Deciphering the structure of heterogeneous catalysts across scales using pair distribution function analysis

Heterogeneous catalysts are complex materials, often containing multiple atomic species and phases with various degrees of structural order. The identification of structure–performance relationships that rely on the availability of advanced structural characterization tools is key for rational catalyst design. Structural descriptors in catalysts can be defined over different length scales from several angstroms up to several nanometers (crystalline structure), requiring structural characterization techniques covering these different length scales. Pair distribution function (PDF) analysis is a powerful method to extract structural information spanning from the atomic to the nanoscale under in situ or operando conditions. We discuss recent advances using PDF to provide insight into the atomic-to-nanoscale structure of heterogeneous catalysts.ISSN:2589-597

Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO2 to Methanol

Supported, bimetallic catalysts have shown great promise for the selective hydrogenation of CO2 to methanol. In this study, we decipher the catalytically active structure of Ni-Ga-based catalysts. To this end, model Ni-Ga-based catalysts, with varying Ni:Ga ratios, were prepared by a surface organometallic chemistry approach. In situ differential pair distribution function (d-PDF) analysis revealed that catalyst activation in H2 leads to the formation of nanoparticles based on a Ni-Ga face-centered cubic (fcc) alloy along with a small quantity of GaOx. Structure refinements of the d-PDF data enabled to determine the amount of both alloyed Ga and GaOx species. In situ X-ray absorption spectroscopy experiments confirmed the presence of alloyed Ga and GaOx and indicated that alloying with Ga affects the electronic structure of metallic Ni (viz. Ni-). Both the Ni:Ga ratio in the alloy and the quantity of GaOx are found to minimize methanation and to determine methanol formation rate and the resulting methanol selectivity. The highest formation rate and methanol selectivity are found for a Ni-Ga alloy having a Ni:Ga ratio of ~ 75:25 along with a small quantity of oxidized Ga species (0.14 molGaOx molNi-1). Furthermore, operando infrared spectroscopy experiments indicate that GaOx species play a role in the stabilization of for-mate surface intermediates, which are subsequently further hydrogenated to methoxy species and ultimately to methanol. Notably, operando XAS shows that alloying between Ni and Ga is maintained under reaction conditions and is key to attain a high methanol selectivity (by minimizing CO and CH4 formation), while oxidized Ga species enhance the methanol formation rate

Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO2 to Methanol

Supported, bimetallic catalysts have shown great promise for the selective hydrogenation of CO2 to methanol. In this study, we decipher the catalytically active structure of Ni–Ga-based catalysts. To this end, model Ni–Ga-based catalysts, with varying Ni:Ga ratios, were prepared by a surface organometallic chemistry approach. In situ differential pair distribution function (d-PDF) analysis revealed that catalyst activation in H2 leads to the formation of nanoparticles based on a Ni–Ga face-centered cubic (fcc) alloy along with a small quantity of GaOx. Structure refinements of the d-PDF data enabled us to determine the amount of both alloyed Ga and GaOx species. In situ X-ray absorption spectroscopy experiments confirmed the presence of alloyed Ga and GaOx and indicated that alloying with Ga affects the electronic structure of metallic Ni (viz., Niδ−). Both the Ni:Ga ratio in the alloy and the quantity of GaOx are found to minimize methanation and to determine the methanol formation rate and the resulting methanol selectivity. The highest formation rate and methanol selectivity are found for a Ni–Ga alloy having a Ni:Ga ratio of ∼75:25 along with a small quantity of oxidized Ga species (0.14 molGaOx molNi–1). Furthermore, operando infrared spectroscopy experiments indicate that GaOx species play a role in the stabilization of formate surface intermediates, which are subsequently further hydrogenated to methoxy species and ultimately to methanol. Notably, operando XAS shows that alloying between Ni and Ga is maintained under reaction conditions and is key to attaining a high methanol selectivity (by minimizing CO and CH4 formation), while oxidized Ga species enhance the methanol formation rate.ISSN:2691-370

Nature of GaOx Shells Grown on Silica by Atomic Layer Deposition

Gallia-based shells with a thickness varying from a submonolayer to ca. 2.5 nm were prepared by atomic layer deposition (ALD) using trimethylgallium, ozone, and partially dehydroxylated silica, followed by calcination at 500 degrees C. Insight into the atomic-scale structure of these shells was obtained by highfield Ga-71 solid-state nuclear magnetic resonance (NMR) experiments and the modeling of X-ray differential pair distribution function data, complemented by Ga K-edge X-ray absorption spectroscopy and Si-29 dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) studies. When applying one ALD cycle, the grown submonolayer contains mostly tetracoordinate Ga sites with Si atoms in the second coordination sphere (Ga-[4](Si)) and, according to N-15 DNP SENS using pyridine as the probe molecule, both strong Lewis acid sites (LAS) and strong Bronsted acid sites (BAS), consistent with the formation of gallosilicate Ga-O-Si and Ga-mu(2)-OH-Si species. The shells obtained using five and ten ALD cycles display characteristics of amorphous gallia (GaOx), i.e., an increased relative fraction of pentacoordinate sites (Ga-[5](Ga)), the presence of mild LAS, and a decreased relative abundance of strong BAS. The prepared Ga1-, Ga5-, and Ga10-SiO2-500 materials catalyze the dehydrogenation of isobutane to isobutene, and their catalytic performance correlates with the relative abundance and strength of LAS and BAS, viz., Ga1-SiO2-500, a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10SiO(2-500). In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO2-500 materials do not catalyze the dehydrogenation of isobutane and this correlates with the lack of strong LAS in these materials that instead feature abundant strong BAS formed via the atomicscale mixing of Al sites with silica, leading to Al-mu(2)-OH-Si sites. Our results suggest that Ga-[4]( Si) sites provide strong Lewis acidity and drive the dehydrogenation activity, while the appearance of Ga-[5]( Ga) sites with mild Lewis activity is associated with catalyst deactivation through coking. Overall, the atomic-level insights into the structure of the GaOx-based materials prepared in this work provide a guide to design active Ga-based catalysts by a rational tailoring of Lewis and Bronsted acidity (nature, strength, and abundance).ISSN:0897-475