Nature and Distribution of Cu and Pd Species in CuPd/TiO₂–Na Bimetallic Catalysts for Glycerol Hydrodeoxygenation

Abstract

We demonstrated recently that CuPd/TiO 2 –Na bimetallic catalysts synthesized by sequential wet impregnation are active, selective, and stable for the hydrodeoxygenation (HDO) of glycerol into propylene glycol at low H 2 pressure. The present study reports on the nature and distribution of Cu and Pd surface species in CuPd/TiO 2 –Na bimetallic catalysts using different scanning transmission electron microscopy techniques that supply cluster-specific alloying details. In particular, we used atomic-resolution Z -contrast imaging, X-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy. We also include X-ray photoelectron spectroscopy results. Our analysis shows that the metallic nanoparticles adopt mainly five different structures according to how the Cu and Pd atoms coordinate among themselves: a homogeneous CuPd alloy structure (45–61%), a Cu shell/CuPd core (15–23%), a smaller number of particles formed by Cu on the surface and Pd in the nucleus (10–17%), and there are also nanoparticles formed only by Pd (4–7%) or by Cu (8–13%). We determined that there is a inhomogeneous distribution of Cu and Pd in the bimetallic nanoparticles, with Cu being predominant on the surface (between 76 and 90% of the total area analyzed for each particle). Most bimetallic nanoparticles have sizes below 6 nm, the Pd monometallic nanoparticles are in the 2–4 nm range, whereas the monometallic Cu nanoparticles are larger than 8 nm. Bimetallic nanoparticles with sizes smaller than 6–7 nm are fundamentally made up of Cu 0 –Pd 0 and Cu 1+ –Pd 0 . The nanoparticles with sizes greater than 7 nm consist of Cu 2+ and Cu 2+ –Pd 2+ . Our obtained results also help describe reports about the activation of HDO by Pd–Cu in the absence of H 2 , an effect apparently not observed with other bimetallic systems.

Kinetic Study of Liquid-Phase Glycerol Hydrodeoxygenation into 1,2-Propanediol over CuPd/TiO2-Na

“…We also demonstrated that the high selectivity to 1,2-PDO of the bimetallic catalysts synthesized is due to the formation of CuPd alloy nanoparticles with sizes smaller than 6–7 nm with their surface enriched with Cu and basically made up of Cu 0 -Pd 0 and Cu 1+ -Pd 0 . 15 However, according to our experimental observations and literature data, obtaining 1,2-PDO from glycerol is not an elementary reaction with 1,2-PDO as the only product formed in the reaction. 9 , 14 − 20 Thus, depending on the reaction conditions, different intermediates and/or side products could affect the kinetics of the reaction and the selectivity to 1,2-PDO.…”

“…We also demonstrated that the high selectivity to 1,2-PDO of the bimetallic catalysts synthesized is due to the formation of CuPd alloy nanoparticles with sizes smaller than 6–7 nm with their surface enriched with Cu and basically made up of Cu 0 -Pd 0 and Cu 1+ -Pd 0 . However, according to our experimental observations and literature data, obtaining 1,2-PDO from glycerol is not an elementary reaction with 1,2-PDO as the only product formed in the reaction. ,− Thus, depending on the reaction conditions, different intermediates and/or side products could affect the kinetics of the reaction and the selectivity to 1,2-PDO. Nevertheless, to use our catalyst in a specific industrial application for the production of 1,2-PDO from glycerol, it is necessary to understand the reaction mechanism and to know some kinetic models; moreover, to the best of our knowledge, these aspects have not been discussed in the previous studies.…”

“…The loading of Pd and of Cu was set at 5 wt %. The details of the catalyst synthesis and activation procedure and the characterization results were published in the previous work. , …”

“…The details of the catalyst synthesis and activation procedure and the characterization results were published in the previous work. 14,15 Swelling Experiments. Swelling is a phenomenon commonly observed in different kinds of supported heterogeneous catalysts, especially when they are in contact with polar solvents.…”

Kinetic Study of Pd-Promoting Effect on Cu/ZnO/Al2O3 Catalyst for Glycerol Hydrogenolysis to Produce 1,2-Propanediol at Low Hydrogen Pressure

“…Xia et al reported that by adding 1.3 wt% Pd to a Cu/MgO/Al 2 O 3 catalyst, the glycerol conversion can be increased from 56.7% using an unpromoted catalyst to 79.6% at 2 MPa hydrogen pressure [21]. Ardila et al investigated the synergetic effect of a Cu-Pd catalyst (5 wt% each) supported on Na-treated TiO 2 ; the experimental results suggested that under a low hydrogen pressure (0.7 MPa) the glycerol conversion and 1,2-PD selectivity were all significantly higher when using the supported bimetallic catalyst than when using any supported monometallic (Pd or Cu) catalyst [22,23]. Nevertheless, most of the works focused on the improvement of glycerol conversion and 1,2-PD selectivity by addition of Pd; the kinetics of the promoting effect of Pd have not been clearly identified on the elementary reactions of the process, so that the promoting effect of Pd cannot be mechanistically understood.…”