Selective Hydrogenation of CO₂ to CH₃OH on Supported Cu Nanoparticles Promoted by Isolated Ti^IV Surface Sites on SiO₂

Abstract

Small and narrowly distributed Cu nanoparticles, supported on SiO 2 decorated with isolated Ti IV sites, prepared through surface organometallic chemistry,s howeds ignificantly improved CO 2 hydrogenationa ctivity and CH 3 OH selectivity compared to the corresponding Cu nanoparticles supportedo nS iO 2 .T hese isolated Lewis acid Ti IV sites, evidenced by UV/Vis spectroscopy, are proposed to stabilize surface intermediates at the interface between Cu nanoparticlesa nd the support.The selective hydrogenation of CO 2 to CH 3 OH, together with the production of H 2 from renewable energy sources (e.g.,i ntermittent excesse nergy generatedf rom wind or solarp ower) is essential to av irtuous sustainable closed-carbon fuel cycle. [1][2][3][4] Its practice, however,i sc omplicated by the parasitic reversew ater-gas-shift (RWGS) reaction, which forms CO instead. The most prominent catalysts for selective CO 2 hydrogenation to CH 3 OH are Cu-based, although their activity and CH 3 OH selectivityd ramatically depend on the oxide support. [5][6][7][8] Cu supportedo nZ rO 2 (Cu/ZrO 2 ,i nw hich "Cu/X"d enotes Cu nanoparticles dispersed on support X) has shown promising activity and high CH 3 OH selectivity compared to Cu/ SiO 2 . [9][10][11][12] Whereas SiO 2 can be considered as an inert support for the Cu nanoparticles that catalyzet he reaction, ZrO 2 provides Zr IV sites interfacing Cu nanoparticles that act as Lewis acid sites andp romote CH 3 OH synthesis. [9] In fact, Cu nanoparticles supported on SiO 2 decorated with isolated Zr IV sites show CO 2 hydrogenationa ctivity andC H 3 OH selectivity nearly identicalt ot hose for Cu/ZrO 2 , [13] underscoring the importance of these Lewis acid sites at the periphery of Cu nanoparticles for the selective hydrogenation of CO 2 to CH 3 OH.In contrast, Cu/TiO 2 has been observed to be av ery poor CO 2 hydrogenation catalystw ith low reaction rates and low CH 3 OH selectivity, [7,8,14] in spiteo ft he ability of Ti IV metal cen- [a] Dr.

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

“…This approach has allowed the generation of highly active and selective CO 2 hydrogenation catalysts by supporting Cu nanoparticles on SiO 2 containing isolated Zr IV and Ti IV sites. 34,35 These catalysts show high activity and CH 3 OH selectivity, but also suffer from the decrease of CH 3 OH selectivity at high conversion as observed for other CO 2 hydrogenation catalysts. 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.…”

“…These nding contrast with what was observed for Cu-Ti/SiO 2 (ref. 35) and Cu-Zr/SiO 2 (ref. 34) prepared in a similar fashion from Ti IV and Zr IV single-sites that remained isolated upon Cu nanoparticle 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. 40 Here, we reasoned that the SOMC/TMP approach would be a useful tool to investigate the behavior of gallium promoters in CO 2 hydrogenation catalysts (Scheme 1c).…”

CO2 Hydrogenation on Cu/Al2O3: Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst

“…In fact, introducing surface Zr IV or Ti IV sites on SiO 2 at the interface with Cu nanoparticles via surface organometallic chemistry (SOMC) increases the CH 3 OH activity and selectivity,f urther supporting the importance of Lewis acid sites in this reaction. [13,14] Regarding CO,i th as mainly been proposed to be formed on copper-based catalysts via two different mechanisms:T he so-called redox and formate pathways.T he former consists of the direct conversion of CO 2 to CO (that may involve M À COOH intermediates) and the subsequent reduction of Cu 2 Osurface species to Cu 0 metal forming H 2 O, while the latter involves the decomposition of formate into CO. [9,[15][16][17][18][19][20][21][22][23][24][25][26][27] Additionally,the formation of not only CO but also CH 3 OH is often proposed to be related to the presence of oxygen defect sites originating from the oxide support. [28][29][30][31][32][33][34][35][36] ForC u/ZnO/Al 2 O 3 ,t he origin of the promotional effect towards selective CH 3 OH formation is still amatter of debate.…”

CO2 Hydrogenation on Cu/Al2O3: Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst

“…For comparison, Cu/SiO 2 that does not contain Lewis acidic sites shows significantly lower activity and selectivity favoring CO formation. In fact, introducing surface Zr IV or Ti IV sites on SiO 2 at the interface with Cu nanoparticles via surface organometallic chemistry (SOMC) increases the CH 3 OH activity and selectivity, further supporting the importance of Lewis acid sites in this reaction . Regarding CO, it has mainly been proposed to be formed on copper‐based catalysts via two different mechanisms: The so‐called redox and formate pathways.…”