Langmuir 1991, 7, 497-502
497
X-Ray Photoelectron Spectroscopic Investigation of Oxidative Coupling of Methane over Samarium Oxide Promoted with Sodium Pyrophosphate Ranjani V. Siriwardane
US.Department
of Energy, Morgantown Energy Technology Center, P.O. Box 880,
Morgantown, West Virginia 26507-0880 Received May 17,1990. I n Final Form: September 25, 1990 Samarium oxide promoted with sodium pyrophosphate was found to be an active and selective catalyst for the oxidative coupling of methane to ethane and ethylene in the presence of molecular oxygen. C2 yields as high as 22% were obtained over Na4P207/Sm203at 1101 K and the catalyst was stable during 23 h of reaction. Experimental conditions such as temperature, treatment with helium, dilution factor, and method of preparation affected the Cz selectivity and Cp yield. X-ray photoelectron spectroscopic (XPS) analysis indicated that the oxygen in Na4Pz0,/Smz03 was in a more electron deficient state than in Sm2O3. More than one type of phosphorus was identified in N%P207/Sm~03by XPS, which was different from that of N%PzO7 in which only one type of phosphorus was present. The surface of the 0.5N~Pz07/Sm203prepared by the solid-state mixing method was found to be significantly different from that prepared by the solution deposition technique.
Introduction Methane is the major component of natural gas and is primarily used as fuel. Transport of natural gas has been a problem in the exploitation of some natural gas resources. It would be extremely valuable to be able to convert methane to more readily manageable or transportable products. However, high molecular stability of methane makes it difficult to convert into other useful chemicals. Reasonable progress has been made in the past in catalytic oxidative coupling of methane to form higher hydrocarbons which could be used as gasoline precursors. Several oxide catalysts1-I2 promoted with alkali metals have shown to be promising catalysts for this process. Smz03 is one of the active catalysts for oxidative coupling of methane.5 Several alkali-metal carbonate promoters’ have been utilized to improve the catalytic performance of Sm203, but the stabilities of these promoted catalysts are not reported. Metal oxides promoted with sodium pyrophosphatell have been shown to be stable and active catalysts for oxidative coupling of methane. The objective of this work is to obtain further information on the catalytic oxidative coupling of methane. The results of both reactor studies and surface characterization studies with X-ray photoelectron spectroscopy (XPS) on samarium oxide promoted with sodium pyrophosphate, which could be used for selective conversion of methane, are reported in the present study. Experimental Section a. Reactor Studies. Smz03was prepared by decomposition of samarium nitrate (Aldrich) at 1123K for 16-19 h. Promoted
samarium oxide was prepared by adding samarium oxideto deion(1)Keller, G.E.;Bhasin, M. M. J. Catal. 1982,73,9. (2)Ito, T.; Wang, J. X.; Lin, C.; Lunsford, J. H. J. Am. Chem. Soc. 1985,107,5062. (3) Ali Emesh, I. T.; Amemoiya, Y . J. Phys. Chem. 1986,90,4785. (4)Bytyn, W.; Baerns, M. Appl. Catal. 1986,28,199. (5)Otauka, K.;Jinno, K.; Morikawa, A. J. Catal. 1986,100, 353. (6)Otsuka, K.; Said, A. A. Znorg. Chim. Acta 1987,132,123. (7)Otsuka, K.;Lui, Q.;Hatano, M.; Morikawa, A. Chem. Lett. 1986, 467. (8) Sofranko, J. A.;Leonard, J. J.; Jones,C. A. J. Catal. 1987,103,302. (9)Lin, C. H.; Wang, J. X.; Lunsford, J. H. J. Catal. 1987,111, 317. (10)Iwamatsu, E.;Moriyama,T.; Takasaki, N.; Aika, K. J.Catal. 1988, 113,25. (11)Siriwardane, R. V. J. Catal. 1990,123,496. (12)Siriwardane, R.V,;Shamsi, A. Appl. Catal. 1990,60,119.
ized water containing sodium pyrophosphate (Alfa) and slowly evaporating water while stirring. The resulting thick solid paste was heated in an oven at 773 K for 16h followed by a 2-h heating at 973 K. The catalyst of 28-48 mesh and 0.25 g was placed in a highpurity quartz tube (length = 23.3 cm, diameter = 4.5 mm, heated zone length = 9 cm). Ultrahigh-purity (99.999%)helium, grade zero oxygen gas (99.8%),and methane (99.99%)were obtained from Matheson. The catalytic experiments were carried out in a fixed-bed reactor operated at 1atm. Reacting gas mixtures of methane and oxygen diluted with helium to achieve a total pressure of 1 atm were introduced after heating the catalyst to the desired temperature. The gaseous products were analyzed with two gas chromatograph columns (Porapak Q and molecular sieve) which were connected to a thermal conductivity detector. The mole ratios that are used in the discussion of the results in the present paper are the moles of the sodium promoter per mole of the oxide. The term ‘yield” used in this paper was obtained by multiplying the selectivity for each carbon containing product and the total CHI conversion (based on mole percentages). The “percentage” of CzH4,CzHe, CO, and COPshown in the graphs is the percentage ‘yield” of each compound. b. Surface Characterization by X-ray Photoelectron Spectroscopy. X-rayphotoelectronspectral3were recorded with a CylindricalMirror Analyzer and a 15-kVX-ray source (Physical Electronics Division of Perkin-Elmer). Charge correction of the XPS data was accomplishedby assumingthat the binding energy of C(1s) was at 284.6 eV. In samples where hydrocarbon was not present, the binding energy of Sm(3d) (1083.5 eV) was used as the standard for charge correction. The experimental uncertainty in the binding energy was f0.5 eV. The XPS system consisted of a separately pumped (10-8-10-7 Torr; 1Torr = 133.3Pa) sample preparation chamber that was equipped with a resistively heated sample probe and a leak valve for gas exposures. The preparation chamber was separated from the analyzing chamber by a gate valve. The catalyst samples were heated in the preparation chamber to 1073 K and were exposed to pure oxygen and methane/oxygen mixture (2:l) at 1073 K at a constant pressure for 30 min. Then the gate valve was opened and the sample was transferred to the analyzing chamber for data acquisition at 1073K. This procedure ensured that there was no rehydroxylation or any other contamination of the samples due to the exposure to atmosphere during the transfer of samples from reactor to spectrometer. (13)Siriwardane, R. V. J. Colloid Interface Sci. 1985,132,200.
This article not subject to US. Copyright. Published 1991 by the American Chemical Society
Siriwardane
498 Langmuir, Vol. 7, No. 3, 1991
5-1 0
,
I
1
I
02
04
05
OB
v c 1 5 0 ' Y 3 4 ~ ' o ,YcearrfS-,C,
/I I
1 Pi I C ha.P.0,
Figure 1. Effect of sodium concentration on the performance of 0.5Na4P20,/Sm203.
Results and Discussion a. Reactor Studies. The maximum yield of C2 compounds on Na4P207/Sm203 was obtained by varying reaction conditions such as sodium concentration, temperature, treatment with other gases (helium), and partial pressure of methane and oxygen (dilution factor). The maximum C2 yield of 22 % was obtained over 0.5Na4P207/ Sm203 a t 1101 K a t a methane-to-oxygen ratio of 2 (flow rates of methane, oxygen, and helium were 5.0, 2.5, and 42.5 mL/min, respectively). Effect of Sodium Concentration. Variation of catalytic performance at 1101 K as a function of Na4P207 content on Sm2O3 is shown in Figure 1. The flow rates of methane, oxygen, and helium were 21,10.5, and 20.4 mL/ min, respectively. Under these conditions, the empty reactor at 1101 K produced 5 % C2 yield with 46% selectivity. As shown in Figure 1,the maxima of C2 yield, C2 selectivity, and CHI conversion were obtained when the Na4P207 content on Sm203was 0.5 mol. A mixture of 0.5NadP207/Sm203 was also prepared by mixing 0.5 mol of Na4P207 of solid and 1 mol of Sm2O3 (rather than the solution technique which is dissolving NadP207 in water and mixing with Sm2O3 as was explained in the Experimental Section) and was heated in an oven at 773 K for 16 h followed by a 2-h heating at 973 K. When this mixture was tested for catalytic performance, a C2 yield of 13.1% with a C2 selectivity of 33% was obtained, which was lower than those obtained (17.9% CZyield, 43 % selectivity) for the catalyst 0.5Na4Pz07/Smz03 prepared by the solution technique. Effect of Helium Treatment and Dilution Factor. The catalyst was exposed to flowing helium (20.4 mL/ min) when it was heated from room temperature to the reaction temperature 1101 K for 2 h. As shown in Table I, the catalysts treated with helium had higher C2 yield, C2 selectivity, and CH4 conversion as compared to those over the untreated catalyst. Similar observations have previously been made with the GdMn03 catalyst.12 When the combined flow rate of CHI and 0 2 was changed from 31.5 to 7.5 mL/min keeping both the total (CH4,02, and He) flow rate and methane-to-oxygen ratio constant, there was a significant increase in the C2 selectivity, leading to an increased C2 yield as shown in Table I. Effect of Temperature. The catalytic performance of 0.5Na4P207/Sm203 as a function of temperature is shown in Figure 2. Flow rates of CH4, 0 2 , and He were 5.0, 2.5, and 42.5 mL/min, respectively. The catalyst seemed to perform better a t higher temperatures. Stability of the Catalyst. The catalyst was found to be stable during 23 h of reaction. The yields of CzH4, CzHs, CO, and C02 obtained over the untreated
0.5Na4P207/Sm203 sample were 13.3,4.3,3.1, and 20.1%, respectively, after 3 h of reaction, and they were 14.5,3.3, 4.3, and 19.0%, respectively, after 23 h of reaction. Surface Characterization by X-ray Photoelectron Spectroscopy (XPS). XPS provides analysis of the surfaces of the samples up to a depth of about 50 A. The binding energy of elements obtained by XPS can generally be used to differentiate the chemical states of the elements on the surface. XPS analyses were performed on Sm203, Na4P207, 0,4N~Pz07/Sm203,and 0.5Na4P207/Sm203, both before and after reaction, and on 0.5Na4P207/Sm203, prepared by solid mixing method. Results of the XPS analysis of Sm2O3 at room temperature, at 1073 K, after exposure to 5400 langmuirs (1 langmuir = 10-6 Torr s) of oxygen at 1073 K, and after exposure to 5400 langmuirs of methane and oxygen mixture (CH4:Oz = 2:l) at 1073 K are shown in Table I1 and Figure 3. The oxygen (O(1s))peak on Sm203 at room temperature was broad, indicating either two or more types of oxygen present. At room temperature, the surface may be hydroxylated and there could be oxygen corresponding to -OH in addition to 0 2 - from Sm2O3. When the temperature was increased to 1073 K, only one type of oxygen was observed as also indicated by the decrease in both full width a t half maximum and O/Smratio at 1073K as shown in both Figure 3 and Table 11. The oxygen peak a t 1073 K was not perfectly symmetric and there could be a very small amount of additional oxygen a t the high binding energy side of the major peak. Significant changes in the oxygen peak were not observed after oxygen exposures or methane/oxygen exposures. The oxygen-to-samarium ratio was 1.2 at 1073 K, which is (slightly) less than the value of 1.5 that is predicted by the formula Sm203. XPS analyses of Na4P207 are shown in Table I11 and Figures 4 and 5. There is only one major type of oxygen present at room temperature which had a higher binding energy than that of the oxygen a t 1073 K. The oxygen peak at 1073 K is not perfectly symmetrical, indicating that there may be a small amount of oxygen a t high binding energy of the major peak. The binding energy of oxygen in Na4P207 a t 1073 K was higher than that of Sm2O3. There was also only one type of phosphorus observed at any of the reaction conditions on N ~ P z 0 7 .According to the formula Na4P2O7, the ratio of P/O should be 0.18 and the O/Na should be 1.7. The ratio of P / O a t 1073 K was higher than was expected from the formula while the O/Na on the surface was lower than was expected from the formula. This indicates that sodium and phosphorus preferentially segregate near the surface of Nap207 a t 1073 K. (The elemental ratios reported here are semiquantitative since they were calculated considering only photoelectron cross sections, but the results would provide a relative understanding of how the elemental ratios vary with both temperature and composition.) When Na4P207 was heated from room temperature to 1073 K, there was an increase in the O/Na ratio, indicating a loss of sodium compared to oxygen from the surface. After methane and oxygen exposures, there was a further increase in the O/Na ratio, indicating either an incorporation of oxygen or further loss in sodium. The XPS analysis of 0.4NadP207/Sm203 is shown in Table IV and Figures 6 and 7. The oxygen peak for 0.4Na4P207/Sm203 at room temperature was broad, indicating that there was more than one type of oxygen on the surface. However, when it was heated to 1073 K, only one type of oxygen was observed. The binding energy of this oxygen was between the binding energies of the oxygen of Smz03 and Na4P207 but it was closer to that of
Oxidative Coupling of Methane
Langmuir, Vol. 7, No. 3, 1991 499
Table I. Effect of Helium Treatment and Dilution Factor on the Catalytic Activity of O . S N ~ ~ P ~ O . I /atS ~1101 ~ OK~ CH4:Op:He flow CH4 conversion, Cp selectivity, yield, % description rate, mL/min % % total Cp CpH4 CpHc total C1 (COz + CO)
untreated
21:10.5:20.4 21:10.5:20.4 5.0:2.5:42.5 5.02.5:42.5
He treated He treated empty reactor
p
40.8 43.3 44.4 5.3
40
m C, Yield t Conversion
35
a
C
43.1 45.4 50.6 67.6
13.3 15.1 16.0 1.6
17.6 19.7 22.5 3.6
4.3 4.6 6.5 2.0
23.2 23.7 21.9 1.7
Selectivity
5
30 25
10
I
1
I
1048
1101
1123
TemperatureiK)
Figure 2. Effect of temperature on the performance of
0.5Na4P~07/Smz03. Table 11. X-ray Photoelectron Spectroscopic Analysis of SmzOa
0 ~
exptl condition
room temp 800 O C 800 ‘C/O2
800 OC/Me/02 a
~~~
BE,O eV
O/Sm
fwhmb
531.5 529.1 529.3 528.6
7.9 1.2 1.3 1.2
5.5 2.9 3.0 2.9
J b
(d) 1073 KICH4102
*
Binding energy. fwhm is full width at half maximum.
~~
516
Na4P207. Thus, a new type of oxygen had been formed on the surface of 0.4Na4P207/Sm203 which was more electron deficient than that of Sm2O3. There were no changes in the binding energy of oxygen after either oxygen or methane/oxygen exposures. The phosphorus peak on 0.4Na4P207JSm203 was completely different from that of pure Na4P207. The phosphorus peak of 0.4Na4P207/ Smz03 was broad, indicating there were more than one type of phosphorus present. The binding energy of the phosphorus of 0.4Na*P207/Sm203 was also less than that of Na4Pz07. This value is not close to the binding energy of phosphide (128.3-129.0 eV14)or elemental phosphorus (129.8 eVI4)but closer to Na3P04. This further indicates that Na4P207 did not simply reside on the SmzO3, but it interacted with SmzO3 to form different types of phosphorus on the surface. The shape of the phosphorus peak indicates that there may be additional phosphorus corresponding to both low binding energy and high binding energy of the major phosphorus peak. Significant changes in the phosphorus peak were not observed after oxygen and methane/oxygen exposures a t 1073 K. In addition to the differences in binding energy of elements in Na4P207 and 0.4Na4P207/Sm203, differences in elemental ratios were also observed in the two compounds. The ratio of P / O was also found to be higher on 0.4Na4P207/Sm203 compared to that of Na4P207, indicating that phosphorus compared to oxygen is preferentially concentrated on the surface of 0.4Na4P207/Sm203. O/Na is also higher in0.4N~P207/Sm203than in Na4P207, (14) Wagner, C. D. Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy;Briggs, D., Ed.; Heyden and Son, Ltd.: Philadelphia, PA, 1978.
520
523
526
529
532
535
536
541
544
547
550
Binding Energy (eV)---->
Figure 3. 0 1s spectral region of Sm&
indicating less sodium concentration on the surface of 0.4Na.J‘207/Sm203. When 0.4Na4P207/Sm203was heated to 1073 K from room temperature, there was a decrease in Na/Sm ratio, indicating a loss of sodium from the surface during heating. The chemical formula 0.4Na4Pz07/Sm203 indicates a ratio of Na/Sm to be 0.8 while the experimentally determined ratio was about 10 times more than the theoretical value. This indicates that all of the Nap207 is segregated on the surface of the Sm2O3, and since only 50 A of the surface is analyzed by XPS, it would indicate a high value of Na/Sm. The XPS analysis of 0.5Na4P207/Sm203 is shown in Table V. The phosphorus peaks of 0.5N~Pz07/Sm203 were similar to that of 0.4N~P207/Sm203.However, some differences in elemental ratios were noted. The ratio of P / O was less on 0.5Na4PzO,/Sm203 than on 0.4Na4P207/ Sm203 but was still higher than that of pure Na4Pz07. Thus, adding more Nap207 to Sm203 had decreased the concentration of P relative to oxygen on the surface. The ratio of Na/Sm has also doubled on 0.5Na4P207/Sm203 as compared to 0.4Na4Pz07/Sm203, which was more than is expected theoretically if sodium is evenly distributed in the bulk. Thus, this indicates that addition of sodium as Na4P207 mainly concentrates sodium on the surface of SmzO3. XPS analysis of O . ~ N ~ ~ P ~ O , /after S ~ 24 ZO h of ~ reaction is shown in Table VI and Figure 8. The Na/Sm ratio at 1073 K has decreased compared to that of the unreacted
500 Langmuir, Vol. 7, No. 3, 1991
Siriwardane
Table 111. X-ray Photoelectron Spectroscopic Analysis of NarP2O.r exDtl condition
room temp 800 OC 800 oc/oz 800 "C/CH4/02
0
Na BE, eV
Na (KLL) KE, eV
BE, eV
P PI0
fwhm
BE, eV
O/Na
fwhm
1073.5 1072.4 1072.6 1072.6
989.2 990.1 990.1 990.2
134.2 132.7 133.4 133.2
0.63 0.44 0.41 0.50
2.8 2.8 2.8 2.6
532.1 531.0 531.3 531.0
0.12 0.43 0.49 0.58
2.9 2.7 3.0 3.2
'/A\-
-
4 -
A
I 113
1lR
123
(d) 1073 KICH4102
\M
I
1
1
I
I
I
I
128
133
138
143
148
153
158
Binding Energy ---->
I
I
l
l
I
l
l
I
l
l
I
519
522
525
528
531
534
537
540
543
547
550
Binding Energy (ev)---->
Figure 4. 0 1s spectral region of NhP207,
sample. However, this did not affect the activity of the catalyst. The binding energy of the oxygen peak has also not changed after reaction. However, there was a broadening of the phosphorus peak on the reacted sample, indicating either the formation of additional forms of phosphorus or an increase in the secondary forms of phosphorus (in addition to the major peak) that were present in the unreacted sample. The CZselectivities were 36 and 43% over SmzO3 and O . ~ N ~ ~ P Z O ~ /respectively, S ~ Z O ~ , under similar experimental conditions. Thus, addition of Na and P has only suppressed the formation of C02 leading to a higher CZ selectivity. Thus, the removal of oxygen a t a lower binding energy (529 eV), which corresponds to oxide (02-) oxygen, by the addition of Na4P207 and formation of a new type of oxygen (530.5 eV) may have contributed to the higher CZselectivity. XPS analyses of the catalyst 0.5Na4P207/Sm203 prepared by using the solid-state mixing method rather than the solution deposition technique are shown in Table VI and Figure 9. The surface is significantly different from that prepared by the solution deposition technique. The phosphorus peak on the solid-state mixture was broader at 1073 K compared to the catalyst prepared by the solution deposition technique, and this indicated the presence of several types of phosphorus on the solid-state mixture.
Figure 5. P 2p spectral region of Na&07.
The Na/Sm ratio on the solid-state mixture was considerably lower than that of the sample prepared by solution deposition. This indicates that the sodium was not fully deposited on the surface of Smz03. Both CZselectivity and Cz yield over 0.5Na4Pz07/Smz03prepared by the solidstate mixing method were even lower than those of pure Sm2O3. Thus, if proper quantity of sodium was not deposited on the surface of Sm203, CZselectivity would not be improved. Some Important Implications of the Data. Methyl radical formation from methane on rare-earth metal oxides has been observed by Lin et al.I5 Thus, it is reasonable to assume that the reaction proceeds via methyl radical formation. The main role of the sodium pyrophosphate promoter seems to be the enhancement of the Cp selectivity (not the increase in total carbon conversion). If the formation of carbon oxides exclusively take place in the gas phase, the selectivity to CZ compounds cannot be greatly enhanced by the addition of the promoter to the surface. Thus, the majority of the deep oxidation reactions may be taking place on the surface. XPS data indicated that the oxygen on SmzOa was in the form 02-, and these oxygens were not observed on the surface of 0.5Na4Pz07/ Sm2O3. Thus, the enhancement in CZselectivity may be due to the depletion of these 02-ions. It is only possible to detect about 50 A into the surface with XPS. Thus the (15) Lin, C.H.; Campbell,C.D.;W a g , J. X.;Lunsford, J. C.J . Phys. Chem. 1986,90, 534.
Oxidative Coupling of Methane
Langmuir, Vol. 7, No. 3, 1991 501
Table IV. X-ray Photoelectron Spectroscopic Analysis of 0.4NarPzOdSmzO~ Na BE, eV 1073.5 1072.4 1072.2 1072.1
exptl condition room temp 800 "C
oc/oz
800 800 OC/CH4/02
Na (KLL) KE, eV 988.5 990.1 990.1 990.5
Na/Sm 12.5 7.7 7.9 8.0
BE,eV 132.4 132.2 132.0 132.1
0
P P/O 0.77 1.11 1.02 1.15
fwhm
BE,eV
O/Na
fwhm
5.7 5.0 4.8 4.3
531.2 530.8 530.5 530.4
0.76 0.67 0.66 0.71
4.2 2.9 2.9 2.9
(a) 298 K
-.0 c
C
3
2.
.-P c
(b) 1073 K
e
5 .-cz v)
c
-C c
(c) 1073 WO,
(d) 1073 K/CH4/0,
I \
I 518
d
L
(d) 1073 K/CH,/O,
I
l
l
I
I
I
521
524
527
530
533
536
I
l
/
I17
122
127
l
/
132
137
/ 142
/ 146
/ 152
l 157
I I 162
167
Binding Energy (ev)---->
I
I
I
I
539
542
545
546
Binding Energy (eV)---->
Figure 6. 0 1s spectral region of 0.4Na4Pz07/Smz03.
02-that may be present in the bulk (>50 A) of Na4P207/ Sm2O3 could still participate in the deep oxidation of methane to form carbon oxides. Furthermore, deep oxidation of methane was found to be very low over pure Na4Pz07. Phosphorus in Na4P207/Smz03 may be contributing to the stability of the catalyst, as was observed with CaO and Gd2O3,l1 by retaining more sodium on the surface during the reaction. Additional forms of phosphorus were observed on 0.5Na4Pz07/Sm203 than on pure Na4Pz07. Contribution of these phosphorus to the reaction is not clear. The binding energy of oxygen (530.5eV) in 0.4NadP207/ Smz03 is close to the value of oxygen in a Na3P04 (530.3 eV). However, the binding energies of oxygen in peroxide dianions such as NazOz and Ba0216 are reported to be 530.4 and 530.0 eV, respectively. Therefore, it is difficult to identify the type of oxygen on 0.4Na4P~07/Sm~03. When the catalyst samples treated with helium (5400 L) were analyzed by XPS, significant changes in the binding energies of oxygen, phosphorus, and sodium were not observed. It is possible that some of the contaminated (16) Kharas, K. C. C.; Lunsford, J. H. J . Am. Chem. SOC.1989, 111, 2336.
Figure 7. P 2p spectral region of 0.4Na4P2O7/Smz03. carbon may have been removed from the samples during the treatment with helium, which may have contributed to the higher selectivity. Dilution by helium (lowering the total pressure of methane and oxygen) contributed to an increase in CZ selectivity. Similar effects have been observed by Lane and W0lP7in methane oxidative coupling studies in empty reactors. This may be related to the decrease in gas phase reactions, which was caused by increased dilution.
Conclusions SmzO3 promoted with Nap207 was found to be a selective catalyst for oxidative coupling of methane. CZ yields as high as 22% with CZ selectivity of 51% were obtained over the catalyst which was also found to be stable for 23 h of reaction. XPS analysis showed that the oxygen in 0.5Na4Pz07/Sm203 was in a more electron deficient state than in SmzO3. Only one kind of phosphorus was observed on pure Na4P2O7 while more than one type of phosphorus were observed over 0.5Na4Pz07/Sm203, indicating that there was interaction of Na4P20, and SmpOa to produce new types of phosphorus. The high Na/Sm ratios on the promoted SmzO3 indicated that sodium was deposited mainly on the surface, not distributed in the bulk of SmzO3. Significant changes in either oxygen and phosphorus peaks were not observed after exposure to (17) Lane, G.; Wolf, E. E. J. Catal. 1988, 113, 144.
502 Langmuir, Vol. 7, No. 3, 1991
Siriwardane
Table V. X-ray Photoelectron Spectroscopic Analysis of 0.5Na4PzO,/Sm208
exptl condition 800 OC 800 OC/He 800 "C/O2 800 "C/Me/02
BE, eV
Na
Na/Sm
Na (KLL) KE, eV
BE, eV
P p/o
fwhm
BE, eV
fwhm
16.6 16.3 16.0 16.3
990.3 990.3 989.9 990.2
132.5 132.6 132.4 132.6
0.62 0.60 0.60 0.40
3.8 4.2 4.0 3.8
531.5 531.3 531.3 531.0
3.3 3.3 3.5 2.9
1072.5 1072.6 1072.9 1072.6
0
Table VI. X-ray Photoelectron Spectroscopic Analysis of 0.5Na4PzO,/SmzO~
exDtl condition
BE, eV
Na Na/Sm
room temp 800 "C
1072.1 1072.3
13.6 7.1
3.5 2.5
room temp
1071.9 1072.0
1.9 1.8
4.2 2.8
800 O
C
P
fwhm
Na (KLL) BE,eV a. After 24 h of reaction
0
P/O
fwhm
BE, eV
O/Na
fwhm
1.01 1.08
6.4 5.0
531.5 531.0
0.49 0.86
3.8 3.2
b. Prepared by Solid-state Mixing 132.7 1.37 - 990.0 133.4 1.95 990.2
9.2 8.2
531.8 530.8
1.58 1.39
4.5 3.6
989.6 989.6
133.7 133.2
'
?2 ,,',
A
m!
J,
A
,;*
I 5,;'
'
,!#
,;, ,,'* ,;,
,;
4
Binding Energy ->
Figure 8. 0 1s and P 2p spectral regions of 0.5N%P207/Sm203 after 24 h of reaction.
Figure 9. 0 1s and P 2p spectral regions of 0.5NaPz07/Smz03 prepared by solid-state mixing method.
oxygen and methane/oxygen at 1073 K. Decrease in Na/ Sm was observed over 0.5N~P20,/Smz03after 23 h of reaction. The samples of NadP~O,/Sm203 prepared by the solid-state mixing method rather than the solution deposition technique were found to be less selective to C2 hydrocarbons than the Smz03 sample. The surface of the samples prepared by the solution deposition technique
Acknowledgment. The author thanks Dr. Jason Cook and Dr. Steven Miller for writing the program for data acquisition in XPS. Review of the manuscript by Dr. Jan Wachter and Dr. Larry Headley is appreciated. Registry No. SmzO3, 12060-58-1; NaP207, 7722-88-5; CHI,
was also found to be significantly different from that of samples prepared by the solid-state mixing technique.
74-82-8; CH8CH3, 74-84-0; HzC=CHz, 74-85-1.
