Chemisorption area measurement

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

The Heterogeneity of Catalyst Surfaces for Chemisorption Hugh S. Taylor Alkylation of Isoparaffins V. N. Ipatieff and Louis Schmerling Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. Emmett... 

In principle any standard catalytic metal surface area measuring technique, such as H2 or CO chemisorption can be used to measure the metal/gas interface area Aq or Nq. This is because solid electrolytes such as YSZ chemisorb practically no H2 or CO at any temperature. 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

The observed ratio aaiioy/aNi agreed well with the Ni content predicted.) As discussed later, it was believed that hydrogen chemisorption was proportional to the surface nickel concentration (see Section IV). It is clear, however, that chemisorption as a method of surface area measurement must be used with discretion in the case of alloy films. 

The bulk crystal structure of the samples was determined by XRD (Rigaku D-max B) using a filtered Cu Ka radiation. Surface structure and composition were monitored by XPS (Perkin-Elmer PHI 5400). The N2 BET surface area and CO chemisorption were measured in a constant-volume adsorption apparatus. For the latter, two successive isotherms separated by evacuation were obtained at RT for a sample and the difference extrapolated to zero pressure was taken as chemisorbed CO. 

Figure 14 gives the decrease of accessible area, measured by chemisorption, as a function of mileage. One /amole of CO adsorbed per gram is equivalent to a combined surface area of Pt plus Pd of 0.05 m2/g. Data from car fleets and from laboratory experiments are incorporated. The laboratory data were obtained with both lead free and lead containing fuels. The steep drop below 4000 miles, obtained with lead-sterile isooctane, is due to the initial loss of area by particle growth. 

These results are consistent, with those reported by Lin et al. [7] who used hydrogen chemisorption to measure the free metal area on Pt/Al203 and Pt-Sn/Al20- catalysts after deactivation. They found that the free metal surface decreased rapidly in the beginning when hydrocarbons were deposited on the catalyst, but reached a minimum level of coverage. At this level 10% and 30% of the metal remained uncovered, for the Pt and Pt-Sn catalyst... 

The dramatic increase in irreversible CO adsorption on presulfided supported nickel catalysts at moderate pressures (162) has significant, practical implications in regard to the use of CO chemisorption to measure nickel dispersion. For example, it is often desirable to determine nickel surface areas for catalysts used in a process where sulfur impurities are present in the reactants. Substantial differences in the measurements of nickel surface area by H2 or CO adsorption are possible depending upon the catalyst history and choice of adsorption conditions. In view of the ease with which catalysts may be poisoned by sulfur contaminants at extremely low concentrations in almost any catalytic process, and since large CO uptakes may be observed on supported Ni not necessarily representative of the unpoisoned nickel surface area, the use of CO adsorption to measure nickel surface areas is highly questionable under almost any circumstance. 

As shown in Table I, complete deactivation for these three catalysts occurs around 0.6 to 0.8 wt% sulfur, based on the active site content. These values are typical for complete deactivation in a commercial reactor. The metal surface area measured by hydrogen chemisorption is almost three times the active site concentration determined from the fit of the model to the accelerated aging data. Some of this difference may be due to a poor separation of the product kg C" into the individual constants. How-... 

D. J. C. Yates (Esso Research Co., Linden, N. J. 07036) It is interesting that you find the same migration effect of nickel on NaX zeolites that I found some time ago on samples reduced under milder conditions than you used. Did you find less migration with nickel on Zeolon I am also concerned with the use of CO for area measurement with nickel, as it is easy to make nickel carbonyl with reduced nickel even at quite low CO pressures. Did you check your results with hydrogen chemisorption ... 

The catalysts were characterized by BET surface area measurement, XRD, in-situ CO2 H2 chemisorption measurements, and Temperature Programmed Reduction (TPR). CO2 hydrogenation was carried out in a fixed bed flow reactor made of stainless steel. Prior to the activity studies, the catalysts were reduced in 99.99 % H2 flow at 723K for 12hrs. After this, the reaction gas (H2/CO2 = 3) was introduced into the reactor at 573K at 10 atm. The gas phase effluents were analyzed by on-line GC. 

Based on surface area measurements (hydrogen chemisorptions) the catalyst with 1 % alumina did not sinter as a result of reaction. This was in distinct contrast to the alumina-free catalyst initially used that sintered so completely upon re-reduction in hydrogen as to lose nearly 100% of its surface area. As for the potassium-promoted catalyst, SA measurements indcale a decrease in active metal sites of around 35% but it is not known whether this is due to sintering or unreduced carbon/carbide. 

A multifaceted characterization effort to stndy these materials as a function of thermal treatment has been undertaken. The techniques include BET surface area measurements. X-ray diffraction, chemisorption, scanning and high-resolution transmission electron microscopy, analytical electron microscopy, neutron activation analysis, atomic absorption spectroscopy, FTIR and isotopic tracer studies. The details of catalyst preparation have been previously... 

One of the best ways of characterizing a supported catalyst is determination of dispersion and effective surface area of the catalyticaUy active component. The dispersion of metal oxide catalysts can be determined by selective chemisorption of oxygen at appropriate temperatures [14-16]. The dispersions obtained from oxygen chemisorption measurements on various catalysts are presented in table 1. The N2 BET surface areas of various samples are also shown in this table. As can be noted, dispersion for 20 wt% catalyst is similar, within experimental limitations, irrespective of their origin. The BET surface area measurements also reveal that both the preparation methods yield similar type of catalysts in terms of physico-chemical characteristics. These catalysts were further evaluated for selective oxidation of / -methox doluene to p-... 

The catalysts were then characterized by XRD, chemisorption and surface area measurements and stepwise temperature programmed reduction (STPR) in hydrogen. All the catalysts were reduced in flowing hydrogen at 573K (low temperature of reduction, LTR) and 773K (high temperature of reduction, HTR), respectively, prior to these measurements. 

Brunauer and Emmett 120), in their extensive studies on synthetic ammonia catalysts have concluded, by a comparison of the CO uptakes and nitrogen adsorption surface area measurements, that on pure iron at temperatures between —78 and — 183°C CO chemisorbs up to one molecule per surface atom. Beebe and Stevens 121) from measurements of differential heats of adsorption confirmed that chemisorption rather than physical adsorption was occurring in this system. 

Initially, catalyst rates were expressed as rates per weight of catalyst. With the advent of surface area measurement using physisorption (BET), rate per total surface area of the catalyst could be determined, although a comparison of catalysts based on such a measure in general was grossly flawed. Use of chemisorption, especially where one of the reactants of interest was used, offered a more reasonable way to compare rates. In 1968, Michel Boudart [1] proposed the use of turnover frequency (TOFchem), based on chemisorption, as an excellent way for comparing catalysts. This quantity has units of sec" and represents, ostensibly, the number of molecules reacting per (chemisorption) site per second. This measure has become a standard in fundamental studies of catalytic reactions, especially for metal catalysts. 

Surface area measurements and precious metal dispersion determinations by CO chemisorption were reported previously [2], The chemical composition of the catalysts and their textural characteristics are given in Table 1. Prior to the BET or CO chemisorption measurements, the samples were either reduced by H2 at... 

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