Adsorption of CO on MgO Surface. Characterization of Adsorbed Species

Abstract

Characterization of CO species adsorbed on well-degassed surfaces of polycrystalline MgO at low CO pressures (< 1300 Pa) and at and below room temperature was studied by using temperature-programmed desorption (TPD) and infrared (IR) spectroscopies. Mutual transformation between adsorbed species stable at room temperature, and their reactivities with 0 2 were also investigated. Adsorbed species are classified into five groups, KO, K1 (and K1' ), Kz, K3, and &. Among them species KO and K1 (and K1' ) are stable only below room temperature while species KZ to & are stable above this temperature. Species KO is a linear-type CO monomer with the C atom bonded to surface Mg2+ and has five subspecies depending on its circumstance, while all the other species K1 to & are formed on surface 02-to which the C atom is linked. A chained-type CO monomer, K1, is reversibly transformed at ca. 230 K in the presence of gaseous CO into a linear-type CO monomer, K1'. Species KZ is a chained-type tetramer of CO with a carbonyl CO bond and resonanced bonds. There are two subspecies, K~A and Km, which are formed by the addition of one CO molecule to corresponding trimer species, K~A and K~x , respectively, in the presence of gaseous CO. K~A and Kzx are reversibly transformed into K~A (at 400 K) and K~x (in 300-400 K), respectively, by the elimination of the CO added.There are two more subspecies, K~B and K~c , in species K3, but they have no corresponding KZ subspecies, and hence no transformations occur. All the K3 species seem to be a linear-type trimer with a ketenic group, >C=O=O, and are desorbed in 520-580 K. Species &, though detailed information is not obtained, consists of three subspecies &A, &B, and &c, but no transformations between species K3 and & are observed.

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

“…[22], each group of five bands has been ascribed to the five (n 1 , n 2 , n 3 , n 3a and n 4 ) modes of trimeric (C 3 O 4 ) 2À species hosted on two slightly different surface sites (see Table 1). It is worth noticing that the bandñ 3a was not assigned in older works [5,24,69,70]. According to the relative intensities of the five components of the two sets of five bands, the relative abundance of C and C 0 species has been estimated to be %60 and %40%, respectively.…”

“…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.…”

FTIR Study of Low-Temperature CO Adsorption on MgAl-Hydrotalcite and Its Calcined Forms

“…10 However, bands below 2190 cm -1 have been recorded only with MgO 11-18 and CoO-MgO, 19 mainly at low temperatures. The strong basic character of the MgO surface is responsible for the formation of carbonite anions 11,20 or similar polymeric structures 11, [16][17][18]20 upon CO adsorption. The Al 3+ -CO carbonyls for aluminumcontaining systems have been detected in the region 2245-2142 cm -1 .…”

Formation and Characterization of Magnesium Bisozonide and Carbonyl Complexes in Solid Argon

“…The CO stretching frequency of Mg(O 3 ) 2 (CO) 2 is blue-shifted 50.1 cm −1 from that of free CO isolated in solid argon. This value is comparable with that of CO adsorption on the three-coordinated Mg 2+ site of MgO particles but is larger than those of CO adsorbed on MgO surfaces. − …”

“…This value is comparable with that of CO adsorption on the threecoordinated Mg 2+ site of MgO particles but is larger than those of CO adsorbed on MgO surfaces. [37][38][39][40] Reaction Mechanism. The mechanisms for the formation of MgO 2 and MgO 3 in the Mg + O 2 and Mg + O 3 reactions have been discussed in detail previously.…”