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Oxygen adsorption on a Ge(100) surface
A460 Surface Science 123 (1982) 49l 504 North-Holland Publishing Company
491
C H A R A C T E R I Z A T I O N OF THE A D S O R P T I O N A N D REACTION OF ACETYLENE O N CLEAN A N D OXYGEN C O V E R E D Ag(110) BY EELS E.M. S T U V E and R.J. M A D I X * Department of Chemical Engineering. Stanford Unit~ersit~, A'tap~ford, California ~4305, USA
and B.A. S E X T O N ** Pt~vsical Chemistry Department, General Motors Research Laboratories, Warren, Mtchigan 4b;090, USA Received 8 December 1981; accepted for publication 22 September 1982 The adsorption and reaction of C2H~ on clean and oxygen covered Ag(110) surfaces was studied with high resolution electron energy loss spectroscopy (EELS). Acetylene adsorbs at 100 K onto the clean surface without rehybridization and desorbs without reaction between t00 and 160 K. Acetylene reacts with adsorbed oxygen atoms at 100 K to form adsorbed CCH and HzO. In contrast to interactions between acetylene and other metals the carbon-carbon triple bond is preserved, as determined by the CH stretching frequency of 3270 cm 1. H 2 0 is displaced from the surface at 100 K, but a small amount (about 5% of a monolayer) remains weakly adsorbed. Unreacted acetylene and the remaining water desorb from 100 to 160 K and from 170 to 200 K, respectively. Although the C--~-C stretching mode was not observed for CCH, the C ~ C bond order was determined to be three from the CH stretching frequency of 3250 cm ~. A model for CCH is proposed with the C:~C bond nearly parallel to the surface. The CCH formed at low temperatures disproportionates at 280 K to CzH2tg ) and CC~a~, which in turn dissociates into CI,. at 550 K. The C C ~ species exhibited only one EELS loss at 240 cm ~, whereas no losses were observed for C~,.
Surface Science 123 ( t 982) 505 -518 North-Holland Publishing Company
505
O X Y G E N A D S O R P T I O N O N A Ge(100) S U R F A C E l. Clean surfaces L. S U R N E V and M. T I K H O V Institute of General and Inorganic Chemistry, Academy of Sciences of Bulgaria, 1040 S~ff~ia,Bulgaria Received 25 May 1982: accepted for publication 4 October 1982 Oxygen adsorption on a Ge(100) surface in the temperature range from 300 to 600 K has been studied by means of thermal desorption (TD), work function (WF) measurements, Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS), It was found that the desorption kinetics order decreases from 1 at low coverages, 0, to zero at high 0. The analysis of the adsorption kinetics shows that oxygen adsorption proceeds via a precursor state. The WE versus 0 plots consist of an initial very steeply increasing part (up to 0 = 0.05-0.1) followed by a sloping part. Two new losses involving a chemically shifted (by - 1.2 eV) Ge 3d core level appear in the EELS after high-temperature oxygen adsorption. The oxygen-related loss features observed in the valence spectra are quite different from those obtained earlier with a G e ( l l l ) surface at the same oxidation conditions.
491
C H A R A C T E R I Z A T I O N OF THE A D S O R P T I O N A N D REACTION OF ACETYLENE O N CLEAN A N D OXYGEN C O V E R E D Ag(110) BY EELS E.M. S T U V E and R.J. M A D I X * Department of Chemical Engineering. Stanford Unit~ersit~, A'tap~ford, California ~4305, USA
and B.A. S E X T O N ** Pt~vsical Chemistry Department, General Motors Research Laboratories, Warren, Mtchigan 4b;090, USA Received 8 December 1981; accepted for publication 22 September 1982 The adsorption and reaction of C2H~ on clean and oxygen covered Ag(110) surfaces was studied with high resolution electron energy loss spectroscopy (EELS). Acetylene adsorbs at 100 K onto the clean surface without rehybridization and desorbs without reaction between t00 and 160 K. Acetylene reacts with adsorbed oxygen atoms at 100 K to form adsorbed CCH and HzO. In contrast to interactions between acetylene and other metals the carbon-carbon triple bond is preserved, as determined by the CH stretching frequency of 3270 cm 1. H 2 0 is displaced from the surface at 100 K, but a small amount (about 5% of a monolayer) remains weakly adsorbed. Unreacted acetylene and the remaining water desorb from 100 to 160 K and from 170 to 200 K, respectively. Although the C--~-C stretching mode was not observed for CCH, the C ~ C bond order was determined to be three from the CH stretching frequency of 3250 cm ~. A model for CCH is proposed with the C:~C bond nearly parallel to the surface. The CCH formed at low temperatures disproportionates at 280 K to CzH2tg ) and CC~a~, which in turn dissociates into CI,. at 550 K. The C C ~ species exhibited only one EELS loss at 240 cm ~, whereas no losses were observed for C~,.
Surface Science 123 ( t 982) 505 -518 North-Holland Publishing Company
505
O X Y G E N A D S O R P T I O N O N A Ge(100) S U R F A C E l. Clean surfaces L. S U R N E V and M. T I K H O V Institute of General and Inorganic Chemistry, Academy of Sciences of Bulgaria, 1040 S~ff~ia,Bulgaria Received 25 May 1982: accepted for publication 4 October 1982 Oxygen adsorption on a Ge(100) surface in the temperature range from 300 to 600 K has been studied by means of thermal desorption (TD), work function (WF) measurements, Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS), It was found that the desorption kinetics order decreases from 1 at low coverages, 0, to zero at high 0. The analysis of the adsorption kinetics shows that oxygen adsorption proceeds via a precursor state. The WE versus 0 plots consist of an initial very steeply increasing part (up to 0 = 0.05-0.1) followed by a sloping part. Two new losses involving a chemically shifted (by - 1.2 eV) Ge 3d core level appear in the EELS after high-temperature oxygen adsorption. The oxygen-related loss features observed in the valence spectra are quite different from those obtained earlier with a G e ( l l l ) surface at the same oxidation conditions.