In the Laboratory
Advanced Chemistry Classroom and Laboratory
edited by
Joseph J. BelBruno Dartmouth College Hanover, NH 03755
Spectroscopic Monitoring of the Heterogeneous Catalytic Decomposition of Gaseous Ammonia
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Jonathan D. Fischer and James E. Whitten* Department of Chemistry, University of Massachusetts-Lowell, Lowell, MA 01854;*
[email protected] The decomposition of ammonia or phosphine on a hot tungsten surface is often cited in physical chemistry textbooks (1–3) and lectures as an example of a reaction that follows zero-order kinetics at relatively high pressures and moderate surface temperatures. The reaction is limited not by the number of molecules but by the available sites on the metal surface. The ammonia reaction, 2NH3(g) → N2(g) + 3H2(g)
(1)
with a hot tungsten filament used as the catalyst, is an example of heterogeneous catalysis. The implementation of this reaction in teaching laboratories, using a reaction vessel constructed by modifying a light bulb and monitoring the rate of the reaction by measuring the change in pressure, has previously been discussed (4). Because of data storage and telecommunication needs, solid-state, electo–optic devices such as lasers, light-emitting diodes (LEDs), and photodiodes are being developed that operate at a variety of wavelengths. As part of our efforts to modernize the physical chemistry laboratory curriculum by using low-cost optical spectroscopy components (5, 6), we report an updated version of this classic experiment. Ammonia has a moderately strong near-IR absorbance band at 2.2 microns (4545 cm᎑1). A relatively inexpensive infrared light emitting diode (LED) and photodiode operating in this spectral region may be used to spectroscopically follow the
Figure 1. FTIR spectrum of gaseous ammonia at approximately atmospheric pressure, with the wavelength used in this study indicated by an arrow.
progress of this reaction. To our knowledge, this is the first reported use of an infrared LED to detect ammonia. The design of this experiment is instructive for undergraduate physical chemistry students; it brings together several aspects of the curriculum, including rotational–vibrational spectroscopy, kinetics, the Beer–Lambert law, heterogeneous catalysis, and the use of modern instrumentation. Furthermore, the entire apparatus may be constructed at minimal expense. Excluding the mechanical pump, mercury manometer, and standard electronics, such as a function generator, this experiment may be assembled for approximately $2,500. Experimental Setup A portion of the IR absorbance spectrum of ammonia using a commercial FTIR spectrometer and a gas cell is shown in Figure 1. The spectral features in the 4300–4600 cm᎑1 range may be assigned to υ1 + υ3 and υ2 + υ3 combination bands (7) that lead to an appreciable extinction coefficient at 2.2 microns. The experimental setup for the spectroscopic ammonia decomposition experiment is depicted in Figure 2. It consists of the 2.2 micron LED (emission FWHM of 0.17 microns) and a photodiode sensitive to this wavelength (active area of 1.0 mm2). Both of these were purchased from Anadigics, Inc (formerly Telcom Devices; as part numbers 2.2LED-TO and 2.2PD1M, respectively). The light from the
Figure 2. Schematic of the experimental setup, including the optics and electronics used for the detection of ammonia. A direct current power supply (not shown), operating in constant current mode, is used to heat the tungsten filament. The dots with circles around them indicate connections via coaxial BNC-type cables.
JChemEd.chem.wisc.edu • Vol. 80 No. 12 December 2003 • Journal of Chemical Education
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Experimental Procedure To measure the decomposition of NH3, the cell is filled with a specific pressure of ammonia and placed in the infrared path, and the detector voltage is recorded. A power supply, operating in constant current mode, is then used to heat the filament. Currents between 0.5 and 3.0 A are used, depending on the desired filament temperature, length of the tungsten wire, and the resistance of the filament-feedthrough connections. Once current is applied, the lock-in output is monitored as a function of time. This is repeated for a variety of different ammonia pressures and filament temperatures. For an experiment showing the heterogeneous catalytic decomposition of ammonia and adherence to zero-order kinetics, it is not necessary to know the filament temperature. A filament current is chosen such that ammonia decomposition is achieved. The same filament may be reused for many different ammonia decomposition runs. To clean the filament between runs, it may be flashed (i.e., heated white hot) momentarily at high current while under vacuum. A pressure dependence to the filament temperature as a result of the convective cooling has been observed: the same current (e.g., 0.65 A) that gives a reasonable rate of ammonia decomposition, without burning out the filament, has been observed to do so when run in vacuum. Hazards The electrolytic cleaning of the tungsten wire should only be performed by an instructor familiar with electricity and its hazards. Even then, it is strongly recommended that a variac transformer plugged into an isolation transformer be 1452
0.20
y = 0.00040581 + 0.00021441x 0.15
−log(T )
LED is focused by a standard 25-mm focal length, 25-mm diameter BK-7 biconvex lens (Edmund Industrial Optics, #32490) into a stainless steel reaction vessel fitted with two glass windows. A second, similar lens collects the light passing through the vessel and focuses it onto the photodiode. The use of phase detection allows the experiment to be run in ambient light and without interference from radiation from the filament. The LED is pulsed by a transistorswitched driver circuit receiving its on–off signal from a function generator outputting a 50% duty cycle square wave at ca. 100 Hz and having an adjustable amplitude. Depending on the desired LED intensity, the amplitude may be adjusted; a 5 V p–p square wave (bipolar) pulse is typically used. The choice of frequency is arbitrary, but 60 Hz and its multiples (i.e., line noise) should be avoided. A low-cost lock-in amplifier (ThorLabs LIA100) receives its reference signal from the TTL output of the function generator and its input from the photodiode amplifier; its output is fed to a voltmeter that displays a dc voltage proportional to the intensity of inphase light impinging on the photodiode. The filament consists of 0.004-in. diameter, electrolytically cleaned, tungsten wire (see Hazards section). The cleaning procedure is optional and unnecessary if only a simple demonstration of the catalytic decomposition of ammonia is desired. The tungsten wire is attached to the electrical feedthrough by means of two medium-sized aluminum alligator clips. A parts list and details related to the construction of the apparatus are included in the Supplemental Material.W
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Ammonia Pressure / mm Hg Figure 3. Plot of the negative logarithm of 2.2 micron transmittance of ammonia versus ammonia pressure measured on two separate occasions. The window-to-window distance is 15.2 cm: Run 1—䊊, Run 2—䊉.
used. Ammonia is a hazardous, corrosive gas. It is recommended that the filling and pumping out of the chamber be carried out in a fume hood and under supervision of the instructor. The mechanical pump should not be exhausted into the laboratory. Results
Pressure Dependance of the Absorbance The dependence of absorbance on pressure may be demonstrated by filling the reaction vessel with various ammonia pressures and recording the output displayed on the digital voltmeter. The results from two different runs, with the negative log of transmittance (i.e., absorbance) plotted versus pressure are shown in Figure 3. Transmittance (T ) is equal to the voltage observed for the ammonia-containing chamber (V ) divided by that for the evacuated chamber (Vo ). Absorbance = ᎑log (T ) = ᎑log (V兾Vo ) = εP l
(2)
where ε is the extinction coefficient, P is the ammonia pressure, and l is the window-to-window distance. A slightly different setup geometry from that illustrated in Figure 2 was used for the data in Figure 3, with l equal to 15.2 cm. The plot includes a linear fit through the low pressure segment of the data, with the observed slope of 2.14 × 10᎑4 (mm Hg)᎑1 corresponding to an extinction coefficient of 1.41 × 10᎑5 (mm Hg)᎑1 cm᎑1. A separate set of experiments with an FTIR spectrometer shows a linear response over the entire pressure range shown in Figure 3. The curvature above ca. 300 mm Hg in Figure 3 is apparently the result of a nonlinear response of the photodiode detector. The detector circuit is operating by monitoring the voltage produced by the photodiode; improved linear response could be achieved by a more sophisticated detector circuit that monitors current (8).
Ammonia Decomposition Typical ammonia decomposition curves are shown in Figure 4 for three different initial pressures. The plot of
Journal of Chemical Education • Vol. 80 No. 12 December 2003 • JChemEd.chem.wisc.edu
In the Laboratory 0.10
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Po = 453 mm Hg, slope = 1.4 × 10ⴚ5 sⴚ1
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Time / s Figure 4. Plot of the negative logarithm of 2.2 micron transmittance versus time for the decomposition of ammonia on a tungsten filament at three different initial ammonia pressures. The estimated temperature of the filament is 900 oC, the window-to-window distance is 17.5 cm, and the chamber volume is 270 cm3. The initial linear behavior and equivalence of the slopes, within experimental error, is consistent with zero-order kinetics with respect to ammonia.
Figure 5. Plot of the negative logarithm of 2.2 micron transmittance versus time for the decomposition of ammonia on a tungsten filament. The estimated temperature of the filament is 1200 ⬚C, the window-to-window distance is 15.2 cm, and the chamber volume is 358 cm3. The lack of nonlinearity indicates deviation from zeroorder kinetics at this temperature.
᎑log (T ) versus time shows the expected decrease, consistent with catalytic decomposition of ammonia to hydrogen and nitrogen. If desirable, a four-way cross (MDC no. 405002), along with a third window, instead of a tee, may be used. An optical pyrometer can then be used to determine the filament temperature during the run or in a separate calibration experiment; this procedure yields an estimated temperature of 900 ⬚C for the data in Figure 4. A reaction that obeys zeroorder kinetics should give a linear plot of ammonia partial pressure versus time; by eq 2, a plot of ᎑log(T ) versus time should also be linear. Furthermore, the rate constant should be independent of ammonia pressure. Only the initial 420 s of data in Figure 4 have been used to calculate the slopes owing to some nonlinearity at longer times. This nonlinearity may be the result of the buildup of contamination on the tungsten surface (such as the formation of tungsten nitride). The rate constants in units of pressure can be calculated from the slopes of the fits in Figure 4 by dividing by the quantity εl: rate constants of 7.3 × 10᎑2, 7.3 × 10᎑2, and 5.7 × 10᎑2 mm Hg s᎑1 were calculated for initial ammonia pressures of 157, 256, and 453 mm Hg, respectively. While the rate constants are not identical, owing to convective cooling and the difficulty of maintaining a constant filament temperature for all three pressures, the difference in the measured rate constants is small relative to the difference in initial pressure. An average of the three rate constants can be used to calculate the molecular rate of ammonia decomposition on a tungsten filament. The volume of the chamber for the data in Figure 4 is ca. 270 cm3 and the surface area of the filament, estimating a glowing length of 0.40 cm, is 0.013 cm2. With this information, and assuming that the gas is at room temperature, the decomposition rate is calculated to be 4.5 × 1019 molecules cm᎑2 s᎑1. This value is in reasonable agreement with experimentally measured and theoretically calculated rates of decomposition of ammonia on tungsten (3, 9).
Discussion The experiment has been repeated for a variety of filament temperatures and ammonia pressures. While lower temperatures, such as 900–1100 ⬚C, yield plots of ᎑log (T ) versus time that deviate only moderately from linear behavior during the first 15 minutes of decomposition, higher temperatures show strong nonlinearity. Data obtained for a filament temperature of ca. 1200 ⬚C is shown in Figure 5. It is only linear for less than 200 s into the reaction. Grosman and Löffler (9) have performed detailed catalytic measurements of ammonia by hot tungsten and report that zero-order kinetics is only obeyed for lower temperatures, consistent with our results. The reaction transitions to higher-order kinetics for temperatures greater than ca. 1100 ⬚C. In fact, the temperature used in Figure 5 is apparently in the complicated regime between zero- and first-order kinetics. Zero-order kinetics with respect to ammonia concentration (i.e., pressure) is observed when the rate is limited not by the number of NH3 molecules but by the number of available surface sites. This should be the case at high ammonia pressures and moderate temperatures. However, at either low ammonia pressures or high temperatures, the rate becomes limited by the ammonia concentration. In the high temperature case, the lower sticking coefficient of ammonia may limit its adsorption and concentration on the surface. Conclusions It is important to note that the tungsten surface used in our experiment is poorly defined. Even though it has been electrolytically cleaned, the mechanical pump produces such a poor vacuum that the surface is immediately covered with reactive contaminants such as water or residual hydrocarbons. Atomically-clean tungsten could only be obtained by heating in oxygen and flashing the filament in ultrahigh vacuum
JChemEd.chem.wisc.edu • Vol. 80 No. 12 December 2003 • Journal of Chemical Education
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conditions. Furthermore, during ammonia decomposition, tungsten nitride may form. Hence, even the hot tungsten surface is undoubtedly contaminated by carbon, oxygen, and nitrogen. While ultrahigh vacuum surface science studies can be used to carry out detailed catalytic experiments on clean surfaces, industrially important processes may actually occur on ill-defined surfaces such as the one used in this paper. This experiment serves as an introduction to heterogeneous catalysis. More detailed runs could be carried out aimed at understanding the transition to higher-order kinetics, and activation energies could be measured by creating Arrhenius plots. The experimental results may also be combined with gas–surface theoretical calculations, as discussed in standard introductory kinetics textbooks such as ref 3.
Chemical Sciences and by the National Science Foundation under grant DMR-0089960. The authors acknowledge the artistic assistance of Susan Whitten in preparing the figures. Literature Cited 1. Levine, I. N. Physical Chemistry, 5th ed.; McGraw Hill: New York, 2002; p 585. 2. Hawes, B. W. V.; Davies, N. H. Calculations in Physical Chemistry; John Wiley & Sons: New York, 1962; p 162. 3. Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper Collins: New York, 1987; p 264. 4. Pryce-Jones, T. J. Chem. Educ. 1972, 49, 848–849. 5. Whitten, J. E. J. Chem. Educ. 2001, 78, 1096–1100.
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Supplemental Material
Instructions for the students, details related to the construction of the apparatus, and a parts list are available in this issue of JCE Online. Acknowledgments Support for this project was provided by the Camille and Henry Dreyfus Foundation Special Grant Program in the
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6. Tran, Y.; Whitten, J. E. J. Chem. Educ. 2001, 78, 1093– 1095. 7. Hertzberg, G. Molecular Spectra and Molecular Structure: II. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1945; p 296. 8. Graeme, J. Photodiode Amplifiers; McGraw-Hill: New York, 1996; pp 21–22. 9. Grosman, M.; Löffler, D. G. J. Catal. 1983, 80, 188–193.
Journal of Chemical Education • Vol. 80 No. 12 December 2003 • JChemEd.chem.wisc.edu
