We investigate the production of ammonia from nitrogen and hydrogen in a dielectric barrier discharge packed with glass spheres and magnesium oxide pellets close to atmospheric pressure. In particular, the influence of the discharge power and the gas composition is studied. The maximum ammonia production yields obtained is 4.9% conversion efficiency of H2 to NH3 and the maximum energy efficiency is 0.47 g/kWh. Ammonia synthesis from gas phase reactions only (without a catalyst) was also tested. We find that catalytic surface reactions dominate, allowing relatively high concentrations of ammonia to be produced despite the low electron energies present in a low-temperature non-equilibrium plasma.