ARTICLE pubs.acs.org/ac
Gravimetric Analysis of CO2 Adsorption on Activated Carbon at Various Pressures and Temperatures Using Piezoelectric Microcantilevers Yusung Jin,† Dongkyu Lee,† Sangkyu Lee,‡ Wonkyu Moon,‡ and Sangmin Jeon*,† † ‡
Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-794 South Korea Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 790-794 South Korea
bS Supporting Information ABSTRACT: We investigated the adsorption and desorption of CO2 on activated carbon using piezoelectric microcantilevers. After coating the free end of a cantilever with activated carbon, variations in the resonance frequency of the cantilever were measured as a function of CO2 pressure, which is related to mass changes due to the adsorption or desorption of CO2. The pressure-dependent viscous damping effects were compensated in the calculation of the CO2 adsorption capacity of the activated carbon by comparing the frequency differences between the coated and uncoated cantilevers. The mass sensitivity of the piezoelectric cantilever was found to be better than 1 pg. The fractional coverage of CO2 agreed with a Langmuir adsorption isotherm, indicating that a submonolayer of adsorbed CO2 occurred on the surface of the activated carbon under the experimental conditions. The heat of adsorption was determined using the ClausiusClapeyron relation and the fractional coverage of CO2 at various temperatures and pressures.
C
arbon dioxide is a major greenhouse gas that induces undesired global climate changes.1 International efforts toward reducing CO2 generation have been undertaken, but it is challenging to achieve meaningful CO2 reduction unless our lifestyle, based on fossil fuel-derived energy, is adjusted to rely instead on renewable energy sources, such as solar and wind power. Because changes are realized over time, it is essential to develop CO2 capture technologies to manage atmospheric CO2 concentrations in the short term. Pressure swing adsorption (PSA) is a promising process that could be applied immediately for the removal of CO2 emitted from industrial plants because it is energy efficient and suitable for a large-scale CO2 capture system.2,3 The PSA process is based on the preferential adsorption of a target gas on a porous sorbent at high pressures and the desorption of the gas at low pressures. The adsorption capacity of a sorbent toward CO2 is generally calculated from measurements of the pressure changes during adsorption of CO2 using an equation of state. This approach is convenient, although indirect. In contrast, mass sensors, such as microcantilevers and quartz crystal microbalances (QCM), can directly measure the adsorbed mass of CO2 with better than 1 ng sensitivity. Wu et al. used QCM to investigate the adsorption of CO2 on quartz crystals.4 They found that the resonance frequency of quartz crystals was affected not only by the mass of the adsorbed gas but also by the viscosity and density of the surrounding medium and the surface roughness of the quartz crystals. Quantitative analysis using r 2011 American Chemical Society
QCM results, however, is not straightforward unless the surface roughness of quartz crystals is determined after sorbent coating.4 In contrast, we reported in a previous study that the CO2 adsorption behavior (kinetics and adsorption capacity) was not affected by the surface roughness of the cantilever if the sorbent was coated on the free end of the cantilever.5 Furthermore, the mass sensitivity of the microcantilevers was found to be far superior to that of the QCM sensors. However, the previous study used a conventional optical beam technique to measure changes in the resonance frequency of the cantilever, which was not easily integrated into a high-pressure system. This problem can be solved by using piezoelectric or piezoresistive microcantilevers because they detect changes in the resonance frequency electrically. In spite of this advantage, piezoelectric and piezoresistive microcantilevers have mainly been used so far as simple sensors for the detection of gases and biomolecules under ambient conditions.69 In this study, we fabricated piezoelectric microcantilevers and investigated the in situ adsorptiondesorption behavior of CO2 onto the activated carbon at various temperatures and pressures. The kinetics of CO2 adsorptiondesorption on the sorbent were measured based on the changes in the resonance frequency of the cantilever during adsorption or desorption at various pressures. Received: July 11, 2011 Accepted: August 17, 2011 Published: August 17, 2011 7194
dx.doi.org/10.1021/ac201786n | Anal. Chem. 2011, 83, 7194–7197
Analytical Chemistry
ARTICLE
Figure 1. Electron microscopy image of an activated carbon-coated piezoelectric microcantilever.
The heat of adsorption of CO2 onto the sorbent was additionally determined based on the CO2 adsorption capacities at different temperatures. This study presents, to the best of our knowledge, the first use of piezoelectric microcantilevers for the investigation of the adsorptiondesorption of CO2 onto solid sorbents at high pressures.
’ EXPERIMENTAL PROCEDURES Experimental Setup. Piezoelectric microcantilevers were fabricated as described elsewhere.9 The dimensions of the trapezoidal cantilever were 30 μm in width at the clamped end of the cantilever, 100 μm in length, and 5 μm in thickness. The PZT (lead zirconate titanate) film was deposited on the clamping region of the cantilever, and its thickness was 2.5 μm. The typical resonance frequency and Q factor of the cantilevers were measured to be 1.17 MHz and 400, respectively, at reduced pressure (15 Torr) using a function generator (NI-PXI 5422, National Instrument Co., Texas) and a home-built LABVIEW program. Whereas the theoretical mass sensitivity of the cantilever was calculated to be 2.1 fg/Hz,9 in practice, only 0.1 pg changes in mass could be measured due to the noise of the cantilever (∼100 Hz). Activated carbon was selected as a CO2 sorbent due to its high surface area, thermal and chemical stability, and hydrophobic surface properties.10 A working cantilever with an activated carbon coating and a reference cantilever without the coating were mounted inside a home-built pressure cell (cell volume 11 mL). The reference cantilever was used to avoid common mode noises, such as viscous damping effects. A multiplexer (NI-PXI 2593, National Instrument Co., Texas) was used to measure the resonance frequencies of each cantilever every 5 s. The differential frequency changes between the reference and working cantilevers were used to calculate the mass changes due to the adsorption or desorption of CO2. The temperature of the cell was controlled using a resistance heater with a programmable temperature controller (Hanyoung, Incheon, Korea). Measurements of CO2 Adsorption. Activated carbon (untreated activated carbon, C3345) was purchased from Aldrich (St. Louis, MO) and used as received. The specific surface area of the activated carbon was determined to be 1060 m2/g by the BrunauerEmmettTeller (BET) method. The cantilever was coated with activated carbon by immersing a piezoelectric microcantilever in activated carbon-dispersed ethanol solutions for 1 min. The immersion depth was controlled to coat the activated carbon only on the free end of the cantilever to avoid stress-induced modulus changes due to the coating.11 The
Figure 2. Variations in the resonance peaks of the bare piezoelectric cantilevers as a function of pressure under (a) carbon dioxide, (b) nitrogen, (c) helium (black, 1.5; red, 2; green, 3; blue, 4; cyan, 5; pink, 6; yellow, 7; dark yellow, 8; navy, 9; purple, 10 bar) at 298 K. (d) Variations in the normalized frequency as a function of molecular mass of the gases.
cantilever was then heated at 100 °C for 30 min to evaporate away the ethanol and fix the sorbent onto the cantilever surface.
’ RESULTS AND DISCUSSION Figure 1 shows a scanning electron microscopy (SEM) image of an activated carbon-coated piezoelectric microcantilever, showing that the activated carbon coating is present only on the free end of the cantilever. Assuming that the cantilever is a simple harmonic oscillator, the frequency changes (Δf) of the trapezoidal cantilever due to the coating of activated carbon are related to the mass change (Δm) according to9 Δf ¼ f0 f # rffiffiffiffiffiffiffiffiffiffiffiffi" 1 k 1 ¼ 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2π 0:07m 1 þ Δm=0:07m
ð1Þ
where k is the spring constant and f0 (1.174 28 MHz at 15 Torr) and f (1.150 28 MHz at 15 Torr) are the resonance frequencies of the cantilever before and after coating, respectively. The mass of the activated carbon coated on the cantilever was calculated from eq 1 to be 52 pg. To investigate the effects of viscous damping on the resonance behavior of the cantilever, the types and pressures of gas in the pressure cell were varied. Parts a, b, and c of Figure 2 show variations in the resonance peak of a bare cantilever in the presence of CO2, N2, and He, respectively. The resonance peaks shifted to lower frequencies at increased pressures due to increases in the mass and viscosity. The largest shift was observed for CO2 adsorption, whereas an almost negligible shift was observed for He adsorption, indicating that the normalized frequency shift was proportional to the molecular mass of the gases as shown in Figure 2d. With dependence on the damping mechanism of the resonator, the pressure range is divided into three characteristic regions:12 an intrinsic region, a molecular region, and a viscous region. Because the pressure range in this study was situated 7195
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Analytical Chemistry
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Figure 3. Variations in the Q factor of the cantilever with pressure (blue circle, CO2; red square, N2; black triangle, He).
Figure 4. Variations in the frequency and mass of a bare cantilever (black) and an activated carbon-coated cantilever (red) as a function of pressure at 298 K.
within the viscous region, the gases acted as a viscous fluid and hydrodynamic inertial forces affected the resonance frequency of the cantilever. Assuming that a microcantilever is a string of spheres, its relative frequency shift in the viscous region due to the inertial force of the gas on the cantilever is given by13,14 ! pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi πr 3 9 μRT=πf pffiffiffiffiffiffiffi MP þ Δf =f ¼ MP ð2Þ 2 r 3mRT where R, m, T, P, μ, M, and r are the gas constant, the mass of the cantilever, the absolute temperature, pressure, dynamic viscosity, molar mass of the gas, and the radius of one of the oscillating spheres, respectively. Equation 2 indicates that the frequency change increased with the pressure due to viscous damping, as shown in Figure 2ac. Compared with the resonance frequency, the quality factor of the cantilever (Q factor) was more directly related to the pressure (or viscous damping). The Q factor was defined as the ratio of the energy stored to the energy dissipated and could be calculated according to Q factor ¼
f fwhm
ð3Þ
where fwhm is the full width at half-maximum. A lower Q factor indicates a higher rate of energy dissipation (i.e., larger viscous damping) relative to the oscillation frequency. Alternatively, the Q factor can be obtained as a function of pressure in the viscous regime using13 Q ¼
2
length of the cantilever, respectively. A is a parameter related to molecular weight, viscosity, and temperature [A = (πfr2M/μRT)1/2]. conditions (∼100), the Q Because AP1/2 > 1 under our experimental √ factor was proportional to 1/ P. Figure 3 shows that the inverse of √ the Q factor increased linearly with P. Because the viscous damping depended on the density and viscosity of the surrounding gases, larger changes in the Q factor were observed in the order CO2, N2, and He. The viscous damping effects on mass changes were compensated using measurements conducted on a reference cantilever. Figure 4 shows the frequency changes of the reference cantilever and activated carbon-coated cantilever as a function of CO2 pressure at 298 K. The measurements were conducted five times, and almost identical results were obtained, indicating that the experimental results were highly reproducible. The resonance frequencies of both cantilevers decreased with pressure due to an increase in the adsorbed mass and viscosity. Larger changes in frequency were observed for the coated cantilever due to the adsorption of CO2 onto the activated carbon. The mass of CO2 adsorbed onto the activated carbon could be calculated from the differential resonance frequencies of the reference and activated carbon-coated cantilevers using eq 1. Because the surface area of the coated activated carbon (5.5 × 108 m2/52 pg) was 2 orders of magnitude greater than that of the bare cantilever (3 × 1010 m2), the mass of CO2 adsorbed onto the cantilever surface could be neglected in calculating the CO2 adsorption capacity of the activated carbon. Variations in the pressure-dependent fractional coverage of CO2 (θ) were measured at various temperatures and are plotted in Figure 5. The fractional coverage could be calculated by dividing the number of adsorbed CO2 molecules (or the mass of adsorbed CO2 molecules) by the maximum value that could be adsorbed on the sorbent. Assuming that each CO2 molecule occupied 0.1 nm2 of the sorbent surface, the number of CO2 molecules adsorbed onto 52 pg of sorbent corresponded to 4.2 × 1011. In contrast, the mass change due to the adsorption of CO2, even at 10 bar, was only 7 pg, which corresponded to 9.5 × 1010 CO2 molecules. This result indicated that the surface coverage of CO2 molecules on the sorbent was less than a monolayer of CO2, even at the highest pressures tested in our experiment. The fractional coverage was fit according to the Langmuir adsorption isotherm, θ¼
1=2
kn bd ðFs E=12Þ 6πμrlð1 þ AP1=2 Þ 2
Figure 5. CO2 adsorption isotherms on activated carbon at different temperatures: (a) 298 K, (b) 307 K, (c) 323 K, and (d) 358 K. The experiments were conducted three times, and the data were fit to the Langmuir adsorption isotherm.
ð4Þ
where kn, Fs, E, b, d, and l are a constant for the nth order mode of resonance, the density, Young’s modulus, width, thickness, and
KP 1 þ KP
ð5Þ
where P and K are the pressure and Langmuir constant, respectively. The good agreement confirmed the submonolayer adsorption of CO2 onto the activated carbon. 7196
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Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant 20110011246).
’ REFERENCES
Figure 6. A plot of the logarithm of pressure as a function of the inverse of the absolute temperature at different fractional coverages (9, 0.02; 0, 0.04; 2, 0.06; 4, 0.08; and b, 0.1). The data are the same as that presented in Figure 5.
The heat of adsorption of CO2 onto the activated carbon (Q) was determined using the ClausiusClapeyron equation, dðln PCO2 Þ Q ¼ dð1=TÞ R
ð6Þ
Figure 6 shows the changes in pressure as a function of temperature at various fractional coverages. The equilibrium pressure and temperature for the CO2 adsorption were obtained from Figure 5. The data were fit to straight lines, and the slopes (1.66 K) were found to be independent of the fractional coverage. The negative slope indicated that the CO2 adsorption process was exothermic. The heat of adsorption was calculated from eq 6 to be 14 ( 0.4 kJ/mol, which was similar to the value in the carbon-based sorbent reference.15
’ CONCLUSIONS In summary, we used piezoelectric microcantilevers to investigate the in situ adsorptiondesorption of CO2 onto activated carbon at various temperatures and pressures with sub picogram sensitivity. In addition, the heat of adsorption of CO2 onto the activated carbon was determined, which was found to be similar to the reported values. Although not implemented in this study, high-throughput analysis with nanogram samples may be possibly achieved using arrayed structures of piezoelectric cantilevers. This unique feature of piezoelectric cantilevers may be exploited to provide promising analytical tools for investigating the gas adsorption properties of a variety of nanomaterials.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Variations in the resonance frequencies of the bare and activated carbon-coated cantilevers over time as a function of pressure at 298 K and variations in the resonance frequencies of the activated carbon-coated piezoelectric cantilever over time at 298 K. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: (82) 54 279 5578. Phone: (82) 54 279 2392.
’ ACKNOWLEDGMENT This research was supported by the Future-based Technology Development Program (Nano Fields) through the National 7197
dx.doi.org/10.1021/ac201786n |Anal. Chem. 2011, 83, 7194–7197
