Operando Spectroscopic Evidence of the Induced Effect of Residual Species in the Reaction Intermediates during CO₂ Hydrogenation over Ruthenium Nanoparticles

Abstract

In this work, we present a highly active catalyst based on Ru nanoparticles dispersed on alumina, which showed an unexpected activity for CO2 methanation. This exceptional catalytic behavior was attributed to the presence of residual species that remained on the surface after synthesis. Furthermore, through Operando DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) measurements it was demonstrated that these remaining species provoked an induced effect on the nature of the surface intermediates spectroscopically observed, and consequently on their mechanistic role during the pathway of the CO2 hydrogenation to methane.

Mechanistic Connections between CO2 and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

“…Ru nanoparticles are among the most active and selective catalysts for CO 2 methanation, leading to their preeminence as state-ofthe art catalysts in mechanistic studies of CO 2 hydrogenation. [5][6][7]10,12,13 Several recent studies on Ru-based catalysts concur that CO and CH 4 formation pathways occur on the same catalytic surfaces and that they are linked through the intervening formation of chemisorbed CO (CO*, where * denotes bound species). 6,10,12 Steady-state and transient infrared spectroscopy and temperature-programmed desorption data 14−16 show that surfaces are significantly covered by CO* as most abundant reactive intermediates during CO and CO 2 hydrogenation reactions.…”

“…[5][6][7]10,12,13 Several recent studies on Ru-based catalysts concur that CO and CH 4 formation pathways occur on the same catalytic surfaces and that they are linked through the intervening formation of chemisorbed CO (CO*, where * denotes bound species). 6,10,12 Steady-state and transient infrared spectroscopy and temperature-programmed desorption data 14−16 show that surfaces are significantly covered by CO* as most abundant reactive intermediates during CO and CO 2 hydrogenation reactions. The minimum-energy paths that activate the first CO bond in CO 2 and the second CO bond (in CO), which differ markedly in strength (bond- dissociation enthalpies of 532 kJ mol −1 for OCO and 1075 kJ mol −1 for CO in the gas phase, 298 K), 17 and their respective kinetic relevance on working catalytic surfaces, however, remain unclear.…”

“…Metal nanoparticles, and even single metal atoms, have been shown to activate CO bonds in CO 2 , thus providing direct catalytic routes for its hydrogenation to more valuable C 1 intermediates or products, such as CO, CH 4 , and CH 3 OH, at modest temperatures (443–673 K). − The thermodynamics of CH 4 and CO formation from CO 2 and H 2 (methanation and reverse water-gas shift reactions, respectively) are far more favorable than CH 3 OH synthesis reactions, which give low equilibrium yields at the temperatures required for practical rates (Δ G rxn = +75 kJ mol –1 for CH 3 OH synthesis, −40 kJ mol –1 for methanation, +14 kJ mol –1 for reverse water-gas shift; per mole CO 2 , 673 K). The significance of methanation and reverse water-gas shift in CO 2 utilization efforts and the promising catalytic routes for such reactions have attracted substantial attention in the recent literature. − …”

“…Previous attempts at relating catalytic properties to CO 2 hydrogenation rates and selectivities (to CO and CH 4 ) have led to persistent mechanistic questions that remain unresolved. ,, These enduring controversies include the connections between the elementary steps that form CO and CH 4 during CO 2 –H 2 catalysis on transition metal surfaces and which ones among those steps limit rates and selectivities. Ru nanoparticles are among the most active and selective catalysts for CO 2 methanation, leading to their preeminence as state-of-the art catalysts in mechanistic studies of CO 2 hydrogenation. − ,,, Several recent studies on Ru-based catalysts concur that CO and CH 4 formation pathways occur on the same catalytic surfaces and that they are linked through the intervening formation of chemisorbed CO (CO*, where * denotes bound species). ,, Steady-state and transient infrared spectroscopy and temperature-programmed desorption data − show that surfaces are significantly covered by CO* as most abundant reactive intermediates during CO and CO 2 hydrogenation reactions. The minimum-energy paths that activate the first CO bond in CO 2 and the second CO bond (in CO), which differ markedly in strength (bond-dissociation enthalpies of 532 kJ mol –1 for OCO and 1075 kJ mol –1 for CO in the gas phase, 298 K), and their respective kinetic relevance on working catalytic surfaces, however, remain unclear.…”

Effect of Pr in CO2 Methanation Ru/CeO2 Catalysts

“…CO 2 conversion to methane by H 2 is a practical strategy for clean energy utilization with the dual benefit of reducing CO 2 emissions while meeting the increasing energy demand forecasted for the next few decades. , Metals supported on ceria can activate CO 2 molecules at lower temperatures compared to analogue catalysts with other supports. Ceria improves the efficiency of the reaction because oxygen vacancies serve as active sites for CO 2 dissociation. − Up to now, the research on this reaction has shed light on the catalytic role of each active center by means of in situ spectroscopies and other advanced techniques. − It is known that CO 2 methanation requires a bifunctional catalyst able to undertake both H 2 and CO 2 activation processes on one or two different type of sites. In heterogeneous catalysts based on inert supports (such as Al 2 O 3 or SiO 2 ), both events take place on reduced metal sites. − However, ceria-based CO 2 methanation catalysts are proven to be more efficient because ceria contributes to CO 2 chemisorption and dissociation in cooperation with the metal.…”