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Reactive sites on oxygenated metal surfaces
N3
observed with Co and Ni, whereas Cu and Zn have only a minor effect. XPS data show that in the particular case of S/Ni/Mo(110), the admetal has a strong tendency to remain in a metallic state while Zn or Cu on Mo(110) become fully sulphided. Extrapolating to XSy/MOS2 catalysts would suggest that the probability of finding a S vacancy around a Ni atom is greater than around a Cu or Zn atom. Then, in agreement with a previous literature proposal, the high HDS activity may be attributable to the existence of Ni centres associated with S vacancies where S-containing molecules can adsorb and readily undergo desulphurization. C.S. McKee Reactive Sites on Oxygenated Metal Surfaces
It has been known for some time that oxygen preadsorbed on various metal surfaces, including the close-packed planes of Ru, Rh and Pt and the (110) planes of Ni and Cu, can be titrated with dissociatively adsorbed hydrogen at around 450 K. An interesting kinetic feature - - an induction period of length increasing with increasing oxygen pre-coverage - - has remained without a completely satisfactory explanation; however, a very direct view of the mechanism on Ni(110) has now been provided by scanning tunneling microscopy [P.T. Sprunger et al., Surf. Sci., 344 (1995)
98]. Fcc (110) surfaces are highly corrugated, making them favourable candidates for investigation by STM, and the details of their interactions with oxygen are well established, particularly in the case of Ni(110). With increasing coverage up to applied catalysis A: General
2/3 ML, the surface passes through a series of reconstructions consisting of parallel - N i - O - N i - O - chains sitting on top of an unreconstructed metal layer. If a surface with an oxygen pre-coverage, 8o, less than 1/2 ML is exposed to hydrogen, the two adsorbed species exist in separate domains and the titration rate is proportional to the overall length of the domain boundaries. The induction period is negligible because clean Ni areas are available for hydrogen dissociation. At 8o = 1/2 ML, however, the surface is completely covered by double-spaced - N i - O chains and the induction period becomes significant. STM shows that the titration reaction is initiated much more readily at steps (inevitably present as defects on any surface) than on flat terraces, with most nucleation sites located along step edges running at 90 ° to the direction of the-N i-Ochains and only very few at steps lying parallel to the chains. Once a small fraction of the oxygen is reacted off, the remainder on the adjoining terraces is removed rapidly. These observations allow formulation of a detailed mechanism. The point of initial attack is the O atom terminating a - N i - O Ni-O- chain at a step, the induction period lasting until this oxygen has been removed by hydrogen dissociating at the step. The Ni atom which then forms the end of the chain breaks away and becomes incorporated in the step edge, exposing the next O in the chain to attack and also creating two new vacant Ni sites for hydrogen dissociation, one on the lower and the other on the upper terrace. The reaction now accelerates as progressively larger areas of nickel are exposed. Thus, the induction period is linked to the density of defects (steps), to the enhancement of hydrogen
Volume 137 No. 1 - - 2 8 March 1996
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dissociation at step edges, and to the release of Ni atoms from - N i - O - chains as oxygen is removed. A further increase in oxygen pre-coverage of just 1/6 ML, to eo = 2/3 ML, increases the induction period by an order of magnitude and the total reaction time by more than two orders of magnitude. The - N i - O - chains now occur in pairs and the initial reaction, again nucleating preferentially at steps, is seen to proceed by oxygen removal and chain rearrangement to give the "single chain" structure corresponding to eo = 1/2 ML. No areas of clean nickel are exposed in the process and it is only when the transition is complete that hydrogen can begin to attack the halfmonolayer structure in the way described above - - hence the much slower overall reaction with the high coverage phase. Reactivity of oxygen atoms terminating -metal-O- chains has also been identified in two other processes: the formation of methoxy intermediates during methanol dehydrogenation to formaldehyde on oxygen predosed Cu(110), and the oxydehydrogenation of ammonia on Cu(110) and Ni(110). Ammonia is stable on Ni(110) in the presence of high coverages of preadsorbed oxygen but low coverages promote reaction. The rate is maximum at eo = 1/6 ML, where STM shows a new structure forming preferentially near the ends of - N i - O - chains, which are simultaneously consumed [L. Ruan et al., Surf. Sci., 314 (1994) L873]. The new features are taken to be OH species produced by hydrogen bonding between ammonia and chain-terminating O, followed by N-H bond cleavage. -Ni-O-Ni-O...H-NH2 ~ -Ni-O-Ni + OH(a) + NH2(a)
applied catalysis A: General
When 80 is low, the chains are short, highly mobile, and break and reform continuously. Hence, all O atoms are accessible for reaction. In contrast, at high coverages, mobility and segmentation are severely restricted and the few terminal O-atom sites are rapidly blocked by adsorbed NH2. The activity of -metal-O- chains for splitting N-H and O-H bonds raises the question as to whether C-H bonds are attacked in a similar way, a possibility suggested by some previous evidence. STM shows that in the case of benzene, at least, no reaction occurs on Ni(110) precovered with 1/6 ML of oxygen (where oxydehydrogenation of NH3 is maximum). A new surface structure does appear with eo = 1/3 ML but no OH species are involved. Benzene simply compresses the oxygen layer and occupies the gaps between the chains which are opened up in the process [1. Stensgaard et al., Surf. Sci., 337(1995)190]. C.S. McKee
Recent Polish Papers
In a recent article entitled 'Structure of hydrorefining catalysts' [Przem. Chem., 75 (1996) 5], M. Lewandowski and Z. Sarbak describe methods of obtaining aluminium oxide and the surface properties of the resultant materials. They also discuss the most important models for the activities of hydrorefining catalysts, most often compositions of Co or Ni and Mo or W, supported on alumina. The authors pay particular attention to the Topsee model of the structure of these catalysts which has gained great interest in recent years. The Volume 137 No. 1 - - 28 March 1996
observed with Co and Ni, whereas Cu and Zn have only a minor effect. XPS data show that in the particular case of S/Ni/Mo(110), the admetal has a strong tendency to remain in a metallic state while Zn or Cu on Mo(110) become fully sulphided. Extrapolating to XSy/MOS2 catalysts would suggest that the probability of finding a S vacancy around a Ni atom is greater than around a Cu or Zn atom. Then, in agreement with a previous literature proposal, the high HDS activity may be attributable to the existence of Ni centres associated with S vacancies where S-containing molecules can adsorb and readily undergo desulphurization. C.S. McKee Reactive Sites on Oxygenated Metal Surfaces
It has been known for some time that oxygen preadsorbed on various metal surfaces, including the close-packed planes of Ru, Rh and Pt and the (110) planes of Ni and Cu, can be titrated with dissociatively adsorbed hydrogen at around 450 K. An interesting kinetic feature - - an induction period of length increasing with increasing oxygen pre-coverage - - has remained without a completely satisfactory explanation; however, a very direct view of the mechanism on Ni(110) has now been provided by scanning tunneling microscopy [P.T. Sprunger et al., Surf. Sci., 344 (1995)
98]. Fcc (110) surfaces are highly corrugated, making them favourable candidates for investigation by STM, and the details of their interactions with oxygen are well established, particularly in the case of Ni(110). With increasing coverage up to applied catalysis A: General
2/3 ML, the surface passes through a series of reconstructions consisting of parallel - N i - O - N i - O - chains sitting on top of an unreconstructed metal layer. If a surface with an oxygen pre-coverage, 8o, less than 1/2 ML is exposed to hydrogen, the two adsorbed species exist in separate domains and the titration rate is proportional to the overall length of the domain boundaries. The induction period is negligible because clean Ni areas are available for hydrogen dissociation. At 8o = 1/2 ML, however, the surface is completely covered by double-spaced - N i - O chains and the induction period becomes significant. STM shows that the titration reaction is initiated much more readily at steps (inevitably present as defects on any surface) than on flat terraces, with most nucleation sites located along step edges running at 90 ° to the direction of the-N i-Ochains and only very few at steps lying parallel to the chains. Once a small fraction of the oxygen is reacted off, the remainder on the adjoining terraces is removed rapidly. These observations allow formulation of a detailed mechanism. The point of initial attack is the O atom terminating a - N i - O Ni-O- chain at a step, the induction period lasting until this oxygen has been removed by hydrogen dissociating at the step. The Ni atom which then forms the end of the chain breaks away and becomes incorporated in the step edge, exposing the next O in the chain to attack and also creating two new vacant Ni sites for hydrogen dissociation, one on the lower and the other on the upper terrace. The reaction now accelerates as progressively larger areas of nickel are exposed. Thus, the induction period is linked to the density of defects (steps), to the enhancement of hydrogen
Volume 137 No. 1 - - 2 8 March 1996
N4
dissociation at step edges, and to the release of Ni atoms from - N i - O - chains as oxygen is removed. A further increase in oxygen pre-coverage of just 1/6 ML, to eo = 2/3 ML, increases the induction period by an order of magnitude and the total reaction time by more than two orders of magnitude. The - N i - O - chains now occur in pairs and the initial reaction, again nucleating preferentially at steps, is seen to proceed by oxygen removal and chain rearrangement to give the "single chain" structure corresponding to eo = 1/2 ML. No areas of clean nickel are exposed in the process and it is only when the transition is complete that hydrogen can begin to attack the halfmonolayer structure in the way described above - - hence the much slower overall reaction with the high coverage phase. Reactivity of oxygen atoms terminating -metal-O- chains has also been identified in two other processes: the formation of methoxy intermediates during methanol dehydrogenation to formaldehyde on oxygen predosed Cu(110), and the oxydehydrogenation of ammonia on Cu(110) and Ni(110). Ammonia is stable on Ni(110) in the presence of high coverages of preadsorbed oxygen but low coverages promote reaction. The rate is maximum at eo = 1/6 ML, where STM shows a new structure forming preferentially near the ends of - N i - O - chains, which are simultaneously consumed [L. Ruan et al., Surf. Sci., 314 (1994) L873]. The new features are taken to be OH species produced by hydrogen bonding between ammonia and chain-terminating O, followed by N-H bond cleavage. -Ni-O-Ni-O...H-NH2 ~ -Ni-O-Ni + OH(a) + NH2(a)
applied catalysis A: General
When 80 is low, the chains are short, highly mobile, and break and reform continuously. Hence, all O atoms are accessible for reaction. In contrast, at high coverages, mobility and segmentation are severely restricted and the few terminal O-atom sites are rapidly blocked by adsorbed NH2. The activity of -metal-O- chains for splitting N-H and O-H bonds raises the question as to whether C-H bonds are attacked in a similar way, a possibility suggested by some previous evidence. STM shows that in the case of benzene, at least, no reaction occurs on Ni(110) precovered with 1/6 ML of oxygen (where oxydehydrogenation of NH3 is maximum). A new surface structure does appear with eo = 1/3 ML but no OH species are involved. Benzene simply compresses the oxygen layer and occupies the gaps between the chains which are opened up in the process [1. Stensgaard et al., Surf. Sci., 337(1995)190]. C.S. McKee
Recent Polish Papers
In a recent article entitled 'Structure of hydrorefining catalysts' [Przem. Chem., 75 (1996) 5], M. Lewandowski and Z. Sarbak describe methods of obtaining aluminium oxide and the surface properties of the resultant materials. They also discuss the most important models for the activities of hydrorefining catalysts, most often compositions of Co or Ni and Mo or W, supported on alumina. The authors pay particular attention to the Topsee model of the structure of these catalysts which has gained great interest in recent years. The Volume 137 No. 1 - - 28 March 1996