Studying the Surface Chemistry of Catalytic Reactions at Realistic Pressures:<br>Silver Catalyzed Epoxidation of Ethylene

D. J. Stacchiola and W. T. Tysoe
Department of Chemistry
Laboratory for Surface Studies
University of Wisconsin-Milwaukee


Contents


<ul>
<li><a href=#intro>Introduction</a>
<li><a href=#s1>Ethylene & Ethylene Oxide on Ag(111) Ultra High Vacuum (UHV)</a>
<li><a href=#s2>Reaction at High-Pressure (1 Torr)</a>
<li><a href=#s3>Identification of Surface Species - Oxametallacycle</a>
<li><a href=#s4>Identification of Surface Species - Acetaldehyde</a>
<li><a href=#con>Conclusions</a>
</ul>

<hr>
<a name=intro>
<font size="+1"><b>Introduction</b></font></a>

<ul>
<li>Silver is a uniquely effective catalyst for the <i>heterogeneous selective oxidation of olefines to epoxides</i>. Although the subject has been extensively studied, a detailed mechanism, including intermediates or transition states, remain unsolved.<br><br>
<li>In the present work, Reflection-Absorption Infrared Spectroscopy (<i>RAIRS</i>) and Temperature-Program Desorption (<i>TPD</i>) are used to scrutinize surface species involved in the epoxidation of ethylene. The overall behavior of this reaction is:<br>

<img src=intro.gif width=255 height=75 border=0 alt="Epoxidation of Ethylene Scheme" align=center>
</ul><hr>

<br clear="all">
<b>Background</b>

<p>R. Grant and R. Lambert, <i>J. of Catal.</i>, <b>92</b> (1985) 364<br>
<img src=graph1.gif width=600 height=335 border=3 alt="Grant and Lambert" align=center><br><br>
<b>TPR of C<sub>2</sub>H<sub>4</sub> oxidation on active Ag(111)</b><br>
<img src=graph2.gif width=500 height=85 border=3 alt="C2H4 Oxidation" align=center><br>
<hr>

<b>Reflection/Absorption Infrared Spectroscopy (RAIRS)</b>

<img src=graph3.gif width=500 height=156 border=0 alt="R/A Diagram" align=center><br>
<p>Only p-polarized radiation is absorbed, &theta; = 80&deg;<br><br><br>
<img src=graph4.jpg width=600 height=225 border=1 alt="Lab Photo" align=center><br>

<hr>

<a name=s1>
<b><font size="+1">Ethylene & Ethylene Oxide on Ag(111) Ultra High Vacuum (UHV)</font></b>
</a>
<b>Reflection/Absorption Infrared Spectroscopy (RAIRS)</b><br>
<img src=graph5.gif width=300 height=400 border=3 alt="RAIRS" align=center><br>
<hr>

<b>RAIRS of C<sub>2</sub>H<sub>4</sub><br>UHV</b><br>
<img src=graph6.gif width=475 height=412 border=3 alt="RAIRS of C2H4" align=center><br>
<hr>

<b>RAIRS of C<sub>2</sub>H<sub>4</sub>O<br>UHV</b><br>
<img src=graph7.gif width=450 height=390 border=3 alt="RAIRS of C2H4O" align=center><br>


<hr>

<a name=s2>
<b><font size="+1">Reaction at High-Pressure (1 Torr)</font></b>
</a>


<b>RAIRS of 1 Torr C<sub>2</sub>H<sub>4</sub>/O/Ag(111)<br>High Pressure</b><br>
<img src=graph8.gif width=450 height=395 border=3 alt="RAIRS of C2H4/O/Ag(111)" align=center><br>
<hr>

<b>RAIRS of 1 Torr C<sub>2</sub>H<sub>4</sub>O/O/Ag(111)<br>High Pressure</b><br>
<img src=graph9.gif width=450 height=400 border=3 alt="RAIRS of C2H4O/O/Ag(111)" align=center><br>
<hr>

<b>RAIRS of 1 Torr C<sub>2</sub>D<sub>4</sub>/O/Ag(111)<br>High Pressure</b><br>
<img src=graph10.gif width=450 height=400 border=3 alt="RAIRS of C2D4/O/Ag(111)" align=center><br>
<hr>

<b>RAIRS of 1 Torr C<sub>2</sub>H<sub>4</sub>/O<sup>18</sup>/Ag(111)<br>High Pressure</b><br>
<img src=graph11.gif width=450 height=430 border=3 alt="RAIRS of C2H4/O18/Ag(111)" align=center><br>


<hr>

<a name=s3>
<b><font size="+1">Identification of Surface Species Oxametallacycle</font></b>
</a>

<b>TPD of 3L ICH<sub>2</sub>CH<sub>2</sub>OH/Ag(111)</b><br>
<img src=graph12.gif width=350 height=450 border=3 alt="TPD of 3L ICH2CH2OH/Ag(111)" align=center><br>
<hr>

<b>Relative Molar Yield <i>vs.</i><br>Iodoethanol Exposure</b><br>
<img src=graph13.gif width=350 height=460 border=3 alt="Relative Molar Yield vs Iodoethanol Exposure" align=center><br>
<hr>

<b>RAIRS of ICH<sub>2</sub>CH<sub>2</sub>OH/Ag(111)<br>Dose at 185&deg; K</b><br>
<img src=graph14.gif width=330 height=460 border=3 alt="RAIRS of ICH2CH2OH/Ag(111) at 185K" align=center><br>
<hr>

<b>Reaction Scheme for ICH<sub>2</sub>CH<sub>2</sub>OH<br>Adsorbed on Ag(111)</b><br>
<img src=graph15.gif width=450 height=130 border=0 alt="Reaction Scheme for ICH2CH2OH Adsorbed on Ag(111)" align=center><br>

<hr>

<a name=s4>
<b><font size="+1">Identification of Surface Species Acetaldehyde</font></b>
</a>

<table width=100% border=0><tr>
<td align=center>
<b>TPD of CH<sub>3</sub>CHO/Ag(111)<br>29 amu</b><br>
<img src=graph16.gif width=325 height=450 border=3 alt="TPD of CH3CHO/Ag(111) - 29 amu" align=center></td>

<td align=center>
<b>RAIRS of<br>CH<sub>3</sub>CHO/Ag(111)<br>Dose at 80&deg;K</b><br>
<img src=graph17.gif width=335 height=450 border=3 alt="RAIRS of CH3CHO/Ag(111)-Dose at 80K" align=center></td>
</tr></table>

<hr>

<a name=con>
<b><font size="+1">Conclusions</font></b>
</a>

<b>RAIRS Assignments of Species A</b><br>
<img src=graph18.gif width=650 height=250 border=0 alt="RAIRS Assignments of Species A" align=center></td>

<b>RAIRS Assignments of Species B</b><br>
<img src=graph19.gif width=650 height=235 border=0 alt="RAIRS Assignments of Species B" align=center>