J. Phys. Chem. 1984, 88, 3209-3215
Spectroscopic Characterization of ThNi, Fe,,
3209
Intermetallic Catalysts
Tuan A. Dang,t Leonidas Petrakis;t and David M. Hercules*t Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, and Gulf Research and Development Company, Pittsburgh, Pennsylvania 15230 (Received: October 14, 1983)
ThM5 (M = Fe, Ni) compounds have a hexagonal structure with two nonequivalent crystallographic sites (2c and 3g) for the M atoms. Replacement of Fe by Ni in ThFe5 shows strong Ni preference for the c sites. It has been shown by ESCA, AES, and static SIMS that the reactivities of the transition metals in these two sites are different. For example, the g-site Ni is found to segregate to the surface more easily and to react with oxygen more readily. Oxidation of ThNi,Fe5_, in air at 350 OC for 24 h causes complete transformation of the original alloy structure. The decomposition products are composed of Thoz, Ni, NiO, Fe203and an NiFe alloy. The surfaces of these oxidized ternary alloys are Fe rich and Ni poor with respect to the bulk. Comparisonbetween the methanation activities and the relative surface Ni concentrationsof the ThNi,Fe5, intermetallic catalysts shows a correlation between the two. Catalysts having larger Ni surface concentrations generally possess higher activity for CO hydrogenation.
Introduction
TABLE I: Percent Nickel Oxide in Total Ni for ThNi-Fe,, Allovs
Treatment of a binary intermetallic, consisting of a rare-earth or an actinide element and a first-row transition metal, with O2 or synthesis gas (CO/H,) forms very active supported catalysts.'" For example, treatment of ThNi5 in O2 or CO/H2 results in Ni/Th02, which is more active than conventional supported Ni catalysts prepared by wet chemical techniques.' Use of this new method for the preparation of bimetallimupported catalysts would be desirable since addition of a second metal can affect significantly the catalytic activity and s e l e ~ t i v i t y . ~ ~ ~ In the present work, we report the use of electron spectroscopy for chemical analysis (ESCA), Auger electron spectroscopy (AES), static secondary ion mass spectroscopy (SIMS), and electron microscopy to study a series of ThNi,Fe5, alloys (x = 0-5) before and after oxidation. Correlation between the results of surface characterization and catalytic activity of these ternary catalysts is also included. Experimental Section
The alloys were prepared by melting a stoichiometric mixture of the metals in a water-cooled copper boat under argon. ThNi,FeS-, (x = 2-5) alloys were prepared without special procedures. A special annealing step was required for ThNiFe, and ThFe5. These samples were sealed in an evacuated ( torr) quartz tube and heated at 1000 OC for 4 days for ThNiFe, and 1125 OC for 2 weeks for ThFe5.6 The crystal structures of all alloys were confirmed with a Diano X-ray diffractometer. The intermetallics were then ground to a fine powder (100-200 pm). Studies of the fresh intermetallics were carried out immediately after grinding to avoid extensive air oxidation. Oxidation of the alloys was achieved by heating the alloy powder under air in a furnace at 350 OC for 24 h. Reduction treatments of the alloys were carried out in a flow-type microreactor. ESCA spectra were obtained with an AEI ES-200A electron spectrometer equipped with an aluminum anode and an AEI DS-100 data system. The spectrometer was operated at 12 kV and 22 mA; the base pressure was below torr. An Apple I1 microcomputer was used to process the digital data from the DS-100 data system. Binding energies were referenced to the C 1s line at 284.6 eV. Sealable ESCA probes were used to transfer air-sensitive samples from the reaction chamber to the spectr~meter.~ Auger analysis was carried out using a Physical-Electronics Model 545 Auger electron spectrometer. The energy and emission current of the primary electron beam were set at 5 keV and 2 mA, respectively. The base pressure was below torr. SIMS studies were performed at the Center for Research in Surface Science (CRISS) (Montana State University) using a University of Pittsburgh. *Gulf Research and Development Co.
0022-3654/84/2088-3209$01.50/0
alloy ThNiFe, ThNi2Fe2 ThNi3Fe2 ThNi4Fe ThNi5
NiO, % untreated treated 20 35 45 50 58
80 82 80 87 100
TABLE I 1 Surface Concentrationsof ThNi,Fe+, Catalysts
compd ThNi5 ThNiiFe ThNi3Fe2 ThNizFe3 ThNiFe, ThFe5
oxidized Ni/Th" Fe/Th" 13.2 (12) 3.0 (2.0) 1.4 (1.1) 0.80 (0.80) 0.53 (0.34)
5.0 (2.3) 5.4 (3.4) 5.6 (4.7) 5.7 (4.8) 11 (10)
oxidized and reduced Ni/Th" Fe/Th" 3.8 1.3 0.50 0.30 0.060
1.o 1.6 2.1 2.4 4.2
"Atomic ratios obtained from ESCA data. Numbers in parentheses are calculated from AES data. Leybold-Heraeus Model SSB/ 10 secondary ion mass spectromtorr was obtained. The eter. A base pressure below 5 X instrument was back-filled with argon to 5 X lo-' torr and operated with a beam voltage of 2.5 keV; the current density was 8 nA/cm2. A JEOL JSM 35C scanning electron microscope was used to obtain the electron micrographs. The electron beam was set at 25 kV. Chemical composition of the samples was identified by a Kevex energy-dispersive X-ray spectrometer (EDX) attached to the microscope. Results
Untreated Alloys. Surface characterization of the untreated alloys was done using ESCA, AES, and SIMS. ESCA binding energies of the elements on the surfaces of the fresh alloys were (1) Chin, R.; Elatter, A.; Wallace, W. E.; Hercules, D. M. J . Phys. Chem. 1980,84, 2895.
(2) Imamura, H.; Wallace, W. E. J. Catul. 1980, 65, 127. (3) Baglin, E. G.; Alkinson, G.; Nicks, L. Ind. Eng. Chem. Prod. Res. Deu.
1981, 20, 87.
(4) Wallace, W. E. CHEM TECH 1982, 12, 752. (5) Dang, T. A.; Petrakis, L.; Kibby, C.; Hercules, D. M. j . Cutal., in press. ( 6 ) France, J. Ph.D. Thesis, University of Pittsburgh, 1982. (7) Sinfelt, J. H. Acc. Chem. Res. 1977, 10, 15. (8) Unmuth, E. E.; Schwartz, L. H.; Bult, J. B. J . Cutal. 1980, 61, 242. (9) Ng, K. T.; Hercules, D. M. J . Phys. Chem. 1976, 80, 1094.
0 1984 American Chemical Society
3210
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 Ni2+
~
i
Dang et al.
’
ESCA
Ni 2 p 3 , 2 0 f
a
Untreated Alloys
\(
c i
u
ThNi5
... ...
Th N i 4 F e
‘\
T h Ni3 Fez
L,
I
I
1
0
I
2
3
i
4
5
X
ThNil Fe3
T hNi, Fe 5 - x ThNi Fed
b
A ES
I
8 67
I
1
86 3
I
I
859 BINDING
I
I
855 ENERGY,
85 1
*”
Figure 1. ESCA spectra of the Ni 2pjjZregion for untreated ThNi,FeSFx alloys.
compared with those of known chemical compounds for chemical-state determination. Examination of the Ni 2p3/2, Fe 2pSi2, and Th 4f,,* lines indicates the existence of Ni, NiO (Ni 2p3j2 = 852.2 and 854.4 eV), Fe, F e 2 0 3(Fe 2p3I2,= 706.7 and 710.4 eV), and T h o z (Th 4fTj2= 333.6 eV) as major surface species. The percent of metal oxide for each element was determined by comparing the measured spectra with composite spectra derived from known combinations of metal and metal oxide spectra.1° An accuracy of *4% (absolute) can be achieved by using this method. Although the fraction of Fez03 in the total surface Fe did not vary significantly with bulk Fe concentration (85 4%), the percent of surface NiO increased with increasing bulk Ni content as shown in Figure 1 and Table I. Quantitative ESCA results were achieved using integrated peak areas for the Th 4f7/2,Ni 2 ~ 3 /and ~ , Fe 2p,,* lines. There is some overlap between Th 4d3/2and Fe 2p3/*,thus the use of Fe 2pli2. Surface concentrations of untreated intermetallics were calculated from ESCA data by using Scofield photoionization cross sections.ll These data are presented in Table 11. A plot of the atomic ratios, M/Th (M = Fe, Ni) vs. substitution coefficient x in ThNi,Fe5-, is shown in Figure 2a. All untreated intermetallics were transition-metal poor on the surface with respect to the bulk. This is clear because all points fall below the dashed diagonal lines in Figure 2a, which correspond to equal surface
..
\
w-.
*
(10) Proctor, A,; Hercules, D. M . , submitted for publication in Appl. Spectrosc. ( 1 1 ) Scofield, T . H . J . Electron Spectrosc. Relar. Phenom. 1976, 8, 129.
1
2
3
4
5
X T h N i , Fe5-, Figure 2. Atomic ratios calculated from (a) ESCA and (b) AES data for ThNi,FeS_xalloys before and after oxidation: 0,Ni/Th untreated; 0 , Ni/Th oxidized; 0,Fe/Th untreated; W, Fe/Th oxidized. The diagonal lines indicate equal surface and bulk concentration.
and bulk concentrations. Both Fe and Ni were found to increase linearly with increasing bulk content of the respective metal. No discontinuity was observed in the Fe/Th curve. However, the
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3211
ThNixFe5-x Intermetallic Catalysts
ThFe5
SIMS
2 8 i
\
R 0
1
2 X ThNi,
3
4
10pm
5
FeS-,
Figure 3. SIMS intensity ratios for ThNi,Fe+, alloys before and after
oxidation: 0,Ni/Th untreated; 0 , Ni/Th oxidized; 0,Fe/Th untreated; m, Fe/Th oxidized. I IIIYI
5
Figure 4. Electron micrographs of (a) untreated and (b) oxidized ThNiS. The bar corresponds to 10 hm.
curve for the Ni/Th atomic ratio shows a distinct change of slope a t ThNi3Fe2. The AES intensity ratios, M/Th, were calculated from peak to peak heights of Ni LMM, Fe LMM, and Th NOV Auger lines.
Figure 5. Electron micrographs of (a) untreated and (b) oxidized ThFe,. The bar corresponds to 10 wm.
They were converted to atomic ratios (Table 11) by using sensitivity factors supplied by the Physical Electronic Division of PerkinElmer and then plotted against x as shown in Figure 2b. The AES results are consistent with the ESCA data. Surface enrichment of Th was observed for all untreated alloys. A straight line was obtained for the Fe/Th curve, and Ni/Th was again found to increase linearly and then show a larger increase after ThNi3Fe2. Both ESCA and AES data were confirmed by static SIMS data as shown in Figure 3. SIMS quantitative analysis shown in this figure was achieved by measuring peak heights of the Fe+ ( m / z 5 6 ) , Ni+ ( m / z 58), and Th+ ( m / z 242) peaks. Figures 4 and 5 show typical electron micrographs of the untreated intermetallics. As can be seen in Figures 4a and 5a, the intermetallics have smooth or polished surfaces with only a few distinct particles evident. Chemical compositions of the alloys were determined by using EDX. Quantitative EDX results were carried out using peak heights of the X-ray lines of Th (3400 eV), Ni (7450 eV), and Fe (6400 eV). The intensity ratios M/Th (M = Ni, Fe) were constant across the sample (f 10%) and did not vary even when small surface particles were examined. A plot of EDX intensity ratio, M/Th ( M = Ni, Fe), vs. the substitution coefficient x in ThNixFe5, (Figure 6) shows that both Ni and Fe signals are linear with their corresponding bulk concentrations. Treated Alloys. Treatment of the alloys in air at 350 O C for 24 h resulted in complete transformation of the original intermetallic structure. X-ray diffraction (XRD) lines corresponding to the intermetallic were not found in any oxidized sample. yFe203and T h o 2 were detected in XRD of oxidized ThFe,; Tho2, primarily Ni, and some NiO were observed in oxidized ThNi5. In the case of the ternary alloys, ThNi,Feq-, (x = 1-4). XRD line assignments were n i t straightforward.- Although 'Tho2, y-Fe203,and NiO were observed in all cases, a peak at 28 = 44.5 f 0.1 was observed, corresponding to Ni, Fe, or FeNi alloy diffraction lines or some combination of them (Ni, 44.5O; Fe, 44.6O; FeNi alloy, 44.5-44.6O). Surface characterization of the treated alloys was carried out using ESCA, AES, and static SIMS. Chemical states of the
3212
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 EDX
Dang et al.
[Untreated A l l o y s ]
, a
0
X
T h N I X Fe,-,
Figure 6. EDX intensity ratios for untreated ThNi,Fe,, Fe/Th; A, Ni/Th. N i 2P3/2
alloys:
0,
Ni 0 NI
C
d
e
Figure 8. SIMS spectra of ThNi,Fe2 as a function of sputtering time: (a) no sputtering (current density 8 nA/cmZ);(b) after 20 rnin of sputtering at a current density of 8 nA/cmZ (removal of 0.01 A/min);. (c) after 105 rnin of sputtering under conditions similar to (b); (d) conditions similar to (c) + 15 rnin of sputtering at a current density of 1.2 pA/cm2 (removal of 1 A/min); (e) conditions similar to (c) + 25 min of sputtering under conditions similar to (d).
L Q
ThNi4Fe
(ox.)
ThNiZFe3 ( o x . )
"Current density of sputtering ion beam is 5 nA/cmZ. bCurrent density of sputtering ion beam is 1.2 pA/cmZ. Corrected for preferential sputtering.
I
-
in H 2
869
863
857
TABLE III: SIMS Intensity Ratio Fe/Ni for Different Sputtering Times approx depth condition I J INi removal. .A no sputtering 15 0 20 rnin" 11 2 105 mid 9.4 10 105 mid + 15 minb 9.4c 25 105 min" + 25 minb 9.4c 35
851
B I N D I N G ENERGY, e v
Figure 7. ESCA spectra of the Ni 2p,,* region for ThNixFe5, catalysts. The alloys were oxidized in air at 350 OC for 24 h. Reduction of the oxidized alloys was carried out at 400 "C for 3 h with H2. elements present on the surfaces of the intermetallic catalysts were detemined by ESCA. Th exists as T h o 2 (Th 4f7/2= 333.6 eV) for all samples; Fe is present as Fe203(Fe 2p3/, = 710.4 eV), and Ni as NiO (Ni 2p3/*= 854.5 eV). In the case of the ternary alloys (x = 1-4), an additional peak corresponding to metallic Ni (Ni = 852.2 eV) was observed as shown in Figure 7. In general, the metallic Ni peak was weaker in alloys containing higher percentages of nickel. The percentages of NiO in the treated alloys
are shown in Table I. NiO varied from 80% to 100%. A plot of the atomic ratio, M/Th, calculated from ESCA data vs. x for the treated alloys (Figure 2a) shows that the transition metals (Ni and Fe) segregate to the surface when the alloys are oxidized, as evidenced by increases in the M/Th atomic ratios for the oxidized alloys. This is also shown in Table I1 which includes the atomic ratios of the oxidized alloys. The surface concentrations of Fe in all of the treated alloys are significantly larger than those of the bulk. On the other hand, except for ThNiS which is Ni rich, the surfaces of the other intermetallics are Ni poor with respect to the bulk even after oxidation. In all cases, the surface concentrations of Fe and Ni increase with increasing bulk metal content. ESCA results were confirmed by both AES (Figure 2b, Table 11), and static SIMS (Figure 3). In both cases, surface segregation of metals was observed for the treated alloys. Both AES and SIMS intensity ratio (M/Th) curves showed trends similar to those observed by ESCA. A SIMS depth profile study of the treated alloys was carried out; the results for ThNi3Fe2are shown as an example in Figure 8. When oxidized ThNi3Fe2 was sputtered by a low current density ion beam (8 nA/cm2) for 20 rnin (removing approximately
ThNi,Fes-, Intermetallic Catalysts
Th Ni4 F e \oxidized I
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3213 TABLE IV. Comparison of Ni Site Occupancy with Surface Oxidation for Untreated Alloys no. of Ni atoms chemical form alloy ThNiFe, ThNizFe3 ThNi3Fez ThNi,Fe ThNi, a
oxide 0.20 0.70 1.3 2.0 2.9
metal 0.80 1.3 1.7 2.0 2.1
site occupancy' g sites 0.26 0.48 1.o 2.0 3.0
c sites 0.74 1.12 2.0 2.0 2.0
From ref 16.
c 121
1 A' 0 Th
0
F e , NI
Figure 10. Hexagonal structure of ThMS (M = Fe, Ni). Reprinted with permission from ref. 13. Copyright 1951 International Union of Crystallography.
Figure 9. Electron micrographs of ThNi,Fe catalysts: (a) at 3000X magnification (the bar corresponds to 10 pm); (b) a t lOOOOX magnification (the bar corresponds to 1 pm).
2 A), the Fe signal decreased significantly while the Ni signal was essentially unchanged (Figure 8). The intensity ratio ZFe/ZNi was found to decrease from 15 to 11 after 20 min of sputtering (Table 111). Additional sputtering (105 min, removing approximately 10 A) caused further removal of Fe, which resulted in a further decrease of ZF,/ZN, (9.4) (Figure 8c, Table 111). The alloy was then subjected to a higher ion current density (1.2 bA/cmZ) for 15 min (removing a total of 25 A) and 25 min (removing a total of 35 A). Although both Fe and Ni signals were found to decrease significantly (Figure 8d,e), the Fe/Th intensity ratios after correcting for relative sputtering yields l 2 were found to be unchanged (9.4) (Table 111). The oxidized catalysts were then subjected to hydrogen treatment for 3 h at 400 "C. The surfaces of the reduced catalysts were composed of T h o 2 (Th 4f7/*= 333.6 eV), Ni" (Ni 2p3/, = 852.2 eV), and Fe" (Fe 2p3i2= 706.7 eV) (Figure 7). The atomic ratios, M/Th, of these catalysts were found to decrease slightly after H2 reduction as a result of sintering (Table 11). Electron micrographs of the oxidized alloys are shown in Figures 4b, 5b, and 9. Although the sizes of the oxidized particles were similar to those of the original intermetallics, their morphology changed significantly. In the case of the ThNi5 (Figure 4b), small nodules (0.5 Fm) were observed on top of the polished surface. Figure 5b shows an electron micrograph of oxidized ThFeS. In contrast to the nodule structure observed for ThNi5, the surface of oxidized ThFeS showed the presence of fine needles. These needles were also found on the surfaces of all ternary ThNi,Fe,, alloys ( x = 1-4). N o nodules were detected in any of the oxidized (12). Carter, G.; Colligon, J. S. "Ion Bombardment of Solids"; Heinemann Educational Books, Ltd.: London, 1968; Chapter 7.
ternary samples even in the nickel-rich system ThNi4Fe as shown in Figure 9. The chemical composition of the oxidized particles was determined by EDX. The EDX intensity ratios IM/Im showed large variation across the sample. Nodules (needles) usually showed larger relative intensities of Ni (Fe) than did the polished region. In general, the ZM/Zm EDX intensity ratios of the oxidized alloys were significantly higher than those of the corresponding fresh alloys. A NiFe alloy (4/1) oxidized in air at 350 "C for 24 h was also subjected to ESCA analysis. Examination of the Ni 2~312and Fe 2p3l2regions of this sample showed two types of Ni with binding energies corresponding to Ni" and NiO (Ni 2~312= 852.2 and 854.5 eV); only Fe corresponding to Fez03 was detected (Fe 2~312 = 710.4 eV). In addition, the fraction of surface NiO formed is 85% and the atomic ratio Ni/Fe is 1.6. Thus, significant segregation of Fe to the surface was obtained. These data are compared with similar data for ThNi4Fe, as shown in Table V.
Discussion Untreated Alloys. The intermetallic compounds ThNixFeS-x form a series of complete solid solutions. They all have the hexagonal CaCus structure with P6/mmm symmetry as illustrated in Figure 10. In this structure there are two nonequivalent crystallographic sites, c and g, for the transition metals.13 The ratio of c sites to g sites in a unit cell is 2/3. The c sites lie in the plane of the Th atoms, and the g sites occur in layers containing only transition metals, located between the Th ~ 1 a n e s . l ~ It has been found that the distribution of transition metals between the c and g sites is not random. Rather, there is preferential occupancy of the c sites by transition metals in the order Ni > Co > Fe.16*17 Elemans and BuschowI6 calculated the (13) Wilson, A. J. C.; Barrett, C. S.; Bijvoet, J. M.; Robertson, J. M. "Structure Reports for 1947-1978"; International Union of Crystallography, N. V. A. Oosthock's Viteevers MIJ: Utrecht. Netherlands. 1951: Vol. 11. (14) Van Diepen, A. h,;Buschow, K. H. J.; Wieringen,' J. A&. Phys. 1972, 43, 645. (15) Lemaire, R. Cobalr (Engl. Ed.) 1966, 32, 132.
3214
The Journal of Physical Chemistry, Vol, 88, No. 15, 1984 0
Csite
A
g s~te
...
-
0
1
Ni Fe
2 3 X ThNi,Fe5-x
4
5
Figure 11. Site occupancy for ThNi,Fe5-x alloys. Reprinted with per-
mission from ref 16. Copyright 1976 Academic Press. TABLE V: Comparison of ESCA Results of Oxidized NiFe (4/1) and Oxidized ThNi,Fe Catalyst
NiFe (4/1) % of Fe20, % of NiO % of Ni
Ni/Fea
100 87 15
1.6
ThNiPFe 100 85 13 1.7
Atomic ratio. number of transition metals in c and g sites for the Th Ni,Fe5, system. The results are plotted in Figure 11. As can be seen, replacement of Fe by Ni in ThFes shows strong preference of Ni for the c sites. In ThNi3Fezall c sites are filled with Ni; thus, the remaining Ni in ThNi4Fe and ThNi, can go only into the g sites. Since the atomic environments of the c and g sites differ, the two sites may be expected to behave differently. Different physical properties have been reported for c and g sites.14J5 For example, the Fe Mossbauer spectrum of ThFe5 exhibits two distinct hyperfine fields, one for c sites and one for g sites, with g sites having a hyperfine field 25% larger than that of the c sites. Therefore, it is reasonable that the chemical properties of the two sites might differ also. Since the untreated alloys have been exposed to air at room temperature, their surfaces have been oxidized. Comparison of the NiO fraction formed on the surfaces of the untreated alloys with the number of Ni atoms in g sites shows a strong correlation between the two. From comparison of Table I and Figure 1, it is clear that the percent of Ni involved in NiO formation increases with increasing bulk Ni content. Since only the number of g-site Ni atoms increases with increasing bulk Ni concentration over the range of x = 0-5 for ThNi,Fe5-x, it is readily seen that more Ni in g sites results in more NiO formation. Stated differently, the g sites are more reactive to oxygen than the c sites. A more quantitative correlation can be obtained by assuming that Ni in the c sites of the untreated alloys remains as Ni metal while all Ni in the g sites is oxidized to NiO. On the basis of this assumption, the number of Ni metal atoms can be related to c sites and the number of Ni atoms present as NiO can be related to g sites, as shown in Table IV. As can be seen, these numbers agree excellently with those calculated by Elemans and Buschow.16 Because Ni g sites are more reactive than Ni c sites, Ni in the g sites should segregate to the surface more readily. Indeed, the number of g-site Ni atoms is found to correlate very well with (16) Elemans, J. B. A. A.; Buschow, K. H. J. Phys. Status Solidi A 1976, 34, 355. (17) Atoji, M.; Atoji, I.; Do Dinh, C.; Wallace, W. E. J . Appl. Phys. 1973,
dd . ., 1_1 _ .
(18) Buschow, K. H. J.; Brouha, M.; Elemans, J. B. A. A. Phys. Starus Solidi A 1978, 30, 177. (19) Elemans, J. B. A. A,; Buschow, K. H. J. Phys. Sratus Solidi A 1974, 24, 393.
Dang et al. the surface concentration of Ni. As can be seen from Figures 2, 3, and 11, the Ni/Th atomic ratios (or intensity ratios) measured from ESCA, AES, and SIMS and the number of Ni atoms in the g sites show similar variation with bulk Ni content. They all increase linearly with x and then drastically increase after x = 3. Note that the c-site Ni curve also increases linearly with x but levels off after x = 3. It is also worth mentioning that surface enrichment of Th was observed for all untreated alloys. This surface segregation of Th should not affect the change in slope of the Ni/Th curve observed by all techniques (ESCA, AES, an SIMS), since an equivalent amount of Th is present in all alloys. Thus, the influence of Th should be similar for all alloys. In contrast to the case of Ni, although the number of Fe atoms in g sites increases with increasing bulk Fe content, there is no significant difference in the fraction of Fez03 formed between alloys (85 f 4%). Since the affinity of Fe for oxygen is significantly greater than that of Ni (A"f(FeZO3) = -100 kcal/(g mol); AHf(NiO) = -58 kcal/(g mol)), it would be anticipated that a larger fraction of the Fe would be oxidized. Note that 85% Fe203 was observed even in low Fe compounds like ThNi4Fe. As a result, the difference in reactivity of c and g sites would cause only a small change in the amount of Fez03 formed which is within the experimental error of our measurements. Since the difference in reactivity of c and g sites causes an insignificant effect on the reactivity of I; 2 for oxygen, it would not influence the surface segregation of Fe. Therefore, a linear relationship between Fe surface concentration and bulk Fe content was observed as shown in Figures 2 and 3. Note that disruption of the hexagonal structure of the ThNi,Fe5, alloys due to the different reactivities of c and g sites is only a surface phenomenon, since results of EDX analysis (sampling depth of 1 pm) show linearity for both Ni/Th and Fe/Th ratios over the entire region ( x = 0-5). Treated Alloys. Decomposition Products. ThNiXFe5-, ( x = 0-5) intermetallics were found to undergo complete transformation when they are treated in air for 24 h at 350 'C. The binary intermetallics (ThFe5 and ThNi5) were found to behave differently from the ternary intermetallics ( x = 1-4). The surfaces of the oxidized binary intermetallic catalysts consisted of T h o z and only metal oxides (FezO3, NiO) (Figure 7, Table I). Both T h o z and the transition-metal oxides (Fez03, NiO) were observed in the bulk of the binary catalysts as shown by XRD. In the case of ThNi5, an additional XRD phase (bulk) corresponding to Ni metal was also detected. Oxidation of the ternary intermetallics resulted in formation of an NiFe bulk alloy in addition to I' le oxides (NiO, Fe203,and Thoz). The presence of the NiFe alloy could not be confirmed by XRD because of the small difference in 28 of the Ni (44.5'), Fe (44.6'), and NiFe alloy (44.5-44.6') diffraction lines, which is within instrumental error (fO.l). However, existence of an alloy has been confirmed by thermomagnetic analysis (TMA) obtained for the same ThNi,Fe5, catalysts.6 Indeed, the Curie temperatures determined from TMA of all oxidized ternary intermetallic catalysts ( x = 1-4) are different from the characteristic Curie temperatures of either Fe or Ni. These results can be taken as firm evidence for the formation of a NiFe alloy in the bulk of all ternary catalysts. ESCA analysis of the ternary catalysts has shown that the surface of the NiFe alloy in the intermetallic catalysts is oxidized. However, Fe is oxidized to a greater extent than Ni as a result of its stronger affinity for oxygen (A"f(FeZO3) = 100 kcal/(g mol), AHf(NiO) = -58 kcal/(g mol)); it forms a passivating layer of Fez03 which prevents complete oxidation of Ni. Therefore, although Fez03 is the only iron species observed, 13-20% of elemental Ni is detected on the surfaces of the oxidized ternary intermetallic catalysts (Figure 7, Table I). Similar behavior was obtained for a NiFe alloy when it is oxidized. Indeed, as shown in Table V, two types of Ni corresponding to NiO and NiO and only one type of Fe (corresponding to Fez03) were observed on the surface of an air-oxidized NiFe alloy (4/1) (350 OC, 24 h) as in the case of ternary catalysts. Note that the fraction of Ni metal observed in the oxidized Ni/Fe alloy (4/1) is 15%, which
-
ThNi,Fe5,
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3215
Intermetallic Catalysts
I
I
,
1
2
3
4
X T h NixFe5-x
Figure 12. CO hydrogenation rates for ThNi,Fe5, alloys (CO/H2 = 3; 275 "C). Reprinted with permission from ref 6 .
is comparable to that observed in the ThNi4Fe catalyst (13%) (Table V). Results from depth profiling of oxidized ThNi3Fe2are consistent with the presence of the passivating Fe2O3 layer in ThNi,Fe5, catalysts. As shown in Figure 8 and Table 111, sputtering of the ThNi,Fe2 catalyst under static conditions removes the cover layer of Fez03 which subsequently results in a decrease in the SIMS intensity ratio, ZFe/ZNi. A steady-state intensity ratio is obtained after all of the passivating Fez03 layer is removed (- 10 %.)(Table 111). Surface Segregation. In addition to alloy decomposition, significant surface segregation of the transition metals (Ni, Fe) was observed when the ThNi,Fe5.., intermetallics were oxidized. As shown in Figure 2 and Table 11, the surfaces of both oxidized ThFe5 and ThNi5 are significantly enriched in Fe and Ni, respectively, relative to both the bulk and the untreated alloys. In the case of the ternary intermetallics, although both Fe and Ni segregate to the surface (Figures 2 and 3), Fe segegrates preferentially, due to its stronger affinity for oxygen. Thus, some of the Ni is covered by Fe. As a result, the surfaces of the oxidized ternary intermetallics are Fe rich and Ni poor with respect to the bulk (Figure 2, Table 11). The preferential segregation of Fe is confirmed by the ESCA results for the oxidized NiFe alloy (4/1). Indeed, the surface atomic ratio, Ni/Fe, of the oxidized NiFe alloy (1.6) is within experimental error of that obtained from oxidized ThNi4Fe (1.7). Both ratios are significantly smaller than the bulk atomic ratio (4.0) (Table IV). Thus, Fe-rich and Ni-poor surfaces, relative to the bulk, were obtained for both samples. Surface enrichment of Fe is again shown in the electron micrographs of the ternary catalysts (x = 1-4). All micrographs show the presence of fine needles, a characteristic of ThFe5 (Figure 5). Note that the Ni nodule structure which is observed for oxidized ThNi5 (Figure 4) is not detected in any ternary intermetallic catalyst. This is true for highly magnified micrographs of the Ni-rich alloy: ThNi4Fe (Figure 9). Correlation of Oxidation Results with Alloy Structure. It is noteworthy that, although the original hexagonal structure of the ThNi,Fe5, alloys is completely destroyed as a result of oxidation, the high reactivity for oxygen of the precursor g-site Ni appears to have some influence on oxidation results of the ThNi,Fe5-, system. As discussed in previous sections, the decomposition
products of binary alloys and ternary alloys differ. Thus, the influence of g-site Ni can only be compared between ternary intermetallic catalysts (x = 1-4). From Figures 2 and 3 it is seen that in the range of x = 1-4 the slope of the Ni/Th curves for the oxidized alloys obtained by all techniques (ESCA, AES, and SIMS) slightly increased after x = 3 (ThNi3Fe2). Note that the slope of the Ni g-site curve also increases after x = 3 (Figure 11). Since g-site Ni segregates to the surface preferentially relative to c-site Ni as a result of its higher reactivity for oxygen, the larger increase in g-site Ni concentration in ThNi4Fe explains its larger accumulation of Ni in this oxidized alloy. In addition, the large g-site Ni content in ThNi4Fealso accounts for the higher percentage of NiO formed in ThNi4Fe (87 f 4%) relative to other ternary intermetallic catalysts (80-82 f 4%). Catalytic Activity and Surface Characterization. C O hydrogenation activity for the same oxidized ThNi,Fe,-, catalysts has been studied by France.6 For ThNi,, about 52% methane was obtained. The remaining 48% corresponds to products having the compositions C2 (28%), C, (12%), C4 (5%), and C5 (3%). Selectivity to methane formation was found to increase with increasing bulk Fe content, up to 100% methane for ThFe5.6 The rates of C O hydrogenation for the oxidized ThNi,Fe5, catalysts calculated from the turnover numbers and C O chemisorption capacities6 are plotted in Figure 12. As can be seen, ThNi5 is significantly more active than any of the other catalysts. For example, ThNi5 is -700 times more active than ThFe5. The low activity of ThFe5 was accounted for by the formation of Fe5C2 in ThFe5 after synthesis gas treatmente6 From Figures 2, 3, and 12 it is readily seen that both the activities and the surface Ni content show behavior similar to that of bulk Ni content. Thus, there is strong correlation between the surface concentration of Ni of these catalysts and their catalytic activity data. In general, catalysts having higher Ni surface concentrations possess higher catalystic activity. Indeed, a linear increase in catalytic activity of ThNiFe4, ThNi2Fe3,and ThNi3Fe2 is accounted for by a similar increase in the surface Ni content of these compounds. A change in slope of the activity curve at ThNi3Fe2(Figure 12) corresponds to a change in slope of the Ni/Th curves (Figures 2 and 3). Finally, the exceptionally high activity of ThNi5 relative to that of other ThNiFe alloys is readily explained by the particularly large enrichment of Ni on the surface of ThNi,. Summary 1 . The reactivities for oxygen of c sites and g-site Ni in the ThNiXFe5-, alloys differ. This difference was observed by ESCA, AES, and SIMS. 2. Treatment of the ThNi,Fe,, in air at 350 "C for 24 h results in complete transformation of the original structure and causes significant segregation of the transition metal (Ni, Fe) to the surface. The decomposition products of binary alloys include T h o 2 and metal (Ni) and/or metal oxides (NiO, Fe20,). In the case of ternary alloys, a NiFe alloy is also formed. Although the surfaces of oxidized binary alloys are enriched with transition metal relative to the bulk (Fe or Ni), the surfaces of oxidized ternary alloys are only enriched with Fe. The Ni surface concentrations of these alloys are still smaller than those of the bulk. 3. Correlations between the C O hydrogenation activity and the surface characterization has shown that a catalyst having a larger surface Ni concentration shows higher catalytic activity for C O hydrogenation. Acknowledgment. This work was supported by the National Science Foundation (NSF) Grant CHE-8020001. We also thank the N S F for the financial support to the Center for Research in Surface Science (CRISS) (Montana State University). Registry No. ThNi,Fe5-,, 89959-17-1.
