Competitive Adsorption of Nitrite and Hydrogen on Palladium during Nitrite Hydrogenation

Abstract

Nitrite hydrogenation is studied in steady‐state as well as transient operation using a Pd catalyst in a tubular membrane contactor reactor. A negative reaction order in hydrogen in steady state operation proofs that hydrogen and nitrite adsorb competitively. In transient operation, feeding nitrite to the Pd surface fully covered with hydrogen results initially in very low conversion of nitrite, speeding up once hydrogen is removed from part of the Pd surface. Additional proof for competitive adsorption between hydrogen and nitrite is provided by the observation that exposure of a nitrite‐covered catalyst to hydrogen induces desorption of nitrite. Formation of ammonia in these experiments proceeds via two pathways, first via a fast reaction followed by extremely slow hydrogenation of adsorbed N atoms, which is kinetically not relevant. This information is relevant for designing effective and selective catalysts when operating at very low nitrite concentration.

Formic acid generating in situ H2 and CO2 for nitrite reduction in the aqueous phase

“…Thus, nitrogen is the only significant product of reduction of nitrite with formic acid, according to the overall redox reaction: 45 3HCOOH + 2NO 2 − → 3CO 2 + N 2 + 2H 2 O + 2OH − This remarkable selectivity is ascribed to the relatively slow decomposition of formic acid, keeping the effective concentration of H 2 and chemisorbed H low, which is favourable for preventing ammonia formation. 3,57 In addition, CO 2 formed during formic acid decomposition acts as a local pH buffer, keeping the pH low which is also known to suppress the formation of ammonium. 58,59…”

“…This remarkable selectivity is ascribed to the relatively slow decomposition of formic acid, keeping the effective concentration of H 2 and chemisorbed H low, which is favourable for preventing ammonia formation. 3,57 In addition, CO 2 formed during formic acid decomposition acts as a local pH buffer, keeping the pH low which is also known to suppress the formation of ammonium. 58,59 Fig.…”

Heterogeneous Continuous Flow Hydrogenation of Hexafluoroacetone Trihydrate and Its Kinetic Modeling

“…A similar dissociative competitive adsorption was found by Postma et al in a Pd-catalyzed nitrite hydrogenation in a tubular membrane contactor reactor. 38 Two hydrogenation reactions catalyzed by Raney nickel and Pd zeolite catalysts also showed such dissociative competitive adsorption. 21,39 Based on our results and other similar cases, it is possible that noble metal catalysts such, as palladium and Raney nickel, with stronger adsorption properties, higher catalytic activity, and selectivity for catalytic hydrogenation, are more likely to cause competitive adsorption.…”

Increased Hydrogenation Rates in Pd/La-Al2O3 Catalysts by Hydrogen Transfer O(-La) Sites Adjacent to Pd Nanoparticles

“…However, because of the high chemisorption energies, competitive adsorption of reactants and hydrogen occurs on the Pd surface. 14 To promote the efficient conversion of the reactant, the partial pressure of hydrogen is often much higher than the stoichiometric ratio and sometimes is extremely high, 15,16 causing excess hydrogen surface coverage, which inhibits the adsorption of the reactant. 17 Spillover, which is more likely to occur on reducible supports, 18−21 is one way to solve this problem.…”

“…For many hydrogenation processes, Pd nanoparticles (NPs) are the active component, activating reactants and dissociating hydrogen. However, because of the high chemisorption energies, competitive adsorption of reactants and hydrogen occurs on the Pd surface . To promote the efficient conversion of the reactant, the partial pressure of hydrogen is often much higher than the stoichiometric ratio and sometimes is extremely high, , causing excess hydrogen surface coverage, which inhibits the adsorption of the reactant .…”