Adsorption and hydrogenation of carbon monoxide on polycrystalline rhodium at high gas pressures

Article abstract of DOI:10.1021/jp963429e

Surface-enhanced Raman spectroscopy (SERS) in conjunction with mass spectroscopy (MS) has been utilized to investigate the adsorption and hydrogenation of carbon monoxide on polycrystalline rhodium surfaces. The SERS-active Rh substrates were prepared by electrodeposition of ultrathin films on electrochemically roughened gold and display remarkably robust SERS activity over a wide range of temperatures (up to 400 °C) and pressures (here up to 1 atm). The SER spectra reveal that CO adsorbed primarily on atop sites (v_Rh-C = 470 cm^-1) and desorbed by about 250-300°C under all gas-phase conditions examined. Partial dissociation of the CO adlayer, however, was obtained at temperatures as low as 100°C, most likely facilitated by the large number of steps and kinks present on these roughened surfaces. This was evidenced by a partial removal of CO at temperatures (ca 100°C) well below those expected for thermal desorption (200-250°C) and supported by the observed formation of surface carbonate (665 cm^-1) under these conditions. The CO dissociation, however, is hampered at lower temperatures (<200°C) when gas-phase H₂ and/or CO are present, most likely due to blocking of site ensembles necessary for decomposition to proceed. Interestingly, heating a CO adlayer in pure H₂ resulted in the formation of an adsorbed oxygen species (v_Rh-O = 295 cm^-1) at temperatures above 250°C. The CO hydrogenation reaction was examined over a wide range of gas-phase conditions (H₂:CO = 99:1 to 4:1 at 1 atm), with methane being the only hydrocarbon product detectable with MS. In addition to the presence of adsorbed CO observed up to 250°C under all H₂/CO reaction ratios, the adsorbed oxygen species noted above was detected at higher temperatures (>250°C) when a low percentage of CO (≤1%) in the feed reactant stream was used. The influence of the adsorbed species on the overall methanation rates is discussed in light of these findings. Utilizing the seconds time-scale resolution of SERS, the exchange between gas-phase and adsorbed CO was also studied. The results of such transient ¹³CO/¹²CO exchange experiments reveal that this desorption pathway is weakly activated (≈1 kcal mol^-1), first order with respect to CO coverage, yet independent of CO partial pressure in the regime studied (8-760 Torr). This contrasts the first-order pressure dependence for much lower CO partial pressures (≤10^-5 Torr) reported earlier in the literature. A rate law and mechanism are proposed which account for these differences and rationalize the observed behavior.