**1. Introduction**

Approximately 95% of industrial hydrogen in the United States is currently produced through natural gas reforming [1,2]. The product of natural gas reforming is a mixture of H2, CO, and CO2, (referred to as syngas), which is a versatile and useful feedstock for producing a number of industrially relevant chemical commodities such as methanol, dimethyl ether [3], and Fischer-Tropsch products [4,5]. Natural gas reforming involves a series of reactions:

$$2\text{ }CH\_4 + H\_2O \leftrightarrow CO + 3H\_2\tag{1}$$

$$nC\_nH\_m + nH\_2O \to nCO + \left(\frac{m+2n}{2}\right)H\_2\tag{2}$$

$$CO + H\_2O \leftrightarrow CO\_2 + H\_2 \tag{3}$$

Reaction 1 is the steam methane reforming (SMR) reaction, which is endothermic and therefore requires high temperatures (e.g., 600–850 ◦C) for operation. Reaction 2 is the reaction for steam reforming of higher hydrocarbons (C2+). Both the SMR (Reaction 1) and water-gas-shift (Reaction 3) reactions are subject to thermodynamic equilibrium. Supported metal catalysts (e.g., Rh, Ru, and Ni metals supported on alumina or alumina spinel type formulations) are typically employed for steam reforming reactions [6,7].

Solar concentrators are a prospective method to provide the thermal requirements necessary to carry out the reforming process, with the added advantage of reduced carbon dioxide emissions since fossil fuel is not used to generate process heat [8–10]. Pacific Northwest National Laboratory has demonstrated an integrated solar thermochemical reaction system that combines solar concentrators with micro- and meso-channel reactors and heat exchangers that accomplish more than 20% solar augmen<sup>t</sup> of methane higher heating value. Further, a solar-to-chemical energy conversion e fficiency slightly over 70% has been achieved [8]. At PNNL, the solar concentrators being utilized include those that provide concentration ratios high enough to achieve the temperatures required for methane steam reforming. These include parabolic dish concentrators and central receivers [8]. PNNL solar reformers typically operate in the 700–800 ◦C temperature range—with some portions of the reactor falling to less than 700 ◦C near the entry point to the reaction channel. PNNL solar reformers start reacting at modestly high temperatures (>600 ◦C) and exit the catalyst zone at substantiality at higher temperatures [8]. Typical steam/carbon (S/C) molar feed ratios utilized are in the 2.5–3.0 range. We routinely achieve 90% or greater conversion in on-sun testing under these conditions [8]. Catalysts used in these highly efficient compact reactors must meet the following requirements:


Conventional Ni-based steam reforming catalysts have drawbacks pertaining to activity and durability that make them unsuitable for solar-driven applications. Noble metal-based catalysts, while generally more expensive than Ni-based catalysts, o ffer higher catalytic activities that enable faster throughput rates and smaller reactor hardware [11]. Furthermore, catalyst durability is improved, with reduced sintering and deactivation by coke when operating at the high temperatures required for typical operation [9]. Additionally, we note how, unlike with conventional fixed-bed reactor systems, with microreactors the use of more expensive precious metal catalysts may be made economical. Monolith-type substrates utilizing highly active catalysts are integrated in microreactors to minimize

heat- and mass-transfer resistances and maximize catalyst efficiency. The use of smaller, more efficient systems may compensate for the higher cost of the catalyst material [12].

SMR catalysts commonly employ Ni or Rh metals and use alumina or alumina spinel (e.g., MgAl2O4) type supports [13–15]. SMR is a structure-sensitive reaction in which turnover rates increase with decreasing metal particle size [16,17]. The support choice influences both the resulting metal particle size and catalyst stability [17,18]. Facilitating increased metal dispersion and enabling improved stability are among the reasons for using MgAl2O4 over other common supports (e.g., Al2O3, SiO2, and ZrO2). Noble metal based catalysts are known to be more resistant to carbon fouling and metal sintering compared to Ni [19–21]. We have previously reported MgAl2O4-supported Rh and Ir catalysts to have improved activity and stability compared to Ni and other metals (e.g., Pd, Pt) [20]. Catalysts with very well dispersed Rh (2 nm) and Ir (1 nm) metal clusters were obtained with the use of a MgAl2O4 support. High dispersion was maintained even after high temperature operation (e.g., 850 ◦C) and with the use of high metal loadings (e.g., 5–10 wt.%). Taken together, well dispersed Rh and Ir catalysts with high metal loadings were previously reported by our group to be highly active and stable for the SMR reaction.

While methane is the predominant compound in natural gas, natural gas also contains up to 20 vol.% higher C2+ constituents, with the exact composition depending on the source [22,23]. The activation of methane requires a higher temperature than required for activation of C2+ natural gas components [24]. Thus, C2+ hydrocarbons are typically converted more readily when compared to methane. However, C2+ hydrocarbons facilitate coke formation [25,26]. Regardless of the hydrocarbon used, upon reaction the metal and metal oxide support surfaces are populated by a variety of reactive species (e.g., C\*, H\*, CHx\*, CO\*, O\*, OH\*) [8,27–33]. While SMR has been studied extensively, relatively few studies have been dedicated to studying its reaction mechanism when higher hydrocarbons are present in the feed, which would be more representative of real feed mixtures [19,21,31,34,35]. The presence of higher hydrocarbons facilitate catalyst deactivation [25,26]. Previously, we investigated steam reforming of biomass gasifier-derived hydrocarbons, which includes tar (polyaromatic hydrocarbons) species. We evaluated steam reforming of benzene, as a model tar species, over MgAl2O4-supported Rh and Ir catalysts. The Rh catalyst was more active than Ir on a turnover basis due to differences in the C–C bond breaking step (which was found to be rate limiting) [19].

The objective of this study is to assess the catalytic performance of Rh-, Ir-, and Ni-supported catalysts for steam reforming of natural gas. Both Rh- and Ir-based catalysts are known to be more active and durable than conventional Ni-based formulations, and recently Ir has been reported to be more active than Rh for methane steam reforming on a turnover basis [20,36]. Thus, the effectiveness of all three metals to perform natural gas steam reforming was evaluated in this study. Steam reforming kinetic and mechanistic comparisons were elucidated using both discrete methane and ethane feeds. While the catalytic performance of various precious metal based catalysts (i.e., Rh, Ir, and Ni) have been widely studied for the methane steam reforming reaction, we are not aware of any studies focused on their comparison when under ethane steam reforming conditions. Finally, we comparatively assess both activity and stability when using a simulant natural gas feedstock that was a mixture of methane and C2+ hydrocarbons. We note that in order to investigate the kinetics of the catalysts we are operating at higher throughputs and lower conversions than what are typically utilized in solar thermochemical application.

#### **2. Results and Discussion**

The performance of MgAl2O4-supported Rh, Ir, and Ni catalysts was investigated for steam reforming of natural gas under industrially relevant conditions. The methods explored included the use of (1) simulated natural gas feedstock, (2) separate methane and ethane model feeds to elucidate contributions from each, (3) catalyst durability tests, and (4) evaluation over a wide temperature range. These conditions are all of particular interest for solar thermochemical applications where highly active and durable catalysts are required for a range of conditions.
