ADSORPTION AND HYDROGENATION OF CARBON-MONOXIDE ON POLYCRYSTALLINE RHODIUM AT HIGH GAS-PRESSURES

Surface-enhanced Raman spectroscopy (SERS) in conjunction with mass sp ectroscopy (MS) has been utilized to investigate the adsorption and hy drogenation of carbon monoxide on polycrystalline rhodium surfaces. Th e SERS-active Rh substrates were prepared by electrodeposition of ultr athin films on electrochemically roughened gold and display remarkably robust SERS activity over a wide range of temperatures (up to 400 deg rees 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 desorbe d by about 250-300 degrees C under all gas-phase conditions examined. partial dissociation of the CO adlayer, however, was obtained at tempe ratures as low as 100 degrees C, most likely facilitated by the large number of steps and kinks present on these roughened surfaces. This wa s evidenced by a partial removal of CO at temperatures (ca 100 degrees C) well below those expected for thermal desorption (200-250 degrees C) and supported by the observed formation of surface carbonate (665 c m(-1)) under these conditions. The CO dissociation, however, is hamper ed at lower temperatures (<200 degrees C) when gas-phase H-2 and/or CO are present, most likely due to blocking of site ensembles necessary for decomposition to proceed. Interestingly, heating a CO adlayer in p ure H-2 resulted in the formation of an adsorbed oxygen species (v(Rh- 0) = 295 cm(-1)) at temperatures above 250 degrees C. The CO hydrogena tion reaction was examined over a wide range of gas-phase conditions ( H-2:CO = 99:1 to 4:1 at 1 atm), with methane being the only hydrocarbo n product detectable with MS. In addition to the presence of adsorbed CO observed up to 250 degrees C under all H-2/CO reaction ratios, the adsorbed oxygen species noted above was detected at higher temperature s (>250 degrees C) when a low percentage of CO (less than or equal to 1%) in the feed reactant stream was used. The influence of the adsorbe d species on the overall methanation rates is discussed in light of th ese findings. Utilizing the seconds time-scale resolution of SERS, 1 t he exchange between gas-phase and adsorbed CO was also studied. The re sults of such transient (CO)-C-13/(CO)-C-12 exchange experiments revea l that this desorption pathway is weakly activated (approximate to 1 k cal mol(-1)), first order with respect to CO coverage, yet independent of CO partial pressure in the regime studied (8-760 Torr). This contr asts the first-order pressure dependence for much lower CO partial pre ssures (less than or equal to 10(-5) Torr) reported earlier in the lit erature. A rate law and mechanism are proposed which account for these differences and rationalize the observed behavior.