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Isotope mixing between chemisorbed CO molecules on a K promoted Ni(111) surface: an LITD study
Surface Science Letters 243 (1991) L67-L69 North-Holland
L67
Surface Science Letters
Isotope mixing between chemisorbed CO molecules on a K promoted Ni(111) surface: an LITD study K u m a r Sinniah, W i l l i a m D. Sands, J a n H r b e k 1, J o h n T. Y a t e s , Jr. a n d K e n n e t h C. J a n d a Department of Chemistry and Surface Science Center, University of Pittsburgh, Pittsburgh, PA 15260, USA
Received 8 August 1990; accepted for publication 24 October 1990
The laser induced thermal desorption technique (LITD) is used to monitor the extent of isotope mixing between CO moleculeson a potassium promoted, (0 K = 0.28) Ni(lll) surface. The onset of isotope mixing is found to occur at 400 K, well below the CO or potassium thermal desorption temperature. The maximum mixing yield is about 60% of the statistical limit over the temperature range of 450 to 600 K, suggesting that not all CO on the surface participates in the isotopic mixing reaction due to limited mobility. It is shown that the LITD technique can be used to measure the rate of mixing at temperatures well below that at which thermal desorption occurs. At 475 K the observed isotope exchange rate is consistent with a second order rate constant of k = 6 × 10 -2 ML-1 s - I
The use of chemisorbed potassium to promote the catalytic activity of transition metal surfaces has received significant attention over the past decade [1-5]. One of the more striking examples of such catalytic enhancement is the isotope mixing reaction between adsorbed CO molecules on a N i ( l l l ) surface partially covered with potassium [3,6]. In the absence of potassium, CO desorbs from N i ( l l l ) at 420 K with no scrambling of isotopes between CO molecules. Partial coverage of the Ni surface with potassium gradually increases the CO desorption temperature until, at OK = 0.28, the CO desorbs at 630 K with significant isotope scrambling. Several intriguing observations are associated with this process. Although isotope mixing is extensive, it does not proceed to the statistical limit [3,6]. The C - O vibrational frequency is very sensitive to both potassium and CO coverage. It ranges from -1400 cm -1 for CO molecules near the potassium site to ~ 1700 cm -1 for the CO farther away from the potassium site [6]. Remarkably, although 1 Permanent address: Brookhaven National Laboratory, Chemistry Department, Upton, NY 119733, USA.
the potassium induces the isotope mixing, no effect due to isotope mixing could be observed in the 1400 c m - 1 band [6]. At 0 K = 0.28, the thermal desorption of all CO molecules, including those which have undergone isotope mixing, occurs over the same narrow temperature range where potassium desorbs. Although several possible mechanisms have been proposed for this reaction, m a n y of the basic phenomena associated with it are still unknown [2,6-8]. Until now, the extent of isotope mixing has only been observed by thermal desorption [3,6]. This meant that there was no way to monitor the extent of the potassium catalyzed reaction below the desorption temperature. In this letter we report the application of the laser induced thermal desorption technique ( L I T D ) to measure the extent of isotope mixing as a function of time and surface temperature. The thermal transient induced by the laser pulse does not cause isotope mixing below a surface temperature of 400 K and does not appear to cause isotope mixing at higher surface temperature. It is found that the onset of mixing occurs at 400 K. At 475 K mixing is fast enough that it reaches its completion limit in
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
K Smniah et al. / LITD study o/ iaotope mixing between CO molecules on K promoted su([aces
L68
200 s. The final extent of isotope mixing at 475 K, 0.6 of the statistical limit, is the same as at 575 K, the high temperature limit imposed by thermal desorption. The experiments were carried out in a stainless steel UHV apparatus that has been described previously [9]. The surface cleanliness and order of the 1.7 cm diameter N i ( l l l ) crystal were verified by LEED and Auger spectroscopy. Potassium was first delivered to the surface at 250 K by evaporation from a commercial getter source (SAES) and potassium coverage, OK = K a t o m / N i surface atoms, was calibrated by temperature programmed desorption and LEED. A value of 0~ = 0.28 was chosen for these studies because it results in a narrow temperature range of CO desorption as well as the highest yield of CO isotope mixing. The surface was then dosed at 155 K with a 1 : 1 mixture of ~2ct80 and L~C~60 using a collimated array doser. The mixture was contaminated by 8% 12C1~'O and 5% 13ct80 as measured by the mass spectrometer in the experimental chamber. All studies reported here employed a Oco = 0.3 [10]. L I T D was performed by using the 308 nm output of a XeCl excimer laser with line-of-sight mass
t50
0K=0.28 "~
~3C~e0 : taC~a0
120
=
1:1
o
";
o
9o
12Cla0
0
O~
o m
60
~
30
o o
o~ 0
o
~2 0
1O0
0
o 200
13C~a0•
,e
o
B
eee 300
400
500
800
700
Temperature, K Fig. l. t2C180 and 13Ca80 LITD signals as a function of surface temperature on the N i ( l l l ) surface measured by laser induced thermal desorption. The surface was initially dosed with a 1 : 1 mixture of 1 3 C 1 6 0 and 12C~80 and then heated at a rate of 0.25 K / s .
,3 5
04
~_~
0.2
0,9
I
I
I
1oo
200
300
Time,
s
Fig. 2. Isotope mixing ratio as a function of time for a surface temperature of 475 K. OK = 0.28 and the initial 1 3 C 1 6 0 : 1 2 C I 8 0 = 1 : 1 dose was 0.3. The solid line is a fit to a second-order rate law resulting in a rate constant k = 6 × i 0 - 2 ML-1 s ! Note that complete mixing would have yielded a ratio of 0.5, whereas the maximum observed ratio is 0.3.
spectrometric detection of desorbing species. The laser output was apertured and focused to form an elliptical spot approximately 4 m m by 1 m m on the surface. The laser pulse energy was chosen such that it was 10% below the energy at which ablation of the Ni could be observed. Fig. 1 shows the results of an experiment in which, using OK = 0.28 and 0co = 0.3, the surface temperature was increased from 100-700 K at a rate of 0.25 K / s . Every 25 K, a laser pulse was directed at a new spot on the surface and the amount of desorbed CO was observed. The onset of L I T D for 12ct80, one of the isotopes initially dosed on to the surface, is seen to be about 200 K. The onset of L I T D for ~3ct80, which is formed by catalytic mixing on the surface is slightly above 400 K. This clearly indicates that the isotope mixing reaction does not proceed below 400 K but does proceed for surface temperatures between 400 and 550 K. Thus, isotope mixing takes place as a reaction on the surface at temperatures well below those which desorb the CO and potassium. Fig. 2 shows the results of an experiment to measure the rate of the isothermal isotope mixing
K. Sinniah et al. / LITD study of isotope mixing between CO molecules on K promoted surfaces
reaction. The K + CO layer was prepared as described above. For each data point shown on the figure the following procedure was used to measure the extent of isotope mixing as a function of reaction time at 475 K. The surface temperature was raised to 475 K for a time as indicated along the abscissa, then cooled to 350 K. Next, many laser desorption shots were performed (each shot impinging on a different spot on the surface) and the yield of one of the CO isotopes was measured for each shot. These measurements were made at 350 K because at this temperature the isotope mixing reaction is effectively quenched, but the laser induced thermal desorption is quite efficient as indicated in fig. 1. The data were averaged, and the isotope mixing ratio, 2131/(2131 + 130 + •29) is plotted as a function of reaction time in fig. 2. I N represents the average intensity of the CO isotope of mass number N. This particular ratio is used because it is a measure of the fraction of isotope mixing and minimizes the effect of background 12C160 contamination. The isotope mixing ratio is seen to gradually rise from a value of 0.09, reflecting the initial isotopic composition of the adsorbed CO gas, to 0.3. Complete mixing would yield a value of 0.5. These data show that mixing proceeds at a moderate rate at 475 K as opposed to instantaneous mixing either upon adsorption or desorption. The incomplete mixing limit, which has been found to be independent of temperature, indicates that all CO on the surface cannot participate equally in the isotopic mixing process due to limited mobility. The solid curve shown in fig. 2 is a fit of the data to a second order rate law. The second order rate constant at 475 K obtained from the fit is 6 × 10 -2 ML -1 s -1. (ML - monolayer, referenced to the number of Ni atoms in the Ni(111) surface.) Experiments are in progress to measure the rate law for other surface temperature in order to obtain Arrhenius parameters for the reaction.
L69
In summary, we have shown that the isotope mixing reaction on a potassium promoted Ni(111) surface can be monitored by the laser induced thermal desorption technique. The onset of reaction occurs at 400 K, and the rate constant at 475 K has been measured to be 6 × 10 -2 ML -1 s -1. The mixing reaction does not proceed to completion at long times, indicating that the CO has limited mobility on the surface. This work was supported by the United States Department of Energy (Contract No. DE-FG0288ER13933). We also wish to thank Mr. Z. Xu and Mr. Xingcai Guo for their help in preparation of the N i ( l l l ) crystal. J.T.Y. would like to acknowledge support from the United States Department of Energy (Contract No. DE-FG0288ER13864). J.H. would like to acknowledge support from the Brookhaven National Laboratory (Contract No. DE-AV02-76CH00016).
References [1] H.P. Bonzel, Surf. Sci. Rep. 8 (1987) 43. [2] F. Solymosi and A. Berko, Surf. Sci. 201 (1988) 361. [3] J. Lee, J. Arias, C.P. Hanrahan, R.M. Martin and H. Metiu, J. Chem. Phys. 82 (1985) 485. [4] R.A. dePaola, J. Hrbek and F.M. Hoffmann, J. Chem. Phys. 82 (1985) 2484. [5] K.J. Uram, L. Ng and J.T. Yates, Jr., Surf. Sci. 177 (1986) 253. [6] L. Ng, K.J. Uram, Z. Xu, P.L. Jones and J.T. Yates, Jr., J. Chem. Phys. 86 (1987) 6523. [7] J. Arias, J. Lee, J. Dunaway, R.M. Martin and H. Metiu, Surf. Sci. 159 (1985) IA33. [8] D. Lackey, M. Surman, S. Jacobs, D. Grider and D.A. King, Surf. Sci. 152/153 (1985) 513. [9] K. Sinniah, M.G. Sherman, L.B. Lewis, W.H. Weinberg, J.T. Yates, Jr. and K.C. Janda, J. Chem. Phys. 92 (1990) 5700. [10] The coverage of CO was calibrated by comparison of the CO TPD area to that of CO desorption from clean Ni at CO saturation. The saturated CO coverage on clean Ni was taken to be 0.57 ML as quoted in ref. [5].
L67
Surface Science Letters
Isotope mixing between chemisorbed CO molecules on a K promoted Ni(111) surface: an LITD study K u m a r Sinniah, W i l l i a m D. Sands, J a n H r b e k 1, J o h n T. Y a t e s , Jr. a n d K e n n e t h C. J a n d a Department of Chemistry and Surface Science Center, University of Pittsburgh, Pittsburgh, PA 15260, USA
Received 8 August 1990; accepted for publication 24 October 1990
The laser induced thermal desorption technique (LITD) is used to monitor the extent of isotope mixing between CO moleculeson a potassium promoted, (0 K = 0.28) Ni(lll) surface. The onset of isotope mixing is found to occur at 400 K, well below the CO or potassium thermal desorption temperature. The maximum mixing yield is about 60% of the statistical limit over the temperature range of 450 to 600 K, suggesting that not all CO on the surface participates in the isotopic mixing reaction due to limited mobility. It is shown that the LITD technique can be used to measure the rate of mixing at temperatures well below that at which thermal desorption occurs. At 475 K the observed isotope exchange rate is consistent with a second order rate constant of k = 6 × 10 -2 ML-1 s - I
The use of chemisorbed potassium to promote the catalytic activity of transition metal surfaces has received significant attention over the past decade [1-5]. One of the more striking examples of such catalytic enhancement is the isotope mixing reaction between adsorbed CO molecules on a N i ( l l l ) surface partially covered with potassium [3,6]. In the absence of potassium, CO desorbs from N i ( l l l ) at 420 K with no scrambling of isotopes between CO molecules. Partial coverage of the Ni surface with potassium gradually increases the CO desorption temperature until, at OK = 0.28, the CO desorbs at 630 K with significant isotope scrambling. Several intriguing observations are associated with this process. Although isotope mixing is extensive, it does not proceed to the statistical limit [3,6]. The C - O vibrational frequency is very sensitive to both potassium and CO coverage. It ranges from -1400 cm -1 for CO molecules near the potassium site to ~ 1700 cm -1 for the CO farther away from the potassium site [6]. Remarkably, although 1 Permanent address: Brookhaven National Laboratory, Chemistry Department, Upton, NY 119733, USA.
the potassium induces the isotope mixing, no effect due to isotope mixing could be observed in the 1400 c m - 1 band [6]. At 0 K = 0.28, the thermal desorption of all CO molecules, including those which have undergone isotope mixing, occurs over the same narrow temperature range where potassium desorbs. Although several possible mechanisms have been proposed for this reaction, m a n y of the basic phenomena associated with it are still unknown [2,6-8]. Until now, the extent of isotope mixing has only been observed by thermal desorption [3,6]. This meant that there was no way to monitor the extent of the potassium catalyzed reaction below the desorption temperature. In this letter we report the application of the laser induced thermal desorption technique ( L I T D ) to measure the extent of isotope mixing as a function of time and surface temperature. The thermal transient induced by the laser pulse does not cause isotope mixing below a surface temperature of 400 K and does not appear to cause isotope mixing at higher surface temperature. It is found that the onset of mixing occurs at 400 K. At 475 K mixing is fast enough that it reaches its completion limit in
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
K Smniah et al. / LITD study o/ iaotope mixing between CO molecules on K promoted su([aces
L68
200 s. The final extent of isotope mixing at 475 K, 0.6 of the statistical limit, is the same as at 575 K, the high temperature limit imposed by thermal desorption. The experiments were carried out in a stainless steel UHV apparatus that has been described previously [9]. The surface cleanliness and order of the 1.7 cm diameter N i ( l l l ) crystal were verified by LEED and Auger spectroscopy. Potassium was first delivered to the surface at 250 K by evaporation from a commercial getter source (SAES) and potassium coverage, OK = K a t o m / N i surface atoms, was calibrated by temperature programmed desorption and LEED. A value of 0~ = 0.28 was chosen for these studies because it results in a narrow temperature range of CO desorption as well as the highest yield of CO isotope mixing. The surface was then dosed at 155 K with a 1 : 1 mixture of ~2ct80 and L~C~60 using a collimated array doser. The mixture was contaminated by 8% 12C1~'O and 5% 13ct80 as measured by the mass spectrometer in the experimental chamber. All studies reported here employed a Oco = 0.3 [10]. L I T D was performed by using the 308 nm output of a XeCl excimer laser with line-of-sight mass
t50
0K=0.28 "~
~3C~e0 : taC~a0
120
=
1:1
o
";
o
9o
12Cla0
0
O~
o m
60
~
30
o o
o~ 0
o
~2 0
1O0
0
o 200
13C~a0•
,e
o
B
eee 300
400
500
800
700
Temperature, K Fig. l. t2C180 and 13Ca80 LITD signals as a function of surface temperature on the N i ( l l l ) surface measured by laser induced thermal desorption. The surface was initially dosed with a 1 : 1 mixture of 1 3 C 1 6 0 and 12C~80 and then heated at a rate of 0.25 K / s .
,3 5
04
~_~
0.2
0,9
I
I
I
1oo
200
300
Time,
s
Fig. 2. Isotope mixing ratio as a function of time for a surface temperature of 475 K. OK = 0.28 and the initial 1 3 C 1 6 0 : 1 2 C I 8 0 = 1 : 1 dose was 0.3. The solid line is a fit to a second-order rate law resulting in a rate constant k = 6 × i 0 - 2 ML-1 s ! Note that complete mixing would have yielded a ratio of 0.5, whereas the maximum observed ratio is 0.3.
spectrometric detection of desorbing species. The laser output was apertured and focused to form an elliptical spot approximately 4 m m by 1 m m on the surface. The laser pulse energy was chosen such that it was 10% below the energy at which ablation of the Ni could be observed. Fig. 1 shows the results of an experiment in which, using OK = 0.28 and 0co = 0.3, the surface temperature was increased from 100-700 K at a rate of 0.25 K / s . Every 25 K, a laser pulse was directed at a new spot on the surface and the amount of desorbed CO was observed. The onset of L I T D for 12ct80, one of the isotopes initially dosed on to the surface, is seen to be about 200 K. The onset of L I T D for ~3ct80, which is formed by catalytic mixing on the surface is slightly above 400 K. This clearly indicates that the isotope mixing reaction does not proceed below 400 K but does proceed for surface temperatures between 400 and 550 K. Thus, isotope mixing takes place as a reaction on the surface at temperatures well below those which desorb the CO and potassium. Fig. 2 shows the results of an experiment to measure the rate of the isothermal isotope mixing
K. Sinniah et al. / LITD study of isotope mixing between CO molecules on K promoted surfaces
reaction. The K + CO layer was prepared as described above. For each data point shown on the figure the following procedure was used to measure the extent of isotope mixing as a function of reaction time at 475 K. The surface temperature was raised to 475 K for a time as indicated along the abscissa, then cooled to 350 K. Next, many laser desorption shots were performed (each shot impinging on a different spot on the surface) and the yield of one of the CO isotopes was measured for each shot. These measurements were made at 350 K because at this temperature the isotope mixing reaction is effectively quenched, but the laser induced thermal desorption is quite efficient as indicated in fig. 1. The data were averaged, and the isotope mixing ratio, 2131/(2131 + 130 + •29) is plotted as a function of reaction time in fig. 2. I N represents the average intensity of the CO isotope of mass number N. This particular ratio is used because it is a measure of the fraction of isotope mixing and minimizes the effect of background 12C160 contamination. The isotope mixing ratio is seen to gradually rise from a value of 0.09, reflecting the initial isotopic composition of the adsorbed CO gas, to 0.3. Complete mixing would yield a value of 0.5. These data show that mixing proceeds at a moderate rate at 475 K as opposed to instantaneous mixing either upon adsorption or desorption. The incomplete mixing limit, which has been found to be independent of temperature, indicates that all CO on the surface cannot participate equally in the isotopic mixing process due to limited mobility. The solid curve shown in fig. 2 is a fit of the data to a second order rate law. The second order rate constant at 475 K obtained from the fit is 6 × 10 -2 ML -1 s -1. (ML - monolayer, referenced to the number of Ni atoms in the Ni(111) surface.) Experiments are in progress to measure the rate law for other surface temperature in order to obtain Arrhenius parameters for the reaction.
L69
In summary, we have shown that the isotope mixing reaction on a potassium promoted Ni(111) surface can be monitored by the laser induced thermal desorption technique. The onset of reaction occurs at 400 K, and the rate constant at 475 K has been measured to be 6 × 10 -2 ML -1 s -1. The mixing reaction does not proceed to completion at long times, indicating that the CO has limited mobility on the surface. This work was supported by the United States Department of Energy (Contract No. DE-FG0288ER13933). We also wish to thank Mr. Z. Xu and Mr. Xingcai Guo for their help in preparation of the N i ( l l l ) crystal. J.T.Y. would like to acknowledge support from the United States Department of Energy (Contract No. DE-FG0288ER13864). J.H. would like to acknowledge support from the Brookhaven National Laboratory (Contract No. DE-AV02-76CH00016).
References [1] H.P. Bonzel, Surf. Sci. Rep. 8 (1987) 43. [2] F. Solymosi and A. Berko, Surf. Sci. 201 (1988) 361. [3] J. Lee, J. Arias, C.P. Hanrahan, R.M. Martin and H. Metiu, J. Chem. Phys. 82 (1985) 485. [4] R.A. dePaola, J. Hrbek and F.M. Hoffmann, J. Chem. Phys. 82 (1985) 2484. [5] K.J. Uram, L. Ng and J.T. Yates, Jr., Surf. Sci. 177 (1986) 253. [6] L. Ng, K.J. Uram, Z. Xu, P.L. Jones and J.T. Yates, Jr., J. Chem. Phys. 86 (1987) 6523. [7] J. Arias, J. Lee, J. Dunaway, R.M. Martin and H. Metiu, Surf. Sci. 159 (1985) IA33. [8] D. Lackey, M. Surman, S. Jacobs, D. Grider and D.A. King, Surf. Sci. 152/153 (1985) 513. [9] K. Sinniah, M.G. Sherman, L.B. Lewis, W.H. Weinberg, J.T. Yates, Jr. and K.C. Janda, J. Chem. Phys. 92 (1990) 5700. [10] The coverage of CO was calibrated by comparison of the CO TPD area to that of CO desorption from clean Ni at CO saturation. The saturated CO coverage on clean Ni was taken to be 0.57 ML as quoted in ref. [5].