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Effect of surface contamination on the heat of adsorption of gases on metals
Surface Science 0 North-Holland
50 (1975) 253-256 Publishing Company
LETTERS TO THE EDITOR
EFFECTOFSURFACECONTAMINATlONONTHEHEATOF ADSOR~IONOFGASESONM~TA~S Received
6 January
1975; revised manuscript
received
24 February
1975
The values published by various authors for heats of adsorption obtained by calorimetric measurements in the same gas-solid systems usually differ quite substantially from each other. So far there does not exist a system whose heat of adsorption could be used as a reasonably reliable standard for c~ibration. Differences in the crystalline state of the adsorbent, in the degree and nature of the surface contamination, and in some cases also in the reliability of the experimental technique and of the data treatment, are responsible for this situation. The present note deals with the effect of the surface cleanliness of molybdenum films on the heat of adsorption of hydrogen as determined by calorimetric measurement. The characteristics of the films and of the hydrogen adsorption are given in table 1. The films were evaporated from resistively heated wire produced by Radium Elektrizitatsgesellschaft, Wipperfiirth, West Germany. Mass spectrometric analysis has Table 1 Characteristics
of the molybdenum
films and of the adsorption -~
Film No.
of hydrogen ___l_____. I
2
-Weight (mg) Time of deposition
(min)
Mean rate of deposition Vacuum
during
X lo-l5
deposition
(atom
mm-’
cme2)
(torr)
Vacuum between the end of deposition sion of the first dose (torr)
7.4
75
57
6.1
8.1
2 x 1o-9
9 x 1o-9
4 x Id’O
3 x 1o-9
88 44.5
215 635
and the admis-
Time from the end of deposition to the first dose (min) to the last dose (min) Number
8.1
of doses up to n*
27
21
n* per 100 mg of the film (micromoles)
34.7
33.3
Integral heat of adsorption (kcal/mole)
24.9
25.5
between
n = 0 and n = n*
254
S. 6ernjjSurface
contamination
dnd heat of adsorption
shown that it contains 0.91 wt% of admixtures, the main constituents being Si (0.67%), Sn (0.12%) W (0.04%) and Cr (0.03%). Hydrogen prepared by electrolysis was purified by diffusion through a hot palladium thimble. The calorimeter and adsorption apparatus have been described in ref. [l] The two traps protecting the section with the calorimeter and the ion gauge against mercury vapor from the apparatus, were cooled in the case of film 1 with liquid nitrogen, and in the case of film 2 with an ethanol-solid CO, bath. The area of the calorimeter wall covered by film was in both cases about 100 cm2. The films were deposited at 285 K and no subsequent thermal treatment was applied. The heat measurement was ended up when an equilibrium pressure of about 1 x 10m4 torr was detected above the film. The corresponding number of adsorbed micromoles of hydrogen has been denoted by IT*. Results of the measurements are given in fig. 1. For fitting the experimental points by a smooth curve the standard processing of the experimental data by correlation was applied. The procedure, outlined in more detail elsewhere [l] , consists essentially in finding the simplest correlation function which exhibits the lowest standard deviations both of the over-all fit and of the individual coefficients. In this way the probability of objective and accurate fitting of the measured points is enhanced as
n/n+
0.4
0.2
.O
0.6
1
/
10
0
5
10
15
20 XA”
Fig. 1. Differential o -. - film 2; -
/ 700
heat-coverage curves for hydrogen films prepared under substantially
25
30
mg
$_lmok)
on molybdenum poorer vacuum
35
films: l - - - film 1; (after ref. [ 21).
S. ~er~~~~~~rface contami~ti~n
and heat #f adsorptive
255
compared to the traditional drawing a smooth curve through the experimental points by hand only. In the present runs, straight lines were found to be the best fits up to n/n* - 0.85, expressed by dQ/dn (kcal/mole)
= 29.79 (* 0.31) - 3.29 (* 0.22) I’I
for film 1, and by dQ/dn (kcal/mole)
= 30.71 (* 0.37) - 4.00 (* 0.34) n
for film 2 (n = XAn denotes the sum of micromoles adsorbed in the successive doses). The over-all standard deviations of the fits were f 0.83 and i: 1.11 kcal/mole for films 1 and 2, respectively. The standard deviations of the coefficients are given in the parentheses. Thus, the initial differential heats are 29.8 and 30.7 kcal/mole for films 1 and 2, respectively. From the basic kinetic theory of gases it follows that other things being equal, the number of impacts of the contaminating species from the gas phase prior to the admission of the first hydrogen dose was markedly higher for film 2 as compared to film 1; viz. 6 times higher in the total period between the beginning of the film deposition and admission of the first dose, and 18 times higher if the period from the end of film deposition to the first dose admission only is considered. The initial heat of adsorption for film 2 was by 3% higher than for film 1, and the differential heat versus coverage curve was slightly steeper. It is interesting to compare this trend with our earlier results for the MO-H, system [2] obtained in a very similar apparatus and with films evaporated from the wire from the same reel as in the present case, so that the nature of the film contamination was very likely to be much the same. Vacuum conditions in those experiments led to approximately 10-100 times more impacts of gas particles onto the film surface prior to the first hydrogen dose as compared to the present film 2. Markedly higher initial heat (40 kcalfmole) resulted and the differential heat-coverage curve was rather steeply falling (fig. 1). Thus it seems likely that the higher the contamination of the film with species occurring in the type of experiments under consideration, the higher the initial heat of hydrogen adsorption, and the steeper the fall in the differential heat-coverage curve. Our recent results for the Pt-H, system [l] suggest the same conclusion. This is in general agreement also with the earlier findings of Wedler and coworkers for films of Ti [3] and Ni [4-71 prepared again in a mercury apparatus, and approximately 5-10 times thinner and with a roughness factor several times lower as compared to the MO films in our present case. The heats of hydrogen adsorption on the said thin films of Ti and Ni deposited under I Om9torr were reported to be about 20-30% higher and more steeply decreasing with the increasing surface coverage than the heats for films deposited in the 1O-1o torr region. (Moreover, even different character of the differential heat-coverage curves has been claimed for the said films of Ni deposited in the lo-’ and in the 10-l’ torr region.) Leaving apart the indirect methods of estimation of the heat of adsorption from adsorption isosteres, from the kinetics of thermal desorption, etc., the direct calori-
256
S. &rn$/Surface contamination and heat of adsorption
metric determination of the heat of adsorption of hydrogen and other simple gases on “clean” metal surfaces has been accomplished in a few cases on low-area filaments under UHV conditions [8-lo], and in a number of cases on films in mercury adsorption apparatus [ 111. With a few exceptions [ 1,3--71, the films for calorimetric measurements have been prepared hitherto under the residual pressure of 10m8 torr or higher. The degree of the contamination obviously depends also on the surface area of the adsorbent. The relevant data enabling to make an estimate of the number of impacts of the contaminating species onto the unit surface before and during the heat measurement are often not given. Nevertheless it seems probable that the most calorimetric values so far available for the magnitude and coverage dependence of the heat of adsorption of hydrogen and likely also of other gases on “clean” polycrystalline metal surfaces are more or less uncertain on account of an appreciable surface contamination. Sincere thanks are expressed to Dr. M. Smutek for his kindly performing the processing of the experimental data to find the best fit curves, and to Dr. M. Cukr for mass spectrometric analysis of the molybdenum wire.
Slavoj CERNY
Czechoslovak Academy and Electrochemistry,
of Sciences, J. Hey?ovsky Institute of Physical Chemistry Ma’chova 7, 121 38 Prague 2, Czechoslovakia,
References [1 ] S. &rnf, M. Smutek and F. Buzek, J. Catalysis, in press. [2] S. Cernq, V. Ponec and L. HlLdek, J. Catalysis 5 (1966) 27. [3] [4] [S] [6] [7] [S] [9]
G. Wedler and H. Strothenk, Ber. Bunsenges. Physik. Chem. 70 (1966) 214. F.J. Brbcker and G. Wedler, Discussions Faraday Sot. 41 (1966) 87. G. Wedler, Discussions Faraday Sot. 41 (1966) 104. G. Wedler and F.J. Brbcker, Surface Sci. 26 (1971) 454. G. Wedler and G. Fisch, Ber. Bunsenges. Physik. Chem. 76 (1972) 1160. V. Ponec, Z. Knor and S. Cerny, Adsorption on Solids (Butterworths, London, 1974) p. 131. P.R. Norton and P.J. Richards, Surface Sci. 44 (1974) 129. [lo] H. Yamazaki, T. Oguri and I. Kanomata, Japan. J. Appl. Phys. 10 (1971) 1105. [ 111 S. tern< and V. Ponec, Catalysis Rev. 2 (1968) 249.
50 (1975) 253-256 Publishing Company
LETTERS TO THE EDITOR
EFFECTOFSURFACECONTAMINATlONONTHEHEATOF ADSOR~IONOFGASESONM~TA~S Received
6 January
1975; revised manuscript
received
24 February
1975
The values published by various authors for heats of adsorption obtained by calorimetric measurements in the same gas-solid systems usually differ quite substantially from each other. So far there does not exist a system whose heat of adsorption could be used as a reasonably reliable standard for c~ibration. Differences in the crystalline state of the adsorbent, in the degree and nature of the surface contamination, and in some cases also in the reliability of the experimental technique and of the data treatment, are responsible for this situation. The present note deals with the effect of the surface cleanliness of molybdenum films on the heat of adsorption of hydrogen as determined by calorimetric measurement. The characteristics of the films and of the hydrogen adsorption are given in table 1. The films were evaporated from resistively heated wire produced by Radium Elektrizitatsgesellschaft, Wipperfiirth, West Germany. Mass spectrometric analysis has Table 1 Characteristics
of the molybdenum
films and of the adsorption -~
Film No.
of hydrogen ___l_____. I
2
-Weight (mg) Time of deposition
(min)
Mean rate of deposition Vacuum
during
X lo-l5
deposition
(atom
mm-’
cme2)
(torr)
Vacuum between the end of deposition sion of the first dose (torr)
7.4
75
57
6.1
8.1
2 x 1o-9
9 x 1o-9
4 x Id’O
3 x 1o-9
88 44.5
215 635
and the admis-
Time from the end of deposition to the first dose (min) to the last dose (min) Number
8.1
of doses up to n*
27
21
n* per 100 mg of the film (micromoles)
34.7
33.3
Integral heat of adsorption (kcal/mole)
24.9
25.5
between
n = 0 and n = n*
254
S. 6ernjjSurface
contamination
dnd heat of adsorption
shown that it contains 0.91 wt% of admixtures, the main constituents being Si (0.67%), Sn (0.12%) W (0.04%) and Cr (0.03%). Hydrogen prepared by electrolysis was purified by diffusion through a hot palladium thimble. The calorimeter and adsorption apparatus have been described in ref. [l] The two traps protecting the section with the calorimeter and the ion gauge against mercury vapor from the apparatus, were cooled in the case of film 1 with liquid nitrogen, and in the case of film 2 with an ethanol-solid CO, bath. The area of the calorimeter wall covered by film was in both cases about 100 cm2. The films were deposited at 285 K and no subsequent thermal treatment was applied. The heat measurement was ended up when an equilibrium pressure of about 1 x 10m4 torr was detected above the film. The corresponding number of adsorbed micromoles of hydrogen has been denoted by IT*. Results of the measurements are given in fig. 1. For fitting the experimental points by a smooth curve the standard processing of the experimental data by correlation was applied. The procedure, outlined in more detail elsewhere [l] , consists essentially in finding the simplest correlation function which exhibits the lowest standard deviations both of the over-all fit and of the individual coefficients. In this way the probability of objective and accurate fitting of the measured points is enhanced as
n/n+
0.4
0.2
.O
0.6
1
/
10
0
5
10
15
20 XA”
Fig. 1. Differential o -. - film 2; -
/ 700
heat-coverage curves for hydrogen films prepared under substantially
25
30
mg
$_lmok)
on molybdenum poorer vacuum
35
films: l - - - film 1; (after ref. [ 21).
S. ~er~~~~~~rface contami~ti~n
and heat #f adsorptive
255
compared to the traditional drawing a smooth curve through the experimental points by hand only. In the present runs, straight lines were found to be the best fits up to n/n* - 0.85, expressed by dQ/dn (kcal/mole)
= 29.79 (* 0.31) - 3.29 (* 0.22) I’I
for film 1, and by dQ/dn (kcal/mole)
= 30.71 (* 0.37) - 4.00 (* 0.34) n
for film 2 (n = XAn denotes the sum of micromoles adsorbed in the successive doses). The over-all standard deviations of the fits were f 0.83 and i: 1.11 kcal/mole for films 1 and 2, respectively. The standard deviations of the coefficients are given in the parentheses. Thus, the initial differential heats are 29.8 and 30.7 kcal/mole for films 1 and 2, respectively. From the basic kinetic theory of gases it follows that other things being equal, the number of impacts of the contaminating species from the gas phase prior to the admission of the first hydrogen dose was markedly higher for film 2 as compared to film 1; viz. 6 times higher in the total period between the beginning of the film deposition and admission of the first dose, and 18 times higher if the period from the end of film deposition to the first dose admission only is considered. The initial heat of adsorption for film 2 was by 3% higher than for film 1, and the differential heat versus coverage curve was slightly steeper. It is interesting to compare this trend with our earlier results for the MO-H, system [2] obtained in a very similar apparatus and with films evaporated from the wire from the same reel as in the present case, so that the nature of the film contamination was very likely to be much the same. Vacuum conditions in those experiments led to approximately 10-100 times more impacts of gas particles onto the film surface prior to the first hydrogen dose as compared to the present film 2. Markedly higher initial heat (40 kcalfmole) resulted and the differential heat-coverage curve was rather steeply falling (fig. 1). Thus it seems likely that the higher the contamination of the film with species occurring in the type of experiments under consideration, the higher the initial heat of hydrogen adsorption, and the steeper the fall in the differential heat-coverage curve. Our recent results for the Pt-H, system [l] suggest the same conclusion. This is in general agreement also with the earlier findings of Wedler and coworkers for films of Ti [3] and Ni [4-71 prepared again in a mercury apparatus, and approximately 5-10 times thinner and with a roughness factor several times lower as compared to the MO films in our present case. The heats of hydrogen adsorption on the said thin films of Ti and Ni deposited under I Om9torr were reported to be about 20-30% higher and more steeply decreasing with the increasing surface coverage than the heats for films deposited in the 1O-1o torr region. (Moreover, even different character of the differential heat-coverage curves has been claimed for the said films of Ni deposited in the lo-’ and in the 10-l’ torr region.) Leaving apart the indirect methods of estimation of the heat of adsorption from adsorption isosteres, from the kinetics of thermal desorption, etc., the direct calori-
256
S. &rn$/Surface contamination and heat of adsorption
metric determination of the heat of adsorption of hydrogen and other simple gases on “clean” metal surfaces has been accomplished in a few cases on low-area filaments under UHV conditions [8-lo], and in a number of cases on films in mercury adsorption apparatus [ 111. With a few exceptions [ 1,3--71, the films for calorimetric measurements have been prepared hitherto under the residual pressure of 10m8 torr or higher. The degree of the contamination obviously depends also on the surface area of the adsorbent. The relevant data enabling to make an estimate of the number of impacts of the contaminating species onto the unit surface before and during the heat measurement are often not given. Nevertheless it seems probable that the most calorimetric values so far available for the magnitude and coverage dependence of the heat of adsorption of hydrogen and likely also of other gases on “clean” polycrystalline metal surfaces are more or less uncertain on account of an appreciable surface contamination. Sincere thanks are expressed to Dr. M. Smutek for his kindly performing the processing of the experimental data to find the best fit curves, and to Dr. M. Cukr for mass spectrometric analysis of the molybdenum wire.
Slavoj CERNY
Czechoslovak Academy and Electrochemistry,
of Sciences, J. Hey?ovsky Institute of Physical Chemistry Ma’chova 7, 121 38 Prague 2, Czechoslovakia,
References [1 ] S. &rnf, M. Smutek and F. Buzek, J. Catalysis, in press. [2] S. Cernq, V. Ponec and L. HlLdek, J. Catalysis 5 (1966) 27. [3] [4] [S] [6] [7] [S] [9]
G. Wedler and H. Strothenk, Ber. Bunsenges. Physik. Chem. 70 (1966) 214. F.J. Brbcker and G. Wedler, Discussions Faraday Sot. 41 (1966) 87. G. Wedler, Discussions Faraday Sot. 41 (1966) 104. G. Wedler and F.J. Brbcker, Surface Sci. 26 (1971) 454. G. Wedler and G. Fisch, Ber. Bunsenges. Physik. Chem. 76 (1972) 1160. V. Ponec, Z. Knor and S. Cerny, Adsorption on Solids (Butterworths, London, 1974) p. 131. P.R. Norton and P.J. Richards, Surface Sci. 44 (1974) 129. [lo] H. Yamazaki, T. Oguri and I. Kanomata, Japan. J. Appl. Phys. 10 (1971) 1105. [ 111 S. tern< and V. Ponec, Catalysis Rev. 2 (1968) 249.