In its more than 25 years of existence, STM has predominantly brought us extremely detailed images of matter at the molecular and atomic level. An Figure 1.2 Design of the torsional harmonic cantilever with an off-axis tip. Left: A SEM image of a torsional harmonic cantilever. Right: An illustration of the THC interacting with the surface. The offset position of the tip results in a torque around the axis of the cantilever. (Image and illustration:Ozgur Sahin)STM—not to be confused with a scanning electron microscope (SEM)6—is a nonoptical microscope that scans an electrical probe over a surface to be imaged to detect a weak electric current flowing between the tip and the surface.The STM allows scientists to visualize regions of high electron density and hence infer the position of individual atoms and molecules on the surface of a lattice.
Researchers have advanced this technique to a point where they can use STM to perform real-time single-molecule imaging of an entire chemical reaction.Many chemical reactions are catalyzed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. Applying the STM approach to studying chemical reactions in a dynamic environment can provide valuable information about reaction mechanisms and rates as well as
catalyst activity and stability.
‘‘A couple of years ago, we took up the idea of imaging all the steps of a chemical reaction in an STM,’’ says Hans Elemans. ‘‘Some examples of imaging chemical reactions have been described before, but only under extreme conditions—for example low temperature, ultra-high vacuum—and we wanted to make it work in an environment familiar to chemists, such as the interface of a liquid and a solid.’’ Elemans, a member of the Nolte Group for Supramolecular
Chemistry at Radboud University in Nijmegen, The Netherlands,together with members from his group and collaborators from the University of Sydney and the Institute for Molecules and Materials at Radboud University, are the first team to use an STM to image a complete catalytic reaction at the interface of a liquid and a solid by following the catalyst molecules, when they are in action, at the single-molecule level.
As the catalysts for their chemical reactions the researchers used manganese porphyrins. These are dye molecules, whose naturally occurring magnesium analogues are involved in processes such as light-harvesting in plants. In their experiments, manganese porphyrin was added to a liquid-cell STM set-up and the researchers were able to observe the formation of extended monolayer domains at the liquid–solid interface, with the porphyrins adsorbed face-on to the surface in regular patterns.
‘‘The reaction that we investigated led to several unexpected new insights,’’ explains Elemans. ‘‘First, the gold surface we used appeared to activate the adsorbed manganese porphyrin catalysts to react with molecular oxygen. When the catalysts are in their dissolved state in a solution, this reaction does not occur. The second surprise was that each oxygen molecule appeared to have a preference to oxidize two adjacent catalysts. This perfectly illustrates that STM is a unique technique, because it measures so locally, i.e. at the single-molecule level, that such a process can be identified. A final surprise was that the surface bound catalysts were extremely stable. In similar reactions in solution, manganese porphyrin catalysts cluster together and oxidize themselves. At the surface, they remain well-separated from each other and many turnovers per single catalyst could be observed.’’
The researchers’ motivation for this work was that they wanted to develop an additional technique to study a chemical reaction. Nearly all conventional techniques are ensemble techniques and measure the average behavior of millions of molecules at a particular time. STM measures the behavior of single molecules, and as such it might be used in the future as an additional analytic technique to the conventional ones.
Elemans points out that there are drawbacks to the new technique. One of the problems still is that STM is slow. Scanning takes seconds, and if the processes to be studied are faster than that they might not be captured by the instrument. ‘‘For that reason, we chose an oxidation reaction that was known to be very slow, so we were sure that we were looking at real-time processes,’’ he says. ‘‘However, it limits the use of this technique for studying many other, faster reactions. Solving the problem relies on the development of faster STM equipment, which is in fact currently ongoing.’’
Being able to gain a highly detailed insight into chemical reaction mechanisms might one day enable chemical engineers to improve the reaction, and, on an industrial scale, maybe improve entire chemical synthesis and catalysis processes. At present this is only a vision, but the possibilities are almost endless.
‘‘This is only the first chemical reaction investigated with STM at a liquid– solid interface, and there might be thousands of other reactions that can be investigated in a similar manner,’’ says Elemans. ‘‘All of these might provide new insights. Our group is investigating other types of reactions with similar porphyrin catalysts. A particular challenge is to study cascade reactions in this way. These are reactions in which two or more different catalysts are bound to the surface, and in which the product of a catalytic reaction at a catalyst A is the starting material of the reaction at catalyst B. It is of great interest to see how such reactions are coupled in time and space, at the single-molecule level.’’
Featured scientist: Hans Elemans
Organization: Cluster for Molecular Chemistry, Radboud University,
Nijmegen, The Netherlands
Relevant publication: Bas Hulsken, Richard Van Hameren, Jan W.
Gerritsen, Tony Khoury, Pall Thordarson, Maxwell J. Crossley,
Alan E. Rowan, Roeland J. M. Nolte, Johannes A. A. W. Elemans,
Sylvia Speller: Real-time single-molecule imaging of oxidation
catalysis at a liquid–solid interface. Nat. Nanotechnol., 2, 285–289.
Source
Nano-Society
www.rsc.org/nanoscience
Researchers have advanced this technique to a point where they can use STM to perform real-time single-molecule imaging of an entire chemical reaction.Many chemical reactions are catalyzed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. Applying the STM approach to studying chemical reactions in a dynamic environment can provide valuable information about reaction mechanisms and rates as well as
catalyst activity and stability.
‘‘A couple of years ago, we took up the idea of imaging all the steps of a chemical reaction in an STM,’’ says Hans Elemans. ‘‘Some examples of imaging chemical reactions have been described before, but only under extreme conditions—for example low temperature, ultra-high vacuum—and we wanted to make it work in an environment familiar to chemists, such as the interface of a liquid and a solid.’’ Elemans, a member of the Nolte Group for Supramolecular
Chemistry at Radboud University in Nijmegen, The Netherlands,together with members from his group and collaborators from the University of Sydney and the Institute for Molecules and Materials at Radboud University, are the first team to use an STM to image a complete catalytic reaction at the interface of a liquid and a solid by following the catalyst molecules, when they are in action, at the single-molecule level.
As the catalysts for their chemical reactions the researchers used manganese porphyrins. These are dye molecules, whose naturally occurring magnesium analogues are involved in processes such as light-harvesting in plants. In their experiments, manganese porphyrin was added to a liquid-cell STM set-up and the researchers were able to observe the formation of extended monolayer domains at the liquid–solid interface, with the porphyrins adsorbed face-on to the surface in regular patterns.
‘‘The reaction that we investigated led to several unexpected new insights,’’ explains Elemans. ‘‘First, the gold surface we used appeared to activate the adsorbed manganese porphyrin catalysts to react with molecular oxygen. When the catalysts are in their dissolved state in a solution, this reaction does not occur. The second surprise was that each oxygen molecule appeared to have a preference to oxidize two adjacent catalysts. This perfectly illustrates that STM is a unique technique, because it measures so locally, i.e. at the single-molecule level, that such a process can be identified. A final surprise was that the surface bound catalysts were extremely stable. In similar reactions in solution, manganese porphyrin catalysts cluster together and oxidize themselves. At the surface, they remain well-separated from each other and many turnovers per single catalyst could be observed.’’
The researchers’ motivation for this work was that they wanted to develop an additional technique to study a chemical reaction. Nearly all conventional techniques are ensemble techniques and measure the average behavior of millions of molecules at a particular time. STM measures the behavior of single molecules, and as such it might be used in the future as an additional analytic technique to the conventional ones.
Elemans points out that there are drawbacks to the new technique. One of the problems still is that STM is slow. Scanning takes seconds, and if the processes to be studied are faster than that they might not be captured by the instrument. ‘‘For that reason, we chose an oxidation reaction that was known to be very slow, so we were sure that we were looking at real-time processes,’’ he says. ‘‘However, it limits the use of this technique for studying many other, faster reactions. Solving the problem relies on the development of faster STM equipment, which is in fact currently ongoing.’’
Being able to gain a highly detailed insight into chemical reaction mechanisms might one day enable chemical engineers to improve the reaction, and, on an industrial scale, maybe improve entire chemical synthesis and catalysis processes. At present this is only a vision, but the possibilities are almost endless.
‘‘This is only the first chemical reaction investigated with STM at a liquid– solid interface, and there might be thousands of other reactions that can be investigated in a similar manner,’’ says Elemans. ‘‘All of these might provide new insights. Our group is investigating other types of reactions with similar porphyrin catalysts. A particular challenge is to study cascade reactions in this way. These are reactions in which two or more different catalysts are bound to the surface, and in which the product of a catalytic reaction at a catalyst A is the starting material of the reaction at catalyst B. It is of great interest to see how such reactions are coupled in time and space, at the single-molecule level.’’
Featured scientist: Hans Elemans
Organization: Cluster for Molecular Chemistry, Radboud University,
Nijmegen, The Netherlands
Relevant publication: Bas Hulsken, Richard Van Hameren, Jan W.
Gerritsen, Tony Khoury, Pall Thordarson, Maxwell J. Crossley,
Alan E. Rowan, Roeland J. M. Nolte, Johannes A. A. W. Elemans,
Sylvia Speller: Real-time single-molecule imaging of oxidation
catalysis at a liquid–solid interface. Nat. Nanotechnol., 2, 285–289.
Source
Nano-Society
www.rsc.org/nanoscience
No comments:
Post a Comment