After carbon, hydrogen, and oxygen, nitrogen is the next most abundant element in the human body. Inorganic and organic compounds of nitrogen feature prominently in many biological and environmental, as well as industrial, processes. In nature, the inorganic compounds of nitrogen are controlled by a reaction cycle called the nitrogen cycle (Figure 1).1 The denitrification pathway in this cycle, that is, the conversion of nitrate to dinitrogen, is employed by certain bacteria to produce ATP anaerobically and gain energy for cell growth. All organisms use ammonia as one of the starting building blocks for the synthesis of amino acids, nucleotides, and many other important biological compounds. Nitric oxide is * Corresponding author. E-mail: m.koper@chem.leidenuniv.nl. † Present address: Energy research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands. ‡ Present address: Research, Development & Innovation, AkzoNobel Chemicals bv, Velperweg 76, P.O. Box 9300, 6800 SB Arnhem, The Netherlands. Chem. Rev. 2009, 109, 2209–2244 2209

Nitrogen cycle electrocatalysis.

Results of the last decade with respect to the selective hydrogenation of hydrocarbons with multiple unsaturation (dienes and alkynes) over heterogeneous palladium catalysts are reviewed. Factors such as metal dispersion, carbon deposits, and the use of promoters and additives controlling catalytic activities and chemo-, regio- and stereo-selectivity are discussed. A detailed treatment of the status of the selective removal of hydrocarbon impurities with multiple unsaturation from industrial feedstocks is also given.

Hydrogenation of carbon–carbon multiple bonds: chemo-, regio- and stereo-selectivity

Developments in the last three decades of kinetics of selective hydrogenation of ethyne in ethene‐rich streams on palladium catalysts are reviewed. Most of the studies can be described comprehensively by a model that assumes carbonaceous deposits (i) create irreversibly on the palladium surface small A types of active site (selective to ethene) and large E types of active site (selective to ethane), and (ii) are involved in hydrogenation of ethene on E s sites on the support. The relative importance of these sites, with varying (i) reaction conditions, (ii) palladium dispersion, (iii) process modifiers, and (iv) promoters, is discussed.

Selective Hydrogenation of Ethyne in Ethene‐Rich Streams on Palladium Catalysts, Part 2: Steady‐State Kinetics and Effects of Palladium Particle Size, Carbon Monoxide, and Promoters

An overview about the effect of co-metal on the performance of Pd in bimetallic catalysts is presented. In the first part, the promotion of monometallic catalysts by addition of a second metal is described from a general point of view in terms of: (i) electronic effects; (ii) geometric effects; (iii) the occurrence of mixed sites; and (iv) the disappearance of the β-PdH phase. The second part presents the main methods to prepare bimetallic catalysts through the unselective and selective depositions of the co-metal precursor and the use of heterobinuclear complexes. The influence of preparation methods on the chemical state and the spatial distribution of both components are highlighted. In the last part, a short overview is given about the influence of co-metal addition on some important reactions in which Pd is the reference active phase: (i) selective hydrogenation of highly unsaturated aliphatic hydrocarbons; (ii) hydrogenation of aromatics or hydrodearomatisation; (iii) hydrogenation of nitrogen-containing compounds; (iv) reactions involving CO; and (v) hydrodechlorination.

Ivo Paseka

Papers

Reduction of no on smooth, porous platinum and platinum-carbon electrodes in acidic solutions

Competitive hydrogenation of unsaturated hydrocarbons on palladium catalysts modified with lead and thallium

Reduction of nitrogen oxide on platinum electrodes in HClO4 and HCl

Hydrogenation of aromatic nitro compounds on copper-modified platinum catalysts

Liquid-phase catalytic hydrogenation of unsaturated compounds: A new view on the causes of the zero order of the reaction