Elsevier

Journal of Catalysis

Volume 287, March 2012, Pages 37-43
Journal of Catalysis

Correlation of Pt–Re surface properties with reaction pathways for the aqueous-phase reforming of glycerol

https://doi.org/10.1016/j.jcat.2011.11.015Get rights and content

Abstract

The surface properties of Pt–Re catalytic nanoparticles supported on carbon following exposure to a hydrogen reducing environment and subsequent hydrothermal conditions have been studied using in situ X-ray photoelectron spectroscopy (XPS) and ammonia temperature-programmed desorption (TPD). These properties have been correlated with the catalyst selectivity for the aqueous-phase reforming of glycerol. We show that Pt in reduced Pt–Re/C becomes electron deficient, and a fraction of the Re becomes oxidized when the catalyst is subsequently exposed to hydrothermal reaction conditions. Oxidation of Pt–Re generates surface acidity, which drastically affects the reaction pathways. The acid site concentration, but not acid site strength, increases with Re loading. This acidity increase with Re addition favors C–O over C–C cleavage, which results in higher selectivity to liquid products and alkanes at the expense of hydrogen selectivity. We propose a model for the Pt–Re active site and the origin of acidity enhanced by the addition of Re.

Graphical abstract

The major reaction pathways for aqueous-phase reforming of glycerol show the dependence of product selectivity on C–C bond cleavage (metal site-based) vs. C–O bond cleavage (acid site-based).

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Highlights

► Addition of Re to Pt/C results in a change in product selectivity, consistent with an increase in C–O bond cleavage reactions. ► A Pt–Re alloy forms on initial catalyst reduction, but some Re oxidizes under aqueous-phase reforming conditions. ► Oxidation of Re results in generation of acidity, as verified by NH3 TPD. ► Acidity amount correlates with Re concentration, and infrared studies suggest formation of Brønsted acid sites.

Introduction

Production of hydrogen through the aqueous-phase reforming (APR) of biomass-derived oxygenated hydrocarbons (sugars, sugar alcohols, polyols, etc.) is considered a promising catalytic process and has attracted academic and industrial interest. Important advantages of the APR process include its low energy consumption, since APR proceeds at lower temperatures than conventional steam reforming, and its compatibility with the use of wet feedstocks, avoiding any predrying step [1]. A variety of catalyst formulations have been proposed and tested by several groups for the APR process [2], [3], [4], [5], [6], [7], [8], [9], [10]. Precious metals such as Pt are preferred catalysts due to their high selectivity in breaking C–C bonds [11]. The catalyst activity in the APR reaction can be enhanced by doping precious metals with other transition metals [9], [10], [12], [13]. For example, our recent work has shown that addition of Re to Pt/C substantially increases conversion of glycerol in the aqueous phase by at least one order of magnitude [14]. However, the role of Re and nature of active sites are still not fully understood.

APR of biomass-derived liquids normally involves reforming followed by water gas shift (WGS) of the CO produced. Literature reports indicate that Re addition to Pt-based catalysts supported on metal oxides increases activity for the WGS reaction. Two theories have been provided for the observed enhanced activity. The first is Pt–Re alloy formation [15], [16], [17], and the second involves the role of ReOx species [18], [19]. One effect of Pt–Re alloy formation that has been in debate is the strength of CO adsorption, with the idea that too strong CO chemisorption increases CO site coverage and decreases the availability of operating WGS sites. Therefore, weaker adsorption strength could result in higher activity. There have been both reports of stronger [15] and weaker CO adsorption [13], [20] compared to pure Pt. These contradictory reports could be due to particle size effects. Ruettinger et al. reported that CO adsorption is weaker on small Pt–Re particles and stronger on the larger Pt–Re particles [16]. An alternative theory has been proposed wherein ReOx species are present and provide a redox route for the WGS reaction, in which ReOx is reduced by CO, generating CO2, and re-oxidized by H2O, forming H2 [18].

Analogous to the examination of Re promotion of Pt catalysts for the WGS reaction, Kunkes and coworkers studied the structure of reduced Pt–Re supported on carbon for the glycerol steam reforming reaction. They proposed formation of a Pt–Re alloy phase or a surface having close contact between the Pt and Re phases and measured a lower CO binding energy compared with Pt alone, thus allowing more sites to be available for reforming [13], [21]. However, the lower CO binding energy was measured on partially oxidized Pt–Re using 2% O2 at 100 °C followed by reduction at 70 °C prior to the CO-TPD [13].

In this work, we provide characterization of Pt–Re under conditions similar to that encountered during APR in an effort to obtain an improved understanding of the role of Re. We specifically focus on investigation of the structure of Pt–Re/C catalysts under environments that simulate APR reaction conditions and on the correlation of catalyst structure to product distribution. XPS results suggest that hydrogen reduction results in the formation of bimetallic Pt–Re with Pt slightly positively charged. Under an environment similar to APR reaction conditions, XPS shows that the Re in the bimetallic is substantially oxidized. NH3 adsorption further reveals that the Pt–Re interaction following exposure to water vapor results in acidity generation that affects the reaction pathways. We show that the prevalence of the dehydration pathways, relative to decarbonylation (C–C bond cleavage), is related to the strength and number of surface acid sites. Surface acidity on bimetallic catalysts such as Pt–Re and Rh-Re has been recently reported [22], [23].

Section snippets

Materials and methods

The catalysts were prepared by sequential incipient wetness impregnation of tetra-amine platinum nitrate and perrhenic acid (Alfa Aesar). A high surface area activated carbon support (Engelhard, SSA 540 m2/g, pore volume 0.42 ml/g) was selected for this work. Specifically, for synthesis of the 3 wt%Pt/C catalyst, 20 g of the dried carbon support was impregnated with a solution prepared by dissolving 1.24 g of Pt(NH3)4(NO3)2 into 8.4 g of DI water in a 30-ml glass vial, with shaking for at least 2 h

APR of glycerol on Pt/C and Pt–Re/C

As reported previously [13], Pt/C and Pt–Re/C behave differently in aqueous-phase reforming of glycerol. Addition of Re to Pt significantly enhances conversion of glycerol as evidenced by more than 10 times increase in TOF of glycerol (based on CO chemisorption), as shown in Fig. 1. As also can be seen in Fig. 1, addition of Re to Pt changes the distribution of products. Specifically, the presence of Re increases the selectivity toward C2+ alkanes and alcohols (ethanol and propanol), along with

Conclusions

We have investigated the structure of Pt–Re/C catalysts with varying Re loadings following reduction and exposure to a simulated hydrothermal environment. When exposed to steam at elevated temperature, the Pt–Re catalyst undergoes oxidization. Our results suggest that the active site during the APR reaction is not the Pt–Re alloy but is more likely to be based on oxidized Pt–Re. The Pt–Re interaction under hydrothermal conditions results in local surface acidity. The number of acid sites is

Acknowledgements

The authors acknowledge financial support from the US Department of Energy, Office of Energy Efficiency and Renewable Energy. The XPS experiments were carried out at the Environmental and Molecular Sciences Laboratory, a user facility of the Department of Energy, of the Pacific Northwest National Laboratory.

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