Converting solar radiation into chemical energy is an attractive method to increase the energy content of methane by driving endothermic upgrading processes, such as methane steam reforming. Although such solar thermochemical conversions would increase natural gas product yields and reduce greenhouse gas emissions, current costs of solar facilities are prohibitively high. Parabolic troughs, which are a mature and relatively inexpensive form of solar concentrator, are limited to temperatures of ca. 400-600 degrees C. In this regime, commercial methane steam reforming Ni-based catalysts have low activity and rapidly deactivate due to oxidation and coking. Catalysts based on platinum group metals are much more active and stable but their high cost limits their use. Here, we investigate Ru-based catalysts with ultra-low metal loadings for low-temperature methane steam reforming. We show that a Ru/gamma-Al2O3 (0.15 wt% loading) catalyst outperforms the commercial (12 wt% Ni) catalyst by two orders of magnitude in terms of methane conversion normalized by metal loading. Systematic evaluation of catalytic performance over a range of Ru loadings and operating conditions shows a pronounced optimum in catalytic activity versus metal loading. Characterization studies reveal a strong correlation between the catalytic activity and the Ru nanoparticle size distribution. The catalyst features excellent stability at industrially relevant conditions and low steam-to-carbon ratios, making it an attractive option for low-temperature solar reforming of methane.(C) 2015 Elsevier B.V. All rights reserved.