Lukas Rochlitz

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Rochlitz

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Lukas

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0000-0002-3287-6088

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Publications 1 - 10 of 16

Heterogeneous alkane dehydrogenation catalysts investigated via a surface organometallic chemistry approach
Docherty, Scott; Rochlitz, Lukas; Payard, Pierre-Adrien; et al. (2021)
Chemical Society Reviews
The selective conversion of light alkanes (C2–C6 saturated hydrocarbons) to the corresponding alkene is an appealing strategy for the petrochemical industry in view of the availability of these feedstocks, in particular with the emergence of Shale gas. Here, we present a review of model dehydrogenation catalysts of light alkanes prepared via surface organometallic chemistry (SOMC). A specific focus of this review is the use of molecular strategies for the deconvolution of complex heterogeneous materials that are proficient in enabling dehydrogenation reactions. The challenges associated with the proposed reactions are highlighted, as well as overriding themes that can be ascertained from the systematic study of these challenging reactions using model SOMC catalysts.
Assessing the structure of SiO₂-supported Ni-Ga nanoparticles under CO₂ hydrogenation to methanol conditions via in-situ X-ray absorption, total scattering and infrared experiments
Zimmerli, Nora; Rochlitz, Lukas; Abdala, Paula Macarena; et al. (2023)
CO₂ Hydrogenation to Methanol over Silica Supported Ni-Ga Alloy Nanoparticles: On the Structural Factors Driving Catalyst Activity and Selectivity
Zimmerli, Nora K.; Rochlitz, Lukas; Abdala, Paula Macarena; et al. (2023)
Online Abstracts: 28th North American Catalysis Society Meeting
Here, we present a structure-performance analysis of Ni-Ga alloy nanoparticles (ca. 2 nm) supported on silica for the hydrogenation of CO2 to methanol. We combine in-situ/operando X-ray total scattering with X-ray absorption and IR spectroscopies to resolve the structures of the catalysts at the nm and sub-nm scales.
Dynamics and Site Isolation: Keys to High Propane Dehydrogenation Performance of Silica-Supported PtGa Nanoparticles
Payard, Pierre-Adrien; Rochlitz, Lukas; Searles, Keith; et al. (2021)
JACS Au
Nonoxidative dehydrogenation of light alkanes has seen a renewed interest in recent years. While PtGa systems appear among the most efficient catalyst for this reaction and are now implemented in production plants, the origin of the high catalytic performance in terms of activity, selectivity, and stability in PtGa-based catalysts is largely unknown. Here we use molecular modeling at the DFT level on three different models: (i) periodic surfaces, (ii) clusters using static calculations, and (iii) realistic size silica-supported nanoparticles (1 nm) using molecular dynamics and metadynamics. The combination of the models with experimental data (XAS, TEM) allowed the refinement of the structure of silica-supported PtGa nanoparticles synthesized via surface organometallic chemistry and provided a structure-activity relationship at the molecular level. Using this approach, the key interaction between Pt and Ga was evidenced and analyzed: the presence of Ga increases (i) the interaction between the oxide surface and the nanoparticles, which reduces sintering, (ii) the Pt site isolation, and (iii) the mobility of surface atoms which promotes the high activity, selectivity, and stability of this catalyst. Considering the complete system for modeling that includes the silica support as well as the dynamics of the PtGa nanoparticle is essential to understand the catalytic performances.
Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of 195Pt Chemical Shift Anisotropy
Venkatesh, Amrit; Gioffrè, Domenico; Atterberry, Benjamin A.; et al. (2022)
Journal of the American Chemical Society
Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of Pt-based heterogeneous catalysts. 195Pt solid-state NMR spectroscopy could be an ideal tool for this task because 195Pt isotropic chemical shifts and chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the characterization of Pt surface-sites is complicated by the typical low Pt loadings that are between 0.2 and 5 wt% and broadening of 195Pt solid-state NMR spectra by CSA. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection using a sensitive spy nucleus (1H or 31P) to enable the rapid acquisition of 195Pt MAS NMR spectra. We demonstrate that high-resolution wideline 195Pt MAS NMR spectra can be acquired in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt%. Low-power, long-duration, sideband-selective excitation, and saturation pulses are incorporated into t1-noise eliminated dipolar heteronuclear multiple quantum coherence, perfect echo resonance echo saturation pulse double resonance, or J-resolved pulse sequences. The complete 195Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the 195Pt pulses is varied in increments of the MAS frequency. Analysis of the 195Pt MAS NMR spectra yields the 195Pt chemical shift tensor parameters. Zeroth order approximation density functional theory calculations accurately predict 195Pt CS tensor parameters. Simple and predictive orbital models relate the CS tensor parameters to the Pt electronic structure and coordination environment. The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum surface-sites.
Revisiting Edge Sites of γ-Al2O3 Using Needle-Shaped Nanocrystals and Recoupling-Time-Encoded {27Al}-1H D-HMQC NMR Spectroscopy
Völker, Laura A.; Meyet, Jordan; Berkson, Zachariah J.; et al. (2022)
The Journal of Physical Chemistry C
Despite being widely used in numerous catalytic applications, our understanding of reactive surface sites of high-surface-area γ-Al2O3 remains limited to date. Recent contributions have pointed toward the potential role of highly reactive edge sites contained in the high-field signal (−0.5 to 0 ppm) of the 1H NMR spectrum of γ-Al2O3 materials. This work combines the development of well-defined, needle-shaped γ-Al2O3 nanocrystals having a high relative fraction of edge sites with the use of state-of-the-art solid-state NMR to significantly deepen our understanding of this specific signal. We are able to resolve two hydroxyl sites with distinct isotropic chemical shifts of −0.2 and −0.4 ppm and different positions within the dipole–dipole network from 1H–1H single-quantum double-quantum NMR. Moreover, the use of recoupling-time-encoded arbitrary-indirect-dwell dipolar heteronuclear multiple quantum coherence allows us to partially revise previous assignments for surface-aluminum sites in the proximity of these hydroxyl sites. Although previous work has ascribed the high-field signal to be correlated with a single four-coordinate Al-site with a substantial quadrupolar broadening of >10 MHz, we can identify the presence of two four-coordinate Al-sites with similar isotropic chemical shifts but different quadrupolar coupling constants of approximately 7 and >10 MHz, respectively. Recoupling-time-encoded data are thus able to differentiate sites that would otherwise only be achievable with access to multiple fields or usage of highly advanced NMR techniques.
Grafting of Group-10 Organometallic Complexes on Silicas: Differences and Similarities, Surprises and Rationale
Gioffrè, Domenico; Rochlitz, Lukas; Payard, Pierre-Adrien; et al. (2022)
Helvetica Chimica Acta
Surface organometallic chemistry (SOMC) represents a unique synthetic platform for the preparation of model heterogeneous catalysts resembling those broadly applied in industry. SOMC techniques usually rely on the grafting of tailored molecular precursors onto the surface OH groups of oxide supports. The development of such precursors and the understanding of their reactivity with the supports are therefore crucial for the development of well-defined surface species. While a large number of organometallic precursors of early transition metals are known, only few examples of group-10 metal complexes are reported, in spite of the great interest for heterogeneous catalysts based on the Pt-group elements. Herein, we report the reactivity of a family of group-10 (Ni, Pd and Pt) alkyl complexes, towards partially dehydroxylated SiO2 yielding well-defined supported species. We studied the effect of the metal, ligand, and support on the grafting mechanism of such precursors through a combined experimental and computational approach. Ultimately, we showed that at least two grafting pathways are possible for these compounds, namely the protonolysis of the M-alkyl bond by surface OH groups and the opening of strained siloxane bridges: the proportion of the two depending on the nature of the metal and its ancillary ligand.
A Molecular Analogue of the C−H Activation Intermediate of the Silica-Supported Ga(III) Single-Site Propane Dehydrogenation Catalyst: Structure and XANES Signature
Rochlitz, Lukas; Searles, Keith; Nater, Darryl; et al. (2021)
Helvetica Chimica Acta
Propane dehydrogenation is an important field of research due to an increasing world-wide demand of propene while classical production routes through naphtha cracking are in decline. In that context, silica-supported Ga(III) sites, synthesized from surface organometallic chemistry principles, show high selectivity and stability in the propane dehydrogenation reaction. This performance is in significant contrast to the reported fast deactivation and lower selectivity of most Ga2O3 and CrO3 based materials. The Ga-catalyzed propane dehydrogenation reaction is proposed to proceed through the formation of Ga alkyl intermediates for which it would be desirable to have detailed structural and spectroscopic information. Here, we prepare a consistent series of Ga(III) molecular complexes with varying numbers of alkyl and siloxide ligands; they are characterized by single crystal X-Ray diffraction and X-Ray Absorption Near Edge Structure analysis, which is known to be highly sensitive to the Ga coordination environment. We report in particular the structure and the spectroscopic signatures of [Ga(iPr)(OSi(OtBu)3)2(HOSi(OtBu)3)], a molecular mimic of the key proposed reaction intermediates in the Ga-catalyzed PDH reaction.
Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO2 to Methanol
Zimmerli, Nora K.; Rochlitz, Lukas; Checchia, Stefano; et al. (2024)
JACS Au
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.
Natural abundance ¹⁹⁵Pt-¹³C correlation NMR spectroscopy on surfaces enabled by fast MAS dynamic nuclear polarization
Wang, Zhuoran; Robinson, Thomas C.; Gioffrè, Domenico; et al. (2024)
Journal of Magnetic Resonance Open
Surface organometallic chemistry has developed as an effective strategy for the rational design and synthesis of well-defined, single-site Pt-based heterogeneous catalysts. Given its high sensitivity to changes in electronic structure, ¹⁹⁵Pt solid-state NMR spectroscopy offers a unique approach to investigate the chemical structure and local environment of Pt surface sites, providing invaluable insights for establishing structure-activity relationships. However, this approach is typically hindered by severe sensitivity issues, due to the low loading of Pt sites and the often-encountered large ¹⁹⁵Pt chemical shift anisotropies. To overcome this limitation, ¹⁹⁵Pt NMR signature of surface metal centers can be indirectly detected through protons. Indirect detection on ¹³C spins, has also been demonstrated to be feasible by combining isotopic labeling with dynamic nuclear polarization (DNP). Here, we extend this methodology to a supported Pt complex at natural abundance. The material was prepared by grafting (COD)PtMeOSi(OtBu)₃ (COD = 1,5-cyclooctadiene, Me = methyl and tBu = tert‑butyl) onto partially dehydroxylated silica. DNP enhanced two-dimensional through-bond ¹³C{¹⁹⁵Pt} heteronuclear correlation experiments were successfully implemented at fast magic angle spinning. They enabled the detection of the 0.37 % NMR-responsive surface species, thereby showcasing the remarkable sensitivity of this approach and its broad applicability. Key bonding information was obtained by measuring the correlated ¹³C and ¹⁹⁵Pt isotopic chemical shifts as well as ¹J(¹³C-¹⁹⁵Pt) coupling constants, confirming directly the coordination structure of the surface Pt sites.

Publications 1 - 10 of 16

Lukas Rochlitz

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