Research
The theme of KInetics and Dynamics at Surfaces (KIDS) proposal
Reaction Rates
The rates of chemical reactions determine the chemical environment within which humanity exists and place boundaries on our ability to perform material transformations to solve society’s problems. To appreciate that this is not an overstatement, consider gas phase reactions that produce and consume ozone in the atmosphere. Obviously, the atmosphere is not an equilibrium system; rather, rate processes dictate its chemical behaviour. Through intensive efforts to obtain the temperature dependent rates of many relevant gas-phase reactions, we have obtained understanding of both the over production of ozone in urban air pollution as well as ozone depletion in the stratosphere.
Kinetic models helped prove that NOx emissions lead to urban ozone pollution and Freon emissions result in stratospheric ozone loss. In the meantime, both of these chemicals are under worldwide legal restrictions; there are few, if any, areas of chemical research that have had a larger and more positive impact on society.
Key to these advances is catalysis, possibly the single greatest discovery coming out of the study of chemistry, the essence of which resides in the catalyst’s ability to accelerate (normally slow) reactions. As a technological tool, it has taken on an outsized significance. A recent workshop report commissioned by the European Union commented that heterogeneous catalysis is “a key enabling technology for most of the seven societal challenges identified in the European Research Framework Programme Horizon 2020.” The report also points out that “…as evidence for global climate change continues to grow, catalysis has moved to the front line of the struggle to obtain new, sustainable technologies for the future.” and that “Catalysis and catalytic processes account, directly or indirectly, for 20-30% of world Gross Domestic Product (GDP).” Many of the most important industrial catalytic processes involve reactions at the surfaces of solid catalysts. In an analogous fashion to the example of atmospheric chemistry, knowledge of the rates of the elementary reactions at surfaces underlies our ability to develop a fundamental understanding and construct realistic models of industrially important catalytic processes. Clearly, progress of this sort has an enormous potential for societal impact.
The key questions for research on surface reaction rates are:
1) How can we measure rates accurately?
and
2) How can we calculate rates from first principles when they are impossible to measure?
The Methodology of velocity-resolved kinetics
The new method borrows from experience gained from imaging gas-phase processes, extending imaging to surface scattering as illustrated in Fig. 1. Temporally short-pulsed molecular beams are skimmed and collimated to about 3 mm in diameter. When studying reactions, we use two beams, crossing at the surface with a relative angle of ~30°. We vary the repetition rates of the pulsed nozzles, thus controlling the relative concentrations of the reactants. The reactant beams strike the surface whose temperature is controlled using radiative and electron bombardment heating. A laser beam intersects the scattered molecules/reaction products ionizing them either using resonance enhanced multiphoton ionization (REMPI) or non-resonant multiphoton ionization (MPI) with a powerful fs laser pulse. Ions are extracted using a pulsed homogeneous electric field perpendicular to the plane of the molecular beams, and detected using a position sensitive imaging detector. Scanning the timing between the laser pulse and the molecular beam records: (a) the entire in-plane scattered molecule velocity distribution for each delay and (b), the velocity-resolved kinetic trace for the reaction. Calibration of both the incoming and scattered velocity distributions achieved by recording images at various delays between the pulsed extraction field and the ionizing laser pulse. The method is a combination of laser and ion slicing, while velocity mapping can also be realized using a large aperture cylindrical lens.
The theme of KInetics and Dynamics at Surfaces (KIDS) proposal
Reaction Rates
The rates of chemical reactions determine the chemical environment within which humanity exists and place boundaries on our ability to perform material transformations to solve society’s problems. To appreciate that this is not an overstatement, consider gas phase reactions that produce and consume ozone in the atmosphere. Obviously, the atmosphere is not an equilibrium system; rather, rate processes dictate its chemical behaviour. Through intensive efforts to obtain the temperature dependent rates of many relevant gas-phase reactions, we have obtained understanding of both the over production of ozone in urban air pollution as well as ozone depletion in the stratosphere.
Kinetic models helped prove that NOx emissions lead to urban ozone pollution and Freon emissions result in stratospheric ozone loss. In the meantime, both of these chemicals are under worldwide legal restrictions; there are few, if any, areas of chemical research that have had a larger and more positive impact on society.
Key to these advances is catalysis, possibly the single greatest discovery coming out of the study of chemistry, the essence of which resides in the catalyst’s ability to accelerate (normally slow) reactions. As a technological tool, it has taken on an outsized significance. A recent workshop report commissioned by the European Union commented that heterogeneous catalysis is “a key enabling technology for most of the seven societal challenges identified in the European Research Framework Programme Horizon 2020.” The report also points out that “…as evidence for global climate change continues to grow, catalysis has moved to the front line of the struggle to obtain new, sustainable technologies for the future.” and that “Catalysis and catalytic processes account, directly or indirectly, for 20-30% of world Gross Domestic Product (GDP).” Many of the most important industrial catalytic processes involve reactions at the surfaces of solid catalysts. In an analogous fashion to the example of atmospheric chemistry, knowledge of the rates of the elementary reactions at surfaces underlies our ability to develop a fundamental understanding and construct realistic models of industrially important catalytic processes. Clearly, progress of this sort has an enormous potential for societal impact.
The key questions for research on surface reaction rates are:
1) How can we measure rates accurately?
and
2) How can we calculate rates from first principles when they are impossible to measure?
The Methodology of velocity-resolved kinetics
The new method borrows from experience gained from imaging gas-phase processes, extending imaging to surface scattering as illustrated in Fig. 1. Temporally short-pulsed molecular beams are skimmed and collimated to about 3 mm in diameter. When studying reactions, we use two beams, crossing at the surface with a relative angle of ~30°. We vary the repetition rates of the pulsed nozzles, thus controlling the relative concentrations of the reactants. The reactant beams strike the surface whose temperature is controlled using radiative and electron bombardment heating. A laser beam intersects the scattered molecules/reaction products ionizing them either using resonance enhanced multiphoton ionization (REMPI) or non-resonant multiphoton ionization (MPI) with a powerful fs laser pulse. Ions are extracted using a pulsed homogeneous electric field perpendicular to the plane of the molecular beams, and detected using a position sensitive imaging detector. Scanning the timing between the laser pulse and the molecular beam records: (a) the entire in-plane scattered molecule velocity distribution for each delay and (b), the velocity-resolved kinetic trace for the reaction. Calibration of both the incoming and scattered velocity distributions achieved by recording images at various delays between the pulsed extraction field and the ionizing laser pulse. The method is a combination of laser and ion slicing, while velocity mapping can also be realized using a large aperture cylindrical lens.