In Situ Determination of Hydrogen Inside a Catalytic Reactor Using

Jul 1, 2008 - Institute of Isotopes, H-1525 Budapest, Hungary, and Fritz-Haber-Institut, Berlin D-14195, Germany. Roger Zepernick. BESTEC GmbH, Berlin...
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Anal. Chem. 2008, 80, 6066–6071

In Situ Determination of Hydrogen Inside a Catalytic Reactor Using Prompt γ Activation Analysis Zsolt Re´vay,* Tama´s Belgya, La´szlo´ Szentmiklo ´ si, Zolta´n Kis, and Attila Wootsch Institute of Isotopes, H-1525 Budapest, Hungary Detre Teschner, Manfred Swoboda, and Robert Schlo ¨ gl Fritz-Haber-Institut, Berlin D-14195, Germany Ja´nos Borsodi Institute of Isotopes, H-1525 Budapest, Hungary, and Fritz-Haber-Institut, Berlin D-14195, Germany Roger Zepernick BESTEC GmbH, Berlin D-12489, Germany Prompt γ activation analysis (PGAA) has been further developed to analyze reacting components inside a chemical reactor. The new method, in situ PGAA, was used to determine the hydrogen-to-palladium molar ratio under various conditions of palladium-catalyzed alkyne hydrogenation. The H/Pd molar ratio was successfully measured in the range of 0.1-1.0 in an ∼2-g catalytic reactor containing a few milligrams of palladium catalyst. The amount of hydrogen was only a few tens of micrograms, and the detection limit was ∼5 µg, i.e., at ppm level compared to the whole reactor. The description of the device, methodological developments, a feasibility study, and results of a series of catalytic measurements are presented. Prompt γ activation analysis (PGAA) is a rapidly developing chemical analytical method utilizing nuclear techniques. It is based on the radiative neutron capture of nuclei, in other words, the (n,γ) reaction.1 Each isotope of every chemical element (with the exception of 4He) is capable of absorbing a neutron, followed by the immediate release of the binding energy in the form of γ radiation. The probability of the process is characterized by the so-called partial γ-ray production cross section. The lower the energy of the neutrons, the higher is the partial cross section. Thus, the highest reaction rates are achieved in cold neutron beams, i.e., with neutrons having energy of less than 0.025 eV. The emitted prompt γ radiation is characteristic; i.e., the detected energies identify the nuclides, and the intensities are proportional * To whom correspondence should be addressed. Phone: +361-392-2539. Fax: +361-392-2584. E-mail: [email protected]. (1) Re´vay, Zs.; Belgya, T. In Prompt Gamma Activation Analysis with Neutron Beams; Molna´r, G. L., Ed.; Kluwer Academic Publishers: Dordrecht, 2004; pp 1-31.

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to their amounts in the analyzed sample. PGAA is mainly used for the chemical analysis of major, and minor components, and in some special cases for the determination of trace elements, especially when no sample preparation is possible. The method is also capable of determining isotopic composition. The highest-performance PGAA facilities are located at neutron guides of high-power research reactors. The best laboratory systems use high-purity germanium (HPGe) detectors for the collection of the gamma spectra,2 but systems using neutron generators or radioactive neutron sources together with scintillator detectors are also being used,3 especially in industrial or in-field applications. PGAA is nondestructive and is insensitive to the chemical form of the sample. It yields the average composition of the observed volume. Because of the deep penetration of both the neutrons, and the high-energy γ radiation, the sample can be placed in a container, whose wall, in some cases, can be as thick as a few centimeters, as was demonstrated with uranium hidden in lead shielding.4 PGAA has been used for the analysis of a great variety of materials in many scientific fields, from archeology through geology to material science. One of its most important applications is the determination of hydrogen in different samples.5 A wide range of hydrogen-containing materials has been analyzed using this technique, ranging from water solutions (as was shown in (2) Lindstrom, R. M.; Re´vay, Zs. In Prompt Gamma Activation Analysis with Neutron Beams; Molna´r, G. L., Ed.; Kluwer Academic Publishers: Dordrecht, 2004; pp 31-58. (3) Alfassi, Z.; Chung, Ch. Prompt Gamma Neutron Activation Analysis; CRC Press: Boca Raton, FL, 1995. (4) Re´vay, Zs. J. Radioanal. Nucl. Chem. 2008, 276, 825–830. (5) Anderson, D. L., Kasztovszky, Zs. In Prompt Gamma Activation Analysis with Neutron Beams; Molna´r, G. L., Ed.; Kluwer Academic Publishers: Dordrecht, 2004; pp 137-172. 10.1021/ac800882k CCC: $40.75  2008 American Chemical Society Published on Web 07/01/2008

the case of the standardization of the technique6) to solids containing small amounts of hydrogen (e.g., fullerenes7). The determination of reacting components during a catalytic reaction has been an important challenge for analytical chemists for many years. A selection of the previous efforts is listed here. Hydrogen is essential both in keeping the catalyst active and in the catalytic reaction. Therefore, a radiotracer method was developed to study the chemisorption and the absorption of hydrogen on platinum black catalyst. The activity of tritium was counted in the reaction products and the desorbing hydrogen emerging from the catalytic reactor.8 The hydrogen contents of platinum catalysts were directly determined using PGAA.9 The compositions of supported Ag and Pd catalysts were also analyzed using this technique.10,11 Hydrogen desorption from amorphous metal alloys was studied with PGAA, demonstrating its capabilities in the investigation of dynamic processes or chemical reactions.12 However, this is the first time that PGAA has been used for monitoring components inside a chemical reactor in situ. The new method, in situ PGAA, helped in clarifying the mechanism of the hydrogenation of alkynes (tested with 1-pentyne) catalyzed by palladium metal. It was shown that unselective alkyne hydrogenation proceeds on hydrogen-saturated palladium β-hydride, whereas selective hydrogenation requires the exclusion of subsurface hydrogen from participating in the reaction. Further, it was concluded that, during the selective hydrogenation process, the hydrogen content of palladium is not a direct linear function of the actual reaction mixture, and the activity is independent of the amount of hydrogen dissolved in palladium.13 In this paper, the methodological development of the new technique is presented. EXPERIMENTAL SECTION The experiments were performed at the PGAA facility of the Budapest Research Reactor, operated by the Institute of Isotopes. The 10-MW nuclear reactor is equipped with a cold neutron source that contains liquid hydrogen to slow down thermal neutrons. The cold neutrons are then transmitted by supermirror guides to a separate instrumental hall. The PGAA system is located at the end of a cold beam ∼35 m away from the neutron source in a low-radiation environment. The flux at the sample position was ∼7 × 107 cm-2 s-1 at the time of the experiment, and the equivalent temperature of the beam was ∼35 K; i.e., the average energy of the particles was ∼0.003 eV. The γ radiation was detected by a 144-cm3 (27% relative efficiency) HPGe detector, which was surrounded by a Compton suppressor to reduce the spectral baseline and by lead shielding to attenuate the radiation background. The detector system is covered by a 6Li-containing polymer2 to avoid the activation of the shielding and the detectors. (6) Re´vay, Zs.; Molna´r, G. L. Radiochim. Acta 2003, 91, 361–369. (7) Re´vay, Zs.; Belgya, T.; Molna´r, G. L.; Rausch, H.; Braun, T. Chem. Phys. Lett. 2006, 423, 450–453. (8) Paa´l, Z.; Thomson, S. J. J. Catal. 1973, 30, 96–108. (9) Kasztovszky, Zs.; Re´vay, Zs.; Molna´r, G. L.; Wootsch, A.; Paa´l, Z. Catal. Commun. 2002, 3, 553–556. (10) Sa´rka´ny, A.; Re´vay, Zs. Appl. Catal., A: Gen. 2003, 243, 347–355. (11) Sa´rka´ny, A.; Beck, A.; Horva´th, A.; Re´vay, Zs.; Guczi, L. Appl. Catal. A: Gen. 2003, 253, 283–292. (12) Tompa, K.; Ba´nki, P.; Bokor, M.; Lasanda, G.; Vasa´ros, L. J. Alloys Compd. 2003, 350, 52–55. (13) Teschner, D.; Borsodi, J.; Wootsch, A.; Re´vay, Zs.; Ha¨vecker, M.; KnopGericke, A.; Jackson, S. D.; Schlo ¨gl, R. Science 2008, 320, 86–89.

Figure 1. Temperature-controlled aluminum housing for tube reactors. The different-size reactor tubes fit in the axial hole. Four smaller holes around it accommodate heating cartridges. The small oval window is for letting in the neutrons, the large one for letting out the γ radiation.

The cold neutron beam passing through the sample, which is in front of the detector, normally has a cross section of 2 × 2 cm2. The viewing solid angle of the detector is defined by a coaxial aperture in the lead shielding having a diameter of 2.4 cm. Thus, the active volume, from where the prompt γ radiation of the activated sample can be detected, is ∼8 cm3 at maximum. The facility is described in detail elsewhere.14,15 When analyzing hydrogen, it is crucial to correct for the hydrogen background properly. The structural materials (such as plastic-based shielding material, sample holders) and the humidity of the air all contribute to the background to a small extent. In these experiments, the amount of hydrogen-containing material was minimized. The background count rate was found to be constant throughout the measurements. Small-size continuous-flow reactors were placed in the active volume of the PGAA system after removing the sample chamber that is used for routine analyses. In our experiments, the neutron beam was collimated to the width of the reactor tube to reduce the activation of the structural materials in its vicinity. The reactor tubes and other structural components had to consist of elements with low neutron capture cross sections to minimize their activation, since the hydrogen signal was expected to be relatively low. Hence, aluminum, its oxide, silicon carbide, and quartz (SiO2) were chosen. For the feasibility study a 20-cm-high, 10-cm-wide, cylindershaped aluminum house was designed and manufactured containing vertical tubes for heating or cooling. The total mass of the aluminum tank was ∼1.6 kg. It contained an axial hole to accommodate reactor tubes with different inside diameters. Two radial channels were also made in the directions of the beam and the detector to eliminate the attenuation of the radiations. Figure 1 shows the photograph of the tank. The smaller channel is the inlet of the neutrons; the bigger one is the outlet of the γ radiation. (14) Re´vay, Zs.; Belgya, T.; Szentmiklo´si, L.; Kis, Z. J. Radioanal. Nucl. Chem. In press. (15) Re´vay, Zs.; Belgya, T.; Kasztovszky, Zs.; Weil, J. L.; Molna´r, G. L. Nucl. Instrum. Methods 2004, B 213, 385–388.

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Figure 2. Radiography images of the reactor, and its house before and after the fine horizontal alignment. The dark oval shape is the window for the neutron beam. The fuzzy silhouette in the first picture is a result of improper angular alignment. The pale vertical band close to the axis of the window is the reactor tube. (The other horizontal and the vertical stripes originate from neutron reflections on the walls of the neutron guide). Table 1. Masses of the Observed Components and Quantification Limits for Hydrogen in the Feasibility Testa

Pd in N2 Pd in H2

QL for H (mg)

H (mg)

Al (mg)

Pd (mg)

0.08 0.05

0.25 ± 0.02 0.56 ± 0.03

1040 ± 20 1040 ± 20

53 ± 2 37 ± 3a

a The drop in the detected mass of palladium was due to gas bubbles in the catalyst bed, lowering the illuminated mass. Alternating dark and bright layers clearly showed it in radiographic images, similar to the ones in Figure 2. The absorption effect was significant in spite of that.

The first alumina-ceramic reactor tube had a length of 20 cm, a total mass of ∼3 g, and its inner diameter was 3 mm. Catalyst or inert material in the form of powder was loaded into the tube. The reactor was put in the axial hole of the house, and then the whole device was located in the center of the active volume of the PGAA system. For fine positioning, a high-resolution neutron radiography camera was used.16 Figure 2 demonstrates the process of the positioning with radiography images. After positioning, 6Li-containing neutron absorbers were placed at the end of the guide to narrow down the beam so as to minimize the activation of the aluminum house. In the first series of experiments, 440 mg of palladium metal powder was loaded into the reactor tube. Gas streams at slightly over atmospheric pressure were passed through the catalyst layer. High-purity nitrogen gas was used first to determine the hydrogen background originating from the aluminum house, reactor tube, and other materials in the vicinity of the reactor. In the measurement with hydrogen gas, the amount of absorbed hydrogen could then be determined. The results are given in Table 1. In the catalytic hydrogenation experiments, a smaller reactor tube containing a diluted catalyst bed was used without the aluminum house to reduce the spectral background. The smaller amount of material observed by the γ-ray detector enabled the (16) Belgya, T.; Kis, Z.; Szentmiklo´si, L.; Kasztovszky, Zs.; Kudejova, P.; Schulze, R.; Materna, T.; Festa, G.; Caroppi, P. A. J. Radioanal. Nucl. Chem. In press. Ancient Charm Collaboration.

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detection of a lower level of hydrogen. This alumina-ceramic tube had an inner diameter of 2 mm and a total mass of ∼2 g. Seven milligrams of palladium black metal powder homogeneously mixed into 100 mg of silicon carbide was loaded into the tube. The composition of the gas mixture was regulated by mass flow controllers. 1-Pentyne, as alkyne reactant, was introduced into the hydrogen stream via a nitrogen flow bubbling through liquid pentyne in a saturator held at 273 K. The flow rate of the hydrogen was varied from 2 to 64 cm3 min-1, while that of the nitrogen was kept constant at 8 cm3 min-1. According to the vapor pressure of 1-pentyne, nitrogen carried 1.6 cm3 min-1 pentyne flow. The hydrogenation was carried out at near room temperature, and the reactor was operated adiabatically. The reaction was monitored by an online micro gas chromatograph at the exit of the reactor tube. Prompt γ spectra were acquired for 1-2 h, depending on the counting statistics. The counting started after a 15-30 min delay to let the reaction reach equilibrium.13 After the catalytic experiments, calibration PGAA measurements were performed to determine the detection efficiency for the given counting conditions. To accomplish this, sodium chloride powder was loaded in the same alumina tube. The sample was irradiated in the neutron beam, and the prompt γ spectrum was acquired under the same conditions as for the catalytic reactions for 20 min. Safety Considerations. Hydrogen gas and hydrocarbon vapors leaving the reaction cell had to be guided out of the instrumental hall. Data Analysis. Prompt γ spectra were evaluated using the Hypermet-PC code,17 which has proved to be a proper tool in PGAA.18 An efficiency function specific to this setup for the energy range of 500-2500 keV was determined with our standardized method,19 based on the accurate spectroscopic data for chlorine,20 using the efficiency routine of the above software. Spectral interferences were carefully examined for the investigated elements (Al, Si, Pd, H) using our library of spectroscopic data and prompt γ spectra.21 The detectability of H depends on the fit of its γ-ray peak. In our case, it is characterized using a type of quantification limit, which is defined as the mass derived from the fitted peak area that has a relative uncertainty of 50%. This modified QL proved to be close to the detection limit calculated from the statistical fluctuations of the baseline but also characterizes the difficulty of peak fitting in the region. (The use of a Currie-type detection limit is incompatible with the logic of the spectrum analysis software used here, where the limit of detection depends on the digital filter of the peak search algorithm.) The net H signal was calculated as the peak area obtained during measurements of ¨ sto (17) Fazekas, B. ; O ¨r, J. ; Kis, Z. ; Molna´r, G. L. ; Simonits, A. In Proc. 9th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics, Budapest, Hungary, October 8-12, 1996, Molna´r, G., Belgya, T.,. Re´vay, Zs., Eds.; Springer Verlag: Budapest, 1997; pp 774-778. (18) Re´vay, Zs.; Belgya, T.; Molna´r, G. L. J. Radioanal. Nucl. Chem. 2005, 265, 261–265. (19) Molna´r, G. L.; Re´vay, Zs.; Belgya, T. Nucl. Instrum. Methods 2002, A 489, 140–159. (20) Molna´r, G. L.; Re´vay, Zs.; Belgya, T. Nucl. Instrum. Methods 2004, B 213, 32–35. (21) Re´vay, Zs.; Firestone, R. B.; Belgya, T.; Molna´r, G. L. In Prompt Gamma Activation Analysis with Neutron Beams; Molna´r, G. L., Ed.; Kluwer Academic Publishers: Dordrecht, 2004; pp 173-364.

Figure 3. Region from 500 to 2500 keV in the PGAA spectrum of a catalytic reactor. Peaks marked: Pd, Al, and H.

absorption, and catalytic reaction, minus that in the background spectrum, both normalized to unit acquisition time. The peak areas, and their uncertainties were determined using HypermetPC. The uncertainty of the background-subtracted H signal was calculated using the standard rules of error propagation. The corrected signal was regarded as significant if the relative uncertainty was less than 50%, which typically meant a few tens of counts. The corresponding mass was regarded as the minimum detectable amount of hydrogen above the background, which was slightly higher than the above-defined quantification limit. The acquisition times were set to attain the relative statistical uncertainty of less than 5%. The masses of the components can be determined from the fundamental activation equation of PGAA:1 A(E) ) nΦσγ(E) ε(E)t

(1)

where A(E) is the net peak area at the γ-ray energy E, ε(E) is the counting efficiency at this energy, t is the acquisition time, n is the number of atoms (n ) NA m/M, i.e., mass over the atomic weight, times the Avogadro number), Φ is the neutron flux (cm-2 s-1), and σγ(E) is the partial γ-ray production cross section in cm2, i.e., the number of emitted photons with energy E for unit neutron flux. When calculating the molar ratio of two elements in the same sample, the flux, and the acquisition time cancel, thus giving n1 A1 ⁄ ε1 σγ,2 ) n2 A2 ⁄ ε2 σγ,1

(2)

In this relative approach, other physical processes, like neutron self-shielding, and scattering also cancel, since these corrections apply to all components to the same extent. γ self-absorption is also negligible at the energies used here. Partial γ-ray production cross sections for palladium have been accurately redetermined. The comparator was chlorine in a sample of PdCl2. The method of standardization is based on the above equations. In greater detail, it is described elsewhere.22 The new values were used here, instead of the ones published in earlier databases,21 which were determined in the 30-times weaker (22) Re´vay, Zs.; Molna´r, G. L. Radiochim. Acta 2003, 91, 361–369.

thermal neutron beam with a higher spectral background and under less controlled conditions. RESULTS AND DISCUSSION In the feasibility study, the detectability of hydrogen was investigated in the presence of palladium, the chemical reactor, and the aluminum house. PGAA measurements lasting 2 h were made in each case shown in Table 1. Hydrogen has one prompt γ line at 2223.248 keV with the partial cross section of 0.3326 ± 0.0007 barn21 (1 barn ) 10-24 cm2), which is regarded as the primary reference value in the determination of every other cross section in PGAA.22 The peak areas and counting rates for this peak were determined under various conditions. The amount of gaseous hydrogen in the active volume of the first reactor tube was estimated to be a few micrograms. According to the abovementioned criteria, the quantification limit for hydrogen in the feasibility study was found to be 1 order of magnitude higher, ∼0.050 mg. The hydrogen background was measured to be 0.29 counts/s, which is equivalent to 0.25 mg with a relative uncertainty of 8%. Its major source must have been the hydrogen content of the materials in the vicinity of the reactor tube that were activated by the scattered neutrons. The count rate from the hydrogen in the absorption measurement was 0.47 couts/s, which corresponds to 0.56 mg (rel unc 6%). Consequently, the hydrogen signal proved to be significantly higher than the background. The mass of palladium in the detected volume was 40-50 mg, while the effective mass of the aluminum appearing in the spectrum was ∼1 g. This was mainly due to the activation of the wall of the house behind the reactor. The minimum detectable mass of hydrogen above the background in the feasibility study was calculated to be ∼0.07 mg. (The approximate masses were determined based on eq 1 using the data in ref 21.) The objective of the catalytic experiment was to determine the H/Pd molar ratio in the catalyst under different reaction conditions. The chemical aspects of the experiment are discussed elsewhere.13 The masses and the more accurate molar ratios were calculated according to eqs 1 and 2 using γ-ray peak areas from Hypermet-PC. Figure 3 shows the relevant spectrum region. To get accurate molar ratios for palladium and hydrogen, two interference-free peaks were restandardized relative to chlorine. For the peaks at 616.192 ± 0.020 and 717.356 ± 0.022 keV, the new partial cross sections were found to be 0.653 ± 0.007 and 0.844 ± 0.009 barn, respectively. Analytical Chemistry, Vol. 80, No. 15, August 1, 2008

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Table 2. Count Rates, Approximate Masses, and Molar Ratios for H and Pd in the Catalytic Experimenta

SiC in H2b SiC+Pd in H2c SiC+Pd in 2H2+1.6C5d SiC+Pd in 64H2+1.6C5e

count rate of Pd peak at 717 keV

count rate of H peak at 2223 keV

0.49 ± 0.02 0.48 ± 0.02 0.490 ± 0.015

0.103 ± 0.003f 0.166 ± 0.007 0.113 ± 0.004 0.193 ± 0.006

approximate net mass of H (mg) (0) 0.041 ± 0.004 0.007 ± 0.003 0.058 ± 0.002

H/Pd molar ratio 0 0.72 ± 0.08 0.12 ± 0.06 1.03 ± 0.10

a Second row characterizes the adsorption of H in Pd, while the last two rows give two extreme conditions for the catalytic reaction study. b SiC fill in H2 gas stream, used for hydrogen background. c Seven milligrams of Pd mixed with 100 mg of SiC in a H2 gas stream. d Seven milligrams ofPd mixed with 100 mg of SiC in a gas stream of 2 cm3 min-1 H2 + 1.6 cm3 min-1 pentyne in N2. Catalytic performance according to the GC data: ∼9% conversion; selectivity, 98% 1-pentene and 2% pentane. e Seven milligrams of Pd mixed with 100 mg of SiC in a gas stream of 64 cm3 min-1 H2 + 1.6 cm3 min-1 pentyne in N2. Catalytic performance according to the GC data: ∼85% conversion; selectivity, 71% pentane, 20% 1-pentene and 9% 2-pentene isomers. f equivalent mass of hydrogen background 0.067 ± 0.002 mg.

First, a measurement was carried out to determine the hydrogen background. The reactor tube was filled with silicon carbide, and a hydrogen stream was passed through it. In this case, the hydrogen gas filled the pores among the solid particles. The estimated amount of hydrogen gas in the reactor was a few micrograms, as above. The total mass of the hydrogen background measured in this experiment was, however, 1 order of magnitude higher: 0.067 ± 0.002 mg determined from the peak with a count rate of 0.103 ± 0.003 counts/s. Thus, the dominant part of the hydrogen background did not originate from the gas stream in the reactor, but can be related to structural materials and moisture around the tube. This value of 0.067 mg was used for a background correction in the catalytic measurements, because the solid phase in the reactor was similar (more than 90% SiC). The variation in the composition of the nearly atmosphericpressure gas stream was neglected. The approximate masses of the major constituents in the active volume were as follows: 280 mg of Al2O3 (the alumina tube), 100 mg of SiC (the inert filling), and 7 mg of Pd. (Using the average neutron flux of 7 × 107 cm-2 s-1, the mass for palladium was found to be ∼6 mg. The discrepancy was due to the inhomogeneity of the neutron beam; see Figure 2. The actual flux at the reactor cell was not determined, because this deviation did not affect the accuracy of molar ratios.) In agreement with the known sensitivities, the PGAA spectra contained lines from the following elements: H, Al, Si, and Pd. The typical count rates were ∼0.5 counts/s for the analyzed palladium peaks, while that for the hydrogen peak was a maximum of 0.2 couts/s, of which ∼0.1 counts/s was the hydrogen background. Measurements of 1-2 h yielded hydrogen peaks with the areas of more than 1000 counts on a spectral baseline of ∼50 counts, giving statistical peak-area uncertainties of ∼3-5%. The H/Pd molar ratios after the background subtraction could be determined with the uncertainties of 10-50%. This precision seems poor at first sight, but considering the mass of the hydrogen present in the solid phase (∼7-60 µg), it is much better than any other method can provide, and it proved to be enough for the investigation of the chemical processes in the reactor. The quantification limit (i.e., the mass having a relative uncertainty of 50%) for hydrogen in the catalytic experiments was found to be as low as 5 µg. The minimum detectable amount of hydrogen above the background in this experiment proved to be ∼7 µg. The H/Pd molar ratio was calculated for both Pd lines from the ratios of the peak areas using the efficiencies and the cross sections, and they agreed within uncertainty limits. Forty-five measurements were made with 6070

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varying reaction parameters. Subtracting the experimentally determined hydrogen background, the H/Pd molar ratio was determined as a function of the gas composition, etc. Table 2 shows a few typical count rates, mass, and molar ratio values. As can be seen, in the feasibility study, the quantification limit achieved for H was 1 order of magnitude higher than in the catalytic studies. This was due to the increased baseline caused by the activation of the aluminum housing. In the catalytic experiment, a smaller amount of catalyst was used with less total material in the active volume. Thus, a smaller amount of hydrogen was detectable. The H-to-Pd molar ratios varied between 0.1 and 1 with the relative uncertainty of between 50 and 10%, respectively. Though the experiments were successful, and clear conclusions could be made on the reaction mechanism of the hydrogenation,13 a new catalytic cell was designed for further in situ measurements. This improved design can be seen in Figure 4. This device accommodates a similar reactor tube. It is designed to eliminate anything,except the reactor tube from the active volume, with the main functionality of temperature control of the catalyst bed, in the range of -50 to 300 °C. It is rationalized using metal thermal contacts below and above the beam accommodating heating cartridges, and cooling. The part of the reactor tube holding the catalyst bed is in a sealed box that will be evacuated to lower the hydrogen background by removing moisture from the viewing angle of the detector. The capability to decrease reaction temperature will be used to slow down reaction rates, and thus increase catalyst loading, while the lower hydrogen background will allow us to increase signal-to-noise ratio or decrease acquisition time. The combination of lower reaction temperature and lower hydrogen background is expected to open up the possibility to study temporal evolution of H/Pd in the time scale of 10 min instead of hours. Two windows, one to let the neutrons into the cell the other on the perpendicular side of the box to let the γ radiation out toward the detector, are made of quartz glass. The neutron beam will be collimated before reaching the window to the size of the tube, and the transmitted beam will be stopped behind the tube by neutron absorbers. The experiences with the new device will be presented in future papers. In situ PGAA is a feasible technique to follow changes in the elemental composition of catalytic materials during chemical reaction. In a more general view, however, any chemical transformation of matter can be investigated if the composition changes. Restriction applies to elements like oxygen or carbon, their detection limits being too high for such purpose.

the in situ investigation of catalytic reactions. Alumina-ceramic reactor tubes were filled with palladium catalysts, and a gas stream containing hydrogen was passed through them. The conditions of the measurement were chosen so as to maximize the useful counts from the chemical reactor and to suppress the counts from its environment. Using aluminum housing, the quantification limit for hydrogen in the reactor proved to be 0.05 mg, while in the case of a naked reactor tube this value was as low as 0.005 mg. The minimum detectable amount of hydrogen above the background was found to be ∼40% higher in both cases. Thanks to these low limits, an amount of hydrogen in the reaction cell ranging from 7 to 60 µg could be determined with a reasonable accuracy. In a few days’ beam time, break-through results could be obtained in the description of the catalytic hydrogenation of alkynes.

Figure 4. Design of the new temperature-controlled catalytic reactor for in situ PGAA. The lower part is placed in a vacuum house made of aluminum with two quartz windows at the height of the catalyst bed to let the neutrons in and the γ radiation out (not shown). (1) feed gas inlet, (2) feed gas outlet, (3) position of catalyst bed, (4) cartridge heating, (5) cooling, (6) cold gas inlet, and (7) electrical feedthrough of heating.

CONCLUSIONS Two series of experiments were performed at the Budapest PGAA facility to demonstrate the suitability of the technique in

ACKNOWLEDGMENT The authors thank the supporters of the GVOP project (contract GVOP-3.2.1-2004-04-0268/3.0.), the NAP VENEUS 2005 project (contract OMFB 00184/2006), and the EU FP6 NMI3 project (contract RII3-CT-2003-505925). The authors acknowledge the co-operation project between the Fritz-Haber Institute and the Institute of Isotopes funded by the Max-Planck Gesellschaft. The help of Jesse L. Weil is appreciated. The suggestions of Tibor Braun are also acknowledged.

Received for review April 30, 2008. Accepted June 2, 2008. AC800882K

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