CO interaction with the surface of thermally activated CaO and MgO

New frontier in transmission IR spectroscopy of molecules adsorbed on high surface area solids: Experiments below liquid nitrogen temperature

“…It is worth underlining that, if T is increased from 60 up to 100 K, at constant P CO , the IR spectrum of adsorbed species changes dramatically and becomes similar to that observed and discussed in previous Table 1 IR frequencies of the five modes (n 1 , n 2 , n 3 , n 3a and n 4 ) of the trimeric (C 3 O 4 ) 2À species formed at 60 K on three different basic surface sites (species labeled as C, C 0 and C 00 ) of high surface area MgO, compared with those obtained with ab initio calculations, from Ref. [22] IR mode Site C (cm À1 ) contributions [9,24,52,53,67]. The species formed at higher temperature (100 K) are irreversible and cannot be removed by pumping.…”

“…Experimental data on powdered MgO samples demonstrated that a great number of species are formed upon interaction with carbon monoxide at 300 K, through a complex sequence of surface reactions involving the most basic surface O 2À ions. Some of these reactions are activated and, at RT, they need considerable time to be completed, with formation of oligomeric pink colored species [5,9,22,24,25,52,53,56,[66][67][68]. The detailed description of these processes is of high It is generally accepted that all the olygomeric species are originated from a common CO 2 2À precursor generated from a primary attack of carbon monoxide on the low coordinated O 3c 2À and O 4c 2À oxygen ions present at defect sites like corners, edges, kinks, etc.…”

“…However, notwithstanding the fact that commercial cryostats able to reach liquid helium temperatures were available since decades and widely employed in the field of solid state physics [11][12][13][14][15], the lower temperature value at which experiments were performed was limited for a long period to 77 K, mainly due to practical problems. This was mainly due to the www.elsevier.com/locate/cattod Catalysis Today 113 (2006) [65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] incompatibility of the materials used to reach and confine very low temperatures with the high temperatures usually needed to activate the surfaces of catalysts, that has prevented for long time the overcoming of the 77 K barrier in FT-IR experiments of species adsorbed on active surface sites. Note that when speaking about 77 K IR experiments we mean a nominal temperature, being the actual one on the sample always slightly higher (typically in the 100-110 K range) owing to the temperature gradient between the cooled part of the cell and the sample itself and to the heating power of the IR beam.…”

Methanol carbonylation catalyzed by TiO2–supported gold: An in-situ infrared spectroscopic investigation

“…A schematic representation of methanol-derived surface species on the TiO 2 and TiO 2 -supported gold samples formed upon methanol adsorption is shown in Figure 6. Also, bands in the range between 1220 and 1270 cm -1 have been assigned to the  asCOO vibration mode of surface carbonates on metal oxides [45][46][47]. Those surface species might be formed by the reactive adsorption of CO on the sample.…”

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism

“…Bands arising below 2000 cm À1 are attributed to carbonite anions or similar polymeric structures reflecting the strong basic character of the MgO surface [12,13]. These species are formed by interaction of the CO molecule with coordinatively unsaturated oxygen ions [12].…”