CO₂‐to‐Methanol Hydrogenation on Zirconia‐Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal–Support Interface

Abstract

Methanol synthesis via CO2 hydrogenation is a key step in methanol-based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO2 to methanol still remain unknown. Here, we present a mechanistic study of the hydrogenation of CO2 on Cu/ZrO2. Using kinetics, in situ IR and NMR spectroscopies and isotopic labeling strategies, we examined the surface intermediates during CO2 hydrogenation at different pressures. Combined with DFT calculations, we show that formate species is the reaction intermediate and that the zirconia/copper interface is a key for its conversion to methanol.The catalytic hydrogenation of carbon dioxide to methanol is a key process in the sustainable methanol-based economy. [1] While copper-based catalysts are highly active for this transformation, [2] their activity and selectivity strongly depend on the support and/or the promoters. Understanding the copper-support interaction -its effect on the activity and product selectivity -has been a very intensive field of research over the last decade. While the reaction mechanisms and the nature of the active sites on Cu/ZnO systems have been extensively investigated, [3] copper supported on zirconia and related materials also exhibits high activity and selectivity in CO2 hydrogenation to methanol (Eq. 1) by minimizing the formation of CO, a byproduct often resulting from the competitive reverse water-gas shift reaction (Eq. 2). [4] CO2 + 3H2 = CH3OH + H2O ∆rH° (500 K) = -62 kJ.mol -1 (1) CO2 + H2 = CO + H2O ∆rH° (500 K) = +40 kJ.mol -1 (2)Although the copper-zirconia interface was proposed to play a key role in the selective formation of methanol, [4c, 4e-g] the active site and the reaction mechanism, including the role of the interface on methanol selectivity, are still not understood. In fact, mechanistic investigations using Diffuse Reflectance IR Fourier Transform spectroscopy (DRIFTS) led to opposite conclusions: formate is an intermediate in methanol formation [4c, 4d] vs. CO2 is first reduced to CO that is in turn hydrogenated to methanol through a carboxyl intermediate. [4f] Herein, by using a combined experimental and computational approach on realistic models, we investigated the reaction mechanism of CO2 hydrogenation to methanol on a Cu/ZrO2 catalyst. Kinetic investigation, in situ and ex situ spectroscopies -FTIR and NMRtogether with isotopic labeling and computational modelling showed that methanol is a primary product formed by the hydrogenation of formate as a reaction intermediate. First, narrowly dispersed copper nanoparticles supported on monoclinic zirconia were prepared by a molecular approach. [5] Grafting of [Cu(O t Bu)]4 on the surface hydroxyl groups of the support ( Figure S1-S2, Scheme S1) followed by a treatment under H2 at 500 °C for 5 h [6] yields smal...

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

“…Similarly,molar CH 3 OH selectivities decreased from 85 %f or Cu/Ti@SiO 2 to 49 %f or Cu/SiO 2 .I n contrast, initial CO formation rates were nearly the same for Cu/Ti@SiO 2 andC u/SiO 2 (3.1 and 3.8 mmol s À1 g Cu À1 ,r espectively). Nearly identical decreases in CH 3 OH rates with increasing residence time were observed for Cu/ ZrO 2 [9] and Cu/Zr@SiO 2 , [13] suggesting that the same pathways mediate CO 2 hydrogenation on these materials. www.chemsuschem.org tion rates are nearly af actor of two higherf or Cu/Ti@SiO 2 than Cu/Zr@SiO 2 (18 and 10.8 mmol s À1 g Cu À1 ,respectively).…”

“…[1][2][3][4] Its practice, however,i sc omplicated by the parasitic reversew ater-gas-shift (RWGS) reaction, which forms CO instead. [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. [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 .…”

“…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. [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. [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.…”

A Critical Look at Direct Catalytic Hydrogenation of Carbon Dioxide to Olefins

“…[87,93,95,98] An excellent example is the study of CO 2 hydrogenation on aC u/ZrO 2 catalyst performed by Larmier et al [107] Using kinetic studies, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and NMR spectroscopy combinedw ith isotopic labeling strategies, as well as DFT,t hey showed that formate and methoxy species were formed during the reaction. [87,93,95,98] An excellent example is the study of CO 2 hydrogenation on aC u/ZrO 2 catalyst performed by Larmier et al [107] Using kinetic studies, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and NMR spectroscopy combinedw ith isotopic labeling strategies, as well as DFT,t hey showed that formate and methoxy species were formed during the reaction.…”

“…[107] The results showed that carbonate and bicarbonate species formed if CO 2 adsorbed on the surface. Experimental and computational studies were used to identify the predominant intermediates during CO 2 hydrogenation on the Cu/ZrO 2 catalyst.…”