Effect of Pt Promotion on the Ni-Catalyzed Deoxygenation of Tristearin to Fuel-Like Hydrocarbons

Abstract

Pt represents an effective promoter of supported Ni catalysts in the transformation of tristearin to green diesel via decarbonylation/decarboxylation (deCOx), conversion increasing from 2% over 20% Ni/Al2O3 to 100% over 20% Ni-0.5% Pt/Al2O3 at 260 °C. Catalyst characterization reveals that the superior activity of Ni-Pt relative to Ni-only catalysts is not a result of Ni particle size effects or surface area differences, but rather stems from several other phenomena, including the improved reducibility of NiO when Pt is present. Indeed, the addition of a small amount of Pt to the supported Ni catalyst dramatically increases the amount of reduced surface metal sites, which are believed to be the active sites for deCOx reactions. Further, Pt addition curbs the adsorption of CO on the catalyst surface, which decreases catalyst poisoning by any CO evolved via decarbonylation, making additional active sites available for deoxygenation reactions and/or preventing catalyst coking. Specifically, Pt addition weakens the Ni-CO bond, lowering the binding strength of CO on surface Ni sites. Finally, analysis of the spent catalysts recovered from deCOx experiments confirms that the beneficial effect of Pt on catalyst performance can be partially explained by decreased coking and fouling.

Promotional Effect of Cu, Fe and Pt on the Performance of Ni/Al2O3 in the Deoxygenation of Used Cooking Oil to Fuel-Like Hydrocarbons

“…profile for 20% Ni-5% Fe/Al2O3 also shows four (but less distinct) reduction events, namely: (1) a small signal with a maximum at 235 °C corresponding to large (10-50 nm) NiO ensembles (vide infra); (2,3) a very large and broad peak ranging from 260 to 675 °C with a maximum at 350 °C, commingling the reduction of nickel and iron oxides (leading to the formation of a Ni-Fe alloy) [17,19]; and (4) a high-temperature tail of the latter peak, assigned to NiAl2O4 reduction. Lastly, as discussed in a recent report [13] the TPR profile for 20% Ni-0.5% Pt/Al2O3 also displays several reduction events, including (1) a small and broad peak between 300 and 350 °C attributed both to the reduction of surface Pt and of large NiO particles in close proximity to Pt [13,33]; (2) an intense and well-defined signal with a maximum at 460 °C assigned to the Pt-assisted reduction of smaller NiO particles [13]; and (3) a broad peak above 500 °C with a high temperature (>700 °C) shoulder attributed to the reduction of NiO and NiAl2O4, respectively. Given that Ni-based formulations used in the deoxygenation of FOG to fuel-like hydrocarbons are known to be particularly susceptible to coking [1], the spent catalysts were subjected to thermogravimetric analysis (TGA) in air, the resulting profiles being shown in Figure 5.…”

“…This suggests that the most coordinatively unsaturated Ni sites-which are the most active cracking sites [44]-are irreversibly deactivated during reaction/regeneration. Similarly, while the fresh Ni-Pt catalyst shows a well-defined peak at~2180 cm −1 and a broad feature at~2120 cm −1 associated with CO on metallic Ni sites, as well as a large and well-defined peak at~2077 cm −1 assigned to CO on Pt sites [13], none of these signals are observed post-regeneration. Since XPS results demonstrate the presence of Ni 0 on the regenerated Ni-Pt formulation, the dearth of signals can be explained by residual coke blocking the sites responsible for low-temperature CO adsorption.…”

“…A detailed description of the instrumentation and procedures employed for catalyst characterization-by means of N 2 physisorption, XRD, TGA, TEM-EDS, TPR, and DRIFTS-can be found in previous contributions [13,15,45]. Briefly, XRD measurements were performed on a Phillips X'Pert diffractometer using Cu Kα radiation (λ = 1.5406 Å) and a step size of 0.02 • .…”

“…Since the high activity of Ni in C-C hydrogenolysis can decrease the carbon yield and the hydrogen efficiency of deCO x processes, the incorporation of a second metal has been investigated as a means to modify the electronic and geometric properties of Ni to ultimately improve its activity and selectivity [17]. Indeed, Ni/Al 2 O 3 promoted with Cu or Pt can afford near quantitative diesel yields in the conversion of both model and realistic lipid feeds to fuel-like hydrocarbons [13,18]. The promotion effect displayed by these bimetallic catalysts is in large part attributed to the ability of Cu and Pt to facilitate NiO reduction at relatively low temperatures since metallic Ni sites constitute the active site for the deCO x reaction.…”

“…The promotion effect displayed by these bimetallic catalysts is in large part attributed to the ability of Cu and Pt to facilitate NiO reduction at relatively low temperatures since metallic Ni sites constitute the active site for the deCO x reaction. Moreover, Pt addition also curbs the adsorption of CO on the catalyst surface, helping to avoid catalyst inhibition by any CO evolved via decarbonylation and the catalyst coking resulting from the disproportionation of CO via the Boudouard reaction [13].…”