885
Anal. Chem. 1984, 56,685-689
04),-8Hz0would be consistent with the solubility produd (log K , = -49) recently reported by Markovic and Pavkovic (8) in solutions of similar pH. The phosphorus content of solid potassium thiocyanate reagent, determined by inductively coupled plasma emission spectrometry, varied from 40 to 750 ppm. Although recrystallization is reported to produce high-purity potassium thiocyanate (9),a single recrystallization from water was not sufficient to purify the reagent for our application. The treatment with zirconyl nitrate described in the Experimental Section removed essentially all the phosphorus from the potassium thiocyanate solutions. Automation. The simplicity of the experimental procedure makes the method suitable for automation in an integrated system. We are currently automating the analysis for laboratory use and on-line analysis of process streams. Registry No. Al, 7429-90-5;Cr, 7440-47-3; Fe, 7439-89-6;Hg, 7439-97-6;Ni, 7440-02-0; Th, 7440-29-1; U, 7440-61-1; Pu, 744007-5; potassium thiocyanate, 333-20-0; nitric acid, 7697-37-2;
phosphorus, 7723-14-0; zirconyl nitrate, 13826-66-9.
LITERATURE CITED (1) Baumann, E. W. US. Department of Energy Report DP-1632, Aiken, SC, June 1982. (2) Rossotti, F. J. C.; Rossottl, H. J. Chem. Educ. 1985, 42, 375-378. (3) Baes, C. F.; Mesmer, R. E. "The Hydrolysis of Cations"; Wiley: New Yark. 1976. (4) S k n , L. G.; Martell, A. E. "Stability Constants" and "Stability Constants Supplement No. 1"; The Chemical Society: London, 1964 and 1971. (5) Meites, L., Ed. "Handbook of Analytlcal Chemistry"; McGraw-Hill: New York, 1963. (8) Davies, W.; Gray, W. Talanta 1964, 1 1 , 1203-121 1. (7) Brand, M. J. D.; Rechnitz, G. A. Anal. Chem. 1970, 42, 1172-1177. (8) Markovic, M.; Pavkovlc, N. Inorg. Chem. 1983, 22, 978-982. (9) Kolthoff, I. M.; Sandell, E. B.; Meehan, E. J.; Bruckenstein, S . "Quantitative Chemical Analysis", 4th ed.; Macmillan: London, 1969.
RECEIVED for review August 22, 1983. Accepted December 23, 1983. The information contained in this article was developed during the course of work under Contract No. DEAC09-76SR00001 with the U S . Department of Energy.
Surface Analysis by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy of Molybdenum-Doped Raney Nickel Catalysts Joseph C. Klein and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Surface analysls of Mo-doped commercial Raney alloys and actlvated catalysts has been carried out by uslng X-ray photoelectron spectroscopy (ESCA) and Auger electron spectroscopy (AES). The thickness of the oxide layer on the Raney alloys was observed to decrease wlth lncreaslng Mo content; the surface concentratlon of metallic Mo and Ai Increases with Increasing bulk Mo content. A maxlmum In the surface NI content for the actlvated catalysts was observed for - 2 % bulk Mo content. Thls correlated wlth actlvlty of the activated catalysts for reductlon of acetone. An Increase In actlvatlon temperature causes greater surface NJ content In the activated catalysts. Because the surface concentratlons of the activated Raney nlckei catalysts determined by AES dld not change after 10 min of actlvatlon and hydrogen was still belng evolved from the actlvatlon vessel, the activation Is complete on the surface wlthin 10 mln but Is still continuing below the surface monitored by AES.
than 2 wt % molybdenum is added to the bulk alloy, a decrease in catalytic activity is observed. Previous studies have shown that the total residual Mo content after NaOH activation remains constant for the doped Raney alloys (7). Also, the mean particle size of the activated catalysts decremes when 1-3% Mo is added to the Ni/A1 alloy; for greater than 3 wt % Mo, the particle size of the activated catalysts remains constant (7). These results do not demonstrate why the Mo-doped catalysts increase and then decrease in activity as the bulk Mo content is increased. An omitted area concerns the surface characterization of Mo-doped Raney alloys and their respective activated catalysts, to determine how the surface characteristics change as a function of bulk Mo content. T o investigate variations of surface properties, a surface study using X-ray photoelectron spectroscopy (ESCA) and Auger electron spectroscopy (AES) was carried out on Mo-doped h e y alloys and their respective activated catalysts.
Much work performed on Raney catalysts to date has focused on the variation of catalytic properties based on different activation parameters (I,2). Two variables which have been shown to alter the catalytic activity of b e y nickel catalysts are the temperature and time of activation ( 3 , 4 ) . Addition of a third component such as Mo to Raney alloys has been employed to enhance catalytic activity (5-7). However, doped Raney catalysts have displayed unpredictable and interesting changes in catalytic activity as a function of the original bulk concentration of the dopant (6). Molybdenum-doped Raney catalysts display significant promotion of activity a t low bulk levels (1-2 wt % Mo) (7). When greater
ESCA spectra were obtained by use of an AEI ES200 electron spectrometer with a DSlOO data system. An aluminum anode (A1 K a 1486.6 eV) was operated at 12 kV and 22 mA. The base pressure was below 2.0 X lo* torr. The digital data were processed with an Apple I1 Plus microcomputer. Binding energies were measured with a precision of k0.15eV referenced to the C 1s line at 284.6 eV. Overlapping peaks were deconvoluted with a nonlinear least-squares fitting routine (8). The ESCA relative sensitivity factors for Ni, Al, and Mo were determined from NiA1204 and Al2(MoO4),. Surface concentrations were determined by correcting integrated signal intensities with the experimentally measured sensitivity factors. Auger spectra were obtained with a Physical Electronics Model 545 Auger electron spectrometer using a 5-keV primary electron
EXPERIMENTAL SECTION
0003-2700/84/0356-0685$01.50/0
0 1984 American Chemical Society
666
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
Table I. Bulk Content of Mo-Doped Raney Nickel Alloys" sample
wt % A1
wt % N i
wt % M o
1 2 3
57.27 54.64 57.66 58.16 52.99
41.53 43.81 39.18 35.45 32.55
1.26 2.11 4.40 13.50
4 5 a
Bulk composition determined by atomic adsorption.
beam. The electron beam resolution is 5 pm. The operating pressure was below 5.0 X torr. Peak to peak heights of the Ni LMM and the A1 KLL lines were measured to determine the relative intensities of the signals. For AES depth profiles, 5.0 X 10" torr of argon was used with an ion beam voltage of 5 keV and an emission current of 30 mA. These settings produced a beam current density of 1 pA/cm2 measured by a Faraday cup positioned on the sample holder. Commercial Raney alloys were obtained from the Davison Chemical Division of W. R. Grace & Co. Five different alloys were used in this study, a control and four Mo-doped alloys. Table I lists the bulk concentrations of the alloys. Activation reactions were performed with 5 g of fresh Raney alloy and 100 mL of 20% NaOH. Activation reactions were carried out in a boiling water bath unless otherwise specified. The Raney alloy was added slowly and allowed to react at least 1 h for complete activation. The base solution was decanted and the catalyst was washed with deionized water until the wash water did not turn phenolphathalein red. The wet catalyst was placed in the insertion lock of a glovebox which was evacuated and back-filled with argon three times to remove the major portions of moisture. The catalyst was then placed in the glovebox and allowed to dry under argon. After drying, the catalyst was placed on a sealable probe in the glovebox. The probe was sealed and transferred to the ESCA spectrometer for analysis (9). Activation of the catalysts as a function of time and temperature was carried out on Raney nickel plates having the same composition as the control sample. Raney nickel plates were used for ease of mounting in the AES spectrometer. The activations were performed at three different temperatures (20 "C, 55 "C, and 100 "C) and six times (0,1, 10, 20,40, and 60 min). After activation the samples were washed with deionized water and dried in a vacuum desiccator. After drying, the samples were immediately placed in the Auger electron spectrometer for analysis. This method of sample handling will not prevent the surface from being oxidized during transfer. qowever, because no in situ system was available, this procedure sufficed to show the surface concentration effects caused by activation for different times and temperatures.
RESULTS AND DISCUSSION Mo-Promoted Raney Nickel Alloy and Catalysts. The Ni 2p3,2, Mo 3d, and A1 2p photoelectron peaks were used to determine the chemical states of Ni, Mo, and A1 on the surfaces of the Raney alloys. For all the alloys a Ni 2p3p photoelectron signal was observed at 852.2 f 0.15 eV which can be assigned to metallic nickel. Also, for the control (sample no. 1) a signal was observed a t 856.4 & 0.15 eV which corresponds to Ni2+(7). The partial surface oxidation of the control alloy is consistent with the ESCA results previously obtained for freshly polished NiA13 intermetallic (7). The control sample Ni/Al bulk concentration (41.5 wt % Ni) is approximately the same as NiA13 (42 wt % Ni). The Al2p photoelectron signal indicated that two different chemical states of A1 exist on the surface of the alloys except for the control (sample 1). Two A1 2p peaks a t binding energies of 74.4 f 0.15 eV and 71.8 f 0.15 were observed; these correspond to A1(3+) and A1(0), respectively. The control (sample 1) did not show metallic aluminum on the surface. The Mo 3d photoelectron spectra of the molybdenum-doped alloys showed that two different chemical states of Mo exist on the surface of the Mo-doped alloys. The two chemical
A! 2.11%
1 i 4.40%
13.50%
80
78
llNOlNO l N l R O Y / L V
Figure 1. AI 2p and Ni 3p ESCA spectra for Modoped Raney alloys. The polnts are the experlmental data. The solid line running through the points is the fitted total spectra. The solM line curves are the fitted
indlvldual peaks. states are Mo(6+) and Mo(O), having Mo 3dsIz binding energies of 232.2 f 0.15 eV and 226.6 0.15 eV, respectively. Because there is significant overlap between the photoelectron lines for the different chemical states of Mo and Al, and because the Ni 3p and Al2p lines overlap, it was necessary to curve-resolve the spectra to obtain accurate intensities of the individual components. Figures 1 and 2 show the curve-resolved A1 2p, Ni 3p, and Mo 3d ESCA spectra of the molybdenum-doped alloys. A few comments should be made about Figure 1to explain the different peaks in the spectra. The A1 2p lines for the oxide and metal are indicated by the vertical lines running through the peaks marked A3+and Al'. The Ni 3p region for nickel metal consists of four lines. The Ni line is designated by the vertical line marked Ni. The Ni 3pIl2line is the signal located between the A1 2p of Al(0) and the Ni 3p3/2 signals. The other two peaks located at lower binding energy than the Ni 3p lines are the A1 K a 3,4 X-ray satellite lines from A1(3+) and Al(0). The Al Ka 3,4 satellite from Al(0) is a t the lowest binding energy. The A1 Ka 3,4
*
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
687
-
Table 11. Relative Surface Concentration of Mo, Al, and Ni for the Raney Alloys' bulk Mo
consam- tent,* ple wt % 1 2 3 4 5
0 1.26 2.11 4.40 13.50
NiC
Mo
6.5 3.7 3.5 2.8 5.6
0.1 0.8i 0.3 1.0 f 0.3 1.9 * 0.3
0.5
i
i i t i: i
4.5 2.9 2.3 2.1 3.3
AI 93.4 i 95.8 i 95.7 i 96.2 i 92.5 i
4.5 2.8 2.0 2.4 3.0
The relative surface concentration is in %, Le., Mol Bulk composition determined by The uncertainty atomic absorption; mesh size -140. values associated with the surface concentration are a result of three analyses of separate samples of the same bulk concentrations. a
( M o t A1 t Ni).
k,
si 0.00
2.00
4.00
6.00
8.00
10.00
12.00
WTZ MO I N BULK
Figure 3. Relative Intensities of the AI metal to the total AI ESCA signal for Mo-doped Raney alloys.
240
236
232
228
224
BlNDlNO ENEROY / E V
Figure 2. Mo 3d ESCA spectra for MAoped Raney alloys. The points are the experimental data. The solid llne running through the points is the fHted total spectra. The solid llne curves are the fled individual peaks.
X-ray satellite signal is about 11% of the intensity of the A1 K a 1,2 X-ray line. The Mo 3d312,512spectra in Figure 2 also have A1 K a 3,4 satellite lines in the spectra. The Mo 3d spectra of the alloy consist of two sets of doublets, Mo 3d312,slzfor Mo(6+) and Mo(0). The vertical lines in Figure 2 marked Mo(6-F) and Mo(0) are the positions of the Mo 3d5I2line for each chemical state. The extremely weak signals under the Mo 3d5I2signal of Mo(0) and at lower binding energies are the A1 Ka 3,4 satellites. Table I1 summarizes the relative surface concentrations of Ni, Al, and Mo for the Mo-doped alloys measured by ESCA. The relative concentrations were calculated by correcting the signal intensities using experimentally measured sensitivity factors for the Mo 3d, Ni 3p, and A1 2p photoelectron lines. The surface concentrations show that A1 is the most concentrated species on the surface of the alloys. Also, the molybdenum surface concentration p a d d y increases as the Mo bulk content increases. The Al2p and Ni 3p spectra of the Mo-doped Raney alloys (Figure 1)indicate that the amount of Al metal on the surface
0 00
2 00
4 00
6 00
8 00
10 00
12 00
WTZ MO I N BULK
Flgure 4. Relative Intensities of the Mo metal to the total Mo ESCA signal for Modoped Raney alloys.
increases as the bulk Mo content is increased. The increase in the amount of surface Al(0) is observed in Figure 1 by the growing intensity of the Al(0) signal for the alloys with greater Mo bulk content. Figure 3 is a plot of the percent A1 metal on the surface of the alloys as a function of bulk Mo. The increase in surface concentration of A1 metal as the bulk Mo content of the alloy increases is clearly evident. The Mo 3d spectra of the doped alloys also show an increase in metallic Mo as the bulk Mo content increases. Figure 2 illustrates this point. The intensity of the Mo 3d doublet of Mo metal increases relative to the Mo 3d doublet of Mo6' for the alloys with greater bulk Mo content. This result is better shown in Figure 4 which is a plot of the relative Mo metal signal to the total Mo ESCA signal. The ESCA results for Mo-doped Raney alloys indicate that the surface becomes more metallic with increasing bulk Mo
668
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984 N
c $ ?
0"s 0 I
w
L
0 00
& ... l . . . . . . . . . . . . . . . . . . . , . . . , . . . . . . , . . . . . . . , . . . . . . . . . . . . . . . . . . . . . ~
L
ii LUL
3 -i "
1 .
5
f 4.0
WEIGHT 7.
Flgure 8. Auger depth
6.0
0.0
10.0
I 00
6 00
8 00
10 00
12 00
MOLY I N BULK ALLOY
Figure 7. ESCA results of activated Mo-doped Raney catalysts: (0) NI; (R) AI; (A)Mo.
".B il..
2.0
2 00
WT%
I
:2.0
MO I N BULK A L L O Y
profile results of Modoped Raney alloys: ( 0 )
Ni LMM determined oxide Interface; (R) 0 KLL determined oxlde In-
terface. content. This is shown by the absence of oxidized nickel in the doped alloys and the increasing percentages of metallic A1 and Mo. The increase in metallic character of the surface with an increase in bulk Mo content requires that the oxide film be thinner for alloys with greater bulk Mo content. The thickness of the alloy oxide films was probed by Auger depth profiling the series of Mo-promoted alloys. Figure 5 shows a typical Auger depth profile of the 2.11 wt % Mo alloy. Figure 5 is a plot of the peak to peak heights of 0 KLL, Ni LMM, A1 KLL, and Mo MNN Auger lines as a function of sputtering time. The approximate time at which the ion beam sputtered through the oxide layer is determined at the point where the oxygen signal stopped decreasing and the nickel stopped increasing in intensity. By measurement of the time needed to sputter to the interface between the oxide and the underlying element (i.e., Ni) an estimate of the relative oxide thickness for the different alloys can be obtained (8). Figure 6 is a plot of sputtering time to the oxide interface as a function of bulk Mo content of the alloys. The results in Figure 6 show that the oxide thickness decreases by the decrease in sputtering time to the metal oxide interface with increasing Mo content. The Ni and 0 Auger signals were used to determine the oxide interface because each displays a significant concentration gradient between the oxide layer and the bulk. The oxygen signal decreased in intensity while the nickel signal increased in intensity during ion etching. Inspection of the depth profile could infer that the oxide interface was never
reached because the oxygen signal did not completely disappear during sputtering. The oxygen signal in this study could not be completely removed because the depth profiles were obtained on powdered samples, which have an oxide layer around the powder particles. Powders can be considered in general to have a spherical shape for all concentrations of Mo, which was observed by the use of an optical microscope. When sputtering a spherical powder the area exposed to the ion beam will have the oxide layer removed. The sides and underlying section of the powder which are shielded from the ion beam will not have the oxide film removed. Therefore, even though the section of the powder not shielded from ion beam had the oxide film removed during sputtering, AES analysis will still detect the oxygen from the oxide film on the sides and underlying section which was not eliminated by ion sputtering. While sputtering a powder with an oxide layer, the oxygen AES signal will decrease and the underlying element (i.e., Ni) AES signal will increase because of the removal of the oxide film from exposed surface. After the oxide film is removed from the area facing the ion beam, the AES intensity of the oxygen and Ni will become constant. Therefore, the oxide interface is determined for powders a t the point when the oxygen signal decreases and the nickel signal increases to a constant intensity. MQ and A1 signals were not used to determine the oxide interface because they displayed no significant intensity change as a function of sputtering time. This effect has been reported previously for A1 when sputtering Ni/A1 alloys (IO). The surfaces of the activated Mo-doped Raney catalysts were examined by ESCA. The relative concentrations of the surface species of the activated catalysts after leaching in 20% NaOH for 1h are plotted in Figure 7 . The surface concentrations of the activated catalysts were determined in the same manner as for the Raney alloys. The ESCA results of the activated catalysts (Figure 7 ) indicate that the surface concentrations of Ni, Al, and Mo vary as a function of bulk Mo content in the unactivated alloys. At low Mo bulk content of the original alloy (1-2 wt %), the activated catalyst has greater amounts of Ni on the surface than the unpromoted activated catalyst. At 2 wt % Mo of the original alloy, the nickel surface concentration for the activated catalysts reaches a maximum. At greater than 2 wt % Mo in the original alloy, the nickel surface concentration of the activated catalysts begins to decrease. Also, the amount of Mo on the surface of the activated catalysts increases linearly with greater Mo bulk content of the original alloy. The binding energy for Mo 3d5,2for all the alloys was 232.4 i= 0.15 eV which corresponds to Mo(6+) oxide. This result indicates the Mo is being oxidized during the activation reaction. Figure 7 indicates that the amount of A1 on the surface of the activated catalysts decreases and becomes essentially constant at greater than -2 wt % Mo bulk content of the original alloys. The A1 2p
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
689
r
0 0
10 0
20 0
30 0
40 0
50 0
60 0
T I M E 3F L E A C H I N G l h MINUTES
-_ - _ _ ---- - - - - -- ----_ ---------Ref. Catalyst
Flgure Q. AES results of different activation times. Activations were carried out at 55 OC in 20% NaOH on 42 wt % Ni Raney alloys.
al. have also indicated that the bulk composition of the activated catalysts contains more nickel at high activation temperatures (100 "C) (4). The time dependence of activation is illustrated in Figure 9. Figure 9 is a plot of the relative nickel LMM Auger intensity to the total Ni LMM -t A1 KLL Auger intensity for activations carried out at 55 O C for various times. Figure 9 shows that a constant amount of nickel on the surface is obtained within 10 min of activation. However, complete activation was not obtained within 10 min because evolution of Hzgas from the activation vessel was still observed up to 1h of activation. This result indicates that the AES is only sensitive to the activation reaction occurring on the surface which is completed in a short period of time (Le., 10 min). However, because Hzgas is still being generated after 10 min, the leaching reaction continues to tunnel into the bulk of the alloy forming a skeletal type structure. The inner surfaces of the porous activated catalysts are obscured from the electron beam of Auger analysis. Therefore Auger analysis will only monitor the outer physical surface of the activated Raney catalysts.
ACKNOWLEDGMENT The authors wish to thank Stuart Montgomery of W. R. Grace, Inc., for supplying the Mo-doped Raney catalysts and his helpful discussions. Registry No. A1 57.Ni 42 alloy, 88610-60-0;A1 55.Ni 44.Mo 1 alloy, 88610-61-1;A1 58.Ni 39.Mo 2 alloy, 88610-62-2;A1 58.Ni 35.Mo 4 alloy, 88685-53-4; A1 53.Ni 33.Mo 14 alloy, 88610-63-3; acetone, 67-64-1.
LITERATURE CITED Robertson, S. D.; Anderson, R. B. J . Catal. 1976, 41, 405-411. Ishikawa, J. Nippon Kagaku Zasshi 1960, 87, 837-842. Ishlbawa, J. Nippon Kagaku Zasshi 1960, 81, 1179-1187. Fred, J.; Robertson, S. D.; Anderson, R. 8. J. Catal. 1970, 18, 243-248. (5) Paul, R. Bull. SOC. Chlm. F r . 1946, 73,208-211. (8) Lyubarski, G. D.; Ivanovskaya, L. N.; Isaeva, G. G.; Lainer, D. I.; Kagen, N. M. Kinet. Katal. 1960, 1, 385-392. (7) Montgomery, S. R. I n "Catalysis of Organlc Reactions"; Moser, W. R., Ed.; Marcel Dekker: New York, 1981; pp 383-409. (8) Proctor. A.; Sherwood, P. M. A. Anal. Chem. 1980. 52,2315-2321. (9) Patterson, T. A,; Carver, J. C.; Leyden, D.E.;Hercules, D.M. J. Phys. Chem. 1976, 80, 1700-1708. (IO) Klein, J. C.; Hercules, D. M. Anal. Chem. 1981, 53,754-758. (11) Freel, J.; Pieters, W. J. M.; Anderson, R. B. J . Cafal. 1969, 74, 247-256. (1) (2) (3) (4)
RECEIVED for review January 5,1983. Resubmitted December 2, 1983. Accepted December 27, 1983. This work was supported, in part, by the National Science Foundation under Grant No. CHE-8108495.
