(424g) A New Approach to Selective CO Removal from Hydrogen: Chemical Looping Preferential Oxidation and In-situ Immobilization

Hydrogen, a clean energy carrier with abundant sources and high energy intensity, is regarded as an indispensable part of the transition to a sustainable energy system. It is also a critical feedstock for key chemical processes, such as ammonia synthesis. However, H­₂ produced via reforming of hydrocarbons typically contains trace amounts of CO, which is highly detrimental to downstream applications. For proton exchange membrane fuel cells (PEMFCs), CO concentrations above 100ppm can poison the platinum-based anode, leading to rapid performance degradation. In ammonia synthesis via the Haber-Bosch process, CO acts as a catalyst poison for iron-based catalysts, reducing activity and selectivity. Therefore, the selective removal of trace amounts of CO from the hydrogen-rich gas is essential. Among available methods, preferential oxidation of CO (PROX) in the presence of excess H₂ offers a promising route due to its operational simplicity and potential for integration into compact reactor systems. However, achieving high CO conversion with minimal H₂ loss remains a major challenge. Besides, PROX faces a series of process challenges, including the requirement for energy-intensive air separation units, the potential risk of explosion, and catalyst deactivation, which hinder its practical application.

A novel chemical looping preferential oxidation (CL-PROX) process is proposed and demonstrated for eliminating trace CO from a H₂-rich stream. CL-PROX is intrinsically safe and significantly alleviates the deactivation problem due to the superior reduction/reoxidation pattern. The proposed process exhibited an excellent CO removal performance (CO concentration < 100 ppm) and a high H₂ recovery (>96 %) with a ceria-supported γ-Fe₂O₃ oxygen carrier. The structure and evolution of iron species in the oxygen carrier were investigated through experimental characterizations during activation and redox cycles. TPR characterizations illustrated a significant difference in the reducibility of the active species, γ-Fe₂O₃, under H₂ and CO atmospheres, which could be the evidence for its relatively high selectivity to CO oxidation. In-situ FTIR characterization also pointed out that the desorption of reaction intermediates to CO₂ is facilitated via the reduction of the surface γ-Fe₂O₃, which might contribute to the outstanding CO oxidation performance of the oxygen carrier.

The process is further intensified by dual function materials (DFM), enabling both catalytic CO preferential oxidation and in-situ immobilization. This enhancement leads to a near-complete CO conversion with a CO concentration below 20 ppm while achieving a hydrogen recovery of 97 % in the purification step. Characterizations indicate that CO can be converted into surface formate and carbonate on the DFM, which can readily decompose to CO₂ during regeneration step.

In summary, this work introduces an intensified chemical looping strategy for hydrogen purification, addressing the stringent CO purity requirements for critical downstream applications such as fuel cells and ammonia synthesis. The integration of catalytic CO oxidation with in-situ immobilization through multifunctional redox materials not only enhances CO removal efficiency but also provides valuable insights into the rational design of advanced oxygen carriers and dual-function materials for clean energy processes.