(5o) Structural Requirements On Catalyst Surfaces During Chemical Turnovers: Hydrogen Production and C1 Conversion Chemistry

Authors: 
Chin, Y. (., University of California, Berkeley
Iglesia, E., University of California at Berkeley


Chemical conversions of natural gas (primarily methane) to transportation fuel and high energy density chemicals are promising technologies important for harvesting stranded gas at remote reservoirs. Strong carbon-hydrogen bond in methane relative to other hydrocarbons renders the direct conversion pathway unselective. The indirect methane conversion pathway, for this reason, remains to date as the commercially viable gas-to-liquid route. The economic feasibility of this route is determined by the efficiency of the initial methane conversion step to synthesis gas as the molecular building blocks for downstream chemical synthesis. Additionally, methane conversion is important for applications in natural gas catalytic combustor and for on-board hydrogen production at distributed sites to deliver hydrogen as a clean energy carrier for next generation power systems. These methane reactions (reformings, catalytic partial oxidation, and combustion) required high temperatures (> 873 K); they are energetically intensive and in the case of reformings and apparent partial oxidation, constrained by thermodynamics. These technology challenges continue to spark interests in understanding the fundamental reaction dynamics and the structural requirement of catalytic surface during CH4 chemical turnovers.

Kinetics for methane steam and dry reformings have been previously established. The mechanistic interpretation of the reaction pathways and selectivity during CH4?O2 reaction, however, remain contradictory despite significant research efforts. The disagreement is in part related to the highly exothermic and endothermic nature of combustion and reforming reactions, which occurred at reaction zones within the length scales of heat conduction. Rate and selectivity data, as a consequence, are corrupted by the inherent temperature and concentration gradients across the catalyst pellets and reactor bed, which lead in turn to disagreements on the microscopic reaction pathways and on the kinetic functions of active sites during CH4 reactions. The mechanistic interpretation of rate and selectivity data are made even more complex as the identity of the most abundant surface intermediates and the thermodynamic phase stability of metal or oxide vary along the reactors in response to changing local reactant pressures and temperatures. Understanding the surface kinetics is nevertheless one of the requirements to predict the overall reactor performance. Our approach is to carry out kinetic measurements under strict kinetic control at relevant operating temperatures on a series of supported Group VIII metal cluster catalysts. This, combined with quantum chemical calculations of transition states, reaction energies and barriers, allows us to derive a comprehensive description of reaction paths at the molecular scale level, to elucidate the site-requirements, periodic trends, and cluster size effects of CH4 reactions on metal clusters.

We propose here, based on rigorous kinetics and isotopic studies, a simple yet complete sequence of elementary reaction steps to describe the CH4 combustion and the apparent partial oxidation on metal clusters. CO is not detected before the complete O2 consumption, which indicates that the direct partial oxidation does not occurred at the molecular scale. CO and H2, by inference, formed via sequential reforming of CH4 with combustion products (H2O and CO2). Four distinct kinetic regimes, each with unique reactant pressures dependences and kinetic isotope effects, were detected depending on the coverages and reactivity of the chemisorbed oxygen. Kinetic analyses show that the seemingly complicated response in CH4 conversion rates with reactant pressures is a single-valued function of the virtual oxygen pressure (O2)v, a surface oxygen chemical potential term that is a function of gas phase composition and the elementary rate constants. The (O2)v determines the relative concentrations of oxygen-oxygen, oxygen-oxygen vacancy, and vacancy-vacancy site-pairs available on metal surfaces for the kinetically-relevant C-H dissociation steps; it is therefore an appropriate descriptor which allows us to predict both the catalytic activity and also the thermodynamically stable state of the metal clusters during reactions. Density functional theoretical calculations are used to provide molecular level details on the catalytic functions of chemisorbed oxygen and oxygen-vacancy during the kinetically-relevant C-H bond activation steps. C-H bond dissociation proceeds on oxygen-vacancy sites via a metal-atom insertion transition state similar to that on metal surfaces. In contrast, the methyl group on surfaces saturated with oxygens acquired free-radical like character in the transition state which indicates a homolytic C-H bond cleavage route. Detailed analyses show that the elementary C-H and O=O bond activation barriers for methane and oxygen are functions of the oxygen binding energy on metal following the prevalent BrØnsted-Evans-Polanyi relation. Combining experimental and theoretical investigations allow us to correlate the CH4 oxidation activity to oxygen binding strength on metal clusters and derive the periodic trend to describe the reactivity Group VIII metal and their cluster size effects during CH4 conversion reactions.

CH4-O2 reactions on Group VIII metal clusters are described by an identical catalytic cycle consisted of a closed sequence of elementary steps. The chemical turnover rates, selectivity, and structure sensitivity vary with the surface oxygen chemical potential because it dictates the identities of the kinetically-relevant step and of the most abundant surface intermediates. The chemistry is of practical significance and can be extend to describe the C-H bond activation on metal clusters involving chemisorbed oxygen species.