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Adsorption and desorption of NO from Rh{111} and Rh{331} surfaces
A462 184
Surface Science 159 (1985) 184-198 North-Holland, Amsterdam
TEMPERATURE ACETYLENE
PROGRAMMED
DESORPTION
ON A CLEAN,
H-COVERED,
P. B E R L O W I T Z ,
J.B. B U T T
STUDY
AND
OF
O-COVERED
Pt(lll)
SURFACE C.E. MEGIRIS,
and H.H. KUNG
*
Chemical Engineering Department, and the Ipatieff Laboratory, Northwestern Universi(~', Evanston, Illinois 60201, USA Received 26 September 1984; accepted for publication 7 March 1985
The reactions of acetylene on a clean, a H-covered and an O-covered P t ( l l l ) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH 4, C2H 6, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO 2, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.
Surface Science 159 (1985) 199-213 North-Holland, Amsterdam
199
ADSORPTION AND DESORPTION OF NO FROM Rh( 111 } AND Rh{331 } SURFACES Lisa A. DeLOUISE
* and Nicholas WINOGRAD
152 Davey Laboratory, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Received 11 December 1984; accepted for publication 15 February 1985 The adsorption and desorption chemistry of NO on the clean R h ( l l l } and Rh{331} single crystal surfaces was followed with SIMS, XPS, and LEED. Results suggest dissociative NO adsorption occurs at step a n d / o r defect sites. At saturation coverage there was - 10 times more dissociated species on the Rh{331) surface at 300 K than on the R h { l l l } surface. On both surfaces two molecular states of NOad s have been identified as fll and f12 which possess different chemical reactivity. Under the condition of saturation coverage the/31 and f12 states are populated on the R h { l l l } surface in a different proportion than on the Rh{331} surface. Further, their population on both surfaces is coverage and temperature dependent. When the sample is heated to desorb the saturation overlayer formed on the R h { l l l } and Rh{331} crystal surfaces, approximately 50% of the overlayer is found to desorb below = 400 K primarily from the /32 state, molecularly as NO(g). Between 300 and 400 K the/31 state dissociates as binding sites necessary to coordinate Nad s and Oads are freed by desorption of NO(g).
Surface Science 159 (1985) 184-198 North-Holland, Amsterdam
TEMPERATURE ACETYLENE
PROGRAMMED
DESORPTION
ON A CLEAN,
H-COVERED,
P. B E R L O W I T Z ,
J.B. B U T T
STUDY
AND
OF
O-COVERED
Pt(lll)
SURFACE C.E. MEGIRIS,
and H.H. KUNG
*
Chemical Engineering Department, and the Ipatieff Laboratory, Northwestern Universi(~', Evanston, Illinois 60201, USA Received 26 September 1984; accepted for publication 7 March 1985
The reactions of acetylene on a clean, a H-covered and an O-covered P t ( l l l ) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH 4, C2H 6, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO 2, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.
Surface Science 159 (1985) 199-213 North-Holland, Amsterdam
199
ADSORPTION AND DESORPTION OF NO FROM Rh( 111 } AND Rh{331 } SURFACES Lisa A. DeLOUISE
* and Nicholas WINOGRAD
152 Davey Laboratory, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Received 11 December 1984; accepted for publication 15 February 1985 The adsorption and desorption chemistry of NO on the clean R h ( l l l } and Rh{331} single crystal surfaces was followed with SIMS, XPS, and LEED. Results suggest dissociative NO adsorption occurs at step a n d / o r defect sites. At saturation coverage there was - 10 times more dissociated species on the Rh{331) surface at 300 K than on the R h { l l l } surface. On both surfaces two molecular states of NOad s have been identified as fll and f12 which possess different chemical reactivity. Under the condition of saturation coverage the/31 and f12 states are populated on the R h { l l l } surface in a different proportion than on the Rh{331} surface. Further, their population on both surfaces is coverage and temperature dependent. When the sample is heated to desorb the saturation overlayer formed on the R h { l l l } and Rh{331} crystal surfaces, approximately 50% of the overlayer is found to desorb below = 400 K primarily from the /32 state, molecularly as NO(g). Between 300 and 400 K the/31 state dissociates as binding sites necessary to coordinate Nad s and Oads are freed by desorption of NO(g).