Muons Provide Insights into the Mechanisms of Catalysis in Copper-Loaded Zeolites

Understanding the active site dynamics and redox behavior of copper species in zeolite catalysts is critical for advancing the understanding of catalytic methane-to-methanol conversion. These catalysts are also used for the selective catalytic reduction of NOx in diesel engines. Here, we present the first application of muon spin spectroscopy (μSR) to study transition metal-exchanged SSZ-13 (Cu-SSZ-13) zeolites and highlight the potential of μSR. This technique reveals unique insights into the local magnetic and electronic environments of Cu species, inaccessible via conventional spectroscopies. Temperature-dependent transverse field μSR measurements show a clear conversion of paramagnetic muonium (Mu0) to diamagnetic muon (Mu+) states, with distinct differences between Cu-loaded and pure SSZ-13 systems. This transformation is thermally activated, with Arrhenius analysis yielding activation energies of ∼3.3–5 meV, consistent with ionization processes of shallow donor states. Longitudinal field measurements confirm 2D muonium diffusion within Cu-SSZ-13 and support a model where muonium reacts with mono(μ-oxo)dicopper species, inducing comproportionation (2Cu2+ → 2Cu1.5+). DFT simulations validate this mechanism, reproducing the experimentally determined hyperfine coupling constants. At low temperatures (≤25 K), μSR also detects the onset of static magnetism in Cu clusters, consistent with Cu(II)-based multinuclear motifs. These results establish μSR as a powerful, underutilized probe for catalytic systems and provide compelling evidence for a multistep oxidation mechanism involving the initial reduction of Cu centers prior to methanol formation. This approach opens new avenues for real-time, local investigation of redox-active catalytic materials.