The nuclearity and accessibility of platinum-group metals (PGMs) are decisive factors in heterogeneous catalysis and are typically controlled through metal-support interactions on oxide surfaces. However, such approaches are often energy-intensive and are primarily effective for reducible oxide supports. Here we demonstrate that precursor chemistry, rather than metal-support interactions, can tune the density of accessible Pd cluster sites on irreducible γ-Al2O3 by an order of magnitude. By controlling the ligand environment of Pd2+ in solution or the aging time prior to impregnation, the reaction rate for CH4 oxidation varies from 0.03 to 0.33 〖mol〗_(CH_4 )/mol_Pd/s at 380 oC. Mechanistic analysis reveals that aquo-ligated Pd2+ species undergo kinetically limiting olation reactions in solution, forming Pd cluster units (~3 nm) that serve as active sites after deposition. Increasing the aging time enhances the surface density of these cluster units without significantly altering their size, whereas excessive aging leads to aggregation and reduced accessibility. In contrast, strongly coordinating ligands such as NH3 or Cl- suppress olation and limit cluster formation. These findings identify precursor solution chemistry as an underexplored design dimension in catalyst synthesis, enabling a simple route to tune active-site density and catalytic performance.

Methane (CH4) is a high-energy-density fuel with abundant global reserves, making it a valuable energy resource. However, its global warming potential is approximately 21 times that of carbon dioxide (CO2), highlighting the urgent need for effective CH4 emission control. Catalytic oxidation of CH4 into CO2 using Pd-based catalysts is a promising strategy due to their high activity. However, their practical application is hindered by rapid deactivation in humid environments, primarily caused by active site transformations and hydroxyl accumulation. Given the high cost of Pd, enhancing catalyst stability is essential for industrial viability. In this study, we demonstrate that gamma-Al2O3 mixed with 0.5 wt% m-ZrO2 significantly improves the stability of Pd-based catalysts under humid conditions. While conventional Pd/gamma-Al2O3 suffered a significant decline in CH4 conversion, dropping from 70% to 39% over 20 h in a 10% H2O atmosphere, the Zr-doped Pd/gamma-Al2O3 maintained its catalytic performance. This enhancement is primarily attributed to the inhibition of Pd sintering and suppression of inactive Pd site formation, facilitated by strong Pd-Zr interactions. The Zr were uniformly dispersed onto the Al2O3 surface via a solvent-free ball-milling method, stabilizing Pd active sites and restricting their mobility. Additionally, the hydrophobic nature of m-ZrO2 mitigated hydroxyl accumulation, promoting the effective reoxidation of Pd active sites. These findings highlight that oxide support physically mixed with secondary oxides is a simple yet effective approach for enhancing the durability of Pd-based catalysts, advancing the development of more sustainable catalytic technologies for industrial applications.

One of the main goals of catalysis research is to improve the reaction efficiency by using platinum-group metals (PGMs) more effectively, given their high cost. PGMs are typically dispersed on oxide supports to maximize their surface area, under the assumption that catalytic activity arises primarily from the PGMs and their immediate oxide surroundings, while oxide surfaces located further away from PGMs are often considered catalytically irrelevant. However, a growing body of research on spillover phenomena suggests that PGMs can influence the catalytic properties of oxide surface sites located several nanometers away from PGMs, prompting the question of whether distant oxide surfaces can play a more active, or even dominant, role in catalytic kinetics. A shift in understanding, from viewing the oxide surface as merely a passive support to recognizing it as an active promoter of the rate-limiting step (RLS), would offer an alternative framework for optimizing PGM utilization. In this contribution, we investigated the role of distant oxide surfaces in CO oxidation, using Pt/CeO2 as a model system. Our findings show that distant CeO2 surfaces are not inert but can promote the CO oxidation reaction via oxygen spillover. Interestingly, when the CeO2 content in Pt/CeO2 is high, the catalytic activity across catalysts with varying distributions of Pt single atoms and clusters is identical. Kinetic analysis reveals that, in CeO2-rich Pt/CeO2 catalysts, the RLS is the activation of oxygen on the distant CeO2 surface. Further investigation indicated that the alignment of CeO2 grains during reductive treatment facilitates the oxygen supply to Pt, boosting catalytic activity. This study suggests that leveraging the catalytic function of the distant oxide surface offers a promising strategy to enhance the efficiency of PGMs, providing an alternative perspective on catalyst development.

Knowledge of how trace amounts of more reactive metals influence the oxidation rate and mechanism of Cu surfaces is essential for developing strategies to optimize the performance of Cu-based catalysts. We find that the addition of 1% Rh to Cu(111) increases the initial O2 dissociation rate by approximately 9-fold. CO poisoning experiments reveal that single Rh atoms activate O2 and facilitate the spillover of atomic oxygen to Cu sites. Scanning tunneling microscopy (STM) and in situ X-ray photoelectron spectroscopy (XPS) support this mechanism, showing enhanced surface oxygen near Rh atoms. A density functional theory (DFT)based model demonstrates that Rh binds the O2 precursor 0.15 eV more strongly than Cu(111) and lowers the O2 dissociation barrier by 0.02 eV. Both single-crystal and nanoparticle experiments show that at low oxygen pressures, Rh enhances Cu oxidation, whereas at higher pressures, it inhibits deeper oxidation, as evidenced by in situ ultraviolet-visible (UV-vis) spectra.

Single-atom catalysts (SACs) with atomically dispersed platinum-group metals (PGMs), such as Pd on γ-Al2O3, offer high metal utilization and distinct catalytic behavior. However, achieving atomic dispersion remains challenging due to metal aggregation during deposition or post-treatment, even at low PGM loadings (≪1 wt %). Developing reliable SACs thus requires clear guiding principles for promoting atomic dispersion. This study demonstrates the influence of metal precursor characteristics (nature of counterions and coordinating ligands) on the resulting structure of deposited Pd domains on γ-Al2O3 during scalable impregnation synthesis. It is shown that Pd-nitrate, a commonly used precursor, forms polynuclear hydroxo complexes in aqueous solution, resulting in Pd cluster deposition on γ-Al2O3. In contrast, Pd-ammonia precursors, such as tetraammine Pd nitrate, inhibit aggregation via NH3 ligand stabilization, leading to improved atomic dispersion. This study underscores the necessity of optimizing precursor characteristics to maximize the atomic dispersion of metals, enabling more reproducible development of SACs.

Redox reactions on the surface of transition metal oxides are of broad interest in thermo, photo, and electrocatalysis. H2 temperature-programmed reduction (H2-TPR) is commonly used to probe oxide reducibility by measuring the rate of H2 consumption during temperature ramps, assuming that this rate is controlled by oxide reduction. However, oxide reduction involves several elementary steps, such as H2 dissociation and H-spillover, before surface reduction and H2O formation occur. In this study, we evaluated the kinetics of H2 consumption over CeO2 and Pt/CeO2 with varying Pt loadings and structures to identify the elementary steps probed by H2-TPR. Literature often attributes changes in H2-TPR characteristics with Pt addition to increased CeO2 reducibility. However, our analysis revealed that the H2 consumption rate is measurement of the rate of H-spillover at Pt-CeO2 interfaces and is determined by the concentration of Pt species on Pt nanoclusters that dissociate H2. Therefore, lower temperature H2 consumption observed with Pt addition does not indicate higher CeO2 reducibility. Measurements on samples with mixtures of Pt single-atoms and nanoclusters demonstrated that H2-TPR can effectively quantify dilute Pt nanocluster concentrations, suggesting caution in directly linking H2-TPR characteristics to oxide reducibility while highlighting alternative material insights that can be gleaned.

The addition of platinum-groupmetals (PGMs, e.g., Pt) to CeO2 is used in heterogeneouscatalysis to promote the rate ofredox surface reactions. Well-defined model system studies have shownthat PGMs facilitate H-2 dissociation, H-spillover ontoCeO(2) surfaces, and CeO2 surface reduction. However,it remains unclear how the heterogeneous structures and interfacesthat exist on powder catalysts influence the mechanistic picture ofPGM-promoted H-2 reactions on CeO2 surfaces developedfrom model system studies. Here, controlled catalyst synthesis, temperature-programmedreduction (TPR), in situ infrared spectroscopy (IR), and in situ electronenergy loss spectroscopy (EELS) were used to interrogate the mechanismsof how Pt nanoclusters and single atoms influence H-2 reactionson high-surface area Pt/CeO2 powder catalysts. TPR showedthat Pt promotes H-2 consumption rates on Pt/CeO2 even when Pt exists on a small fraction of CeO2 particles,suggesting that H-spillover proceeds far from Pt-CeO2 interfaces and across CeO2-CeO2 particleinterfaces. IR and EELS measurements provided evidence that Pt changesthe mechanism of H-2 activation and the rate limiting stepfor Ce3+, oxygen vacancy, and water formation as comparedto pure CeO2. As a result, higher-saturation surface hydroxylcoverages can be achieved on Pt/CeO2 compared to pure CeO2. Further, Ce3+ formed by spillover-H from Pt isheterogeneously distributed and localized at and around interparticleCeO(2)-CeO2 boundaries, while activatedH(2) on pure CeO2 results in homogeneously distributedCe(3+). Ce3+ localization at and around CeO2-CeO2 boundaries for Pt/CeO2 isaccompanied by surface reconstruction that enables faster rates ofH(2) consumption. This study reconciles the materials gapbetween model structures and powder catalysts for H-2 reactionswith Pt/CeO2 and highlights how the spatial heterogeneityof powder catalysts dictates the influence of Pt on H-2 reactionsat CeO2 surfaces.

Single-atom catalysts (SACs) offer efficient metal utilization and distinct reactivity compared to supported metal nanoparticles. Structure-function relationships for SACs often assume that active sites have uniform coordination environments at particular binding sites on support surfaces. Here, we investigate the distribution of coordination environments of Pt SAs dispersed on shape-controlled anatase TiO 2 supports specifically exposing (001) and (101) surfaces. Pt SAs on (101) are found on the surface, consistent with existing structural models, whereas those on (001) are beneath the surface after calcination. Pt SAs under (001) surfaces exhibit lower reactivity for CO oxidation than those on (101) surfaces due to their limited accessibility to gas phase species. Pt SAs deposited on commercial-TiO 2 are found both at the surface and in the bulk, posing challenges to structure-function relationship development. This study highlights heterogeneity in SA coordination environments on oxide supports, emphasizing a previously overlooked consideration in the design of SACs.

Journal Article Atomistic Understanding of CO and H2 Influence on Pt Sintering in Pt/CeO2 Get access Peter Tieu, Peter Tieu Department of Chemistry, University of California, Irvine, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Wenjie Zang, Wenjie Zang Department of Materials Science and Engineering, University of California, Irvine, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Jaeha Lee, Jaeha Lee Department of Chemical Engineering, University of California, Santa Barbara, USA Search for other works by this author on: Oxford Academic Google Scholar Xingxu Yan, Xingxu Yan Department of Materials Science and Engineering, University of California, Irvine, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Phillip Christopher, Phillip Christopher Department of Chemical Engineering, University of California, Santa Barbara, USA Search for other works by this author on: Oxford Academic Google Scholar Xiaoqing Pan Xiaoqing Pan Department of Materials Science and Engineering, University of California, Irvine, CA, USAIrvine Materials Research Institute, University of California, Irvine, CA, USA Corresponding author: xiaoqinp@uci.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 1573–1574, https://doi.org/10.1093/micmic/ozad067.809 Published: 22 July 2023

CeO2 has been extensively studied as a support material for platinum group metals because of the high catalytic activity of CeO2-supported catalysts in oxidation reactions. In particular, it has been reported that the Pt/CeO2 catalyst exhibits high CO oxidation activity after reductive treatment, thereby lowering the temperature of 50 % CO conversion to 150 celcius or less. Such high activity of Pt/CeO2 has been attributed to the formation of active sites at the interface between the Pt nanoparticles (NP) and CeO2 surface. In this study, we attempt to deepen our understanding of the catalytic activity of Pt/CeO2 by studying the spillover of CO and O species at the interface. Combined kinetics and in situ CO-diffuse reflectance infrared Fourier transform spectroscopy studies reveal that CO spills over from Pt NPs to the CeO2 surface; this contributes to circumventing Pt poisoning by CO at low temperatures. Moreover, the CO oxidation activity of the Pt/CeO2 catalyst was considerably enhanced by simply mixing CeO2 particles with the Pt/CeO2 catalyst. This is explained by the formation of more reactive O atoms on the CeO2 surface after the reductive treatment. This study demonstrates that understanding the dynamic mobility of reactants is vital for discerning catalyst activity and designing more reactive catalysts.(c) 2022 Elsevier Inc. All rights reserved.

A catalyst composed of platinum-group metals supported on an oxide exhibits high activity in a low-temperature water-gas shift (LT-WGS) reaction; however, the reaction rate is greatly reduced when H2 or CO2, the product gases of the WGS reaction, are included in the reactant stream. In this study, we attempted to understand the origin of this activity inhibition by analyzing the kinetic data with in-situ CO-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The WGS reaction rate decreases more severely by H2 than CO2. The CO-DRIFTS spectra indicate that this can be explained by H2 preoccupying the active sites for the WGS reaction. In addition, by comparing the kinetic data with the literature, it could be inferred that a similar inhibition mechanism is operating in other oxide-supported Pt catalysts. Considering this inhibition mechanism will be important for the development of catalysts with high WGS activity in reformate gas.

Zeolites have been widely applied as support materials for metal nanoparticles. However, when developing a metal/zeolite catalyst for hydrogenation reactions, the Br phi nsted acid sites of zeolites can promote side reactions and the zeolite structure can inhibit the access of reactants to the metal, resulting in low activity and selectivity. In this study, these challenges are addressed by preparing a metal/zeolite catalyst via a modified ion-exchange followed by reduction method, and subjecting the catalysts to two test reactions, hydrogenation of vanillin and fatty acid methyl esters (FAMEs). After forming Pd nanoparticles within ZSM-5, it was treated with a NaOH solution in a controlled manner to form mesopores around Pd and replace hydronium ions with sodium ions to remove Br phi nsted acid sites. Pd/ZSM-5 catalysts prepared with controlled NaOH treatment showed higher hy-drogenation reactivity and improved selectivity toward the desired hydrogenation products than those prepared without NaOH treatment. This study presents a simple strategy to prepare metal/zeolite catalysts to promote hydrogenation without side reactions.

Supported single metal atoms have been extensively investigated in heterogeneous catalysis due to the ultimate utilization efficiency of the noble metal, resulting in cost savings.However, all these single atom catalysts (SACs) studied so far have non-

During thermal treatment of Pt/CeO2 catalysts, aggregation of CeO2 particles can facilitate the sintering of supported Pt, especially when the neighboring lattice-aligned CeO2 particles aggregate.

Pt/CeO2 has gained much attention for their high activity in low-temperature (LT) water-gas shift (WGS) reaction. However, the inclusion of H-2 in the feed as in the practical reaction condition significantly degrades the LT-WGS activity of the Pt/CeO2 catalysts. In this contribution, the activity of Pt/CeO2 catalyst under the feed gas containing excess H-2 (20 vol% of H-2) was enhanced more than three times by forming CeO2 nano-patches on Pt nano-particles. Both in-situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculation results indicate that dissociated H-2 on the Pt nano-particle inhibits the activity of the Pt/CeO2 catalysts by occupying the active sites (Pt nano-particle-CeO(2 & nbsp;)interface). On the other hand, thin CeO2 nano patches on Pt nano-particle suppressed the H-2 dissociation. As a result, the WGS reactivity of the active Pt nano-particle-CeO2 interface was less affected by H-2, granting the catalysts the high activity under the practical reaction conditions.

A significant amount of NO is emitted from advanced gasoline vehicles during preheating three-way catalysts (TWCs) to operating temperature (>150°C). Pd nanoparticles (NPs) loaded on CeO2 are studied as a NO abatement material to mitigate NO emissions during the cold-start period. The air-fuel equivalence ratio is systematically switched from high to low while increasing temperature to promote NO storage below the operating temperature of TWCs and NO reduction at high temperature. Combined experimental and theoretical studies indicate that the Pd-NP–CeO2 interface modified by oxygen vacancies (VOs) plays an important role in converting NO∗ to NO2. NO2 is captured by VOs formed on the CeO2 surface, enabling the use of Pd/CeO2 as a NO storage material. Consequently, NO emission decreases by 67.6% during the cold-start period under practical conditions, which has been unattainable with conventional TWCs, thus bringing us a step closer to zero harmful emissions.

Decrease in the metal dispersion of noble-metal-loaded catalysts with high metal loading is a major factor reducing noble-metal efficiency. Herein, we investigated the enhancement in metal efficiency of Ru/TiO2 catalyst by utilizing hydrogen spillover in the liquid-phase. Ru was highly dispersed at low loadings (0.1 and 0.5 wt%), while larger nanoparticles were formed at higher loadings (1-5 wt%). The hydrogen spillover in liquid phase was activated at reaction temperature (100 degrees C) as the Ru dispersion decreased, which was confirmed through physical dilution experiments, hydrogen temperature-programmed-reduc tion, and kinetic analysis. Isotope experiment was conducted using D2O, observing inverse kinetic isotope effect (IKIE) for the high-Ru-loading catalysts. Based on the understanding of the hydrogen spillover in the liquid phase, the low metal efficiency of high-Ru-loading catalysts resulting from low dispersion could be compensated simply by physically mixing pristine TiO2, which played a role as new active sites when liquid-phase hydrogen spillover was activated. (C) 2022 Elsevier Inc. All rights reserved.

Journal Article Spatial Decorrelation of Ceria Surface Reduction and Platinum Surface Loading Site Get access Peter Tieu, Peter Tieu Department of Chemistry, University of California, Irvine, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Jaeha Lee, Jaeha Lee Department of Chemical Engineering, University of California, Santa Barbara, USA Search for other works by this author on: Oxford Academic Google Scholar Chaitanya Gadre, Chaitanya Gadre Department of Physics and Astronomy, University of California, Irvine, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Xingxu Yan, Xingxu Yan Department of Materials Science and Engineering, University of California, Irvine, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Wenjie Zang, Wenjie Zang Department of Materials Science and Engineering, University of California, Irvine, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Phillip Christopher, Phillip Christopher Department of Chemical Engineering, University of California, Santa Barbara, USA Search for other works by this author on: Oxford Academic Google Scholar Xiaoqing Pan Xiaoqing Pan Department of Physics and Astronomy, University of California, Irvine, CA, USADepartment of Materials Science and Engineering, University of California, Irvine, CA, USAIrvine Materials Research Institute, University of California, Irvine, CA, USA Corresponding author: xiaoqinp@uci.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2426–2428, https://doi.org/10.1017/S143192762200931X Published: 01 August 2022

Passive NOx adsorbers (PNA) adsorb NOx at low temperatures and release NOx at high temperatures, at which downstream catalytic systems, such as selective catalytic reduction catalysts, operate efficiently and can remove NOx. Pd/CeO2 is a promising catalyst for PNA applications. In this study, the promoting effect of CO on the NOx adsorption ability of Pd/CeO2 at low temperatures was investigated. The amount of NOx adsorbed significantly increased when CO was added to the feed gas. Specifically, during the first 10 min, the amount of adsorbed NOx increased more than six times in the presence of CO. A mechanism for the promoting effect of CO on low-temperature NOx adsorption was suggested by considering the decreased CO oxidation ability with increasing amount of NOx adsorbed over Pd/CeO2. The model proposes that NOx was readily adsorbed by the oxygen vacancies of CeO2 adjacent to the Pd particles, which were generated during CO oxidation, resulting in an increase in NOx adsorption. A better understanding of the CO and NO interactions would contribute to the development of ceria-based PNA materials.