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Direct observation of hydrogen-deuterium exchange in ethylidyne adsorbed on Pt(111)
Ll37
Surface Science 138 (1984) L137-L141 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
DIRECT OBSERVATION OF HYDROGEN-DEUTERIUM IN ETHYLIDYNE ADSORBED ON Pt(ll1) * J.R. CREIGHTON,
K.M. OGLE
Department
The University of Texas at Austin, Austin,
Received
of Chemistry,
3 October
1983; accepted
EXCHANGE
and J.M. WHITE
for publication
7 December
Texas 78712, USA
1983
The hydrogen isotope exchange reaction in ethylidyne adsorbed on Pt(ll1) was followed using static secondary ion mass spectrometry. The results are adequately described using a model in which exchange occurs sequentially.
Ethylene is known to dehydrogenate to an ethylidyne species (>C-CH,) on Pt(Il1) near room temperature. Conclusive evidence for the ethylidyne structure comes from TPD [l-3], LEED [4], UPS [5], HREELS [l] and SIMS [2] experiments. The pioneering work was done by Somorjai and coworkers [4]. It is of interest to investigate alkylidyne chemistry since such species may represent an important class of intermediates in hydrocarbon catalysis. Isotope exchange is one method of probing the kinetic properties of these adsorbates. Most studies have used the isotopic analysis of gas phase products to monitor the exchange process since most surface-sensitive spectroscopies cannot differentiate between adsorbates with different isotopic composition. In many situations the principle species may remain adsorbed under the conditions of interest, making it advantageous to monitor the adsorbed phase. One recent example of this approach was the study of hydrogen-deuterium exchange in benzene adsorbed on Pt(ll0) [6]. The time dependence of the relative amounts of C-H and C-D bonds on the surface was measured with HREELS. In this letter we demonstrate that static secondary ion mass spectroscopy (SSIMS) is capable of monitoring the hydrogen-deuterium exchange reaction in ethylidyne adsorbed on Pt(ll1). The outstanding feature of the SSIMS technique is that all four ethylidyne isotopes (C,H,, C,H,D, C,HD, and C,D,) can be distinguished from each other and their time dependence can be recorded simultaneously.
* Supported in part by the National Science Foundation (CHE80-01507), the Petroleum Fund of the American Chemical Society, and the Robert A. Welch Foundation.
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Research
Ll3X
J.R. Creighton et al. / H-D
exchange in ethylidyne on Pt(ll1)
The experiments were performed using a Pt(ll1) surface and a UHV chamber which has previously been described [2]. The surface was probed with a defocused 500 eV argon ion beam with a typical current density of 4-10 nA/cm’. This assured that the loss of material due to sputtering was < 10% over the time scale of the experiments. The key to monitoring ethylidyne concentrations is the presence of an intense CH: (m/e = 15) in the positive SSIMS spectrum [2]. In all experiments the intensities of the positive secondary ions at m/e = 15, 16, 17 and 18 (denoted as I,,, I,,, I,, and I,,, respectively) were recorded by multiplexing the mass spectrometer. These ions serve as monitors of the C,H,(a), C,H,D(a), C,HD,(a) and C,D,(a) concentrations. For C,H,(a) there is a significant CH: intensity in the cracking pattern, typically 25%40% of the CH: signal. Likewise, the deuterated ethylidynes will contribute a signal component to I,, and Z,, because of the CHD+ and CD: ions produced. While these effects can
15
300
TIME/set Fig. 1. Upper panel: A plot of the secondary ion intensities during the isotope exchange reaction at 383 K; curves a-d are for m/e = 15, 16, 17 and 18, respectively. Incident Ar+ current density is 7 nA/cm’. Lower panel: Calculated results for the model discussed in the text using (kO,,-’ = 87 s. Curves e-h are for 8X0, Ox,, Ox, and Ox,, respectively.
J.R.
Creighton et al. / H-D
exchange
in ethylidyne on Ftfl II)
L139
be accounted for with extensive calibration, the preliminary results presented here utilized the uncorrected ion signals. It should be noted that it, and I,, are not complicated by these cracking pattern problems (neglecting 13C effects) and thus are uniquely derived from CHD: and CD: ions, respectively. The Pt(ll1) sample was exposed to 2.2 L of ethylene at 200 K, and the temperature was raised to 3.50-400 K and held constant within f0.2 K. During this process the ethylene that does not desorb is converted to ethylidyne by loss of a hydrogen atom which recombines with another surface hydrogen and desorbs as Hz [l--3]. After the temperature stabilized, the ion beam was turned on, and ZiS, Zi6, I,, and Zi8 were recorded. Deuterium was then introduced at a pressure of 1.3 x 10e6 Torr to initiate the isotope exchange reaction. The initial coverage of ethylidyne was calculated as 0.21 4 0.03 ML on the basis of the total amount of H, desorbed from the complete decomposition of ethylene [2] as compared to the same total from a Pt(ll1) surface saturated with H(a) at 100 K. The latter is defined as 1 ML. A typical experimental result is shown in fig. 1 (upper panel). Immediately following D, introduction, Z,5 begins to decay and, in sequence, I,,, I,, and Zis begin to increase. After I,, has decayed to less than one-half its original value, I,, peaks and begins to decay. Then, I,, peaks and Zis increases to a saturation value. It is important to distinguish ion beam effects from thermal effects. As noted above, the direct sputtering of the surface species is less than 10% of the initial coverage during the course of the measurements shown in fig. 1. We also note that the isotope exchange cannot be due to the formation of D+ in the ion source. If the beam were 100% I)+ and it reacted with unit probability, the currents used correspond to only 0.01 ML over 500 s. Further tests have been done in these and other experiments involving ethylene [2] and there is no evidence for any contribution by D’. On this basis we proceed to interpret the results of fig. 1 as the result of thermal processes. The results can be modeled qualitatively using the following scheme: X,+D2X,+H,
X,+D
2x,+H,
X,+0
k, +X,+H,
Xl = C,Hs_;D,. We shall assume that D(a) quickly reaches a steady-state concentration after D, is introduced into the system. Since the sample temperature (350-400 K) is higher than the desorption temperature of hydrogen on the ethylidyne-covered surface [l-3], the steady-state coverage is small and quickly reached. For 8, = constant and 0, = 0, the model can be easily solved explicitly. For k, = k, = k, = k, the solution is: Bx, =
eioe-keD',
X0 = C,H,,
8, = k@,@&t emk8f+,
Xl = C,H,D,
Ll40 8 x2
J. R. Crerghton ei al. / H - D exchange in et+lidyne = ‘kZ@2@ t2 e-X8,r 2 D X,
on Pt(I I I)
X,=C,HD,,
7
e,l=e~o-B,,~-B,,-8,1,
X,=C,D,.
This solution is plotted in fig. 1 (lower panel) for (kB,)-’ = 87 s, The qualitative behavior of this model compares very favorably with the experimental results of fig. 1. Quantitative differences are due in part to the cracking pattern overlap which is most noticeable when comparing I,, with 8, late in the reaction. One fundamental question about the isotope exchange mechanism is whether or not the dehydrogenation step occurs before, during or after the deuteration step, i.e., either C,H,
+ C,H,
+ H %,H,D
+ H,
or C,H,
+ D -+ H--C,H,--D
-+ C,H,D
+ H,
or C,H,
+ D -+ C,H,D
-+ C,H,D
+ H.
Preliminary evidence favors one or the other of the latter two because the dehydrogenation of ethylidyne appears to be irreversible under conditions similar to those used in the isotope exchange experiments. This was determined in an experiment where ethylidyne was formed in the usual manner and then allowed to dehydrogenate at temperatures reaching - 450 K until about half of the ethylidyne remained (as evidenced by SSIMS). Then 1.3 X 10d6 Torr of H, was admitted into the vacuum chamber, and the sample temperature was scanned over a 300-400 K range while the m/e = + 15 SIMS signal was recorded. This signal did not increase over a 20 min period, indicating that the ethylidyne dehydrogenation step was essentially irreversible. Therefore, we
or
Fig. 2. Schematic
paths for isotope
exchange
in ethylidyne.
J. R. Creighron et al. / H - D exchange in ethylidyne on Pi(l1 I)
believe the isotope exchange reaction ethylidyne to form either (1) ethylidene deuterated ethylidyne or (2) H,C-CDare illustrated in fig. 2.
L141
involves the addition of deuterium to which then dehydrogenates leaving the in a concerted step. These two paths
The thermal behavior of the isotope exchange reaction rate has not yet been thoroughly investigated. Preliminary experiments indicate that the deuterium adsorption-desorption rates, and hence the steady-state deuterium coverage, are very sensitive to the initial ethylidyne coverage and/or the presence of small amounts of CO(a) or other impurities. An investigation of these variables is underway. In conclusion, SSIMS can be used to study the time dependence of the hydrogen-deuterium exchange reaction in ethylidyne on Pt(ll1) by simultaneously monitoring secondary ion signals characteristic of the four possible isotopes. The isotope exchange mechanism is believed to proceed through an ethylidene intermediate since the ethylidyne dehydrogenation step is irreversible under reaction conditions. A simple kinetic model qualitatively reproduces the observed time dependence of the reaction. Further improvements require better knowledge of the cracking patterns of the ethylidyne isotopes. Determination of the temperature dependence awaits refinements in procedures for controlling initial conditions.
References [l] [2] [3] [4]
H. Steininger, H. Ibach and S. Lehwald, Surface Sci. 117 (1982) 685. J.R. Creighton and J.M. White, Surface Sci. 129 (1983) 327. M. Salmeron and G.A. Somotjai, J. Phys. Chem. 86 (1982) 341. (a) L.L. Kesmodel, L.H. Dubois and G.A. Somojai, J. Chem. Phys. 70 (1979) 2180; (b) L.L. Kesmodel, L.H. Dubois and G.A. Somojai, Chem. Phys. Letters 56 (1978) 267. [5] M.R. Albert, L.C. St&don, W. Eberhardt, F. Greuter, T. Gustafson and E.W. Plummer, Surface Sci. 120 (1982) 19. [6] M. Surman, S.R. Bare, P. Hofmann and D.A. King, Surface Sci. 126 (1983) 349.
Surface Science 138 (1984) L137-L141 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
DIRECT OBSERVATION OF HYDROGEN-DEUTERIUM IN ETHYLIDYNE ADSORBED ON Pt(ll1) * J.R. CREIGHTON,
K.M. OGLE
Department
The University of Texas at Austin, Austin,
Received
of Chemistry,
3 October
1983; accepted
EXCHANGE
and J.M. WHITE
for publication
7 December
Texas 78712, USA
1983
The hydrogen isotope exchange reaction in ethylidyne adsorbed on Pt(ll1) was followed using static secondary ion mass spectrometry. The results are adequately described using a model in which exchange occurs sequentially.
Ethylene is known to dehydrogenate to an ethylidyne species (>C-CH,) on Pt(Il1) near room temperature. Conclusive evidence for the ethylidyne structure comes from TPD [l-3], LEED [4], UPS [5], HREELS [l] and SIMS [2] experiments. The pioneering work was done by Somorjai and coworkers [4]. It is of interest to investigate alkylidyne chemistry since such species may represent an important class of intermediates in hydrocarbon catalysis. Isotope exchange is one method of probing the kinetic properties of these adsorbates. Most studies have used the isotopic analysis of gas phase products to monitor the exchange process since most surface-sensitive spectroscopies cannot differentiate between adsorbates with different isotopic composition. In many situations the principle species may remain adsorbed under the conditions of interest, making it advantageous to monitor the adsorbed phase. One recent example of this approach was the study of hydrogen-deuterium exchange in benzene adsorbed on Pt(ll0) [6]. The time dependence of the relative amounts of C-H and C-D bonds on the surface was measured with HREELS. In this letter we demonstrate that static secondary ion mass spectroscopy (SSIMS) is capable of monitoring the hydrogen-deuterium exchange reaction in ethylidyne adsorbed on Pt(ll1). The outstanding feature of the SSIMS technique is that all four ethylidyne isotopes (C,H,, C,H,D, C,HD, and C,D,) can be distinguished from each other and their time dependence can be recorded simultaneously.
* Supported in part by the National Science Foundation (CHE80-01507), the Petroleum Fund of the American Chemical Society, and the Robert A. Welch Foundation.
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Research
Ll3X
J.R. Creighton et al. / H-D
exchange in ethylidyne on Pt(ll1)
The experiments were performed using a Pt(ll1) surface and a UHV chamber which has previously been described [2]. The surface was probed with a defocused 500 eV argon ion beam with a typical current density of 4-10 nA/cm’. This assured that the loss of material due to sputtering was < 10% over the time scale of the experiments. The key to monitoring ethylidyne concentrations is the presence of an intense CH: (m/e = 15) in the positive SSIMS spectrum [2]. In all experiments the intensities of the positive secondary ions at m/e = 15, 16, 17 and 18 (denoted as I,,, I,,, I,, and I,,, respectively) were recorded by multiplexing the mass spectrometer. These ions serve as monitors of the C,H,(a), C,H,D(a), C,HD,(a) and C,D,(a) concentrations. For C,H,(a) there is a significant CH: intensity in the cracking pattern, typically 25%40% of the CH: signal. Likewise, the deuterated ethylidynes will contribute a signal component to I,, and Z,, because of the CHD+ and CD: ions produced. While these effects can
15
300
TIME/set Fig. 1. Upper panel: A plot of the secondary ion intensities during the isotope exchange reaction at 383 K; curves a-d are for m/e = 15, 16, 17 and 18, respectively. Incident Ar+ current density is 7 nA/cm’. Lower panel: Calculated results for the model discussed in the text using (kO,,-’ = 87 s. Curves e-h are for 8X0, Ox,, Ox, and Ox,, respectively.
J.R.
Creighton et al. / H-D
exchange
in ethylidyne on Ftfl II)
L139
be accounted for with extensive calibration, the preliminary results presented here utilized the uncorrected ion signals. It should be noted that it, and I,, are not complicated by these cracking pattern problems (neglecting 13C effects) and thus are uniquely derived from CHD: and CD: ions, respectively. The Pt(ll1) sample was exposed to 2.2 L of ethylene at 200 K, and the temperature was raised to 3.50-400 K and held constant within f0.2 K. During this process the ethylene that does not desorb is converted to ethylidyne by loss of a hydrogen atom which recombines with another surface hydrogen and desorbs as Hz [l--3]. After the temperature stabilized, the ion beam was turned on, and ZiS, Zi6, I,, and Zi8 were recorded. Deuterium was then introduced at a pressure of 1.3 x 10e6 Torr to initiate the isotope exchange reaction. The initial coverage of ethylidyne was calculated as 0.21 4 0.03 ML on the basis of the total amount of H, desorbed from the complete decomposition of ethylene [2] as compared to the same total from a Pt(ll1) surface saturated with H(a) at 100 K. The latter is defined as 1 ML. A typical experimental result is shown in fig. 1 (upper panel). Immediately following D, introduction, Z,5 begins to decay and, in sequence, I,,, I,, and Zis begin to increase. After I,, has decayed to less than one-half its original value, I,, peaks and begins to decay. Then, I,, peaks and Zis increases to a saturation value. It is important to distinguish ion beam effects from thermal effects. As noted above, the direct sputtering of the surface species is less than 10% of the initial coverage during the course of the measurements shown in fig. 1. We also note that the isotope exchange cannot be due to the formation of D+ in the ion source. If the beam were 100% I)+ and it reacted with unit probability, the currents used correspond to only 0.01 ML over 500 s. Further tests have been done in these and other experiments involving ethylene [2] and there is no evidence for any contribution by D’. On this basis we proceed to interpret the results of fig. 1 as the result of thermal processes. The results can be modeled qualitatively using the following scheme: X,+D2X,+H,
X,+D
2x,+H,
X,+0
k, +X,+H,
Xl = C,Hs_;D,. We shall assume that D(a) quickly reaches a steady-state concentration after D, is introduced into the system. Since the sample temperature (350-400 K) is higher than the desorption temperature of hydrogen on the ethylidyne-covered surface [l-3], the steady-state coverage is small and quickly reached. For 8, = constant and 0, = 0, the model can be easily solved explicitly. For k, = k, = k, = k, the solution is: Bx, =
eioe-keD',
X0 = C,H,,
8, = k@,@&t emk8f+,
Xl = C,H,D,
Ll40 8 x2
J. R. Crerghton ei al. / H - D exchange in et+lidyne = ‘kZ@2@ t2 e-X8,r 2 D X,
on Pt(I I I)
X,=C,HD,,
7
e,l=e~o-B,,~-B,,-8,1,
X,=C,D,.
This solution is plotted in fig. 1 (lower panel) for (kB,)-’ = 87 s, The qualitative behavior of this model compares very favorably with the experimental results of fig. 1. Quantitative differences are due in part to the cracking pattern overlap which is most noticeable when comparing I,, with 8, late in the reaction. One fundamental question about the isotope exchange mechanism is whether or not the dehydrogenation step occurs before, during or after the deuteration step, i.e., either C,H,
+ C,H,
+ H %,H,D
+ H,
or C,H,
+ D -+ H--C,H,--D
-+ C,H,D
+ H,
or C,H,
+ D -+ C,H,D
-+ C,H,D
+ H.
Preliminary evidence favors one or the other of the latter two because the dehydrogenation of ethylidyne appears to be irreversible under conditions similar to those used in the isotope exchange experiments. This was determined in an experiment where ethylidyne was formed in the usual manner and then allowed to dehydrogenate at temperatures reaching - 450 K until about half of the ethylidyne remained (as evidenced by SSIMS). Then 1.3 X 10d6 Torr of H, was admitted into the vacuum chamber, and the sample temperature was scanned over a 300-400 K range while the m/e = + 15 SIMS signal was recorded. This signal did not increase over a 20 min period, indicating that the ethylidyne dehydrogenation step was essentially irreversible. Therefore, we
or
Fig. 2. Schematic
paths for isotope
exchange
in ethylidyne.
J. R. Creighron et al. / H - D exchange in ethylidyne on Pi(l1 I)
believe the isotope exchange reaction ethylidyne to form either (1) ethylidene deuterated ethylidyne or (2) H,C-CDare illustrated in fig. 2.
L141
involves the addition of deuterium to which then dehydrogenates leaving the in a concerted step. These two paths
The thermal behavior of the isotope exchange reaction rate has not yet been thoroughly investigated. Preliminary experiments indicate that the deuterium adsorption-desorption rates, and hence the steady-state deuterium coverage, are very sensitive to the initial ethylidyne coverage and/or the presence of small amounts of CO(a) or other impurities. An investigation of these variables is underway. In conclusion, SSIMS can be used to study the time dependence of the hydrogen-deuterium exchange reaction in ethylidyne on Pt(ll1) by simultaneously monitoring secondary ion signals characteristic of the four possible isotopes. The isotope exchange mechanism is believed to proceed through an ethylidene intermediate since the ethylidyne dehydrogenation step is irreversible under reaction conditions. A simple kinetic model qualitatively reproduces the observed time dependence of the reaction. Further improvements require better knowledge of the cracking patterns of the ethylidyne isotopes. Determination of the temperature dependence awaits refinements in procedures for controlling initial conditions.
References [l] [2] [3] [4]
H. Steininger, H. Ibach and S. Lehwald, Surface Sci. 117 (1982) 685. J.R. Creighton and J.M. White, Surface Sci. 129 (1983) 327. M. Salmeron and G.A. Somotjai, J. Phys. Chem. 86 (1982) 341. (a) L.L. Kesmodel, L.H. Dubois and G.A. Somojai, J. Chem. Phys. 70 (1979) 2180; (b) L.L. Kesmodel, L.H. Dubois and G.A. Somojai, Chem. Phys. Letters 56 (1978) 267. [5] M.R. Albert, L.C. St&don, W. Eberhardt, F. Greuter, T. Gustafson and E.W. Plummer, Surface Sci. 120 (1982) 19. [6] M. Surman, S.R. Bare, P. Hofmann and D.A. King, Surface Sci. 126 (1983) 349.