4, 18 As for adsorption of gas molecules in ambient atmosphere, competitive adsorption relationship is existent among different kinds of gas molecules. On the other hand, environmental factors in real atmospheric air, e.g., humidity, have strong interference to adsorption process. The interference of H2O molecules in atmosphere can also be considered as a kind of competitive adsorption effect to the target gas molecules. Along the conventional “trial-and-error” route, commercially available gas-sorption analyzers are normally used in lab to evaluate the developed adsorption materials.1,19,20 Based on the conventional analysis method, the CO2 adsorption performance of zeoliticimidazolate-framework (ZIF) of ZIF-8 has been widely studied.21,22 However, the previous reported gassorption experiments are normally assessed under anhydrate atmosphere, thereby, the interference was not taken into account. In addition, high humidity in real atmosphere possibly damages the structure of some MOF materials, as was reported in literature.23 The MOF material with damaged structure will exhibit degraded adsorption capacity. From this point of view, high humidity may have a negative effect on

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CO2 adsorption by using some MOF materials. Therefore, as for adsorption to CO2 in real air, the environmentally ubiquitous moisture and other gases may bring about strong competitive influence to CO2 adsorption. By simply using conventional gas-sorption analysis, it is really hard to differentiate such competitive molecule-adsorption sequence. Conventional sorption analysis is not only timeconsuming and costly, but also requires quite a large amount of material sample (e.g., at least 0.1g level) 20 and not less than mbar-level pressure (i.e., about 103 ppm-level concentration) of the adsorbed gas. Thus, the traditional gas-sorption analysis method is not suitable for trace-amount adsorption processes, such as those for fixation of toxic gas or detection of agricultural pesticide-residue. In addition, using the traditional gas-sorption analyzers can only detect the adsorbed amount at equilibrium state, but cannot be used to continually record the whole adsorbing process. Nowadays, there is really lack of in situ detection tool to continually record the adsorbed molecule number during the whole trace-amount adsorption process. The shortage in method for elucidating the competitive adsorption mechanism (e.g., identify the competitive adsorption order) seriously hinders highefficiency development of adsorbing materials. In order to thoroughly clarify competitive adsorption of the target gas molecules under complex atmospheric environment, it is needed to see the inherent nature through the apparent adsorbing phenomenon. From the perspective of “material genome”, the sorption performance of a material is eventually governed by the thermodynamic parameters like enthalpy ∆H° (i.e., isosteric heat).14,17,24-27 If such thermodynamic parameters can be quantitatively extracted from the adsorption process, development of sorption material will benefit quite a lot. Based on ultra-sensitive mass-type transducer of resonant-cantilevers, herein we proposed a three-step resonant-gravimetric experimental method for competitive adsorption research. By sequentially implementing the three-step resonant-gravimetric identification process for the adsorbing material, three-level corresponding findings can be obtained. With the first-step analysis, the adsorbability of material with target gas can be qualitatively judged by reading the resonant-cantilever frequency-shift signal that directly proportional to the mass of the adsorbed gasmolecules. Under real air atmosphere, the adsorption behaviour can be influenced by the environmental factors like humidity. In this case the target gasmolecules may exist in hydrate state. By using the second-step analysis, accurate resonant-gravimetric data can be obtained to quantitatively determine sorp-

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tion details, like the crystal-water number combined to the hydrate NO2 molecule. This quantitative determination is based on the nature that the detected mass directly reflects the adsorbed molecule number. For elucidating the competitive adsorption mechanism between the target gas-molecule and the environmental interfering molecules like H2O, the thermodynamic parameter of ∆H° can be quantitatively obtained through the third-step temperature-varying resonantgravimetric experiment. By comparing the obtained ∆H° values, we can finally reveal the competitive adsorption relationship between the target gas and the interfering molecules like the herein H2O. For demonstrating the effectiveness of the proposed method, the nanoporous material of ZIF-8,28-36 which is a subclass of MOFs, is herein selected as the representative adsorbent to environmental gases. Our resonantgravimetric analysis will clarify that, ZIF-8 is not suitable for capturing CO2 in real atmosphere, but it is able to efficiently capturing/fixing/sensing environmental NO2. EXPERIMENTAL SECTION ZIF-8 synthesis. ZIF-8 nanocrystals are synthesized with a modified procedure in literatures 28, 29 and detailed as follows. Firstly, 0.30 g of Zn(NO3)2⋅6H2O (Aldrich) is dissolved in 11 g of anhydrate methanol to form stock solution I. Then, 0.66 g of 2methylimidazole (Aldrich) is added into 11 g of methanol to form stock solution II. After 2methylimidazole is dissolved completely in methanol, stock solution II is quickly poured into stock solution I under vigorous stirring, and a homogeneous suspension is obtained after 5 min. After that, the abovementioned suspension is transformed into a Teflon lined stainless steel autoclave and kept aging statically at 423 K for 3 hours. After cooling to room temperature, the suspension is transformed into a centrifuge tube with 50mL volume and the solid product is collected by high-speed centrifugation (10,000 rpm) for 2 min. Then, the solid product is washed and purified by repeating the following procedure for three times: dispersed in 20 mL of methanol and centrifugation. Finally, the ZIF-8 nanocrystals are obtained after dry overnight at 343 K. Ink-jet printing technology for ZIF-8 sample loading on resonant micro-cantilever. Featuring 1.5 pg/Hz mass sensitivity and pg-level resolution, our lab-fabricated resonant micro-cantilevers37 are employed to investigate the adsorption properties of ZIF8 to CO2 molecules. About 10 mg of the ZIF-8 sample is added into 1 mL deionized water (under ultra-sonic) to form a crude suspension which is used as ink in the


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following material deposition experiment. Then, several drops of the inks are printed onto the microcantilever top-surface by using a commercial GIX II Microplotter (Sonoplot Inc.). After that, the microcantilever detector is dried in an oven at 333 K for about 2 hours. Material characterization. Powder X-ray diffraction pattern is obtained with a Rigaku D-MAX/IIA X-ray diffractometer with Cu Kα radiation. The scanning range is 5-30° (2θ) and the scanning rate is set as 1.2° min-1. Transmission electron microscopy (TEM) is taken with a FEI Tecnai G20 microscope, where 200 kV accelerating voltage is used. Scanning electron microscopy (SEM) images are taken using an FEI Magellan 400 XHR ultrahigh resolution cold field emission scanning electron microscope. Experimental set-up. The target gas with desired concentration is diluted by high purity N2 and is supplied by Shanghai Shenkai Gases Technology Co., LTD. All the MFC (mass flow controller) are calibrated by using digital soap-film flow-meter. The temperature-controlled oven can supply a precise temperature in the range of 233 K to 423 K, with a negligible temperature fluctuation of less than 0.5 K. In order to control the temperature of the flowing gas, a helical-coil tube with a length of 10 m, which is put inside into the temperature controlled oven, is used prior the testing chamber. To diminish the nonspecific adsorption of the gas molecules with ultralow concentration, all the tubes and joints as well as valves used in the set-up are made by high-grade Teflon with ultra-low surface-energy. A lab-developed digital phase-locked loop circuit is used as the key component to construct the data acquisition system. RESULTS AND DISCUSSION

tem. In order to generate simulant ambient gas with various relative humidity, a piece of wet sponge is placed in the bottle-shaped container that is connected with the gas line. The concentration of moisture is determined by both the water amount in the sponge and the gas flow-rate. The value of relative humidity is real-time measured by a commercial humidometer that is placed at the end of the gas line. When the bottle with wet sponge is disassembled from the gas line, the sample of adsorbing material can also be examined under dry gas. An integrated silicon resonant micro-cantilever is used as the core detecting component of the temperature-varying resonant-gravimetric system. Both an electro-thermally micro-heater for resonance excitation and a piezoresistive Wheatstone bridge for frequency-shift signal readout are integrated in the cantilever.37 Due to the adsorbed gasmolecule mass (∆m) being much smaller than that of the silicon cantilever (m0), the real-time recorded frequency-shift signal can be accurately proportional to ∆m, i.e., ∆f/f0=0.5∆m/m0 and the mass sensitivity is defined as S=∆f/∆m.38 By previously implementing calibration experiment, where the micro-beads with known-mass were loaded on the cantilever for sensing, S=1.5 Hz/pg has been obtained.39 According to the experimentally achieved lower than 0.5Hz noise-floor of the frequency signal,40 the mass detection resolution of the cantilever is finer than 1 picogram in atmospheric ambience. Thanks to the precise MEMS (microelectromechanical systems) technology for batch fabrication of the cantilevers, the mass sensitivity of the batch-fabricated cantilevers is quite uniform. It is deserved to mention another resonant transducer of quartz crystal microbalance (QCM) that has been market available. Theoretically QCM could also be utilized to perform the three-step resonant-gravimetric experiment. By using the temperature-varying experimental method as we described previously,27 the ∆f data obtained from QCM sensing experiments can be used to calculate the value of ∆H°.44 As for QCM, the relationship between frequency-shift ∆f and mass change ∆m follows Sauerbrey equation of ∆

Figure 1. Experimental setup for the resonant-gravimetric analysis method.

Figure 1 shows the schematic setup for the resonantgravimetric experiment. The setup consists of three parts: gas generator, temperature-varying resonantgravimetric detection system and data acquisition sys-

∆

(1)

where n, f0, ρ, µ, and A are harmonic overtone, fundamental resonance frequency, quartz crystal density, elastic modulus of quartz crystal and material coating area, respectively.41-44 According to Eq. (1), the ∆f signal is determined by not only the ∆m (i.e., molecule uptake) but also the material coating area A. In order to accurately obtain the thermodynamic parameters, the A value and the uniformity of the coated material thickness should be precisely processed and controlled. More or less, it is difficult to secure this



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during the material coating process on the QCM surface. As for the cantilever however, the material loaded at the cantilever end can be mathematically treated as a concentrated mass-loading point, and the frequency signal is only proportional to the adsorbed mass ∆m. From this point of view, resonant microcantilever is advantageous in simple operation and accurate acquisition of the parameter.

Figure 2. Characterization results of the synthesized ZIF-8 nanocrystals: (A) XRD pattern, (B and C) TEM images, (D and E) SEM images. (F and G) SEM images of one resonant microcantilever before and after deposition of the ZIF-8 nanocrystals.

The required material sample for loading on the cantilever is generally in nano-gram level, thus the molecule adsorbing process can be completed quite fast. Therefore, the adsorption induced mass adding process can be real-time detected by recording the frequency-shift signal. The whole evaluation experiment is performed in a temperature programmable oven. Based on the resonant-gravimetric detection data obtained at two different temperatures, the key thermodynamic parameters like enthalpy ∆H° can be calculated based on classical physical-chemistry theories.27 By using the setup schematically drawn in Figure 1, both dry gas and simulated ambient gas (i.e. with humidity) can be generated for adsorbing experiment. The method can be used to examine various kinds of adsorbing materials to various gases, including the herein exampled nanoporous MOF of ZIF-8 crystals to CO2 and NO2. Figure 2 shows the characterization results of the herein exampled ZIF-8 material. As shown in Figure 2A, the sharp diffraction peaks of the

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corresponding samples are in accordance with the standard XRD pattern of ZIF-8 material that was previously reported.28 TEM images in Figures 2B-2C clearly show the tiny nanoporous structure of the well-dispersed ZIF-8 nanocrystals. According to the SEM results in Figures 2D-2E, the ZIF-8 nanocrystals are prepared with the candy-like polyhedron profile. Both TEM and SEM characterization results indicate that the diameter of the ZIF-8 nanocrystals is approximately 70nm. To conduct following three-step material-“genome” evaluation, our lab-made resonant cantilever (shown in Figure 2F) is used as the microgravimetric tool. As is shown in Figure 2G, the prepared ZIF-8 nanocrystals can be easily deposited on the end region of the cantilever by using ink-jet printing technology.

Figure 3. Qualitative analysis of ZIF-8 material to adsorb CO2 and NO2, under both dry atmosphere (i.e. 0% RH) and moisture-containing atmosphere (70% RH).

Figure 3 shows the first-step qualitative judgment by implementing resonant-gravimetric experiment. The adsorption function of the herein exampled ZIF-8 material to CO2 and NO2 is preliminarily examined under both absolute dry atmosphere (i.e. 0% RH) and moisture-containing atmosphere. At roomtemperature of 298 K, the test chamber for the ZIF-8 loaded cantilever is firstly introduced to dry atmosphere for sequentially adsorbing CO2 and NO2. Under dry atmosphere, the adsorbing response of the resonant cantilever to 400 ppm CO2 and 0.5 ppm NO2 are sequentially obtained and shown in Figure 3. Thereafter, the chamber is switched to wet atmosphere of 70% RH. It can be seen in Figure 3 that an enormous frequency drop occurs when the moisture-containing air is introduced to replace previous dry air. Although ZIF-8 is considered as hydrophobic material, the very large frequency-drop signal in Figure 3 indicates that there still exists a strong mass adsorption of H2O onto the ZIF-8 material. After the frequency base-line gets stable, the adsorption experiment to the same 400 ppm CO2 and 0.5 ppm NO2 is repeated again. Under the moisture-containing atmosphere, no signal re-


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sponse to CO2 can be observed. In contrast, the response to 0.5 ppm NO2 is still clearly detected. In order to double-check NO2 adsorption under humid atmosphere, NO2 concentration is then increased to 23 ppm. The measured very large response signal indicates a large amount of NO2 molecules captured by ZIF-8. Then, after NO2 is cut off and the atmosphere is switched back to humid air, only a small fraction of the frequency response can recover, which implies many NO2 molecules having been fixed by the adsorbing material of ZIF-8. Only when we raise the temperature from 298K to 303K, the frequency can return to the origin value, indicating desorption of the NO2 molecules from ZIF-8. Based on the first-step assessment results in Figure 3, a qualitative conclusion can be preliminarily made. Although some studies declared that ZIF-8 shows potential usage to capture environmental CO2, it is herein verified to be able to adsorb CO2 only in absolute dry atmosphere, which however is inexistent in reality. In fact the ZIF-8 material cannot adsorb CO2 in real ambient atmosphere where H2O molecules are inevitably existent. In contrast, our experiment indicates that ZIF-8 can adsorb NO2 in both dry air and the moisture-containing real atmosphere.

concentration) can be performed, respectively. The obtained two data can be compared together. Figure 4 compares the results for ZIF-8 to adsorb NO2 of 300ppb, 500ppb and 1ppm, respectively. Under wet atmosphere (70% RH), the mass adsorption induced frequency-shift is obviously higher than that under dry air, and the ratio between them is always around 1.4. The moisture enhanced mass adsorption can be quantitatively attributed to the crystal-water attached on the target gas molecule. Under wet atmosphere, NO2 molecule tends to adsorb H2O to form hydrate of NO2⋅xH2O, where x is the unknown number of the crystal water. The hydrated NO2 can be adsorbed on ZIF-8 to induce larger frequency response compared to the anhydrous NO2. By using the second-step experimental results, the quantity of x can be determined based on the obtained frequency-shift ratio that directly represents the ratio of adsorbed mass. The results in Figure 3 have shown that, when dry air is replaced by wet air atmosphere, a very large frequency-drop signal is generated. The phenomenon indicates that, in wet air, the surface of ZIF-8 should have been already covered by the adsorbed H2O molecules. Considering that one hydrous NO2⋅xH2O will substitute one adsorbed H2O molecule, the net mass addition should substrate the mass of one H2O. According to the results in Figure 4, the mass-ratio of approximate 1.4 is always obtained for the three gas concentrations. By solving the equation of

Figure 4. Resonant-gravimetric frequency-shift signals for ZIF-8 to adsorb NO2 of 300ppb, 500ppb and 1ppm, respectively. The experiment is conducted under both dry air (the signals plotted with thick black curves) and 70% RH wet air (the signals plotted with thin red curves).

The second-step resonant-gravimetric analysis can be further used to quantitatively determine adsorbing properties of the materials under both dry and wet atmospheres. With the obtained quantitative data, the molecular structure of the gas hydrated in humid air can be determined. The second-step resonantgravimetric experiment can be considered acting as an analytic tool like mass-spectrum (MS), but no ionization process is required here.45 Under dry air and wet atmosphere, the experiment on NO2 (with identical

1.4

(2)

x = 2.02 ≈ 2 is obtained. Thus, the molecule formulae of the hydrated NO2 in humid air can be precisely determined as NO2⋅2H2O. As a typical automobile exhaust, NO2 has nowadays attracted much attention due to its high toxicity and strong corrosivity.46-48 More importantly, NO2 can also bring about acid-rain or form PM2.5 particles in real air atmosphere.49 Hence, identification of the hydrated molecule structure of NO2 in ambient atmosphere is helpful for in-depth understanding the molecular interaction mechanism between the adsorbing material and the hydrous NO2 molecule. If without this experimental method, it is difficult for using available analysis instruments to identify the hydrated crystal-water number, due to the thermal instability of such kind of crystal-water. Still with the resonant-gravimetric experiment, the third-step analysis by quantitatively extracting the key thermodynamic parameter of ∆H° is explored to reveal the interfacial interaction mechanism between the adsorbing material and the target gas molecules. The value of ∆H° can directly reflect the interfacial molecule capture ability of the examined material to the target gas. Knowing this material inherent nature



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(i.e., the so-called “material genome”) is very helpful for clarifying the competitive adsorption mechanism of the gas molecules on the ZIF-8 material. The resonant-gravimetric signals obtained for different gas concentrations can be transformed into moleculeadsorption induced mass-addition. With the known molecular weight, the mass-addition can be quantitatively converted into adsorbed molecule number. According to the experimental data, the relationship between molecule uptake and gas partial pressure can be quantitatively obtained and plotted into a curve of thermodynamic sorption isotherm. With the two isotherms obtained in two experiments at two temperatures of T1 and T2, the value of ∆H° can be obtained by solving Clausius-Clapeyron equation27,50 of °

R ! $ ln $! - !

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kJ/mol and 73±2 kJ/mol, respectively. The quantitatively extracted -∆H° values clearly show the order of (-∆H°)NO2 > (-∆H°)H2O > (-∆H°)CO2. According to the definition of enthalpy -∆H°, which is also called adsorption heat, its value directly determines the type and degree of strength of the interfacial interaction. As for the competitive adsorption of ZIF-8 to the three gases, from strong to weak the adsorptioncapability order is NO2 > H2O > CO2.

(3)

with the nano-porous MOF material of ZIF-8 loaded on the resonant-gravimetric cantilever, the experimentally obtained data for adsorbing NO2, CO2 and H2O are shown in Figure 5.

Scheme 1. By comparing the quantitatively obtained thermodynamic ∆H° values for adsorption of ZIF-8 to CO2, NO2 and H2O, all the experimentally observed gas adsorption behavior of the ZIF-8 nanocrystal material can be clearly explained. In dry air, both CO2 and NO2 can be captured by ZIF-8. In real moisture-containing atmosphere, where H2O molecules have already been adsorbed on the surface of ZIF8, NO2 molecules can replace the H2O molecules to realize gas adsorption/fixation, due to the much higher -∆H° value for NO2 than that for H2O. In contrast, since the -∆H° value for CO2 is much smaller than that for H2O, the H2O molecules occupied at the surface of ZIF-8 are no way to be replaced by CO2, thereby refuting the ZIF-8 as adsorbent of environmental CO2. Figure 5. The third-step resonant-gravimetric experiment for quantitatively extracting the thermodynamic-parameter data of enthalpy (∆H°). At two different temperatures, the resonant-gravimetric frequency signals of the ZIF-8 loaded resonant-cantilever are obtained for the three kinds of target gases: NO2 (A and B), CO2 (D and E) and H2O (G and H). The two isotherms for the two temperatures are plotted in (C), (F) and (I), respectively.

The various gas concentrations (i.e. the gas partial pressures) are formed by pure-nitrogen dilution to the original vapour introduced from a standard gas generator. Shown in Figures 5C, 5F or 5I, the horizontal dotted-line has two intersection points with the two isotherms. The two points correspond to identical mass uptake but different gas pressures of p1 and p2. By substituting the data of the two points into Eq.2, the -∆H° values of the ZIF-8 nanocrystals to NO2, CO2 and H2O are calculated as 164±5 kJ/mol, 32±1

So far, the competitive molecule-adsorption mechanism is thoroughly revealed and illustrated in Scheme 1. Based on thermodynamic theories, all the interesting adsorbing phenomena observed in Figure 3 can be reasonably explained. In moisture-containing real atmosphere, CO2 with much lower -∆H° value cannot substitute the already adsorbed H2O molecules at the surface of the ZIF-8 nanocrystal material. Thus, ZIF-8 cannot be used for CO2 fixation in atmospheric environment, since H2O is inevitably existent there. In contrast, even under humid atmosphere, the ZIF-8 nano-material can be used to capture the harmful NO2 due to the much higher -∆H° value of NO2 than that of H2O. By using the resonant-gravimetric experiment method, the ZIF-8 nano-crystal material is, for the first time, sufficiently proved being able to efficiently capturing/fixing/sensing environmental NO2 and rea-


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sonably proposed for the applications of automotiveexhaust treatment. CONCLUSIONS In summary, by sequentially implementing the threestep resonant-gravimetric identification process for the ZIF-8 nanoporous material, three-level corresponding findings have been obtained, which are: (1) Qualitative judgment. The resonant-gravimetric experiment qualitatively indicates that ZIF-8 cannot be used as CO2 adsorbent in moisture-containing real atmospheric air but, for the first time, the material is found being able to high-efficiency capture the typical automobile exhaust of NO2 under the real atmosphere. (2) Quantitative identification. Further resonantgravimetric analysis is performed to accurately determine the crystal-water number in a hydrated NO2 molecule in moisture-containing atmosphere, which helps to reveal NO2 adsorbing mechanism in real environmental air. (3) Thermodynamic-level evaluation. The adsorption ∆H° value is extracted through a varying-temperature resonant-gravimetric experiment. The obtained quantitative thermodynamic parameter is used as a criterion to in-depth understand and elucidate the competitive adsorption mechanism among different gas molecules including the environmentally ubiquitous H2O. After the three-step process, the ZIF8 is thoroughly identified being unable to adsorb CO2 under real humid atmosphere. In contrast, for the first time we find that ZIF-8 is an excellent adsorbent to environmental NO2. Based on the three-step resonantgravimetric analysis method, we can give all the reasons behind the observed phenomena. Each of the proposed three experimental steps can be used either independently or combined with each other. Therefore, the three-step analysing paradigm can be used for thoroughly understanding the adsorption nature on material surface. It is expected that the resonantgravimetric method will be widely used in development of adsorbing materials. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Fax: +86-21-62131744; Tel: +86-21-62131794.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research is supported by MOST of China (2016YFA0200803), NSF of China (91323304, 61527818, 61604163, 61571430, 61321492) and NSF of Shanghai (15ZR1447300). P.C.X appreciates the financial support of the Youth Innovation Promotion Association CAS (2016213).

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for TOC only


9

Resonant-gravimetric Identification of Competitive Adsorption of

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