Article pubs.acs.org/IECR
Comparative Study on Adsorption of Iodine Vapor by SilicaSupported Cu Nanoparticles and Micronized Copper Mohammad Outokesh,*,† Arezoo Saket,† Seyed Javad Ahmadi,‡ Morteza Hosseinpour,† and Ali Reza Khanchi‡ †
Dept. of Energy Engineering, Sharif University of Technology, Azadi Ave. P.O.Box 113658639, Tehran, Iran Jaber Ebne Hayyan Research Laboratory, NSTRI, Tehran, Iran
‡
S Supporting Information *
ABSTRACT: The current study is aimed at comparison of adsorption behaviors of silica-supported Cu nanoparticles (Si−N− Cu) and micrometric copper powder (Mi-Cu) for uptake of iodine vapor. The Si−N−Cu was synthesized by the decomposition of aqueous Cu(NO3)2 solution at supercritical condition, followed by reduction of the sample by H2−N2 mixture. The Si−N−Cu sample with 29.4 nm Cu particles adsorbed 95% of I2 at partial pressure 10−5 bar in 1 h, while the 1 μm Mi-Cu adsorbed 51% of iodine in 6 h, indicating higher yield and faster kinetics of the nanometric sample. Theoretical analysis revealed the existence of a strong thermodynamic size effect in the Cu−I2 reaction system, so that molar |ΔG| for 2 nm Cu particles was 2.5 times larger than |ΔG| for 1 μm particles. For the Mi-Cu, kinetics obeyed a three-dimensional diffusion model, while in the case of Si−N−Cu, diffusion did not play any role in the kinetics. Apparently, no passivation mechanism was operative in the iodination. curvatures.13−18 This phenomenon is often referred to as the thermodynamic size effect and, so far, has been investigated only for a few solid−gas reactions.19,20 One of the objectives of the current study is to theoretically examine these effects on the reaction system of copper−iodine. As for the fabrication of the Cu nanoparticles, we have used the supercritical hydrothermal (SCH) method, which has proved its effectiveness in rapid synthesis of inorganic nanoparticles.21,22 Another advantage of the SCH method is its ability in simultaneous synthesis and deposition of nanoparticles on a porous support.23,24 Utilization of the supports is particularly important in the fixed bed operations, where the great pressure drop of nanopowders so far has hindered their widespread industrial application. The final objective of the current study is the development of an efficient adsorbent for separation and purification of iodine radioisotopes. This new adsorbent is to combine the high selectivity of copper for uptake of iodine, and the great thermodynamic and kinetics performances of a supported nanomaterial. If this adsorbent could pass all required tests successfully, it can be used in production lines of the iodine medical radioisotopes. However, to arrive at such a decision point, a long experimental, as well as economical, study is needed, of which we are now just in its initial steps.
1. INTRODUCTION Iodine isotopes are among the most significant medical radioisotopes with a wide range of applications in therapy and diagnosis.1−3 The principal usage of these isotopes is in the imaging of thyroid and (in the case of I-131) destroying its malignant nodules and tumors. However, with the progress of the radio-medicine, now some 131I bearing medicines (e.g., metaiodobenzylguanidine) are available that can be sequestered in other organs and kill their dysfunctional tissues by beta emission of the radioiodine.4,5 The 131I isotope is usually synthesized by irradiation of natural tellurium in atomic reactors,6,7 but there is an alternative route in which this isotope is produced along with two other significant medical radioisotopes (i.e. 99Mo, 131Xe) by neutronic irradiation of uranium in the nuclear reactors.8 In such a process, 131I is separated from its undesired companions by selective adsorption on copper wool, followed by desorption and transfer into an alkaline solution.9 In addition to the aforementioned medical applications, the copper also has been investigated as a potential adsorbent of hazardous radioactive iodine in the nuclear reprocessing plants.10 In those facilities, copper can eliminate the iodine isotopes that are released into the gas stream by dissolution of spent nuclear fuel in nitric acid.11 The current study is aimed at the comparison of the uptake behavior of copper nanoparticles with micronized copper for adsorption of nonradioactive iodine vapor. The reason for testing nanoparticles is evident; the small size of these particles brings about an increased amount of surface area,12 which in turn results in an enhanced uptake kinetics. Despite such apparent superiority, the application of copper nanoparticles for removal of iodine has not been reported yet. Besides their appreciable kinetics behavior, nanoparticles are characterized by an elevated level of chemical affinity that arises from their high amount of surface energy and greater © 2012 American Chemical Society
2. EXPERIMENTAL METHODS 2.1. Synthesis. Two forms of copper metal were synthesized and examined in the current study: silica-supported copper nanoparticles and micronized copper. In both cases, the Received: Revised: Accepted: Published: 15315
December 20, 2011 November 5, 2012 November 5, 2012 November 5, 2012 dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
Figure 1. Schematic illustration of the apparatus used for study of adsorption of iodine vapor: (1) Flow meter, (2) gas heater, (3) iodine sublimation chamber, (4) copper sample, (5) oil thermostatic bath, (6 and 7) caustic washing bottles for removal of iodine, (8, 9, and 10) stopcocks. TC: temperature transducer and controller.
Size and morphology of the silica-supported and plain nanosamples were observed by transmission electron microscopy (TEM, LEO 912AB). In the case of Cu micropowder, scanning electron microscopy (SEM, LEO 1455VP) was used for the same purpose. The surface area of the silica-supported samples was determined by the nitrogen adsorption method (Brunauer−Emmett−Teller (BET), Quantachrome Instruments, Nova 2000e, Boynton Beach, FL). 2.3. Iodine Adsorption Experiments. Figure 1 illustrates the flow type apparatus that was used for the study of the iodine adsorption. Iodine crystals were sublimated by the effect of a hot carrier gas (Ar) whose flow rate and temperature were adjusted on 10 L/min and 250 °C, respectively. The iodinebearing hot gas then was divided into two streams. The main stream bypassed the adsorption chamber (B), and exited the system, while the minor stream (about 10 cm3/min) entered the B chamber, and contacted the copper. Because of the high reactivity of iodine, the whole experimental system was made of the borosilicate glass. For the absorption of unreacted iodine, two washing bottles containing 0.1 mol·dm −3 sodium hyposulfite solution were put at the end of the gas circuit. The kinetics of the iodine adsorption was investigated by a semibatch method. In this scheme, first, the steady flow of the iodine-bearing gas was established through the bypass route AA, and then, the gas stream was directed into the adsorption chamber (B). The temperature of the copper in the adsorption chamber was adjusted on the desired level using a silicon oil bath. After elapsing a definite period, the gas flow was stopped and the reaction chamber was removed from the circuit. When this experiment was repeated for different time intervals up to 12 h, it could result in a complete picture of the uptake kinetics. The amounts of iodine and copper adsorbent at the onset of reaction were about 4 and 0.1 g, respectively, but the rate of I2 sublimation hardly exceeded 0.065 g/h, corresponding to I2 partial pressure 10−5 bar. For quantitative determination of the adsorbed iodide (as CuI), the copper samples after adsorption were dissolved in ammonia. According to reaction 1, this led to liberation of iodine as iodide ions. Thereafter, concentrated hydrochloric acid was added to this ammoniacal solution, to neutralize it and lower pH to the acidic range.25 Under this condition, the obtained solution was titrated with standard iodate solution in the presence of chloroform.25,26
fabrication procedure was comprised of (1) preparation of copper(II) oxide from copper(II) nitrate trihydrate (Fur synthesis, Merck AG, Germany) and (2) reduction of the formed CuO by a mixture of nitrogen and hydrogen. Preparation of silica-supported CuO nanoparticles was carried out in a 200 cm3 stainless steel batch reactor, especially designed for enduring working pressure and temperature of 610 atm and 550 °C, respectively. The reactor was filled up to onethird of its volume with 0.1 mol·dm−3 Cu(NO3)2 solution, and then, to this solution was added about 2g of highly porous silica (surface area: 500 m2/g). The reactor was heated at 500 °C for about 1 h. Afterward, it was removed from the furnace and quenched by cold water. The CuO-deposited silica particles were separated from the solution by decantation and underwent a triple washing-decantation procedure using distilled water. Subsequently, they were spread on some Petri dishes and dried at ambient temperature. The CuO micropowder was synthesized by a two-step calcination procedure. In this process, first the crystalline water of copper(II) nitrate trihydrate was removed by gentle heating on a hot plate, and then, the dehydrated Cu(NO3)2 sample was decomposed by high temperature heating at 400 °C for about 22 h. At last, the obtained CuO granules were ground into a fine micronized product. The system used for reduction of the CuO samples was a flow type apparatus in which a dilute stream of the hydrogen (H2 (5%) + N2 (95%)) flowed over the specimens. Complete conversion of CuO was assured by performing the reaction at 600 °C for about 8 h. The extent of loading of copper on the silica support was estimated by dissolving its Cu content in nitric acid, followed by measurement of copper concentration by Atomic Absorption Spectrometry (AAS, Varian 150 AX Turbo). As a basis for estimation of size of the silica-supported CuO nanoparticles, some amount of plain CuO nanoparticles was also synthesized.22 The preparation procedure of this sample was very similar to the silica-supported CuO, but in contrast to it, no silica powder was added to the Cu(NO3)2 solution on the time of its loading into the supercritical reactor. Furthermore, separation of CuO nanoparticles from solution was achieved by centrifuge. The plain CuO nanoparticles were not reduced to the copper. 2.2. Physical Characterization. The crystal structure and composition of the CuO samples before and after the reduction were examined by the X-ray diffractometry (XRD, Philips PW 1800) using the Cu Kα radiation line. 15316
Cu I(s) + 2NH3(aq) → [Cu(NH3)2 ]+ (aq) + I−(aq)
(1)
IO3− + 5I− + 6H+ → 3I 2 + 3H 2O
(2)
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
IO3− + 2I 2 + 10Cl− + 6H+ → 5ICl 2− + 3H 2O −
(3)
The KIO3 solution, at first, oxidized I ions to I2 and transferred it to the chloroform phase, but afterward, the addition of the excess iodate solution (in the presence of strong hydrochloric acid) oxidized the extracted I2 to the colorless ICl2− ion.25−27 This latter reaction (reaction 3) caused the disappearance of the purple color of iodine in the chloroform. The end point was attained when the purple color completely disappeared. The preliminary tests using analytical grade CuI samples demonstrated that the standard deviation of the above titration method was less than 1%. The extent of proceeding of iodine adsorption was determined by “fractional uptake” and “fractional uptake percent” parameters that are defined as α= α, % =
26−28
Figure 2. XRD pattern of a nano-CuO sample that was synthesized by supercritical decomposition of Cu(NO3)2 solution within 1 h.
m0(Cu) − m(Cu) m0(Cu)
m0(Cu) − m(Cu) × 100 m0(Cu)
where m0(Cu) and m(Cu) show the initial mass of copper and its unreacted mass at time t, respectively. The initial mass of copper, as mentioned before, was determined by the dissolution of the entire copper of the sample in nitric acid, followed by its measurement by atomic absorption spectrometry. The reacted mass, m(Cu), was estimated from amount of the adsorbed iodine by the above-mentioned iodate method. The extent of adsorption of iodine on the plain silica matrix (i.e., unloaded with Cu) was also determined by repeating the experimental procedure.
3. RESULTS AND DISCUSSION 3.1. Synthesis of the CuO and Cu Samples. Hydrothermal conversion of copper(II) nitrate into copper(II) oxide at supercritical conditions proceeds through a complicated pathway, in which first copper hydroxyl nitrate (Cu2(OH)3NO3) is formed in a two stage mechanism (eqs 4 and 5), and then, this complex is decomposed into final CuO product.21,22 Cu 2 + + 2OH− → Cu(OH)2
(4)
2Cu(OH)2 + HNO3 → Cu 2(OH)3 NO3 + H 2O
(5)
Cu 2(OH)3 NO3 → HNO3(g) + H 2O(g) + 2CuO
(6)
Figure 3. XRD pattern of the silica-supported Cu nanoparticles and micronized copper powder, both after the reduction by hydrogen, indicate the high purity of the samples.
the pure Cu metal by the action of the reducing gas mixture (Figure 3). The weight percent of the deposited copper on the silica support was estimated (see the experimental section) to be around 7%, which corresponded to 10% loading of CuO on the silica support. 3.2. Morphology. Transmission electron microscopy of the plain and silica-supported CuO and the silica support alone indicated the nanometric sizes of their particles (Figure 4a−c). Although in Figure 4b the silica support cannot be differentiated from the CuO particles, there are three following pieces of evidence that confirm that nanometric particles on the upper part of that image are of CuO. • The first evidence is provided by Figure 4a, in which the plain CuO nanoparticles, which were prepared under the same conditions with silica-supported sample (but in the absence of silica), present an average size of 60 nm. During formation of the CuO nanoparticles, the porous
Reviewing this mechanism suggests that the final CuO product is likely contaminated with the intermediate Cu2(OH)3NO3 complex. In the current study, in which solution containing silica granules were heated to 450 °C, there was no sign of Cu2(OH)3NO3 peak after 1 h heating (Figure 2). However, in the case of synthesis of the plain CuO nanoparticles (i.e., in the absence of silica in the hydrothermal reactor), such an intermediate complex was actually formed and accompanied the CuO product.22 This new finding shows that the existence of silica somehow catalyzes decomposition of Cu2(OH)3NO3 to CuO in less than 1 h. After obtaining the pure supported nano-CuO product, it was reduced by a mixture of nitrogen and hydrogen. Figure 3 represents the results of the XRD test on this specimen after the reduction process, which indicates an appreciable level of copper purity. Likewise to the supported nano-CuO, the micronized CuO sample also underwent a 100% conversion to 15317
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
Figure 4. TEM images of (a) plain CuO nanoparticle, (b) silica-supported CuO nanoparticles prior to reduction, (c) the silica support alone, (d) the silica-supported sample after reduction. (e) Morphological SEM image of the micronized CuO sample.
• Calculation of sizes of the CuO particles from their XRD peak by the well-known Sherrer’s29 formula composes our third evidence. The Sherrer’s equation is stated as
structure of the silica (that exists in the reactor) acts as a scaffold for precipitation of the Cu2(OH)3NO3 nanoparticles and keeps them relatively apart from each other. Decomposition of the Cu2(OH)3NO3 to CuO in the later stages of the process does not change the size of the nanoparticles, and CuO takes a size comparable or smaller than the plain CuO nanoparticles (i.e., ≤ 60 nm). • The TEM image of the pure silica support in Figure 4c shows that the size of its particles is less than 10 nm; thus, particles with sizes around 37 nm in Figure 4b cannot be of the silica. This argument serves as the second evidence.
d=
kλ FWHM × cos θmax
(7)
where k denotes the shape factor, λ is the X-ray wavelength, full width at half maximum (FWHM) shows line broadening at half of the maximum intensity, and θ designates the Bragg angle. In the formula, d is the mean size of the crystalline domains, which is normally considered as the size of the nanoparticles.29 Using the eq 7 and Figure 2, the size of the CuO nanoparticles in the silica-supported sample was estimated to be around 85 nm. 15318
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
Putting the above pieces of evidence in a frame leads to the conclusion that the distinct particles with the average size 37 nm in Figure 4a are CuO ones. During the reduction process, the CuO nanoparticles shrank to the smaller Cu particles (Figure 4b and d). This was expected because the density of Cu (8.9 g/cm3) is greater than CuO (5.6 g/cm3) and particles may in addition experience some kind of breakage and cracking. Since Figure 4d is not sufficiently lucid for accurate estimation of size of the Cu nanoparticles, we instead used the following equation for that purpose: ⎛ ρ MCuO ⎞1/3 dCu ⎟⎟ = ⎜⎜ Cu dCuO ⎝ ρCuO MCu ⎠
(8)
where d, ρ, and M stand for diameter, density, and molecular weight, respectively, and subscripts Cu and CuO refer to the corresponding nanoparticles before and after the reduction. The equation takes into account effects of both (1) shrinking of the size of the particles by loss of a part of their masses in the reduction process and (2) increasing the density of the particles from CuO to Cu. Using eq 8, the average size of the Cu nanoparticles was found to be 29.4 nm, provided that size of the CuO nanoparticles in Figure 4b was 37 nm. Shrinking of the size of the CuO nanoparticle is not the only effect that the reduction process causes. Indeed, from the standpoint of the adsorption activity, the more significant phenomenon is the sintering of the silica support at elevated temperature. To estimate the extent of such sintering, we measured the surface area of the silica-supported samples prior and after reduction by the BET method and recorded the results in Table 1.
Figure 5. Uptake behavior of (a) silica-supported Cu nanoparticles and (b) micronized copper, in adsorption of iodine vapor at partial pressure 10−5 atm.
Table 1. Surface Areas of Different CuO and Cu Samples Measured by Nitrogen Adsorption (BET) Test silica-supported nanoparticles
surface area (m2/g)
In Figure 5, in all cases, the uptake rate slowed down over the long-term, and consequently, the adsorption tails became very long in the neighborhood of the equilibrium. Hence, to make a basis for comparison, a new parameter “t80” was devised that signified the time for 80% approach to the equilibrium. Here, “equilibrium” refers to the maximum employed reaction time (i.e., 12 h); thus, t80 shows the time in which the ratio of α to αequ becomes 0.8. Note that, in most of the cases, t80 was found by interpolation of the obtained data. Table 2 lists the t80 values of different experiments along with their equilibrium uptakes percent (i.e., αequ, % ). According to this table, the silicasupported sample with an appreciably shorter t80 values
micronized powder
before reduction (CuO)
after reduction (Cu)
before reduction (CuO)
after reduction (Cu)
382
336
0.49
0.4
According to Table 1, the impact of the reduction process on the porosity and, hence, the uptake ability of the supported samples were not excessive. Thus, much of the catalytic activity was saved. Morphology of the micrometric Cu samples is given in Figure 4d, and the size of its particles was estimated to be around 1066 nm (radius ≈ 533 nm). 3.3. Iodine Adsorption. Previous studies on the adsorption of iodine demonstrated that copper(II) iodide is thermodynamically unstable and immediately decomposes to CuI, the sole product of the Cu−I2 reaction. In addition, they showed that there is an inflection temperature as 307 °C, above which significant vaporization of CuI substantially diminishes formation of this material.9,30 Figure 5 exhibits the kinetic behavior of the micronized and silica-supported copper samples in adsorption of iodine vapor. In this figure, adsorption of iodine on the pure silica sample (with zero Cu content) was taken as zero, because in preliminary experiments, the solution obtained by treatment of this sample was titrated by the first droplet of the reagent (see eqs 1−3).
Table 2. Comparison of the Uptake Data of the SilicaSupported and Micronized Copper Samples on Adsorption of Iodine Vapor silica-supported nanoparticles temp. (°C) time for 80% completion of reaction (t80, min) αequ, % thickness of iodide layer at time t80 (δt80, nm) 15319
micronized powder
130 58.3
180 51.25
230 50.6
130 229.25
180 216
230 212.7
62 11.25
82 14.89
95 15.72
40 269.39
46 307.4
51 344.7
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
which both lattice and surface energies were available.35−37 Details of such calculations are given in part 2 of the Supporting Information, and the result was γ2,CuI = 0.244 J/ m2. Since the absolute value of γ3 had a negligible effect on the overall response of the system, by employing the same proportion between γ2 and γ3 that was used by Bi et al.,18 we took a value of 0.061 J/m2 for the γ3. Likewise, by considering the arguments of refs 18 and 19, a value of 0.5 nm was taken for d. Panels a and b of Figure 6 were obtained by applying these data and the following relations
possessed much faster kinetics. In addition, the equilibrium uptake of the nanometric (i.e., silica-supported) copper was higher, and it even reached a level of 95% at 230 °C. 3.3.1. Thermodynamics of the Iodine−Copper Reaction. Probably the most significant difference between thermodynamics of the nano and large particles is the great contribution of surface energy and curvature effect in the total free energy change of the systems. According to the analysis given by Bi et al.18 and Kofman et al.,19 the free energy change of reaction of spherical copper particle with initial radius R0 can be expressed as ΔG = N2ΔG0 + ΔGSC
(9)
Panel a:Ordinate =
N ΔG + ΔGSC ΔG = 2 0 N0 N0
(15)
Panel b:Ordinate =
ΔGSC ΔG = ΔG0 + N2 N2
(16)
The parameters ΔG0 and ΔGSC are defined as ΔG0 = (μ2 − μ0 ) −
1 RT ln(PI2) 2
ΔGSC = 4πγ2(R1 + δ)2 + 4πγ3R12 +
(10)
In calculations corresponding to the Figure 6, initial radii of the nano- and micro-Cu particles were taken at 29.4/2 = 14.7 nm and 533 nm, respectively (see section 3.2.). Both panels a and b of Figure (6) indicate existence of a size effect in the reaction system Cu + I2, and they both point out that this effect
2γ2V2N2
R1 + δ ⎛ ⎛ 2γ2 2γ3 ⎞ 2γ V0N0 ⎞ +⎜ + ⎟ ⎟V0N1 − ⎜4πγ0R 02 + 0 R1 ⎠ R0 ⎠ ⎝ R1 + δ ⎝ + Se−δ / d
(11)
where μi, Ni, Ri, Vi, and γi denote chemical potential (J/atom), substance amount (in atoms or molecules), particle radius (m), molecular volume (m3/molecule), and surface energy (J/m), respectively. Subscript i = 0, 1, 2 represents initial copper particle, copper in core, and copper iodide in shell, respectively; γ3 is the interfacial energy between Cu and CuI. Also, PI2 represents partial pressure of iodine (in atm), and R and T are the gas constant and the absolute temperature, respectively. Stochiometric balance is preserved by N0 = N1 + N2, and interaction between the outer surface of CuI and the CuI/Cu interface is given by the term Se−δ/d in which δ stands for thickness of the iodide layer and d is interaction distance. Parameter S is given by18 ⎛ 2V N ⎞ S = ⎜4πR 02 + 0 0 ⎟(γ1 − γ2 − γ3) R0 ⎠ ⎝
(12)
Of the above quantities, the Vi parameters were calculated by dividing molar volumes of the corresponding substances (m3/ mol) to Avogadro’ number; ΔG0 was obtained as a function of temperature by using the available data31,32 in part 1 of the Supporting Information. The core diameter of the reacting particles (i.e., R1) was obtained from eq 13: (R1+δ)3 − R13 =
V2 (R 03 − R13) ⇒ R 2 = R1+δ 2V0
(13)
The γ0 parameter was obtained from an experimental source by extrapolation as 1.857 J/m.33 The surface energy of CuI (i.e., γ2), was calculated by Walton’s method,34 which for two ionic compounds, a and b, from a crystal family is expressed as γa γb
=
Ua xb̅ 2 Ub xa̅ 2
(14)
where U denotes the lattice energy of the crystal and x̅ is the bond length (i.e., sum of cation and anion radii). For estimation of the surface energy of the CuI from the formula, we used the data of cadmium selenide from the same crystal family, for
Figure 6. Effect of size of the nanoparticles on the Gibbs free energy change of the Cu + I2 reaction: (a) in ΔG/N0 coordinates and (b) in ΔG/N2 coordinates. 15320
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
Table 3. Fitting of the Uptake Data of Micronized and Silica-Supported Copper Samples by Different Kinetics Models regression R2 values micronized copper 130 °C
name of the applied model power law (P2) power law (P3) power law (P4) Avarami−Erofe’ev (A2) Avarami−Erofe’ev (A3) Avarami−Erofe’ev (A4) Prout−Tompkins (B1) contracting area (R2) contracting vol. (R3) 1-D diffusion (D1) 2-D diffusion (D2) 3-D diffusion Jandereq (D3) Grinstling−Brounshtein (D4) zero-order (F0/R1) first-order (F1) second-order (F2) third-order (F3)
180 °C
silica-supported Cu nanoparticles 230 °C
Group 1: Nucleation Models α1/2 0.645 0.738 0.759 α1/3 0.605 0.721 0.748 α1/4 0.585 0.712 0.742 [−Ln(1 − α)]1/2 0.941 0.924 0.955 [−Ln(1 − α)]1/3 0.896 0.897 0.936 [−Ln(1 − α)]1/4 0.868 0.881 0.925 Ln[α(1 − α)] 0.930 0.924 0.964 Group 2: Geometrical Contraction Models 1 − (1 − α)1/2 0.901 0.897 0.911 1 − (1 − α)1/3 0.948 0.931 0.946 Group 3: Diffusion Models α2 0.866 0.870 0.852 [(1 − α) Ln(1 − α)] + α 0.942 0.939 0.926 [1 − (1 − α)1/3]2 0.999 0.985 0.989 1 − (2α/3) − (1 − α)2/3 0.974 0.964 0.959 Group 4: Reaction Order Models (Mass Action Law) α 0.744 0.786 0.792 −Ln(1 − α) 0.996 0.975 0.985 (1 − α)−1 −1 0.860 0.907 0.884 [(1 − a)−2 −1]/2 0.809 0.829 0.817
is more pronounced when size of the particles is in the range of a few nanometer. However, in Figure 6a, ordinate corresponds to overall change of the free energy (G(δ) − G(0)) and hence is an indication of the spontaneity of the reaction. In this sense, the reaction of 1 nm particles is the most spontaneous one. On the other hand, by progress of the reaction, the amount of reacted copper (i.e., N2) gradually increases, and except for very small particles (i.e., 1 nm), the second term of eq 16 that takes the surface effects into the account gradually vanishes. The consequence of such behavior is an initial increase in the algebraic value of ΔG/N2 followed by its long-term level off (Figure 6b). This phenomenon is particularly important from a kinetic standpoint because it shows that, for the large particles, besides kinetics mechanisms (e.g., reducing the rate of diffusion by thickening of the CuI layer), the thermodynamic driving force also decreases with the progress of the reaction. 3.3.2. Kinetics of Adsorption. In the current study, which shall be considered as a preliminary work (rather than a rigorous treatment of the kinetics data), at first, the kinetics of iodine adsorption was investigated through applying different phenomenological solid−gas models.38 For this purpose, for every curve of Figure 5, the α values were retrieved; then, all different α-functions of column 2 of Table 3 were plotted versus time for each of the aforementioned curve (e.g., micronized copper, 180 °C). The results that are tabulated in Table 3 show that for the micronized Cu sample the best regression (i.e., the maximum R2 value for curve fitting) is provided by a three-dimensional diffusion formula,16,17 while for the supported nanoparticles, the only applicable model is the zero-order reaction one. To discover the basis of such difference, more mechanistic details are needed, which are outlined in the succeeding paragraphs. Historically, the first theory of metal oxidations that accounted for the gradual slowdown of this process was conceived by Wagner on his study on the reaction system Cu− I2.30 He postulated two important assumptions, as follows:39
130 °C
180 °C
230 °C
0.981 0.961 0.947 0.887 0.936 0.954 0.897
0.952 0.919 0.899 0.937 0.967 0.972 0.945
0.957 0.921 0.897 0.898 0.940 0.953 0.899
0.927 0.873
0.953 0.912
0.941 0.888
0.924 0.848 0.680 0.794
0.941 0.872 0.722 0.825
0.94 0.867 0.697 0.814
0.994 0.726 0.496 0.483
0.991 0.790 0.520 0.485
0.998 0.738 0.497 0.483
(1) The rate controlling step of the oxidation (or halidation) is either the outward diffusion of metal ions or the inward migration of oxidizing atom or positive hole. (2) All other ions, holes, and electrons in the oxide (halide) film adjust their motion with the aforementioned ions to bring about a condition of zero net current. In the case of the Cu−I2 reaction, the diffusion of Cu+ ions controls the kinetics.13 Mathematically, the Wagner or thick film theory is expressed by
Kp dδ = dt 2δ
(17)
where δ denotes thickness of the CuI layer and Kp is a constant that can be theoretically determined.39 Integration yields δ 2 = K pt
(18)
Wagner’s original work and the aforementioned formula were derived for planar geometry. For a description of the reaction of micronized copper there are more pertinent forms of the diffusion equation. In this way, the current study applied four different diffusion models (see Table 1), and the threedimensional model provided the best fitting. Cabrera and Mott, in their successful thin film theory, showed that, at the initial steps of formation of an halide (or oxide),39,40 the free passage of electron from metal surface to the halide−gas interface sets up an electric field E that facilitates the intralayer migration of the metal ions. The width of the electrical field is equal to the thickness of the iodide layer. By the progress of the oxidation process, the E field gradually vanishes and diffusion becomes less favorable. On the other hand, in cases in which the width of the electrical field is comparable to the size (radius) of Cu nanoparticles, until nearly end of the reaction, the accelerating action of E persists, and it can compensate for the deceleration effect of intrafilm resistance. This may explain why intrafilm diffusion does not retard the Cu−I2 reaction in Cu nanoparticles considerably. 15321
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
Figure 7. Schematic illustration of the change of the reaction area of the Cu + I2 reaction during its progress.
more than 2.5 times larger than for a 1 μm (i.e., R0 = 0.5 μm) specimen. When plotted in terms of ΔG/N2, the algebraic value of free energy of all but 2 nm particles greatly increased, and as a result, the thermodynamic driving force of the process reduced. This trend was in a close accordance with the chronological declining of the reaction rate (kinetic effect). Perhaps a deeper analysis of this harmony between thermodynamic and kinetics by the transition state theory (Eyring’s theory) can bring about more insight about the involved molecular mechanisms. The kinetics study indicates that the rate of iodination of large Cu particles (i.e., R0 = 533 nm) follows a threedimensional diffusion model. This finding is in a close accordance with the Wagner’s theory, which demonstrated that the rate controlling step of Cu−I2 reaction was the slow diffusion of the Cu+ ions. In contrast to the micrometric copper, for the 29.4 nm silicasupported sample, apparently diffusion did not play a major role in kinetics. Among the different possible mechanisms, the Cabrera−Mott theory, which considers existence of an electrical field in the initial stage of the reaction, turns out to make the most satisfactory explanation. Besides this effect, by the progress of iodination process, the core of a reacting Cu particle gradually shrinks, and as a result, its momentary reaction surface becomes smaller. Consequence of this phenomenon is nothing but the deceleration of kinetics on the long period.
An important feature of the Cabrera−Mott theory that should be taken into the consideration is the passivation mechanism.39 When aluminum (or some other metal) is exposed to the air, an impermeable oxide layer as thin as 4 nm is formed on its surface, which protects metal from further oxidation.39,40 One may, by looking at Figure 5b, imagine that such a passive layer is also formed over the copper during its iodination. However, if such a protecting halide layer had existed, likewise with the aluminum, its thickness would have been independent of the size of the Cu particle. In practice, at time t80, the thickness of the iodide layer formed on micronized copper was an order of magnitude larger than the same layer formed on the Cu nanoparticles (Table 2), implying that no passivation mechanism was operative. In the Cabrera−Mott theory, for every metal-oxidant couple, there is a critical temperature above which the halide layer grows limitlessly (with no passivation).41 In the Cu−I2 system, in which the CuI layer thickens is up to hundred nanometers, temperature is likely above such a critical point. With the progress of the iodination process, the reaction front moves inward and the momentary surface area of the Cu core decreases (Figure 7). Shrinking the area of reaction might explain the long term decrease of the rate of reaction between Cu nanoparticles and I2 in Figure 5b, where according to Table 3 diffusion resistance is nearly absent.
■
4. CONCLUSION Supercritical hydrothermal synthesis presents a simple and efficient method for fabrication of the supported nanoparticles. The current study demonstrates that existence of silica in the synthesis reactor also leads to formation of the purer CuO nanoparticles in a shorter period. The silica-supported CuO nanoparticles indicates a slight sintering effect during the reduction, and as a result, they preserved major portion of their surface area and activity. In uptake of iodine vapor, the Cu nanoparticles are superior to the micronized copper in terms of a faster kinetics and a higher reaction yield. The results also show that, for both of the samples, the kinetics and reaction yeild were improved by increasing the temperature, but in all cases, the rate of reaction leveled off on the long periods. The thermodynamic analysis revealed the existence of a significant size effect in the reaction of Cu + I2, so that the absolute value of ΔG for a 2 nm (i.e., R0 = 1 nm) sample was
ASSOCIATED CONTENT
S Supporting Information *
Details of the thermodynamic and surface energy calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors would like to expresses their gratitude and appreciation to Dr. Tahereh Mousavand of McGill University, Montreal, and Prof. Seyed Vaghef Hosein of Tarbiat Moallem 15322
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
Industrial & Engineering Chemistry Research
Article
(24) Zhang, Y.; Erkey, C. Preparation of Supported Metallic Nanoparticles Using Supercritical Fluids: A Review. J. Supercrit. Fluids 2006, 38, 252. (25) Ettre, L. S.; Snell, F. D. Encyclopedia of Industrial Chemical Analysis; Vol.14. Wiley−Interscience: New York, 1971. (26) Smith, G. F.; Wilcox, C. New Indicators for Iodate-Iodine Monochloride Andrews Analytical Procedures. Ind. Eng. Chem. Anal. Educ. 1942, 14, 49. (27) Kipling, J. J.; Grimes, G. Note on the Titration of Iodide by the Andrews Method. TALANTA. 1960, 5, 278. (28) Adachi, M.; Eguchi, W.; Tohdo, F.; Yoneda, M. Reactions of Iodine in Aqueous Solutions Containing Sodium Hydroxide. J. Chem. Eng. Jpn. 1974, 7, 360. (29) Skoog, D. A.; West, D. M. Principles of Instrumental Analysis; Holt-Saunders: Tokyo, 1987. (30) Elliott, P.; Shelton, R. A. J. The Kinetics of Growth of Copper Iodide on Copper Metal. Oxid. Met. 1975, 9, 451. (31) Atkins, P. William, P. Physical Chemistry, 7th ed.; Oxford University Press: London, 1978. (32) Green, D. W.; Perry, R. H. Perry’s Chemical Engineers’ Handbook; McGraw-Hill: New York, 2008. (33) Udin, H.; Shaler, A. J.; Wulff., J. The Surface Tension of Solid Copper. Trans. Am. Inst. Min. Metall. Eng. 1949, 185, 186. (34) Walton, A. G. Calculation of Ionic Crystal Surface Energies from Thermodynamic Data. J. Am. Ceram. Soc. 1965, 3, 151. (35) Mayer, J. E.; Levy, R. B. The Lattice Energies of the Cuprous Halides. J. Chem. Phys. 1933, 1, 647. (36) Verma, A. S.; Sharma, D. Lattice Energy of Zinc Blende (AIIIBV and AIIBVI). Phys. Status Solidi, B. 2008, 4, 678. (37) Liu, L.; Zhuang, Z.; Xie, T.; Wang, Y. G.; Li, J.; Peng, Q.; Li, Y. Shape Control of CdSe Nanocrystals with Zinc Blende Structure. J. Am. Chem. Soc. 2009, 131, 16423. (38) Hosseinpour, M.; Outokesh, M.; Ahmadi, S. J.; Mousavand, T. Production of Granulated Copper Oxide Nanoparticles for Catalytic Application. J. Mater. Res. 2010, 25, 2025. (39) Atkinson, A. Transport Processes during the Growth of Oxide Films at Elevated Temperature. Rev. Mod. Phys. 1985, 57, 437. (40) Cabrera, N.; Mott, N. F. Theory of the Oxidation of Metals. Rep. Prog. Phys. 1949, 12, 163. (41) Chernavskii, P. A.; Peskov, N. B.; Mugtasimov, A. V.; Lunin, V. V. Oxidation of Metal Nanoparticles: Experiment and Model. Russ. J. Phys. Chem. B. 2007, 4, 394.
University, Tehran, for their helpful discussions on reactions mechanisms.
■
REFERENCES
(1) Wilson, L. M.; Barrington, S. F.; Morrison, I. D. Therapeutic Implications of Thymic Uptake of Radioiodine in Thyroid Carcinoma. Eur. J. Nucl. Med. 1998, 5, 622. (2) Becciolini, A.; Porciani, S.; Lanini, A. Serum Amylase and Tissue Polypeptide Antigen as Biochemical Indicators of Salivary Gland Injury During Iodine-131 Therapy. Eur. J. Nucl. Med. 1994, 1, 1121. (3) Aktay, R.; Rezai, K.; Seabold, J. E. Four- to Twenty-four Hour Uptake Ratio: An Index of Rapid Iodine-131 Turnover in Hyperthyroidism. J. Nucl. Med. 1996, 37, 815. (4) Kimmig, B.; Brandeis, W. E.; Eisenhut, M. Scintigraphy of Neuroblastoma with I-131 Metaiodobenzylguanidine. J. Nucl. Med. 1984, 25, 773. (5) Khagaghi, F. A.; Shapiro, B. M. Labetalol Reduces I-131 MIBG Uptake by Phaeochromocytoma and Normal Tissues. J. Nucl. Med. 1989, 30, 481. (6) U.S. Environmental Protection Agency. Home Page. http://www. epa.gov (accessed Sept. 2012). (7) Phan, T.; Wanich, R. Practical Nuclear Pharmacy; Banyan Enterprises: Honolulu, HI, 1981. (8) Soenarjo, S.; Gunawan, A. H; Purwadi, B.; Wisnukaton, K.; Sukmana, A. Analysis of Radioiodine Fraction Separated from 99 Mo Production Process of Fission. Atom Indonesia 1999, 25, 101. (9) Morris, J. B.; Stinchcombe, R. A.; Gray, M. D.; Logsdail, D. H. Removal of Iodine Vapor from Air by Metallic Copper. U.K. Atomic Energy Authority A.E.R.E. Report; United Kingdom Atomic Energy Authority: Oxfordshire, 1962. (10) Chamberlain, A. C. Experiments on the Deposition of Iodine 131 Vapor onto Surfaces. Phil. Mag. 1953, 44, 1145. (11) Benedict, M.; Pigford, T. Nuclear Chemical Engineering; McGraw-Hill: New York, 1957. (12) Lee, B. I.; Lu, S. W. Synthesis of Nanoparticles via Surface Modification for Electronic Applications. J. Ceram. Process. Res. 2000, 1, 20. (13) Fromhold, A. T. Growth Rate of Low-Space-Charge Oxides on Spherical Metal Particles. J. Phys. Chem. Solids 1988, 49, 1159. (14) Zhdanov, V. P.; Kasemo, B. Cabrera−Mott Kinetics of Oxidation of Nm-sized Metal Particles. Chem. Phys. Lett. 2008, 452, 285. (15) Ermoline, A.; Dreizin, E. L. Equations for the Cabrera−Mott Kinetics of Oxidation for Spherical Nanoparticles. Chem. Phys. Lett. 2011, 501, 47. (16) Zhou, L.; Rai, A.; Piekiel, N.; Ma, X.; Zachariah, M. R. IonMobility Spectrometry of Nickel Nanoparticle Oxidation Kinetics: Application to Energetic Materials. J. Phys. Chem. C 2008, 112, 16209. (17) Kanervo, J. Kinetic Analysis of Temperature-Programmed Reactions. Ph.D. Dissertation, Helsinki University of Technology, Espoo, Finland, 2003. (18) Bi, H.; Cai, W.; Kan, C.; Zhang, L.; Martin, D.; Trager, F. Optical Study of Redox Process of Ag Nanoparticles at High Temperature. J. Appl. Phys. 2002, 92, 7491. (19) Kofman, R.; Cheyssac, P.; Aouaj, A.; Lereah, Y.; Deutscher, G.; Ben-David, T.; Penisson, J. M.; Bourret, A. Surface Melting Enhanced by Curvature Effects. Surf. Sci. 1994, 303, 231. (20) Murzin, D. Y. Thermodynamic Analysis of Nanoparticle Size Effect on Catalytic Kinetics. Chem. Eng. Sci. 2009, 64, 1046. (21) Adschiri, T.; Hakuta, Y.; Sue, K.; Arai, K. Hydrothermal Synthesis of Metal Oxide Nanoparticles at Supercritical Conditions. J. Nanopart. Res. 2001, 3, 227. (22) Outokesh, M.; Hosseinpour, M.; Ahmadi, S. J.; Mousavand, T.; Sadjadi, S.; Soltanian, W. Hydrothermal Synthesis of CuO Nanoparticles: Study on Effects of Operational Conditions on Yield, Purity, and Size of the Nanoparticles. Ind. Eng. Chem. Res. 2011, 50, 3540. (23) Sawai, O.; Oshima, Y. Deposition of Silver Nanoparticles on Activated Carbon Using Supercritical Water. J. Supercrit. Fluids 2008, 47, 240. 15323
dx.doi.org/10.1021/ie301263j | Ind. Eng. Chem. Res. 2012, 51, 15315−15323
