Direct measurement of key molecule will increase accuracy of combustion models
08 February 2015
Sandia National Laboratories researchers are the first to measure directly hydroperoxyalkyl radicals—a class of reactive molecules denoted as “QOOH”—that are key in the chain of reactions that controls the early stages of combustion. This breakthrough has generated data on QOOH reaction rates and outcomes that will improve the fidelity of models used by engine manufacturers to create cleaner and more efficient cars and trucks. A paper describing the work, performed by David Osborn, Ewa Papajak, John Savee, Craig Taatjes and Judit Zádor at Sandia’s Combustion Research Facility, is published in the journal Science.
As the Science Editor notes, the net combustion reaction is simple to describe (oxygen reacts with hydrocarbons to form water and carbon dioxide) but the details are much more complicated. Thousands of chemical reactions are involved in the conversion of a fuel’s chemical energy into mechanical work in an automobile engine. The fleeting molecules that initiate, sustain and then increase combustion are radicals: short-lived molecules that readily react and form new chemical bonds.
Oxidation of organic compounds in such disparate processes as secondary organic aerosol (SOA) formation in Earth’s troposphere and fuel autoignition in internal combustion engines is governed by a surprisingly similar set of reactive intermediates. At the heart of low-temperature oxidation are chain reactions initiated by peroxy radicals, ROO. These oxygen-centered radicals are formed when O2 adds to an organic radical, R, initially formed by either hydrogen abstraction from RH or addition of radicals (primarily OH) to π bonds between carbon atoms. It is well established that isomerization of ROO to a carbon-centered hydroperoxyalkyl radical (typically denoted as QOOH) can occur via intramolecular hydrogen abstraction, and in many cases this isomerization step profoundly influences the rate and effect of the oxidation reaction.
Unimolecular decomposition of QOOH can produce a reactive OH radical and therefore plays an important role in radical chain propagation. More important, reaction of QOOH with O2 can form OOQOOH intermediates that may subsequently decompose, yielding two OH radicals. This mechanism is regarded as the most important radical chain-branching step in hydrocarbon oxidation below ~900 K [~627 ˚C]. In combustion, understanding low-temperature chain branching is critical for improving efficiency through new engine designs in which ignition is controlled by fuel oxidation chemistry (autoignition).
… Despite this awareness of the importance of QOOH intermediates, they have eluded direct experimental detection. With one exception studies of QOOH reactivity rely on measurements that necessarily involve complex assumptions about the reaction mechanism. Routine theoretical calculations of QOOH + O2 kinetics are hindered by the large size of these systems and the absence of a saddle point separating QOOH + O2 from OOQOOH, necessitating extensive and computationally demanding sampling of the potential energy surface (PES). Theoretical predictions of QOOH + O2 rate coefficients are thus scarce, and direct experimental measurements combined with theory and modeling are critical for a detailed understanding of this class of chemical reactions. … Here, we report direct detection of a resonance-stabilized QOOH intermediate formed during oxidation of the cyclic unsaturated hydrocarbon 1,3-cycloheptadiene (c-C7H10, c-hpd).
—Savee et al.
Decades of research worldwide have shown that QOOH must be a central connection in the network of ignition reactions. Researchers learned this by studying the products of ignition chemistry, looking at this web of reactions from its perimeter and working inward, gradually deducing the nature of the “reactive intermediate” molecules that must lie at the center.
Nearly 10 years ago, Sandia researchers designed a new instrument, the Multiplexed Photoionization Mass Spectrometer (MPIMS), to directly probe all kinds of intermediates, including the species that are at the center of important webs of reactions. In 2012, the Sandia team, together with colleagues from the University of Manchester and Bristol University in England used the MPIMS to directly measure reaction rates and products of the “Criegee intermediate,” a crucial reactive molecule in the web of reactions that occur in atmospheric chemistry. (Earlier post.)
We not only measured the Criegee intermediates and provided fundamental knowledge about Criegee reactions, we also disclosed tTo other researchers the process for generating and measuring the intermediates on their own. The impact has been enormous, as others have taken this knowledge and put it to work.
—Craig Taatjes, manager of Sandia’s combustion chemistry department
The Sandia team needed to devise a strategy to create enough QOOH radicals to detect, and a way to determine the spectral fingerprint of a QOOH molecule. Chemist John Savee helped pinpoint the best fuel for producing a detectable QOOH: cycloheptadiene, a molecule with seven carbon atoms arranged in a ring.
Initial experiments seemed to prove Savee’s ideas were right, and the team turned to its computational experts, Ewa Papajak and Judit Zádor, who used quantum chemistry to predict what the experimentalists should have observed. For the direct measurements, the team moved the MPIMS to the Advanced Light Source, a synchrotron user facility at Lawrence Berkeley National Laboratory. The intense tunable light created by the synchrotron allowed the team to measure spectral fingerprints of molecules, deducing the particular arrangement of atoms that gives a molecule its identity.
They confirmed that the spectrum of the radical they observed matched that predicted by Papajak and Zádor, showing that it was in fact a QOOH molecule, rather than some other possible arrangement of the same atoms.
The results establish that resonance stabilization dramatically changes QOOH reactivity and, hence, that oxidation of unsaturated organics can produce exceptionally long-lived QOOH intermediates.
—Savee et.al
The impact of this class of QOOH radicals is not yet clear; the data will be incorporated into the latest combustion models to test its impact.
We’ve been working on this reaction network from all sides for many years. Now that we have directly measured reaction rates for a QOOH radical, we’ve filled in a large part of the picture.
—Craig Taatjes
The researchers acknowledged there is still much to do to create a complete and accurate model of ignition or atmospheric oxidation. For example, measurements of other, more reactive QOOH species will be important for predicting ignition and oxidation behavior of a range of fuels.
We know from our experience with the Criegee intermediate that researchers around the world will make great use of this information. And because these oxidation processes are important in many areas, including atmospheric studies, the impacts are likely to reach far beyond combustion.
—David Osborn
This research was funded by the Office of Basic Energy Sciences within the US Department of Energy’s Office of Science.
Resources
John D. Savee, Ewa Papajak, Brandon Rotavera, Haifeng Huang, Arkke J. Eskola, Oliver Welz, Leonid Sheps, Craig A. Taatjes, Judit Zádor, David L. Osborn (2015) “Direct observation and kinetics of a hydroperoxyalkyl radical (QOOH)” Science doi: 10.1126/science.aaa1495
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