Potential energy curve chemisorption

The minimum of the chemisorption potential energy curve (C) corresponds to the sum of the Ni and H atomic radii (y), a result of the formation of the Ni-H bonds. Fig. 2.7 also shows the atomic arrangements at the various... 

Figure 19.2 Potential energy curves for physical (II) and chemisorption (I).

Only monomolecular chemisorbed layers are possible. Chemisorption is a specific process which may require an activation energy and may, therefore, be relatively slow and not readily reversible. The nature of physical adsorption and chemisorption is illustrated by the schematic potential energy curves shown in Figure 5.2 for the adsorption of a diatomic gas X2 on a metal M. 

Figure 14 Potential energy curves for the H-graphite interaction for three cases physisorption on a rigid lattice (dotted line, filled squares), chemisorption where the lattice is allowed to relax (solid line, open circles), and chemisorption where the bonding carbon is fixed in the puckered position (dashed line, filled circles). The symbols correspond to the DFT calculations, and the lines correspond to the model PES. Taken from Ref. [90],...

Figure 1.2 Potential energy curves for the approach of a hydrogen molecule and of two hydrogen atoms to a metal surface E is the activation energy — AH is the heat of adsorption subscripts p and c are, respectively, physical adsorption and chemisorption.

Figure 33 Schematic drawing showing how the interactions of levels (bottom) can lead to a potential energy curve (top) which has a substantial barrier to chemisorption. R measures the approach of a molecule, symbolized by a single interacting electron pair, to a surface. At large R repulsive four-electron interactions dominate. At some R (second point from left), die antibonding combination crosses the Fermi level and dumps its electrons. At shorter R there is bonding.

Figure 39 Some calculated characteristics of H2 on Mg(0001), after Ref. 87. Top schematic potential energy curve. P = physisorption minimum M = chemisorbed molecule B = chemisorbed atoms A and D are transition states for chemisorption and dissociation. Bottom development of the one-electron density of states at certain characteristic points. M and M2 correspond to two molecular chemisorption points, different distances from the surface. The dashed line is the au density, moving to lower energy as the dissociation proceeds.

The activation energies of most chemisorptions are very low, sometimes even zero. The reason for this low activation energy is shown in Fig. 2.7 which illustrates the potential energy curves for both the physisorption (curve P)... 

Fig. 2. Crossed potential energy curves for physisorption and chemisorption, (a) Non-activated adsorption (b) activated adsorption.

Figure 2.23. Potential energy curves for chemisorption and physisorption.

Fig. 4.11 Schematic potential energy curves for activated and non-activated chemisorption of hydrogen on a clean metal surface and exothermic or endothermic solution in the bulk. A more pronounced minimum just below the surface allows for subsurface hydrogen (onedimensional Lennard-Jones potential, Somorjai (1987) Ref [33]).

For simple systems, the potential energy curves, activation energy and chemisorption energy can be calculated quite accurately, and satisfactory approximations are available for clean surfaces of simple and noble metals and for clean transition metal surfaces [37, 52, 111, 112]. 

FIGURE 23 One-dimensional potential energy curves for dissociative adsorption through a precursor or physisorbed state (a) adsorption into the stable state with no activation energy and (b) adsorption into the chemisorption well with activation energy A E. ... 

I This conclusion is implicit in Butler s treatment in 1936, using potential energy curves for chemisorption at electrodes. (See also Ref. 10.)... 

The adsorption of H is conveniently described in terms of simplified one dimensional potential energy curves for an H2 molecule and for 2H atoms on a metal surface (Fig. 1). Far from the surface the two curves are separated by the heat of dissociation Ed = 218 kJ/mol H. The flat minimum in the H2 + M curve corresponds to physisorbed H2 (heat of physi-sorption Ep 10 kJ/mol H) and the deep minimum in the 2H + M curve describes chemisorbed H (heat of chemisorption E 50 kJ/mol H). If the two curves intersect above the zero energy level, the chemisorption requires an activation energy E. In further steps the chemisorbed H atoms penetrate the surface and are then dissolved exothermically or endothermically in the bulk where hydrides can be formed. There is now experimental and theoretical evidence that not all chemisorbed H necessarily stays on top of the first metal atom layer, but also below it as a so called subsurface H (two step chemisorption). 

Fig. 4.1 Potential energy curves for (7) physical and (2) chemical adsorption (a) non-activated (b) activated. Epot - potential energy, Qc - heats of chemisorption, Qp - heats of physisorption, Ead -energy of activation for desorption, Ediss - dissociation energy for the diatomic molecule. The sum AEdes = Ead + Qc is the the heat of hemisorption, in the activated processes [8]...

For chemisorption, the potential energy curve is dominated by a much deeper chemisorption minimum at shorter values of d, which is the distance between the surface and adsorbed species (Fig. 2.31). 

The transition from physical adsorption to chemisorption occurs at point A. The potential energy at A is in excess of that for the adsorbate and the adsorbent when separated and represents the activation energy required for chemisorption, A fl. If curve I resided more to the right or curve II more to the left, then the transition from physical to chemical adsorption would occur with no activation energy since the crossover point would reside beneath zero potential energy. 

In these cases, the standard free energy of adsorption can be obtained from the equilibrium condition and is a simple exponential function of the potential which does not depend significantly on the charge distribution at the interface for an uncharged adsorbate. The chemisorption thus corresponds to a vertical shift in the free energy curves as depicted in Fig. 12 and affects the energy of activation [76]. 

The difference between physisorption and chemisorption can be explained using a potential-energy diagram. The potential-energy diagram for physisorption and chemisorption of an A-A molecule (e.g., H2) is shown in Figure 10.1. Curve P in... 

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