Oxidation behavior of Ni/Al₂O₃ catalyst in nonthermal plasma-enabled catalysis

Abstract

The oxidation behavior of Ni/Al 2 O 3 catalyst in CO 2 plasma was investigated compared with CO 2 thermal oxidation, aiming for providing new insight into oxidationreduction cycle of plasma catalysis of dry methane reforming (DMR). Temperature programmed reaction spectrometry was applied to enable a quantitative analysis of gas consumption which is related to the oxidation and reduction of Ni/Al 2 O 3 catalysts. The nonthermal plasma oxidation mechanism was further analyzed by Raman spectroscopy, showing the crosssectional distribution of NiO over the 3 mm spherical catalyst pellets. Nonthermal plasmaexcited CO 2 oxidizes Ni only near the external surface of spherical pellets with 20 μm depth: both dielectric barrier discharge generation and plasmaexcited species diffusion into the internal pores are inhibited. The partially oxidized Ni catalyst in the external surface would promote plasmaenabled oxidationreduction cycle, which was further correlated with coke formation behavior in plasma catalysis of DMR.

Review of Plasma-Assisted Catalysis for Selective Generation of Oxygenates from CO2 and CH4

“…45,46 As a result, during plasma-assisted CO 2 + CH 4 conversion over Ni/Al 2 O 3 spheres, the active oxygen can diffuse into the relatively small pores (pore size < 1 μm < the Debye length), resulting in the formation of NiO even at 20 μm beneath the Al 2 O 3 surface. 99 In the case of micropores (<2 nm), which are characteristic of conventional zeolites, Knudsen diffusion significantly reduces the λ acc of plasma-induced species. 45,46 Inside these micropores, few plasma-derived radicals are sufficiently longlived to travel a large distance.…”

“…It is worth noting that pure metals can be oxidized rapidly in CO 2 plasma. , CH x adsorbed on supported metal nanoparticles may also spill over onto the metal oxide supports during reactions . Therefore, the involvement of metal oxides in CH 3 O formation should be considered.…”

Quantifying the impact of vibrational nonequilibrium in plasma catalysis: Insights from a molecular dynamics model of dissociative chemisorption

“…In addition, beam studies have suggested that dissociative chemisorption of methane is promoted more efficiently by vibrational energy than translational energy [18][19][20]. As a result, reactions of vibrationally excited molecules have been postulated to be important in a wide range of plasma-catalytic processes, including CO 2 splitting, CH4 reforming, and N 2 activation [13,[21][22][23][24][25][26]. However, no detailed mechanistic evidence is available that conclusively supports these claims.…”

Ensemble-Based Molecular Simulation of Chemical Reactions under Vibrational Nonequilibrium

“…In particular the ability of certain plasmas to carry out thermodynamically highly unfavorable processes such as CO 2 dissociation with very high efficiency can be traced back to their strong nonequilibrium properties . In addition, when interfaced with a solid catalyst, a wealth of additional plasma-induced chemistry is obtained; here, too, stimulation of reactions by vibrational excitation has been postulated to be crucial for processes such as plasma-enabled catalytic ammonia synthesis, methane reforming, , and CO 2 reduction . However, the precise mechanisms of this type of vibrationally stimulated processes and their chemical ramifications are poorly understood.…”

“…11 In addition, when interfaced with a solid catalyst, 12 a wealth of additional plasma-induced chemistry is obtained; here, too, stimulation of reactions by vibrational excitation has been postulated to be crucial for processes such as plasma-enabled catalytic ammonia synthesis, 13 methane reforming, 14,15 and CO 2 reduction. 16 However, the precise mechanisms of this type of vibrationally stimulated processes and their chemical ramifications are poorly understood. Detailed atomic-level simulation approaches are therefore desirable.…”