H2/M ratios

The area under a TPR or TPO curve represents the total hydrogen consumption, and is commonly expressed in moles of H2 consumed per mole of metal atoms (H2/M). The ratios of almost 1.5 for rhodium in the right panel of Figure 2.4 indicate that rhodium was present as RI12O3. For iron, the H2/M ratios are significantly lower, indicating that this metal is only partially reduced. 

XPS also provides evidence that, at its endpoint, the reaction of metal ions in LB films with H2S is not stoichiometric as depicted in Eq. (4). For example, XPS analysis for a number of MBe films (M = Cd or Hg) to H2S for 1 h gave an average S M ratio of 0.76 instead of 1 as predicted by Eq. (4). Both QCM and UV/visible absorbance measurements indicate that 1 h of H2S exposure is more than enough for the reaction to reach its endpoint. In another XPS investigation of films of calixerenes containing Cd2+ ions, S Cd ratios of 0.84 0.1, on average, were obtained (37). 

Figure 1. The effects of a series of Mo/AljOj catalysts with varying Mo loading (0-10%(m/m)) on the coke selectivity and the hydrodesulfurization activity. The HDS activity has been expressed as a second-order rate constant ( hds)< Process conditions temperature 450 C, pressure 30 bar, H2/oil ratio 900 Nl/kg, WHSV 1.3 kg/(kg h) and run length 180 h.

Further problems can arise because of uncertainties concerning the stoichiometry of the adsorption reaction. For most metals it is assumed that the surface stoichiometry with H2 is H/M = 1. However, there is evidence especially for very small metal particles (of the order of 1 -5 nm) that the stoichiometry can exceed H/M = 1. For quantitative measurements of surface area it is necessary to establish the chemisorption stoichiometry and structure. In practice it is usually possible to achieve approximate estimate of the surface area by some other independent method (for example, from particle size analysis by X-ray line broadening or by TEM). In the case of CO, the CO/M ratio is generally taken as 1.0, but the true value may depend on the particle size and on the particle morphology. With N2O the N2O/M ratio at monolayer coverage is usually assumed to be 0.5, but once again there is no certainty about the validity of this particular assumption. 

Year Catalyst Syngas pressure fbar) H2/CO ratio Temp. rc> Time (h> Methanol conv. m Ethanol selectivity (%) Ref. 

The regioselectivity of HRh(CO)(PPh3)2 is strongly related to the concentration of PPhs in solution (up to a certain point) and the H2/CO ratio used. Coimnercial hydroformylation reactions are run using solutions that have PPhs concentrations of 0.3 M or higher. This corresponds to PPhs weight percentages of 8 50% of the total solution in coimnercial reactors. The effect of PPhs concentration on the rate and selectivity for the hydroformylation of 1-hexene can be seen in Table 2. 

Table 2 Equilibrium H2 and CO Uptakes over the M/Ti02(D) Catalysts Expressed as H/M and CO/M Ratios...

Sample pmol H2/ g T <950K Total H/M ratios Corrected H/M ratios... 

Of course, in normal circumstances a variation of a factor of four in the metal dispersion would not seem unusual. However, in the context of SMSI, since H/M ratios represent the evidence that SMSI exist, it is critically important to know whether the observed range of H/M ratios are due to real differences in the particle size or to the suppression of H2 adsorption. No clear answer can be given, but it is perhaps significant that Haller et al. ... 

Fe, Co, and Ni on T1O2. Table 6 summarizes the information available on the chemisorption of H2 and CO on Fe, Co, and Ni. The results for Fe and Co are very sparse. It appears that even a mild reduction at 723 K is sufficient to suppress the adsorption of H2. However, these results should be treated with caution for, as will be seen later, there is evidence that Fe can migrate into the support at fairly low temperatures. It is possible, therefore, that for Fe the very low H/M ratios indicate that there is only a small fraction of the Fe present on the surface of the support. 

Structural Information on TMO-supported Catalysts X-Ray Measurements. - The chemisorption results show that, for most metals on most supports, reduction at temperatures above about 700 K leads to a dramatic suppression of the adsorption of H2 and CO. It is natural to enquire why this should be, and to try to eliminate the obvious, trivial explanations, such as sintering, etc. Contamination, encapsulation, etc., have been considered and rejected (but see earlier discussion of Pd). For the Pt metals. X-ray diffraction lines due to metal particles are invariably absent, thus indicating that the average particle size must be less than about 5 nm. Consequently, the low H/M ratios cannot be due to sintering. This is supported by the fact that in some cases the adsorption of O2 at room temperature is sufficient to restore the normal H2 chemisorption capacity. 

---