H2 chemisorption measurements

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... 

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. 

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). 

Experimental details of H2 chemisorption measurements are described elsewhere33. Typically, the reduced samples are evacuated at 200°C and the H2 uptake is measured at 35°C. 

H2 chemisorption measurements (at Chevron Research Center, Richmond, CA) showed that by increasing the Pt content from 1 to 3.2%, the Pt surface area Increased from 4.8% to 10.9% of the support area. Pt crystallite size measurements were conducted by TEM (Hitachi Electron Microscope type HU-125). Crystallites were found in the range of 10 to 100a. Further details of sample preparation techniques can be found elsewhere [16]. 

Table 1. Surface Area (BET) and H2 Chemisorption Measurements on two Ni Catalysts...

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. 

The in sim characterization of catalysts was earned out in an apparatus which included a quadiupole mass-spectrometer and a gas chromatograph for TPO and H2 chemisorption measurements. In situ coking was performed by injecting a mixture of He and n-hexane vapor over the reduced catalysts at 500 C, In TPO experiments, ihe coked sample was heated at a rate of 8 C/min in a stream of 2 voL% O2 + 98% He. The amount of CO2 produced was recorded. The chemisorption of H2 was carried out in the same appanitus by a flow method after reduction or caking. The flow rate of carrier gas (Ar) was maintained at 25 ml/min and the volume of H2 injected was 0.062 ml/pulse. Since the partial piessiire of H2 was very low in this system, the hydrogenation of coke was never observed. Isobaric H2 chemisorption measurements with fresh catalysts were carried out in a static adsorption apparatus. Dehydrogenation of n-butane was carried out in a flow micro-rcactor in H2 atmosphere at LHSV = 3 h-l and H2/HC=1. Reaction products were... 

Hydrogen and oxygen chemisorption measurements were performed in a conventional glass system at 22°C. Before a H2 chemisorption measurement, the catalyst was reduced, or oxidized and reduced, at temperatures and during times to be specified under Results. These temperatures were reached with a heating rate of 5°C min-1. 

That the time dependence of the H2 chemisorption was anyway caused by a slow attainment of equilibrium, was established by performing the H2 chemisorption measurement in a modified way. Hydrogen was admitted to the evacuated catalyst at ca. 200 C and subsequently the reactor was slowly cooled down to room temperature and H2 chemisorption was measured. In this case equilibrium was quickly established and the chemisorption value was substantially higher (about 20%) than that reached during room temperature measurements for 18 h. Furthermore, since there was no difference between the H2 chemisorption measured after admission of H2 at 190 C during 10 min and that measured after admission at 205 C during 30 min, a temperature of about 200 C during 10 min seems sufficient to quickly reach equilibrium. 

The characterisation of the colloids both in the free and in the embedded state was first performed using transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and atomic absorption spectroscopy (AAS). In addition, nitrogen adsorption-desorption curves at 77 K, H2-chemisorption measurements, solid state Si-NMR, XRD, SAXS, XPS, MAS-NMR, NH3-FTIR, and Au Mossbauer spectroscopy were applied. For the embedded triflates, the catalysts were characterized by nitrogen adsorption-desorption isotherms at 77K, TG-DTA, H, C, and Si solid state MAS-NMR, XRD, TEM, SEM, XPS and, FTIR after adsorption of NH3. 

Temperature programmed reduction (TPR), temperature programmed Ar heating (TPAr) and extent of reduction experiments were all performed in a Perkin Elmer thermogravimetric analyzer (TGA) Model TGA 7 described elsewhere [22]. H2 chemisorption measurements were eonducted using a flow chemisorption method and apparatus described by Jones and Bartholomew [24]. Activity measurements and high-pressure steam treatments were conducted in a fixed-bed microreactor described elsewhere [22]. A Micromeritics Gemini 2360 surface analyzer was used to measure N2 adsorption at liquid N2 temperature for BET surface area measurements. A Micromeritics Tri-Star 3000 analyzer with N2 adsorption was used to obtain the pore size distribution. 

Though not reported in [5], our measurements on PQ silica indicated that the BET surface area is close to 365 m /g, with a pore volume of 2.44 cm /g and an average pore diameter of 27.8 nm. The metal dispersion, based on H2 chemisorption measurements at 100°C after reduction at 325°C, was reported to be 5.8%. However, it is not clear if this is the true dispersion of the metal, or an uncorrected dispersion, as it is necessary for cobalt catalysts to determine the extent of cobalt reduction. 

Relative activities in CO hydrogenation measured for supported rhodium catalysts are listed in Table 5-36. These experimental findings are supported by H2 chemisorption measurements and active rhodium centers. 

H2 chemisorption measurements were performed at 298K using a pulse flow system. Before measurements, catalysts were heated (lOK/min) in a H2 stream (40 ml/min) at 673K for 2 h. After reduction the siirface was cleaned for 2 h with an Ar stream, then cooled at 298K. The chemisorption measurements were performed with 5% diluted H2 in Ar. 

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