About

Shabnam Akhtari is a professor at the Department of Mathematics, located in the McAllister Building. Her research interests include Number Theory, Geometry of Numbers, and Diophantine Analysis. Her work involves exploring fundamental questions in these areas, contributing to the understanding of mathematical structures and their properties.

Research topics

• Chemistry
• Photochemistry
• Organic chemistry
• Chemical physics
• Inorganic chemistry
• Physical chemistry
• Computational chemistry
• Thermodynamics

Selected publications

• Long-Range Metal–Sorbent Interactions Determine CO₂ Capture and Conversion in Dual-Function Materials

  ACS Nano · 2025-01-08 · 11 citations

  article

  Carbon capture and utilization involve multiple energy- and cost-intensive steps. Dual-function materials (DFMs) can reduce these demands by coupling CO2 adsorption and conversion into a single material with two functionalities: a sorbent phase and a metal for catalytic CO2 conversion. The role of metal catalysts in the conversion process seems salient from previous work, but the underlying mechanisms remain elusive and deserve deeper investigation to achieve maximum utilization of the two phases. Here, preformed colloidal Ru nanoparticles were deposited onto a “NaOx”/Al2O3 sorbent to prepare prototypical DFMs with controlled phases for CO2 capture and hydrogenation to CH4. Ru addition was found to double the high-temperature CO2 adsorption capacity by activating the “NaOx”/Al2O3 sorbent phase during a reductive pretreatment step. Most importantly, low Ru loadings were sufficient to ensure maximum CO2 adsorption and conversion. This was attributed to the key role of the metal–sorbent interactions, wherein Ru was required to hydrogenate strongly bound CO2 on the “NaOx”/Al2O3 sorbent to CH4 via the H2 activated on Ru. This interaction facilitated rate-determining carbonate migration and subsequent hydrogenation at the metal–sorbent interface. Overall, Ru controlled the CO2 hydrogenation reaction rate, while the “NaOx”/Al2O3 sorbent dictated the CO2 uptake capacity. By controlling metal–sorbent interactions at the molecular level, we demonstrate the critical role of the two phases and their synergy, facilitating the design of DFMs with maximum CO2 capture and conversion efficiency.

  PublisherDOI

• High-Temperature Size Exclusion Chromatography for Advanced Recycling: Systematic Errors in Analysis of Polyethylene─Alkane Mixtures

  ACS Applied Polymer Materials · 2025-08-01

  articleOpen access

  Polyolefins, which dominate the plastics marketplace, require high-temperature size exclusion chromatography (HT-SEC) to characterize their molar masses. Chemical recycling methods designed to deconstruct plastic waste into smaller molecules (i.e., depolymerization methods) rely on HT-SEC to characterize their products, but depolymerization methods often yield a complex mixture, including components below the typical separation range of commercial HT-SEC columns. Herein, we report the accuracy and limitations of triple detection HT-SEC to analyze model chemical recycling products. We examined the chromatographic separation and quantification of individual components and mixtures of short polyethylenes (apparent Mn = 465 and 2722 g/mol), hexatriacontane (C36H74, M = 507 g/mol), dotriacontane (C32H66, M = 451 g/mol), and octadecane (C18H38, M = 254 g/mol). Despite short alkanes exhibiting molar masses below the lower molar mass limit of the columns (500 g/mol), the separation of their mixtures was resolved via refractive index (RI) detection. However, the determined molar masses of these alkane mixtures did not agree with the known molar masses of the short alkanes or their prepared mixtures. Similarly, the SEC analysis of mixtures containing discrete alkanes with low molar mass PE revealed that their individual components are easily resolved in the HT-SEC chromatogram. No obvious shift in the elution volume of the blend components versus the elution volume of the pure components was observed. However, inaccurate molar masses and molar mass distributions for these alkane-PE mixtures are calculated from the HT-SEC data. These inaccuracies are attributable to two factors: (1) molar mass-dependent ∂n/∂c for low molar mass components and (2) the small size of the molecules in solution that limits light scattering. These results highlight the importance of considering measurement limitations for quantitative interpretation of triple detection HT-SEC data obtained from chemical recycling and depolymerization experiments that commonly contain complex mixtures of polymeric, oligomeric, and small molecule alkanes.

  PublisherDOI

• Surface Entropy Mediated Hydrogen Spillover on Au/TiO₂: Influences of Strongly Adsorbed Water on H₂ Adsorption Thermodynamics

  Journal of the American Chemical Society · 2025-08-05 · 12 citations

  articleSenior authorCorresponding

  Hydrogen spillover, a poorly understood adsorption phenomenon, plays an important role in hydrogen storage, catalytic hydrogenation, and energy conversion processes. While widely invoked to explain anomalous observations, the fundamental mechanisms underlying spillover remain under debate, particularly regarding the influence of surface adsorbates, such as water. In this study, we investigate how strongly adsorbed water (SAW) impacts hydrogen spillover (H*) on Au/TiO2 catalysts using in situ Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). By carefully correlating IR and TGA data, we quantify the relationship between water coverage and spillover. At low to moderate temperatures (<200 °C), SAW resides primarily on Ti Lewis acid sites, while hydrogen spillover is associated with surface hydroxyl groups. Our findings reveal that even though H* and SAW do not directly compete for surface adsorption sites, SAW suppresses H*. Van’t Hoff studies indicate SAW suppresses spillover by modifying the surface entropy of the titania, presumably by perturbing multiple proton transfer equilibria across the support surface. Maintaining constant water and hydroxyl coverage over a modest temperature range allowed for the determination of reliable thermodynamic parameters for hydrogen spillover on titania, yielding a slightly exothermic heat of adsorption (−7 ± 1 kJ/mol H*). These insights highlight the indirect role that surface water can play in catalytic reactions involving hydrogen spillover and offer a new perspective on catalyst design and optimization for hydrogen-involved reactions. This work also highlights the importance of considering the entropy of oxide surfaces in understanding catalysis over oxides.

  PublisherDOI

• Hydrocarbon Deposition during Polyolefin Upcycling: Irreversible Adsorption and Surface Reactions of Polyethylene and Ethylene Oligomers on Silica Supports

  Industrial & Engineering Chemistry Research · 2025-03-12 · 1 citations

  article

  Catalytic conversion of polyolefins to value-added products offers an alternative route to capture value from plastic waste. Here we initially examine reactions of a polyethylene model (hexatriacontane, C36H74) on a Pt/SiO2 catalyst under typical hydrogenolysis and hydrocracking temperatures, which leads to irreversibly adsorbed surface hydrocarbons identified after extraction of hexatriacontane with excess hot toluene. The IR spectra of these catalysts after extraction reveal only aliphatic C–H stretches. SiO2 alone leads similar hydrocarbon adsorption on the surface where extended extraction fails to fully remove the adsorbed hydrocarbons from neat silica. The amount of hydrocarbon irreversibly adsorbed increases nearly 10-fold when the reactant is changed from hexatriacontane to polyethylene (Mn = 4000 Da), but the adsorbed quantity is insensitive to reaction temperature (200–300 °C). These results demonstrate significant, nonextractable hydrocarbon deposition on catalyst support surfaces without dehydrogenation catalyst present at temperatures typical of catalytic deconstruction of polyolefin waste, which may limit catalyst turnover and impact the product distribution.

  PublisherDOI

• Molar-Mass-Dependent Partitioning of Polyethylene in Nanopores of Model Catalyst Supports from Small-Angle Neutron Scattering

  Langmuir · 2025-10-13

  article

  Heterogeneous catalysis offers opportunities to enhance valorization of plastic waste via chemical recycling through control of the upcycled product distributions. Minimizing low-value light hydrocarbons is desired; however, fundamental insights into how to control selectivity are lacking. Here we use contrast variation with small-angle neutron scattering (SANS), model perdeuterated polyethylenes (dPEs), and a model liquid hydrocracking product (tetradecane) to quantify polymer partitioning within mesoporous silica (SBA-15). Polyethylene concentration within the mesopores is increased relative to the bulk solution, and this partitioning increases as the temperature increases. However, this polyethylene partitioning is maximized when the radius of gyration of the polymer chains is comparable to the SBA-15 pore size (10 nm). An increased partitioning at higher temperatures is attributed to entropically driven adsorption of PE within the mesopores. There is no observed preferential partitioning of hexatriacontane (a model oligomer) within the mesopores at the temperatures examined. These results suggest that pore size could promote the selective partitioning of polymer species into the mesopores by size. For plastic upcycling, pore-size-dependent partitioning should increase the probability for the reaction of long polymers over oligomeric and small-molecule polyolefin depolymerization products.

  PublisherDOI

• Recommendations for improving rigor and reproducibility in site specific characterization

  Journal of Catalysis · 2024-03-18 · 18 citations

  articleOpen access
  PublisherOA PDFDOI

• Upside-Down Adsorption: The Counterintuitive Influences of Surface Entropy and Surface Hydroxyl Density on Hydrogen Spillover

  Journal of the American Chemical Society · 2024-10-24 · 12 citations

  articleSenior authorCorresponding

  Although hydrogen spillover is often invoked to explain anomalies in catalysis, spillover remains a poorly understood phenomenon. Hydrogen spillover (H*) is best described as highly mobile H atom equivalents that arise when H2 migrates from a metal nanoparticle to an oxide or carbon support. In the 60 years since its discovery, few methods have become available to quantify or characterize H*-support interactions. We recently showed in situ infrared spectroscopy and volumetric chemisorption can quantify reversible H2 adsorption on Au/TiO2 catalysts, where adsorbed hydrogen exists as H* and interacts with titania surface hydroxyl (TiOH) groups. Here, we report parallel thermogravimetric analysis and Fourier transform infrared spectroscopy methods for systematically manipulating the surface TiOH density. We examine the role of surface hydroxylation on spillover thermodynamics using van't Hoff studies to determine apparent adsorption enthalpies and entropies at constant H* coverage, which is necessary to maintain constant H* translational entropy. Although surface TiOH groups are the likely adsorption sites, the data show removing hydroxyl groups increases spillover. This surprising finding─that adsorption increases as the adsorption site density decreases─is associated with improved thermodynamics on dehydroxylated surfaces. A strong adsorption enthalpy–entropy correlation implicates the changing surface entropy of the titania support itself (i.e., an initial state effect) is deeply intertwined with the H* configurational entropy. These effects are surprising and should apply to all low-coverage adsorbates where entropy terms dominate more traditional enthalpic considerations. Moreover, this study points toward a kinetic test for invoking spillover in a reaction mechanism: namely, in situ dehydroxylation should enhance spillover processes.

  PublisherDOI

• The role of surface hydroxyls in the entropy-driven adsorption and spillover of H2 on Au/TiO2 catalysts

  Nature Catalysis · 2023 · 143 citations

  Senior authorCorresponding
  • Chemistry
  • Chemical physics
  • Inorganic chemistry
  PublisherDOI

• Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid–Base Parameters

  ACS Applied Materials & Interfaces · 2023-01-25 · 40 citations

  articleSenior authorCorresponding

  Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

  PublisherDOI

• Kinetics of H₂ Adsorption at the Metal–Support Interface of Au/TiO₂ Catalysts Probed by Broad Background IR Absorbance

  Angewandte Chemie · 2021-01-07 · 11 citations

  articleSenior authorCorresponding

  Abstract H 2 adsorption on Au catalysts is weak and reversible, making it difficult to quantitatively study. We demonstrate H 2 adsorption on Au/TiO 2 catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H 2 adsorption; adsorption equilibrium constants are comparable to volumetric adsorption measurements. H 2 adsorption kinetics measured with the BBA show a lower E app value (23 kJ mol −1 ) for H 2 adsorption than previously reported from proxy H/D exchange (33 kJ mol −1 ). We also identify a previously unreported H‐O‐H bending vibration associated with proton adsorption on electronically distinct Ti‐OH metal‐support interface sites, providing new insight into the nature and dynamics of H 2 adsorption at the Au/TiO 2 interface.

  PublisherDOI

Recent grants

• RUI: Preparation and Kinetic Characterization of New Bimetallic Au-M Selective Hydrogenation Catalysts

  NSF · $357k · 2018–2021

• RUI: Preparation and Characterization of Dendrimer Templated Au-M Nanoparticles and Catalysts

  NSF · $260k · 2010–2014

• Water-Assisted Oxygen Insertion Reactions Over Supported Gold Catalysts

  NSF · $161k · 2015–2019

• CAREER: Chemical and Catalytic Characterization of Dendrimer Templated Bimetallic Nanoparticles

  NSF · $415k · 2005–2012

• RUI: Preparation and Characterization of New Heterogeneous Bimetallic Au-M Catalysts for Selective Oxidations and Hydrogenations

  NSF · $330k · 2013–2018

Frequent coauthors

• John D. Gilbertson

  26 shared

• Christopher J. Pursell

  22 shared

• Todd N. Whittaker

  University of Colorado Boulder

  16 shared

• Keith J. Stevenson

  Lomonosov Moscow State University

  15 shared

• G. V. Vijayaraghavan

  B.S. Abdur Rahman Crescent Institute of Science & Technology

  14 shared

• K. B. Sravan Kumar

  University of Houston

  13 shared

• Akbar Mahdavi‐Shakib

  Pennsylvania State University

  12 shared

• Lars C. Grabow

  University of Houston

  12 shared

Labs

• Bert Chandler LabPI

Education

• Ph. D., Chemistry

  University of Minnesota Twin Cities

  1999

• BS, Chemistry

  Georgia Southern University

  1994