CO₂ Hydrogenation to CH₃OH on Supported Cu Nanoparticles: Nature and Role of Ti in Bulk Oxides vs Isolated Surface Sites

Abstract

The selective hydrogenation of CO 2 to CH 3 OH is a crucial part of efforts to mitigate climate change via the methanol economy. Understanding the nature and role of active sites is essential for designing highly active and selective catalysts. Here, we examine Cu nanoparticles dispersed on TiO 2 and Ti-containing SiO 2 supports, where the Ti moieties of these materials are reducible to different extents, using a surface organometallic chemistry approach, together with state-of-the-art characterization methods (UV− vis, infrared (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies and in situ transmission electron microscopy (TEM)). Cu nanoparticles are small and well-dispersed on isolated, dimeric, and oligomeric Ti moieities on SiO 2 when reduced or when the material has been oxidized via exposure to air, but they are small only in the latter case for Cu dispersed on the bulk oxide TiO 2 . Large Cu nanoparticles, present on TiO 2 when reduced, redisperse upon exposure to air, likely associated with the facile oxidation of the reduced TiO 2 surface, and agglomerate again when reduced in situ within the electron microscope. CH 3 OH formation rates and selectivities on Cu/TiO 2 are low as a result of these large Cu nanoparticles and the ability of TiO 2 to catalyze the hydrogenation of CO 2 to CO. After accounting for the CO formation rates of the support itself, the CH 3 OH selectivities are similar for all catalysts (and greater than that for Cu/SiO 2 , where SiO 2 is considered as an innocent support), suggesting that Ti sites in all materials have a similar nature and role. These Ti sites act as Lewis acid sites, whose presence is evidenced by pyridine adsorption studies using IR and NMR spectroscopies, that stabilize the same surface intermediates at the interface of Cu nanoparticles and the support, despite differences in the reducibility of these Ti species.

Heterogeneous alkane dehydrogenation catalysts investigated via a surface organometallic chemistry approach

“…This approach has been exploited for metals which possess greater 'noble' character (Ir, Rh, Pt, Cu, Ag, and Au), though the lower M-O bond strength reduces the thermodynamic impetus for grafting, and renders the range of relevant precursors more limited. [25][26][27][28][29] These two approaches can also be combined, exploiting the stability of doped oxide materials generated by SOMC (Scheme 2a), and further functionalizing these materials through the grafting of a second molecular precursor, through remaining/regenerated surface hydroxyls. 14 Upon treatment under H 2 , one of two outcomes occurs: (i) the reduction of the second grafted species to form monometallic nanoparticles, of uniform size, atop silica containing isolated metal ions (Scheme 2c); or (ii) the reduction of the second grafted species to form metallic nanoparticles, of uniform size, atop oxide supports containing isolated metal ions, with gradual reduction and intercalation of the second metal to the nanoparticle, forming an alloy (Scheme 2c).…”

Enhanced CH3OH selectivity in CO2 hydrogenation using Cu-based catalysts generated via SOMC from GaIII single-sites

“…34 The outstanding activity and CH 3 OH selectivity of copper supported on silica containing Ti IV isolated sites is particularly noteworthy, considering that Cu/TiO 2 performs very poorly in CH 3 OH synthesis by favoring CO formation. [36][37][38][39] This difference of catalyst performance has been ascribed to the site isolation of Ti IV and the use of a non-reducible support, SiO 2 , thus allowing Ti IV to play exclusively the role of a Lewis acid, that stabilizes reaction intermediates at the interface with Cu particles. 35 Using a similar approach, i.e., the treatment under H 2 of a graed platinum(II) molecular precursor on isolated Ga III sites generates small and narrowly distributed PtGa x nanoparticles stabilized by remaining Ga III sites that show high activity, selectivity and stability for propane dehydrogenation.…”

Lewis Acid Strength of Interfacial Metal Sites Drives CH3OH Selectivity and Formation Rates on Cu‐Based CO2 Hydrogenation Catalysts

“…[16] In CO 2 -to-CH 3 OH hydrogenation reactions,i nterfacial sites-Zr IV or Ti IV ,d ispersed on SiO 2 or present in ZrO 2 and TiO 2 -at the periphery of Cu nanoparticles act as Lewis acid sites to stabilize surface formate and methoxide intermediates. [17][18][19][20][21] These sites thereby lead to increased CH 3 OH formation rates and selectivities compared to Cu/SiO 2 (where the "Cu/X" nomenclature denotes Cu nanoparticles supported on X support). However,ac lear relationship between Lewis acid strength of the interfacial sites and CH 3 OH formation rates has not yet been established for these bifunctional systems.…”