Infra-red studies of carbon monoxide and hydrocarbons adsorbed on silica-supported nickel

Abstract

Infra-red studies of NH3, HC1, CO, C2H4, and butene adsorbed on transparent aerogels were made to elucidate the surface chemistry of nickel/silica and the nature of adsorbed species. In addition to the hydroxyl band found on silica at 3750 cm-1, most " dry " nickel/silica aerogels exhibited a band near 3620 cni-1, apparently caused by silanol groups near nickel ions. Reduction of ail nickel ions by 1-12 treatment proved very difficult. Nickel ions remaining on "reduced" gels were apparently the acid sites that adsorbed NH3 strongly. Chemisorption of HC1 showed that reactive oxide ions, not found on silica, were also present. Bands between 1800 and 2210 cm-1 showed at least 7 types of adsorbed CO, varying in relative importance with the extent of reduction. Three distinct ionic sites for CO adsorption appeared to exist. Bands near 2200 cm-1 arose from CO on the sites which held NH3 most strongly, implying binding to a nickel ion. Reaction of HCl with these sites, however, showed that they also contain oxide ions.Adsorption of ethylene on " reduced " nickel/silica produced 1-and 2-butenes which were quickly converted to butane on subsequent addition of H2. Spectra ascribed by others to associatively adsorbed ethylene (CH2-CH2) and to ethyl groups on reduced nickel probably instead show butenes and butane.

FTIR Study of CO Adsorption on Ni−ZSM-5

“…Usually, the Ni 2+ -CO species are easily decomposed by evacuation at room temperature. [20][21][22][23][24][25][26][27][28] In our case, however, these species are characterized by a remarkable stability. This is in line with the expected high electrophilicity of cations in a ZSM-5 matrix.…”

“…[20][21][22][23][24][25][26][27][28] In this case, CO is bonded predominantly by a σ bond which leads to a high stretching frequency of the carbonyls. The stronger the bond, the higher the frequency of the σ carbonyls.…”

Adsorption of CO on Defined Co/SiO2: Influence of the Degree of Reduction and the Nature of Support Studied by Infrared Spectroscopy

“…The second interpretation originates from the picture that CO is bonded to incompletely coordinated Co2+ ions in tetrahedral and octahedral sites similar to the assignment of Peri [17] and Borello et al [12] in order to explain the appearance of two bands in the HF region on Ni2 + /Si02 or Ni2+/Al203, respectively. But on the basis of these arguments we would also expect two high frequency bands in case of Co/Aerosil.…”

“…A general survey over the HF -CO surface species observed on transition metals like iron [3, 5, 15], cobalt [2][3][4][5]7], nickel [2,4,8,16,17], copper [2,18] In the meantime it seems to be accepted that species with stretching frequencies above that of gaseous CO must be attributed to CO attached to positively charged metal sites [24]. A general survey over the HF -CO surface species observed on transition metals like iron [3, 5, 15], cobalt [2][3][4][5]7], nickel [2,4,8,16,17], copper [2,18] In the meantime it seems to be accepted that species with stretching frequencies above that of gaseous CO must be attributed to CO attached to positively charged metal sites [24].…”

Surface titration of supported Ni catalysts by O2-pulse thermal analysis

“…This phenomenon only allows the titration of Ni 0 species by CO at low temperature to avoid the tetra-carbonyl formation. Furthermore, the stoichiometry of CO adsorption on metallic Ni adds to the complexity of the task, making any titration method useless without spectroscopic is demanding, making any titration method useless without spectroscopic quantification of the linear/bridging adsorption ratio [9,67,68].…”