Abstract
Methane steam reforming (MSR) plays a key role in the production of syngas and hydrogen from natural gas. The increasing interest in the use of hydrogen for fuel cell applications demands development of catalysts with high activity at reduced operating temperatures. Ni-based catalysts are promising systems because of their high activity and low cost, but coke formation generally poses a severe problem. Studies of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) indicate that CH4/H2O gas mixtures react with Ni/CeO2(111) surfaces to form OH, CHx, and CHxO at 300 K. All of these species are easy to form and desorb at temperatures below 700 K when the rate of the MSR process is accelerated. Density functional theory (DFT) modeling of the reaction over ceria-supported small Ni nanoparticles predicts relatively low activation barriers between 0.3 and 0.7 eV for complete dehydrogenation of methane to carbon and the barrierless activation of water at interfacial Ni sites. Hydroxyls resulting from water activation allow for CO formation via a COH intermediate with a barrier of about 0.9 eV, which is much lower than that through a pathway involving lattice oxygen from ceria. Neither methane nor water activation is a rate-determining step, and the OH-assisted CO formation through the COH intermediate constitutes a low-barrier pathway that prevents carbon accumulation. The interactions between Ni and the ceria support and the low metal loading are crucial for the reaction to proceed in a coke-free and efficient way. These results pave the way for further advances in the design of stable and highly active Ni-based catalysts for hydrogen production.
Highlights
Methane steam reforming (MSR, CH4 + H2O ⇄ 3H2 + CO) is the main route for the large-scale industrial manufacture of hydrogen, primarily used for the synthesis of ammonia and methanol, among other commodities,[1] as well as the hydrocracking of long-chain hydrocarbons in petroleum refineries.[2]
Using a combination of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and molecular modeling based on density functional theory (DFT), we present a comprehensive study of the MSR reaction on the surface of model Ni/CeO2(111) catalysts and compare with results reported for the extended Ni(111) surface in the literature.[21−24,47−54] We show that lowloaded Ni/CeO2 catalysts have sites with unique properties that result from the nature of both the metallic phase and the support and their interactions, which enable the facile activation of C−H and O−H bonds from CH4 and H2O, respectively
CH4/H2O gas mixtures with the Ni/CeO2(111) surfaces were performed using instruments located at the Chemistry Division in Brookhaven National Laboratory (BNL) and at the Advanced Light Source (ALS) in Berkeley.[27−30] In both instruments, characterized the Ni/CeO2(111) following standard surfaces were prepared and procedures.[27−29] Ce metal was first evaporated onto a Ru(0001) substrate at 700 K under a background pressure of 5 × 10−7 Torr of O2, and the sample was annealed at 800 K for a period of 10 min at the same O2 pressure
Summary
Methane steam reforming (MSR, CH4 + H2O ⇄ 3H2 + CO) is the main route for the large-scale industrial manufacture of hydrogen, primarily used for the synthesis of ammonia and methanol, among other commodities,[1] as well as the hydrocracking of long-chain hydrocarbons in petroleum refineries.[2]. Several alternative reactions have been proposed, such as methane dry reforming and partial oxidation, but their lower H2/CO ratio compared to that of MSR makes them unfit for fuel cell applications that require high-purity H2.4−6 it is necessary to improve MSR technology to reduce heating and steam requirements and achieve cost-efficient H2 manufacture. In this sense, the capability to operate fuel cells at ambient pressure[7] and the development of hydrogen-selective membrane reactors[8−10] represent an opportunity to increase the thermodynamically limited conversion imposed by the endothermicity of the MSR reaction,[11] allowing for both lower operating temperatures (500−600 °C) and lower steamto-methane ratios while maintaining good H2 yield.
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