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Anal. Chem. 1991, 63,508-517
Mechanisms of Selenium Vaporization with Palladium Modifiers Using Electrothermal Atomization and Mass Spectrometric Detection David L. Styris,* Laurie J. Prell,' and David A. Redfield2 Pacific Northwest Laboratory, P.O. Box 999, Richland, Washington 99352 James A. Holcombe, Dean A. Bass,3and Vahid Majidi4 Department of Chemistry, The University of Texas, Austin, Texas 78712
Mass spectrometric sampling of gaseous species that evolve from graphite fiats and tubes that are pulse heated in vacuo and from graphite tubes that are pulse heated at 1 atm of pressure was employed in order to elucidate mechanisms controlling selenium electrothermal atomization. Investlgations were performed (I) without modifier, (ii) with thermally reduced palladium modifier, and (ill) with nonreduced palladium modifier. Selenium oxides, carbides, hydroxides, dimer, free atom, and palladium were observed as a function of time and temperature during the atomization cycle. The reduced palladium modifier eliminated formation of the molecular selenium species except selenium monoxide. Both SeO and SeO, were observed when the nonreduced palladium modifier was involved. I n both cases, palladium forms a stoichiometric compound with selenium that retains the selenium until higher temperatures are reached.
INTRODUCTION The determination of selenium at the ultratrace level has traditionally been quite difficult. Hydride generation and electrothermal atomization atomic absorption spectrometry (ETA-AAS) are the methods that are often employed because of their sensitivity and relative speed of analysis. ETA-AAS is rarely attempted without the use of chemical modifiers to reduce selenium loss during thermal pretreatment stages and to maximize the free atom formation during the atomization cycle. Although nickel has been the traditional modifer for selenium, the use of palladium is increasing. Early reports on the potential utility of palladium emerged from Shan and Ni (1-3). Voth-Beach and Shrader ( 4 ) found that when palladium was used charring temperatures could be elevated 700 K above those when nickel was used. Their approach recommended the use of a "reduced" palladium modifier in which the palladium was either reduced chemically (e.g., H2, ascorbic acid, or hydroxylamine hydrochloride) or thermally (treating the modifier a t ca. 1270 K ( 4 , 5 )after injection into the furance). Welz et al. (6) also found improved performance with palladium but employed magnesium nitrate as an additional chemical additive and did not attempt to reduce palladium. The chemical and physical role of palladium in retaining the selenium during the char step while enhancing free atom formation during the atomization cycle have not been studied
* Author to whom correspondence should be addressed.
'Current address: Lockheed, 1050 E. Flamingo Rd, Las Vegas, NV 89119. *Current address: Department of Chemistry, Northwest Nazarene College, Nampa, ID 83636. Current address: Argonne National Laboratory, Argonne, IL 60439. *Current address: Department of Chemistry, University of Kentucky, Lexington, K Y 40506.
in depth. Teague-Nishimura e t al. (7) obtained scanning electron micrographs with fluorescence detection to conclude that the selenium and palladium are nearly coincident in their spatial position on the surface of the graphite and suggested the existence of a palladium selenide alloy as the means by which selenium is stabilized. Others have reported on the pragmatic utility of using various quantities of palladium and its effect on the appearance temperature and peak areas of the selenium absorption signal (8-10). However, none of these reports have presented supporting evidence for potential chemical processes responsible for the stabilization. Recent investigations of the mechanisms of palladium-induced stabilization of arsenic indicate that stabilization involves formation of arsenic-palladium compounds or mixed solutions (11). Styris (12) and Droessler and Holcombe (13) have independently studied the selenium atomization mechanism using mass spectrometry (MS) simultaneously with AAS but focused on simple selenium systems and selenium systems employing the nickel modifier. They have shown low-temperature losses of selenium oxides, carbides, and dimers in the absence of nickel and an attendant decrease in these species with the presence of nickel, although they disagreed on the exact mechanism responsible for the improvements. This paper combines efforts of these two laboratories (Styris and Holcombe) to focus on the selenium system with and without the presence of palladium, These efforts strive to develop an understanding of palladium-induced stabilization processes based on arguments that are consistent with data from three differently configured experimental apparatus. This study relies on MS data from these three apparatus and on the imaginations of six amateur scientists working late in smoke-filled rooms to arrive a t a semblance of truth!
EXPERIMENTAL SECTION Apparatus. Three different AAS-MS systems were used in this study: flat-in vacuum, tube-in vacuum, and tube-in atmosphere. All employed resistive heating of a pyrolytically coated graphite substrate and detection of the gas-phase species, in real time, using quadrupole mass analyzers with electron impact ionization. A minimum of two data sets were collected for any given set of conditions; additional data sets were collected in most cases. Since a majority of the data are to be used in a qualitative manner to deduce the chemical processes occurring in the system, the peak profiles (Le., appearance temperature, peak location, etc.) are of greater importance than absolute signal magnitude variations. Such variations between different species were observed, but extreme caution is necessary in attributing quantitative significance to such observations. This is because mass spectrometers exhibit intrinsic mass discrimination effects. For the present experiments,these effects are difficult to evaluate because of the transient nature of the evolving species; continuous sources of selenium molecular species are not available for such evaluations. Signal magnitudes are therefore used in this investigation only when significant differences (e.g., orders of magnitude) are noted. Isotope ratios were used to verify whether other isobaric
0003-2700/91/0363-0508$02.50/00 1991 American Chemical Society
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species evolved from the surface during heating of the atomizer. Flat-In Vacuum. The sample in this system is loaded onto a pyrolytically coated graphite flat substrate (10 mm X 15 mm X 3 mm thick) (Ringsdorff Werke; Bonn, FRG), which is made from high-density graphite that has been pyrolytically coated by using the same procedure employed in the manufacturing of commercial furnaces. Details of this system have been described previously (14).In brief, it consists of a two-chamber high-vacuum system, a quadrupole mass analyzer (UTIInstruments, Milpitas, CA, Model lOOC), a UTI programmable peak selector, and and Apple 11+ microcomputer. The quadrupole employed an electron impact ionizer operated a t 70 eV and an electron multiplier detector. The analyzing chamber contains the quadrupole mass analyzer and is maintained in the Pa range prior to atomization. The other chamber, which is isolated from the analyzing chamber by a gate valve, can be brought to atmospheric pressure for sample introduction and then evacuated during analysis. Samples are typically dried a t ca. 350 K and, for selected experiments, thermally pretreated at 1 atm of N2 in this chamber before being transferred to the analyzing chamber for subsequent pulse heating (i.e., “atomization”) beneath the quadrupole mass analyzer. Timing, heating, and data collection are computer controlled. A linear temperature heating ramp is provided by a power supply designed at the University of Texas laboratory (12). While heating rates up to 800 K/s were available for the “atomization cycle”, 500 K / s was typically used. Temperatures are indicated by a photodiode output that is calibrated against a disappearing filament pyrometer focused into a blackbody cavity drilled in the side of the flat. Temperature accuracies of 150 K are typically expected. Temperatures below the output of the photodiode were calculated from heat-transfer parameters of the atomizer system (15). No attempt was made to monitor the Se atomic absorption (AA) signal in this system due to the low intensity of the Se hollow cathode lamp and the relatively weak absorbance signal observed when such a measurement was attempted. Both single ion monitoring and mass hopping were employed with this system. Data were collected a t a minimum rate of 20 Hz per given mass. Typical collection rates using the PPS involved 5-ms delay between masses and a 5-channel scan about the peak with 1-ms measurement period per scan. Each mass signal contained a minimum of 20 data points, depending on the peak shape, to define the signal profile. Single ion monitoring was used when better time resolution was desired. Tube-In Vacuum. This system, described in detail in ref 16, employs vacuum atomization from within a commercially available pyrolytically coated graphite tube (Thermo Jarrell Ash, Franklin, MA). Mass sampling of the gaseous species in the tube is conducted with the quadrupole analyzer located directly over the dosing hole. The tube was pulse heated by an Instrumentation Laboratories (Thermo Jarrell Ash) Model 455 furnace power supply using 80% power for OS and 5s. Briefly, the system consists of gate-valve-isolated ultrahigh-vacuum chambers, an electron impact ionizer and quadrupole mass analyzer (Extrel, Pittsburgh, PA) and a triple-beam Tektronix 5110 (Beaverton, OR) oscilloscope with a Tektronix C51 camera for recording outputs of the mass spectrometer, a Instrumentation Laboratories Model 353 atomic absorption spectrometer, and Ircon 1100 series automatic pyrometer (Nile, IL). The pyrometer was calibrated to 2600 K against a 0.08-mm-diameter W-5%Re:W-28%Re thermocouple (Omega-Engineering, Stamford, CT) that monitored inner-tube surface temperatures. Samples are deposited on the tube wall in lo5 Pa of N, and allowed to evaporate a t approximately 300 K in the first chamber; any additional thermal pretreatment is conducted in vacuo in the second chamber. The base pressure for the system before atomization is typically 7 X Pa. The mass spectrometer is set to monitor a single mass of interest during the atomization cycle. Ambiguities in mass identification of species were resolved by examining isotope ratios involving naturally occurring 77Se,80Se,and %e; a separate experiment was made for each isotope. In those cases where ionizer-induced fragmentation could conceivably introduce daughter product artifacts, the ionization electron energy was adjusted about 2 eV above the known ionization potential of the daughter. Fragmentation is eliminated if this energy is less than the dissociation energy of the parent; otherwise, this energy provides
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a condition of operation that allows daughter identification under minimal probability for fragmentation plus ionization. Appearance potentials provided additional confirmation of species identification when necessary and when ionization potentials for the species of interest were available. Atomic absorption measurements were attempted with a Se hollow cathode lamp focused through the sapphire entrance and exit windows on the vacuum chamber using quartz lenses positioned between the chamber and the hollow cathode and between the chamber and spectrophotometer. The wavelength and slit width were adjusted to 196 nm and 320 pm, respectively. These AA measurements were terminated when it was determined that the free selenium was absent in the mass spectra during vacuum vaporization. Tube-In Atmosphere. The atmospheric pressure system uses the same type graphite tube that is employed in the tube-in vacuum system. Molecular beam sampling of the region inside the graphite tube near the dosing hole is achieved with tungsten sampling and nickel skimmer cones; the sampling cone with a 500-pm-diameter orifice protrudes through this hole approximately 500 pm beyond the inside tube circumference. Details concerning this system can be found in ref 9. The system consists of an atmospheric pressure chamber for vaporization, two differentially pumped ultrahigh-vacuum chambers (intercone and quadrupole regions, respectively), and Extrel electron impact ionizer and quadrupole mass analyzer, and an AT-IBM computer as well as a triple-beam Tektronix 5110 oscilloscope to record the mass analyzer, Thermo Jarrell Ash Video I1 atomic absorption spectrometer, and Ircon 1100 series automatic pyrometer outputs. The pyrometer was calibrated as described for the tube-in vacuum system. The AA spectrometer computer and oscilloscope were triggered by the atomization cycle “read” output from the HGA 400 furnace power supply (Perkin-Elmer, Norwalk, CT). This cycle was programmed for 3020 K, 1-s ramp, 5-s hold. A typical Pa; in the pressure in the intercone region is about 7 X Pa. Samples are loaded in 1atm quadrupole region, it is 7 x of N2 (99.97%) and allowed to evaporate. The graphite tube is then positioned in the appropriate proximity to the sampling cone. All thermal treatment is done a t 1 atm of N,. Three preselected masses can be monitored from the output of the Extrel Model 031-3 fast electrometer using the computer control to rapidly peak hop between these masses during a given thermal cycle. A given mass channel is separately integrated 5 times in 0.2-ms intervals (2500 samples s-l). The average of these five samples is stored in an array, and after a 5-ms stabilization period, the averaging is repeated for the second mass. This sequence is continued for the third mass, and the entire cycle is then repeated, allowing for an additional 5-ms fly back time; the totalcycle time is 23 ms. Thus, for each second of data acquisition, 43 averaged data points are obtained for each of the 3 masses. This provides a minimum of 20 data points for each of the observed masses. Th AA spectrometer (196-nm wavelength, 0.3-nm band width) involves using the optical arrangement discussed above for the tube-in vacuum. A mask with a 2.6-mm-diameter orifice is positioned at the chamber exit window to mask the direct viewing of the tube walls. The AA and mass spectrometer are monitored simultaneously. Reagents. Palladium solutions for the flat-in vacuum desorption experiments were prepared from 10000 mg L-’ Pd stock solution, that was prepared by dissolving palladium metal in a minimal amount of HNO,. Selenium solutions for these experiments were prepared from a 1000 mg L-’ Se stock solution that was prepared by dissolving SeO, in water. The tube experiments used Pd stock solutions (tube-in vacuum, 1000 mg L-’; tube-in atmosphere, 115 mg L-’) made from dissolving palladium nitrate (Aldrich, Milwaukee, WI) in deionized water. Selenous acid (Alfa Products, Danvers, MA) was used to prepare the selenium stock solutions for these latter experiments by dissolution in deionized water. Procedure. Sample aliquots of 2-7 pL were used in all cases. When the reduced palladium (designated in this text as PdO) modifier conditions were investigated, palladium solution was introduced onto the graphite, dried, and thermally pretreated between 1100 and 1300 K in either 1atm of N, (the flat-in vacuum and tube-in atmosphere) or in vacuo (the tube-in vacuum). After
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Table I. Experimental Summaryc
species Se
m/z 80
SeO
96
SeO,
112
Se2
160
SeCz
104
Se(OH)," 114 Pd
106
modifier form none PdO PdO none PdO PdO none PdO PdO nnne PdO PdO nnne PdO PdO none PdO PdO PdO PdO
temp profiles (Tspp/Tpeak,K) flat-in tube-in tube-in vacuum vacuum atm ND 1200/1400 1200/1400 400/450 400/410 350/425 400/450 400/410 500/725 410/470 400/600 ND
ND 1150/1350 1150/1400 420/520 470/530 450/575 420/520 ND 700/850 520/960 680/770 ND 520/640
1600/ 2650 1550/1940 1675/2100 410/480 450/520 415/490 410/480 ND 435/480 440/550 ND ND ND 500/700 ND ND ND ND ND 410/480 ND ND ND ND ND ND 1200/1500 1200/1400 1900/2610 1200/1500 1200/1400 1920/2310 40:60006 40:6O0Ob 25:115*
Identical thermal profile for SeOHt. Typical quantities Se(ng):Pd(ng)when Pd is present. 'ND = not detected. cooling, the selenium solution was pipetted onto the same location and dried. The substrate was then pulse heated, and MS data were collected. In the case of the nonreduced palladium (designated in this text as PdO), the modifier was introduced onto the graphite substrate followed immediately by the introduction of the appropriate aliquot of the selenium sample. No measurable difference cnuld he discerned between this latter method of introduction and that resulting from mixing the modifier and selenium sample solutions prior to their introduction onto the graphite surface. The typical quantities of selenium deposited for each experiment were 40 and 25 ng for the vacuum and for the atmospheric pressure vaporization experiments, respectively. These quantities were sometimes varied from 10 to 200 ng in order to identify species by varying ionizer electron energies or to determine the influence of load variations. For each species investigated, a minimum of two (in most cases more than two) replicate experiments, each followed by a water blank, were performed. Background corrections were negligible for most of the experiments. Ionizer electron energies were generally maintained at 70 eV for the flat-in vacuum and for the atmospheric pressure vaporization experiments; 45.5 eV was typically used for the tube-in vacuum experiments. The mass analyzers were calibrated with water, nitrogen, oxygen, argon, carbon dioxide, and trichlorotrifluoroethane. The m / z values listed in the second column of Table I were used to determine the profiles of the species in the first column. Other m / r values were used, of course, in the isotope ratio determinations discussed above. The mass spectrometers were optimized over the mass range of interest for given ionizer electron energies by adjusting ion extraction energies and electrostatic focusing lenses for various calibration masses after tuning the radio frequency (rf) and the capacitive coupling for maximum rf power input. Mass resolutions/sensitivities were adjusted to provide unit widths for the largest mass monitored during a given experiment.
RESULTS AND DISCUSSION The composite of all results from these experiments make up a five-dimensional array consisting of the three experiment types (flat-in vacuum, tube-in vacuum, tube-in atmosphere), t h e three samples types (Se/no P d , Se/Pd, Se/PdO), the species identified in the mass spectra, and the thermal history and magnitudes of these signals. The two geometries, flat and tube, help to define the influence of graphite enclosure (long-range secondary wall) on atomization. Examples of
SeO'
100
Se;
80
-
60
II
x 100
1
I
1
700
800
< 0
Temperature, K
Selenium oxide and dimer thermal profiles from the flat-in vacuum system, without palladium modifier. Ionizer electron energy: 70 eV. m l z : 96, 112, and 160 for SeO, SeO,, and Se,, respectively. All signals monitored simultaneously by mass hopping. Figure 1.
140 C
0
I
400
'.
600
500
700
800
900
1000
1100
1200
Temperature, K
Selenium oxide and dimer thermal profiles from the tube-in vacuum system, without palladium modifier. Ionizer electron energy: 45.5 eV. m l z : 96, 109 and 160 for SeO, SeO,,and Se,, respectively. Figure 2.
I
%\
I
20
500
600
700
800
'0
Temperature, K
Flgure 3. Selenium oxide and dimer thermal profiles from the tubain atmosphere system without palladium modifier. Ionizer electron energy: 70 eV. m l z : 96, 109, and 160 for SeO, SeO,, and Se,, respectively. The two oxides are monitored simultaneously by mass hopping.
thermal profiles obtained from the three types of experiments are shown in Figures 1-3. The overall results are capsulated
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in Table I, which lists all of the species observed (column 1) for each of the experiments (columns 4-6) for each sample type (column 3). The bottom row lists the quantities of Se and Pd used in the experiments. Appearance and peak temperatures are shown in columns 4-6 also. For purposes of discussion, we define “precursors” as gas-phase species appearing in the mass spectra prior to the “usual” higher temperature appearance of atomic selenium in conventional ETA-AAS. Note that appearances of species in the vacuum experiments and a paucity of the same species in the atmospheric pressure studies are insufficient evidence to categorically exempt these species from being released from surfaces contained in a 1-atm inert sheath gas. Probabilities for involvement of these species in homogeneous and heterogeneous gas-phase reactions in the vacuum experiments are minimal. Such reactions are, however, more likely to occur at atmospheric pressure and can inhibit the lifetime or even the appearance of these gaseous species. G e n e r a l Overview. The selenium dimer appears as a precursor in all cases where palladium is not introduced in the sample. Dimer formation is inhibited in all cases where palladium is introduced but not thermally pretreated. Thermal pretreatment of the palladium in the tube prior to introduction of the selenium sample also inhibits dimer formation in the atmospheric pressure vaporization but not in the vacuum vaporization experiments. Hydroxides and oxides of selenium also appear as precursors in the atmospheric pressure experiments; hydroxides are never observed in the vacuum vaporization spectra. The presence of palladium inhibits the formation of these hydroxides during atmospheric pressure vaporization. Selenium dioxide is inhibited by palladium, but only in the graphite tube experiments and then only if the palladium has been thermally pretreated. In all cases, where palladium is present, the monoxide signal is attenuated. Selenium dicarbide appears as a precursor in the tube-in vacuum experiments only; PdO inhibits dicarbide formation. Finally, free selenium is not present in the vacuum experiments unless palladium is contained in the sample. Hence, the general effect of the palladium is to modify the vaporization by inhibiting formation of the hydroxide, dimer, carbide, and dioxide of selenium. It is essential, for purposes of brevity and clarity in discussing these data, to view the results in a form most amenable to direct comparisons. Bar-graph thermal representations are used for these purposes. This is reasonable because, as discussed in the Experimental Section, it is necessary that most of the data be treated qualitatively. Figures 4-6 are composites of these representations for each sample type, Se/no Pd, Se/Pdo, and Se/PdO, respectively. The three experimental systems are defined as A, B, and C, respectively, for flat-in vacuum, tube-in vacuum, and tube-in atmosphere. The proposed step-by-step reaction pathways for the tube-in atmosphere are shown at the bottom of each figure. The widths and the peaks and the location of the maxima are shown through shading. Relative intensities cannot be extracted from this presentation, but they will be introduced in the discussion when needed. S e / N o Pd. The Se/no P d data are presented in Table I and in Figure 4. Oxides. Figures 1-3 are examples of thermal profiles resulting from the traces for Se,+, SeO+, and SeOz+for each of the three experimental systems. In the case of the flat-in vacuum, the three traces were collected by using mass hopping. In the case of the tube-in atmosphere, the Se2+ and SeO+ profiles were also collected during a single experiment using mass hopping. The relative temperature displacement of the Se, and oxide signals, to be discussed later, is therefore real and independent of any uncertainty in the data collection or heating rate of the furnace. As seen in the thermal profile
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composite (Figure 4), SeO,,, and SeOz are the dominant precursors observed in all of the experiments when palladium is absent. They appear in the same temperature region ( ~ 4 0 0 K) regardless of the vaporization environment (vacuum or atmospheric pressure). The monoxide cannot be attributed to ionizer dissociation since it is observed when ionizer electron energies are below 10 eV (an energy sufficient to ionize but insufficient to induce significant fragmentation), and its exists in the absence of the dioxide. This monoxide is therefore created in the furnace and is from dissociative adsorption processes or from thermal decomposition, not reduction. If reduction were occurring, the dioxide should not be observed. The concurrent appearance of SeO and SeO, implies that sublimation from condensed SeO, is not solely responsible for these oxides. Furthermore, it suggests that solid SeOz,which sublimates at 590 K (27), is not formed on the surface. Desorption of adsorbed selenium oxides from the graphite surface is therefore suggested. This is supported by the fact that increased quantities of analyte result in oxide signals having relatively constant peak temperatures while maintaining signal symmetry, which is in agreement with the peak shapes expected for a first-order release process (18). The 600 K widths of the oxide signals resulting from tube experiments (vacuum and atmospheric pressure) indicate that strong interactions between the oxides and the tube walls (viz. readsorption) may be controlling the peak shapes. This is substantiated by the narrow 100 K width observed for these same species vaporized in vacuo from the flat where secondary wall collisions are much less likely. Longer residence times resulting from multiple collisions with and sticking to the tube wall at these relatively low temperatures are also consistent with the previous first-order desorption model. Polymers. It is well established that gaseous polymeric species for selenium are a result of sublimation from the condensed phase of the element (17). Similarly, the extremely large mean free paths associated with the vacuum studies exclude the possibility of gas-phase dimer formation. Hence, condensed elemental species must result from thermal decomposition or from dissociative adsorption of the oxides, discussed above, with subsequent formation of condensedphase selenium. The higher temperatures associated with the Sepsignal relative to the oxides imply that the dimer can be released even in the absence of oxide desorption. Unfortunately, limitations inherent in the quadrupole mass spectrometers used in these studies do not allow detection of the higher polymers in these transient heating experiments. However, these polymers must be present if this model is correct. Hydroxides. Selenium hydroxide appears only a t low temperature and only in the atmospheric pressure experiments. This implies a gas-phase reaction between H 2 0 and the oxides. This gas-phase reaction explains the paucity of hydroxides in both vacuum experiments; there is little chance of vapor-phase involvement in these cases. For all three experiments, adsorbed and interstitial water is available for homogeneous condensed-phase reactions, but the hydroxides appear only undr atmosphereic pressure conditions. Thus, hydroxide formation must involve a t least one gas-phase reactant. It is not clear whether the hydroxide is formed homogeneously with water vapor and selenium oxide gas species or whether a heterogeneous reaction is occurring. The latter possibility cannot be dismissed since it is well established that H 2 0 will remain available for desorption even at temperatures well above the 950 K extent of the observed selenium hyroxide (19). Free Selenium. Free selenium, which appears nar 1650 K at atmospheric pressure only, is the sole species in the gas phse beyond 1200 K. Neither thermal pretreatment nor atomizer geometry is responsible for the absence of the selenium in the
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1400
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1400
1600
1800
2000
2200
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vacllym
seo,seo2
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€,
1200
1200
atmosDhere
I Se2
Figure 4. Composite of thermal profiles for the Seho Pd case for (A) flat-in vacuum, (B) tube-in vacuum, and (C) tube-in atmosphere systems. Relative intensities cannot be extracted from this presentation. The circled, numbered notation is described in Chart I. Pd
Flat in vacuum
A.
% m
s
8po
1000
2
1200
1400
1690
I
W,seo, 1800
2000
2290
2400
260
Flgure 5. Composite of thermal profiles for the Se/Pdo case for (A) flat-in vacuum, (B) tube-in vacuum, and (C) tube-in atmosphere systems. Relative intensities cannot be extracted from this presentation. The circled numbered notation is described in Chart I.
vacuum experiments. This is based on the observation that thermal pretreatment steps identical with those used in either vacuum experiment failed to inhibit the appearance of Se in the atmospheric pressure experiments. The presence of the
sheath gas is therefore contributig to this appearance, probably through heterogeneous reactions involving gas-phase selenium oxides and dimer and the graphite surface. The probability for redeposition of the oxides and dimer is enhanced by
ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991 513 A.
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1400
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1800
2000
2200
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Figure 6. Composite of thermal profiles for the Se/PdO case for (A) flat-in vacuum, (B) tube-in vacuum, and C) tube-in atmosphere systems. Relative intensities cannot be extracted from this presentation. The circled numbered notation is described in Chart I .
Chart I.
The following refer to the circled number notation in Figures 4-6. (1) While some bulk (crystalline) SeO, likely exists on the surface, the presence of various other selenium species in vacuum experiments suggests adsorbed SeO,. (2) These are species observed in vacuum experiments. ( 3 ) The carbide was detectable above the baseline in vacuum only when increased amounts (>80 ng) of sample were used. (4) Due to quadrupole characteristics, only the dimer was observed. The other polymeric species (e.g., Se4, Se6, etc.) should be present if the model is correct. ( 5 ) Although the sample is dried, considerable adsorbed water is retained by the graphite and represents a potential reactant at elevated temperatures, particularly at 1 atm. Residual oxygen may also be affecting the species observed in the atmospheric system. (6) These are species observed in atmospheric pressure experiments. (7) This is probably a consequence of gas-phase reactions involving residual H20. (8) These are active sites that trap selenium. (9) High-temperature atomic selenium observed only at 1 atm of N,. (10) Since gaseous precursors are reduced in intensity and shaped differently relative to the Se/no Pd case, some reaction between palladium and the analyte must occur at T < 400 K. The composition of the condensed-phase intermediate is not known and may be the [Pd,Se,O] compound proposed in ref 14. Some SeOZ(ad) that does not initially have any contact with the palladium must also be present. (11) These are intermediate species seen in the vacuum vaporization experiments. SeO, is seen desorbing from the flat-in vacuum surface only in the reduced palladium case. (12) The SeO(,, signal decreases with increasing amounts of palladium. In the atmospheric system, it must be in equilibrium with the intermediate species in ref 11. (13) Some of the intermediate species may react with the palladium to produce the compound given in ref 14. The vacuum data suggest this may be a heterogeneous gas-solid reaction. (14) A \Pd,Se,O] compound is hypothesized; however, the stoichiometry of the compound is unknown. (15) The compound in ref 14 decomposes and is vaporized in the vacuum system. However, in the atmospheric system, the palladium and selenium recondense, and both are stabilized to higher temperatures. (16) The noncoincidence of selenium and palladium in the atmospheric pressure system suggests that palladium is retained on the surface. (17) The carbide and dimer formation is inhibited because in the PdO experiments solution mixing ensures that the byproducts of the decomposed SeO, will react immediately with the palladium and/or this oxygen-rich modifier readily forms the [Pd,Se,O] compound. (18) SeO.] desorution is not influenced significantly bv the presence of PdO modifier. The x values are 1 and 2. trapping of the oxide in the Langmuir film above the hot graphite surface in the presence of the 1-atm sheath gas; Le., multiple collisions with the surface occur. The oxides and/or dimer must be dissociatively adsorbed at high-energy desorption sites, b u t these sites would have to be inaccessible to the condensed phase of selenium; otherwise, free selenium would appear during vacuum vaporization. This indicates that temperatures necessary to overcome the activation energy of
dissociative adsorption of the oxides and dimer exceed their vaporization temperatures in vacuum. Consequently, the condensed phases are unable to participate directly in the dissociative adsorption process. The resulting adsorbed selenium desorbs when a 1600 K surface temperature is achieved. The possibility €or the carbide t o contribute to free selenium formation by reacting with residual oxygen in the sheath gas or on the surface to form Se and CO or C 0 2 must
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also be considered. Unfortunately, large CO and CO, background prevents substantiation of this reaction in these experiments. The carbide may only be a minor contributor; however, the carbide signal intensity is significantly smaller than that of the oxides (see Carbides discussion). Carbides. The carbide appears only in the vacuum experiments. It is therefore a result of the condensed or adsorbed selenium species reacting with the graphite surface. As discussed in the preceding paragraph, adsorbed selenium is the result of heterogenous reactions of the oxides and dimer with the surface. Such reactions are severely limited in vacuo, so adsorbed selenium cannot contribute to carbide formation. Carbides do not appear in the atmospheric pressure case because the gas-phase carbide trapped in the Langmuir film reacts with 0, in the film or on the surface. Carbides therefore disappear from the system shortly after desorption. It should be noted also that the carbide signal intensity was close to the noise level in the vacuum experiments even when relatively large quantities of selenium (i.e., 80 ng) were used. Summary. The proposed reaction pathways associated with the Se/no Pd experiments are illustrated at the bottom of Figure 4. The reactions relate to selenium introduced as an aqueous oxide, dried with no modifier, and atomized in 1 atm of N,.The following descriptions of the mechanism for atmospheric pressure atmoization are implied by the above discussion of the flat-in vacuum, tube-in vacuum, and tube-in atmosphere experiments. 1. Selenium dioxide is adsorbed to the graphite surface preceding the release of selenium-containing species. 2. At some temperature below 400 K, some of the selenium dioxide is either thermally decomposed or adsorbed dissociatively. This results in monoxide and elemental selenium being formed in adsorbed phqses. Desorption of the oxides begins at 400 K. 3. Gas-phase polymeric selenium forms as a result of the adsorbed selenium coalescing. The carbide can also be formed from the condensed selenium-graphite interaction, but it is short-lived a t atmospheric pressure because of the presence of residual oxygen in the sheath gas. 4. Selenium species along with HzO undergo heterogeneous and perhaps homogeneous reactions to form the hydroxides. These and the above precursors are vehicles for low-temperature selenium losses from the analysis volume at atmospheric pressure. 5. Oxides and/or polymers of selenium trapped in the Langmuir film become dissociatively adsorbed on the graphite a t sites that are unavailable to selenium species during the initial drying of the aqueous solution. 6. Desorption of the resulting adsorbed selenium occurs near 1600 K. 7 . The long-range secondary wall has no influence on the atomization or stabilization mechanisms. These mechanisms are described by the reactions
where m L 2, y = 1, x = 1, 2 or y = 2, x = 0, (ad) refers to an adsorbed state, and C* refers to high activation energy sites on the graphite surface. Se/Pdo. The data associated with experiments using selenium and thermally pretreated palladium are shown in Table I and Figure 5. Recall that the selenium sample for these experiments was introduced after thermally pretreating the palladium nitrate solution (6000 ng of P d in the vacuum
experiments; 115 ng of P d in the atmospheric pressure experiment) in vacuum and in 1 atm of Nz at 1100-1300 K. Oxides. The intensities of all the low-temperature oxide spectra that were observed in the case of Se/no P d were diminished, and thermal profiles were altered by the presence of the reduced palladium modifier. This implies that modifier involvement in the vaporization of analyte species occurs a t temperatures below 400 K. Only a portion of Se02(ad)reacts with the palladium at these low temperatures, since SeOk) still appears for all three experimental conditions. Se02(g)also persists for the flat-in vacuum. At atmospheric pressure, the dioxide that is not in initial contact with Pd(,) probably interacts with the condensed palladium via multiple collisions in the Langmuir film; the SeO, signal is quenched. This heterogeneous interaction is not possible in vacuum, of course. Thus, the SeO, not contacting and interacting with palladium in the vacuum experiments contributes to the production of many of the same species that are observed in the absence of the palladium. The presence of SeO and the absence of SeO, at atmospheric pressure suggest that the dioxide is more reactive with the reduced palladium than is the monoxide. SeO also disappears under atmospheric pressure vaporization of 25 ng of Se when large quantities (400 ng) of palladium are used; lower limits of P d needed for this effect were not determined. These observations suggest the formation of [Pd,Se,O], a condensed-phase Se-Pd compound that may contain oxygen. The presence of SeO and absence of SeO, in the tube-in vacuum experiments can be explained by the enhanced reducing environment that results from heating in vacuum only; this is discussed below. The entrapment of oxides by the reduced palladium and the subsequent depleting of these oxides from the gas phase are dependent on the reducing capability of the surface and on the probability of the selenium oxides interacting with palladium, particularly the sparingly small amount used in the atmospheric pressure experiments. Even though vacuum minimizes gas/ solid interactions when the palladium is thermally pretreated at 1100-1300 K in vacuo, less oxygen is chemisorbed onto the graphite and the general nature of the surface is more reducing in character-hence, more likely to convert the dioxide to the monoxide and/or elemental selenium. This explains the paucity of SeO, in the tube-in vacuum experiments. In contrast, pretreatment of palladium at 1 atm provides a less reducing surface; Le., more Oz is chemisorbed. Consequently, the presence of SeO, remains significant even though vaporization occurs in vacuo. Polymers. The thermally pretreated palladium, in the atmospheric pressure experiments (25 ng of Se/115 ng of Pd), is evidently inhibiting the appearance of the dimer through heterogeneous reactions since Se, is still detected in the vacuum experiments. The dimer in the atmospheric pressure experiments has multiple chances of interacting with palladium on the surface during transport in the Langmuir film. This is not possible in either of the vacuum experiments. Under vacuum, residual SeO, not contacting the palladium is either thermally decomposed or dissociatively adsorbed to form a condensed phase of selenium, Se(,]);sublimation provides the dimer product as discussed for the Se/no P d results. It will be shown in the Carbides section that PdO interaction with condensed-phase selenium is insignificant. Hence, PdO reacts principally with SeO,. The exact role of Pdo in inhibiting Se, appearance in the atmospheric pressure case is not entirely clear. It is known that palladium readily forms solid solutions with selenium (17). However, compound formation is also possible. This is discussed in the section describing the Se/PdO results. The existence of stoichiometric compounds with P d S e atom ratios
ANALYTICAL CHEMISTRY, VOL.
extending from 0.5 to 4.5 have been determined by Olsen et al. (20). While a lower Pd:Se ratio limit for complete dimer inhibition was not determined in the present experiments, it was observed that the dimer was always absent for P d S e ratios greater than 3. Recent scanning electron microscopy studies show spatial coincidence between palladium and selenium (7). This evidence is not, however, sufficient to clarify whether a Se-Pd compound or solid solution exists prior to detection of the analytical signal. It will become clear in the discussion under Free Selenium and Palladium that, in fact, stoichiometric compound formation is most probable. Hydroxides. The absence of the hydroxides suggests that the palladium is interfering with hydration of the oxides. One possible way palladium could interfere is by catalyzing the water gas shift reaction (21, 22) or the reaction of CO and water (23). This is reasonable when palladium's affinity for hydrogen is considered along with the ability of graphite to dissociatively adsorb water and preferentially retain the hydrogen (24). As discussed earlier, there is little change of any significant heterogeneous reaction occurring in the vacuum experiments, so hydroxides are not formed in vacuo with or without palladium present; the data substantiate this. Free SdPnium and Palladium. Free selenium in both vacuum experiments appears at approximately 1150 K, almost 400 K below its appearance temperature in the atmospheric pressure experiments. However, the vacuum appearance temperature is only 200 K above the highest melting point reported for any palladium selenide (20). The palladium appearance temperature in the vacuum experiments is identical with that observed for selenium. This is 700 K below the palladium appearance temperature in the atmospheric pressure experiments. The 50 K temperature displacement between Se and Pd, noted in Table I for the two separate tube-in vacuum experiments, is negligible relative to the accuracy of the temperature measurements. A difference of this magnitude is significant, however, if temperature comparisons are made within a single experiment (see General Overview/Oxides). The simultaneous gas-phase appearances of selenium and palladium in the vacuum experiments indicate decomposition of a stoichiometric compound, [Pd,Se,O]. Both species would not vaporize simultaneously into the gas phase if a solid solution were involved. Thus, the compound versus solid solution dilemma resulting from the dimer observations is resolved. The exact description of the compound is unknown, however. It is unlikely to be a selenide because of the above discrepancies concerning palladium selenide melting points and appearance temperatures of free selenium. This stoichiometric compound, [Pd,Se,O], decomposes near 1150 K in the vacuum experiments; the selenium and palladium are released into the gas phase. In contrast, an increase in the appearance temperature of Se and P d occurs in the atmospheric pressure experiments. This results from the trapping of these species a t higher energy retention sites; release from these sites occurs a t 1550 K where Se(,) is detected. Four possibilites exist for this retention of selenium: (1)simple adsorption of selenium onto the graphite, (2) dissolution of selenium into a palladium melt, (3) trapping of selenium at pitting and channeling sites created by palladium-catalyzed oxidation of graphite, and (4) selenium adsorption onto or in the proximity of palladium. The first possibility does not require the presence of palladium, and hence the selenium appearance temperature should remain constant with increasing quantities of palladium. This is contrary to the shift to higher appearance temperatures with increasing palladium concentrations as reported by Rettberg and Shrader (25). The second possibility was discussed in the previous section and discounted as a likely
63,NO. 5,
MARCH 1, 1991
515
mechanism at these low palladium concentrations. The last two possibilities remain, but the most probable of these cannot be determined from these data. Carbides. Selenium dicarbide from 40 ng of Se is not influenced significantly by 6000 ng of reduced palladium; larger Pd concentratiohs were not attempted. T o be consistent with the homogeneous condensed-phase carbide formation mechanisms proposed earlier for the Se/no Pd case, the Pdo-condensed selenium interaction must also be insignificant. The vacuum experiments yield the dicarbide because sufficient quantities of selenium oxide do not physically contact the palladium. This argument agrees with the above discussions explaining the behavior of the oxides and the polymers. The dicarbide is absent at atmospheric pressure for the same reason presented for Se/no Pd; i.e., because of competitive reactions with residual oxygen to form oxides. Summary. The proposed reaction pathway associated w$h the Se/Pdo experiments are illustrated at the bottom of Figure 5. The reactions relate to the case where aqueous palladium nitrate solution is introduced into a graphite tube furnace and is then thermally pretreated between 1100 and 1300 K and cooled. Aqueous selenium oxide is then introduced, allowed to dry, and atomized in 1 atm of Nz. The following descriptions of the mechanism for atmospheric pressure atomization are implied by the above discussion of the flat-in vacuum, tube-in vacuum, and tube-in atmosphere experiments. 1. Selenium is partially adsorbed onto the graphite walls. Prior to 400 K (possibly upon sample solution introduction), some selenium species become associated with palladium metal on the surface. 2. Palladium inhibits the hydration of the selenium oxides by catalytically promoting either the water gas shift reaction or the reaction of CO and water. 3. Near 400 K, the selenium species are desorbed from the graphite as oxides, polymeric species, and carbides. Only SeO persists sufficiently long to leave the Langmuir film and be lost from the furnace. The balance of the selenium-containing species in the film (excluding thefcarbide that reacts with residual oxygen to form oxides) react with the condensedphase palladium through multiple collision to form more [Pd,Se,O] compound. 4. At approximately 1200 K, the [Pd,Se,O] compound begins thermal dissociation. Selenium and palladium are released a t 1200 K from [Pd,Se,O] and are trapped at highenergy retention sites associated with the presence of palladium; desorption from these sites occurs near 1550 and 1900 K, respectively. These mechanisms are described by the reactions
Pdo(,)
-
+ Se02(g) >400 K
[Pd,Se,O]
+ SeO,,)
(5b)
1900 K Pd(ad)
Pd(g)
where x = 1 or 2, [Pd,Se,O] refers to a selenium-palladium compound possibly containing oxygen, and (Se-Pd)ad refers to Se and P d readsorbed on the graphite surface. Se/PdO. Data from the vacuum and atmospheric pressure experiments where the selenium samples include palladium (6000 and 115 ng of Pd, respectively, for the vacuum and the atmospheric pressure experiments) that has not been ther-
516
ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991
mally pretreated are shown in Figure 6. The selenium solution was loaded immediately after the palladium solution. Note that palladium nitrate thermally decomposes (26)most likely to PdO, which is stable up to 1140 K (27);a stable form of the nitrite has not been reported. Thus, the absence of any thermal pretreatment of the palladium modifier implies that palladium oxide can interact with selenium. The PdO in subsequent discussions refers to this untreated oxide. Oxides. The monoxide and dioxide are the prinicipal selenium species that leave the furnace when PdO is used as a modifier in vacuo and at atmospheric pressure. The atomic absorption signal, being weaker than that observed for the Se/Pdo case, indicates that these oxides contain a measurable fraction of the selenium. This might be expected since PdO is the less reducing form of the modifier. It explains why the more highly oxidized SeO, was not observed in vacuo when elemental palladium was present in the tube. A t 1 atm, the S e 0 2signal is relatively short-lived (Le., narrow half-width). This perturbation of the SeO, indicates either ultimate readsorption of SeO, to available sites on the tube surface or reaction of SeO, with the palladium oxide. Polymers. The paucity of gas-phase dimer species for all of these experiments implies that PdO is inhibiting either formation of condensed-phase selenium or sublimation of this phase. Low-temperature formation of a [Pd,Se,O] compound is the most obvious inhibiting mechanism. Hydroxides. Hydroxides do not appear at atmospheric pressure because hydration of PdO is competing with that of SeO. Again, the hydroxides are not formed in vacuo because of the relatively low probability of heterogeneous reactions occurring. Free Selenium and Palladium. The general thermal histories of the high-temperature selenium and palladium mass spectra are consistent with those observed with the reduced palladium experiments discussed previously. The [Pd,Se,O] compound must, therefore, be identical in both cases. The mechanism for analyte retention and release is therefore similar to that noted in the previous discussion of the free selenium and palladium. Carbides. Gaseous dicarbide formation is inhibited in the experiments done in vacuo and in those done in atmospheric pressure. The low-temperature formation of a [Pd,Se,O] compound that inhibits dimer formation must also inhibit formation of the carbides; Le., the elemental selenium necessary for SeC, formation cannot be produced at low temperatures. Summary. Proposed reaction pathways associated with Se/PdO experiments are illustrated at the bottom of Figure 6. The reactions pertain to introduction of palladium nitrate solution into the graphite tube furnace, subsequent introduction of aqueous selenium oxide, drying, and atomization in 1 atm of N?. The following descriptions of the mechanism for atmospheric pressure atomization are implied by the above discussion of the flat-in vacuum, tube-in vacuum, and tube-in atmosphere experiments. 1. The unreduced form of palladium contributes to a less reducing environment than the elemental form of the modifier within the furnace. An SeOzcp,precursor still evolves, although not to the extent seen in the absence of modifier. 2. The PdO forms a compound with selenium at low temperature, and this compound remains stable to approximately 1200 K. The compound then undergoes thermal dissociation and free selenium is desorbed near 1675 K as described previously for the Se/Pdo case (see previous summary, item 4). These mechanisms are described by the reactions
where [Pd,Se,O] is a selenium-palladium compound that probably contains oxygen and is the same as that compound formed in the case of reduced palladium, and (Se-Pd),d refers to the recondensed stabilized species.
CONCLUSION From the above self-consistent mechanisms associated with the two vaporization environments and two geometries and three analyte-modifier treatments with two forms of the modifier, it is concluded that palladium performs two functions in enhancing selenium analysis by ETA-AAS. It first forms a compound with selenium on the surface and thus prevents low-temperature formation of gas-phase selenium species; this [Pd,Se,O] compound thermally dissociates near 1200 K. Released elemental selenium is trapped or adsorbed at high energy retention sites created by the presence of palladium. Release from these sites occurs at temperatures greater than 1500 K (25),depending on the quantity of palladium employed. Principal losses of analyte occur at low temperatures in the form of oxides, dimers, and hydroxides when palladium is absent. Introduction of palladium inhibits formation of the dimer and hydroxide and reduces oxide losses at low temperatures. The vacuum vaporization results imply that surface reactions induced by the modifier and heterogeneous gas-solid reactions are of major importance in controlling atomization. Homogeneous gas-phase reactions are not a major factor except in providing the means of inducing readsorption or vapor loss. Gas-phase selenium carbides are observed in vacuo only if sufficient analyte is introduced. These appear a t low temperatures, but they are not concurrent with the free selenium. This suggests that, for selenium, gaseous carbide formation is a result of the bonding of the analyte to the graphite surface and represents neither a major route for analyte loss nor a controlling mechanism that governs the manner in which free selenium is generated in the furnace. The above mechanisms can be compared to those associated with arsenic and arsenic stabilization by palladium (11). For both elements, atomization occurs by dissociative adsorption and stabilization results from compound or solid solution formation with palladium. ACKNOWLEDGMENT We thank D. R. Ells for his invaluable assistance with the apparatus at Pacific Northwest Laboratory, and G. F. Christopher for his assistance at The University of Texas. LITERATURE CITED Acta Chim. Sin. (Engl. Ed.) 1979, 37, 261; Chem. Abstr. 1979, 92. 220474X. Shan. X.-a.: Ni. 2.-m. Huaniino Kexue 1980 1 . 24:. Chem. Abstr. 1980, 93,'12147~. Shan, X.-q.; Ni, 2.-m. Can. J . Spectrosc. 1982. 27, 75. Voth-Beach, L. M.; Shrader, D. E. Spectroscopy 1986. 7 , 49. Voth-Beach, L. M.; Shrader, D. E. J. Anal. At. Spectrom. 1987. 2.
(1) Shan, X.-q.; Ni,
(2) . . (3) (4) (5)
,
I
--.
A4
(6) Welz, B.; Schlemmer, G.; Mudakavi, J. R. J . Anal. At. Soectrom. 1988- 3 ,93. (7) Teague-Nishimura, J. E.; Tominaga, T.; Katsura. T.; Matsumoto, K. Anal. Chem. 1987. 59. 1647. (8) Shan, X.-q.; Hu, K.-J. Talanta 1985, 32, 23
Anal. Chem. 1991, 63,517-519 (9) Schlemmer. G.; Welz, B. Spectrochim. Acta, Part 8 1988, 41, 1157. (10) Lindberg, 1.; Lundberg, E.; Arkhammar, P.; Berggren, P.-0. J . Anal. At. Spctrom. 1988, 3 , 497. (11) Styrls, D. L.; Prell, L. J.; Redfield, D. A. Anal. Chem., precedlng paper in this issue. (12) Styrls, D. L. Fresenius 2.Anal. Chim. 1986, 323, 710. (13) Droessler, M. S.;Holcombe, J. A . Spectrochlm. Acta, Part B 1987, 42, 981. (14) Bass, D. A.; Holcombe, J. A. Anal. Chem. 1987, 5 9 , 974. (15) Christopher, G. F.; Holcombe, J. A. Anal. Instrum. 1988, 3 , 235. (16) Styris. D. L. Anal. Chem. 1984. 56, 1070. (17) Chizikov, D. M.; Shchastllvyi, V. P. Selenium and Selenides; Elkin, E. M., Translator; Collet's Publishers: London, 1968; p 13. (18) Redhead, P. A. Vacuum 1962, 12, 203. (19) Marchon, B.; Carraza, J.; Hienemann, H.; Samorjai, G. A. Carbon 1988, 26, 507. (20) Olsen, T.; Post, E.; Gronvold, F. Acta Chem. Scand. 1979, A33, 251. (21) Ziemecki. S. B. Stud. Surf. Sci. Catal. 1987. 38, 625. (22) McKee, D. W. In Chemistty and Physics of Carbon; Walker, P. L., Thrower. P. A., Eds.; Marcel Dekker: New York. 1981; Vol. 16, pp 77, 88. (23) Pyatnitskii. Yu 1. Kinet. Katal. 1984, 2 5 , 620. (24) Puri, B. R. I n Chemistty and Physics of Carbon; Walker, P. L., Eds.; Marcel Dekker: New York. 1970; Vol. 6, p 200.
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(25) Rettberg, T.; Shrader, D. E. Palladium Modification In GFAA: Establishing Maximum Performance. PMsburgh Conference, New Orleans; Feb 1988; Abstract 815. (26) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 57th ed.; CRC Press: Cleveland, OH, 1976; p 8-158. (27) Turkdogan, E. T. Physical Chemistry of H@h Temperature Technology; Academic Press: New York, 1980: p 19.
RECEIVED for review June 29,1990. Accepted November 21, 1990. The University of Texas research effort was supported by the National Science Foundation (Grant No. CHE-87 04024), and support for the Pacific Northwest Laboratory effort was provided by the Director, Office of Energy Science, Chemical Sciences Division of the U.S. Department of Energy and performed under Contract DE-AC06-76RLO 1830. Financial support for D. A. Redfield was provided in part by the Northwest College and University Association for Science (Washington State University) under Contract DE-AMO676RLO 2225.
CORRESPONDENCE Electrochemical Formation of High Surface Area Carbon Fibers Sir: There is reasonable concensus among electrochemists that the surface of most carbon electrodes needs to be pretreated in some manner to improve their charge-transfer properties. The effect of such pretreatment on the surface and the reason for the enhanced electrochemical activity are still not totally understood. Our laboratory has been involved in the development of pretreatment procedures and in the examination of the structural, chemical, and electrochemical properties, particularly of glassy carbon, for several years. A particularly effective pretreatment for polished glassy-carbon electrodes was found to be vacuum heat treatment by which impurities and surface oxygen functionalities were removed ( I ) . More recently, McCreery and co-workers (2-5) have reported on the effectiveness of laser beam treatments to activate carbon surfaces for electron transfer. The situation with carbon fibers is quite different due to their microsize and fragility that prevents any mechanical manipulations for activation. A particularly effective pretreatment method has been the application of either an anodic potential or a galvanostatic step. In working with various carbon fiber types from different manufacturers, we recently found an unusual phenomenon in which a particular pitchbased fiber underwent extensive fracturing when treated anodically. The result of this fracturing is a dramatic increase in the surface area as reflected by a concurrent increase in the measured capacitance. We are unaware of any previous report of such an observation for electrochemically treated carbon fibers. The degree of fracturing appears to be controllable by the electrochemical treatment method employed. Thus, a "mild" fracture with a 1 or 2 order of magnitude increase in the capacitance, and hence surface area, can be accomplished by a potential-step method. For "severe" fracturing where there is nearly a 4 order change in the area, a galvanostatic method is most convenient. The change in the surface morphology due to the fracturing was readily apparent in the scanning electron photomicrographs.
EXPERIMENTAL SECTION The high modulus (Type E120) carbon fibers with a nominal diameter of 10-12 pm were manufactured by Du Pont Co. (Chattanooga, TN). The fibers are specified as "ultraclean" and have a negligible content of non-carbon elements as evidenced by X-ray analysis. Discussions with the manufacturer suggest that the fibers were subjected to a final heat treatment in the 2500-3000 "C range and that an "onion-skin" or "smooth laminar" graphitic structure may exist at the fiber surface. Fibers used "as received" will be designated as such in the text. Our previous results and experience with glassy-carbon electrodes suggested that a reproducible reference surface may be best produced by the vacuum heat treatment method. Thus, further experimental work with the fibers was preceded by vacuum heat treating them at temperatures of 1000-1100 "C for 30 min at pressures in the low lo-' Torr range. Details of the vacuum heat treatment equipment and protocol were previously described (6). Heated fibers were allowed to cool to room temperature while remaining in the high vacuum. The fibers were removed from the UHV chamber with minimal exposure to the laboratory air and stored under nitrogen in a sealed vial and kept at 1-5 "C until needed. In the potential-step method of "mild" fracturing, a sequence of square-wave potential steps was applied in which the potential was stepped from 0.0 to + L O V for 30 s and then returned to 0.0 V. In each successive step, the upper limit of the potential was increased incrementally by 0.2 V until +2.0 V was reached. Between each step, the electrode was examined for the presence of surface redox functionalities by cyclic and differential pulse voltammetry. The extent of fracturing could be controlled to a certain extent by the type of electrolyte present in the solution. Extensive (severe) fracturing was produced conveniently by the application of a galvanostatic step in which a constant current of 3 mA was applied for ca. 5 s. The current was generated by applying + L O V between the working and reference leads of the potentiostat across a 3304 resistor. The carbon fibers were microcylindrical in shape, and approximately 1.0 cm was immersed in the solution for study. The fiber electrodes were prepared by the attachment of a single fiber to a conducting wire with silver epoxy. Solutions were deoxygenated with dry nitrogen for 5-15 min. During experiments, nitrogen gas covered the solution.
0003-2700/9110363-0517$02.50/0 0 1991 American Chemical Society
