Dry Reforming of Methane on NiCu and NiPd Model Systems: Optimization of Carbon Chemistry

Philipp Zambaldi,Leander Haug,Simon Penner,Bernhard Klötzer

doi:10.3390/catal12030311

Abstract

A series of ultra-clean, unsupported Cu-doped and Pd-doped Ni model catalysts was investigated to develop the fundamental concept of metal doping impact on the carbon tolerance and catalytic activity in the dry reforming of methane (DRM). Wet etching with concentrated HNO3 and a subsequent single sputter–anneal cycle resulted in the full removal of an already existing oxidic passivation layer and segregated and/or ambient-deposited surface and bulk impurities to yield ultra-clean Ni substrates. Carbon solubility, support effects, segregation processes, cyclic operation temperatures, and electronic and ensemble effects were all found to play a crucial role in the catalytic activity and stability of these systems, as verified by X-ray photoelectron spectroscopy (XPS) surface and bulk characterization. Minor Cu promotion showed the almost complete suppression of coking with a moderate reduction in catalytic activity, while high Cu loadings facilitated carbon growth alongside severe catalytic deactivation. The improved carbon resistance stems from an increased CH4 dissociation barrier, decreased carbon solubility in the bulk, good prevailing CO2 activation properties and enhanced CO desorption. Cyclic DRM operation on surfaces with Cu content that is too high leads to impaired carbon oxidation kinetics by CO2 and causes irreversible carbon deposition. Thus, an optimal and stable NiCu composition was found in the region of 70–90 atomic % Ni, which allows an appropriate high syngas production rate to be retained alongside a total coking suppression during DRM. In contrast, the more Cu-rich NiCu systems showed a limited stability under reaction conditions, leading to undesired surface and bulk segregation processes of Cu. The much higher carbon deposition rate and solubility of unsupported NiPd and Pd model catalysts results in severe carbon deposition and catalytic deactivation. To achieve enhanced carbon conversion and de-coking, an active metal oxide boundary is required, allowing for the increased clean-off of re-segregated carbon via the inverse Boudouard reaction. The carbon bulk diffusion on the investigated systems depends strongly on the composition and decreases in the following order: Pd > NiPd > Ni > NiCu > Cu.

Highlights

The steady increase in anthropogenic greenhouse gas (GHG) emissions and the growing energy demand has created a strong interest in CO2 capture and its utilization as a feedstock for valuable chemical and fuel production to mitigate climate change [1,2]
The most convenient method to prepare oxygenand carbon-free Ni foil substrates, which do not exhibit irreversible self-passivation during dry reforming of methane (DRM), is the initial removal of this already existing oxidic/passivation layer via wet etching with concentrated HNO3
The observed rapid onset indicates a fast modification process on the respective catalyst’s surfaces occurring at a specific temperature, which may depend on their bulk composition. We propose that this onset emerges most likely from the sudden breakdown of an inactive/passivating oxide layer that was most likely formed on the catalyst’s surface during exposure to the reaction mixture in the sub-onset temperature regime

Summary

Introduction

The steady increase in anthropogenic greenhouse gas (GHG) emissions and the growing energy demand has created a strong interest in CO2 capture and its utilization as a feedstock for valuable chemical and fuel production to mitigate climate change [1,2]. As part of the reforming technologies, the dry reforming of methane is the most promising and economical way to produce syngas (Equation (1)) [4,5], steam reforming (SMR), partial oxidation (PO) and autothermal reforming (ATR) of methane currently dominate syngas production (Equations (2)–(4), respectively) While these processes yield more hydrogen-rich syngas with a H2/CO ratio ≥ 2, DRM ideally approaches a hydrogen-to-carbon-monoxide ratio of unity [4]: CH4 + CO2 → 2 CO + 2 H2 ∆H0 = 247 kJ mol−1 (1). Extensive investigations revealed that all group VIII transition metal catalysts, except osmium, exhibited catalytic activity towards DRM [10,11] Supported noble metals, such as Rh, Ru and Pt, can provide a high catalytic performance and stability, but base metals are preferred in industrial applications on a large scale, considering their low cost and wider availability. Many Ni-based supported catalysts largely tend to deactivate during DRM due to severe coke formation and subsequent activity loss [12]

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