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Surface adsorption in strontium chloride ammines
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Image of FIG. 1.
FIG. 1.

Sketch of the experimental setup. The chamber volumes are specified in the text.

Image of FIG. 2.
FIG. 2.

Relationship between logarithms of the absorption rate in mol(NH3)/mol(SrCl2)/s and the relative pressure. The saturation degree in the displayed measurements is s = 0.3.

Image of FIG. 3.
FIG. 3.

TGA measurement showing release of ammonia prior to the onset of bulk desorption. This is interpreted as desorption from the surface.

Image of FIG. 4.
FIG. 4.

Van 't Hoff plot for the adsorbed state on the surface and bulk state. The data points for the adsorbed state are the pressures p and temperatures T desorption of Table I . The fit to data yields ΔH surf = 39.2 kJ/mol and ΔS surf = 228 J/(mol K). For the bulk state, the plotted line is based on the literature value of ΔH 8-1 = 41.4 kJ/mol and ΔS 8-1 = 228.8 J/(mol K). 1

Image of FIG. 5.
FIG. 5.

View of the surface atomic structures as calculated with DFT. Green: Cl, blue: N, white: H and Sr is in the center of the yellow coordination polyhedra. The adsorbed NH3 are indicated by having grey hydrogen. The black line indicates the calculational unit cell. (a) The mono-ammine (010) surface with full coverage. The unit cell indicated is 4.69 × 7.72 Å2. (b) The di-ammine (001) surface with half coverage. The unit cell is 6.23 × 8.45 Å2. (c) Top view of the octa-ammine (100) surface with full coverage. Dark and light grey hydrogen indicates θ = 0.5 and θ = 1.0, respectively. The NH3 molecule with black hydrogen is weakly bound as explained in the text. The unit cell indicated is 7.51 × 15.4 Å2. (d) Side view of the octa-ammine surface structure. Half of the calculational vacuum between layers is indicated with the unit cell.

Image of FIG. 6.
FIG. 6.

Model describing absorption of ammonia through a stable adsorbed state.

Image of FIG. 7.
FIG. 7.

The desorption measurements at 235 K with blue circles, 277 K green diamonds, 306 K with red squares and 328 K with cyan triangles. The full lines represent the simple microkinetic model at the corresponding temperature.


Generic image for table
Table I.

List of TGA experiments: For each experiment the pressure, the temperature interval, and the temperature, T desorption , at which desorption from the surface begins, are listed. The asterisk denotes the dataset plotted in Figure 3 .

Generic image for table
Table II.

Energies of the adsorbed NH3 molecules in half of the stable sites (θ = 0.5) and all of the stable sites (θ = 1.0).

Generic image for table
Table III.

Parameters used for the microkinetic model.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Surface adsorption in strontium chloride ammines