The e
ffect o
f Zn addition on the Pd turnover rate (TOR)
for the water–gas shi
ft (WGS) reaction was studied on 2 wt%Pd on alumina with Zn content varying
from 0 to 19%Zn by weight. Over 85%o
f the Zn reacted with alumina to
form zinc aluminate with a
fraction reacting with Pd. The average Pd
![single <font color=](http://www.sciencedirect.com/scidirimg/entities/sbnd)
bond" title="single
bond" border="0">Zn coordination number as measured by extended X-ray absorption
fine structure (EXAFS) increased monotonically
from 0 to 4.5 as the Zn loading on alumina increased. At a loading o
f 2%Pd and 19%Zn on alumina the molar
fraction o
f Zn in the Pd particles was 0.34, and the turnover rate increased by a
factor o
f 13, as compared with the pure metallic Pd supported on alumina, to
f="/science?_ob=MathURL&_method=retrieve&_udi=B6WHJ-4SK0C4K-1&_mathId=mml13&_user=1067359&_cdi=6852&_rdoc=7&_acct=C000050221&_version=1&_userid=10&md5=7cdad2b7185a302a63d1cf09f1e80f22">![View the MathML source View the MathML source](http://www.sciencedirect.com/cache/MiamiImageURL/B6WHJ-4SK0C4K-1-X/0?wchp=dGLbVzb-zSkWb)
(mol exposed Pd)
−1 at 280 °C, 6.8%CO, 8.5%CO
2, 21.9%H
2O, and 37.4%H
2 at 1 atm. The
formation o
f the ordered 1:1 PdZn intermetallic compound by deposition o
f Pd on zinc oxide increased the average Pd
![single <font color=](http://www.sciencedirect.com/scidirimg/entities/sbnd)
bond" title="single
bond" border="0">Zn coordination number to 5.9 and the WGS turnover rate by a
factor o
f 20 to
f="/science?_ob=MathURL&_method=retrieve&_udi=B6WHJ-4SK0C4K-1&_mathId=mml14&_user=1067359&_cdi=6852&_rdoc=7&_acct=C000050221&_version=1&_userid=10&md5=8278dba02efe116e91005f2bb1e3711a">![View the MathML source View the MathML source](http://www.sciencedirect.com/cache/MiamiImageURL/B6WHJ-4SK0C4K-1-19/0?wchp=dGLbVzb-zSkWb)
(mol exposed Pd)
−1. The
bonding o
f adsorbed CO changed
from 80%bridging on pure Pd to 90%linear on the 1:1 PdZn intermetallic compound as determined by di
ffuse re
flectance in
frared spectroscopy per
formed under WGS conditions. The
fraction o
f CO binding linearly to Pd correlated linearly with the WGS rate per Pd on the sur
face. Because the dispersion o
f the samples was about the same, this result implies that the number o
f active sites can be counted by the
fraction o
f CO binding linearly. A simple random substitution model
for PdZn distribution was used to estimate the concentration o
f Pd and Zn on the sur
face, based on the coordination numbers measured by EXAFS. The correlation o
f the reaction rate with the types o
f sites on catalysts with six di
fferent Pd
![single <font color=](http://www.sciencedirect.com/scidirimg/entities/sbnd)
bond" title="single
bond" border="0">Zn compositions shows that the data is consistent with an active site composed o
f sur
face Pd atoms with sur
face coordination to 2 Pd and 4 Zn atoms, which is the coordination expected
for the (111) plane o
f the 1:1 PdZn intermetallic compound. An alternate explanation
for the promotional e
ffect o
f Zn on Pd/alumina is that the
formation o
f zinc aluminate breaks a deleterious Pd–alumina interaction.