Low‐Temperature CO Oxidation over Cu‐Based Metal–Organic Frameworks Monitored by using FTIR Spectroscopy

Low-temperature CO oxidation is among the most interesting reactions in heterogeneous catalysis. It is well known that this reaction can be catalyzed by gold deposited as nanoparticles on metal oxide surfaces (e.g. TiO2, MgO, and ZnO). [1] Developing highly efficient and low-cost catalysts for low-temperature CO oxidation represents a major challenge. Most recently, Cubased metal–organic frameworks (MOFs) have been identified for efficient CO oxidation at ambient pressure and elevated temperatures, namely [Cu3btc2] (btc = benzene-1,3,5-tricarboxylate ) and [Cu5(OH)2(nip)4(H2O)6] (H2O)4.25 (nip = 5-nitroisophthalate) with 100 % conversion at 240 8C and 200 8C, respectively. 3] However, the origin of the catalytic activity of such CuMOFs is not well understood. Coordinatively unsaturated metal ion sites (CUS) at the backbone of MOFs are known to play an important role for catalysis, gas storage, chemical sensing, and other applications. For catalysis, Schlichte et al. reported on the cyanosilylation of benzaldehyde and acetone with [Cu3btc2] as catalyst (HKUST-1, Figure S1) for which its activity was assigned to the Lewis acid properties of intrinsic Cu CUS at the copper-carboxylate paddle-wheel building unit (CuPW) of this prototypical MOF in its dehydrated form. The characterization of reactive CUS of MOFs with infrared spectroscopic techniques and suitable probe molecules, quite similar to classic heterogeneous catalysts, is expected to provide most valuable information for mechanistic understanding and fine tuning of the MOF materials. Herein, we present our studies on the in situ monitoring of the co-adsorption of CO and O2 at [Cu3btc2] (HKUST-1) [9] and its congener [Cu3btb2] (MOF-14, [11] btb = benzene-1,3,5-tribenzoate) by using an ultrahigh vacuum infrared spectroscopy tool (UHV-FTIRS). The high-quality IR data provide unambiguous spectroscopic evidence for the surprisingly high catalytic activity of both Cu-MOFs samples for CO oxidation at temperatures as low as 105 K, according to Scheme 1. Presented in Figure 1 are the UHV-FTIR spectra obtained after exposing activated [Cu3btc2] to CO (1 10 4 mbar) in the temperature range of 105 K to 135 K. Prior to CO adsorption, the thermal stability of the sample was confirmed up to 650 K (Figure S2). The H2O-related bands at ñ= 3647 (O H stretching mode) and 1617 cm 1 (d(H2O) scissoring mode) disappeared completely by heating the sample above 400 K, indicating the formation of Cu + CUS. The adsorption of CO at 105 K leads to the appearance of four IR bands at ñ= 2198, 2175, 2152, and 2128 cm . The dominant CO band at ñ= 2175 cm 1 exhibits a blue shift of 32 cm 1 with respect to the gas-phase value and is assigned to CO adsorbed at the intrinsic Cu + CUS of the CuPW in the axial position. After evacuation to 10 10 mbar, this band shifts to ñ= 2179 cm 1 and decreases in intensity (Figure 1 c). It is removed completely upon heating to 130 K, revealing that the majority CO species is weakly bound to intrinsic Cu + CUS sites. Density functional theory (DFT) calculations were performed in order to support the assignment of the IR bands. Based on our previous studies on copper paddle-wheel (CuPW) MOFs, we found it to be sufficient to use a non-periodic formate model of the inorganic unit to represent the carboxylate-linked Scheme 1. Low-temperature oxidation of CO to CO2 over activated Cu-based MOFs.