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Earth abundant copper oxide (II) is a promising oxygen carrier for CLC and CLOU processes due to its high oxygen content and reaction rate. Nevertheless, CuO has not receive much attention from researchers due to the problem of particle agglomeration at relatively high temperatures used by CLC. In this work, we investigated the impact of several parameters related to CLC, such as CuO particle sizes, CH₄ flow rate, CH₄ concentration and competition between CLC and CLOU as a function of reaction temperature, on the performance of unsupported CuO as an oxygen carrier. The reduction of two commercial unsupported CuO samples with 50 nm and 1 µm particle sizes was investigated in a temperature range of 600 to 800 ^oC using dilute (5%) and pure (99.999%) CH₄ and a flow rate range from 12.5 to 250 h^-1. The reactivity of Cu₂O was also considered as a common intermediate of CuO reduction to Cu metal. To track the phase transformation of CuO during methane oxidation, bulk and surface analysis was performed on several samples with different reaction times at 700 ^oC. We have found that CuO reduction is a multistep process that is initiated by the formation of reduced phase nuclei and then proceeds by a shrinking core mechanism. Increasing the reaction temperature and CH₄ concentration increase the CuO reduction rate. When the reaction temperature exceed that of CuO decomposition, specifically at 800 ^oC, competition between CLC and CLOU resulted in sustained 100% conversion of CH₄ to CO₂ and H₂O, allowing utilization of 50% of the CuO oxygen content with full capturing of CO₂. Collectively, these new fundamental insights into CuO reduction by methane will help to increase its feasibility for CLC or CLOU processes.