Tuning the degree of CO 2 activation by carbon doping Cu n - ( n = 3-10) clusters: an IR spectroscopic study.

Copper clusters on carbide surfaces have shown a high catalytic activity towards methanol formation. To understand the interaction between CO 2 and the catalytically active sites during this process and the role that carbon atoms could play in this, they are modeled by copper clusters, with carbon atoms incorporated. The formed clusters Cu n C m - ( n = 3-10, m = 1-2) are reacted with CO 2 and investigated by IR multiple-photon dissociation (IR-MPD) spectroscopy to probe the degree of CO 2 activation. IR spectra for the reaction products [Cu n C·CO 2 ] - , ( n = 6-10), and [Cu n C 2 ·CO 2 ] - , ( n = 3-8) are compared to reference spectra recorded for products formed when reacting the same cluster sizes with CO, and with density functional theory (DFT) calculated spectra. The results reveal a size- and carbon load-dependent activation and dissociation of CO 2 . The complexes [Cu n C·CO 2 ] - with n = 6 and 10 show predominantly molecular activation of CO 2 , while those with n = 7-9 show only dissociative adsorption. The addition of the second carbon to the cluster leads to the exclusive molecular activation of the CO 2 on all measured cluster sizes, except for Cu 5 C 2 - where CO 2 dissociates. Combining these findings with DFT calculations leads us to speculate that at lower carbon-to-metal ratios (CMRs), the C can act as an oxygen anchor facilitating the OCO bond rupture, whereas at higher CMRs the carbon atoms increasingly attract negative charge, reducing the Cu cluster's ability to donate electron density to CO 2 , and consequently its ability to activate CO 2 .