Surface reaction with hydrogen

J.G. McCarty, P.Y. Hou, D. Sheridan, and H. Wise, Reactivity of Surface Carbon on Nickel Catalysts Temperature-Programmed Surface Reaction with Hydrogen and Water, in Coke Formation on Metal Surfaces, eds. L.G. Albright and R.T.K. Baker, American Chemical Society, Washington D.C., 1982, p. 253. 

Reactivity of Surface Carbon on Nickel Catalysts Temperature-Programmed Surface Reaction with Hydrogen and Water... 

FIGURE 1. The fractional abundances of certain species included in the model are shown as a fraction of time. The sharp fall off in the collapse phase abundances is due to accretion onto the dust as the density rises rapidly towards the end of a free-fall collapse. The grains are heated following the formation of a nearby star and the grain mantles evaporate, returning material to the gas phase. The importance of surface reactions with hydrogen atoms is clearly seen in the enhancement of the H,0 abundance. 

JG McCarty, PV Hou, D Sheridan, H Wise. Reactivity of surface carbon on nickel catalysts temperature-programmed surface reaction with hydrogen and water. In LF Albright, RT Baker, eds. Coke formation on metal surfaces. Washington, DC American Chemical Society, 1982. 

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

About 70% of the western world s supply of nickel comes from iron and nickel sulfide ores that were brought close to the surface nearly 2 billion years ago by the violent impact of a huge meteor at Sudbury, Ontario. The ore is first roasted (heated in air) to form nickel(II) oxide, which is reduced to the metal either elec-trolytically or by reaction with hydrogen gas in the first step of the Mond process ... 

The autocatalytic reaction mechanism apparent at low temperatures is expected to apply to catalytic hydrogen oxidation at high pressures. In addition, the above study is the first to use STM to observe the formation of dynamic surface patterns at the mesoscopic level, which had previously been observed by other imaging techniques in surface reactions with nonlinear kinetics [57]. This study illustrates the ability of in situ STM to visualize reaction intermediates and to reveal the reaction pathway with atomic resolution. 

Catalytic reactions of methanol on an Mo(112)-(lX2)-0 surface under a constant flow of CH3OH and 02 (10 6—10 5 Pa) were monitored as a function of reaction time by the temperature-jump method. Total amounts of the products are summarized in Table 8.3. When only CH3OH was fed, the reaction rate exponentially decayed with reaction time. After the reaction ceased in both conditions, the surfaces were covered with nearly 1 ML of C(a) (Table 8.3) and the sharp (1X2) LEED subspots of the surface before the reaction almost disappeared due to an increase in background intensity. As shown in Table 8.3, the selectivity of the reaction at 560 K is similar to that obtained by TPR (Table 8.2). The C(a) species formed with 26% selectivity cover the surface, resulting in the exponential decay of the reaction rate. O(a) species are also formed on the surface but they are desorbed as H20 by reaction with hydrogen atoms. It should be noted that neither C(a) nor a small amount of O(a) change the selectivity in this case. 

Examples 2-10 are for the hydrogenation of CO to produce CH4. For Example 2, the order and calculated log L values suggest that Step 1 or 6 for hydrogen is the rate-determining step. If it is Step 1, the rate-determining step is the adsorption of H2 on a CO-saturated surface. If it is Step 6, it is a surface reaction between hydrogen and CO, where the surface is saturated with CO but the amount of hydrogen adsorbed corresponds to the linear part of the adsorption isotherm. 

Since group 4 derived species are of particular interest as catalysts for olefin polymerization and epoxidation reactions, the thermal stability of surface metal-alkyl species, as weU as their reactivity towards water, alcohols and water, deserve some attention. On the other hand, mono(siloxy) metaUiydrocarbyl species can be converted into bis- or tris(siloxy)metal hydrides by reaction with hydrogen [16, 41, 46-48]. Such species are less susceptible to leaching and can be used as pre-catalysts for the hydrogenolysis of C-C bonds, alkane metathesis and, eventually, for epoxidation and other reactions. 

The metal forms its oxide, AmO on its surface in contact with air or oxygen. Simdarly, reaction with hydrogen forms the hydride, AmH2. 

Fig. 26. STM images of the oxygen pre-covered platinum(l 1 1) surface during reaction with hydrogen. Images were recorded at a temperature of T = 111 K with a time interval of 625 K. The white ring in the upper right corner is associated with a reaction front of OH intermediates from the autocatalytic reaction. The outside is characterized by an oxygen-terminated surface, whereas water molecules from the reaction are identified inside the ring. Adapted with permission from Reference (757).

There are no real thermodynamic limits in the removal of sulfur from any organic sulfur compound by reaction with hydrogen (1, 2, 5). There are, however, limits on the overall rates of conversion that may be achieved by increasing the temperature of the reaction. A classic limitation in rates is the result of the inverse relationship between adsorption on a catalytic surface and temperature. This may be a problem with dialkyldibenzothio-phenes, which have steric limitations for adsorption. 

C-Tracer studies of acetylene adsorption on alumina- and silica-sup-ported palladium [53,65], platinum [66] and rhodium [53] show the coexistence of at least two adsorbed states, one of which is retained on the surface, the other being reactive undergoing molecular exchange and reaction with hydrogen. Acetylene adsorption exhibits the same general characteristics as those observed with ethylene (see Sect. 3.2). However, there are important differences. The extent of adsorption and retention is substantially greater with acetylene than with ethylene. Furthermore, the amounts of acetylene retained by clean and ethylene-precovered sur-... 

In order to reveal the nature of deactivation, the potential of the catalyst slurry was continuously measured during the partial oxidation of alcohols. Cyclic voltammetric measurements [16] were also performed in the same aqueous alkaline solution with model (unsupported) catalysts for the interpretation of the potential values. The experiments revealed that the oxidation of alcohols may be divided into three groups. The basis of classifying is the oxidation state of proroot-ed catalyst and its surface coverage with hydrogen or oxygen (OH) during reaction. 

The picture that emerges is that the bonding within the majority of thiophene molecules adsorbed on the catalyst surfaces is hardly perturbed, and this contrasts sharply with the situation in the thiophene complexes. The thiophene molecule parallel to the surface does not correspond to a metal f/ -bound thiophene. Rather, it is suggestive of a weakly chemisorbed precursor state of thiophene that lies parallel to the surface. In this state, the molecule interacts indiscriminately with the alumina, the basal or edge planes, or both. Moreover, the weakness of this binding enhances the surface mobility of thiophene and allows molecules to move across the surface to the catalytic site for reaction with hydrogen atoms. The few sulfur-bound thiophene molecules, no more than 5-10%, would then correspond to thiophene at the coordinatively unsaturated Mo (or Co) atoms. 

---