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Transient species in surface chemical reactions
N10
Platinum-Promoted Catalysis by Ceria?
This might be subtitled "Is the Frost hypothesis right after all, if not for methanol synthesis, then for some CO oxidations?". A recent publication by Lambert's group at Cambridge [C. Hardacre, R.M. Ormerod and R.M. Lambert, J. Phys. Chem., 98 (1994) 10901] presents some strikingly unexpected results on carbon monoxide oxidation over model platinum/ceria catalysts. These are clearly relevant to the mechanisms occurring on auto exhaust catalysts, and, if Lambert has uncovered a general phenomenon, to much other catalysis as welt. The model catalyst was made from a Pt(111) crystal face upon which Ce metal was deposited and then oxidized to ceria. This method produced a disordered CeO2 on the platinum which became ordered upon annealing at 700 K or above. Submonolayers of ceria strongly promoted the oxidation of CO - - this was not surprising as similar enhancing effects of partial coverage of metal by a support oxide have been observed before. With increasing coverage of ceria the rate of oxidation fell "to an undetectably low level between 0.8 and 1.3 ML'. However, "Above 1.3 ML, the rate again rises steeply to a value which is much greater than that observed over the clean Pt(111/ surface." This holds for 10 ML of ceria (the highest loading examined) when no bare Pt metal is detectable by CO chemisorption, so the fast oxidation must be occurring on a modified ceria. Annealing the ceria almost eliminated this enhanced rate of reaction. Lambert et al. "conclude that the reaction is occurring at the surface of the thin oxide film whose properties are radically
applied catalysis A: General
altered by the presence of the underlying, fully encapsulated Pt metal". They also suggest that this can be interpreted in terms of the proposal made by Frost [J.C. Frost, Nature, 334 (1988) 577], in which electron transfer from a metal phase to an oxide phase can reduce the enthalpy for oxygen vacancy formation in the oxide, so forming a catalytically active site. This model of the system is the most plausible on offer at present but one wonders if the electron transport is likely to be sufficiently efficient through ten monolayers of ceria. However, as the catalytically active overlayer of ceria is disordered, the active sites may well be rather nearer to the Pt surface than to the top of the ceria. Lambert et al. finish with a note of caution on interpretation: "the techniques at our disposal do not provide absolute and incontrovertible evidence of total encapsulation of the Pt crystal. It could always be argued that some Pt species, undetectable by our methods, are present at the oxide surface and responsible for the observed high catalytic activity: a possibility which would also be of considerable interest.". MIKE SPENCER
Transient Species in Surface Chemical Reactions For workers in catalysis, and especially those concerned with reaction mechanisms, some of the most revealing surface chemistry in recent years has been the discovery of highly reactive, transient surface species in surface reactions. For example, Ertl and co-workers [H. Brune, J. Winterlin, R.J. Behm and G. Ertl, Phys. Rev. Lett., 68 (1992) 624] have observed that the Volume 122 No. 2 -
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Nll
immediate product of the dissociation of molecular oxygen on a metal surface is a transient, "hot" oxygen atom which diffuses rapidly across the surface before reaching equilibrium as O(a). However, almost all the work on this subject has been carried out by Roberts and his group in Cardiff. The key to understanding the details of the reaction of, say, molecular oxygen with a metal surface has been first, the use of a probe species (typically ammonia) and secondly, the coadsorption of oxygen and the probe, when reactions are found which are not seen after successive adsorption of the same reagents [M.W. Roberts, J. Mol. Catal. 74 (1992) 11; M.W. Roberts, in H.-J. Freund and E. Umbach (Editors), Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, Springer Series in Surface Science, Vol. 33, 1993; M.W. Roberts, Studies Surf. Sci. Catal., 48 (1989) 787; C.T. Au and M.W. Roberts, Nature, 319 (1986) 206; J. Chem. Soc., Faraday Trans., 83 (1987) 2047]. With molecular oxygen and ammonia on Cu(110) and on Cu(111) [MW. Roberts and co-workers, Chem. Phys. Lett., 74 (1980) 472; Catal. Lett., 16 (1992) 345; Surf. Sci., 284 (1993) 109] there was little and slow reaction upon successive adsorption, in either order, of the reagents. In contrast, the coadsorption of ammonia and oxygen gave rapid reaction, with a high selectivity to NH(a). Clearly the key step or steps in this reaction must involve some intermediate or transient species quite distinct from either the gas phase species or the equilibrated adsorbed species. This reaction pattern has been seen in reactions over many metals. For the copper surfaces Roberts et al. proposed that the key reaction step was the reaction of a molecularly adsorbed oxygen
applied catalysis A: General
precursor with an ammonia molecule. In other systems oxygen atom surface transients (similar to Ertl's surface "hot" atoms) appear to be the important intermediates. In some ways the present position in this area of surface chemistry is analogous to the study of gas-phase chain reactions in the 1930's and 1940's, before the development of flash-photolysis and other related techniques. Then there was abundant indirect evidence for the existence and the crucial importance of atom/radical chain carriers, but they had not been seen directly in action. Similarly, the presently available surface science techniques cannot identify these transient surface species in action. Under these circumstances it is especially valuable for theoretical chemists to examine the plausibility of the mechanisms put forward by Roberts and his group. The first such study has now been published [M. Neurock, R.A. van Santen, W. Biemolt and A.P.J. Jansen, J. Am. Chem. Soc., 116 (1994) 6860]. Van Santen and his group analyzed the ammonia/oxygen system on copper, using first-principle density functional calculations. They computed adsorption energies for ammonia, molecular oxygen, NHx, NO, and various possible intermediates and adatoms from geometry-optimized calculations on the model Cu(8,3) cluster of the Cu(111) surface. Attractive and repulsive lateral interactions were important in obtaining accurate adsorption energies. The calculations show that atomic oxygen enhances N-H bond activation but also acts as a poison on active sites and inhibits ammonia reaction. Transient molecular oxygen adsorbs weakly in both parallel (-17 kJ/mol) and perpendicular (-10 kJ/mol) orientations to the surface. Parallel adsorption appears to be the most prob-
Volume 122 No. 2 - - 16 February 1995
N12
able precursor for oxygen dissociation, whereas perpendicular adsorption is the precursor for reaction with ammonia. Van Santen et al. examined various reaction paths involving transient atomic and molecular species. The sequential hydrogen abstraction pathway, in which reaction with the molecular oxygen transient leads to an OOH(a) intermediate, is the most likely of all those examined: its apparent activation energy is zero, i.e., it is a non-activated process. All paths controlled by transient atomic oxygen, including those with "hot" O atoms, have an initial activation of at least + 17 kJ/mol. The appearance of an HO2 intermediate will not surprise anyone familiar with gasphase oxidations, but the dominance of molecular rather than atomic oxygen reactions emphasizes (if emphasis is needed) that surface reactions are not the same as homogeneous reactions. The role of theoretical chemistry in this work is very encouraging. It has moved from mere post hoc rationalization of understood chemistry to being an essential predictive and interpretative tool. MIKE SPENCER
Personal News
Professor H.-J. Freund, of the Chemistry Department of the Ruhr-University of Bochum, Germany, has been awarded the Leibnitz Prize for 1994. This is a prize of very high prestige, awarded every two years to the most eminent German scientist. The last time the Leibnitz prize was given to a scientist in the catalysis~surface science area was its award a few years ago [how many? - - do we know?] to Professor
applied catalysis A: General
G. Ertl. Awarded with the prize is a grant of 3,000,000 DM.
Without Comment
"1 realized that the progress of the scientific revolution that we were participating in required that much of the shoddy thinking that had been masquerading as science in the field of surface chemistry should be directly, evenly rudely, confronted. (Less forgivable, though, the arrogance that some of us were learning to display derived also from the realization that we had our hands on a significantly superior approach, and felt the need to rub in the salt.)" D.A. King, Surf. Sci., 299/300 (1994) 680.
Kata-List
A second issue of the bulletin of the Polish Catalytic Club Kata-List has just appeared. This issue includes the second part of a series of entries on the Polish Scientific Institutions dealing with catalysis, giving basic information about the Department of Kinetic Processes (Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw; manager Prof. Wanda Pasiuk-Bronikowska). There are also two interesting notes, one related to Catalysis at the Institute of Industrial Chemistry (Warsaw) and the second to "The History of the Chemical Industry in Poland - - Studies of the Research Chemical Institute on Caoutchouc". There is also information on the forthcoming 27th National Colloquium (Cracow 6th-8th February 1995), as well as further information about EuropaCat-II. Volume 122 No. 2 -
16 February 1995
Platinum-Promoted Catalysis by Ceria?
This might be subtitled "Is the Frost hypothesis right after all, if not for methanol synthesis, then for some CO oxidations?". A recent publication by Lambert's group at Cambridge [C. Hardacre, R.M. Ormerod and R.M. Lambert, J. Phys. Chem., 98 (1994) 10901] presents some strikingly unexpected results on carbon monoxide oxidation over model platinum/ceria catalysts. These are clearly relevant to the mechanisms occurring on auto exhaust catalysts, and, if Lambert has uncovered a general phenomenon, to much other catalysis as welt. The model catalyst was made from a Pt(111) crystal face upon which Ce metal was deposited and then oxidized to ceria. This method produced a disordered CeO2 on the platinum which became ordered upon annealing at 700 K or above. Submonolayers of ceria strongly promoted the oxidation of CO - - this was not surprising as similar enhancing effects of partial coverage of metal by a support oxide have been observed before. With increasing coverage of ceria the rate of oxidation fell "to an undetectably low level between 0.8 and 1.3 ML'. However, "Above 1.3 ML, the rate again rises steeply to a value which is much greater than that observed over the clean Pt(111/ surface." This holds for 10 ML of ceria (the highest loading examined) when no bare Pt metal is detectable by CO chemisorption, so the fast oxidation must be occurring on a modified ceria. Annealing the ceria almost eliminated this enhanced rate of reaction. Lambert et al. "conclude that the reaction is occurring at the surface of the thin oxide film whose properties are radically
applied catalysis A: General
altered by the presence of the underlying, fully encapsulated Pt metal". They also suggest that this can be interpreted in terms of the proposal made by Frost [J.C. Frost, Nature, 334 (1988) 577], in which electron transfer from a metal phase to an oxide phase can reduce the enthalpy for oxygen vacancy formation in the oxide, so forming a catalytically active site. This model of the system is the most plausible on offer at present but one wonders if the electron transport is likely to be sufficiently efficient through ten monolayers of ceria. However, as the catalytically active overlayer of ceria is disordered, the active sites may well be rather nearer to the Pt surface than to the top of the ceria. Lambert et al. finish with a note of caution on interpretation: "the techniques at our disposal do not provide absolute and incontrovertible evidence of total encapsulation of the Pt crystal. It could always be argued that some Pt species, undetectable by our methods, are present at the oxide surface and responsible for the observed high catalytic activity: a possibility which would also be of considerable interest.". MIKE SPENCER
Transient Species in Surface Chemical Reactions For workers in catalysis, and especially those concerned with reaction mechanisms, some of the most revealing surface chemistry in recent years has been the discovery of highly reactive, transient surface species in surface reactions. For example, Ertl and co-workers [H. Brune, J. Winterlin, R.J. Behm and G. Ertl, Phys. Rev. Lett., 68 (1992) 624] have observed that the Volume 122 No. 2 -
16 February 1995
Nll
immediate product of the dissociation of molecular oxygen on a metal surface is a transient, "hot" oxygen atom which diffuses rapidly across the surface before reaching equilibrium as O(a). However, almost all the work on this subject has been carried out by Roberts and his group in Cardiff. The key to understanding the details of the reaction of, say, molecular oxygen with a metal surface has been first, the use of a probe species (typically ammonia) and secondly, the coadsorption of oxygen and the probe, when reactions are found which are not seen after successive adsorption of the same reagents [M.W. Roberts, J. Mol. Catal. 74 (1992) 11; M.W. Roberts, in H.-J. Freund and E. Umbach (Editors), Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, Springer Series in Surface Science, Vol. 33, 1993; M.W. Roberts, Studies Surf. Sci. Catal., 48 (1989) 787; C.T. Au and M.W. Roberts, Nature, 319 (1986) 206; J. Chem. Soc., Faraday Trans., 83 (1987) 2047]. With molecular oxygen and ammonia on Cu(110) and on Cu(111) [MW. Roberts and co-workers, Chem. Phys. Lett., 74 (1980) 472; Catal. Lett., 16 (1992) 345; Surf. Sci., 284 (1993) 109] there was little and slow reaction upon successive adsorption, in either order, of the reagents. In contrast, the coadsorption of ammonia and oxygen gave rapid reaction, with a high selectivity to NH(a). Clearly the key step or steps in this reaction must involve some intermediate or transient species quite distinct from either the gas phase species or the equilibrated adsorbed species. This reaction pattern has been seen in reactions over many metals. For the copper surfaces Roberts et al. proposed that the key reaction step was the reaction of a molecularly adsorbed oxygen
applied catalysis A: General
precursor with an ammonia molecule. In other systems oxygen atom surface transients (similar to Ertl's surface "hot" atoms) appear to be the important intermediates. In some ways the present position in this area of surface chemistry is analogous to the study of gas-phase chain reactions in the 1930's and 1940's, before the development of flash-photolysis and other related techniques. Then there was abundant indirect evidence for the existence and the crucial importance of atom/radical chain carriers, but they had not been seen directly in action. Similarly, the presently available surface science techniques cannot identify these transient surface species in action. Under these circumstances it is especially valuable for theoretical chemists to examine the plausibility of the mechanisms put forward by Roberts and his group. The first such study has now been published [M. Neurock, R.A. van Santen, W. Biemolt and A.P.J. Jansen, J. Am. Chem. Soc., 116 (1994) 6860]. Van Santen and his group analyzed the ammonia/oxygen system on copper, using first-principle density functional calculations. They computed adsorption energies for ammonia, molecular oxygen, NHx, NO, and various possible intermediates and adatoms from geometry-optimized calculations on the model Cu(8,3) cluster of the Cu(111) surface. Attractive and repulsive lateral interactions were important in obtaining accurate adsorption energies. The calculations show that atomic oxygen enhances N-H bond activation but also acts as a poison on active sites and inhibits ammonia reaction. Transient molecular oxygen adsorbs weakly in both parallel (-17 kJ/mol) and perpendicular (-10 kJ/mol) orientations to the surface. Parallel adsorption appears to be the most prob-
Volume 122 No. 2 - - 16 February 1995
N12
able precursor for oxygen dissociation, whereas perpendicular adsorption is the precursor for reaction with ammonia. Van Santen et al. examined various reaction paths involving transient atomic and molecular species. The sequential hydrogen abstraction pathway, in which reaction with the molecular oxygen transient leads to an OOH(a) intermediate, is the most likely of all those examined: its apparent activation energy is zero, i.e., it is a non-activated process. All paths controlled by transient atomic oxygen, including those with "hot" O atoms, have an initial activation of at least + 17 kJ/mol. The appearance of an HO2 intermediate will not surprise anyone familiar with gasphase oxidations, but the dominance of molecular rather than atomic oxygen reactions emphasizes (if emphasis is needed) that surface reactions are not the same as homogeneous reactions. The role of theoretical chemistry in this work is very encouraging. It has moved from mere post hoc rationalization of understood chemistry to being an essential predictive and interpretative tool. MIKE SPENCER
Personal News
Professor H.-J. Freund, of the Chemistry Department of the Ruhr-University of Bochum, Germany, has been awarded the Leibnitz Prize for 1994. This is a prize of very high prestige, awarded every two years to the most eminent German scientist. The last time the Leibnitz prize was given to a scientist in the catalysis~surface science area was its award a few years ago [how many? - - do we know?] to Professor
applied catalysis A: General
G. Ertl. Awarded with the prize is a grant of 3,000,000 DM.
Without Comment
"1 realized that the progress of the scientific revolution that we were participating in required that much of the shoddy thinking that had been masquerading as science in the field of surface chemistry should be directly, evenly rudely, confronted. (Less forgivable, though, the arrogance that some of us were learning to display derived also from the realization that we had our hands on a significantly superior approach, and felt the need to rub in the salt.)" D.A. King, Surf. Sci., 299/300 (1994) 680.
Kata-List
A second issue of the bulletin of the Polish Catalytic Club Kata-List has just appeared. This issue includes the second part of a series of entries on the Polish Scientific Institutions dealing with catalysis, giving basic information about the Department of Kinetic Processes (Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw; manager Prof. Wanda Pasiuk-Bronikowska). There are also two interesting notes, one related to Catalysis at the Institute of Industrial Chemistry (Warsaw) and the second to "The History of the Chemical Industry in Poland - - Studies of the Research Chemical Institute on Caoutchouc". There is also information on the forthcoming 27th National Colloquium (Cracow 6th-8th February 1995), as well as further information about EuropaCat-II. Volume 122 No. 2 -
16 February 1995