*3.2.1. Effect of the reduction temperature*

**Figure 3** presents 1-heptyne total conversion and selectivity to 1-heptene as a function of time for Pd(5%)/Al catalyst pretreated for 1 h in a hydrogen flow at 373 and 573 K, the reaction temperature was 303 K. In the figure, it can be seen that the total conversion increases as the reduction temperature is increased, while the selectivity is slightly lower at the higher reduction temperature (≥90%). From the TOF values displayed in **Table 2**, it may be concluded that the catalyst reduced at higher temperature is nearly twice more active than when it is reduced at 373 K. The activity results can be explained taking into account the electronic state of Pd in each catalyst: the more electron-deficient palladium species the less active is the catalyst for the hydrogenation of 1-heptyne. It is probably that the Pdδ+ species inhibit the interaction between the metal and 1-heptene due to an electronic effect, decreasing its electrondonor character. Therefore, it can say that the presence of electron-deficient Pdδ+OxCly species improves the selectivity to the desired product. Given that the chlorine is not completely removed from the Pd(5%)/Al catalyst after the heat treatments, it seems likely that the role of the remaining chlorine on the catalyst surface could be to stabilize the positively charged palladium structures, resulting in less active but more selective catalysts (electronic and steric factors). Therefore, our results suggest a correlation between the reduction temperature, electron deficiency of palladium species and chloride content, with total conversion and selectivity. In total accordance with our results, an increase in selectivity was found by some authors [38] when smaller and more electron-deficient Pd clusters were used. Other authors [39] found that 1-heptene is more weakly adsorbed than 1-heptyne on electron-deficient Pd species and, once formed, the 1-heptene molecules are more easily desorbed than 1-heptyne. This effect was previously found for other partial hydrogenation reactions using monometallic Ru-supported catalysts [40].

**Figure 3.** *Effect of reduction temperature*: Total conversion of 1-heptyne and selectivity to 1-heptene as a function of time for Pd(5%)/Al catalyst: reduced at 573 K (filled symbols) and reduced at 373 K (opened symbols). Reaction temperature: 303 K.

According to the obtained results, 573 K was selected as the reduction temperature of the monometallic catalysts because higher values of total conversions were obtained.
