T
he effect of Zn addition on t
he Pd turnover rate (TOR) for t
he water–gas s
hift (WGS) reaction was studied on 2 wt%Pd on alumina wit
h Zn content varying from 0 to 19%Zn by weig
ht. Over 85%of t
he Zn reacted wit
h alumina to form zinc aluminate wit
h a fraction reacting wit
h Pd. T
he average Pd
![](<font color=)
http://www.sciencedirect.com/scidirimg/entities/sbnd" alt="single 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 t
he Zn loading on alumina increased. At a loading of 2%Pd and 19%Zn on alumina t
he molar fraction of Zn in t
he Pd particles was 0.34, and t
he turnover rate increased by a factor of 13, as compared wit
h t
he pure metallic Pd supported on alumina, to
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−1 at 280 °C, 6.8%CO, 8.5%CO
2, 21.9%H
2O, and 37.4%H
2 at 1 atm. T
he formation of t
he ordered 1:1 PdZn intermetallic compound by deposition of Pd on zinc oxide increased t
he average Pd
![](<font color=)
http://www.sciencedirect.com/scidirimg/entities/sbnd" alt="single bond" title="single bond" border="0">Zn coordination number to 5.9 and t
he WGS turnover rate by a factor of 20 to
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−1. T
he bonding of adsorbed CO c
hanged from 80%bridging on pure Pd to 90%linear on t
he 1:1 PdZn intermetallic compound as determined by diffuse reflectance infrared spectroscopy performed under WGS conditions. T
he fraction of CO binding linearly to Pd correlated linearly wit
h t
he WGS rate per Pd on t
he surface. Because t
he dispersion of t
he samples was about t
he same, t
his result implies t
hat t
he number of active sites can be counted by t
he fraction of CO binding linearly. A simple random substitution model for PdZn distribution was used to estimate t
he concentration of Pd and Zn on t
he surface, based on t
he coordination numbers measured by EXAFS. T
he correlation of t
he reaction rate wit
h t
he types of sites on catalysts wit
h six different Pd
![](<font color=)
http://www.sciencedirect.com/scidirimg/entities/sbnd" alt="single bond" title="single bond" border="0">Zn compositions s
hows t
hat t
he data is consistent wit
h an active site composed of surface Pd atoms wit
h surface coordination to 2 Pd and 4 Zn atoms, w
hic
h is t
he coordination expected for t
he (111) plane of t
he 1:1 PdZn intermetallic compound. An alternate explanation for t
he promotional effect of Zn on Pd/alumina is t
hat t
he formation of zinc aluminate breaks a deleterious Pd–alumina interaction.