Edmond I. Ko

Control of mixed oxide textural and acidic properties by the sol-gel method

Abstract Preparation of catalytic mixed oxides by sol-gel chemistry affords a degree of control over the intimacy of molecular-scale mixing — and therefore the important acidic and textural properties that depend upon mixing — that is not available by other methods. Literature examples illustrate how careful control of any one of a number of preparative parameters, including, but not limited to, concentrations, addition order and drying conditions, can lead to changes in the microstructural distribution of the components that are, in turn, related directly to the textural and chemical properties of the calcined sample.

Structures of niobium pentoxide and their implications on chemical behavior

Abstract The many structures of bulk niobium pentoxide can be grouped into the low-temperature and high-temperature forms, with the latter being more ordered. The crystallization behavior, however, is influenced by starting materials, impurities, and interactions with another component. These interactions affect both the physical (mobility) and chemical (reducibility, acidity) properties of catalytic systems containing niobium pentoxide.

A study of acidic titania/silica mixed oxides and their use as supports for nickel catalysts

Abstract A series of titania/silica ( TiO 2 SiO 2 ) mixed oxides, containing between 25 and 75 wt% TiO 2 , was prepared by the coprecipitation method. After a calcination temperature of 773 K, these mixed oxides were found to be amorphous by X-ray diffraction and to be acidic by Hammett indicators, n -butylamine titration, and 1-butene isomerization. Both the acid amounts and isomerization activity were higher than those of either of the pure components. When Ni catalysts were prepared onto these acidic oxides, support effects were observed after a reduction at 773 K in the form of suppressions in hydrogen chemisorption and ethane hydrogenolysis. Furthermore, CO hydrogenation showed a catalytic behavior different from what is normally expected from strong metalsupport interactions. These results demonstrated that the environment of TiO 2 could affect its role as an interacting oxide.

Ethane hydrogenolysis and carbon monoxide hydrogenation over niobia-supported nickel catalysts: A hierarchy to rank strong metal-support interaction

Ethane hydrogenolysis and carbon monoxide hydrogenation were studied over two niobia (Nb2O5)-supported nickel catalysts, containing 2 and 10 wt% nickel, which had been reduced in hydrogen at 573 or 773 K for 1 h. Compared to silica-supported nickel catalysts, these samples had lower ethane hydrogenolysis activity but higher CO hydrogenation activity. For some samples a different experimental rate law for ethane hydrogenolysis was observed. In CO hydrogenation, all samples showed a shift in product distribution to hydrocarbons higher than methane, and olefinic products were detected. These observations were attributed to strong metal-support interaction (SMSI). The use of these chemical probes identified different manifestations of SMSI that depend on crystallite size and reduction treatment. On the basis of these manifestations, a hierarchy consisting of five stages was developed to rank the extent of interaction in NiNb2O5 catalysts. A mechanism of SMSI was proposed for a physical explanation of this hierarchy.

Characterization and selective poisoning of acid sites on sulfated zirconia

Microcalorimetric measurements and infrared spectroscopy of ammonia adsorption were used to characterize the acidic properties of sulfated zirconia catalysts. Reaction kinetic measurements forn-butane isomerization were conducted over catalysts that were selectively poisoned with controlled amounts of ammonia. Initial heats of ammonia adsorption on the strong acid sites of sulfated zirconia were 150–165 kJ/mol, and these sites contain Brønsted acid and possibly Lewis acid centers. Sulfated zirconia samples that show high activity for the isomerization ofn-butane possess Bransted acid sites of intermediate strength, with differential heats of ammonia adsorption between 125 and 140 kJ/mol. The results of selective poisoning of sulfated zirconia with ammonia confirm that Bransted acid sites of intermediate strength are active forn-butane isomerization at 423 K while not discounting a possible role of the stronger acid sites.

Structural sensitivity of CO adsorption and H2/CO coadsorption on Ni/SiO2 catalysts

Abstract The surface structure of a series of Ni SiO 2 catalysts was probed by CO adsorption and H 2 CO coadsorption using quantitative chemisorption measurements and IR spectroscopy. Preparation method, pretreatment conditions, and metal crystallite size were found to affect the nature of the metal surface for crystallites in the range 2.5–9.0 nm. CO was found to adsorb in linear, bridge, and multiple-CO forms on these supported nickel catalysts. Multiple-CO adsorbs on “defect sites” of low coordination, which increase in number as the heterogeneity of the nickel surface increases. On large crystallites exhibiting a large fraction of bridge-CO adsorption, coadsorption of hydrogen aided in probing the types of surface planes present on the crystallites. A high frequency peak in the bridge-CO region was assigned to twofold bridge-CO on Ni(111) while a low-frequency peak was assigned to two- or fourfold bridge-CO on Ni(100) planes. The coadsorption of H2 also resulted in an increase in the intensity of the peak for CO adsorption on defect sites, possibly due to increased dipole-dipole interactions between linear and subcarbonyl CO species. CO and CO H 2 adsorption were found to be sensitive probes for both surface smoothness in terms of the presence of defect sites and surface structure in terms of the types of planes exposed.

Acidic properties of oxides containing niobia on silica and niobia in silica

A series of composite oxides containing niobia (Nb2O5) and silica (SiO2) was prepared by either incipient wetness impregnation to form surface phase oxides or by coprecipitation to form mixed oxides. At low Nb2O5 concentrations, both the surface phase and mixed oxides showed strong Lewis acidity as determined by thermogravimetric and infrared studies of adsorbed pyridine. A surface phase oxide containing 0.25 monolayer of Nb2O5 on SiO2 further showed strong Bronsted acidity. With increasing Nb2O5 concentration and/or treatment temperature, Nb2O5 in these composite oxides became more bulk-like and showed a corresponding decrease in acid strength. These acidic properties are discussed in terms of proposed structural models which contain tetrahedral niobia, octahedral niobia, and Nb0 bonds as basic building blocks.

Structural and acidic characterization of niobia aerogels

Abstract A niobia (Nb2O5) gel was prepared by the hydrolysis and condensation of niobium pentaethoxide and subsequently dried by supercritical extraction with carbon dioxide to produce an aerogel. The structural and acidic properties of this aerogel heat-treated at different temperatures were characterized by surface area measurements, X-ray diffraction, Raman spectroscopy, n-butylamine titration, pyridine adsorption, and 1-butene isomerization. These results were then compared to those of a niobia xerogel, a precipitated niobia, and a commercial niobic acid. The synthetic route to produce the aerogel was found to stabilize a porous network consisting of NbO6 octahedra with NbO bonds which gave rise to strong Lewis acid sites. Steady-state activity and selectivity data of 1-butene isomerization suggested that all niobia samples possessed comparable Bronsted acidity.

Preparation, reduction, and chemisorption behavior of niobia-supported nickel catalysts

Abstract Two niobia(Nb 2 O 5 )-supported nickel catalysts, containing 2 and 10 wt% nickel, were prepared by incipient wetness impregnation. Subsequent to reduction in hydrogen at 573 and 773 K for 1 h, these catalysts adsorbed a smaller amount of hydrogen at room temperature than silica-supported nickel catalysts similarly prepared. The suppression in hydrogen adsorption was more pronounced for the 2 wt% sample, which had a smaller average crystallite size as determined by X-ray line broadening measurement. Thermogravimetric analysis showed a facile reduction of the nickel precursor salt to metallic nickel. The chemisorption behavior was thus ascribed to strong metal-support interactions (SMSI). These results compared with similar data previously obtained for titania(TiO 2 )-supported nickel catalysts showed that niobia was a more interacting support than titania for nickel, when parameters such as average crystallite size and reduction treatment were comparable. The extent of interaction appeared to correlate with the reducibility of the oxide support, although quantitative thermogravimetric measurements suggested that the amount of support being reduced was small.

Titania-zirconia mixed oxide aerogels as supports for hydrotreating catalysts

Abstract Supercritical fluid (SFC) extraction was used to make aerogels of TiO 2 , ZrO 2 and two TiO 2 /ZrO 2 mixed oxides, having surface areas from two to five times greater than their conventionally prepared equivalents; additionally the mixed oxides have higher surface acidities than the two single component oxides. Heat treatments, either during catalyst preparation or reactor testing, always resulted in small to significant decreases in surface areas in the aerogel-containing samples. These samples were used as supports for Mo-Ni catalysts for the hydroprocessing of gas oil in a pilot-plant scale reactor. The high ZrO 2 containing materials were found to be unstable under reaction conditions and were nearly inactive; in contrast, the high TiO 2 containing catalysts, while somewhat unstable, are more active on a surface area basis than Al 2 O 3 or conventional TiO 2 equivalent supported Mo-Ni catalysts. This improvement is attributed to properties inherent in the SCF prepared supports; these results also indicate that support acidity contributes to hydrotreating activity.