Nature of the Active Sites on Ni/CeO2 Catalysts for Methane Conversions.

Pablo G Lustemberg,M Verónica Ganduglia-Pirovano,Zhongtian Mao,Agustín Salcedo,Beatriz Irigoyen,Charles T Campbell

doi:10.1021/acscatal.1c02154

Abstract

Effective catalysts for the direct conversion of methane to methanol and for methane’s dry reforming to syngas are Holy Grails of catalysis research toward clean energy technologies. It has recently been discovered that Ni at low loadings on CeO2(111) is very active for both of these reactions. Revealing the nature of the active sites in such systems is paramount to a rational design of improved catalysts. Here, we correlate experimental measurements on the CeO2(111) surface to show that the most active sites are cationic Ni atoms in clusters at step edges, with a small size wherein they have the highest Ni chemical potential. We clarify the reasons for this observation using density functional theory calculations. Focusing on the activation barrier for C–H bond cleavage during the dissociative adsorption of CH4 as an example, we show that the size and morphology of the supported Ni nanoparticles together with strong Ni-support bonding and charge transfer at the step edge are key to the high catalytic activity. We anticipate that this knowledge will inspire the development of more efficient catalysts for these reactions.

Highlights

The recent dramatic increase in methane availability worldwide has inspired a surge of interest in new catalytic processes for methane conversions that could lead to major environmental and economic benefits
These scanning tunneling microscopy (STM) measurements were for larger particle sizes than at the rate maximum here (∼1 nm) and possibly missed seeing many or most of the particles smaller than this size due to particle mobility and the limitations of STM imaging on oxide surfaces at the temperature used
Note that dividing the total Ni coverage by the fractional area covered by the Ni particles measured by LEIS32 gives the average Ni particle thickness (in ML, or atoms per unit area, which we converted to nanometers by dividing by the number of Ni atoms per unit volume in bulk Ni(solid) and converted to particle diameter by dividing by this height/diameter ratio (0.25))

Summary

Introduction

The recent dramatic increase in methane availability worldwide has inspired a surge of interest in new catalytic processes for methane conversions that could lead to major environmental and economic benefits. H and C−H bonds in H2O and CH4, respectively, at room temperature, with lower activation barriers than for extended metallic Ni surfaces, and promote the activation of CO2 at moderate temperatures. Most importantly, this type of catalyst is very active in the DRM in a clean and efficient way[3,4,20] and in the direct conversion of methane to methanol using a mixture of oxygen and water, with a higher selectivity than ever reported for ceria-based catalysts.[9] The activation of CH4 is the first and only step shared by both reactions, whereas, for example, their steps for C−O bond formation are quite different. We further reveal how this nanomaterial escapes the so-called “tyranny of linear scaling”, at least for this

Methods

Results

Conclusion