Chemisorption measurements

The conclusion is that for chemisorption measurements in a CSTR, the matter in the empty space must be minimized, which calls for low (atmospheric) pressure, and low concentration of the chemical, in a low flow of carrier gas. Even at low pressure it will work only for very large surface area materials, like molecular sieves or active charcoals. 

Physical adsorption—surface areas of any stable solids, e.g., oxides used as catalyst supports and carbon black Chemisorption—measurements of particle sizes of metal powders, and of supported metals in catalysts... 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

Static Chemisorption. Measurements were made by two procedures. In the first, the catalyst was evacuated at ca. 250°C for at least 8 hrs and cooled to the measurement temperature under vacuum. Hydrogen was then admitted at progressively higher pressures and the amount of gas adsorbed after 15-30 min at each pressure recorded. The sample was then evacuated for 30 min and the dosing procedure repeated so as to obtain a measure of the reversibly adsorbed gas. In the second (saturation) procedure, after reduction and evacuation, the catalyst was cooled to the... 

Ft. The remainder was assumed to be surface atoms and Ao was allowed to vary to achieve the best fit to the data. The relative fraction of surface and bulk was assigned from the chemisorption measurement, l.e., 0.9 and 0.1, respectively. The results are given... 

Results of the H2 chemisorption measurements after NH3 synthesis based on H/Ru = 1/1. NHs synthesis was run at 773 K with Ru/MgO and RU/AI2OS, and at 673 K with all alkali-promoted catalysts. The mean particle size was calculated assunung spherical particles. 

Chemisorption measurements, combined with oxygen uptake, TPR, and pulsed CO-O2 experiments were employed to determine the source of large differences in dynamometer sweep performance of a series of Pt/Rh, Pd/Rh and Pd-only TWCs after dynamometer aging. The following observations have been made ... 

It is worth to note that this dimension is directly comparable with the specific surface area obtained from chemisorption measurements [38]. 

For illustration we will present some commonly used techniques for chemisorption measurements. Chemisorption can be measured gravimetrically, volumetrically, or spectroscopically. Also, pulse techniques, and Temperature Programmed Desorption (TPD) can be used. 

Barometric chemisorption. Chemisorption on catalysts is measured routinely by the barometric method. The equipment is very similar to that commonly used in texture determination by physical adsorption (see Section 3.6.2), except that for chemisorption measurements facilities for pretreatment of the samples should be present. In particular for metal catalysts often the catalyst is received in a partly or fully oxidized form and, as a consequence, reduction is required when one wants to measure the amount of active sites. Moreover, during storage adsorption of various molecules might occur and evacuation is... 

As always in chemisorption measurements, pretreatment of the samples should be done with care. For metal catalysts prepared from oxides in particular this is experimentally troublesome because a reduction step is always needed in the preparation of the metal catalyst. Hydrogen or hydrogen diluted with an inert gas is usually used for the reduction but it is difficult to remove adsorbed H2 from the surface completely. So, after reduction the metal surfaces contains (unknown) amounts of H atoms, which are strongly retained by the surface and, as a consequence, it is not easy to find reliable values for the dispersion from H2 chemisorption data. 

Besides CO, hydrogen, oxygen, and other gases can also be used in chemisorption, provided suitable conditions are applied for sample pretreatment and chemisorption measurements (Overbury et ai, 1975 Bartley et al., 1988 Scholten et al., 1985 and Van Delft et ai, 1985). 

Instrument. Before the H2 chemisorption, each sample was heated in pure H2 at 300°C for 90 min, subsequently it was heated in He at 290°C for 60 min in order to desorb hydrogen from the sample. The chemisorption measurement was performed at 0°C by several H2 pulses with an Ar purge in between, in order to desorb physisorbed hydrogen. A 1/1 H/Pt ratio was used to estimate the Pt dispersion. Values in the range 50-70% have been obtained on the fresh samples Pt—Ba/ y-Al203 (1/20/100 w/w) catalyst. 

The anchoring and the reduction methods of precious metal precursors influence the particle size, the dispersion and the chemical composition of the catalyst. The results of SEM and H2 chemisorption measurements are summarised in Table 3. The XPS measurements indicate that the catalysts have only metallic Pd phase on their surface. The reduction of catalyst precursor with sodium formate resulted in a catalyst with lower dispersion than the one prepared by hydrogen reduction. The mesoporous carbon supported catalysts were prepared without anchoring agent, this explains why they have much lower dispersion than the commercial catalyst which was prepared in the presence of a spacing and anchoring agent (15). 

Chemisorption measurements (Quantachrome Instruments, ChemBET 3000) were conducted in order to determine the metal (Co) dispersion. Therefore, the nanomaterial catalysts were reduced under a hydrogen flow (10% H2 in Ar) at 633 K for 3 h. The samples were then flushed with helium for another hour at the same temperature in order to remove the weakly adsorbed hydrogen. Chemisorption was carried out by applying a pulse-titration method with carbon monoxide as adsorbing agent at 77 K. The calculation of the dispersion is based on a molar adsorption stoichiometry of CO to Co of 1. 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

The existence of Pt-Ba interactions has been confirmed by FT-1R analysis of CO chemisorption measurements. A band characteristic of CO linearly adsorbed on Pt sites is observed in all cases however, the intensity of the band decreases on increasing the Ba loading (due to the decrease of the Pt dispersion) and shifts towards lower energy (from 2072 to 2049 cm ) according to the increase in the system basicity. Hence the data indicate a strong interaction between Pt and the basic oxygen anions of the Ba phase, thus suggesting that the exposed Pt sites and the Ba component are in close proximity [98]. 

Oxygen chemisorption measurements were performed in the above flow apparatus using He as carrier gas (30 Ncm -min" ). Prior to chemisorption measurements, catalyst samples (0.25 - 1.00 g) were treated "in situ" for 15 min in a flow of CH4/02/He (Pch4 02 He 2 1 7) reaction mixture at 550-... 

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

When using the continuous flow method, however, some additional versatility is available in chemisorption measurements. For example, when data is required at an adsorbate pressure of 0.1 atm, a 10 % mixture of adsorbate, mixed with an inert carrier gas, is passed through the apparatus with the sample cooled to a temperature at which no chemisorption can occur. Upon warming the sample to the required temperature, adsorption occurs producing an adsorbate-deficient peak that is calibrated by injecting carrier gas into the flow stream. Equation (15.9) is then used to calculate the quantity adsorbed. This process is repeated for each concentration required. Caution must be exercised to avoid physical adsorption when the sample is cooled to prevent chemisorption. Should this occur, the adsorption peak due to chemisorption can be obscured by the desorption peak of physically bound adsorbate when the sample is heated. 

Chemisorption measurements have shown that ethylene does not adsorb on pure silver, but only on a silver surface which has been preoxidized [339]. Complete coverage with an oxygen monolayer, however, also seems to destroy the capacity to adsorb ethylene, as was demonstrated by Force and Bell [114,116] (favouring the idea of adsorption on silver). Consequently, partial oxygen coverage seems to be a necessary condition for catalytic activity. 

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