Microkinetic Insights into the Competition between Oxidative Cracking and Dehydrogenation in the Partial Oxidation of n -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 C 2 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/SiO 2 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, C 2 −C 4 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 H 2 O, with C 2 intermediate chemistry prevailing at low temperatures and OH/H 2 O reactions dominating at higher temperatures. Steam and dry reforming contribute to H 2 /CO formation and shape H 2 O/CO 2 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 C 2 intermediates that ultimately yield H 2 and CO. Pathway analyses for minor species, including benzene and toluene, support the proposed kinetic model and confirm their role as coke precursors.