Isolated Platinum Atoms in Ni/γ-Al₂O₃ for Selective Hydrogenation of CO₂ toward CH₄

Abstract

We introduce a catalyst composed of isolated Pt atoms surrounded by Ni atoms in a Ni−Pt alloy (iso-Pt), which was prepared on aluminum oxide (Al 2 O 3 ) obtained by a simple impregnation method with typical hydrogen reduction pretreatment. This catalyst exhibited a relatively high CH 4 -formation rate while maintaining excellent selectivity for CH 4 evolution, when compared to bulk Ni atoms (bulk-Ni), although bulk Pt atoms (bulk-Pt) produced CO rather than CH 4 . Kinetic studies revealed that one of the two roles of the iso-Pt species in the Ni−Pt alloy is to provide H 2 dissociation sites at low temperature, resulting in high CO 2 methanation activity. Fourier transform infrared spectroscopy clarified the second role of the iso-Pt species; tuning of the d-electronic state by alloying to surrounding Ni atoms leads to CO molecules that are strongly anchored to the iso-Pt atoms, which weakens the C−O bond of the CO species attached to the Pt atoms and improves the abilities of the surrounding Ni atoms to promote further methanation with hydrogen.

Bimetallic Ni-Based Catalysts for CO2 Methanation: A Review

“…Therefore, the RDS for CO 2 methanation over NiPt alloys was suggested to be H 2 dissociation rather than H-assisted CO dissociation or hydrogenation of surface carbon species, which are commonly indicated as the RDS over monometallic Ni methanation catalysts [7,122]. The experimental results and reaction mechanism for these two works are summarized in Figure 14 [31,122].…”

“…Combined Ni and Ru catalysts exist mostly in the form of heterostructures, rather than alloys, and Ru can provide additional methanation sites, as well as spillover hydrogen into nearby Ni sites [94]. On the other hand, Pt and Pd mostly modify the electronic properties of Ni via alloy formation [31,32].…”

“…When Pt was finely dispersed, CH4 production was high, whereas higher Pt loadings could lead to increased CO production that blocked the active Ni methanation sites. Kikkawa et al [31] elucidated the promoting role of isolated Pt atoms that are finely dispersed in Ni-rich NiPt alloy nanoparticles supported on Al2O3 for the CO2 methanation reaction. Through Ni and Pt precursor salt impregnation and reduction, Pt atoms could dissolve into the lattice of Ni nanoparticles and were slightly negatively charged as Pt δ-, surrounded by Ni δ+ atoms.…”

“…In short, despite monometallic Pt catalysts being very active for the RWGS reaction, Ni-rich NiPt alloys appear to promote CH 4 formation [31,33]. The dilution of Pt atoms over Ni metallic nanoparticles changes the CO 2 methanation pathway, since Pt sites accommodate adsorbed CO intermediates due to their high affinity with carbonyl [31].…”

“…Noble metals Ru, Rh, Pt, Pd and Re can increase the reaction activity by enhancing the reducibility of the primary Ni phase, by increasing the Ni dispersion, or by changing the reaction pathway [29]. Ru and Ni mostly form monometallic heterostructures that rely on the synergistic effect between the two separate metallic phases, while Pt and Pd mostly lead to the creation of NiPt and NiPd alloys [30][31][32]. It has been shown that an addition of only a miniscule amount of noble metal (e.g., 0.5% or 1%) can greatly enhance the reducibility and low-temperature activity of Ni-based catalysts without the need to use high loadings of precious metals [33].…”