THE J O U R N A L OF
PHYSICAL CHEMISTRY Registered in U.S.Patent Office 0 Copyright, 1980, by the American Chemical Society
VOLUME 84, NUMBER 21
OCTOBER 16,1980
LETTERS Infrared Elllpsometric Spectroscopy of CO on Ni( 1I O ) during the Catalytlc Methanation Reaction Paul Mahaffy and Mlchael J. Dlgnam" Department of Chemistry, Universtty of Toronto, Toronto, Canada M5S 1A 1 (Received: May 12, 1980)
Recent studies have pointed toward the surface "carbide" methanation mechanism as being important on nickel catalysts. Our infrared spectra of the CO stretching bands on Ni!llO) obtained by the ellipsometrictechnique under methanation conditions support this view for reaction at moderately high temperature (2200 "C), but not for reaction at lower temperatures, suggesting therefore an explanation for the hydrocarbon product distxibution as a function of temperature. We also find that adsorption of H2with CO has very little effect on the CO stretching bands at low temperatures. We recently reported vibrational spectra for CO on Ni(llO).l These spectra were obtained by the infrared ellipsometric technique2 in a single reflection from a ( N 1 cm2) Ni(ll0) disk which was mounted in a ultrahigh vacuum cell. Our small stainless steel chamber' allows any pressure from ultrahigh vacuum to atmospheric to be readily established. This cell can also be used, therefore, to obtain vibrational spectra of surface species during catalytic reactions and we herein give the results of a preliminary study of the behavior of the CO stretching bands under methanation conditions. It is anticipated that considerable additional valuable information with respect to this important reaction will be obtained when the ellipsometric technique is extended to the far IR where the Ni-C and Ni-0 stretching frequencies occur. Figure 1exemplifies the type of behavior observed when CO adsorbs on a clean Ni(ll0) surface. Various pressures of pure CO are established over the Ni(ll0) surface held at 60.0 "C, in this case, and both linear (-2080-2150 cm-') and bridge (-1800-1950 cm-l) CO species are formed. In the model which we have proposedl as being consistent 0022-3654/80/208~2683$01.OO/O
with all our IR spectra and with previous LEED studies,Bp there is, above room temperature, a continuous and reversible transformation as coverage increases, from a structure where rows of linear and bridge CO molecules are adsorbed on top of the Ni(ll0) ridges to a structure where half the CO molecules are on long bridge sites and the rest are on linear sites. Coadsorption of hydrogen with CO appears to have little effect on the CO stretching bands at low temperatures as evidenced in Figure 2. In each set of experiments in this figure, CO was first dosed and a spectrum obtained, then the pressure of hydrogen in the cell raised to 1.5 X torr before additional scans were taken. In fact, even under the conditions of the first (lowest) spectrum of Figure 3 (60.0 "C, 0.03 torr CO and 0.11 torr H2),there was no substantial change in the CO stretching bands due to the presence of hydrogen. Much above 60 "C,however, pronounced changes occur (Figure 3). In the series of spectra shown in Figure 3 the behavior of the CO bands is followed as the temperature of the nickel disk is raised to the point at which methanation 0 1980 American Chemical Society
2884
Lettem
The ,bournel of PhySIcBI C b M f r y , Vd. 84, No. 21, 1980 I
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CO a n d H, gn Nt (110 Pco = .03 torr
C O on N i (110)
n
0.3 %
I
T = 60.0"C
PH,..II
torr
T = 60.0*C I
/ T.270.0
166t o r r
4. I a
*C
1.240.0 *C
-7
T = 2 10.0
7.8 a I O torr 2.2 a
16' torr
5.7 x
16' t o r r
5.7 a
1 6t o~ rr
'C
T=IBO.O * C
e
T = 120.0*C
BACKGROUND I
1
1
I
I
I
I
1*60.0*C
L400 2300 2200 2100 2000 1900 le00 WAVEN UMBER ( cm-I )
-1. AdsaptknspectraareshomatvarbusCOpressves. The Nishgle crystaldlsk is heid at 00.0 "C h Wsset of expehmts. The shtft to high wavenumbers with Increasing CO wesswe is readUy reversible on "110) at Ws temperahre.
2 1 . 400 2300 22CO 2100 2000 1900 I800 W A V E NUM B E R ( cm-') -0 3. The CO stretching bands observed on "110) under methenah codtfom. Thetime sequence h Wsserlesof MS te from bottom to top.
I I 1 I 1 I CO and H,m N i (110) 10.3% 2.2x IO-? torr co 1.4 x torr H2
CO a n d H2 on N i ( I IO) 2 7 L CO T - 30O.C I 5 I IO-' torr
HZ
h
T = 60.0' C
A
%
T = 150 *C
&
T=120'C
T = 9 0 *C
T=60
'C
T=60'C
no H2
up-.. I I -11100
2200
2100
2000
1900
WAVE Nu MBER
1800
h
1700
00
( c m-'
Fb4ro 2. T h e m sequence hl this set of runs is from bottom to top. In each of the three sets of If3 scans the "110) disk was Rrst dosed with CO, a spectrum obtained, then a steady presswe of 1.5 X lod tar of bedded tothe cel forseveralhudredsecondsbefore the next
scan was started.
readily occurs on Ni catalysts. In the T = 240 "C scan both the temperature and reactant pressures are quite close to conditions in experiments carried out by Arak and Ponec6 In their study, combined kinetic and isotope labeling experiments over Ni films led to the conclusion that the
2200
2100
2000
1900
W A V E NuM BE R -0
I800
1700
(ern-')
4. Data slmllar to that of Figure 3. but for dmeremt preswe
CondttbflS.
surface carbide mechanisms dominated. Madey, Goodman, and Kelley' provide further support for the carbide methanation mechanism on Ni by demonstrating the existence of "carbidic" carbon but no oxygen on a Ni(100) surface subsequent to a 450 "C exposure to a methanation reactant mixture (4/1 H2/C0 ratio and a total pressure of 120 torr). In our experiments the greatly reduced CO
J. Phys. Chsm. 1960, 84, 2685-2688
adsorption after heating to 300.0 "C then cooling to 60.0 "C is consistent with the latter observations. Madey et alS7also report that methane's fraction of the hydrocarbon product at 425 "C (0.99) is considerably higher than this fraction a t 175 "C (0.90). It has been suggested that while the methanation mechanism may involve the hydrogenation of a surface carbon species produced by the decomposition of CO, the formation of higher hydrocarbons may require oxygen-containing species as intermediates. This would certainly be consistent with the results of Figure 3. At T = 180.0 "C,for example, considerable adsorbed CO is available from which oxygen-containing species could be formed while at 240.0 "C there is little adsorbed CO and carbon atoms appear to be the predominant surface species.
2685
Care must be taken, however, in extrapolating our results of Figure 3 to other reactant pressure regions, since the corresponding experiments carried out in the torr range showed a similar type of behavior but the CO band disappears at a temperature approximately 70 OC lower (Figure 4). References a n d Notes
(1) P. Mahaffy and M. J. Dlgnam, Surf. Scl., In press. (2) M. J. Dlgnam and J. Fedyk, A&. Spectrasc. Rev., 14, 249 (1978). (3) H. H. Madden, J. Kuppers, and Q. Ertl, J. Chem. Phys., 58, 3401 (1973). (4) T. N. T a w and P. J. Estnp, J. Vec. a/. Technol., 10, 26 (1973). (5) M. Arakl and V. Ponec, J. Catel., 44, 439 (1976). (6) (3. A. Mills and F. W. Steffgen, Catel. Rev., 8, 159 (1973). (7) ~ . ~ . ~ , ~ . w . ~ o o d m a n , a n d ~ . ~ . ~ e l e y , ~ . v e c . ~ d . r e d n 16, 433 (1979).
A Laser-Induced Transient Photovoltaic Effect Using Blocked Electrodes William F. Coleman,' C w p 8 m n t of Chemktry, Unhrersily of New Msxico, Ahquerque, New Mxkw 87131
Michael 0. Prlrant, and Rkhard N. Zare' oepartment of chemktry, Stanford Unhrslty, Stanford, CaUfm4 94305 ( R w M : May 21, 1980; In F h l Form: August 12, 1980)
An aqueous solution of KMnO, is irradiated with a pulsed laser source. In the absence of an initially applied voltage, a transient photopotential is observed along the axis of excitation. The sign, magnitude, and temporal dependence of the photopotential are shown to be dependent on, and characteristic of, the nature of the solute. It is shown that the peak potential is linear in the excitation power of the laser source and in the quantum yield for the photoreduction of MnO; as a function of excitation wavelength. It is suggested that the transient potential originates from the photochemical perturbation of the electrode-electrolyte double layer.
Introduction Traditional electrochemical techniques apply electrical perturbations to a solution and measure the current and/or voltage response of the solution.' This study explores the electrical response of solutions to photochemical perturbations in the absence of an initially applied field. Photovoltaic phenomena are hardly new: but the introduction of the laser to these experiments allows us to reexamine these phenomena in a radically new light?v4 We see the following advantages in using a laser to induce a photochemical perturbation in a solution. First, the perturbation is induced in a very short time-on the order of 6-10 ns-with a nitrogen-pumped dye laser. This fine temporal resolution allows us to study rapidly decaying transients formed near the electrode-solution interface with relative ease compared to traditional techniques. Secondly, the laser provides a tunable, narrow excitation frequency. Spectral resolution allows us to deposit a precise amount of energy into a solute and follow its electrochemical behavior as a function of excitation wavelength. To begin our study of laser-induced photovoltaic phenomena, we chose to measure a transient potential using a blocked electrode because such measurements allow one to isolate the perturbative influence of the light. We selected the permanganate system for detailed study because, after cursorily examining a number of solutes in aqueous solution, we found that the permanganate ion produced the largest value of the peak photopotential. This was a 0022-3654f 80f2084-2685$0 1.OO/O
fortunate choice, The photochemistry of Mn04- under steady-state photolysis has been studied by several workersKp6and, although the mechanism of the photoprocess remains in doubt, Zimmerman' has demonstrated that the quantum yield for &04- photoreduction varies markedly as a function of excitation wavelength. Experimental Section Reagents. Reagent grade KMn04 (Matheson Coleman and Bell) was used without further purification. Solutions were prepared with triply distilled water. All of the results which have been obtained indicate that (1) the permanganate solutions used in this work were stable8 with respect to decomposition for at least several days and (2) the water used as the solvent contained no species that contributed to the transient photopotentials being measured. In spite of the above, the measurements reported here were made on freshly prepared KMn04 solutions. Cell Design. The basic cell design which has been used in this work is shown in Figure 1. The electrodes in the cell are made of NESA glass-Sn02 coated on borosilicate glass (Practical Products Co.). These electrodes are conducting and optically transparent at wavelengths greater than310 nm. The electrodes have been coated by vacuum deposition (UVIRA Optics, Mountain View, CA) with a 1-2-pm layer of quartz to prohibit direct contact between the solution under investigation and the SnOz surface. Electrical connections are made to evaporated silver contacts on each electrode. The solution being studied is 0 1980 American Chemlcal Soclety
