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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.