CO2 activation by copper oxide clusters: size, composition, and charge state dependence

The interaction of CO2 with copper oxide clusters of different size, composition, and charge is investigated via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. Laser ablation of a copper target in the presence of an O2/He mixture leads to the preferred formation of oxygen-rich copper oxide cluster cations, CuxOy+ (y > x; x ≤ 8), while the anionic cluster distribution is dominated by stoichiometric (x = y) and oxygen-deficient (y < x; x ≤ 8) species. Subsequent reaction of the clusters with CO2 in a flow tube reactor results in the preferred formation of near-stoichiometric CuxOy(CO2)+/− complexes. IR-MPD spectroscopy of the formed complexes reveals the non-activated binding of CO2 to all cations while CO2 is activated by all anions. The great resemblance of spectra for all sizes investigated demonstrates that CO2 activation is largely independent of cluster size and Cu/O ratio but mainly determined by the cluster charge state. Comparison of the IR-MPD spectra with DFT calculations of the model systems Cu2O4(CO2)− and Cu3O4(CO2)− shows that CO2 activation exclusively results in the formation of a CO3 unit. Subsequent CO2 dissociation to CO appears to be unfavorable due to the instability of CO on the copper oxide clusters indicating that potential hydrogenation reactions will most likely proceed via formate or bicarbonate intermediates.


Figure S2
: IR-MPD spectrum (in gray) of Cu2O4(CO2) -together with calculated vibrational spectra (in blue) of several isomeric structures (relative energies are given in brackets in eV).The experimental spectrum was measured in two independent runs (1800-660 cm -1 and 800-240 cm -1 , while the first one was obtained at reduced FELICE fluence).To obtain an intensity match of the peaks in the overlapping spectral region the intensity of the 800-240 cm -1 spectrum was multiplied by a factor 0.6.The intensities of the calculated spectrum were mutliplied by the wavelangth.The red dashed peak corresponds to the O-O stretching mode resulting from the scaling with a factor of 0.921.Isomers 2,4,1-e and 2,4,1-f are similar to isomer 2,4,1-a shown in the main text and also exhibit similar vibrational spectra.Isomers 2,4,1-g and 2,4,1-h provide an inferior match with the IR-MPD spectra, in particular for band I. Cu, C, and O atoms are depicted as yellow, black, and red spheres, respectively.CO2) -together with calculated vibrational spectra (in blue) of several isomeric structures (relative energies are given in brackets in eV).The experimental spectrum was measured in two independent runs (1800-660 cm -1 and 800-240 cm -1 , while the first one was obtained at reduced FELICE fluence).To obtain an intensity match of the peaks in the overlapping spectral region the intensity of the 800-240 cm -1 spectrum was multiplied by a factor 0.6).The intensities of the calculated spectrum were multiplied by the wavelength.Isomer 3,4,1-h is similar to isomers 3,4,1-a,b of the main text; isomer 3,4,1-j is similar to isomer 3,4,1-e; isomers 3,4,1-k-m correspond to the same isomer class as 3,4,1-e.Isomers 3,4,1-g,i do not contain an O2 unit.While the co-existence of some of these isomers in the molecular beam cannot be completely excluded, none of them provide a superior fit as those discussed in the main text.Cu, C, and O atoms are depicted as yellow, black, and red spheres, respectively.

Figure S3 :
Figure S3: DFT calculated harmonic characteristic frequencies of the CO3 unit in Cu2O4(CO2) -(blue) together with calculated vibrational spectra of an isolated carbonate ion CO3 -2 (black).Cu, C, and O atoms are depicted as yellow, black, and red spheres, respectively.

Figure S4 :
Figure S4: IR-MPD spectrum (in gray) of Cu3O4(CO2) -together with calculated vibrational spectra (in blue) of several isomeric structures (relative energies are given in brackets in eV).The experimental spectrum was measured in two independent runs (1800-660 cm -1 and 800-240 cm -1 , while the first one was obtained at reduced FELICE fluence).To obtain an intensity match of the peaks in the overlapping spectral region the intensity of the 800-240 cm -1 spectrum was multiplied by a factor 0.6).The intensities of the calculated spectrum were multiplied by the wavelength.Isomer 3,4,1-h is similar to isomers 3,4,1-a,b of the main text; isomer 3,4,1-j is similar to isomer 3,4,1-e; isomers 3,4,1-k-m correspond to the same isomer class as 3,4,1-e.Isomers 3,4,1-g,i do not contain an O2 unit.While the co-existence of some of these isomers in the molecular beam cannot be completely excluded, none of them provide a superior fit as those discussed in the main text.Cu, C, and O atoms are depicted as yellow, black, and red spheres, respectively.

Table 1 :
Calculated frequencies and (unscaled) intensities of the Cu2O4(CO2) -isomers shown in Figure3of the main text.

Table 2 :
Calculated frequencies and (unscaled) intensities of the Cu3O4(CO2) -isomers shown in Figure4of the main text.