Chemisorption pulse method

The electrical double-layer structure of a Pt/DMSO interface has been investigated using the potentiostatic pulse method.805 The value of C at E = const, as well as the potential of the diffuse layer minimum, have been found to depend on time, and this has been explained by the chemisorption of DMSO dipoles on the Pt surface, whose strength depends on time. Eg=Q has been found11 at E = -0.64 V (SCE in H2O). 

Metal dispersions were obtained by the dynamic pulse method using either H2, CO or O2 chemisorption at 298 K (.J) ... 

A word should be said regarding the use of O2 chemisorption to measure Ru-Ir metal dispersions. The stoichlonetry of the CO adsorption on Ir (CO/Irjgj) was taken from the literature to be 0.5 (9-10). The measured CO/O2 chemisorption ratio on Ir was determined using the dynamic pulse method and found to be 1.55. These results give... 

Fig. 3.51 shows CO chemisorption data using the pulse method for a reduced Pt/A Os sample. It is clear that the peak height gradually increases as more pulses are given. From the... 

The results confirm that the novel metal nitrate conversion method using nitric oxide in place of air advocated by Sietsma et al. in patent applications WO 2008029177 and WO 2007071899 leads to, after activation in H2, catalysts with smaller cobalt crystallites, as measured by EXAFS and hydrogen chemisorption/ pulse reoxidation. In spite of the lower extent of cobalt reduction for H2-activated nitric oxide calcined catalysts, which was recorded by TPR, XANES, EXAFS,... 

Hydrogen chemisorption results, obtained by the pulse method, indicated that the dispersion of Rh on the calcined 3%Rh/Ti02 catalyst was 56.8 % which corresponded to an average crystallite size of 1.9 nm. It should be noted that this value is rather low, probably because some of the surface was in an oxidized state as shown by the XPS results. 

The dispersion of Pt was determined from the amount of chemisorbed CO using pulse method. The experiments were carried out using a (Micromeritics 2910 AutoChem) instrument. The Pt modified catalysts were reduced with hydrogen in a U- shaped quartz tube and then cooled to 313 K under flow of He and CO pulse chemisorption was performed. More detail description of the R dispersion can be found in reference [11], The dispersion of R measured by CO chemisorption was the highest for R-MCM-41 catalyst prepared by ion-exchange method Table 1. 

The chemisorption of ethylene glycol on platinum has been investigated [59, 60] and the results obtained by pulse methods were interpreted [60] as due to chemisorption process on smooth platinum at low temperatures in the following equation ... 

Step- and impulse-response methods. Chemisorption can conveniently be measured under flow conditions using transient techniques, in particular, step-response and impulse-response measurements. After pretreatment, pulses of probe gas are injected into a carrier gas stream passing through the reactor that contains the pre-treated. sample. The response is detected at the reactor exit. 

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. 

Chromatographic methods are widely used for the study of both physisorption and chemisorption. In its simplest form the technique consists of passing a pulse of the adsorbate through a column of the adsorbent and measuring the retention time and registering the elution curve. Measurement of the variation in the retention time as a function of temperature permits the evaluation of the enthalpy of adsorption, and analysis of the shape of the elution curve provides information about the adsorption isotherm. 

The problem with sulfide catalysts (hydrotreatment) is to determine the active centres, which represent only part of their total surface area. Chemisorption of O2, CO and NO is used, and some attempts concern NIL, pyridine and thiophene. Static volumetric methods or dynamic methods (pulse or frontal mode) may be used, but the techniques do not seem yet reliable, due to the possible modification (oxidation) of the surface or subsurface regions by O2 or NO probe molecules or the kinetics of adsorption. CO might be more promising. Infrared spectroscopy, especially FTIR seems necessary to characterise co-ordinativcly unsaturated sites, which are essential for catalytic activity. CO and NO can also be used to identify the chemical nature of sites (sulfided, partially reduced or reduced sites). For such... 

The fast galvanostatic charging method can only be applied to the study of intermediates in the HER if the arrest due to hydrogen desorption is well separated in potential from the second arrest due to oxide formation or chemisorption of oxygen and/or H2 reoxidation. The shape of the galvanostatic pulse for platinum, exhibiting two separated arrests, is typical for most noble metals. The processes which give rise to the two separate arrests normally seen in these cases (74, 120) (Fig. 7) occur over a common potential... 

The prepared catalysts and the chemical compositions measured by atomic absorption, are listed in Table 1. Complementary characterization experiments such as hydrogen chemisorption in a pulse apparatus and temperature-programmed reduction (TPR) were performed using experimental systems and methods described in detail elsewhere [10]. 

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... 

H2 chemisorption. Metal dispersion was determined by H2 chemisorption performed with a pulse flow method (PulseChemisorb 2700, MICROMERITICS). The samples (0.3-1.0 g) were placed in a Pyrex reactor and heated from 20 to 300°C (heating rate of 10°C/min) in N2 flow (15 ml/min), treated at 300°C for 2 h in H2 flow (35 ml/min), kept under a stream of N2 for 1 hr to clean the surface and eventually cooled to 20°C in the same atmosphere. The ehemisorption experiments were performed at 20+/-1°C. Successive pulses of 86 ml of H2 were sent to the catalyst in a constant stream of N2 (15 ml/min) the time interval between successive pulses was 90 s. The total amount of adsorbed hydrogen was calculated from the difference between the saturation peak area and the area of the peak before saturation. From this amount metal dispersion parameters were calculated [10] (1) the percent of platinum present on the surface with respect to the total amount in Ae catalyst (Pt /Pt, %), (2) 4e catalyst surface covered by metal particles (Pt area, m Pt/g cat) and (3) the average diameter of the Pt particles on the catalyst surface, using a spherical model for the aggregates (d. A). 

Results of pulse chemisorption experiments are shown in Table 1. Using the standard evaluation method and a stoichiometric ratio H2/Metal=2 for both ruthenium and platinum, the active metal dispersion was calculated, and from it the mean metal particle size, L. 

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