The chemical reactivity of gases with solid surfaces is ubiquitous in natural and industrial energy transformation. Cooperative effects that couple gas phase chemistry with surface chemistry are critical for foundational understanding, but they prove challenging to probe experimentally and theoretically. Heterogenous catalysis is an ideal field in which to expose and isolate the fundamental chemical physics of these cooperative effects. We are pursuing a program to characterize gas-surface coupling, through chemically specific, temporally and spatially resolved probes of both reacting surfaces and the near-surface gas phase. The program combines linear and non-linear optical spectroscopies, universal photoionization mass spectrometry, and surface-specific spectroscopies at ambient pressures. Breakthrough coherent nonlinear spectroscopic imaging methods enable mapping of surface-gas exchange, which, combined with in operando surface spectroscopy, will correlate reaction rates with local surface structure, composition, and oxidation state during catalytic chemical transformations. These correlated measurements in self-organized oscillatory reaction systems will directly quantify how different surface domains communicate via chemical transport and are linked by chemical feedback. We have observed reactive intermediates above surfaces, which both establishes critical mechanistic links between surface and gas-phase reactivity and provides detailed species maps to inform microkinetic models. The long-term goal of this project is to elucidate the fundamental mechanisms of cooperative gas-surface chemistry, influencing DOE mission research in catalysis, synthesis, and energy transformation.

Figure 5: We image the gas phase immediately above a reacting surface using three complementary techniques: Planar laser induced fluorescence (lower right), Raman spectroscopy (upper left), and near-surface molecular beam mass spectrometry (center, background). Source: Sandia National Laboratories.

 

Partner: Prof. Johan Zetterberg, Lund University

PIs: Chris J. Kliewer, Nils Hansen, Jonathan H. Frank, Farid El Gabaly, and David L. Osborn

Sandia is developing pynta, a tool to explore reactions of adsorbates on crystal facets automatically.

The goal of the ECC project is to create a computational framework that accelerates discovery and characterization of complex molecular systems. We are targeting gas-phase and coupled heterogeneous/gas-phase reactions and reaction mechanisms with relevance to catalytic conversion of hydrocarbons, oxygenates, and small molecules. We capitalize on recent improvements in theoretical chemistry combined with improved mathematical software for solving complex problems integrating exascale-size supercomputers to develop a uniquely powerful chemical computational toolset for the research community. Learn more at ECC project.

The physical chemistry that enables particles to form and grow from gas phase precursors is highly complex and spans systems from the highly oxygenated particles of secondary organic aerosol to the carbonaceous particles of combustion, i.e., soot. At the CRF, scientists apply advanced mass spectrometric methods to probe the gas phase chemistry of polycyclic aromatic hydrocarbons (PAH)  formation in the high-temperature environment of flames and to study the chain of reactions that lead to highly oxygenated molecules at atmospherically relevant temperatures.

Understanding soot inception requires a chemical mechanism that can covalently bond hydrocarbon precursors at high temperatures rapidly enough to explain particle formation. CRF researchers, in collaboration with Lawrence Berkeley National Laboratory and University of California–Berkeley, highlighted the possibility of clustering of hydrocarbons by radical-chain reactions (CHRCR) driven by resonance-stabilized hydrocarbon radicals. CRF scientists employed tandem mass spectrometry to identify aliphatically bridged polycyclic aromatic hydrocarbons and PAHs with aliphatic side chains, which have been hypothesized to serve as “seeds” for soot particles. Current work at the CRF links controlled experimental kinetics investigations, machine learning methods, and automated theoretical kinetics to understand the role of resonance stabilization in the clustering reactions that lead to soot.

The formation and growth of atmospheric aerosols, important for climate and Earth’s radiation balance, is linked to low-volatility, highly oxygenated molecules (HOM) from gas phase oxidation of hydrocarbons. CRF researchers have investigated the production of HOM via sequential reactions of Criegee intermediates from ozonolysis and multiple additions to molecular oxygen in peroxy-radical driven chemistry. These oxidation processes form a bridge between the high-temperature oxidation important in ignition and combustion and the lower-temperature conditions of tropospheric chemistry.

 

The yellow color at the top of the flame comes from the glow of hot soot. The diagram (upper right) depicts some possible reactions that can grow molecules large enough to lead to the formation of the initial particles that start soot growth in the flame. Source: https://doi.org/10.1021/acs.jpca.8b08947


Potential energy diagram for a reaction sequence, driven by resonance-stabilized radical (RSR) formation, that is representative of the CHRCR mechanism. Source: https://science.sciencemag.org/content/361/6406/997


CRF researchers (left to right) Scott Skeen (now at Dixie State University), Nils Hansen, and postdoc Brian Adamson (now at ThermoFisher) around the tandem mass spectrometer they used to study the structure of large hydrocarbon molecules in a sooting flame.  (Photo credit: Michael Padilla)

Key Contributions

Partners

  • Hope Michelsen (University of Colorado)
  • Kevin Wilson (Lawrence Berkeley National Laboratory)
  • Ralf Kaiser (University of Hawaii)
  • Yiguang Ju (Princeton University)

PIs: Nils Hansen, Habib Najm, Craig A. Taatjes, Judit Zádor

The interaction of gas phase molecules with other phases bridges two systems—condensed phase and gas phase—that are described by different paradigms of chemical physics. Our research in multiphase systems focuses on understanding how these regimes affect each other and the nature of the intermediary region between the phases. Two areas are emphasized at the CRF: emergence of condensed phase particles from gas phase reactive processes, and reactive interactions between the gas phase and other phases. Chemically controlled gas-to-particle conversion depends on chemical reaction sequences that govern the approach to condensation or particle inception. CRF researchers study these reaction sequences as well as the particles themselves. With multiple implications for energy missions, researchers need to understand the nature of these processes such as soot formation in combustion, aerosol formation in Earth’s atmosphere, particle synthesis processes, or deposit formation in catalytic systems. Reactive interactions between the gas-phase and other phases inform our understanding of catalysis. It is increasingly clear that at atmospheric or greater pressures—conditions under which real catalysis occurs—gas-phase and surface reactions are coupled. CRF scientists apply temporally and spatially resolved probes, developed in gas-phase chemical physics, to map directly the gas phase above reacting surfaces. These measurements can be coupled with advanced operando surface characterization to quantify the interactions between gas phase and surface reactions.