Chemisorption hydrogen

Cremaschi P and Whitten J L 1987 The effect of hydrogen chemisorption on titanium surface bonding Theor. Chim. Acta. 72 485-96... 

Hydrogen gas chemisorbs on the surface of many metals in an important step for many catalytic reactions. A method for estimating the heat of hydrogen chemisorption on transition metals has been developed (67). These values and metal—hydrogen bond energies for 21 transition metals are available (67). 

F1 NMR of chemisorbed hydrogen can also be used for the study of alloys. For example, in mixed Pt-Pd nanoparticles in NaY zeolite comparaison of the results of hydrogen chemisorption and F1 NMR with the formation energy of the alloy indicates that the alloy with platinum concentration of 40% has the most stable metal-metal bonds. The highest stability of the particles and a lowest reactivity of the metal surface are due to a strong alloying effect. 

Standard Test Methodfor Surface Area of Catalysts. (D3663—78) Standard Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts. (D3908-80) American Society for Testing and Materials (ASTM), Philadelphia, PA. 

Table III. Hydrogen Chemisorption as an Indicator of Catalytically Active Area...

Hydrogen chemisorption Static H2 chemisorption at 100°C on the reduced cobalt catalysts was used to determine the number of reduced surface cobalt metal atoms. This is related to the overall activity of the catalysts during CO hydrogenation. Gas volumetric chemisorption at 100°C was performed using the method described by Reuel and Bartholomew [6]. The experiment was performed in a Micromeritics ASAP 2010 using ASAP 2010C V3.00 software. 

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

In conclusion, XPS is among the most frequently used techniques in characterizing catalysts. It readily provides the composition of the surface region and also reveals information on both the oxidation state of metals and the electronegativity of any ligands. XPS can also provide insight into the dispersion of particles over supports, vrhich is particularly useful if the more common techniques employed for this purpose, such as electron microscopy or hydrogen chemisorption, can not discriminate between support and active phase. 

The role of the support on hydrogen chemisorption on supported rhodium catalysts was studied using static and frequency response techniques. In all Instances, several klnetlcally distinct H2 cheml-sorptlve sites were observed. On the basis of the kinetics, at least one site appears to sorb H2 molecularly at temperatures below 150°C, regardless of the support. At higher temperatures, a dissociative mechanism may become dominant. Inducement of the SMSI state In Rh/T102 does not significantly alter Its equilibrium H2 chemisorption. 

Similar SSIMS and TDS results were obtained for rhodium on tltanla and fiir hydrogen chemisorption on both substrates. In a blank experiment Involving i o metal over layer, temperature programming while following the T1 and TIO SIMS signals (Fig. 4) shows that the tltanla thin film does not begin to change until the temperature reaches about 760 K, well beyond the 615 K where Tl was first noted to Increase on the systems with thin metal overlayers. ... 

In a series of studies of carefully prepared catalysts of Pt on silica gel (7,10-12) we have shown that the Pt particles are equi-axed, (and de-finitely not cuboidal as is often assumed) that the size (or percent metal exposed) agrees with results from hydrogen chemisorption, and that the particles are free of microstrain faults or twins, except when the average size is similar to the pore size of the support. In this latter case, the particles are elongated, and there is microstrain, probably due to differ-... 

The amount of coke formed as a function of the number of turnovers is shown in Fig. 2. The steeper slopes of these curves for Pt/y-Al203 and Pt/Ti02 indicate the higher selectivity of Pt/y-AI2O3 and Pt/TiOj to form coke than Pt/ZrOj Hydrogen chemisorption capacity decreased nwkedly after some time on stream (see Table 2), but could be completely restored by oxidative treatment. 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

Table 3.9. shows the results of barometric hydrogen chemisorption on Cu/AlaOj, Ni/A Oj, Pt/Al203 and a Ni on alumina-washcoated monolith. 

Newns DM. 1969. Self consistent model of hydrogen chemisorption. Phys Rev 178 ... 

Medvedev IG. 2004. To a theory of electrocatalysis for the hydrogen evolution reaction The hydrogen chemisorption energy on the transition metal alloys within the Anderson-Newns model. Russ J Electrochem 40 1123-1131. 

The Pt dispersion of the fresh samples was measured by dynamic hydrogen chemisorption by using a temperature-programmed desorption (TPD)/R/0 1100 ThermoFisher... 

Highly mesoporous carbon supported Pd catalysts were prepared using sodium formate and hydrogen for the reduction of the catalyst precursors. These catalysts were tested in the enantioselective hydrogenation of isophorone and of 2-benzylidene-l-benzosuberone. The support and the catalysts were characterized by different methods such as nitrogen adsorption, hydrogen chemisorption, SEM, XPS and TPD. 

The dispersion of the catalyst was measured by hydrogen chemisorption at 27 °C in the Coulter Omnisorp 100 CX equipment. The catalysts were subjected to an in situ reduction under hydrogen at 350 °C for 1 h before chemisorption... 

Hydrogen chemisorption has frequently been used for catalyst characterization. Considerable detailed information is available for hydrogen... 

Copper clusters, as reported by the Rice group(lc), do not react with hydrogen. Hydrogen chemisorption on copper surfaces is also an activated process. Surface beam scattering experiments place this barrier between 4-7 kcal/mole(33). This large value is consistent with the activated nature oT hydrogen chemisorption on metal clusters, and the trend toward bulk behavior for relatively small clusters (>25 atoms in size). 

For niobium and cobalt clusters structures have been proposed based upon the elements behavi or (71). Niobium s specific inertness has been associated with structures that are analogous to close-packed surface of W(110) which also has an activation barrier for hydrogen chemisorption. Since the IPs are also expected to be higher for closed packed structures these two sets of observations are in agreement. This model at its current stage of development requires different structures for each system and as yet has not been useful in making predictions. 

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