CO Adsorption on Hydrated Ru/Al₂O₃: Influence of Pretreatment

The adsorption of CO on hydrated H₂-, O₂-, H₂S-, and He-pretreated 5 wt % Ru/Al₂O₃ was investigated using attenuated total reflection IR spectroscopy (ATR-FTIR) to determine how the oxidation state of Ru influences CO-H₂O interactions. The frequencies of the three IR bands (high frequency (HF), midfrequency (MF), and low frequency (LF)) that are observed when CO adsorbs on Ru/Al₂O₃ are influenced by (1) CO coverage (CO−CO interactions), (2) Ru oxidation state, (3) pretreatment gas, and (4) pretreatment time. Water red-shifts the CO features by as much as 116 cm⁻¹and influences the MF/LF ratio in a complex mechanism involving both (1) blockage of CO adsorption on the LF site and (2) the formation of OH---CO interactions. While the LF band position is correlated to the adsorbate electronegativity and the MF band slightly correlates to CO coverage, the HF position is independent of either CO coverage or adsorbate electronegativity. These varying degrees of correlations can be rationalized using the extent of interaction between water and the adsorbed CO species by assuming the LF band is due to CO adsorption on an a-top site that is expected to have minimal interaction with coadsorbed water, while the MF and HF bands most likely arise from Ru(CO)_m(OH)_n (n, m ≥ 1) and Ru(CO)_o(H₂O)_p (o, p ≥ 1), respectively.