Microkinetic Insights into the Competition between Oxidative Cracking and Dehydrogenation in the Partial Oxidation of <i>n</i> -Butane over Ni/SiO ₂ Catalysts

The catalytic partial oxidation (CPO) of n-butane over Ni catalysts offers a promising route for industrial hydrogen and syngas production, yet its performance is governed by the kinetic competition between oxidative C−C bond scission of intermediate alkenes that facilitates syngas formation via C2 fragments and deep dehydrogenation leading to aromatic coke precursors. Balancing these pathways is critical for maximizing hydrogen yield while mitigating catalyst deactivation. In this work, n-butane CPO was investigated in a packed-bed reactor over Ni/SiO2 at 0.1 atm, 573−923 K, and a C/O ratio of 1.0. Gas-phase intermediates were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry coupled with molecular beam sampling. Approximately 20 intermediates were detected, including major oxidation products, C2−C4 alkenes/alkynes, oxygenates (propanal, acrolein, formaldehyde, formic acid, and acetic acid), radicals (allyl and methoxy), and aromatic coking precursors (benzene and toluene). A thermodynamically consistent surface mechanism was developed and integrated with a validated gas-phase mechanism to reproduce the measured species profiles. Microkinetic simulations show that surface reactions dominate CPO, with surface-generated intermediates initiating subsequent homogeneous oxidation. Sensitivity and degree-of-rate-control analyses highlight C−H bond activation and H-abstraction by O(s) as rate-determining steps, while surface coverage evolution underscores the central role of O(s) in oxidation pathways. Hydrogen formation arises mainly from recombination of surface H atoms from hydrocarbon dehydrogenation and H-abstraction from H2O, with C2 intermediate chemistry prevailing at low temperatures and OH/H2O reactions dominating at higher temperatures. Steam and dry reforming contribute to H2/CO formation and shape H2O/CO2 selectivity. Although 1-butene and 2-butene are expected dehydrogenation products from 1-butyl and 2-butyl, only trace amounts were detected, as microkinetic analysis reveals their preferential surface C−C scission to C2 intermediates that ultimately yield H2 and CO. Pathway analyses for minor species, including benzene and toluene, support the proposed kinetic model and confirm their role as coke precursors.