On the effect of hydrogen on the palladium-catalyzed formation of benzene from acetylene

An infrared spectrum of a Pd(111) surface collected in the presence of 5 To rr of acetylene as a function of hydrogen pressure reveals that the ethylid yne coverage increases with hydrogen pressure (P(H-2) between zero and 20 T orr). The amount of CO that can be accommodated onto the surface at a press ure of 5 Torr, measured after evacuating the acetylene and hydrogen, increa ses linearly with hydrogen pressure, and this effect is ascribed to the pre sence of a more open surface produced by the formation of ethylidyne. It is found that acetylene adsorbs in ultrahigh vacuum on ethylidyne-covered Pd( 111) and reacts to form benzene, where the benzene desorbs at similar to 28 0 K. This effect is mirrored in the catalytic chemistry where the rate of b enzene formation from acetylene in the presence of hydrogen increases linea rly with hydrogen pressure.