Chemisorption experiment

All of these results are consistent with the notion that surface migration of titanium oxide species Is an Important factor that contributes to the suppression of carbon monoxide chemisorption. The H2 chemisorption experiments on 1-2 ML of Ft, where no migration Is observed, strongly Indicate that electronic (bonding) Interactions are also occurring. Thus, for the tltanla system, both electronic Interactions and surface site blocking due to titanium oxide species must be considered In Interpreting SMSI effects. 

The observed distribution can be readily explained upon assuming that the only part of polymer framework accessible to the metal precursor was the layer of swollen polymer beneath the pore surface. UCP 118 was meta-lated with a solution of [Pd(AcO)2] in THF/water (2/1) and palladium(II) was subsequently reduced with a solution of NaBH4 in ethanol. In the chemisorption experiment, saturation of the metal surface was achieved at a CO/Pd molar ratio as low as 0.02. For sake of comparison, a Pd/Si02 material (1.2% w/w) was exposed to CO under the same conditions and saturation was achieved at a CO/Pd molar ratio around 0.5. These observations clearly demonstrate that whereas palladium(II) is accessible to the reactant under solid-liquid conditions, when a swollen polymer layer forms beneath the pore surface, this is not true for palladium metal under gas-solid conditions, when swelling of the pore walls does not occur. In spite of this, it was reported that the treatment of dry resins containing immobilized metal precursors [92,85] with dihydrogen gas is an effective way to produce pol-5mer-supported metal nanoclusters. This could be the consequence of the small size of H2 molecules, which... 

For the chemisorption experiments a weighed catalyst sample (wet) was put in a cell and mounted on the Micromeritics 2010 (static) chemisorption instmment. The sample was heated under vacuum to 150°C where it was exposed to hydrogen (0.7 atm) for 0.5 hour. The sample was then evacuated at room temperature, reexposed to hydrogen for 0.5 hour, then evacuated, and cooled to 30°C under vacuum. 

C02 chemisorption experiments on CsOH/Si02 revealed that 1.6 % of the Cs present on the catalyst existed as basic sites. C02-TPD experiments (Figure 1) revealed a major desorption peak at 373 K along with a small peak at 437 K. 

Using Equation (13), the external mass transfer coefficient at 723 K was calculated to be 60 cm/s. Since the reactor operating conditions at th s temperature (723 K, slightly above atmospheric pressure, 247 cm /s) were very similar to those of our transient chemisorption experiments, the external mass transfer coefficient calculated above was used for the simulations. 

Ruthenium and copper are not miscible hence, homogeneous alloy particles will not be formed in supported Ru-Cu catalysts. As copper has a smaller surface free energy than ruthenium, we expect that if the two metals are present in one particle, copper will be at the surface and ruthenium in the interior (see also Appendix 1). This is indeed what chemisorption experiments and catalytic tests suggest [40], EXAFS, being a probe for local structure, is of particular interest here because it investigates the environment of both Ru and Cu in the catalysts. 

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

Ammonia chemisorption experiments were carried out at different temperatures ranging from 25 to 400°C to establish conditions for the adsorption studies on the catalysts. The NH chemisorption experiments were performed on an all glass high vacuum system according to the procedure described by Kanta Rao et al. (31). In a typical experiment, about 0.3 g of the catalyst... 

The SMSI effect in Mn-promoted Ru/Ti02 catalysts was studied in more detail making use of the SSIMS technique, as well as with TEM, and selective chemisorption experiments. The SSIMS technique revealed the presence of TiO c forming two new surface sites, TiO -Ru and TiO-Mn. These species were found to be located at the immediate vicinity of the Ru nanoparticles. These new surface sites were considered to alter the electronic properties of the Ru metal surface and, as a consequence, the product selectivity. 

Reif (37), on the other hand, supports mechanism B. He argues that Gadsby et al. incorrectly interpret their chemisorption experiments (reason one above) and further states that his own chemisorption experiments for carbon monoxide on a coke surface (37, 44) make mechanism A unlikely. Insofar as reason two offered above is concerned, Reif (37) counters with the fact that Wu (40) finds an activation enei of -1-21.4 kcal. for reaction (1) reverse under mechanism B. Reif does not comment on reason three given by Gadsby and co-workers but acknowledges that there is a possibility that the two retarding reactions may be operative for different types of carbon under different conditions of temperature and pressure. 

At 400°C, the whiteline is still decreasing, indicating that reduction is still not complete. This is confirmed by the H2 chemisorption experiments. The autoreduction followed by an additional reduction in H2 for 30 min. at 400°C results in a higher hydrogen uptake (H/Pt = 1.09, Table 1) compared to the autoreduced sample without additional reduction (H/Pt = 0.99). The lower hydrogen uptake indicates that not all Pt is in the reduced state, since admission of H2 at 35°C will not result in a reduction of the Pt complex. 

The TPR and chemisorption experiments were carried out in an apparatus described elsewhere [3] equipped with a thermal conductivity detector. The experiments were performed using a gas mixture of 7 vol% H2 in Ar and a heating rate of 10 G/min. Separate TPR experiments (not shown here) indicate that the degree of reduction of tin, based on the reaction Sn02 + 2 - Sn + 2H20, was about 50 % for the Pt-Sn catalyst. This indicates that most of the tin is in the Sn2+ state. 

The results from the hydrogen chemisorption experiments were for Pt/Al203 13% dispersion and for Pt-Sn/AljO 29% dispersion. Recently Sfinchez et al. [5] showed that hydrogen chemisorption may give an overestimation of the dispersion of a Pt-Sn catalysts using the same procedure as here. Therefore caution should be taken in the interpretation of data based on hydrogen chemisorption (e.g. dispersion and turnover frequency). 

In order to establish the optimum chemisorption temperature, a series of isothermal chemisorption experiments were performed at different temperatures between 200 and 300 °C. The sample was first outgassed in Ar (15 °C min, 1000 C, 5 hours). The temperature was then lowered to the chemisorption temperature, and a flow rate of 75 mL min of oxygen was introduced to the TGA. In this way, an optimised chemisorption temperature of 250 °C was found, so that equilibrium could be achieved in a reasonable period of time, and simultaneous carbon gasification could be avoided. 

Two catalysts were prepared with 1.. ) and 0.38 percent Pt supported on silica. Chemisorption experiments revealed that the percentage of metal exposed was 100 percent on both catalysts. The turnover frequency for liquid-phase cyclohexene hydrogenation (101.3 kPa H2 pressure) was 2.67 s and 2.51 s at 275 K and 9.16 uid 9.02 ai 307 K. The similarity of the turnover frequencies at each of two dilfcreni icMiipcralurc indicates that the measured rates were not influenced by transport limitations. 

Table 4.6. Study of the H chemisorption on a Pt/CcOz catalyst reduced (I h) at 473 K or 773 K evacuated (Ih) ai 773 IC, and further treated with as indicated. Comparison between the quantitative data obtained from TPD-MS and volumetric chemisorption experiments. Data taken from (117).

Our aim in this section has been to prove the existence of a surface CO-oxygen complex, to establish its heat of formation and then to assess the evidence for its participation as the reaction intermediate in CO oxidation. The application of arguments based on isolated chemisorption experiments in discussing the mechanism of a delicately balanced catalytic reaction is always a calculated risk, but we have tried to show here that the method is most powerful if the behavior of all the various possible combinations of preadsorption and dosing can be fitted to a consistent picture. 

To determine the degree of phosphorous accumulation in the pores H2 chemisorption experiments were carried out and the results are summarized in Table 5. The amount of H2 chemisorption decreased with increasing of phosphorous content up to 5 wt% and thereafter was not diangcd significantly. This suggests that the phosphorous in the pores consisted of a monolayer up to a certain value of phosphorous content after which it accumulates as a multilayer, which docs noi influence the loss of chemisorption sites. 

This last quantity is entirely removed by holding at 300° for 4 hours. One may now eool to —78° and repeat the original chemisorption experiments with equivalent results. The three points shown in Fig. 7 at 300° (marked by an arrow) are three successive experiments of this type. However, since the activation that preceded the first point was at 350°, these points probably should be entered at 350° rather than 300°. 

The H2 chemisorption uptakes at increasing sulfur coverage (S/Pt=0-100) of the metal are shown in Figure 3. At higher S/Pt ratios the percentage of platinum particles able to interact with the probe decreased to 40% of the oii i value of dispersion at S/Pl 92. When the catalyst was contacted with laiger quantities of H2S, the H2 chemisorption experiments showed a reverse trend i.e. increasing the S/Pt ratio, the amoimt of H2 adsorbed at room temperature increased (Fig 4). 

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