Heterogeneity in Mixed Cerium Oxides and Its Influence on the

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Heterogeneity in Mixed Cerium Oxides and Its Influence on the Behavior of Gold Catalysts for the Selective Oxidation of Ethanol Gregory M. Mullen, Benjamin C. Siegert, Andrei Dolocan, Nathaniel R. Miller, Benjamin K. Rosselet, Iliya Sabzevari, and Charles Buddie Mullins J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06029 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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The Journal of Physical Chemistry

Heterogeneity in Mixed Cerium Oxides and its Influence on the Behavior of Gold Catalysts for the Selective Oxidation of Ethanol Gregory M. Mullen†, Benjamin C. Siegert†, Andrei Dolocan‡, Nathaniel R. Miller⊥, Benjamin K. Rosselet†, Iliya Sabzevari†, and C. Buddie Mullins†‡§* †

McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin,

Texas 78712, United States ‡

Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United

States ⊥Department

of Geosciences, Jackson School of Geosciences, University of Texas at Austin,

Austin, TX 78712, United States §

Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United

States

ACS Paragon Plus Environment

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ABSTRACT

Mixed metal oxides have been investigated for a host of applications, yet many questions regarding the relationship between their material properties and their functional behavior remain unanswered. Mixed metal oxides are frequently used as support materials for catalytic processes, and the structure/composition of the oxide can significantly influence the behavior of a mixed oxide-supported catalyst. In this article, we study the material properties of binary mixed cerium oxide supports synthesized with various metal additives and their corresponding influences on the behavior of gold catalysts for selective oxidation of ethanol. We have found that, although all of these materials exhibited X-ray diffraction patterns consistent with a pure cerium oxide phase, compositional heterogeneities were present in many of the samples, as indicated by time-offlight secondary ion mass spectrometry and temperature programmed reduction techniques. Furthermore, the mixed cerium oxide-supported gold catalysts demonstrated differences in both activity and selectivity for ethanol oxidation, with the materials that exhibited heterogeneous phases promoting the esterification reaction with higher selectivities than the homogeneous mixtures. Our results show that the composition of mixed metal oxide supports can have an important impact on catalytic behavior, providing a means of influencing activity and tuning selectivity.


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1. INTRODUCTION The investigation of gold catalysts has become a rich field of study since the pioneering work of Haruta1 and Hutchings.2 However, despite several decades of progress, a number of lingering questions remain regarding the catalytic nature of gold, one of which involves the role that the support materials play in gold-catalyzed processes. Previous studies have demonstrated that the support material can influence the behavior of a gold catalyst.3–8 Most of this previous work has focused on carbon monoxide oxidation4,5 or the water-gas shift reaction (WGSR),7,8 processes that do not readily undergo side reactions. However, gold catalysts also display exceptional activity for other reactions such as the selective oxidation of alcohols, which can undergo multiple reaction pathways and generate distributions of products that vary under different sets of conditions. By tailoring the properties of the support material, it may be possible to tune the selectivity of the catalyst for reactions such as these. Metal oxides are the most common support materials used to prepare gold catalysts. In fact, all of the studies mentioned above4–6,8 demonstrating support effects on catalytic behavior employed metal oxides as the support materials. The material properties of mixed metal oxides can differ substantially from their constituent component oxides, and these properties can significantly influence the activity of the resulting catalyst. Introducing an additional element into any compound can change the electronic properties of the mixture, but in addition to exhibiting an altered electronic structure, mixed metal oxides can also exhibit differences in thermal stability,9 acid-base functionality,10 and reducibility.11–13 Leveraging these material property variations provides a powerful tool for tuning catalytic performance. Previous studies have shown that employing mixed metal oxides as supports can alter the behavior of gold catalysts for CO oxidation,14,15 the WGSR,16 and total oxidation of


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hydrocarbons.17–19 Ceria, in particular, has been often used as the support material for these types of studies. Mixed cerium oxides can be prepared easily and frequently display novel material properties and behavior. Sudarsanam et al. demonstrated that modification of CeO2 with Fe, La, and Zr prior to deposition of gold resulted in catalysts with altered activity for CO oxidation.20 Both enhancements and decreases to activity were observed depending on the additive metal employed. Modification of ceria supports can also influence the tolerance of gold catalysts toward deactivation during the preferential oxidation of CO in H2.21,22 Rodriguez and colleagues have conducted a number of enlightening studies of gold supported on mixed ceria-titania surfaces.23,24 Ceria-titania supported gold particles displayed extraordinary activity for the WGSR,23 resulting from synergistic effects brought about by the interaction between the two metal oxides. As a result of this interaction, the activity of Au/CeOx/TiO2(110) far surpassed that of both Au/TiO2(110) and Au/CeO2(111).23 The goldsupport interface is believed to be the location of active sites for the WGSR with these catalysts – the gold and oxide surfaces acting as co-catalysts.24 Recent studies by Chowdhury et al. showed that the activity of Au/CeO2 catalysts for selective oxidation of alcohols was enhanced by modification of the support with Bi,25 Mn,26 Sn,27 or alkali-earth metals.28 The authors proposed that the activity variations observed in these studies resulted from changes to the reducibility and to the acid/base character of the mixed oxide supports. Su et al. found that gold catalysts supported on binary mixed oxides of Ga and Al were more active for alcohol oxidation than Au/Ga2O3 and Au/Al2O3,29 and Haider and Baiker were able to tailor the activity of gold catalysts for alcohol oxidation by varying the composition of ternary Cu-Mg-Al oxide supports.30 We note that each of these studies was carried out under conditions that resulted in nearly 100% selectivity to the corresponding aldehyde products.


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Systems operating in regimes under which parallel and/or sequential reactions can occur may experience shifts to selectivity as well as activity brought about by changes to the support material. As gold-catalyzed alcohol oxidation reactions carried out in both condensed and gas phases frequently generate aldehydes, esters, and/or carboxylic acids,31–35 these reactions may provide useful insights into the influence that the support material can have on reaction selectivity. Despite the clear influence that mixed metal oxide supports can have on the behavior of gold catalysts, insight into the structure and composition of these materials has tended to be quite limited.36 Common techniques used to characterize these materials (e.g. X-ray diffraction) do not provide comprehensive analysis of their structures or compositions, which can lead to mischaracterization and misrepresentation of important material phenomena such as the formation of segregated amorphous regions within the oxides. As catalytic behavior is dictated by the nature of the material surface, effects such as these may play important roles in directing the nature of the catalysts. Therefore, deeper understanding of the structure-function relationships that arise in mixed metal oxide supported catalysts may prove invaluable to the design of future catalytic materials. In this study, we characterized various binary mixed cerium oxide supports (modified with ~10 mol % Al, Bi, Co, Cu, La, Pb, and Zr during synthesis) and investigated the effects that they had on the activity and selectivity of supported gold catalysts for oxidative dehydrogenation and esterification of ethanol in a fixed bed flow reactor system. Despite all of these materials displaying X-ray diffraction (XRD) patterns attributed to a pure phase of CeO2 and exhibiting lattice parameter shifts consistent with incorporation of the additive materials into the crystal structures, we observed compositional heterogeneities in a number of the materials by time-of-


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flight secondary ion mass spectrometry (TOF-SIMS) imaging, indicating that these mixed oxides were not uniform solid solutions, as several previous studies have suggested based on the observation of similar XRD patterns. Furthermore, the mixed oxides exhibiting heterogeneity in TOF-SIMS images also displayed peaks during temperature programmed reduction (TPR) consistent with the presence of separate oxide phases. We also note changes to the reduction behavior of the gold catalysts supported on heterogeneous oxides, suggesting that the segregated additive phases interacted directly with the gold particles. Several of the gold catalysts supported on mixed cerium oxides exhibited different activities and selectivities for the oxidative dehydrogenation and esterification of ethanol, and the appearance of compositional heterogeneity in the mixed oxide samples correlated with shifts to the selectivity for ethyl acetate production. Our findings demonstrate that mixed cerium oxide supports can influence nature of gold catalysts, resulting in variations to activity and selectivity. It follows that catalytic behavior can be “tunable” by customizing the composition of mixed metal oxide supports.

2. METHODS 2.1. Synthesis of ceria supports and ceria-supported gold catalysts. The ceria support materials were prepared by a urea decomposition coprecipitation technique. For each synthesis, a solution of 0.11 M ammonium cerium nitrate and 2.0 M urea in deionized water was prepared. For the mixed cerium oxide supports, the additive precursor (M), a hydrated nitrate salt in each case, was incorporated into the solution at a molar ratio of 1:9 (M:Ce). The solution was then transferred to a round bottom flask and heated to 100 °C under vigorous stirring. A precipitate was formed after ~1 h at 100 °C, at which point the solution was diluted by ~50% with deionized


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water. The resulting mixture was aged at 100 °C for 8 h. After cooling to room temperature, the mixture was centrifuged, and the supernatant was discarded. The remaining solid was washed three times with deionized water. The product was dried for ~24 hours under vacuum at room temperature and then heated in air at 400 °C for 4 h. This synthesis method was chosen to maximize the potential for generating a highly disperse dopant phase within the ceria host matrix. As discussed by McFarland and Metiu in their review of mixed oxide catalysts, preparation methods that begin from homogeneous mixtures of precursor materials and that employ low calcination temperatures are more likely to result in a mixed oxide product that exhibits high dispersion of the additive within the oxide matrix.36 The gold-ceria catalysts employed in this study were synthesized by the deposition precipitation with urea technique developed by Zanella et al.37 For each synthesis, a solution of 6.1x10-3 M HAuCl4 and 0.42 M urea in deionized water was prepared. The ceria support was suspended in this solution under magnetic stirring. The mixture was heated to 80 °C and aged for 16 h to deposit gold on the support surface. After deposition, the suspension was centrifuged, and the supernatant was discarded. The catalyst was washed three times with deionized water. The product was dried for ~24 h under vacuum at room temperature and then heated in air at 300 °C in a box furnace for 1 h. The resulting solid material was crushed and sieved to between 200 and 500 µm before use. 2.2. Characterization. BET surface area analysis was performed with a Quantachrome Instruments NOVA 2200e high-speed surface area BET analyzer at a temperature of 77 K. Before N2 physisorption, the samples were degassed at 100 °C overnight in a vacuum oven. Multiple data points in the pressure range of P/P0 = 0.1 to 0.3 were used to fit a line to the BET plot for each sample, and each fit achieved a correlation coefficient greater than 0.999.


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XRD was conducted with two different diffractometers to investigate the crystal structure of the support materials and supported gold catalysts. A Rigaku R-AXIS SPIDER diffractometer equipped with an image plate detector was used to collect XRD patterns used to determine the lattice parameter of each sample. Patterns from this instrument were collected with radiation from a Cu sealed tube Kα X-ray source operated at 40 kV and 40 mA. The sample was rotated at 10°/sec during X-ray exposure. Due to excessive instrumental peak broadening associated with patterns obtained from the SPIDER diffractometer, which complicated the determination of crystallite sizes by Williamson-Hall analysis, a Rigaku MiniFlex 600 diffractometer (which did not exhibit as much broadening but had a lower angular resolution) was used to collect patterns for determining crystallite sizes. Patterns from this instrument were collected with radiation from a Cu sealed tube Kα X-ray source operated at 40 kV and 15 mA with a step size of 0.01°/step. The wt % of gold incorporated into the catalyst samples for all materials and the mol % of additive incorporated into the samples made with mixed cerium oxides were determined by inductively coupled plasma-mass spectrometry (ICP-MS). A 10 mg aliquot of each material was digested in 10 mL of conc. HCl / 30% H2O2 solution in a 2/1 mixture. The digested samples were each diluted by a factor of 1000 with an aqueous solution containing 2% HCl (v/v) and 1% thiourea (w/v) prior to analysis. Cation concentrations in the diluted samples (Al, Co, Ni, Cu, Zr, La, Ce, Au, Pb, Bi) were determined using an Agilent 7500ce ICP-MS. The instrument was optimized for sensitivity across the AMU range, while minimizing oxide production (CeO/Ce

Heterogeneity in Mixed Cerium Oxides and Its Influence on the

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