AmericanChemicalSociety.com

Dwindling fossil fuel supplies and global warming from rising carbon emissions are the potential cause of economic and environmental crises of global proportion.  During the past century, atmospheric CO₂ levels have increased significantly, and global temperatures are predicted to subsequently rise ~2-4 °C by the end of this century.  A viable solution to these quandaries is the utilization of CO₂ as a primary C₁ source, which could help stave off potential economic and environmental catastrophes by converting CO₂ into a useable commodity such as methanol.  The formation of methanol from CO₂ and syngas proceeds through the equilibria in equations 1-3 and is exothermic.  Optimum reaction conditions typically require low temperatures and increased pressure.

Funded by a two year American Chemical Society Petroleum Research Fund (Type UNI) Grant (May 2009 – May 2011), the synthesis, characterization, and evaluation of Cu-based bimetallic nanoparticle catalysts for carbon dioxide hydrogenation (from CO₂ and H₂) is being investigated.  The catalysts are prepared via the dendrimer templating method (Scheme 1) which allows us to exercise careful control over the particle size, composition, and morphology of the catalysts.  The dendrimer-encapsulated nanoparticles are deposited onto the appropriate support before removing the dendrimer template thermally to produce active catalysts.  Typically the preparation of bimetallic particles of uniform size, composition, and morphology is exceedingly difficult and our approach allows us to overcome the major limitations that are present in the development of current systems; such as metal segregation, agglomeration, and wide particle size distributions. The dendrimer templating method allows us such control and we are currently using it to prepare Cu-M bimetallic nanoparticles (M = Au, Pd, Pt, and Ni initially) deposited onto different oxide supports (ZnO, SiO₂, and Al₂O₃).  We have chosen Au, due to its well behaved nature which allows us to optimize our synthetic techniques.  Pd, Pt, and Ni were chosen due to their propensity for hydrogenation activity.

To date we have successfully prepared a series of Cu/ZnO catalysts ranging from 10 - 50 wt% Cu based on the co-precipitation method.  We have also prepared a series of Cu-based mono- and bimetallic nanoparticle catalysts which include Cu₂₅/ZnO, Ni₂₅Cu₂₅/ZnO, Cu₂₅/SiO₂, and Cu₂₅Au₂₅/SiO₂ (all 2.5 wt % metal) via the dendrimer templating method.  (The  n in Cu_n represents the number of atoms in the nanoparticles after synthesis and before thermal activation to remove the dendrimer.)  Included in the preparation of these catalysts are the proper dendrimer removal conditions (calcination in an O₂/N₂ mixture, followed by H₂/N₂).  A representative XRD pattern of the catalysts are presented in Figure 1a.  The XRD pattern is indicative of particle sizes that lie below the size limit accessible by XRD (< 5 nm).  All of our catalysts synthesized via the dendrimer-templating method to date yield no information in their respective XRD patterns due to small particle size.  Fig 1b confirms that in the case of Cu₂₅/SiO₂ we are indeed producing particles on the order of 2 nm (particle size distribution of 2.44 +/-  nm).  Figure 1c shows a TEM micrograph of CuAu/SiO₂.

                We have recently finished the construction of a high pressure single pass plug flow methanol synthesis microreactor.  This allows us to vary the pressure of the H₂/CO₂ 3:1 feed from atmospheric pressure up to 20 MPa.  We are currently calibrating the reaction and obtaining the optimal temperatures and pressures with which to perform our activity measurements on the synthesized catalysts.  Our initial data (Figure 2) shows that the Cu/ZnO catalysts synthesized from the dendrimer-templating method are more active for the production of methanol from CO₂/H₂ than a control Cu/ZnO catalyst synthesized by the co-precipitation method.