Microgravimetric Analysis Method for Activation-Energy Extraction

Article pubs.acs.org/ac

Microgravimetric Analysis Method for Activation-Energy Extraction from Trace-Amount Molecule Adsorption Pengcheng 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 S Supporting Information *

ABSTRACT: Activation-energy (Ea) value for trace-amount adsorption of gas molecules on material is rapidly and inexpensively obtained, for the first time, from a microgravimetric analysis experiment. With the material loaded, a resonant microcantilever is used to record in real time the adsorption process at two temperatures. The kinetic parameter Ea is thereby extracted by solving the Arrhenius equation. As an example, two CO2 capture nanomaterials are examined by the Ea extracting method for evaluation/optimization and, thereby, demonstrating the applicability of the microgravimetric analysis method. The achievement helps to solve the absence in rapid quantitative characterization of sorption kinetics and opens a new route to investigate molecule adsorption processes and materials.

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a time-consuming process is also used in GC or TG analysis for obtaining the value of E a .11 Temperature-programmed desorption (TPD) is mainly used in desorption processes to obtain desorption activation energy Ed, from which the Ea value for the reverse adsorption process is very hard to obtain.12 An accurate but more expensive method is magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. With the MAS NMR method, the quantity of adsorbed molecules can be measured by intermittently recording sequential chemical-shift signals until adsorption equilibrium is reached.13,14 In addition, the MAS NMR equipment is expensive and bulky, so an Ea value obtaining process is really time-consuming. Generally, the currently available Ea value obtaining technologies are expensive, bulky, and time-consuming. They are not suitable for rapid or portable analysis. Such technologies normally consume quite a lot of hours up to several days to complete the Ea obtaining task. For rapidly and inexpensively extracting the quantitative value of Ea, the adsorbing speed during the nonequilibrium process needs to be recorded in real time. However, the above-mentioned methods cannot continually record the nonequilibrium process to plot a whole curve for the whole adsorption process. In order to obtain a single datum point, conventional methods normally need to intermittently take serial samples and then implement corresponding analyses. After many such data points are obtained, one curve can be fit and plotted to express one adsorption process. For obtaining a series of such curves to

dsorption kinetics reveals the essential nature of interfacial sorption processes and materials that are used in various application fields like catalytic engineering,1 environmental protection,2−5 and bio/chemical sensing.6−10 As a key kinetic parameter, activation energy (Ea) can be employed to quantitatively evaluate the performance of functional materials for sorption. The value of Ea directly indicates sorption rate; that is, the higher the value of Ea, the slower the sorption rate to the targeted molecules. For CO2 fixation as an example, Ea governs the capturing speed to the greenhouse gas. Therefore, quantitatively obtaining Ea value is of great importance in guidance of optimally exploring new materials for desired adsorption functions such as CO2 adsorbing/fixing. So far, there has been a lack of rapid Ea extraction method, especially for adsorption of low-concentration molecules onto tiny material samples. Nowadays, the available Ea obtaining technologies are mainly based on intermittent experiments in solution. Among them, three kinds of instrumental methods UV−vis spectrophotometry, gas chromatography (GC), and thermogravimetric (TG) analysisare normally associated with adsorption experiments in solution environment. These methods are difficult to use for adsorption at a gas/solid interface. For UV−vis spectrophotometry as an example, the adsorption material to be examined should be first merged in a pigment solution with known initial concentration. Then serial pigment solution samples are sequentially taken up, with a certain time interval between adjacent liquid samples. Due to continual adsorption of pigment molecules by the material, pigment concentrations in the serial solution samples are progressively reduced and can be obtained by absorbence measurement. In this way an adsorption process is indirectly reflected by fitting the measured data points into a curve. Such © XXXX American Chemical Society

Received: February 26, 2016 Accepted: April 13, 2016

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DOI: 10.1021/acs.analchem.6b00757 Anal. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Proposed Roadmap for Quantitatively Extracting Ea Value from Real-Time Recorded Data of a Varying-Temperature Microgravimetric Analysis Experiment

pg in ambient air.18 Some reported resonant nanocantilevers even detect yoctogram (10−24 g) mass that corresponds to a limited number of gas molecules.19−24 By using a phase-locked loop (PLL) interface circuit to maintain the resonance,18 the resonant cantilever can record online the frequency-shift curve that directly represents the molecule adsorption process. With the microgravimetric experiment method, the whole adsorbing/desorbing curve can be continually recorded. For certain sorts of adsorbate molecule with known molecular weight, the molecule number adsorbed on the material can be counted in real time by the frequency signal. According to our experimental results, the minimum detectable molecule concentration can be as low as ppb or ppt level.25−29 Thanks to the tiny material sample needed in the experiment (e.g., at nanogram level), the whole adsorption/ desorption process can be completed very fast, which helps to rapidly obtain the Ea value. As shown in Scheme 1, Ea extraction based on a varyingtemperature microgravimetric analysis experiment is proposed. Aided by a commercial micromanipulator, the material sample to be examined is loaded onto a resonant microcantilever that is fabricated in our lab.18 At one temperature T1, the microgravimetric experiment (i.e., the adsorption mass recording process described) is continually performed for serial concentrations of the targeted gas molecules. Then the testing cell of the experimental setup is changed to another temperature T2, and the identical experimental procedure is repeated again. The experimental setup is schematically shown in Figure S1 in Supporting Information. The varying-temperature experiment is carried out in a temperature-controllable chamber. After two curves (at T1 and T2, respectively) for the time-domain sorption process are obtained, the Langmuir sorption equilibrium constant K and the total active-site number N in the material sample can be calculated according to the known mass sensitivity of the resonant cantilever. Mass sensitivity S = 1.5 Hz/pg of our lab-made cantilevers has been calibrated by using commercial microbeads with known mass.30 Thanks to highly reproducible MEMS fabrication technologies, the fabricated silicon integrated cantilevers in one batch feature highly uniform sensitivity.

represent the adsorbing process to serial molecule concentrations, such a complicated procedure should be repeated many times. On the other hand, conventional expensive methods are not suitable for trace-amount material samples. Normally a large amount of sample is needed to match the LOD (limit of detection) of the equipment. In general, the LOD of conventional methods is microgram to milligram level.11 Extraction of Ea from large amounts of sample is timeconsuming because more adsorbed molecules are needed, further resulting in longer times to reach sorption equilibrium. Conventional methods have difficulty detecting ultralow concentration of adsorbed molecules. However, ultra-lowconcentration adsorption is frequently required for applications such as recognizing concentration change of ambient CO2 or finding hidden explosives (in parts per billion, ppb, or parts per trillion, ppt, concentration) for public security. Hence, it is highly demanded to explore novel methods to rapidly obtain Ea, especially in trace-amount adsorption processes.

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BACKGROUND OF MICROGRAVIMETRIC ANALYSIS METHOD Featuring ultrasensitive gravimetric detection capability, silicon resonant microcantilevers can be fabricated at low cost in batches by use of microelectromechanical systems (MEMS) technologies. Such ultrasensitive devices have been used in various applications such as high-resolution tapping-model atomic force microscopy (AFM) and on-site bio/chemical detection.15−17 As long as the adsorbed mass (Δm) at the cantilever free end is much smaller than the effective mass of the cantilever, the resonant frequency shift of the cantilever (Δf) is accurately proportional to Δm.18 The mass sensitivity of the resonant sensor is defined as S = Δf/Δm. With the elements integrated in the cantilever for both resonance excitation and frequency signal readout, the mass increasing process during molecule adsorption can be recorded in real time and in situ.18 Before detection, the material to be evaluated is loaded at the free end of the cantilever. Thanks to the fine frequency signal noise floor lower than 0.5 Hz, our labmade cantilevers can recognize adsorbed mass smaller than 1 B

DOI: 10.1021/acs.analchem.6b00757 Anal. Chem. XXXX, XXX, XXX−XXX

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equation. Pore size distribution (PSD) was calculated by the Barrett−Joyner−Halenda (BJH) method. Transmission electron microscopy (TEM) was performed on a JEOL-2010F microscope, where 120 and 200 kV accelerating voltages were used. Scanning electron microscopy (SEM) and scanning transmission electron micrographs (STEM) images of the samples were taken on an FEI Magellan 400 XHR ultra-highresolution cold field emission scanning electron microscope. Fourier transform infrared (FT-IR) spectra were obtained under vacuum on a Bruker Vertex 70v spectrometer. Thermogravimetric analysis (TGA) was carried out on a Netzsch STA 449 F3 simultaneous thermal analyzer. To characterize the content of organic groups in the sample, TGA was performed in an oxygen-enriched atmosphere (containing 40% O2 and 60% N2 by volume). TGA experiments were performed in the range 30−800 °C, with a 5 °C/min heating rate. Loading Material Sample onto Resonant Microcantilever. Featuring 1.5 pg/Hz mass sensitivity and picogram-level resolution, our lab-made resonant microcantilevers were employed to extract adsorption activation energy of aminefunctionalized MSNs to CO2 molecules. One drop of the MSN aqueous solution (containing about 10−30 ng of MSN material) was loaded onto the cantilever top surface by use of a commercial micromanipulator (PatchMan NP2, Eppendorf) with the help of inspection under a microscope (DM4000, Leica). Then the microcantilever was dried in an oven at 60 °C for about 2 h.

For a certain gas concentration (i.e., certain partial pressure p), the adsorption rate at the initial adsorbing stage can be represented by the slope of the corresponding frequency-shift curve, that is, df/dt = kapN. At the other aspect, based on the experimentally obtained relationship of p/V versus p, Langmuir sorption equilibrium constant K can be calculated by solving the Langmuir equation (eq 1): p p 1 = + V KV∞ V∞ (1) In eq 1, V∞ is the total volume of adsorbed gas corresponding to complete surface coverage. Then the value of Langmuir sorption surface coverage θ can be worked out from another expression of the Langmuir equation (eq 2): θ=

Kp 1 + Kp

(2)

Now that the adsorbed molecule number n can be deduced from the experimentally obtained mass adsorption and the known molecular weight of the gas, the total active-site number in the material, N = n/θ, can be calculated. With known values of df/dt, N, and p, the adsorption rate constant ka can be calculated according to the relationship df/dt = kaNp. On the basis of results at temperatures T1 and T2, the corresponding values of ka1 and ka2 can be obtained. Finally, the Ea value is extracted by solving the Arrhenius equation (eq 3):

ln

ka2 E (T − T1) = a 2 ka1 RTT 1 2

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(3)

RESULTS AND DISCUSSION The proposed microgravimetric analysis method for Ea value extraction was examined and validated with CO2-adsorbing materials as an example. Fixation/sequestration of the greenhouse gas CO2 has recently attracted intensive attention worldwide. It is highly demanded to develop high-performance CO2-adsorption materials with both large adsorbing capacity and rapid adsorbing rate.32 Various materials such as polymers,33 amine−oxide hybrids,3,34 and metal−organic frameworks (MOFs)2,35−37 have been recently reported for CO2 adsorption. However, there is still a lack of quantitative analytical criteria to evaluate the functional materials. By using the proposed new method, two CO2-adsorbing nanomaterials with similar nanostructure are examined, which are monoamine- and dual-amine-modified mesoporous silica nanoparticles (MSNs). By straight-line fitting of the microgravimetric curves, the adsorption rate df/dt can be obtained. With known values of df/dt, N, and p, the adsorption rate constant ka is quantitatively calculated from the relationship df/ dt = kaNp. By performing varying-temperature microgravimetric experiments, the corresponding values of ka1 and ka2 are obtained for temperatures T1 and T2, respectively. Finally, the Ea values of the two nanomaterials are extracted from the Arrhenius equation (eq 3). From the extracted Ea values, we can gain insight into the inherent nature to distinguish the essential differences between the two materials. By translating apparent adsorbing behavior to the backside kinetic parameter, the method is helpful for material optimization toward desired CO2-adsorbing performance. Actually, each of the experimentally obtained gravimetric sensing curves can be quantitatively transformed into its corresponding isotherm to express the relationship between CO2 uptake and partial pressure. The transformation method has been detailed in our previous reports.8,9,25

Named after the Sweden scientist Professor S. A. Arrhenius, the Arrhenius equation elucidates the relationship between rate constant and temperature of a chemical reaction process.31

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EXPERIMENTAL SECTION Material Synthesis. Synthesis details of monoaminefunctionalized mesoporous silica nanoparticles (MSNs) are summarized as follows: 1 g of CTAB (cetyltrimethylammonium bromide, Alfa Aesar) was dissolved into 480 mL of deionized water under stirring at 80 °C. Then 3.5 mL of NaOH solution (2 mol/L) was added and allowed to react for 5 min. Thereafter 5 mL of TEOS (tetraethyl orthosilicate, Sinopharm) was slowly added into the solution for 10 min, and then 200 μL of APTES [(3-aminopropyl)triethoxysilane; Gelest Inc.] was added. Subsequently, the resultant mixture was allowed to react for 2 h at 80 °C. The product was filtered, washed with deionized water, and dried overnight at 80 °C to obtain monoamine-functionalized MSN precursor. Acid extraction of the CTAB surfactant was performed at 90 °C by placing the asobtained monoamine-functionalized MSN precursor in a mixture of methanol (150 mL) and concentrated hydrochloric acid (9.0 mL) for 24 h. Monoamine-functionalized MSNs were then filtered, washed with water/methanol, and dried under vacuum for 12 h at 80 °C. For synthesis of dual-amine functionalized MSNs, N-(2aminoethyl)-3-aminopropyltriethoxysilane (Gelest, Inc.) was employed to introduce dual amine groups. The synthesis procedure for dual-amine-functionalized MSNs was similar to that for monoamine-functionalized MSNs. Material Characterization. N2 sorption isotherms were measured with a surface area and porosity analyzer (ASAP2020, Micromeritics) at 77 K. Specific surface area of the MSNs was calculated according to the Brunauer−Emmett−Teller (BET) C

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Analytical Chemistry The synthesized two nanomaterials are first characterized by TEM. As shown in Figure S2, the two MSNs show similar oval shapes and ordered mesopores. TEM images also indicate that the well-dispersed nanoparticles have two-dimensional hexagonal nanostructure and the mesochannels are oriented parallel to the long axis of the nanoparticles. Figure S3 shows standard N2 sorption measurement results of the two materials. From the BET method, the calculated specific surface areas of the two materials are similarly around 950 m2/g, and the characterized pore-size distributions of the two MSNs show that average pore sizes of the two nanomaterials are both close to 3 nm. The chemical components of the two materials are further characterized by TGA and FT-IR. TGA curves in Figure S4a indicate that the both MSNs feature dual-step weight loss property. The first step corresponds to weight loss of adsorbed water from the material, and the second step is caused by decomposed organic groups at high temperature. Due to the relatively longer molecule chain of the dual-amine MSNs, its total weight loss of 28.4 wt % is larger than that (22.9 wt %) of the monoamine MSNs. FT-IR spectra in Figure S4b show that, compared with monoamine MSNs, dual-amine MSNs feature an additional peak at 1420 cm−1 that is assigned to the vibration of -CH2-NH-CH2- group in the dual-amine molecule chain. Thus, TGA and FT-IR characterization results verify successful modification of the two sorts of amine groups on the mesoporous silica structures. All the characterization results indicate that the two types of MSNs indeed have similar morphology and chemical structure. The SEM image in Figure 1 shows the integrated resonant cantilever, which was formed by using silicon micromachining

Figure 2. Adsorption activation energy Ea of monoamine-modified MSNs to CO2, extracted from experimental curves at (a) T1 = 298 K and (b) T2 = 318 K. (c) To calculate the Langmuir sorption equilibrium constant K, the signals at T1 and T2 are respectively transformed into the two lines of p/V versus p.

Figure 3. For the CO2-adsorbing dual-amine-modified MSNs, the Ea value is extracted from the experimental data: (a) T1 = 298 K, (b) T2 = 318 K, and (c) transformed p/V versus p.

material was previously put inside (see Figure S1), the CO2 molecules are captured by the functionalized amine groups on the MSNs, thereby inducing mass increase. The real-time process is recorded in situ by the frequency-drop curve. Serial adsorbing/desorbing curves correspond to various concentrations of CO2 and the experimental results at T1 and T2 are shown in Figure 2a,b. When each adsorption process to one concentration is completed, i.e. when the frequency drop becomes to level off, CO2 is switched to pure nitrogen and desorption induced signal recovery is also recorded. At the temperature of T1 = 298 K, the measured frequency response to 800 ppm of CO2 is 17 Hz. Similarly, the frequency signals to higher CO2 concentrations are sequentially obtained. Then the temperature of the testing cell is arbitrarily heated to a higher temperature of T2 = 318 K, similar microgravimetric experiment is performed again. It can be observed from Figure 2 that the signal at T2 is always lower than that at T1 for identical CO2 concentration. According to the route in Scheme 1, the signals in Figure 2a,b are transformed into the two lines in Figure 2c to express the relationship of eq 1. By fitting the lines, the

Figure 1. SEM image of the integrated resonant cantilever. (Inset) Dark-field STEM image showing monoamine MSNs loaded at the cantilever end for microgravimetric analysis experiment.

technologies in the clean room of our lab.18 Integrated in the silicon cantilever are both the electric thermally heating resistor for exciting resonance and the piezoresistive Wheatstone bridge for reading the frequency signal. The inset of Figure 1 shows a dark-field STEM image of monoamine-modified MSNs, where the nanopores can be clearly observed. With similar morphology and chemical structure to each other, the two MSNs are respectively loaded onto microgravimetric cantilevers for E a extraction. Thanks to MEMS batch-fabrication techniques, the mass sensitivity is quite uniform among different cantilevers in one batch. Detailed Ea extracting routes for the CO2-adsorbing materials monoamine and dual-amine MSNs are shown in Figures 2 and 3, respectively. When CO2 of a certain concentration is introduced into the testing cell where the cantilever loaded with D

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Analytical Chemistry equilibrium constants K1 and K2 for T1 and T2 are extracted, respectively. It is worth noting that adsorption rate is associated with gas partial pressure p, that is, with gas concentration. In order to gain a high accuracy of quantitative extraction, the adsorption rate for the highest CO2 concentration in the experiment, 2000 ppm, is used for the calculation. With the corresponding CO2 partial pressure p and the obtained K, the fractional coverage θ can be worked out by substituting the data into the Langmuir equation in eq 2. The adsorbed CO2 molecule number n is obtained from the cantilever mass sensitivity of S = 1.5 Hz/pg, the molecular weight of CO2, and the recorded frequency response to 2000 ppm of CO2. With the obtained values of θ and n, the total adsorption-site number N in the material is worked out. The frequency response of the early-stage adsorption velocity to 2000 ppm of CO2 can be represented by the slope of the tangent line (see dashed lines in Figure 2a,b). Then ka is extracted from the relationship df/dt = kapN. With the obtained ka1 for T1 and ka2 for T2, the Ea value is finally calculated by using the Arrhenius equation (eq 3). According to the experimental results in Figure 2, we obtain Ea = 11.5 ± 0.4 kJ/mol for monoamine-functionalized MSNs. By using the same method and experimental data in Figure 3, Ea = 20.8 ± 0.2 kJ/mol is obtained for dual-amine-functionalized MSNs. The adsorption performance of the two nanomaterials is listed and compared in Table 1. At room-temperature T1, Ea of

Scheme 2. CO2-Adsorbing Mechanisms of (a) Monoamine MSNs and (b) Dual-Amine MSNs38,39

be made: monoamine MSNs are suitable for rapidly dealing with CO2 leakage in closed people-working spaces; but dualamine MSNs can be applied for fixation of high-concentration CO2 in flue gas environments. Guided by the proposed microgravimetric analysis method, optimized adsorbing performance can also be achieved. Due to its high Ea value, the dual-amine nanomaterial has the shortcoming of low adsorption speed at room temperature. However, based on the Arrhenius equation (eq 3), high Ea signifies a strong temperature effect on ka. This viewpoint of chemical kinetics sheds light on optimizing the adsorption performance in terms of both capacity and speed. By increasing the working temperature, the CO2-adsorbing rate of the dualamine material should be enhanced. This guidance is verified by the practical results in Table 1. At a higher temperature of 45 °C, the CO2-adsorbing rate of dual-amine MSNs is increased by about 3.3 times, while its large adsorbing capacity is still retained.

Table 1. Comparison of Adsorption Performancea between the Two MSN Materials monoamine MSNs kinetic parameter

T1 = 298 K

activation energy Ea (kJ/mol) response time for adsorbing CO2 of 2000 ppm (min) active-site number N (mol) adsorption capacity to CO2 of 2000 ppm (pg)

11.5 ± 0.4

a

T2 = 318 K

dual-amine MSNs T1 = 298 K

T2 = 318 K

20.8 ± 0.2

28

17

86

20

1.2 × 10−12

9.3 × 10−13

2.7 × 10−12

2.1 × 10−12

33

23

101

74

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CONCLUSIONS In summary, a rapid method for obtaining adsorption Ea is proposed. The method utilizes a resonant-cantilever sensor to perform varying-temperature microgravimetric analysis experiments to rapidly extract the key kinetic parameter of activation energy. With the two CO2-adsorbing nanomaterials monoamine and dual-amine MSNs as examples, the varyingtemperature microgravimetric experiments are performed to record online the trace-amount molecule adsorbing process. The Ea values are rapidly extracted by inputting sensing data into the Arrhenius equation, with which the adsorbance rates of the materials are assessed, and an adsorption-performance optimizing method of raising temperature is also advised.

Based on obtained Ea.

the monoamine-modified MSNs is relatively lower than that of the other material. The lower Ea indicates higher CO2 adsorption rate of the monoamine MSNs. This conclusion is verified by comparing the response time between the two materials. Though featuring higher adsorbing rate, monoamine MSNs unfortunately have a shortcoming of low CO2-adsorbing capacity. In contrast, the dual-amine MSNs have higher Ea (i.e., lower adsorbing rate) but larger adsorbing capacity. Adsorbing capacity is determined by active-site number N of the material. The calculated N = 2.7 × 10−12 mol (at T1) for dual-amine MSNs is almost 2 times the value of 1.2 × 10−12 mol for monoamine MSNs. The calculated 2-fold relationship in N can be explained on the basis of literature-reported adsorbing mechanisms for the two materials, which are illustrated in paths a and b of Scheme 2.38,39 On average, two monoamine groups capture one CO2 molecule, while one dual-amine group can capture one CO2 molecule. Thus, the results extracted from the proposed method are reasonable and accurate. On the basis of the kinetic evaluation presented, the following conclusion can

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b00757. Four figures showing the setup for varying-temperature microgravimetric analysis and TEM images, nitrogen sorption isotherms, and TGA and FT-IR spectra of MSN samples (PDF) E

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AUTHOR INFORMATION

Corresponding Author

*Telephone (+86) 21-62131794; fax (+86) 21-62131744; email [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by NSF of China (61527818, 91323304, 61571430, 61321492), Science and Technology Commission of Shanghai (14521106100), and NSF of Shanghai (15ZR1447300). P.X. appreciates the financial support of the Youth Innovation Promotion Association CAS (2016213).

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DOI: 10.1021/acs.analchem.6b00757 Anal. Chem. XXXX, XXX, XXX−XXX