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Absence of magnetic field influences on the rate of Ni(CO)4 formation on Ni(100) under various surface conditions
L510
Surface Science 109 (1981) L510-L512 North-Holland Publishing Company
SURFACE SCIENCE LETTERS
ABSENCE OF MAGNETIC FIELD INFLUENCES ON THE RATE OF Ni(CO)4 FORMATION ON Ni(100) UNDER VARIOUS SURFACE CONDITIONS G. GREINER and D. MENZEL Physik-Department E 20, TU Miinchen, D-8046 Garching, i~R. Germany Received 21 May 1981
Using UHV techniques and a flow system with continuous monitoring of the reaction rate, we have investigated the rate of Ni(CO)4 formation, R, on Ni(100) in an external magnetic field up to 1000 G, under various surface conditions. No magnetic field effect (zXR/R < 0.01) has been found under any conditions (clean surface with or without roughness; precoverage with C, S, Hg; temperatures up to 200°C; different B orientation; darkness or illumination).
Several years ago Krinchik et al. [1] reported a dramatic oscillatory magnetic field dependence of the rate of Ni(CO)4 formation on Ni single crystal surfaces. As such an effect would be difficult to understand in terms of the current concepts of ferromagnetism at surfaces and would therefore require the introduction of new ideas, several groups including ourselves attempted to duplicate these results [2,3], but without success. The discrepancy between ref. [1] on the one hand and refs. [2] and [3] and our own tests on the other prompted us to search for possible experimental differences in these various set-ups; our experience of the extremely strong influence of surface conditions on the reaction rate [4] suggested the possibility that different surface conditions might have been responsible. As we knew from close collaboration with the authors of ref. [2] that they were centering their attention on steady state conditions after long reaction times, we focussed onto initial reactivities and the influence of promoting and inhibiting surface species. To anticipate the main result, we have not found any conditions under which a magnetic field influence was observable. The experimental set-up emphasized cleanliness and continuous monitoring of the reaction rate. The system used standard UHV procedures and was made of Pyrex whereever possible; the inevitable stainless steel parts were covered internally with colloidal graphite to avoid carbonyl formation from the contained Ni of Fe. The reaction cell of Pyrex contained a Ni(lO0) crystal of about 1 cm diameter, spotwelded to Ta heating rods, and could be inserted between the poles of an electromagnet. A flow of purified [4] CO (1 bar; flow rate 1 cm3/s) was directed onto the crystal; continuous detection of Ni(CO)4 was done downstream by monitoring 0039-6028/81/0000-0000/$02.50 © 1981 North-Holland
G. Greiner, D. Menzel / A bsence of magnetic fleM influences
L511
UV-absorptance at 220 nm in an absorption cell of 50 cm length with a sensitivity of about 10 -s mbar Ni(CO), in 103 mbar CO, i.e. of 1 : 108, and an accuracy of about 1% in the commonly encountered range. After baking, the residual pressure before admission of CO was below 10 -1° mbar. The crystal was cleaned by repeated alternating heating in oxygen and hydrogen. By previous Auger studies and by ESCA investigations [4], this treatment had been shown to result in removal of impurities. For further details see ref. [4]. As reported in more detail in ref. [4], the rate of formation of Ni(CO), on a freshly cleaned surface fell fast in the first minutes and then slowly with time (typically by a factor of 2 in 150 min at room temperature). As this decay was always present, and as we wanted to concentrate on the initial behaviour (for longtime results see ref. [2]), the magnetic field was varied during it. Oscillations would have had to appear as super-imposed variations of the rate which would have easy to detect (Krinchik et al. [ 1] report R changes up to a factor of 6 in fields up to 1200 Gauss, with periods between 10 and 40 Gauss). Because of the transit time of CO through the system, the magnetic field was varied stepwise every 1.5 min, in steps of 10 or 20 Gauss. Starting with a clean surface at room temperature no change of the rate above the noise level of 1% was observed. Variation of crystal temperature up to 470 K (with particularly extended runs at 353 K, the temperature of the rate maximum [4] under these conditions and at 315 K), orientation of magnetic field (with Bu (100) and B± (100)), irradiation (day light; irradiation by incandescent bulb; total shielding against light; see also ref. [2]), and time of reaction after cleaning (up to 211) gave the same negative result. As the absolute rates reported in ref. [1] are very high, and as sulphur and mercury preadsorption are known to promote the carbonyl formation (see ref. [4] and references therein), the same tests were performed with a crystal precovered with S (to about half or full saturation [4]) by exposure to H2S, and with Hg (about 200 Ex [5] ~2 X 1016 cm-2), with the same negative results. We did not change the flow rate or the CO pressure as this has been done extensively in ref. [2]. The initial decrease of reaction rate on a "clean" surface is likely to be due to formation of carbon by CO disproportionation and/or to removal of Ni atoms from surface crystallographic faults. The absence of a B influence throughout the decrease shows that neither presence nor absence of exposed Ni atoms or of carbon can be responsible for a field effect. The same is true for S or Hg preadsorption (which lead to strong increases of the rate at room temperature and to drastically changed T-dependence [4]. On the other extreme, the experiments of de Groot and Dransfeld [2,6] have shown that the "forming" of the surface and its steady state contamination by the carbonyl reaction do not lead to a B effect either. In terms of removed Ni layers, the conditions of ref. [1] were closer to ours (there Ni(CO), detection was done discontinuously by chemical analysis of the amount formed in 3 to 5 min which corresponded to the removal of about 100 layers. Here, the same amount was removed after about 2 h). Apart from the different methods of detection and our supposedly higher emphasis on cleanliness, the only difference in pro-
L512
G. Greiner, D. Menzel / Absence of magnetic fieM influences
cedure of ref. [1] was that there the system was flooded with He in between taking reaction points. In view of the inertness of He it appears unlikely that this should have produced any physical e f f e c t - u n l e s s the He contained some unknown impurity. "Normal" expected impurities (C, S, Hg) have been shown not to lead to a B influence. We must conclude that we cannot give an explanation for the results of ref. [1], and that we join refs. [2] and [3] in the exclusion of magnetic field effects on the Ni(CO)4 formation rate, extending it to S- and Hg-promoted and C-inhibited Ni surfaces as well as to surface roughness. While this negative result is less exciting than a positive one would have been it appears to eliminate the necessity to search for a sophisticated mechanism of the B-influence.
References [1] G.S. Krinchik, R.A. Shvartsmann and A.Ya. Kipnis, JETP Letters 19 (1974) 231; G.S. Krinchik and R.A. Shvartsmann, Soviet Phys.-JETP 40 (1975) 1153; G.S. Krinchik, Umschau Wiss. Tech. 78 (1978) 54. [2] P. de Groot and K. Dransfeld, Z. Anorg. AUgem. Chem. 446 (1978) 39. [3] R.S. Mehta, M.S. Dresselhaus, G. Dresselhaus and A.J. Zeiger, Surface Sci. 78 (1978) L681. [4] G. Greiner, Thesis, TU Mtinchen (1980); G. Greiner and D. Menzel, J. Catalysis, to be published. [5] D. Menzel and J.C. Fuggle, Surface Sci. 74 (1978) 321. [6] P. de Groot, Thesis, TU Miinchen (1980); P. de Groot, M. Coulon and K. Dransfeld, Surface Sci. 94 (1980) 204.
Surface Science 109 (1981) L510-L512 North-Holland Publishing Company
SURFACE SCIENCE LETTERS
ABSENCE OF MAGNETIC FIELD INFLUENCES ON THE RATE OF Ni(CO)4 FORMATION ON Ni(100) UNDER VARIOUS SURFACE CONDITIONS G. GREINER and D. MENZEL Physik-Department E 20, TU Miinchen, D-8046 Garching, i~R. Germany Received 21 May 1981
Using UHV techniques and a flow system with continuous monitoring of the reaction rate, we have investigated the rate of Ni(CO)4 formation, R, on Ni(100) in an external magnetic field up to 1000 G, under various surface conditions. No magnetic field effect (zXR/R < 0.01) has been found under any conditions (clean surface with or without roughness; precoverage with C, S, Hg; temperatures up to 200°C; different B orientation; darkness or illumination).
Several years ago Krinchik et al. [1] reported a dramatic oscillatory magnetic field dependence of the rate of Ni(CO)4 formation on Ni single crystal surfaces. As such an effect would be difficult to understand in terms of the current concepts of ferromagnetism at surfaces and would therefore require the introduction of new ideas, several groups including ourselves attempted to duplicate these results [2,3], but without success. The discrepancy between ref. [1] on the one hand and refs. [2] and [3] and our own tests on the other prompted us to search for possible experimental differences in these various set-ups; our experience of the extremely strong influence of surface conditions on the reaction rate [4] suggested the possibility that different surface conditions might have been responsible. As we knew from close collaboration with the authors of ref. [2] that they were centering their attention on steady state conditions after long reaction times, we focussed onto initial reactivities and the influence of promoting and inhibiting surface species. To anticipate the main result, we have not found any conditions under which a magnetic field influence was observable. The experimental set-up emphasized cleanliness and continuous monitoring of the reaction rate. The system used standard UHV procedures and was made of Pyrex whereever possible; the inevitable stainless steel parts were covered internally with colloidal graphite to avoid carbonyl formation from the contained Ni of Fe. The reaction cell of Pyrex contained a Ni(lO0) crystal of about 1 cm diameter, spotwelded to Ta heating rods, and could be inserted between the poles of an electromagnet. A flow of purified [4] CO (1 bar; flow rate 1 cm3/s) was directed onto the crystal; continuous detection of Ni(CO)4 was done downstream by monitoring 0039-6028/81/0000-0000/$02.50 © 1981 North-Holland
G. Greiner, D. Menzel / A bsence of magnetic fleM influences
L511
UV-absorptance at 220 nm in an absorption cell of 50 cm length with a sensitivity of about 10 -s mbar Ni(CO), in 103 mbar CO, i.e. of 1 : 108, and an accuracy of about 1% in the commonly encountered range. After baking, the residual pressure before admission of CO was below 10 -1° mbar. The crystal was cleaned by repeated alternating heating in oxygen and hydrogen. By previous Auger studies and by ESCA investigations [4], this treatment had been shown to result in removal of impurities. For further details see ref. [4]. As reported in more detail in ref. [4], the rate of formation of Ni(CO), on a freshly cleaned surface fell fast in the first minutes and then slowly with time (typically by a factor of 2 in 150 min at room temperature). As this decay was always present, and as we wanted to concentrate on the initial behaviour (for longtime results see ref. [2]), the magnetic field was varied during it. Oscillations would have had to appear as super-imposed variations of the rate which would have easy to detect (Krinchik et al. [ 1] report R changes up to a factor of 6 in fields up to 1200 Gauss, with periods between 10 and 40 Gauss). Because of the transit time of CO through the system, the magnetic field was varied stepwise every 1.5 min, in steps of 10 or 20 Gauss. Starting with a clean surface at room temperature no change of the rate above the noise level of 1% was observed. Variation of crystal temperature up to 470 K (with particularly extended runs at 353 K, the temperature of the rate maximum [4] under these conditions and at 315 K), orientation of magnetic field (with Bu (100) and B± (100)), irradiation (day light; irradiation by incandescent bulb; total shielding against light; see also ref. [2]), and time of reaction after cleaning (up to 211) gave the same negative result. As the absolute rates reported in ref. [1] are very high, and as sulphur and mercury preadsorption are known to promote the carbonyl formation (see ref. [4] and references therein), the same tests were performed with a crystal precovered with S (to about half or full saturation [4]) by exposure to H2S, and with Hg (about 200 Ex [5] ~2 X 1016 cm-2), with the same negative results. We did not change the flow rate or the CO pressure as this has been done extensively in ref. [2]. The initial decrease of reaction rate on a "clean" surface is likely to be due to formation of carbon by CO disproportionation and/or to removal of Ni atoms from surface crystallographic faults. The absence of a B influence throughout the decrease shows that neither presence nor absence of exposed Ni atoms or of carbon can be responsible for a field effect. The same is true for S or Hg preadsorption (which lead to strong increases of the rate at room temperature and to drastically changed T-dependence [4]. On the other extreme, the experiments of de Groot and Dransfeld [2,6] have shown that the "forming" of the surface and its steady state contamination by the carbonyl reaction do not lead to a B effect either. In terms of removed Ni layers, the conditions of ref. [1] were closer to ours (there Ni(CO), detection was done discontinuously by chemical analysis of the amount formed in 3 to 5 min which corresponded to the removal of about 100 layers. Here, the same amount was removed after about 2 h). Apart from the different methods of detection and our supposedly higher emphasis on cleanliness, the only difference in pro-
L512
G. Greiner, D. Menzel / Absence of magnetic fieM influences
cedure of ref. [1] was that there the system was flooded with He in between taking reaction points. In view of the inertness of He it appears unlikely that this should have produced any physical e f f e c t - u n l e s s the He contained some unknown impurity. "Normal" expected impurities (C, S, Hg) have been shown not to lead to a B influence. We must conclude that we cannot give an explanation for the results of ref. [1], and that we join refs. [2] and [3] in the exclusion of magnetic field effects on the Ni(CO)4 formation rate, extending it to S- and Hg-promoted and C-inhibited Ni surfaces as well as to surface roughness. While this negative result is less exciting than a positive one would have been it appears to eliminate the necessity to search for a sophisticated mechanism of the B-influence.
References [1] G.S. Krinchik, R.A. Shvartsmann and A.Ya. Kipnis, JETP Letters 19 (1974) 231; G.S. Krinchik and R.A. Shvartsmann, Soviet Phys.-JETP 40 (1975) 1153; G.S. Krinchik, Umschau Wiss. Tech. 78 (1978) 54. [2] P. de Groot and K. Dransfeld, Z. Anorg. AUgem. Chem. 446 (1978) 39. [3] R.S. Mehta, M.S. Dresselhaus, G. Dresselhaus and A.J. Zeiger, Surface Sci. 78 (1978) L681. [4] G. Greiner, Thesis, TU Mtinchen (1980); G. Greiner and D. Menzel, J. Catalysis, to be published. [5] D. Menzel and J.C. Fuggle, Surface Sci. 74 (1978) 321. [6] P. de Groot, Thesis, TU Miinchen (1980); P. de Groot, M. Coulon and K. Dransfeld, Surface Sci. 94 (1980) 204.